October 10, 2023 • 43 mins
Daniel and Jorge break down a new study that looks for dark matter using both gravity and light.
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Speaker 1 (00:08):
Hey, Dannie, I'm curious how does a physicist make mental
images of some of the tricky things you do research on.
Speaker 2 (00:14):
You know, I'm not sure you really want to see
inside the brain of physicists. It's a bit of a mess.
Speaker 1 (00:20):
Well, let's find out what's your mental image of dark matter?
Speaker 2 (00:23):
Okay, that's a really tough one because it's invisible. I
guess I sort of imagine it like water, which is,
you know, mostly transparent, but you can definitely tell it's there.
Speaker 1 (00:33):
So we're all swimming in a bath of dark matter.
Speaker 2 (00:36):
I hope there's some dark matter rubber duckies out.
Speaker 1 (00:38):
There, some dark duckies. I wonder if they have a
cute song for that on Sesame Street.
Speaker 2 (00:42):
That would make dark matter bath time lots of fun.
Speaker 1 (00:45):
It's a special episode on the Letter D for dark Matter.
Speaker 3 (01:04):
Hi.
Speaker 1 (01:04):
I am Horam mccartoonist and the author of Oliver's Great
Big Universe. Hi.
Speaker 2 (01:08):
I'm Daniel. I'm a particle physicist and a professor you
see Irvine. And this episode is brought to you by
the taxpayers of California.
Speaker 1 (01:15):
Is it They're not paying me?
Speaker 2 (01:18):
This professor is brought to you by the taxpayers of California.
Speaker 1 (01:21):
I should say I thought our heart worth paying you.
Are you double dipping here, Daniel? Sounds like it. I
am multifaceted, your multi paid? Is that what you're saying
by multiple people? Sounds like a good setup there.
Speaker 2 (01:37):
I got no complaints, but.
Speaker 1 (01:38):
Anyways, welcome to our podcast, Daniel and Jorge Explain the Universe,
a production of iHeartRadio.
Speaker 2 (01:43):
In which we try to bring multiple streams of understanding
together into your brain. There's so many ways to understand
the universe, so many questions to ask about it, so
many things to understand, and the challenge of physics is
to weave all those together into one coherent, comprehend of
story that explains the universe to us and to you
(02:04):
and to our kids. And that is the goal of
this podcast.
Speaker 1 (02:07):
That's right, because it is a vast universe full of
all kinds of things that start with all kinds of letters,
the letter A, the letter B, the letter C, all
of the letters in the universe, even the ones we
haven't discovered.
Speaker 2 (02:17):
Yet, even letters in other alphabets. In particle physics, we
often reach to the Greek alphabet to name our particles.
Speaker 1 (02:24):
Is that why it's all Greek? To me.
Speaker 2 (02:27):
That's exactly right. And sometimes when we run out of
Greek letters, we dip into the Hebrew alphabet.
Speaker 1 (02:32):
Mmm. I thought you were going to say, we're gonna
you're gonna dip into alien alphabets, or one day we
might have variables that are only defined in an alien language.
Speaker 2 (02:40):
That sounds wonderful. I look forward to figuring out how
to write alien letters in word.
Speaker 1 (02:45):
You're going to need like another app like trio Linguo
or something.
Speaker 2 (02:50):
I'm sure this's gonna be a Unicode symbol for alien languages.
Speaker 1 (02:53):
Oh so, I mean they'll be alien emojis.
Speaker 2 (02:57):
What if the first message we get from aliens is
just in emojis that we need our kids to help.
Speaker 1 (03:02):
Us interpret it. Yeah, let's hope it's not like food
icons like hey, hey dinner, Hey hot dog, hamburger, pizza.
Speaker 2 (03:11):
I think the food emojis have other meanings sometimes, you know,
the eggplant for example.
Speaker 1 (03:15):
I don't know what you're talking about, Daniel. What kinds
of websites have you been surfing?
Speaker 2 (03:19):
Ask your kids about it later.
Speaker 1 (03:24):
I doubt it. But anyways, it is a big universe
full of amazing things to study and observe. If we
can actually see them.
Speaker 2 (03:33):
One of the challenges of understanding the universe is first
figuring out what's out there. We begin by using the
natural senses. We're all familiar with our eyes, our ears,
our noses, et cetera. But there's so much more out
there that we can detect with more subtle methods.
Speaker 1 (03:48):
Yeah, because there's a lot of stuff out there that
we can see, but also a lot of stuff we
can't see. The most amazing discoveries in recent decades has
been the idea that most of the universe is out there,
but we can't see it, or touch it or feel it.
Speaker 2 (04:02):
As we progress scientifically and technologically, we build new kinds
of eyeballs, new kinds of sensors, new kinds of ears
that let us detect things happening in the universe that
were otherwise invisible and impossible for us to notice. Sometimes
that means seeing entirely new signals like photons of very
high energy or low energy. Sometimes that means noticing patterns
(04:23):
in other signals, like the rotations of galaxies and gravitational
hints in the motions of other objects that tell us
there's so much more going on out there than we
could immediately see.
Speaker 1 (04:33):
Yep, and one of the biggest pieces of stuff out
there that we can see is called dark matter. It
accounts for about twenty seven percent of the universe, right, Daniel.
Speaker 2 (04:42):
That's right. In any given chunk of the universe, just
over a quarridor of the energy is devoted to dark matter,
and only five percent of it is made of the
kind of stuff that you and I are made out of, baryons, quarks, leptons,
this kind of stuff, which means that there's a lot
more of the invisible stuff out there than the visible stuf.
Speaker 1 (05:00):
Yeah, which can I make you wonder if maybe we're
the ones that are invisible.
Speaker 2 (05:03):
Maybe we needs learn those dark matter emojis pretty quickly,
but could.
Speaker 1 (05:07):
We see them? Maybe you are receiving dark matter emojis,
you just don't know it.
Speaker 2 (05:10):
I've kind of an outdated operating system, so a lot
of the emojis I get are question marks anyway. Maybe
those are the aliens trying to talk to me.
Speaker 1 (05:17):
Mmm, maybe they're actually sending you question marks. They're like, Daniel,
why are you getting paid so many ways for the
same thing?
Speaker 2 (05:25):
You know, I do get lots of questions from listeners,
so maybe some of those are actually coming from the aliens.
Speaker 1 (05:30):
Interesting, are you calling our listeners aliens?
Speaker 2 (05:33):
I'm saying we're inclusive, right, everybody is welcome. We try
to reach everybody, not just humans.
Speaker 1 (05:38):
Well, there you go. If you want to stump a physicists,
just send them a question in the form of emojis.
You know how they summarize movies sometimes with just emojis.
I wonder if you could do that with physics theory.
Speaker 2 (05:48):
What they summarize movies with just emojis?
Speaker 1 (05:52):
Yeah? Or books or stories or news items.
Speaker 2 (05:55):
Oh, I feel really out of touch.
Speaker 1 (05:57):
Speaking of out of touch, that's what dark matter is.
Stuff that's out there that you can't see and you
can't even touch, right, because it doesn't feel the electromagnetic force,
so you can feel it with your fingers.
Speaker 2 (06:08):
That's right. It doesn't emit light, give off light, reflect light,
or interact with light at all, which makes it pretty dark.
And if a huge chunk of the universe is so
dark but so important, then scientists really have to figure
out how to study, how to understand what it is.
So we're doing our best to be creative to find
new ways.
Speaker 1 (06:28):
To look for it so that we can study it
and kind of figure out what it's made out of.
So there are maybe new ideas out there about how
to do this. So today on the podcast, we'll be
tackling the question good dark matter be making flashes of light?
I always figured dark matter was pretty flashy.
Speaker 2 (06:52):
Well, I don't know. The name dark matter is more
mysterious than flashy, isn't it.
Speaker 1 (06:56):
Well, that's just what we'll call it. Maybe it's something
we can but really inside it's fancy and flashy.
Speaker 2 (07:03):
Maybe when we finally meet the beings made of dark matter,
they'll be like annoyed or offended or disappointed that we
call them dark matter.
Speaker 1 (07:10):
Hopefully they won't flash us.
Speaker 2 (07:12):
You mean like zap us with a laser being from
orbit or open the trench code.
Speaker 1 (07:15):
I mean like send this fruit emojis. So, as usually,
we were wondering how many people out there had thought
about the idea of seeing dark matter through flashes of light.
Speaker 2 (07:26):
Thank you very much to our group of volunteers who
answers these questions. We'd love hearing your thoughts on the
topic of the day. If you would like to contribute,
please don't be shy. Everybody is welcome, whether you've been
listening for years or weeks or days or this is
your first episode, just write to me too, questions at
Danielandjorge dot com.
Speaker 1 (07:44):
So think about it for a second. Do you think
dark matter could be making flashes of light that we
could see? Here's what people had to say.
Speaker 2 (07:51):
I don't know, but that'd be cool.
Speaker 3 (07:53):
The concept of dark matter making flashes of light is
quite interesting since dark matter seems to make up a
large part of the universe. I don't see why it
could be giving off light pulses given the right circumstances.
I think that only directly by making stuff made of
regular matter to behave in a certain way due to
(08:16):
gravitational effects, since it doesn't interact with the electra week force, I.
Speaker 1 (08:21):
Think not all right. I think that pretty much summarizes
the episode here. I don't know, but that would be cool.
I feel like that's al those every episode.
Speaker 2 (08:29):
That's the emoji version of the episode. Yeah, if you
have to summarize it, so that would be what shrug
question mark check mark is how you summarize the emojis.
Speaker 1 (08:37):
Yeah, or like a black square for dark matter question
mark trump, and then the emoji with the sunglasses.
Speaker 2 (08:44):
For cool yeah or sparkles. Isn't there a sparkles emoji.
Speaker 1 (08:47):
Well, depends how flashy you want to get.
Speaker 2 (08:49):
Daniel, let's go all out. We got multiple funding sources here.
Speaker 1 (08:52):
There you go. Let's go out with a flash of
jail time. But anyways, let's get down to it, Daniel,
let's recap for listeners. What is dark matter in the
first place.
Speaker 2 (09:02):
It's important that we explain what we mean when we
say dark matter, because I noticed there's lots of different
ideas out there about what dark matter is. Online you
see a lot of people saying dark matter is just
a placeholder. Is just a way to say we don't know.
Other people talk about dark matter as if it was
a very specific theory of a very specific particle. There's
a whole bunch of people in the middle to talk
about dark matter as a sort of general catalog of ideas.
(09:26):
But all of these things are there to explain something
that we don't understand, which is that there's a lot
of gravity happening in the universe that we cannot explain,
Like when you look at how galaxies spin, and when
you look at how the universe formed, and all the
gravity necessary to pull the stars together into galaxies. We
just cannot explain all that galaxy using the stuff that
(09:48):
lights up, using the stuff made of quarks. It either
glows or reflects light or gives off light. We just
cannot tell the story of the universe and have it
make sense without something else out there provid a bunch
of gravity. So much gravity is missing that you need
five times as much of this mysterious stuff we call
dark matter as there is normal matter. So very briefly,
(10:09):
dark matter is just an idea to explain all this
unexplained gravity in the universe.
Speaker 1 (10:14):
Yeah, and there are different ways that scientists have sort
of found or think that dark matter is there and there.
As you said, they all relate to gravity. But I
think basically the main idea is that what we see
of the universe tells us that there's more matter out
there than the stuff that glows or that we can
see and feel right exactly.
Speaker 2 (10:29):
And the story started with the galaxy rotation curves. We
looked at galaxies and measured how fast they spin, and
we notice that they spin really really fast, and that
if you add up all of the stars and the
gas and the dust in those galaxies, they don't provide
enough gravity to hold that galaxy together as it spins.
So that was evidence number one, and for decades we
knew about that, but it was sort of hard to
(10:51):
accept the idea that there could be so much more
missing matter out there. It was just one piece of evidence.
But slowly, over the decades we've pieced together lots of
totally independent measurements that tell us that there is missing
stuff out there, that there's matter out there providing gravity
that we cannot see.
Speaker 1 (11:10):
Like initially, for example, it could have just been that
galaxies out there had a lot of like dark rocks,
right or gas that you couldn't see through the telescope exactly.
Speaker 2 (11:19):
Or it could have been that gravity work differently over
really really long distances, like we've measured gravity in the
Solar System and on Earth, but maybe over hundreds of
thousands of light years gravity operates differently than Newton and
even Einstein suggested that could have been the explanation when
you were just looking at, one example, just at galaxy
(11:39):
rotation curves. But now we have lots of other ways
to probe this, you know, we look for example, at
the structure of the universe. How did it come together?
How did you go from blobs of gas mostly dispersed
through the universe clumping together into stars and galaxies, and
that requires gravity. And if you run the universe without
any dark matter, just with the kind of matter we
can see, you don't get stars and galaxies after fourteen
(12:02):
billion years. There isn't enough gravity to do it. You've
got to add in the dark matter and boom, then
you get a universe that looks just like ours. So
it's another very convincing piece of evidence that there is
matter out there. It's not just like gravity operates differently
that those distance scales. There really is missing matter.
Speaker 1 (12:19):
But could you ask the same question about these large
scale structure theories. Could it be just that gravity works
differently though we thought at different scales, and that might
explain why galaxies form the way they are.
Speaker 2 (12:29):
It is possible, and people who work on these theories
they're called like mond modified Newtonian dynamics, have tried to
tweak them. I've not seen one that can successfully explain
both the galaxy rotation curves and the large scale structure
of the universe. Mostly these theories are tuned to explain
the galaxy rotations, and they don't even try to explain
(12:50):
the other evidence for dark matter.
Speaker 1 (12:52):
What's this other evidence.
Speaker 2 (12:53):
Maybe one of the most compelling and precise piece of
evidence for dark matter is seeing its effect on the
very very early universe plasma. Before stars were formed, we
really had any structure in the universe at all. It
was just huge blobs of hot gas everywhere in the universe.
And we see the glow from that gas in the
cosmic microwave background radiation. When that gas cooled enough to
(13:14):
become transparent, when it formed neutral atoms, so photons mostly
could fly through it. Those photons are still around and
we see them, so they're like the last glow of
this plasma from the very early universe, and we see
ripples in that plasma, ripples that show us how the
normal matter and the dark matter and the photons were
all sloshing around. And it's a very very precise measurement,
(13:35):
and it tells us how much normal matter there is
in the universe, how much radiation there was in the universe,
how much dark matter there was in the universe back then.
It's very very precise because we've taken these very very
detailed maps of this early universe plasma, and that tells
us that there is a component of that plasma that
doesn't interact with photons, it's not buryonic, it's not our
kind of matter at all. So it's another completely independent
(13:58):
measurement that tells us there's a dark but gravitationally active
component to the universe.
Speaker 1 (14:03):
Now, is that also due to gravity? Does that evidence
also depend on our current model of gravity?
Speaker 2 (14:10):
It does, but those are much shorter distances. Those oscillations
ended up seeding the larger scale structure of the universe,
but it was before a lot of the expansion. So
we're talking about things that happened over short distance scales.
Speaker 1 (14:21):
There were a short distance but there are large distance now.
Speaker 2 (14:23):
That's right, But the CMB comes from when it was
short distance. Earlier, you were asking if this could all
just be due to like long distance gravitational effects, and
the CMB proves shorter distance scales gravity because the picture
we have of it comes from back when things were
much more cozy and compact.
Speaker 1 (14:39):
All right, So then the prevailing picture is that there's
a lot of stuff out there, stuff that creates gravity
and feels gravity, but we just can't see it. So
let's get into what it could be, how we might
see it, and what might be new ways to figure
out where it is and what it's doing. So we'll
dig into that, but first let's take a quick break.
(15:11):
All right, we're talking about dark matter now, Daniel. How
many episodes now have we talked about dark matter?
Speaker 2 (15:15):
Oh? Man, so many episodes. But it's still a thing
people are most confused and most interested about. In the
emails we get from people, people are still in the
dark about it. Yeah, And it's a really active area
of research, obviously, because it's one of the biggest open
questions in modern physics. So people are constantly trying to
come up with new theories for what dark matter might be.
(15:36):
Is it this particle, is it that particle, Is it
not a particle at all? Is it something else entirely?
And new ways to spot dark matter. If it's this
kind of particle, how could we see it? Could we
build a detector to observe it. So it's a huge
area of active research.
Speaker 3 (15:51):
Mm.
Speaker 1 (15:51):
Yeah, I guess we can't just wait for the Sesame
Street episode on it.
Speaker 2 (15:55):
We could just sit back and wait for the aliens
to show up and tell us the answers to the universe,
But that kind of embarrassing if they showed up and
we had nothing to contribute.
Speaker 1 (16:03):
Yeah, it'd be more embassing if they have to do
it over dinner while they eat us. I wonder if
they would feel less bad about eating us if they
knew we weren't. You know, dark matter intelligence, All right, Well,
we just kind of define what dark matter is. It's
the stuff we think that's out there to explain a
lot of the gravitational effects we see in how the
universe form and also in the microwave background radiation. Now
(16:25):
there are candidates where what we think this stuff might.
Speaker 2 (16:28):
Be, right Daniel, There are lots of ideas for what
dark matter might be, and people initially thought, well, maybe
dark matter isn't some new, weird kind of stuff. It's
just more of the kind of stuff we already know about.
But you know, it's just hiding or something. People thought,
maybe dark matter is like neutrinos, because neutrinos are tiny
little particles that don't interact with photons, they don't give
(16:49):
off light, they don't reflect light, they pass right through
a lot of stuff. So they seemed initially like a
really good candidate maybe for dark matter, Like maybe the
universe is just filled with accountable numbers of neutrinos that
would have been amazing. But neutrinos don't have a lot
of mass. They're very very low mass particles, which means
they almost always move nearly the speed of light. And
(17:11):
one thing we do know about dark matter is that
it's kind of slow moving. It's not a fast particle.
Speaker 1 (17:17):
Well, I guess that's Those are two questions. First of all,
why can't theatrinos go slow? Even if they're light? Can't
you still stop them from moving?
Speaker 2 (17:24):
You can stop a neutrino because they have very low mass,
but their mass is so low it's like thousands of
times lower than even an electron. That essentially any energy
in putrino has means it's moving at almost the speed
of light. So it's almost impossible to have a natural
process that produces a huge number of neutrinos and have
them be slow moving.
Speaker 1 (17:42):
And you also say dark matter is cold. How do
we know it is cold if we can't feel it.
Speaker 2 (17:45):
Yeah, we know that dark matter can be moving very
fast because of the way it's influenced the structure of
the universe, like, dark matter is mostly responsible for what
we see out there. The reason you have a galaxy
here or a galaxy there is because there's a huge
blob of dark matter that created gravitational attraction that pulled
all that gas in to form the stars and the galaxy. Now,
if dark matter was moving really really fast, if it
(18:08):
was hot, then it wouldn't clump the same way. It
would spread itself out much more. So, we know that
dark matter has to be below a certain speed basically,
or it wouldn't have clumped into these blobs, which then
formed the structure of the universe. Like you run simulations
where dark matter is a fast moving particle, you don't
get the same kind of structure that we see in
our universe. That means dark matter has to be slow
(18:29):
moving or cold, as physicists say, and that means that
it can't be neutrinos.
Speaker 1 (18:33):
Couldn't it be like super massive but hot too?
Speaker 2 (18:36):
Well, hot essentially refers to its velocity regardless of its mass.
The issue is the velocity, and the problem is neutrinos
basically can't have low velocity. We know that dark matter
can't be moving very fast, otherwise it would spread itself
out and wash out the structure of the universe.
Speaker 1 (18:50):
All right, well, then how is it that we can
see it? How can we hope to see it and
study it.
Speaker 2 (18:54):
Then yeah, it's tricky. You know. The one thing that
we do know is that dark matter feels gravity. Vitational
studies are surefire way to detect dark matter. The problem
is that gravity is super duper weak. It's like the
weakest of the fundamental forces. If it even is a
fundamental force, it's weaker than the weak force by like
ten to the thirty, which means that it's basically impossible
(19:16):
to use gravity to detect tiny bits of dark matter,
like we can see like Solar System sized chunks of
dark matter maybe, but anything smaller than that, the gravity
from it is too weak for us to even detect it.
And that means that you couldn't really use gravity to
detect the particle nature of dark matter. Like one of
the deepest questions is what is dark matter made out of?
(19:37):
Is that this kind of particle? Is that kind of particle?
But gravity is really too weak to tell us anything
about the particle nature of dark.
Speaker 1 (19:44):
Matter, and we don't even know if it is a particle, right,
it could be that dark matter is something that doesn't
form into particles. Is that possible.
Speaker 2 (19:51):
It's totally possible. It's not a mainstream idea, like most
of modern physics right now, is focused on the idea
of particle dark matter, because the kind of matter we
know is all made of particles, and so we extrapolate
and we say, well, probably this other kind of matter
is made out of particles. And you might think that
sounds reasonable. It's a pretty basic assumption, but it's also
extrapolating from five percent of the universe to like twenty
(20:13):
five percent of the universe. It's entirely possible that the
physics of this other huge section of the universe is
very different from anything we've imagined. We've talked in the
podcast before about unparticles or other weird kind of theories
that describe it as not made of particles. So like
as you zoom in, it doesn't ever change. It's not
like there's a basic unit of it. You can just
(20:33):
keep zooming in forever and it always looks the same.
That would be really weird but super awesome. But the
mainstream theories are mostly particle dark matter because that's what
we know how to.
Speaker 1 (20:43):
Think about, because that's kind of the only way to
think about things right.
Speaker 2 (20:47):
Well, you know, there are people trying to think outside
the box. That requires a lot more creativity and flashes
of insight. But there definitely are people out there working
on sort of crazier theories of what dark matter is.
But I think most people are working on article dark
matter because yeah, that's what we're good at thinking about.
I mean, you talk to a particle physicist, you're going
to get a particle explanation for everything.
Speaker 1 (21:07):
I guess. I mean, like, we haven't seen anything that
isn't explained by particle theory, right, and we wouldn't even
know what that math would look like.
Speaker 2 (21:16):
Yes and no, we've never seen anything that we've been
able to explain with a non particle based theory. But
remember there's ninety five percent of the universe dark matter
and dark energy that we still can't really explain. So
particle based theories are the only ones that have ever
been successful, but they've only been successful in five percent
of the universe.
Speaker 1 (21:34):
So yes and no, I guess there's a lot out
there that we can't still explain. So maybe our current
theory only covers five percent of the universe.
Speaker 2 (21:42):
Yeah, that's exactly right. So we should definitely keep an
open mind to other crazy theories of what dark matter
might be. But currently we're mostly working on. Is dark
matter a particle? Is it a new kind of particle?
What does it do? How could we possibly spot it
if it is a particle?
Speaker 1 (21:57):
All right, So if it is a particle, how do
we see it? How do we study it? We know
that it feels gravity, what else does it feel or
not feel?
Speaker 2 (22:04):
So the short answer is we have no idea. Like,
it could be that dark matter feels some new kind
of force with itself. It could be the dark matter
feels no forces other than gravity, and like that could
totally be our universe, in which case it's almost impossible
to ever discover the particle nature of dark matter. It
could just be a mystery forever because gravity is just
so weak. There's another possibility that dark matter is kind
(22:27):
of misnamed, that dark matter does actually interact with our
kind of matter through some mechanism we haven't discovered yet
that's not really truly one hundred percent.
Speaker 1 (22:37):
Dark, But I guess you mean using electromagnetic forces or
other kinds of forces, like it dark in terms of
light or dark in terms of all the forces in nature.
Speaker 2 (22:47):
We're imagining maybe there's a new force out there also,
So we're suggesting maybe dark matter is some new kind
of particle, right, And in addition, maybe there's a new
force out there, a force that helps dark matter particles
interact with our kind of particles. So call this a
new dark force or a portal to the dark sector
or whatever. If there is this new kind of force,
(23:08):
then maybe it helps dark matter particles bump into our
kind of particles, or turn into our kind of particles,
or somehow interact with our kind of particles, which would
make them effectively visible.
Speaker 1 (23:20):
But would you need to come up with a new
kind of force. Couldn't interact with us through the weak
force or the strong force somehow?
Speaker 2 (23:27):
It's totally possible that dark matter could have interacted with
us via one of the forces we know already, but
we've basically ruled that out with our experiments.
Speaker 1 (23:35):
Have we really is that official?
Speaker 2 (23:36):
If dark matter interacted with a strong force, it would
be a very powerful interaction because the strong force is
super duper strong, and that would actually be pretty easy
to find. We look for dark matter giving off photons.
We've never seen that, so we don't think that dark
matter actually directly interacts with photons. We've also looked for
dark matter interacting via the weak force. And this is
probably the biggest area of experimental dark matter particle physics
(23:59):
right now, with these huge tanks underground very quiet liquid
like xenon, and we wait for dark matter to pass
through the Earth and interact with one of these xenon
molecules and give it a little kick. So these huge
tanks of underground liquid xenon are just waiting for a
dark matter particle to bump into it via the weak force.
And if it does interact via the weak force, we
(24:20):
can calculate how often that should happen. And we've been
running these experiments for years and years and years, and
we've never seen a blip that looks like dark matter
kicking one of these xenon molecules via the weak force.
So now we can pretty definitely rule it out.
Speaker 1 (24:36):
I see, we've been putting stuff out there to hopefully
interact with dark matter through the weak force, but so
far nothing has been bumped that way. So now you're
saying the dark matter probably doesn't interact with the weak force.
Speaker 2 (24:48):
As time goes on and we don't see any of
those interactions, we more confidently say that they never happen.
If you only listen for a day. It might be
that they only happen once a year and you just
haven't seen one yet. But after you've listened for five years,
ten years, twenty years, either you're getting very very unlucky
or it just doesn't happen. And so now we've had
big enough detectors running for enough years that were pretty
confident in ruling that out. But it doesn't mean that
(25:11):
there isn't a new kind of force, like an even
weaker force, that would allow dark matter to interact with
our detectors. So that's what they're looking for now. Like
they call it the feeble force.
Speaker 1 (25:21):
It's weaker than weak. That's the idea, exactly weaker than weak.
We only we have seen that already with our detectors,
with the tanks of Xenon they're sitting there waiting.
Speaker 2 (25:31):
Depends on how weak it is. If it's super duper
extra week, if it's the feeblest force, you can imagine
it might take a very very long time. It might
be so unlikely they have to run for ten years
or one hundred years in order to see it. And
that's why we're also at the same time using other
methods to try to look for these kinds of interactions,
like we're hoping to use this new feeble force to
(25:51):
create dark matter at particle colliders.
Speaker 1 (25:54):
What do you mean, like when you collide protons or
quarks like that, it might make dark matter.
Speaker 2 (25:59):
Yeah, if this feeble force exists that allows dark matter
to bump into protons and neutrons inside a xenon atom,
then in principle you can reverse that. You can say, well,
what if we smash protons together, maybe sometimes they can
use the feeble force to create dark matter. Because that's
what we do with the particle collider. We annihilate protons
together and create new kinds of stuff. And the cool
(26:22):
thing about a collider is that anything that's out there,
you can make it as long as your protons can
interact with it. So anything that protons interact with we
produce at the collider eventually. Things that interact with protons
a lot, we produce them all the time. Things that
don't interact with protons very often, we produce them more rarely,
So higgs bosons are pretty rare, for example. But we
comb through all of those collisions looking for evidence of
(26:43):
the production of dark matter.
Speaker 1 (26:44):
Wouldn't we have seen evidence of that already? I mean,
you've been running the LHC for a long time and
particle colliders for decades. You know, if there was some
sort of unaccounted for force, wouldn't we have seen it
by now?
Speaker 2 (26:56):
We haven't seen anything. You're right, And again it's a
question to the of that force. If that force was
as strong as the weak force, yeah, we probably would
have seen it. And the longer we run our colliders
and don't see it, the weaker that force has to
be to still be consistent with our data, to still
be hiding from all of these experiments. But we don't
know how weak that force is. Maybe it's really ridiculously weak,
(27:18):
so weak that we haven't seen it underground and we
haven't seen it in our colliders. So we actually turn
to another mechanism to try to see this super duper
weak force in action, which is to look at the
center of the galaxy to see if dark matter is
smashing into itself.
Speaker 1 (27:32):
I see use the center of the galaxy as a
particle collider.
Speaker 2 (27:36):
Exactly because one problem with the particle collider is that
it's not very dense. Right. We have protons smashing into protons,
but it's like very few protons and we try to
run it as often as we can, and it adds
up to zillions of collisions, but it just might not
be enough. But we think that the center of the
galaxy is very very dense with dark matter. We can
(27:56):
map out where the dark matter is in the galaxy
by looking at how things rouse and how fast things
are moving, and we suspect that the center of the
galaxy is very dense with dark matter. And so if
dark matter has this feeble force, this super weak force,
then occasionally two dark matter particles should bump into each
other and produce normal matter particles, like the opposite of
(28:17):
what we think might happen in the collider, which two
protons smash together to make dark matter run that backwards
in time. We hope that's the operation happening in the
center of the galaxy. So we turn these special telescopes
to the wards the center of the galaxy and look
for these characteristic flashes of light that might come from
those collisions.
Speaker 1 (28:35):
Interesting, that sounds a little cheaper than building a thirty
billion dollar collider here.
Speaker 2 (28:40):
Well, it was only ten billion dollars, right, so we
can save you twenty billion off the top right there,
and it wasn't that cheap because you're building a particle
detector and launching it into space, which is never simple.
Speaker 1 (28:51):
All right, Well, let's get into the details of how
we're using the center of the galaxy as a collider
to look for dark matter. Let's dig into that, but
first let's take another quick break. All right, we're looking
(29:13):
for dark matter. I had some right here, Daniel. You
don't know where I put it.
Speaker 2 (29:16):
I can give you emoji based directions to it, Derry,
I'll send you some arrows.
Speaker 1 (29:21):
That might not be helpful there. But yeah, there's a
lot of matter missing in the universe. All twenty seven
percent of the universe is out there, but it's invisible.
We can't see it or touch it called dark matter,
and our only hope for ever studying its particle nature
is that there's some sort of new force that we
haven't discovered yet, some super weak force that you're calling
(29:42):
the feeble force. Although Daniel, I'm kind of disappointing you
didn't call it the dark force.
Speaker 2 (29:48):
Because then forever it would dominate my destiny exactly right.
Speaker 1 (29:52):
Then you get the cool red lightsabers or the cool
laser pointers when you give your lectures.
Speaker 2 (29:57):
Yeah, that's true, but I didn't want to have to
grow those horns.
Speaker 1 (30:00):
Sith have horns.
Speaker 2 (30:01):
That's relief.
Speaker 1 (30:03):
You need to brush up on your Star Wars there apparently. Yeah,
but yeah. Our only hope forever studying the particle nature
of dark matter is that it feels a new kind
of force we haven't seen before, and we can't generate
that force apparently here in our colliders. But there's hope
that maybe you can use the center of the galaxy
as a collider to maybe study this part of this
aspect of dark matter we think we're hoping is there.
Speaker 2 (30:26):
Exactly, And the idea is dark matter smashes into itself
and then via this new feeble force, turns into standard
model particles, maybe tau leptons or be quarks or something,
and those particles can then give you flashes of light
because those particles do interact with light. So what we
do is we turn this telescope to the center of
the galaxy and we look for these flashes of light
(30:48):
that we can't otherwise explain. If you see flashes of
light that look like they come from dark matter turning
into particles, we know, then you can say, oh, there's
evidence for dark matter. There.
Speaker 1 (30:58):
Wait, the idea is that dark matter smashes into itself.
Why does it have to smash into itself.
Speaker 2 (31:05):
It doesn't have to. It's possible for dark matter also
interact with normal matter particles in the sension of the galaxy,
the way it might interact, for example, with our tanks
of Xenon underground. But the signature we're looking for would
come from two dark matter particles smashing into itself, producing
like a pair of bottom quarks or a pair of
tau leptons, which then give off some photons which travel
(31:25):
to our telescope and we observe them.
Speaker 1 (31:27):
Wait, so two dark matter particles can interact with each
other and generate regular kind of matter? Yeah, exactly does
that work the other way too? Like, can regular matter
interact and come up with dark matter?
Speaker 2 (31:39):
That's what we're trying to do with the Large Hadron
collider is smash regular matter together two protons and turn
it into dark matter. We haven't seen that yet, so
we're trying to do the opposite, smash dark matter together
and turn it into regular matter. It's like the inverse collider.
Speaker 1 (31:52):
What's the inverse of a collider? An expander as a separator.
Speaker 2 (31:56):
It just runs it the other direction, you know, instead
of colliding normal matter to make dark matter, and we
collide dark matter to make normal matter.
Speaker 1 (32:02):
So the picture is that in the center of the galaxy,
you're saying, there's a high density of dark matter, and
some dark matter particle just happens to smash head on
with another dark matter particle. Hopefully they're going fast enough
to actually sort of interact and create a lot of energy.
And then out of that comes a regular matter particle
from the center of the galaxy that then we see
(32:24):
here on Earth and go, yep, that came from two
dark matter particles.
Speaker 2 (32:28):
Yeah, And if you want the microparticle physics explanation, two
dark matter particles smashed together form some new kind of
particle that mediates this feeble force, call it a dark
photon or something. And then that dark photon turns into
a pair of normal particles like two bottom corks or
two tau leptons or two muons or something, and then
(32:49):
those produce photons which then we see here.
Speaker 1 (32:51):
But then how can we hope to like discern that
from all the way here the center of the galaxy is,
you know, fifty thousand light years away.
Speaker 2 (32:59):
It's very very tricky, and that's exactly the difficulty of
this method. Number one. It's far away. We think that
these photons would be very high energy, so we think
that gamma rays would survive travel from the center of
the galaxy. But the real problem is that we don't
understand the center of the galaxy. So a lot of
stuff going on there, making flashes of light anyway that
we can't really understand. And so actually we have seen
(33:23):
flashes of light from the center of the galaxy that
we cannot explain that No astrophysical explanation has been provided
to explain these flashes of light, and so some people
are actually pretty excited about that. They say, whoa, maybe
this is dark matter. But you know, the problem is,
we don't really know what's in there in the center
of the galaxy. It's a very weird, dense region. We
just did a whole episode about how weird it is
(33:44):
and how little we understand about what's there. So it
could just be new, weird stuff made of normal matter
in the center of the galaxy flashing lights and ways
we don't understand, or it could be dark matter creating
these flashes of light.
Speaker 1 (33:56):
Wait, wait, wait, what do these flashes look like? Like
You're just looking at at the galaxy center of the
galaxy and suddenly there's like a spike or a pulse
or a train of pulses. What do these flashes look like?
Speaker 2 (34:08):
So these flashes come as individual photons. So this telescope,
a Fermila telescope, can see gamma ray photons, which are
just photons in a certain energy range in this case
from fifty million electron volts up to a trillion electron volts.
And they see a bunch of these gamma ray photons
from the center of the galaxy, and we cannot explain.
Speaker 1 (34:27):
Them otherwise, like one at a time, Well, the.
Speaker 2 (34:29):
Detector can only see one photon at a time.
Speaker 1 (34:32):
Yeah, I guess. I mean, like you see one and
then you don't see one for another year. Or is
it do you see like a whole bunch of them
in a cluster or something.
Speaker 2 (34:38):
So the higher energies are definitely more rare than they
are at the lower energies, but there's a lot of them.
It's not just like one or two or seven photons.
You know, we're talking about thousands of photons here, accumulated
over many, many years and so people are trying to
understand where are these photons coming from. Is there something
going on with normal matter at the center of the
galaxy or is this actually dark matter?
Speaker 1 (34:58):
But I guess, why are they eplainable or why are
they so mysterious. Isn't it just like, oh, here's a
photo with a lot of energy.
Speaker 2 (35:05):
Well, we have an idea for what's in the center
of the galaxy. We think that there's a black hole there,
and we think there's a swirl of stuff around the
black hole, and we think there's a certain density of stars.
We think there's gas and dust and all sorts of stuff,
and we use that model to predict what we should
see from the center of the galaxy, and we see
all that stuff, plus we see more. We see another
spectrum of photons that have sort of like a different shape,
(35:27):
like they emit light in different patterns. They're distributed around
the center of the galaxy, and their brightness and their
wavelengths can't be explained by any of the components that
we do know about in the center of the galaxy.
So it's definitely something new happening in the center of
the galaxy emitting photons in a way that we can't
otherwise explain, and it's consistent with some theories of dark matter.
Speaker 1 (35:48):
All right, so well, let's dig into that part of it. Then,
what's the connection between gravity and or what might be
the connection between gravity and these flashes of light.
Speaker 2 (35:56):
Yes, so we've been seeing these flashes for a while,
and there's been theories of dark matter for decades from
the center of the galaxy, but people have been unsure
about what it means because we don't understand what's going
on at the center of the galaxy very well. So
recently people came up with a really cool clever idea
to combine gravitational information with these flashes of light. They say, look,
(36:17):
if there is dark matter creating these flashes of light,
we should be able to zero in on where these
flashes come from and see if there's dark matter there.
By seeing if there's a gravitational effect from that dark matter,
Like if there's a blob of dark matter there that's
extra dense and making these flashes, we should be able
to see gravitational lensing from that dark matter blob to
tell us, oh, that really is dark matter and not
(36:39):
just some star going crazy.
Speaker 1 (36:41):
Couldn't it just be a star going crazy or maybe
like a quasar or something like that. Doesn't that seem
more likely? But like, how do you know which is
more likely?
Speaker 2 (36:49):
Exactly? We don't really understand what's there, so it is tricky.
But the first thing they did is they tried to
map all the gravitational lensing near the center of the
galaxy and measure that by seeing like how that does
or it's light from behind the galaxy, Like galaxies in
the background far away, how is their light being distorted
as it passes through the center of the galaxy. So
that's the gravitational lensing measurement. They use that as a
(37:11):
way to just see like are there pockets of extra
gravity in the center of the galaxy. So they make
a map of all this gravitational lensing and then they
line that up with the map of these extra flashes
from the center of the galaxy, and boom, boom, boom,
they do line up. They line up very nicely. So
there seems to be this correlation between where there's extra
(37:32):
gravitational lensing and where there's more gamma ray flashes.
Speaker 1 (37:36):
That's pretty interesting, although it could just be something else.
I wonder, like, you know where maybe where there's a
lot of dark matter. There are also a lot of
black holes.
Speaker 2 (37:45):
Exactly, and everybody's very skeptical at first, right, you don't
want to just like jump up and down and claim
you discover dark matter. So then they try to explain
this with other sources, you know, And one idea they
have in this paper is that maybe it's blazars. Blazars
are these super awesome galaxies that are generating a really
powerful beam of light from the center of the galaxy.
If you have a super massive black hole the core
(38:06):
of a galaxy, then its magnetic field can funnel radiation
up and down the north and south pole of that
galaxy and produce a very powerful beam of light. And
if that beam of light is pointed right at the Earth,
the relativistic effects effectively make the Earth a little shorter
and pile up the light beams a little bit to
make it extra bright. So these things are called blazars,
(38:27):
and blazars could also explain these effects that we're seeing.
And so in the paper they analyze like, well, is
this just more blazars than we anticipated, or is it
dark matter? And they can explain part of this effect
with blazars, but there's a part of it that blazars
cannot explain, and that is very nicely explained by dark matter.
And so this is the challenge with studying dark matter.
(38:49):
It's like, is what we're seeing dark matter or is
it something else we don't understand because dark matter is
still kind of a fuzzy idea.
Speaker 1 (38:56):
M yeah, I mean we don't really know or have
a clear idea what it's nature is. I feel like
we're sort of like putting assumptions on top of assumptions
on top of assumptions and then checking that.
Speaker 2 (39:07):
That is what we're doing. We're making hypotheses and we're
checking them, and we're coming up with alternatives, like is
this dark matter? Is this something else? What we know
is that there is something new happening at the center
of the galaxy. Something is generating these flashes of light
that we do not understand. Maybe it's dark matter, maybe
it's some other new astrophysical source. Maybe it's some new
process that's distorting light from behind the galaxy. We don't know.
(39:29):
There's something new to be discovered there. We don't know
if that lines up with these other mysteries we're seeing,
like galactic rotation, curves and the structure of the galaxy.
So maybe this reveals something about dark matter, but maybe
it reveals something about something else. Right, we're so clueless
about the nature of the universe that we're trying to
put these puzzle pieces together. But maybe they don't click.
Maybe they're parts of different puzzles. It's just because we're
(39:49):
at the beginning of understanding the universe that we're so
clueless about how to put this all together.
Speaker 1 (39:54):
Yeah, I guess it could be anything. Could be alien
sending us emojis. Maybe those high speed photons are there
version of.
Speaker 2 (40:01):
Emojis exactly, Maybe there are express emojis, hopefully they're safer work.
This is a very difficult kind of science because the
data is sparse. You just have like these flashes of
life from the center of the galaxy, and we're very
far away from the center, as you said. So I've
been to like whole conferences dedicated just to this signal
and understanding it, and people show completely different interpretations. They say, oh,
(40:23):
I can explain all of it using some astrophysical source,
and other people say, no, that's impossible, it can't describe it.
And other people say, well, you forgot to account for
all these uncertainties in that and if you let these
things float and consider other possible ways that these things
get interact, then maybe it explains it. So we really
are at the beginning days of understanding what it is
we're seeing. But it's cool to see people try to
line these things up, you know, gravity and flashes of
(40:45):
light and other ways to try to get ideas for
what dark matter might be.
Speaker 1 (40:49):
All right, well, it sounds like it's still kind of
an open question and a big mystery, but it is
a pretty exciting idea to kind of like use one
mystery to try to explain another mystery. It's like you're
piling on the mysteries.
Speaker 2 (41:00):
We are piling on the mysteries, and we're trying to
line things up. You know, we're trying to come up
with a coherent explanation for everything we see out there
in the universe. It's not fair to have one story
you tell in one case and a completely different story
you tell in another case. Right, you need a single
theory that explains everything we see. And that's really a challenge,
especially because the data is not as good as we'd
like it to be. You know, we'd like to have
(41:22):
these sensors at the center of the galaxy probing these
things in detail with very fine spatial resolution. But instead
we're limited to these like tiny cameras orbiting very far
from the center of the galaxy trying to get a glimpse.
Like imagine you were trying to understand what was happening
in Manhattan and all you had was like a camera
in Indiana. It'd be pretty tricky.
Speaker 1 (41:40):
Yeah, especially with all those buildings, be hard to kind
of see what's going on between there. So it sounds
like maybe we just need to send a pro to
the center of the galaxy right figure out what's there,
you know, and just wait fifty thousand years or so.
Speaker 2 (41:53):
Yeah, some poor graduate student is going to wait fifty
thousand years to defend their thesis.
Speaker 1 (41:58):
I'll get paid for that. I could get paid for
that while I'm doing this podcast.
Speaker 2 (42:02):
Sounds good.
Speaker 1 (42:02):
I mean, as long as we're double dipping.
Speaker 2 (42:04):
Are your checks going to come from the sense of
the galaxy because you might be waiting a while?
Speaker 1 (42:08):
Yeah, no, no, no, they're going to come in a dollar bill.
Emojis the magically transform into cash into my Venmo or the.
Speaker 2 (42:16):
New x app quantum Cash entanglement.
Speaker 1 (42:18):
There you go. I'll be rich and not rich at
the same time.
Speaker 2 (42:22):
I hope your finances don't collapse.
Speaker 1 (42:24):
All right, Well, another reminder that there are still humongous
mysteries out there in the universe, and that there are
scientists out there actively searching for the answers and searching
for it, trying to find ways to study it and
reveal to the rest of us what its true nature is.
Speaker 2 (42:39):
So remember that dark matter is really a whole suite
of ideas. It's not just like a blank check placeholder
for things we don't understand. It's an empty chalkboard for
us to fill in with details, and scientists are constantly
coming up with ideas for what dark matter might be
and then being creative about how we might discover it
because we hope one day to fill that chalkboard in
with some thing we really do understand.
Speaker 1 (43:01):
Does that mean you have to call it dark matters
or dark's matter? These are the types of questions that
I get paid to the thing about from multiple people.
Speaker 2 (43:11):
That sounds good. Sounds like you are qualified to answer.
Speaker 1 (43:13):
That question, as are we all all right, well, we
hope you enjoyed that. Thanks for joining us. See you
next time.
Speaker 2 (43:28):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hey, Dannie, I'm curious how does a physicist make mental
images of some of the tricky things you do research on.
Speaker 2 (00:14):
You know, I'm not sure you really want to see
inside the brain of physicists. It's a bit of a mess.
Speaker 1 (00:20):
Well, let's find out what's your mental image of dark matter?
Speaker 2 (00:23):
Okay, that's a really tough one because it's invisible. I
guess I sort of imagine it like water, which is,
you know, mostly transparent, but you can definitely tell it's there.
Speaker 1 (00:33):
So we're all swimming in a bath of dark matter.
Speaker 2 (00:36):
I hope there's some dark matter rubber duckies out.
Speaker 1 (00:38):
There, some dark duckies. I wonder if they have a
cute song for that on Sesame Street.
Speaker 2 (00:42):
That would make dark matter bath time lots of fun.
Speaker 1 (00:45):
It's a special episode on the Letter D for dark Matter.
Speaker 3 (01:04):
Hi.
Speaker 1 (01:04):
I am Horam mccartoonist and the author of Oliver's Great
Big Universe. Hi.
Speaker 2 (01:08):
I'm Daniel. I'm a particle physicist and a professor you
see Irvine. And this episode is brought to you by
the taxpayers of California.
Speaker 1 (01:15):
Is it They're not paying me?
Speaker 2 (01:18):
This professor is brought to you by the taxpayers of California.
Speaker 1 (01:21):
I should say I thought our heart worth paying you.
Are you double dipping here, Daniel? Sounds like it. I
am multifaceted, your multi paid? Is that what you're saying
by multiple people? Sounds like a good setup there.
Speaker 2 (01:37):
I got no complaints, but.
Speaker 1 (01:38):
Anyways, welcome to our podcast, Daniel and Jorge Explain the Universe,
a production of iHeartRadio.
Speaker 2 (01:43):
In which we try to bring multiple streams of understanding
together into your brain. There's so many ways to understand
the universe, so many questions to ask about it, so
many things to understand, and the challenge of physics is
to weave all those together into one coherent, comprehend of
story that explains the universe to us and to you
(02:04):
and to our kids. And that is the goal of
this podcast.
Speaker 1 (02:07):
That's right, because it is a vast universe full of
all kinds of things that start with all kinds of letters,
the letter A, the letter B, the letter C, all
of the letters in the universe, even the ones we
haven't discovered.
Speaker 2 (02:17):
Yet, even letters in other alphabets. In particle physics, we
often reach to the Greek alphabet to name our particles.
Speaker 1 (02:24):
Is that why it's all Greek? To me.
Speaker 2 (02:27):
That's exactly right. And sometimes when we run out of
Greek letters, we dip into the Hebrew alphabet.
Speaker 1 (02:32):
Mmm. I thought you were going to say, we're gonna
you're gonna dip into alien alphabets, or one day we
might have variables that are only defined in an alien language.
Speaker 2 (02:40):
That sounds wonderful. I look forward to figuring out how
to write alien letters in word.
Speaker 1 (02:45):
You're going to need like another app like trio Linguo
or something.
Speaker 2 (02:50):
I'm sure this's gonna be a Unicode symbol for alien languages.
Speaker 1 (02:53):
Oh so, I mean they'll be alien emojis.
Speaker 2 (02:57):
What if the first message we get from aliens is
just in emojis that we need our kids to help.
Speaker 1 (03:02):
Us interpret it. Yeah, let's hope it's not like food
icons like hey, hey dinner, Hey hot dog, hamburger, pizza.
Speaker 2 (03:11):
I think the food emojis have other meanings sometimes, you know,
the eggplant for example.
Speaker 1 (03:15):
I don't know what you're talking about, Daniel. What kinds
of websites have you been surfing?
Speaker 2 (03:19):
Ask your kids about it later.
Speaker 1 (03:24):
I doubt it. But anyways, it is a big universe
full of amazing things to study and observe. If we
can actually see them.
Speaker 2 (03:33):
One of the challenges of understanding the universe is first
figuring out what's out there. We begin by using the
natural senses. We're all familiar with our eyes, our ears,
our noses, et cetera. But there's so much more out
there that we can detect with more subtle methods.
Speaker 1 (03:48):
Yeah, because there's a lot of stuff out there that
we can see, but also a lot of stuff we
can't see. The most amazing discoveries in recent decades has
been the idea that most of the universe is out there,
but we can't see it, or touch it or feel it.
Speaker 2 (04:02):
As we progress scientifically and technologically, we build new kinds
of eyeballs, new kinds of sensors, new kinds of ears
that let us detect things happening in the universe that
were otherwise invisible and impossible for us to notice. Sometimes
that means seeing entirely new signals like photons of very
high energy or low energy. Sometimes that means noticing patterns
(04:23):
in other signals, like the rotations of galaxies and gravitational
hints in the motions of other objects that tell us
there's so much more going on out there than we
could immediately see.
Speaker 1 (04:33):
Yep, and one of the biggest pieces of stuff out
there that we can see is called dark matter. It
accounts for about twenty seven percent of the universe, right, Daniel.
Speaker 2 (04:42):
That's right. In any given chunk of the universe, just
over a quarridor of the energy is devoted to dark matter,
and only five percent of it is made of the
kind of stuff that you and I are made out of, baryons, quarks, leptons,
this kind of stuff, which means that there's a lot
more of the invisible stuff out there than the visible stuf.
Speaker 1 (05:00):
Yeah, which can I make you wonder if maybe we're
the ones that are invisible.
Speaker 2 (05:03):
Maybe we needs learn those dark matter emojis pretty quickly,
but could.
Speaker 1 (05:07):
We see them? Maybe you are receiving dark matter emojis,
you just don't know it.
Speaker 2 (05:10):
I've kind of an outdated operating system, so a lot
of the emojis I get are question marks anyway. Maybe
those are the aliens trying to talk to me.
Speaker 1 (05:17):
Mmm, maybe they're actually sending you question marks. They're like, Daniel,
why are you getting paid so many ways for the
same thing?
Speaker 2 (05:25):
You know, I do get lots of questions from listeners,
so maybe some of those are actually coming from the aliens.
Speaker 1 (05:30):
Interesting, are you calling our listeners aliens?
Speaker 2 (05:33):
I'm saying we're inclusive, right, everybody is welcome. We try
to reach everybody, not just humans.
Speaker 1 (05:38):
Well, there you go. If you want to stump a physicists,
just send them a question in the form of emojis.
You know how they summarize movies sometimes with just emojis.
I wonder if you could do that with physics theory.
Speaker 2 (05:48):
What they summarize movies with just emojis?
Speaker 1 (05:52):
Yeah? Or books or stories or news items.
Speaker 2 (05:55):
Oh, I feel really out of touch.
Speaker 1 (05:57):
Speaking of out of touch, that's what dark matter is.
Stuff that's out there that you can't see and you
can't even touch, right, because it doesn't feel the electromagnetic force,
so you can feel it with your fingers.
Speaker 2 (06:08):
That's right. It doesn't emit light, give off light, reflect light,
or interact with light at all, which makes it pretty dark.
And if a huge chunk of the universe is so
dark but so important, then scientists really have to figure
out how to study, how to understand what it is.
So we're doing our best to be creative to find
new ways.
Speaker 1 (06:28):
To look for it so that we can study it
and kind of figure out what it's made out of.
So there are maybe new ideas out there about how
to do this. So today on the podcast, we'll be
tackling the question good dark matter be making flashes of light?
I always figured dark matter was pretty flashy.
Speaker 2 (06:52):
Well, I don't know. The name dark matter is more
mysterious than flashy, isn't it.
Speaker 1 (06:56):
Well, that's just what we'll call it. Maybe it's something
we can but really inside it's fancy and flashy.
Speaker 2 (07:03):
Maybe when we finally meet the beings made of dark matter,
they'll be like annoyed or offended or disappointed that we
call them dark matter.
Speaker 1 (07:10):
Hopefully they won't flash us.
Speaker 2 (07:12):
You mean like zap us with a laser being from
orbit or open the trench code.
Speaker 1 (07:15):
I mean like send this fruit emojis. So, as usually,
we were wondering how many people out there had thought
about the idea of seeing dark matter through flashes of light.
Speaker 2 (07:26):
Thank you very much to our group of volunteers who
answers these questions. We'd love hearing your thoughts on the
topic of the day. If you would like to contribute,
please don't be shy. Everybody is welcome, whether you've been
listening for years or weeks or days or this is
your first episode, just write to me too, questions at
Danielandjorge dot com.
Speaker 1 (07:44):
So think about it for a second. Do you think
dark matter could be making flashes of light that we
could see? Here's what people had to say.
Speaker 2 (07:51):
I don't know, but that'd be cool.
Speaker 3 (07:53):
The concept of dark matter making flashes of light is
quite interesting since dark matter seems to make up a
large part of the universe. I don't see why it
could be giving off light pulses given the right circumstances.
I think that only directly by making stuff made of
regular matter to behave in a certain way due to
(08:16):
gravitational effects, since it doesn't interact with the electra week force, I.
Speaker 1 (08:21):
Think not all right. I think that pretty much summarizes
the episode here. I don't know, but that would be cool.
I feel like that's al those every episode.
Speaker 2 (08:29):
That's the emoji version of the episode. Yeah, if you
have to summarize it, so that would be what shrug
question mark check mark is how you summarize the emojis.
Speaker 1 (08:37):
Yeah, or like a black square for dark matter question
mark trump, and then the emoji with the sunglasses.
Speaker 2 (08:44):
For cool yeah or sparkles. Isn't there a sparkles emoji.
Speaker 1 (08:47):
Well, depends how flashy you want to get.
Speaker 2 (08:49):
Daniel, let's go all out. We got multiple funding sources here.
Speaker 1 (08:52):
There you go. Let's go out with a flash of
jail time. But anyways, let's get down to it, Daniel,
let's recap for listeners. What is dark matter in the
first place.
Speaker 2 (09:02):
It's important that we explain what we mean when we
say dark matter, because I noticed there's lots of different
ideas out there about what dark matter is. Online you
see a lot of people saying dark matter is just
a placeholder. Is just a way to say we don't know.
Other people talk about dark matter as if it was
a very specific theory of a very specific particle. There's
a whole bunch of people in the middle to talk
about dark matter as a sort of general catalog of ideas.
(09:26):
But all of these things are there to explain something
that we don't understand, which is that there's a lot
of gravity happening in the universe that we cannot explain,
Like when you look at how galaxies spin, and when
you look at how the universe formed, and all the
gravity necessary to pull the stars together into galaxies. We
just cannot explain all that galaxy using the stuff that
(09:48):
lights up, using the stuff made of quarks. It either
glows or reflects light or gives off light. We just
cannot tell the story of the universe and have it
make sense without something else out there provid a bunch
of gravity. So much gravity is missing that you need
five times as much of this mysterious stuff we call
dark matter as there is normal matter. So very briefly,
(10:09):
dark matter is just an idea to explain all this
unexplained gravity in the universe.
Speaker 1 (10:14):
Yeah, and there are different ways that scientists have sort
of found or think that dark matter is there and there.
As you said, they all relate to gravity. But I
think basically the main idea is that what we see
of the universe tells us that there's more matter out
there than the stuff that glows or that we can
see and feel right exactly.
Speaker 2 (10:29):
And the story started with the galaxy rotation curves. We
looked at galaxies and measured how fast they spin, and
we notice that they spin really really fast, and that
if you add up all of the stars and the
gas and the dust in those galaxies, they don't provide
enough gravity to hold that galaxy together as it spins.
So that was evidence number one, and for decades we
knew about that, but it was sort of hard to
(10:51):
accept the idea that there could be so much more
missing matter out there. It was just one piece of evidence.
But slowly, over the decades we've pieced together lots of
totally independent measurements that tell us that there is missing
stuff out there, that there's matter out there providing gravity
that we cannot see.
Speaker 1 (11:10):
Like initially, for example, it could have just been that
galaxies out there had a lot of like dark rocks,
right or gas that you couldn't see through the telescope exactly.
Speaker 2 (11:19):
Or it could have been that gravity work differently over
really really long distances, like we've measured gravity in the
Solar System and on Earth, but maybe over hundreds of
thousands of light years gravity operates differently than Newton and
even Einstein suggested that could have been the explanation when
you were just looking at, one example, just at galaxy
(11:39):
rotation curves. But now we have lots of other ways
to probe this, you know, we look for example, at
the structure of the universe. How did it come together?
How did you go from blobs of gas mostly dispersed
through the universe clumping together into stars and galaxies, and
that requires gravity. And if you run the universe without
any dark matter, just with the kind of matter we
can see, you don't get stars and galaxies after fourteen
(12:02):
billion years. There isn't enough gravity to do it. You've
got to add in the dark matter and boom, then
you get a universe that looks just like ours. So
it's another very convincing piece of evidence that there is
matter out there. It's not just like gravity operates differently
that those distance scales. There really is missing matter.
Speaker 1 (12:19):
But could you ask the same question about these large
scale structure theories. Could it be just that gravity works
differently though we thought at different scales, and that might
explain why galaxies form the way they are.
Speaker 2 (12:29):
It is possible, and people who work on these theories
they're called like mond modified Newtonian dynamics, have tried to
tweak them. I've not seen one that can successfully explain
both the galaxy rotation curves and the large scale structure
of the universe. Mostly these theories are tuned to explain
the galaxy rotations, and they don't even try to explain
(12:50):
the other evidence for dark matter.
Speaker 1 (12:52):
What's this other evidence.
Speaker 2 (12:53):
Maybe one of the most compelling and precise piece of
evidence for dark matter is seeing its effect on the
very very early universe plasma. Before stars were formed, we
really had any structure in the universe at all. It
was just huge blobs of hot gas everywhere in the universe.
And we see the glow from that gas in the
cosmic microwave background radiation. When that gas cooled enough to
(13:14):
become transparent, when it formed neutral atoms, so photons mostly
could fly through it. Those photons are still around and
we see them, so they're like the last glow of
this plasma from the very early universe, and we see
ripples in that plasma, ripples that show us how the
normal matter and the dark matter and the photons were
all sloshing around. And it's a very very precise measurement,
(13:35):
and it tells us how much normal matter there is
in the universe, how much radiation there was in the universe,
how much dark matter there was in the universe back then.
It's very very precise because we've taken these very very
detailed maps of this early universe plasma, and that tells
us that there is a component of that plasma that
doesn't interact with photons, it's not buryonic, it's not our
kind of matter at all. So it's another completely independent
(13:58):
measurement that tells us there's a dark but gravitationally active
component to the universe.
Speaker 1 (14:03):
Now, is that also due to gravity? Does that evidence
also depend on our current model of gravity?
Speaker 2 (14:10):
It does, but those are much shorter distances. Those oscillations
ended up seeding the larger scale structure of the universe,
but it was before a lot of the expansion. So
we're talking about things that happened over short distance scales.
Speaker 1 (14:21):
There were a short distance but there are large distance now.
Speaker 2 (14:23):
That's right, But the CMB comes from when it was
short distance. Earlier, you were asking if this could all
just be due to like long distance gravitational effects, and
the CMB proves shorter distance scales gravity because the picture
we have of it comes from back when things were
much more cozy and compact.
Speaker 1 (14:39):
All right, So then the prevailing picture is that there's
a lot of stuff out there, stuff that creates gravity
and feels gravity, but we just can't see it. So
let's get into what it could be, how we might
see it, and what might be new ways to figure
out where it is and what it's doing. So we'll
dig into that, but first let's take a quick break.
(15:11):
All right, we're talking about dark matter now, Daniel. How
many episodes now have we talked about dark matter?
Speaker 2 (15:15):
Oh? Man, so many episodes. But it's still a thing
people are most confused and most interested about. In the
emails we get from people, people are still in the
dark about it. Yeah, And it's a really active area
of research, obviously, because it's one of the biggest open
questions in modern physics. So people are constantly trying to
come up with new theories for what dark matter might be.
(15:36):
Is it this particle, is it that particle, Is it
not a particle at all? Is it something else entirely?
And new ways to spot dark matter. If it's this
kind of particle, how could we see it? Could we
build a detector to observe it. So it's a huge
area of active research.
Speaker 3 (15:51):
Mm.
Speaker 1 (15:51):
Yeah, I guess we can't just wait for the Sesame
Street episode on it.
Speaker 2 (15:55):
We could just sit back and wait for the aliens
to show up and tell us the answers to the universe,
But that kind of embarrassing if they showed up and
we had nothing to contribute.
Speaker 1 (16:03):
Yeah, it'd be more embassing if they have to do
it over dinner while they eat us. I wonder if
they would feel less bad about eating us if they
knew we weren't. You know, dark matter intelligence, All right, Well,
we just kind of define what dark matter is. It's
the stuff we think that's out there to explain a
lot of the gravitational effects we see in how the
universe form and also in the microwave background radiation. Now
(16:25):
there are candidates where what we think this stuff might.
Speaker 2 (16:28):
Be, right Daniel, There are lots of ideas for what
dark matter might be, and people initially thought, well, maybe
dark matter isn't some new, weird kind of stuff. It's
just more of the kind of stuff we already know about.
But you know, it's just hiding or something. People thought,
maybe dark matter is like neutrinos, because neutrinos are tiny
little particles that don't interact with photons, they don't give
(16:49):
off light, they don't reflect light, they pass right through
a lot of stuff. So they seemed initially like a
really good candidate maybe for dark matter, Like maybe the
universe is just filled with accountable numbers of neutrinos that
would have been amazing. But neutrinos don't have a lot
of mass. They're very very low mass particles, which means
they almost always move nearly the speed of light. And
(17:11):
one thing we do know about dark matter is that
it's kind of slow moving. It's not a fast particle.
Speaker 1 (17:17):
Well, I guess that's Those are two questions. First of all,
why can't theatrinos go slow? Even if they're light? Can't
you still stop them from moving?
Speaker 2 (17:24):
You can stop a neutrino because they have very low mass,
but their mass is so low it's like thousands of
times lower than even an electron. That essentially any energy
in putrino has means it's moving at almost the speed
of light. So it's almost impossible to have a natural
process that produces a huge number of neutrinos and have
them be slow moving.
Speaker 1 (17:42):
And you also say dark matter is cold. How do
we know it is cold if we can't feel it.
Speaker 2 (17:45):
Yeah, we know that dark matter can be moving very
fast because of the way it's influenced the structure of
the universe, like, dark matter is mostly responsible for what
we see out there. The reason you have a galaxy
here or a galaxy there is because there's a huge
blob of dark matter that created gravitational attraction that pulled
all that gas in to form the stars and the galaxy. Now,
if dark matter was moving really really fast, if it
(18:08):
was hot, then it wouldn't clump the same way. It
would spread itself out much more. So, we know that
dark matter has to be below a certain speed basically,
or it wouldn't have clumped into these blobs, which then
formed the structure of the universe. Like you run simulations
where dark matter is a fast moving particle, you don't
get the same kind of structure that we see in
our universe. That means dark matter has to be slow
(18:29):
moving or cold, as physicists say, and that means that
it can't be neutrinos.
Speaker 1 (18:33):
Couldn't it be like super massive but hot too?
Speaker 2 (18:36):
Well, hot essentially refers to its velocity regardless of its mass.
The issue is the velocity, and the problem is neutrinos
basically can't have low velocity. We know that dark matter
can't be moving very fast, otherwise it would spread itself
out and wash out the structure of the universe.
Speaker 1 (18:50):
All right, well, then how is it that we can
see it? How can we hope to see it and
study it.
Speaker 2 (18:54):
Then yeah, it's tricky. You know. The one thing that
we do know is that dark matter feels gravity. Vitational
studies are surefire way to detect dark matter. The problem
is that gravity is super duper weak. It's like the
weakest of the fundamental forces. If it even is a
fundamental force, it's weaker than the weak force by like
ten to the thirty, which means that it's basically impossible
(19:16):
to use gravity to detect tiny bits of dark matter,
like we can see like Solar System sized chunks of
dark matter maybe, but anything smaller than that, the gravity
from it is too weak for us to even detect it.
And that means that you couldn't really use gravity to
detect the particle nature of dark matter. Like one of
the deepest questions is what is dark matter made out of?
(19:37):
Is that this kind of particle? Is that kind of particle?
But gravity is really too weak to tell us anything
about the particle nature of dark.
Speaker 1 (19:44):
Matter, and we don't even know if it is a particle, right,
it could be that dark matter is something that doesn't
form into particles. Is that possible.
Speaker 2 (19:51):
It's totally possible. It's not a mainstream idea, like most
of modern physics right now, is focused on the idea
of particle dark matter, because the kind of matter we
know is all made of particles, and so we extrapolate
and we say, well, probably this other kind of matter
is made out of particles. And you might think that
sounds reasonable. It's a pretty basic assumption, but it's also
extrapolating from five percent of the universe to like twenty
(20:13):
five percent of the universe. It's entirely possible that the
physics of this other huge section of the universe is
very different from anything we've imagined. We've talked in the
podcast before about unparticles or other weird kind of theories
that describe it as not made of particles. So like
as you zoom in, it doesn't ever change. It's not
like there's a basic unit of it. You can just
(20:33):
keep zooming in forever and it always looks the same.
That would be really weird but super awesome. But the
mainstream theories are mostly particle dark matter because that's what
we know how to.
Speaker 1 (20:43):
Think about, because that's kind of the only way to
think about things right.
Speaker 2 (20:47):
Well, you know, there are people trying to think outside
the box. That requires a lot more creativity and flashes
of insight. But there definitely are people out there working
on sort of crazier theories of what dark matter is.
But I think most people are working on article dark
matter because yeah, that's what we're good at thinking about.
I mean, you talk to a particle physicist, you're going
to get a particle explanation for everything.
Speaker 1 (21:07):
I guess. I mean, like, we haven't seen anything that
isn't explained by particle theory, right, and we wouldn't even
know what that math would look like.
Speaker 2 (21:16):
Yes and no, we've never seen anything that we've been
able to explain with a non particle based theory. But
remember there's ninety five percent of the universe dark matter
and dark energy that we still can't really explain. So
particle based theories are the only ones that have ever
been successful, but they've only been successful in five percent
of the universe.
Speaker 1 (21:34):
So yes and no, I guess there's a lot out
there that we can't still explain. So maybe our current
theory only covers five percent of the universe.
Speaker 2 (21:42):
Yeah, that's exactly right. So we should definitely keep an
open mind to other crazy theories of what dark matter
might be. But currently we're mostly working on. Is dark
matter a particle? Is it a new kind of particle?
What does it do? How could we possibly spot it
if it is a particle?
Speaker 1 (21:57):
All right, So if it is a particle, how do
we see it? How do we study it? We know
that it feels gravity, what else does it feel or
not feel?
Speaker 2 (22:04):
So the short answer is we have no idea. Like,
it could be that dark matter feels some new kind
of force with itself. It could be the dark matter
feels no forces other than gravity, and like that could
totally be our universe, in which case it's almost impossible
to ever discover the particle nature of dark matter. It
could just be a mystery forever because gravity is just
so weak. There's another possibility that dark matter is kind
(22:27):
of misnamed, that dark matter does actually interact with our
kind of matter through some mechanism we haven't discovered yet
that's not really truly one hundred percent.
Speaker 1 (22:37):
Dark, But I guess you mean using electromagnetic forces or
other kinds of forces, like it dark in terms of
light or dark in terms of all the forces in nature.
Speaker 2 (22:47):
We're imagining maybe there's a new force out there also,
So we're suggesting maybe dark matter is some new kind
of particle, right, And in addition, maybe there's a new
force out there, a force that helps dark matter particles
interact with our kind of particles. So call this a
new dark force or a portal to the dark sector
or whatever. If there is this new kind of force,
(23:08):
then maybe it helps dark matter particles bump into our
kind of particles, or turn into our kind of particles,
or somehow interact with our kind of particles, which would
make them effectively visible.
Speaker 1 (23:20):
But would you need to come up with a new
kind of force. Couldn't interact with us through the weak
force or the strong force somehow?
Speaker 2 (23:27):
It's totally possible that dark matter could have interacted with
us via one of the forces we know already, but
we've basically ruled that out with our experiments.
Speaker 1 (23:35):
Have we really is that official?
Speaker 2 (23:36):
If dark matter interacted with a strong force, it would
be a very powerful interaction because the strong force is
super duper strong, and that would actually be pretty easy
to find. We look for dark matter giving off photons.
We've never seen that, so we don't think that dark
matter actually directly interacts with photons. We've also looked for
dark matter interacting via the weak force. And this is
probably the biggest area of experimental dark matter particle physics
(23:59):
right now, with these huge tanks underground very quiet liquid
like xenon, and we wait for dark matter to pass
through the Earth and interact with one of these xenon
molecules and give it a little kick. So these huge
tanks of underground liquid xenon are just waiting for a
dark matter particle to bump into it via the weak force.
And if it does interact via the weak force, we
(24:20):
can calculate how often that should happen. And we've been
running these experiments for years and years and years, and
we've never seen a blip that looks like dark matter
kicking one of these xenon molecules via the weak force.
So now we can pretty definitely rule it out.
Speaker 1 (24:36):
I see, we've been putting stuff out there to hopefully
interact with dark matter through the weak force, but so
far nothing has been bumped that way. So now you're
saying the dark matter probably doesn't interact with the weak force.
Speaker 2 (24:48):
As time goes on and we don't see any of
those interactions, we more confidently say that they never happen.
If you only listen for a day. It might be
that they only happen once a year and you just
haven't seen one yet. But after you've listened for five years,
ten years, twenty years, either you're getting very very unlucky
or it just doesn't happen. And so now we've had
big enough detectors running for enough years that were pretty
confident in ruling that out. But it doesn't mean that
(25:11):
there isn't a new kind of force, like an even
weaker force, that would allow dark matter to interact with
our detectors. So that's what they're looking for now. Like
they call it the feeble force.
Speaker 1 (25:21):
It's weaker than weak. That's the idea, exactly weaker than weak.
We only we have seen that already with our detectors,
with the tanks of Xenon they're sitting there waiting.
Speaker 2 (25:31):
Depends on how weak it is. If it's super duper
extra week, if it's the feeblest force, you can imagine
it might take a very very long time. It might
be so unlikely they have to run for ten years
or one hundred years in order to see it. And
that's why we're also at the same time using other
methods to try to look for these kinds of interactions,
like we're hoping to use this new feeble force to
(25:51):
create dark matter at particle colliders.
Speaker 1 (25:54):
What do you mean, like when you collide protons or
quarks like that, it might make dark matter.
Speaker 2 (25:59):
Yeah, if this feeble force exists that allows dark matter
to bump into protons and neutrons inside a xenon atom,
then in principle you can reverse that. You can say, well,
what if we smash protons together, maybe sometimes they can
use the feeble force to create dark matter. Because that's
what we do with the particle collider. We annihilate protons
together and create new kinds of stuff. And the cool
(26:22):
thing about a collider is that anything that's out there,
you can make it as long as your protons can
interact with it. So anything that protons interact with we
produce at the collider eventually. Things that interact with protons
a lot, we produce them all the time. Things that
don't interact with protons very often, we produce them more rarely,
So higgs bosons are pretty rare, for example. But we
comb through all of those collisions looking for evidence of
(26:43):
the production of dark matter.
Speaker 1 (26:44):
Wouldn't we have seen evidence of that already? I mean,
you've been running the LHC for a long time and
particle colliders for decades. You know, if there was some
sort of unaccounted for force, wouldn't we have seen it
by now?
Speaker 2 (26:56):
We haven't seen anything. You're right, And again it's a
question to the of that force. If that force was
as strong as the weak force, yeah, we probably would
have seen it. And the longer we run our colliders
and don't see it, the weaker that force has to
be to still be consistent with our data, to still
be hiding from all of these experiments. But we don't
know how weak that force is. Maybe it's really ridiculously weak,
(27:18):
so weak that we haven't seen it underground and we
haven't seen it in our colliders. So we actually turn
to another mechanism to try to see this super duper
weak force in action, which is to look at the
center of the galaxy to see if dark matter is
smashing into itself.
Speaker 1 (27:32):
I see use the center of the galaxy as a
particle collider.
Speaker 2 (27:36):
Exactly because one problem with the particle collider is that
it's not very dense. Right. We have protons smashing into protons,
but it's like very few protons and we try to
run it as often as we can, and it adds
up to zillions of collisions, but it just might not
be enough. But we think that the center of the
galaxy is very very dense with dark matter. We can
(27:56):
map out where the dark matter is in the galaxy
by looking at how things rouse and how fast things
are moving, and we suspect that the center of the
galaxy is very dense with dark matter. And so if
dark matter has this feeble force, this super weak force,
then occasionally two dark matter particles should bump into each
other and produce normal matter particles, like the opposite of
(28:17):
what we think might happen in the collider, which two
protons smash together to make dark matter run that backwards
in time. We hope that's the operation happening in the
center of the galaxy. So we turn these special telescopes
to the wards the center of the galaxy and look
for these characteristic flashes of light that might come from
those collisions.
Speaker 1 (28:35):
Interesting, that sounds a little cheaper than building a thirty
billion dollar collider here.
Speaker 2 (28:40):
Well, it was only ten billion dollars, right, so we
can save you twenty billion off the top right there,
and it wasn't that cheap because you're building a particle
detector and launching it into space, which is never simple.
Speaker 1 (28:51):
All right, Well, let's get into the details of how
we're using the center of the galaxy as a collider
to look for dark matter. Let's dig into that, but
first let's take another quick break. All right, we're looking
(29:13):
for dark matter. I had some right here, Daniel. You
don't know where I put it.
Speaker 2 (29:16):
I can give you emoji based directions to it, Derry,
I'll send you some arrows.
Speaker 1 (29:21):
That might not be helpful there. But yeah, there's a
lot of matter missing in the universe. All twenty seven
percent of the universe is out there, but it's invisible.
We can't see it or touch it called dark matter,
and our only hope for ever studying its particle nature
is that there's some sort of new force that we
haven't discovered yet, some super weak force that you're calling
(29:42):
the feeble force. Although Daniel, I'm kind of disappointing you
didn't call it the dark force.
Speaker 2 (29:48):
Because then forever it would dominate my destiny exactly right.
Speaker 1 (29:52):
Then you get the cool red lightsabers or the cool
laser pointers when you give your lectures.
Speaker 2 (29:57):
Yeah, that's true, but I didn't want to have to
grow those horns.
Speaker 1 (30:00):
Sith have horns.
Speaker 2 (30:01):
That's relief.
Speaker 1 (30:03):
You need to brush up on your Star Wars there apparently. Yeah,
but yeah. Our only hope forever studying the particle nature
of dark matter is that it feels a new kind
of force we haven't seen before, and we can't generate
that force apparently here in our colliders. But there's hope
that maybe you can use the center of the galaxy
as a collider to maybe study this part of this
aspect of dark matter we think we're hoping is there.
Speaker 2 (30:26):
Exactly, And the idea is dark matter smashes into itself
and then via this new feeble force, turns into standard
model particles, maybe tau leptons or be quarks or something,
and those particles can then give you flashes of light
because those particles do interact with light. So what we
do is we turn this telescope to the center of
the galaxy and we look for these flashes of light
(30:48):
that we can't otherwise explain. If you see flashes of
light that look like they come from dark matter turning
into particles, we know, then you can say, oh, there's
evidence for dark matter. There.
Speaker 1 (30:58):
Wait, the idea is that dark matter smashes into itself.
Why does it have to smash into itself.
Speaker 2 (31:05):
It doesn't have to. It's possible for dark matter also
interact with normal matter particles in the sension of the galaxy,
the way it might interact, for example, with our tanks
of Xenon underground. But the signature we're looking for would
come from two dark matter particles smashing into itself, producing
like a pair of bottom quarks or a pair of
tau leptons, which then give off some photons which travel
(31:25):
to our telescope and we observe them.
Speaker 1 (31:27):
Wait, so two dark matter particles can interact with each
other and generate regular kind of matter? Yeah, exactly does
that work the other way too? Like, can regular matter
interact and come up with dark matter?
Speaker 2 (31:39):
That's what we're trying to do with the Large Hadron
collider is smash regular matter together two protons and turn
it into dark matter. We haven't seen that yet, so
we're trying to do the opposite, smash dark matter together
and turn it into regular matter. It's like the inverse collider.
Speaker 1 (31:52):
What's the inverse of a collider? An expander as a separator.
Speaker 2 (31:56):
It just runs it the other direction, you know, instead
of colliding normal matter to make dark matter, and we
collide dark matter to make normal matter.
Speaker 1 (32:02):
So the picture is that in the center of the galaxy,
you're saying, there's a high density of dark matter, and
some dark matter particle just happens to smash head on
with another dark matter particle. Hopefully they're going fast enough
to actually sort of interact and create a lot of energy.
And then out of that comes a regular matter particle
from the center of the galaxy that then we see
(32:24):
here on Earth and go, yep, that came from two
dark matter particles.
Speaker 2 (32:28):
Yeah, And if you want the microparticle physics explanation, two
dark matter particles smashed together form some new kind of
particle that mediates this feeble force, call it a dark
photon or something. And then that dark photon turns into
a pair of normal particles like two bottom corks or
two tau leptons or two muons or something, and then
(32:49):
those produce photons which then we see here.
Speaker 1 (32:51):
But then how can we hope to like discern that
from all the way here the center of the galaxy is,
you know, fifty thousand light years away.
Speaker 2 (32:59):
It's very very tricky, and that's exactly the difficulty of
this method. Number one. It's far away. We think that
these photons would be very high energy, so we think
that gamma rays would survive travel from the center of
the galaxy. But the real problem is that we don't
understand the center of the galaxy. So a lot of
stuff going on there, making flashes of light anyway that
we can't really understand. And so actually we have seen
(33:23):
flashes of light from the center of the galaxy that
we cannot explain that No astrophysical explanation has been provided
to explain these flashes of light, and so some people
are actually pretty excited about that. They say, whoa, maybe
this is dark matter. But you know, the problem is,
we don't really know what's in there in the center
of the galaxy. It's a very weird, dense region. We
just did a whole episode about how weird it is
(33:44):
and how little we understand about what's there. So it
could just be new, weird stuff made of normal matter
in the center of the galaxy flashing lights and ways
we don't understand, or it could be dark matter creating
these flashes of light.
Speaker 1 (33:56):
Wait, wait, wait, what do these flashes look like? Like
You're just looking at at the galaxy center of the
galaxy and suddenly there's like a spike or a pulse
or a train of pulses. What do these flashes look like?
Speaker 2 (34:08):
So these flashes come as individual photons. So this telescope,
a Fermila telescope, can see gamma ray photons, which are
just photons in a certain energy range in this case
from fifty million electron volts up to a trillion electron volts.
And they see a bunch of these gamma ray photons
from the center of the galaxy, and we cannot explain.
Speaker 1 (34:27):
Them otherwise, like one at a time, Well, the.
Speaker 2 (34:29):
Detector can only see one photon at a time.
Speaker 1 (34:32):
Yeah, I guess. I mean, like you see one and
then you don't see one for another year. Or is
it do you see like a whole bunch of them
in a cluster or something.
Speaker 2 (34:38):
So the higher energies are definitely more rare than they
are at the lower energies, but there's a lot of them.
It's not just like one or two or seven photons.
You know, we're talking about thousands of photons here, accumulated
over many, many years and so people are trying to
understand where are these photons coming from. Is there something
going on with normal matter at the center of the
galaxy or is this actually dark matter?
Speaker 1 (34:58):
But I guess, why are they eplainable or why are
they so mysterious. Isn't it just like, oh, here's a
photo with a lot of energy.
Speaker 2 (35:05):
Well, we have an idea for what's in the center
of the galaxy. We think that there's a black hole there,
and we think there's a swirl of stuff around the
black hole, and we think there's a certain density of stars.
We think there's gas and dust and all sorts of stuff,
and we use that model to predict what we should
see from the center of the galaxy, and we see
all that stuff, plus we see more. We see another
spectrum of photons that have sort of like a different shape,
(35:27):
like they emit light in different patterns. They're distributed around
the center of the galaxy, and their brightness and their
wavelengths can't be explained by any of the components that
we do know about in the center of the galaxy.
So it's definitely something new happening in the center of
the galaxy emitting photons in a way that we can't
otherwise explain, and it's consistent with some theories of dark matter.
Speaker 1 (35:48):
All right, so well, let's dig into that part of it. Then,
what's the connection between gravity and or what might be
the connection between gravity and these flashes of light.
Speaker 2 (35:56):
Yes, so we've been seeing these flashes for a while,
and there's been theories of dark matter for decades from
the center of the galaxy, but people have been unsure
about what it means because we don't understand what's going
on at the center of the galaxy very well. So
recently people came up with a really cool clever idea
to combine gravitational information with these flashes of light. They say, look,
(36:17):
if there is dark matter creating these flashes of light,
we should be able to zero in on where these
flashes come from and see if there's dark matter there.
By seeing if there's a gravitational effect from that dark matter,
Like if there's a blob of dark matter there that's
extra dense and making these flashes, we should be able
to see gravitational lensing from that dark matter blob to
tell us, oh, that really is dark matter and not
(36:39):
just some star going crazy.
Speaker 1 (36:41):
Couldn't it just be a star going crazy or maybe
like a quasar or something like that. Doesn't that seem
more likely? But like, how do you know which is
more likely?
Speaker 2 (36:49):
Exactly? We don't really understand what's there, so it is tricky.
But the first thing they did is they tried to
map all the gravitational lensing near the center of the
galaxy and measure that by seeing like how that does
or it's light from behind the galaxy, Like galaxies in
the background far away, how is their light being distorted
as it passes through the center of the galaxy. So
that's the gravitational lensing measurement. They use that as a
(37:11):
way to just see like are there pockets of extra
gravity in the center of the galaxy. So they make
a map of all this gravitational lensing and then they
line that up with the map of these extra flashes
from the center of the galaxy, and boom, boom, boom,
they do line up. They line up very nicely. So
there seems to be this correlation between where there's extra
(37:32):
gravitational lensing and where there's more gamma ray flashes.
Speaker 1 (37:36):
That's pretty interesting, although it could just be something else.
I wonder, like, you know where maybe where there's a
lot of dark matter. There are also a lot of
black holes.
Speaker 2 (37:45):
Exactly, and everybody's very skeptical at first, right, you don't
want to just like jump up and down and claim
you discover dark matter. So then they try to explain
this with other sources, you know, And one idea they
have in this paper is that maybe it's blazars. Blazars
are these super awesome galaxies that are generating a really
powerful beam of light from the center of the galaxy.
If you have a super massive black hole the core
(38:06):
of a galaxy, then its magnetic field can funnel radiation
up and down the north and south pole of that
galaxy and produce a very powerful beam of light. And
if that beam of light is pointed right at the Earth,
the relativistic effects effectively make the Earth a little shorter
and pile up the light beams a little bit to
make it extra bright. So these things are called blazars,
(38:27):
and blazars could also explain these effects that we're seeing.
And so in the paper they analyze like, well, is
this just more blazars than we anticipated, or is it
dark matter? And they can explain part of this effect
with blazars, but there's a part of it that blazars
cannot explain, and that is very nicely explained by dark matter.
And so this is the challenge with studying dark matter.
(38:49):
It's like, is what we're seeing dark matter or is
it something else we don't understand because dark matter is
still kind of a fuzzy idea.
Speaker 1 (38:56):
M yeah, I mean we don't really know or have
a clear idea what it's nature is. I feel like
we're sort of like putting assumptions on top of assumptions
on top of assumptions and then checking that.
Speaker 2 (39:07):
That is what we're doing. We're making hypotheses and we're
checking them, and we're coming up with alternatives, like is
this dark matter? Is this something else? What we know
is that there is something new happening at the center
of the galaxy. Something is generating these flashes of light
that we do not understand. Maybe it's dark matter, maybe
it's some other new astrophysical source. Maybe it's some new
process that's distorting light from behind the galaxy. We don't know.
(39:29):
There's something new to be discovered there. We don't know
if that lines up with these other mysteries we're seeing,
like galactic rotation, curves and the structure of the galaxy.
So maybe this reveals something about dark matter, but maybe
it reveals something about something else. Right, we're so clueless
about the nature of the universe that we're trying to
put these puzzle pieces together. But maybe they don't click.
Maybe they're parts of different puzzles. It's just because we're
(39:49):
at the beginning of understanding the universe that we're so
clueless about how to put this all together.
Speaker 1 (39:54):
Yeah, I guess it could be anything. Could be alien
sending us emojis. Maybe those high speed photons are there
version of.
Speaker 2 (40:01):
Emojis exactly, Maybe there are express emojis, hopefully they're safer work.
This is a very difficult kind of science because the
data is sparse. You just have like these flashes of
life from the center of the galaxy, and we're very
far away from the center, as you said. So I've
been to like whole conferences dedicated just to this signal
and understanding it, and people show completely different interpretations. They say, oh,
(40:23):
I can explain all of it using some astrophysical source,
and other people say, no, that's impossible, it can't describe it.
And other people say, well, you forgot to account for
all these uncertainties in that and if you let these
things float and consider other possible ways that these things
get interact, then maybe it explains it. So we really
are at the beginning days of understanding what it is
we're seeing. But it's cool to see people try to
line these things up, you know, gravity and flashes of
(40:45):
light and other ways to try to get ideas for
what dark matter might be.
Speaker 1 (40:49):
All right, well, it sounds like it's still kind of
an open question and a big mystery, but it is
a pretty exciting idea to kind of like use one
mystery to try to explain another mystery. It's like you're
piling on the mysteries.
Speaker 2 (41:00):
We are piling on the mysteries, and we're trying to
line things up. You know, we're trying to come up
with a coherent explanation for everything we see out there
in the universe. It's not fair to have one story
you tell in one case and a completely different story
you tell in another case. Right, you need a single
theory that explains everything we see. And that's really a challenge,
especially because the data is not as good as we'd
like it to be. You know, we'd like to have
(41:22):
these sensors at the center of the galaxy probing these
things in detail with very fine spatial resolution. But instead
we're limited to these like tiny cameras orbiting very far
from the center of the galaxy trying to get a glimpse.
Like imagine you were trying to understand what was happening
in Manhattan and all you had was like a camera
in Indiana. It'd be pretty tricky.
Speaker 1 (41:40):
Yeah, especially with all those buildings, be hard to kind
of see what's going on between there. So it sounds
like maybe we just need to send a pro to
the center of the galaxy right figure out what's there,
you know, and just wait fifty thousand years or so.
Speaker 2 (41:53):
Yeah, some poor graduate student is going to wait fifty
thousand years to defend their thesis.
Speaker 1 (41:58):
I'll get paid for that. I could get paid for
that while I'm doing this podcast.
Speaker 2 (42:02):
Sounds good.
Speaker 1 (42:02):
I mean, as long as we're double dipping.
Speaker 2 (42:04):
Are your checks going to come from the sense of
the galaxy because you might be waiting a while?
Speaker 1 (42:08):
Yeah, no, no, no, they're going to come in a dollar bill.
Emojis the magically transform into cash into my Venmo or the.
Speaker 2 (42:16):
New x app quantum Cash entanglement.
Speaker 1 (42:18):
There you go. I'll be rich and not rich at
the same time.
Speaker 2 (42:22):
I hope your finances don't collapse.
Speaker 1 (42:24):
All right, Well, another reminder that there are still humongous
mysteries out there in the universe, and that there are
scientists out there actively searching for the answers and searching
for it, trying to find ways to study it and
reveal to the rest of us what its true nature is.
Speaker 2 (42:39):
So remember that dark matter is really a whole suite
of ideas. It's not just like a blank check placeholder
for things we don't understand. It's an empty chalkboard for
us to fill in with details, and scientists are constantly
coming up with ideas for what dark matter might be
and then being creative about how we might discover it
because we hope one day to fill that chalkboard in
with some thing we really do understand.
Speaker 1 (43:01):
Does that mean you have to call it dark matters
or dark's matter? These are the types of questions that
I get paid to the thing about from multiple people.
Speaker 2 (43:11):
That sounds good. Sounds like you are qualified to answer.
Speaker 1 (43:13):
That question, as are we all all right, well, we
hope you enjoyed that. Thanks for joining us. See you
next time.
Speaker 2 (43:28):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of iHeartRadio. For more podcasts
from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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