What's the cosmic gravitational background?

Daniel and Jorge explain how we might be able to listen to a new kind of gravitational hiss and what secrets it could reveal.

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Episode Transcript

Speaker 1 (00:08):
Hey, or Hey, I have a question for you about
drawing cartoons. Hey, finally something that I'm an expert in.
What's the question, Well, it's more about the backgrounds. Actually.
How do you decide whether to leave the cartoon background
blank or fill it in with a color, or like
draw a full scene in the background. M Well, I
guess it can be whatever you want the background. I mean,

(00:30):
you just don't want to take away attention from what's
happening in the panel. So the background is just like
filler to be ignored. Oh, you know, it's all hard.
You know. Sometimes that's where the treasures are hidden in
the background. Oh, sometimes that's true in physics as well. Really,
you find Waldo sometimes in the background, the cosmic Waldo background.

(01:05):
Hi am Orhammed, cartoonists and the creator of PhD comics. Hi.
I'm Daniel. I'm a particle physicist, and I have never
found Waldo in one of those books. Never. I think
maybe you just haven't tried, Daniel. I think that's it.
I get bored after like two seconds, I'm like, why
do I care about finding this guy? And yet finding
the Higgs boson in a bunch of noisy data. That's

(01:26):
somehow more interesting. Yeah, I can write a computer program
that scans it for me automatically. That could write a
computer program that looked for Waldo. That would be more
fun do it? Yeah, I mean, what else are you doing?
What else could you be doing? It's more productive than that.
Welcome to our podcast Daniel and Jorge Explain the Universe,
a production of I Heart Radio, in which we don't
look for Waldo, but we do look for answers to

(01:47):
the deepest questions humans have ever asked. Questions like why
is the universe the way that it is? Could it
have been another way? How big is it anyway? How
did it start, how will it end? And what is
it made out of. We don't shy away from any
of these incredible questions, which inform the entire context of
the human existence, and we admit ignorance. We don't pretend

(02:10):
to know all of the answers, but we try to
explain to you what science does and does not know. Yeah,
we like to talk about all of those big questions
that are at the forefront of science and the exploration
being done by physicists and other scientists. We also like
to talk about what sort of in the background of
science as well. That's right. There are wonderful surprises out
there in the universe. Sometimes you know what you're looking for.

(02:31):
Sometimes you stumble across something accidentally when you were looking
for something else. Sometimes you see an exciting discovery that
jumps right out at you. Sometimes it's treasures hidden in
the background. Yeah. And the crazy thing is that it's
all there right like literally, when you look at the
background of any picture that you take, and you look
at the sky behind you or behind the person that
you're taking the picture of, there's information there about the universe.

(02:54):
There might be hidden secrets and interesting things to discover
right there where you least expected. That's true. Even the
darkest spots in the night sky have some photons coming
to us from very very distant locations. If you pointed
the hubble instead of your iPhone at some point in
the sky, you would see millions and billions of galaxies

(03:14):
so fat and so distant that their photons arrived like
every few seconds or every few minutes, rather than all
the time like the sun. But there is information in
every little tiny patch of the sky about the nature
of the universe and what's going on out there. Yeah.
Can you detect wal dogs. Maybe in those signals standards,
you know, the bosons to their waldenos, they're hard to find.

(03:35):
I guess, yeah, if I discover particle, can I called
the waldo? Like if it's in the background of the
you know, the universe. I think that would disqualify you
from participating in any experiment because they would not want
you to give it that name. I see a pre
screen for a name giving. We don't, but we should
because there's been some bad choices in the past. But yeah,

(03:56):
we like to ask all of the big questions that
are out there that scientists are wondering about and that
scientists are speculating about. There are interesting things that scientists
think might exist and things that we have not yet detected.
That's right. Every time we open up a new kind
of eyeball and look out there in the universe in
a new way, we find all sorts of crazy, exciting stuff,
and sometimes we know what we're looking for. We think

(04:18):
that there might be a hint a message out there
in the universe we haven't detected yet that could give
us fantastic clues as to the nature of the Big Bang,
What happened in the first moment of the universe, or
what's going on with black holes dying all across the galaxy? Yeah,
because you never know where the next big answer is
going to come from, or where the next big question
might come from. For example, we have this great question

(04:39):
from j J, who's five years old, who sent us
a listener question through the internet. That's right, here's a
question that JJ's father sent to us because he couldn't
answer it himself. Here's j J as the son by
that in the middle of our galaxy? Oh so ute,

(05:00):
Wait was it a question or a statement? Is he
saying he's going to make a star goes supernova? This
is not the youngest supervillain listener that we have. He's
asking what would happen if our star win supernova next
to the supermassive black hole in the center of our galaxy. Wow,
it's amazing that a five year old knows the word
supernova and kind of what it means. Yeah, so thank

(05:23):
you Lowell for encouraging the scientific curiosity in your children
in the next generation. You are fueling human progress. So
that's a complex question. Here, JJ is asking what if
our son was somehow next to the black hole in
the middle of our galaxy, and then what if it
suddenly went supernova? Right, that's right, and j J doesn't
have to worry. Our son is not going to go supernova.

(05:46):
Supernova is one of the end stages of a star,
when it gets so massive that it collapses gravitationally. But
in order for that to happen, you have to have
a lot more mass than our son does. You need
like eight times the mass of our son the minimum
threshold for a start to end up as a supernova. Right,
But so it can go supernova on its own, but
it might eat something else and then get bigger, right,

(06:08):
and then go supernova. That's right. There is one possibility
our sun will probably end its life as a white dwarf.
So there will be a phase which becomes a red
giant and blows out a lot of stuff, and then
it collapses again, and then it blows out more stuff,
and at the core would be left a very very
dense blob, a white dwarf, which essentially just a hot
rock and it's white not because this fusion going on

(06:29):
inside it, but because it's glowing white hot. And these
white dwarves usually just sit around for trillions of years
and cool off but sometimes if there's another star that
comes nearby, they're so dense they can eat those stars,
and if they eat enough of them, that can trigger
a supernova. Wow. Yeah, I sometimes feel like that. And Thanksgiving,
don't bring around another pie. I'm gonna blow No more turkey, please,

(06:52):
I'm gonna supernova. Do you gravitationally attract dessert if you
eat enough turkey? No, I'm just easy electromagnetic corps for that. Nice.
But I guess the question is what happens if a star,
or I guess any star blows up next to a
black hole? Like, does it blow away the black hole?
Does it like evaporated? Does it do nothing? If you
explode it next to a black hole? What happens? Well,

(07:12):
first of all, you cannot explode a black hole, like
there's nothing you can do that will destroy a black hole,
because remember, a black hole is a dense blob of
matter that will eat everything else, and that includes other
forms of energy. So you shoot a particle beam, you
shoot antimatter, you shoot dark matter into a black hole.
It eats everything. It's like Jorge at Thanksgiving dinner, right,
It's happy to take anything. And so anything you blow

(07:35):
in there, including the remnants of a supernova will just
make it bigger and stronger. Oh, I see, So it's
not possible to like blow it away or move it,
or at least maybe like push it a little bit.
Is it possible to push a black hole? It is
possible to push a black hole to accelerate. It is
a mass, and so you can tug on it. For example,
if you bring another black hole near it, those two
black holes will accelerate towards each other, swirling around and

(07:57):
creating all sorts of like gravitational radiation. And it is
interesting because the supernova near a black hole, like that
supernova will eject the material really really violently. It's a
huge explosion. The stuff comes out like a significant fraction
of the speed of light. But it won't necessarily just
like get all immediately eaten by the black hole because
it's moving at such high speed unless it pointed exactly

(08:17):
at the black hole. They'll probably like miss a little
bit and then swirl around the black hole and become
part of the accretion disc. That's like stuff around the
black hole that's hot and glowing. Interesting, But the explosion
itself I won't impart any like momentum on the black
hole to push it a little bit, even no, it
will impart some momentum on the black hole. But you know,
supermassive black hole like we're talking about as a mass

(08:39):
that's millions of times the mass of the Sun, and
the kind of supernover we're talking about comes from a
white dwarf, which is like less than the mass of
our Sun. So the energy from it is just going
to be like a pin prick. It's like if a
mosquito lands on you, does it knock you over? What
depends on the mosquito, you know, it depends on what
it's carrying. I mean, I've never been to Panama. I
don't know how big those skeetos they get big. Yeah,

(09:01):
not as big as the Australian ones. Everything is bigger
in Australia. Well up here in the States. I've never
been knocked over by a mosquito. But you know, not yet.
I'm getting older and they're getting bigger. Well that's the
answer for j J. Thanks for sending in the question.
And so today we'll be talking about sort of what's
happening in the background of the universe. We know that
there's the cosmic microwave background radiation. That's light, that's electromagnetic

(09:25):
energy that's out there hanging out from the Big Bang.
But there might be other things in the background that
may or may not be there that we might have
information also about the universe. Right, that's right. The cosmic
microwave background is a really really rich source of information
about what happened in the very early universe and what's
going on now. But it's just another form of light. Right,

(09:46):
Microwaves are electromagnetic radiation. But recently we figured out other
ways to look at the universe, Like we were just
talking about black holes generate gravitational radiation, which is a
completely different form of radiation and another way to look
at the universe. And so now we're wondering about whether
we can see a background to the gravitational radiation in

(10:06):
the universe. So today on the program, we'll be asking
the question what is the cosmic gravitational background and who
will win the Nobel Prize for finding it? Interesting. So,
it's something that we haven't found yet. It is not
something that we have seen exactly. There are some hints,

(10:27):
and we'll talk a little bit about some of the
experiments that claim to maybe see it and have claimed
to see it and been debunked. But so far it's
not an established thing. It's like potentially a discovery in
our future. It's like a treasure chest we haven't dug
up yet. And it has a very similar name to
the cosmic microwave background. This was the cosmic gravitational background. Now,
couldn't you use a different name, like the universal Gravitational background,

(10:50):
just to like set it apart a little bit. No,
we want to use the same name to show the similarities. Like,
it really is very similar in concept to the CMB,
so we wanted to use a name that, you know,
invade the parallelism there. That was a little bit more awkward.
The CGB is harder to say than the c m B.
The cg B. That sounds like a rap group maybe,
or it sounds like a type of monitor. Do you

(11:10):
have an l E ED or a c GB or
maybe a Supreme Court justice? Maybe the r g B. Yeah,
I'm done with c g BRBG right. Yeah. Anyways, it
sounds cosmic, and we were wondering how many people out
there knew what it was or had even heard of
the cosmic gravitational background. So, as usual Daniel went out
there into the wilds of the Internet to ask people

(11:32):
what is the cosmic gravitational background? So thank you to
all of our listeners who do live in the wilds
of the Internet and allow me to visit. If you'd
like to participate and send your speculation to Crazy Physics
topics in the future, please don't be shy. Send me
a message to questions at Daniel and Jorge dot com
and you can show off to all your friends hearing
your voice on the podcast. So think about it for

(11:54):
a second. What do you think it might be. Here's
what people had to say heard of the Cosmic Gravitation
show background. Um, I'm going to guess that it has
to do with the overall mass of the universe or
density I guess of mass in the universe. It makes
me think of cotton candy and when you're making it,

(12:17):
you know, you get the big glob of cotton candy
on the paper cone, but after that there's still fragments
and strings of it blowing around in the machine. And
that's how I picture cosmic gravitational background, just a little
snippets that are left after the planets and the stars congealed.

(12:38):
Shure that the cosmic gravitational background is an option on Zoom. Well,
this is something recent. Um, it might work like a
cosmic microwave background. I think so. Never heard of the
cosmic gravitational background. I guess it might be some low

(13:02):
level gravitational way of background, or some general gravitational pool
of the universe to words a specific point or from
the Big Bang. I have not heard of the cosmic
gravitational background, but since it has a pretty similar name

(13:24):
to the cosmic background radiation, I'll guess it's some sort
of map of the very early universe and how gravity
like began to clump mass after the initial quantum fluctuations
that caused the clumping, just how gravity was distributed in

(13:48):
the very early universe right after the Big Bang and
all that stuff. I have never even heard this term before,
so this is a wild guess. I'm assuming it has
something to do with the gravitational field of the entire cosmos.
I don't know how you would measure it, but that

(14:09):
would be my guest, all right. I like the person
who say it's an option on zoom to set your
background to the cosmic gravitational background. Technically that is true, maybe, right, everything.
I guess everything in the background does have gravity, so
and it's part of the cosmos. So really all Zoom
backgrounds are cosmic gravitational backgrounds. Yeah, I wonder if astronomers

(14:32):
even thought to look on Zoom to find the c GB.
I mean, maybe it's just that easy, right, just an
option on Zoom win Nobel, at least in some other
noble category. Maybe. Well, there's so many options on Zoom.
Who knows what's buried in some of those sub sub menus, right,
there's a Waldo option? No, please no, because we know
it sometimes means are not that exciting. It'd be nice

(14:54):
to have a little a bit of a puzzle there.
Find Waldo in the background of your Zoom speaker. Yeah,
there you that's actually a pretty good idea to build
games in a Zoom to keep people entertained. So this
is pretty interesting, Daniel, I didn't even know there was
a cosmic gravitational background, like most of our listeners and
people who responded so step us three. I'm guessing it

(15:16):
has something to do with gravitational waves and maybe the
Lego experiment that we have to detect them exactly it
does to understand what the cosmic gravitational background is, we
first have to understand what gravitational messages are and with
the gravitational foreground is. So let's start off with what
gravitational wave is. For those of you who don't recall,

(15:36):
space itself can ripple because mass changes the shape of space,
like if you have a black hole or a heavy
object or something that bends space. We know now that
gravity is not just a force between two objects. It's
actually space curving around masses and changing the way things move.
So gravity is this like apparent force. That's just because

(15:58):
we can't see the shape of space, the bending of space.
So we know that mass can bend space. Well, what
happens when mass moves or mass changes, Well, that causes
ripples because if a mass moves, then it's gravitational field
is going to move. So that's what causes ripples in
space time, and that's what we call a gravitational wave.
Right Like, gravitational signals like the effects of gravity take

(16:21):
time and space to move. Right Like if our son
suddenly disappeared or started moving or jiggling, it would take
like eight minutes for us to feel those changes in
its gravity. Exactly because information takes time to propagate. The
gravitational fields don't change immediately. There's a ripple. There's like
a wave that passes through the gravitational field, sort of
updating everything, and that wave moves at the speed of light.

(16:45):
And this is not unique to gravity, right, happens to
lots of things anything in fact, where information takes time
to propagate. Like think about a simple string. Maybe you're
playing jump rope with your friend. You pull the string up,
the whole string doesn't move at once. Right. You see,
first the first bit of the string moves up, and
then that pulls the next bit of the string, which
pulls the next bit of the string, and which you
get is a wave moving through your jump rope. Right. So,

(17:08):
anytime you have a material where information takes time to propagate,
where it's not just like instantaneous transfer, that's because everything
is local, then you get a wave. And so that's
true for a jump rope, and it's also true for electromagnetism. Right.
We can think about photons as electromagnetic waves because if
you take an electron, for example, it has an electric field.

(17:29):
If you shake that electron, then the electric field moves
with it. But again not instantaneously. It goes back and
forth with the electron. And that's what a photon is.
It's just an update in the electromagnetic field. Yeah, it's
like a wave. It's like electromagnetic what a ripple, right,
is what a photon is. Yeah, And that's how you
generate photons. You have an antenna and you wiggle the

(17:50):
electrons up and down inside the antenna changing the electromagnetic
fields around the antenna in a regular pattern, and that
causes waves in those fields, which are photo That's what
light is. That's what light is, yes, exactly. And a
gravitational wave is the same concept, except instead of wiggling
a string or wiggling the electromagnetic fields, you're wiggling space itself,

(18:13):
which is bonkers and super fun to say. Yeah, and
it's a true thing, right, Like we have measurements of
the house space wiggles out there in space out there
in the cosmos, right, Like we've detected gravitation ways from
black holes swirling around each other and things kind of exploding. Right, Yes,
we have seen it, which is amazing. This is an
idea that's a hundred years old. When Einstein developed general relativity,

(18:36):
one of the big steps forward, there was this concept
that information doesn't propagate instantaneously. Right in Newton's gravity, information
was instantaneous, so the Sun disappeared, then gravity and Earth
changed instantaneously. So when Einstein introduced this concept of the
limited speed of information, which of course came from his
special relativity, this was one of the big predictions that

(18:57):
if general relativity was true, we should see gravitational waves.
But everybody thought these things are tiny either gonna be
really really hard to see because gravity is so weak.
And then in the seventies people saw some clues about
gravitational waves because they saw pulsars, and those pulsars were
orbiting each other and slowly falling into each other, and

(19:17):
the only way that happens is if they're losing energy
somehow radiating gravitational waves. We didn't see the waves themselves yet,
but we saw the things were radiating some kind of energy,
so it must have been gravitational That won a Nobel prize.
And then decades later people built machines that actually were
able to measure the gravitational waves directly, like to see

(19:37):
the waves themselves, operate on machines here on Earth to
see the physical effects of these ripples of space and time. Yeah,
that's the big Lego experiment a few years ago that
made a big splash in the science community. And now
we have other observatories to listen for these gravitational waves. Right, yeah,
this is one of the Nobel Prizes I didn't win.

(19:58):
Do you keep track of all the Nobel Prize if
you don't win. I mean there's a lot more Noble prices.
I haven't one than I have one, so it's more
work to keep track of. But I interviewed at cal
Tech and was thinking about going to grad school there
to work on this project. But I remember thinking to myself,
they're never going to see these things. Oh my gosh,
this seems too hard, and I decided to go to
particle physics instead. And you know, they won the Nobel Prize.

(20:20):
Congrats to them. They proved me wrong. So one more
mistake I've made in my career. Well, you were involved
in the discovering the Higgs boson that won a Noble Prize, right, Yeah,
I didn't win a Nobel Prize for that, but that
went to the theorist. But yeah, I was involved there. Anyway,
we have the Lego and the Virgo experiments. These are
observatories around the world that use interferometers to see these wiggles.

(20:41):
And these wiggles come from really big events, dramatic events
out there in the universe that generate gravitational waves. Like
any time any object with mass accelerates, it generates a
little gravitational wave. But you know, gravity is so weak
that in order to see these things you need a
big gravitational wave. Things like two black holes swirling around
each other and then merging and eating each other generates

(21:04):
a lot of gravitational wave energy. That's the kind of
thing that we can see here on Earth. But even
if it's that big, even if it's a huge, dramatic
cataclysmic event, we can just barely see it here on
Earth because the ripples in space and time are very
very small. For example, if you took a rod, then
when a gravitation wave passed, it would get shorter by
one part in ten to the twenty and then we

(21:26):
get longer as the wave passed by one part in
tens of the twenties. So to see these things you
need really really accurate measurements of distance. Yeah, it's a
crazy amount of engineering to be able to measure things
so small and it works. And they've seen a lot
of these crazy events like black holes colliding with each
other or black holes with neutron stars, and we've seen

(21:47):
like maybe a dozen of them, and so they're common
and they sort of form, as you say, the foreground
of the gravitational universe, right, the gravitational field in the universe.
And so there's the question of whether or not there's
also kind of a background in gravitational waves, and whether
or not that has some interesting signals that might tell
us about the origins of the universe. So let's get

(22:10):
into that. But first let's take a quick break. All right,
we're talking about the cosmic gravitational background now, Daniel, we
talked about gravitational waves. That's when like big things like

(22:31):
black holes and neutron stars swirl around each other and collide.
They generate these ripples in space that we can detect here.
That's what you call the foreground of the gravitational universe, right,
that's right. Those are like big shouts in the gravitational spectrum,
huge events that we're listening for specifically in their short
their localizes, they come from one direction, right, then they're

(22:53):
big events were like things happening, but there might be
things happening in the background as well, that's right. Just
like when you look up at the sky you see
photons from stars. Those are like little localized events. You
can see them. But then in between all the stars
is the background, right, is the thing that you're not
looking for usually, And what we're interested in here is like,
is there a background to gravitational radiation? Are there things

(23:16):
between these big events, these big gravitational shouts that we
can listen to and we can hear and maybe learn something.
The concept of a background is something everybody should be
familiar with. Like if you're at a party and you're
trying to talk to your friend, there's sometimes a lot
of background noise, or at a restaurant or a bar,
there can be a lot of background noise, which you know,
isn't the information you're looking for. You're trying to listen

(23:37):
to your friends story, trying to seem interested in it,
but there's always other noise around you, which sometimes makes
it hard to listen to it. Right, that's the background. Yeah,
there's might be stuff that you don't maybe want to
pay attention to, but maybe you do. Maybe there's some
interesting conversations behind your friends. Yeah, that's right. I like
eavesdropping in bars, listen to other people's conversations. In this case,
it's interesting because we only recently became able to serve

(24:00):
the cosmic gravitational four ground. Right, we were listening for
gravitational radiation and didn't hear anything for years and years
and years because we were making our instruments more and
more sensitive. It's only recently we've been able to hear
the loud shouts. Right, Only recently we've been able to
hear your friend at the bar talking in gravitational radiation.
Now we're getting ambitious. We're like, can we hear what's

(24:20):
going on between the shouts? Is there anything there? Also?
And it's interesting that you call it gravitational radiation? Can
I use it? Is it a technical term? Gravitational radiation? Yeah? Absolutely,
it's radiation. It's transmission of energy. And when two black
holes collide, the new black hole that's formed has less
mass than the masses of the two black holes that
formed it combined. And the reason is a lot of

(24:42):
the energy is lost. And it's not lost to light,
it's not lost the particles. It's lost through gravitational waves.
And sometimes it's like, you know, five times the mass
of the Sun is lost in gravitational radiation. So yes,
it's absolutely the radiation of huge amounts of energy. Interesting. Yeah,
like it's quantifiable, like it carries energy. It's not just information.

(25:03):
It has some sort of substance to it, It has
some energy to it. For example, you could take a
seventy solar mass black hole, combine it with a thirty
solar mass black hole, and end up with a ninety
solar mass black hole, which means that ten masses of
the Sun's worth of energy was dumped out into the
universe in gravitational radiation. Now, don't be confused, we're talking

(25:24):
about gravitational waves, not gravitons. Right. Gravitons are not a
thing we know exists. It's the theoretical quantization of the
gravitational field. So we know the gravitational waves exist. Those
are waves in the gravitational field, but we don't know
if that field is quantized into like the smallest piece,

(25:45):
which would be gravitons. So gravitational waves exist, we don't
know yet if they're made of gravitons. Interesting, So then
how would you define as the cosmic gravitational background. Is
it like fainter signals or is it like a hum or?
Is it like actually always in the gravity of the universe. Yeah,
it's all of those things. It wouldn't be localized in
any particular direction. It's not something that comes from one

(26:07):
particular event, like one of these cataclysmic mergers, and it
should be an overall hum And I think it's useful
to go back to the cosmic microwave background radiation as
a sort of comparison because it's very similar in concept.
Like when you look at in the night sky you
see stars and galaxies, but also between those stars and galaxies,
we are getting light from the very early universe. That's

(26:29):
the cosmic microwave background radiation, and it's in every direction,
no matter where you look. And it first appeared as
a hiss in a radio telescope. We had a whole
fun episode about how that was discovered. And it's left
over light from the very early universe from four hundred
thousand years after the Big Bang. The reason we call
that the background radiation is again because it's sort of
everywhere the universe was filled with this plasma and admitted

(26:52):
this light which was going in all direction. It's like
it's not coming from a particular thing like a sun
or a pulsar or something like that. It's like it's
coming from everywhere, like you hear this his his in
the signal of the universe everywhere you look. Yeah, and
it's sort of a funny naming thing because it's called
the background radiation for that reason that it doesn't come
from any particular direction. But you know, now we have

(27:13):
experiments dedicated to just looking for this. So it's like
you're looking for the background. Is it really still the background?
It's sort of like your target, it's your signal, right,
no longer the background. Right, if the background becomes a foreground,
then what's in the background the background background. We have
multiple Nobel Prizes awarded, specifically four experiments measuring and analyzing

(27:33):
ripples and wiggles in the cosmic microwave background. So it's
definitely you know, worth calling it a foreground something. But anyway,
you have to call it a background Nobel Prize, like
a noisy you know, static qui Nobel Prize. Maybe it's
like an off the record Nobel Prize, you know, like
this is on background only. Yeah, there you go unofficial.

(27:53):
The universe doesn't want to speak up about its secrets.
It just wants to like slip it to us on background.
I think that would conveniently reduce the number of noble
prices you haven't want, Daniel. That would be difficult. I
could be like, look, I have this secret about the universe,
but I can't tell you how I know it because
I got it on background from a source. I would
be pretty disappointing. And of course you gotta protect your sources,
right absolutely, even if it's the universe. Yeah, So then

(28:16):
the cosmic microwave background is this sort of leftover light
from the early universe. What would be the cosmic gravitational background.
So we don't know exactly because we haven't seen it.
We only have theoretical ideas for what could be creating it,
what it might look like, but we're excited to see
if it's there. The limitation on seeing this thing is
making our experiments sensitive enough to listen to these very

(28:40):
very quiet signals, And the challenge there is making your
experiment insensitive to other things that look like gravitational waves, right,
that look like this hum or, this hiss. Because the
way these experiments work. For examples, you have like two
mirrors hanging miles apart in a tunnel underground, shooting lasers
back and forth to me sure the distance between them,

(29:01):
and it's very easy to get wiggles in that distance.
It's like the mirrors shake a little bit for a
breeze flows through and moves the mirror. You know, the
size of a gravitational signal is a tiny, tiny fraction,
much smaller than the width of a human hair, and
so it's very hard to isolate these things and just
sort of like experimentally to make these systems work so
you can see these signals, and so the challenge is

(29:23):
making them work even better, making them even more sensitive.
So now we could pick up like a low level
omnidirectional hum if it exists, right, And I guess the
tricky thing is that you know, you have your instrument,
it's listening to the universe, and it's picking up noise
from its environment, from the Earth, from the circuits in
your instruments, but it's also maybe picking up noise from

(29:45):
the universe itself in gravity right in the gravitational spectrum
of the universe. And so you want to be able
to say like Okay, this crazy random noise here that's
from my experiment, and this crazy random noise here is
actually from the universe. Is gravity exactly. It will be
a much more difficult thing to demonstrate we've seen than
the gravitational waves we've seen so far, because it's easier

(30:08):
to see things that are like individual cate exclismic events.
First of all, they're isolated, so you can say it
happened and then it stopped, whereas the gravitational background is
going to be like all the time, so you can't
say like, oh, we turned it on and off. And
here's the difference. And also, the gravitational waves that we
have seen have a very particular signature, like when black
holes swish around each other and create these gravitational waves,

(30:31):
it's not just like a big screen. We know exactly
what it should look like. You should start small, and
should get bigger and bigger and faster and faster as
they swore a closer and closer. So the gravitational waves
we're looking for in the foreground have a very particular
pattern when we first saw them. That's what convinced us
that we had actually seen them right, that they look
just like what we expected. If you do these numerical
relativity calculations that predict the signature of the fingerprint of

(30:55):
these gravitational waves, that makes it much easier to see
they come from particular direction, they're very where it lived,
and they look different from everything else. But as you say,
we're just looking for a hiss, it's much harder to
know if that his is coming from gravitational waves or
from wiggles in your detector. So experimentally it's much much
more challenging. And I think the idea is that this

(31:15):
noise in the gravity of the universe and the gravitational
spectrum of the universe, again, it's not coming from anywhere
in particular, It's coming from everywhere, and it's maybe made
up of you know, what would it be made out of.
Is it made out of like big explosions somewhere else
that have sort of propagated and diffused throughout the universe?
Is there any idea? We don't know exactly what it

(31:35):
might look like, and we can dig in a minute
about the possibilities what could be generating it and what
it would look like. But you know, one possibility is
that it looks sort of like a hiss. Another possibility
is that it's like very slowly changing, you know, like
instead of having a wave that changes over seconds the
way gravitational waves from black holes do, it might be
like a very gradually changing tide, like we're seeing a

(31:58):
gravitational wave that slates over like years or decades or centuries.
And that again, it would be much harder to see
because we like to look for the pattern that tells
us that there was actually something there. But again I
think it is that you would be looking for like
a hum in the distortion of gravity in the universe, right.
We have to make sure that you know you're measuring

(32:20):
the wiggles in gravity really well, and then you'd be
looking for some sort of activity there that wasn't like coherent.
There wasn't like a spike or a particular shape. You
need to establish that you're seeing a signal that is
louder than the noise in your device, that your device
is less noise than the signal you are seeing, which
is tricky. And then what it would look like is,

(32:40):
as you say, it would be like sort of random
ripples in space and time, the way the cosmic microwave
background radiation sort of has like little ripples in it.
You get like a little distortion this direction, a little
distortion not direction, a little distortion the other direction. And
all these distortions would be much much smaller than the
signals we've already seen. It would be even fainter. Right.
And what's cool is that with our instrument snow like
Lego and Virgo, we can tell which way gravitational waves

(33:05):
are coming from. Right, so this would also tell you
that they're coming from everywhere, not just in a particular direction.
That's right. We can tell the direction of gravitational waves
mostly because they take time to propagate, and so we
can see them in different observatories around the world. There's
one in Louisiana, one in Washington State, and one in
Italy does Virgo, and as the gravitational waves sort of

(33:26):
washes over the Earth, it arrives at one place before
another place, and that timing information allows us to figure
out so where in the sky it might have been
coming from. But as you say, is cosmic gravitational background
would be coming from all directions. Cool. Well, let's get
into now whether or not this gravitational background really does
exist and what could be making it and what he

(33:47):
can tell us about the universe. But first, let's take
another quick break. All right, we're talking about finding wads
in the cosmic gravitational background of the universe. Then you know, like,

(34:10):
is there something interesting in the background noise of gravity
in the universe. Yeah, we don't know yet whether it's
out there, right, This is like an idea people have had.
There are theories that suggest it might be out there
and could contain like treasures of the universe, and people
have looked for it, but we don't really have a
conclusive evidence for it so far. Like Lego and Virgo

(34:32):
are best gravitational wave detectors have looked for us very specifically.
But what they see sort of in between the loud
shouts from black holes, looks to them just like noise
in their detector. Like they can estimate how much noise
they expect to have in their detector, and that's what
they see, and their estimate for their noise is above
anything that anybody predicts for cosmic gravitational background. So sort

(34:53):
of like you go into a library and you're listening
for whispers, but you know you can't hear the whispers yet.
All you can hear is like electronic noise in your device,
and that's what you hear, so you don't see anything.
We also haven't really learned anything yet. You just learned
that you need a better listening device. Oh, I see.
It's like, we know we have kind of a crappy microphone,
so we know that with our current set up, we

(35:14):
can't listen to this background radiation, which means that we
maybe don't know if it exists or not. Like this
is a theoretical concept, then this cosmic gravitational background or
does the theory pretty much predict that it exists, we
just can't measure it. Yeah, but first I gotta stand
up for Lego and Virgo, because you just called them
crappy microphones are like the world's most amazing supersensitive microphones,

(35:35):
just not quite supersensitive enough to listen to this incredibly
faint signal. But you're you're absolutely right. So this is
something people have theorized, and you know, there were moments
when people thought they might have seen it. Lego hasn't
seen it yet, but there was a moment people might
remember this experiment BICEP two something on the South Pole,
which thought they saw the effect of this radiation on

(35:57):
the cosmic microwave background radiation they can measure, and they
had this whole claim that they had discovered it and
it was evidence that there was cosmic gravitational background radiation
which was tweaking the cosmic microwave background radiation. But it
turns out it was wrong. It was just that they
didn't understand how much dust there was in the university
caused a fake signal. So there was a real excitement
there for a while, and then that faded. And now

(36:19):
recently there's another experiment which is super cool, which it
uses a galaxy size measuring device and they think they've
seen evidence for this cosmic gravitational background. Why you mean
we have something that big that we can use to
measure the gravitational ways of the universe. We have something
the size of a galaxy. Yeah, well, we have our
galaxy and people have figured out how to use the

(36:41):
galaxy as a gravitational way of observatory. We're gonna do
a whole episode on this because it's super awesome. But
very briefly, if you look at pulsars out there in
the universe, these are fast rotating neutron stars that send
us super super regular pulses of light, and you can
tell if those things get pushed away from us or
pulled towards us when a gravitational wave passes, because it

(37:02):
changes when those light pulses arrive. And there's an experiment
that's looked at some of these very regular pulses. It's
called nano grab and they see a signal which looks
like cosmic gravitational background radiation. But you know, it's early
days and there have been false claims before, so nobody's
really accepted this yet. It's just sort of like an
exciting possibility, all right. So we're actively listening for this

(37:23):
background or trying to listen to it. And it's sort
of a funny thing, right, It's like trying to see
if you can listen to the background noise and in
a noisy bar, right or in a quiet library, it's
sort of like you're trying to get everyone to quiet
down so you can see if there is sort of
a his there or not exactly. If you're curious about
what's going on outside your house, for example, you turn

(37:45):
off the TV and Telbrady shut up and listen to
see if it's just crickets or something else going on. Outside.
Did you turn this into a scary movie? Like what's
out there? What's out there that we we can't hear? Yeah,
And there's a whole range of reasons why we think
the cosmic a gravitational background might exist, and there's some
like kind of boring possibilities to explain it, and then
there's some crazy, exciting, bonkers possibilities. I guess, you know,

(38:08):
maybe as many of our listeners might be wondering, like
does it make sense that there wouldn't be a cosmic
gravitational background? Like things are everything causes gravitational ripples, so
why wouldn't there be a background exactly? And that's the
sort of most boring explanation is everything causes gravitational waves,
Like every time you accelerate in your car, you cause

(38:30):
a little gravitational wave. As the Earth moves around the Sun,
it's generating gravitational waves, and there's a lot of that
stuff going on in the universe. Most of that is
really really small, but the idea is that it sort
of adds up to this like overall stochastic background, and
in particular, black holes that are too far away for
us to like make out individually should add up to

(38:51):
like an overall sort of noise level in the background.
There should be black holes merging and neutron stars colliding
with black holes all over the galaxy. Some of them
are too far away for us to like pick out individually,
but they should turn into like a hum, the way
like a whole crowd in a baseball stadium. You can't
make out their conversations, but you can tell the people
are talking. You hear this like overall hum. So people

(39:12):
expect that that can tell us something about like the
overall rate at which gravitational waves are generated by distant objects. Right,
But I guess is it possible that maybe you know,
I'm just thinking, like maybe the universe could be quantized
enough so that at some point there is no noise,
do you know what I mean? Like, maybe at some
point there's a minimum size to these irritation and ways

(39:33):
at which point and since everything is so faint, everything
just flattens out. Is that possible? It's possible. But you know,
take an analogy to photons coming from distant galaxies. You
look up in the night sky. You can't tell that
there are galaxies there because they're so distant, Because the
photons aren't arriving very frequently because they're quantized, right, It's
not like you can get point o one photon. Every

(39:53):
second you get one and then a minute later you
get another one. But they are still coming, and so
there is a non zero rate there. It's not like
it goes exactly to zero. They just get less and
less frequent. And so if gravitational waves are made of
gravitons and they're super duper distant and faint, we should
still be getting gravitons every once in a while, even
from the most distant sources of gravitational radiation. But yeah,

(40:15):
that would be really challenging to pick up, right. But
I guess you know, photons are sort of discretized in direction,
but gravitational waves kind of goes out in all directions, right,
like a ripple in a pond. Yeah, but you know,
a star sends out photons in all directions. It's really
equivalent when a black hole merger event happens. It sends
out gravitational waves in every direction. We just only pick
them up from one direction, and so we would be

(40:38):
picking up the gravitons from those events. And if these
things are happening everywhere, we should be getting gravitons and
gravitational waves in every direction from all these distant events.
And so this is, you know, what people expect at
a very bare minimum. Because we know the black holes
are generating gravitational waves, we should be able to pick
out at least the gravitational background from these distant events. Interesting,

(40:59):
so we think there's definitely a cosmic gravitational background from
just because we know there's things happening in the universe
that are making them. So it makes sense that we
would be sort of inundated with the faint ripples from
all of these events, even things other than you know,
black hole mergers. Even for example, when a star collapses
into a black hole, you get gravitational waves. When a
supernova goes off, you get gravitational waves. If a supernova

(41:22):
goes off near a black hole, like JJ was suggesting,
you get gravitational waves. So we should be able to
see all these things sort of like added up all together, right,
but it might be so faint that we can't see
them or hear them. They are definitely too faint for
us to see them now. But you know, with future
observatories and improvements in our technology, we should in principle
be able to see it if it's there, right, all right.

(41:42):
So that's one possible source of cosmic gravitational background. What
are some of the more bonkers sources that might be
out there? One really exciting one is a mission of gravitons.
You know, in some theories, gravitons are the quantization of gravity.
Remember that our theory of gravity general real activity is
a classical theory. It says that you can have any

(42:03):
arbitrarily small distance, or any arbitrarilly small amount of energy,
or any arbitrarily small slice of time. The quantum mechanics
says that's probably not true, and the theory of quantum
gravity would have gravitons in it, as we've said before,
like the basic element of gravity, and these things should
be emitted basically any time anything happens, you know, when
a particle gets accelerated, it should emit a graviton, When

(42:26):
an electron jumps down an energy level, it should emit
a graviton. And so in principle the universe is filled
with these gravitons, and they would be very very high frequency.
Like the gravitons we've seen so far, have an oscillation
time of like you know, a few seconds, these would
be much much higher frequency, like you know, mega hurts
sort of gravitons, and so that's exciting because it would

(42:48):
be like a signal of quantum gravity, right, But do
they need to be gravitons, Like when a adom relaxed
us from an excited state doesn't also generate a gravitational
wave like a regular wave. It does, But now we're
talking about a quantum particle, and a quantum particle should
emit quantized bits of radiation, right, it can't admit classical radiation,

(43:09):
And so this is something we don't understand. We don't
know how to do gravitational calculations for a particle, for
a quantum object that requires the theory of quantum gravity.
We just don't have, so general relativity like can't tell
you what happens to the gravity of a particle. And
we can't really do those experiments very easily because particles
have almost no detectable gravity because there's so light in
mass and gravity is so weak. So we just don't know.

(43:32):
But you know, maybe all those particles out there are
sort of like humming in gravity and we can pick
up like the some of them like, maybe all the
particles in the universe sending us gravitons simultaneously would be
something we might be able to detect. Interesting. I think
they're saying that, maybe, like the gravitational background of the
universe is not coming from these giant events and supernovas
and black holes gliding, and it might be being generated

(43:55):
by everything like you and I just sitting here. It
could be generating gravitational noise. Yeah, we should be in theory,
and we should be able to measure it. And if
we see it and we measure its frequency, we can
use that to understand how they're being generated, the same
way we look at the frequency of everything else, and
we see emission from hydrogen, for example, at a particular frequency.
We see transitions in lithium gas at a particular frequency.

(44:17):
So these would be like a fingerprint for what's generating
gravitational waves in the universe. Who tells something about gravity
and the composition of the universe. It would be amazing
to see gravitons emitted as the cosmic gravitational background. And
what about some of the other bonkers possibilities that are
making this cosmic gravitational background. One that gets me really
excited are gravitational waves emitted from the very very early universe.

(44:40):
Our theory of what happened in the very beginning of
the universe is called inflation. It suggests that space was
stretched by a ridiculous amount, by like a factor of
ten to the thirty in a short amount of time,
like tend to the minus thirty. When you do that,
you've got to generate gravitational waves because that's like the
biggest ripple in space has ever seen, right, So there
should be huge gravitational waves left over from that, because,

(45:04):
like the CMB, it happened everywhere all at once, so
it should be like a cacophony of gravitational waves. But
over the fourteen billion years since, they've probably gotten stretched
out and smoothed over. And so this would be exciting
to detect because it would probe the very very very
very early universe. Like people talk about how the CMB
tells us about the Big Bang, it doesn't really. It

(45:26):
tells us about a plasma that's formed four hundred thousand
years later. It's like reading about the birth of Jesus
from somebody who wasn't there, who wrote like hundreds of
years later. Gravitational waves from inflation would come from like
ten to the minus thirty two seconds after the beginning
of the universe. So it's like really a first hand
account of the beginning of the universe. If we could
see it, it would tell us a huge amount about

(45:48):
how the universe actually began. All right, Yeah, interesting, I
guess you know, anything with mass and energy affects gravity, right,
or it creates gravity or and so anytime you have
a change in the universe, or anything really happening in
the universe, it's going to generate a gravitation a ripple, right, Yeah,
even like the beginning of the universe, that must have
generated a bunch of ripples. Yeah, huge ribbles exactly. Tsunamis

(46:10):
of gravitational waves. You could probably serve them. Yeah, with
your friend Walda on a surfboard. Look, I know you
keep trying to include him. I just don't like the guy. Okay,
what do you have against Walda? I don't know. He
just won't let me find him. You know, he's just elusive.
He's a sneaky guy. But sneaky particles are totally cool

(46:32):
with sneaky particles, all right, So then what's the last
possibility here that might be causing a background in the universe.
The last possibility is the most bonkers, and these are
cosmic strings. We talked about this one in the podcast
Now a couple of years ago. They're like cracks in
space and time. We are used as spacetime being sort
of smooth. If you go from here to there, you

(46:53):
can sort of just like slide through space. But we
don't really understand how space was formed, and as the
universe cooled, it might have like formed differently in different regions,
leading to like weird discontinuities, like parts of space are
a little different here and a little different there, and
at the boundaries there can be these cracks where like
space changes from one state to another, And so these

(47:13):
would be like long filaments, maybe even you know, like
thousands of light years long, that have like cracks in
space and time, and as they wiggle, they would generate
gravitational waves. And there's some theories that like the ends
of the wiggle really fast like a whip, causing these
crazy gravitational waves. So if that's happening all over the universe,
we might be able to pick up some of those

(47:35):
signals and detect the existence of these cosmic strings. Whoa,
it's like the universe was a giant guitar, and like
there's giant light years long strings that are you know,
basically making no it's music, right, Yeah, we don't know
if they exist. It's a super cool theory. If we
do find them, we would tell us that we understand
something deep about how space and time sort of formed

(47:56):
in the early universe. You can think about them sort
of like the way ice isn't as clear. If you
take water and you cool it down, you get these
cracks sometimes because it doesn't all crystallize in exactly the
same way. And so this would really tell us something
about how space itself formed in the very early universe.
Would be super awesome. That's my new theory of the
origin of the universe. Dandium. When new religion, it's that

(48:17):
the universe it's she's a giant guitar played by a
giant rock star named Waldo. Waldo is um we just
found it a new religion, all right, So I guess
when one final question is, how would we know which
of these origins of the background might be, Like do
we have any hope at all of ever knowing like, oh,
this noise is coming from inflation or gravitons or cosmic strings,

(48:41):
they would look different, right. Gravitational waves have frequencies, just
like light does. Different frequencies of light look different. You've
got radio, you've got infrared, you've got X rays, you've
got gamma. Raised in the same way, gravitational waves have frequencies.
The ones we've seen have frequencies of about a second,
and we have sensitivities due to our detectors to a
range of frequencies. So they come at very very high frequencies,

(49:03):
we think they might be gravitons. If they come at
lower frequencies, they might be from inflation. If they're sort
of periodic and spastic, then they might be from cosmic strings.
So each one of these has like a different fingerprint.
If there's just nothing there except for like the low
level hum that that should be pretty flat and lots
of frequencies represented. So the sort of spectrum of the
frequencies there is a fingerprint that tells you what generated it.

(49:26):
We just gotta, I guess, tune up those microphones and
turn up the amplifiers and reduce the noise just to
be able to maybe listen to these signals. And be
able to tell which one is which. Yeah, and we
have some exciting plans. The Ligo Observatory is based here
on Earth, and what they'd like to do is build
a much bigger version out in space. They want to
put three satellites out there that managed to somehow keep

(49:49):
a very precise distance from each other and then shoot
lasers back and forth to detect the passing of gravitational
waves or the cosmic gravitational background. It's not science fiction.
There's a real experiment might really happen sometime in the
next ten or fifteen years, and it can really teach
us something deep about the universe. It's Lego in space.
It's called Lisa, all right. Well, that's what the cosmic

(50:11):
gravitational background is. It might be there, it might not
be there. We think it's there, but we don't know
quite what's causing it, right, and we don't know if
it's there, We don't know what's causing it. But we're
desperate to listen to these signals that might contain new
treasures about the early universe. So stay tuned once again,
and maybe think about that when you look up at

(50:31):
the night sky once again, and how much information is
bathing over us as we speak, and that might tell
us about the very origin of the universe. And think
about all the gravitational radiation that you are generating. Every
time you go out there and get exercise or hit
the accelerator in your car. You are contributing to the
cosmic gravitational background, some of us more than others. Like

(50:52):
if you're a couch potato, technically you're just creating less
noise for the universe, right, that's true, But you're getting
more and more massive, So if you ever do get
off the couch, you're gonna be pretty noisy, right, there's
a trade off there, all right. Well, thanks for joining us.
We hope you enjoyed that. See you next time. Thanks

(51:14):
for listening, and remember that Daniel and Jorge Explain the
Universe is a production of I Heart Radio. For More
podcast For my Heart Radio, visit the I Heart Radio app,
Apple Podcasts, or wherever you listen to your favorite shows. Yeah,