June 20, 2025 • 20 mins
Daniel and Katie answer a question about why distant objects can look surprisingly large in the sky
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Speaker 1 (00:07):
Hey, Daniel, what's the most distant object you've ever seen?
Speaker 2 (00:12):
You know, the Andromeda galaxy is pretty far away. It's
like millions of light years. How about you, what's the
most distant thing you've ever seen?
Speaker 1 (00:21):
When I am sleepy, it is my couch.
Speaker 2 (00:24):
Your couch is far away.
Speaker 1 (00:26):
Yeah, it's so fun.
Speaker 3 (00:30):
W want mom.
Speaker 4 (00:50):
Hi.
Speaker 2 (00:50):
I'm Daniel. I'm a particle physicist and a professor at
UC Irvine, and I like to couch all my puns.
Speaker 1 (00:57):
Hi. I am Katie Golden. I a podcast on animals,
and I am cushion for a never mind. I I
thought if I just said cushion that a pun would
come to me, and it didn't. It just made me
(01:17):
want to take a nap.
Speaker 2 (01:18):
You were pushing for a pun, Cushion.
Speaker 1 (01:20):
I thought I was going to be bruising for a
cruise in, but I was cushion for a pushing. I
guess a defenestration.
Speaker 2 (01:28):
Sounds like you're set up for some punishment anyway. Welcome
to the podcast. Daniel and Jorge explain the universe in
which we try not to punish you. We try to
enrapture you. We try to share with you the joy
of understanding the nature of the universe, or when we
are mystified by the puzzles, the curiosity, the open questions
about the nature of this cosmos we find ourselves in,
(01:50):
we seek to understand it, we seek to explain all
of it to you.
Speaker 3 (01:54):
On the podcast, Daniel.
Speaker 1 (01:56):
Is this the one coming out on the summer solstice?
Just the quickest side before we get into our listener question.
I just visited in near Dublin, Bruno Boyne, these funeral mounds,
and it was so interesting because these very very ancient
people created these funeral mounds which had an internal structure
(02:21):
that they're large mounds like you could actually you can
go inside it. They can do ceremonies inside, and they
constructed it such that like during the solstice, the summer solstice,
on the longest day of the year, it illuminates this
entire inner chamber. And I just love that. So long
ago we were already thinking about physics, figuring out the universe,
(02:45):
and like trying to think about how light works and
use it for our interesting little Richard.
Speaker 2 (02:52):
Yeah, exactly. I don't think that's a digression at all.
That's exactly what we're talking about today. People have been
asking questions about how the universe works, trying to make
connections with the cosmos. For as long as people have
been asking questions, all you have to do is look
up at the night sky and wonder, how does that
all work? Why does it seem so regular? Why are
their patterns? And then patterns within those patterns, what do
they reveal about the nature of the universe. Because one
(03:15):
of my favorite things about the universe is that it
does seem to follow laws, and we can figure those
laws out if we pay attention to those patterns and
try to draw dotted lines between them. And as you say,
people have been doing that for a long long time.
We sometimes dismiss our ancient history as like prescience and
only take things seriously if it's been figured out in
the last few hundred years. But you know, the development
(03:36):
of science is much more gradual than is often described
in history textbooks. People a long long time ago. We're
thinking methodically and trying to understand the universe, and that's
what people are still doing. And one of my favorite
things is trying to think deeply about the nature of
time and the universe. How far back in space can
we see, what do we see, and what do we
learn about how big and how strange the universe is
(03:59):
as we look into space. And so on today's episode,
we're going to be doing exactly that. We're going to
be answering a question a listener asked us about the
nature of deep time. So today on the podcast, we'll
be answering listener questions. Summer Solstice Edition. We're so happy
(04:20):
to be back and be back in your ear with
this special episode to celebrate the summer Solstice. Happy Solstice, everybody.
Speaker 1 (04:26):
Happy Solstice. Enjoy your roasted pine combs and your chicken
feather damp and you're huge bonfires.
Speaker 2 (04:36):
I hope you stay up late in the night, stare
at the stars, wonder about the nature of the universe,
and then write to me with your questions about it
questions at Danielanjorge dot com. Don't be shy, send me
your questions. You will get an answer. And on today's show,
we're answering a question from Nick in Florida, who's wondering
about the very oldest things we see in the sky.
(04:57):
Here is Nick's question.
Speaker 4 (04:59):
Hey Daniel, this is Nick from Jacksonville, Florida. I came
across an article this morning that taught me something new
and amazing about the universe. Apparently, beyond the distance of
approximately fifteen billion light years, objects we see with telescopes
start becoming larger again instead of smaller. My question is
(05:19):
how close are we to being able to see this
far and what will be the implications for cosmology once
we get there? Thanks and love the show.
Speaker 1 (05:28):
So this is really interesting. I have never actually heard this,
that objects that you see beyond a certain distance start
to appear larger, and I also don't understand how we
can see this far in the first place.
Speaker 2 (05:45):
Yeah, it's really fascinating. It's one of my favorite facts
about the universe. It really shows us that the universe
is expanding and that we have a pretty good grasp
on how that works, or at least what's been happening,
which is already kind of astounding. The fact that wall
we've been trapped on this tiny little rock in one
corner of the universe, we can measure things that tell
(06:05):
us about the universe on the grandest scale. You know,
what's happening over billions and billions of light years, and
what's been going on over the last few billion years.
It's really incredible what we are able to piece together
from just the few photons that happen to land on
our rock.
Speaker 1 (06:22):
That is wild. I mean, it's also just kind of
weird that in a way we can time travel, but
just with our eyes and some fancy shaped glass.
Speaker 2 (06:33):
Yeah, it is really amazing. So there's a few different
things going on here that all come together to make
this really weird effect that if you can get them
all working together in your mind, show you how beautiful
our explanation is. So let's take them one at a time.
One concept is the one you just measured that looking
up into the sky is basically time travel, and that's
(06:53):
because the speed of light is not instantaneous. You know,
something happens in Andromeda, you don't see it right away.
It takes time for light to go from Andromeda to
our eyeballs. And so when we look up at the
night sky and we see Andromeda, we're not seeing what's
happening in Andromeda right now. We're seeing what's happening in
Andromeda when that light left Andromeda. And because Andromeda is
(07:16):
several million light years away, it means we're seeing photons
that left Andromeda several million years ago. Andromeda is not
even in the same place it was when emitted that light,
and so what we're seeing really is ancient history. And
the further we look out into the universe, the deeper
back in time. We are seeing like things are happening
(07:38):
out there in the deep universe right now, but we
won't see them for millions or billions of years, depending
on how far away they are. So as we look
out into the universe, we are really seeing further and
back in time just because of this simple basic physics
fact that light takes time to get to us.
Speaker 1 (07:54):
So you're saying that if Andromeda has a newspaper that
is really huge that we can see with a telescope,
by the time that light reaches us, we're looking at
a pretty old headline. It is out of date.
Speaker 2 (08:08):
Yeah, exactly. And conversely, it means that if Andromeda has
aliens that point their telescope at us, they're seeing millions
of years ago. They would see the Earth as it
was millions of years ago. They wouldn't even know that
there was an intelligent technological civilization on our planet for
millions of years, because that's only the very briefest part
(08:29):
of our history. And so somewhere out there are photons
from like when the Earth had dinosaurs on it, or
even further away, there are photons from the Earth when
it was like just a ball of magma. All those
photons are still out there in the universe. Somebody could
be picking them up where they're alien eyeballs and seeing
the ancient history of our planet.
Speaker 1 (08:50):
I like to imagine that the reason aliens don't contact
us is they caught us when we had the weird
Anomalocaris that came marine explosion, giant disgusting shrimp looking thing,
and they're like, no, thank you, I'm not gonna get
in touch with that planet.
Speaker 2 (09:07):
It's too icky.
Speaker 3 (09:08):
Nah.
Speaker 1 (09:10):
Yes, don't like shellfish seafood.
Speaker 2 (09:12):
So that's one concept, right, Things in the sky are
old news. Right, We're not seeing the universe as it
is now. We're seeing the universe as it was. And
on one hand, that's frustrating because it means like we're
missing out on the latest, greatest news. On the other hand,
it's a huge gift. It means we can see back
in time and like on Earth, you want to know
(09:33):
what happened during the Roman Era or what happened a
million years ago, you got to look for clues that
happened to be here and be left over. Most of
that stuff is destroyed or buried, but in the history
of the universe, you just look into the sky. It's
all still there. It's just arriving right now, fresh as
the day was emitted. So you want to know what
happened in the universe a million years ago, or a
billion years ago or ten billion years ago, you just
(09:54):
build bigger telescopes to capture more distant, fainter photons, and
you can see it. It's all still they're in the sky.
Speaker 1 (10:01):
So the further back in time we want to go
in understanding our universe, the further way look exactly.
Speaker 2 (10:09):
So you have to imagine these shells, like in a
shell that's very close to the Earth, we're seeing the
more recent past. We can't see the distant past in
a close by shell, but we can't see the ancient
past in a more distant shell. So we can't just
like dial the knob and say I want to see
what happened over here at that time. We can only
see a certain time for every certain location because photons
(10:31):
are just now arriving from that location based on how
far away they are, all right, So that's one concept.
The other concept is how big things look in the sky.
And so to understand this first, let's just imagine the
universe is not expanding. Imagine that you have galaxies and
they're just hanging in space and nothing is moving. Everything
is just hanging there. There's no relative velocity. Then you
(10:54):
would expect things that are further away look smaller, right, because,
for example, if you have some galaxy and it's right
up in your eyeball, then photons from one side of
the galaxy and photons from the other side of the
galaxy are gonna come from very very different angles. Now
you move that galaxy further away in the sky, photons
from two different edges of the galaxy are now getting
(11:15):
closer and closer together, so they just seem smaller in
the sky. This is no magic. It's just like you
hold your finger close to your eyeball, looks big. You
push it further away, it looks smaller.
Speaker 1 (11:25):
Right, It's just perspective, I see, Yes, So I have
been told that people from far away are not tiny people.
They are just far away. So all right, that makes
perfect sense.
Speaker 2 (11:36):
Yeah, exactly, And we have an intuition for this exactly
the way you describe you don't think that people who
are standing next to you are giants and people who
are far away are midgets. You know this, and you
invert it in your mind. When you see people, you
can tell how far away they are, and so you
estimate their actual size because you've taken that into account. Right,
You have this intuition for things looking small far away
(11:58):
and things looking big close up, and you'll count for
this naturally. So if the universe was like that and
nothing was moving, then it would just look like that.
Things that are further away would look smaller. But there's
one more effect you have to throw into the mix,
and that's the expansion of the universe. Things are not
just hanging in space. There's a relative velocity there. Space
(12:20):
is expanding between galaxies, or another way to think about
that is galaxies are running away from us and from
each other, so everything is getting further and further away.
And what that means is that the light we're seeing
from those galaxies is not from where they are now,
it's from where they were when they emitted that light.
(12:40):
Oo okay, right, So, for example, imagine something on the
very distant edge of the universe. We talk about how
something could be forty five billion light years away, the
furthest things in our observable universe, all right, But when
we talk about that, we mean that's how far away
it is now, not how far away it was when
it admitted the light that's now coming to us, that's
(13:02):
now arriving on Earth. It was much much closer when
it emitted that light. What's happened in the intervening period
as that photon has flown through space towards us is
the universe has expanded and it's gotten further and further away.
Speaker 1 (13:15):
Okay, So if I try to think about this on
human scale, you have various people standing out in a field,
and it's a very big field. So I need a
telescope for me to see people, and it's a telescope
with a slight delay, or a camera, let's say a camera.
It's got a delay in it, so I will only
(13:35):
see them after a while. So like, I could be
looking at these people who are further away, but then
the people who are closer to me, let's say they
are running away from me at a faster rate, so
they might with my delayed camera, look further away by
the time I get my snapshot than the ones who
(13:56):
were actually further away from me, but they look bigger
because they were not running away from me as fast
as the other.
Speaker 2 (14:03):
Yeah, exactly, you have old information, and so you're taking
these snapshots and you're seeing people where they were when
the picture was taken. But if the picture takes a
while to get to you, by the time it does,
they could be much further away, right. And that's important
to think about because that affects how big they look
in the sky because when we talk about things that
are really really far away, we're seeing them remember as
(14:26):
they were when they emitted those photons, and if they
were much closer when they emitted those photons, they took
up more space in the sky because they were closer.
So those photons have been flying towards us those millions
or billions of years, but they started pretty close by, right,
So they started further apart because the thing was closer
to us. So there are two effects happening here. One
(14:47):
is things that are further away look smaller, right, But
the other effect is this expansion effect that some of
those things that are now really really far away were
closer when they emitted the photons. So these two competing
effects there. And so what that means is that if
we look at nearby stuff, mostly the first effect dominates
that things that are further away look smaller. So as
(15:09):
you look out into the night sky, stuff that's further
away looks smaller and smaller and smaller. But then there's
a point from about ten billion years ago when the
other effects starts to dominate. When we're seeing stuff that
was much closer when it admitted that light. Stuff looks
smaller and smaller and smaller as it gets further away
until about ten billion years into the past, and then
(15:30):
stuff starts to look bigger and bigger. So the oldest
stuff actually looks bigger in the night sky than some
of the closer stuff, some of the more recent stuff,
because you have these two effects that are competing with
each other.
Speaker 1 (15:42):
Because we're basically seeing like an after image of something
that was going Yeah, it's actually not further from us.
I mean that the actual star is that was there,
but those photons that after image are finally hitting us,
representing where in the sky.
Speaker 2 (16:01):
When we talk about the universe, we talk about where
things are now, that's where we put them in our
mental maps, but we don't think about where they were
when they emitted that light. And so if you just
line things up based on where they were when they
emitted that light, then yes, closer things would look bigger,
and further things would look more distant. But because the
expansion of the universe, not all the photons that are
arriving here are perfectly lined up with where things are now,
(16:25):
So things that are more distant now can actually look
closer than things that actually are closer still, so there's
this amazing effect in cosmology. And so Nick is asking
about this effect, and he's wondering, can we see this yet?
Can we see far enough out into the universe to
see this turnover where the really distant stuff starts to
get larger and larger again, and what does that mean
(16:47):
for cosmology? And the answer Nick is that yes, we
already can see that. We can see much further than
ten billion years into our history. We can see all
the way back to almost fourteen billion in years. So
we have observed this. This is not like a theoretical
concept that people are wondering about and going ooh, maybe
in the future sometime. This is something we see. We've
(17:09):
measured this, and this really nicely confirms our whole model
for how the universe is expanding and how that expansion
is accelerating. It's all encoded in the distances and the
intensity of stuff in the sky.
Speaker 1 (17:22):
Did anyone come up with a theory that this might
be the case before we actually observed it, or did
we observe it and then come up with the theory
behind it.
Speaker 2 (17:32):
Oh, yeah, great question. It al sort came together at
the same moment when we realized that the universe's expansion
was accelerating. We didn't even know that until we had
a solid way for measuring how far away things were
when they are really distant. We only figured that out
when we were able to calibrate type one A supernova
around the turn of the century, like twenty five years ago,
(17:52):
and that's what told us that the expansion of the
universe was different from what we expected. It's actually accelerating,
and that changed the whole picture. At the same time,
gave us a way to measure the distances to these
big things and showed us that distant objects are actually
larger than some nearer objects in our sky. And so
it all sort of came together about the same time
(18:13):
when we had this revolution in our understanding about the
expansion history of the universe. It's really nice confirmation of
that idea.
Speaker 1 (18:20):
That's incredible. I mean, that must be so special as
a scientist to actually see these things that fit into
all of this theory and all all of these things
that you're discovering of like, oh yeah, there is visual
confirmation of this stuff at ward.
Speaker 2 (18:36):
It's amazing that we can look up the night sky
and we can piece the story together from again, just
these photons and the patterns within them. And for thousands
of years, our ancestors desperately wanted to do that. They
looked up at the stars and wondered what they meant,
and found patterns, but never really had an understanding anywhere
close to what we have until very very recently. So
many people lived in such deep ignorance. Unfortunately, real blessed
(19:00):
to live in an era when we know anything about
the universe. But you know, I wonder about the deep
future when people will know so much more about how
the universe works, and wonder what it was like to
be as ignorant as we are today.
Speaker 1 (19:12):
Uh, blissful is what I would say.
Speaker 2 (19:15):
All right, If you're listening to this podcast in the
year three thousand know that we were dumb, but we
were happy.
Speaker 1 (19:20):
We were dumb and happy. Take that future us all right.
Speaker 2 (19:24):
Thanks very much to Nick for your question, and thanks
to everybody who wonders about how the universe works and
wants to figure it out. Please don't be shy to
write to me to questions at Danielandhorgheay dot com. You'll
always hear back. Happy Summer solsis everybody, and you'll hear
from us again soon. For more science and curiosity, come
(19:44):
find us on social media, where we answer questions and
post videos. We're on Twitter, This org, Instant, and now TikTok.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of Iheartwriting. For more podcasts
from my Heart Radio, visit the iHeartRadio app, Apple Podcasts,
or wherever you listen to your favorite shows.
Hey, Daniel, what's the most distant object you've ever seen?
Speaker 2 (00:12):
You know, the Andromeda galaxy is pretty far away. It's
like millions of light years. How about you, what's the
most distant thing you've ever seen?
Speaker 1 (00:21):
When I am sleepy, it is my couch.
Speaker 2 (00:24):
Your couch is far away.
Speaker 1 (00:26):
Yeah, it's so fun.
Speaker 3 (00:30):
W want mom.
Speaker 4 (00:50):
Hi.
Speaker 2 (00:50):
I'm Daniel. I'm a particle physicist and a professor at
UC Irvine, and I like to couch all my puns.
Speaker 1 (00:57):
Hi. I am Katie Golden. I a podcast on animals,
and I am cushion for a never mind. I I
thought if I just said cushion that a pun would
come to me, and it didn't. It just made me
(01:17):
want to take a nap.
Speaker 2 (01:18):
You were pushing for a pun, Cushion.
Speaker 1 (01:20):
I thought I was going to be bruising for a
cruise in, but I was cushion for a pushing. I
guess a defenestration.
Speaker 2 (01:28):
Sounds like you're set up for some punishment anyway. Welcome
to the podcast. Daniel and Jorge explain the universe in
which we try not to punish you. We try to
enrapture you. We try to share with you the joy
of understanding the nature of the universe, or when we
are mystified by the puzzles, the curiosity, the open questions
about the nature of this cosmos we find ourselves in,
(01:50):
we seek to understand it, we seek to explain all
of it to you.
Speaker 3 (01:54):
On the podcast, Daniel.
Speaker 1 (01:56):
Is this the one coming out on the summer solstice?
Just the quickest side before we get into our listener question.
I just visited in near Dublin, Bruno Boyne, these funeral mounds,
and it was so interesting because these very very ancient
people created these funeral mounds which had an internal structure
(02:21):
that they're large mounds like you could actually you can
go inside it. They can do ceremonies inside, and they
constructed it such that like during the solstice, the summer solstice,
on the longest day of the year, it illuminates this
entire inner chamber. And I just love that. So long
ago we were already thinking about physics, figuring out the universe,
(02:45):
and like trying to think about how light works and
use it for our interesting little Richard.
Speaker 2 (02:52):
Yeah, exactly. I don't think that's a digression at all.
That's exactly what we're talking about today. People have been
asking questions about how the universe works, trying to make
connections with the cosmos. For as long as people have
been asking questions, all you have to do is look
up at the night sky and wonder, how does that
all work? Why does it seem so regular? Why are
their patterns? And then patterns within those patterns, what do
they reveal about the nature of the universe. Because one
(03:15):
of my favorite things about the universe is that it
does seem to follow laws, and we can figure those
laws out if we pay attention to those patterns and
try to draw dotted lines between them. And as you say,
people have been doing that for a long long time.
We sometimes dismiss our ancient history as like prescience and
only take things seriously if it's been figured out in
the last few hundred years. But you know, the development
(03:36):
of science is much more gradual than is often described
in history textbooks. People a long long time ago. We're
thinking methodically and trying to understand the universe, and that's
what people are still doing. And one of my favorite
things is trying to think deeply about the nature of
time and the universe. How far back in space can
we see, what do we see, and what do we
learn about how big and how strange the universe is
(03:59):
as we look into space. And so on today's episode,
we're going to be doing exactly that. We're going to
be answering a question a listener asked us about the
nature of deep time. So today on the podcast, we'll
be answering listener questions. Summer Solstice Edition. We're so happy
(04:20):
to be back and be back in your ear with
this special episode to celebrate the summer Solstice. Happy Solstice, everybody.
Speaker 1 (04:26):
Happy Solstice. Enjoy your roasted pine combs and your chicken
feather damp and you're huge bonfires.
Speaker 2 (04:36):
I hope you stay up late in the night, stare
at the stars, wonder about the nature of the universe,
and then write to me with your questions about it
questions at Danielanjorge dot com. Don't be shy, send me
your questions. You will get an answer. And on today's show,
we're answering a question from Nick in Florida, who's wondering
about the very oldest things we see in the sky.
(04:57):
Here is Nick's question.
Speaker 4 (04:59):
Hey Daniel, this is Nick from Jacksonville, Florida. I came
across an article this morning that taught me something new
and amazing about the universe. Apparently, beyond the distance of
approximately fifteen billion light years, objects we see with telescopes
start becoming larger again instead of smaller. My question is
(05:19):
how close are we to being able to see this
far and what will be the implications for cosmology once
we get there? Thanks and love the show.
Speaker 1 (05:28):
So this is really interesting. I have never actually heard this,
that objects that you see beyond a certain distance start
to appear larger, and I also don't understand how we
can see this far in the first place.
Speaker 2 (05:45):
Yeah, it's really fascinating. It's one of my favorite facts
about the universe. It really shows us that the universe
is expanding and that we have a pretty good grasp
on how that works, or at least what's been happening,
which is already kind of astounding. The fact that wall
we've been trapped on this tiny little rock in one
corner of the universe, we can measure things that tell
(06:05):
us about the universe on the grandest scale. You know,
what's happening over billions and billions of light years, and
what's been going on over the last few billion years.
It's really incredible what we are able to piece together
from just the few photons that happen to land on
our rock.
Speaker 1 (06:22):
That is wild. I mean, it's also just kind of
weird that in a way we can time travel, but
just with our eyes and some fancy shaped glass.
Speaker 2 (06:33):
Yeah, it is really amazing. So there's a few different
things going on here that all come together to make
this really weird effect that if you can get them
all working together in your mind, show you how beautiful
our explanation is. So let's take them one at a time.
One concept is the one you just measured that looking
up into the sky is basically time travel, and that's
(06:53):
because the speed of light is not instantaneous. You know,
something happens in Andromeda, you don't see it right away.
It takes time for light to go from Andromeda to
our eyeballs. And so when we look up at the
night sky and we see Andromeda, we're not seeing what's
happening in Andromeda right now. We're seeing what's happening in
Andromeda when that light left Andromeda. And because Andromeda is
(07:16):
several million light years away, it means we're seeing photons
that left Andromeda several million years ago. Andromeda is not
even in the same place it was when emitted that light,
and so what we're seeing really is ancient history. And
the further we look out into the universe, the deeper
back in time. We are seeing like things are happening
(07:38):
out there in the deep universe right now, but we
won't see them for millions or billions of years, depending
on how far away they are. So as we look
out into the universe, we are really seeing further and
back in time just because of this simple basic physics
fact that light takes time to get to us.
Speaker 1 (07:54):
So you're saying that if Andromeda has a newspaper that
is really huge that we can see with a telescope,
by the time that light reaches us, we're looking at
a pretty old headline. It is out of date.
Speaker 2 (08:08):
Yeah, exactly. And conversely, it means that if Andromeda has
aliens that point their telescope at us, they're seeing millions
of years ago. They would see the Earth as it
was millions of years ago. They wouldn't even know that
there was an intelligent technological civilization on our planet for
millions of years, because that's only the very briefest part
(08:29):
of our history. And so somewhere out there are photons
from like when the Earth had dinosaurs on it, or
even further away, there are photons from the Earth when
it was like just a ball of magma. All those
photons are still out there in the universe. Somebody could
be picking them up where they're alien eyeballs and seeing
the ancient history of our planet.
Speaker 1 (08:50):
I like to imagine that the reason aliens don't contact
us is they caught us when we had the weird
Anomalocaris that came marine explosion, giant disgusting shrimp looking thing,
and they're like, no, thank you, I'm not gonna get
in touch with that planet.
Speaker 2 (09:07):
It's too icky.
Speaker 3 (09:08):
Nah.
Speaker 1 (09:10):
Yes, don't like shellfish seafood.
Speaker 2 (09:12):
So that's one concept, right, Things in the sky are
old news. Right, We're not seeing the universe as it
is now. We're seeing the universe as it was. And
on one hand, that's frustrating because it means like we're
missing out on the latest, greatest news. On the other hand,
it's a huge gift. It means we can see back
in time and like on Earth, you want to know
(09:33):
what happened during the Roman Era or what happened a
million years ago, you got to look for clues that
happened to be here and be left over. Most of
that stuff is destroyed or buried, but in the history
of the universe, you just look into the sky. It's
all still there. It's just arriving right now, fresh as
the day was emitted. So you want to know what
happened in the universe a million years ago, or a
billion years ago or ten billion years ago, you just
(09:54):
build bigger telescopes to capture more distant, fainter photons, and
you can see it. It's all still they're in the sky.
Speaker 1 (10:01):
So the further back in time we want to go
in understanding our universe, the further way look exactly.
Speaker 2 (10:09):
So you have to imagine these shells, like in a
shell that's very close to the Earth, we're seeing the
more recent past. We can't see the distant past in
a close by shell, but we can't see the ancient
past in a more distant shell. So we can't just
like dial the knob and say I want to see
what happened over here at that time. We can only
see a certain time for every certain location because photons
(10:31):
are just now arriving from that location based on how
far away they are, all right, So that's one concept.
The other concept is how big things look in the sky.
And so to understand this first, let's just imagine the
universe is not expanding. Imagine that you have galaxies and
they're just hanging in space and nothing is moving. Everything
is just hanging there. There's no relative velocity. Then you
(10:54):
would expect things that are further away look smaller, right, because,
for example, if you have some galaxy and it's right
up in your eyeball, then photons from one side of
the galaxy and photons from the other side of the
galaxy are gonna come from very very different angles. Now
you move that galaxy further away in the sky, photons
from two different edges of the galaxy are now getting
(11:15):
closer and closer together, so they just seem smaller in
the sky. This is no magic. It's just like you
hold your finger close to your eyeball, looks big. You
push it further away, it looks smaller.
Speaker 1 (11:25):
Right, It's just perspective, I see, Yes, So I have
been told that people from far away are not tiny people.
They are just far away. So all right, that makes
perfect sense.
Speaker 2 (11:36):
Yeah, exactly, And we have an intuition for this exactly
the way you describe you don't think that people who
are standing next to you are giants and people who
are far away are midgets. You know this, and you
invert it in your mind. When you see people, you
can tell how far away they are, and so you
estimate their actual size because you've taken that into account. Right,
You have this intuition for things looking small far away
(11:58):
and things looking big close up, and you'll count for
this naturally. So if the universe was like that and
nothing was moving, then it would just look like that.
Things that are further away would look smaller. But there's
one more effect you have to throw into the mix,
and that's the expansion of the universe. Things are not
just hanging in space. There's a relative velocity there. Space
(12:20):
is expanding between galaxies, or another way to think about
that is galaxies are running away from us and from
each other, so everything is getting further and further away.
And what that means is that the light we're seeing
from those galaxies is not from where they are now,
it's from where they were when they emitted that light.
(12:40):
Oo okay, right, So, for example, imagine something on the
very distant edge of the universe. We talk about how
something could be forty five billion light years away, the
furthest things in our observable universe, all right, But when
we talk about that, we mean that's how far away
it is now, not how far away it was when
it admitted the light that's now coming to us, that's
(13:02):
now arriving on Earth. It was much much closer when
it emitted that light. What's happened in the intervening period
as that photon has flown through space towards us is
the universe has expanded and it's gotten further and further away.
Speaker 1 (13:15):
Okay, So if I try to think about this on
human scale, you have various people standing out in a field,
and it's a very big field. So I need a
telescope for me to see people, and it's a telescope
with a slight delay, or a camera, let's say a camera.
It's got a delay in it, so I will only
(13:35):
see them after a while. So like, I could be
looking at these people who are further away, but then
the people who are closer to me, let's say they
are running away from me at a faster rate, so
they might with my delayed camera, look further away by
the time I get my snapshot than the ones who
(13:56):
were actually further away from me, but they look bigger
because they were not running away from me as fast
as the other.
Speaker 2 (14:03):
Yeah, exactly, you have old information, and so you're taking
these snapshots and you're seeing people where they were when
the picture was taken. But if the picture takes a
while to get to you, by the time it does,
they could be much further away, right. And that's important
to think about because that affects how big they look
in the sky because when we talk about things that
are really really far away, we're seeing them remember as
(14:26):
they were when they emitted those photons, and if they
were much closer when they emitted those photons, they took
up more space in the sky because they were closer.
So those photons have been flying towards us those millions
or billions of years, but they started pretty close by, right,
So they started further apart because the thing was closer
to us. So there are two effects happening here. One
(14:47):
is things that are further away look smaller, right, But
the other effect is this expansion effect that some of
those things that are now really really far away were
closer when they emitted the photons. So these two competing
effects there. And so what that means is that if
we look at nearby stuff, mostly the first effect dominates
that things that are further away look smaller. So as
(15:09):
you look out into the night sky, stuff that's further
away looks smaller and smaller and smaller. But then there's
a point from about ten billion years ago when the
other effects starts to dominate. When we're seeing stuff that
was much closer when it admitted that light. Stuff looks
smaller and smaller and smaller as it gets further away
until about ten billion years into the past, and then
(15:30):
stuff starts to look bigger and bigger. So the oldest
stuff actually looks bigger in the night sky than some
of the closer stuff, some of the more recent stuff,
because you have these two effects that are competing with
each other.
Speaker 1 (15:42):
Because we're basically seeing like an after image of something
that was going Yeah, it's actually not further from us.
I mean that the actual star is that was there,
but those photons that after image are finally hitting us,
representing where in the sky.
Speaker 2 (16:01):
When we talk about the universe, we talk about where
things are now, that's where we put them in our
mental maps, but we don't think about where they were
when they emitted that light. And so if you just
line things up based on where they were when they
emitted that light, then yes, closer things would look bigger,
and further things would look more distant. But because the
expansion of the universe, not all the photons that are
arriving here are perfectly lined up with where things are now,
(16:25):
So things that are more distant now can actually look
closer than things that actually are closer still, so there's
this amazing effect in cosmology. And so Nick is asking
about this effect, and he's wondering, can we see this yet?
Can we see far enough out into the universe to
see this turnover where the really distant stuff starts to
get larger and larger again, and what does that mean
(16:47):
for cosmology? And the answer Nick is that yes, we
already can see that. We can see much further than
ten billion years into our history. We can see all
the way back to almost fourteen billion in years. So
we have observed this. This is not like a theoretical
concept that people are wondering about and going ooh, maybe
in the future sometime. This is something we see. We've
(17:09):
measured this, and this really nicely confirms our whole model
for how the universe is expanding and how that expansion
is accelerating. It's all encoded in the distances and the
intensity of stuff in the sky.
Speaker 1 (17:22):
Did anyone come up with a theory that this might
be the case before we actually observed it, or did
we observe it and then come up with the theory
behind it.
Speaker 2 (17:32):
Oh, yeah, great question. It al sort came together at
the same moment when we realized that the universe's expansion
was accelerating. We didn't even know that until we had
a solid way for measuring how far away things were
when they are really distant. We only figured that out
when we were able to calibrate type one A supernova
around the turn of the century, like twenty five years ago,
(17:52):
and that's what told us that the expansion of the
universe was different from what we expected. It's actually accelerating,
and that changed the whole picture. At the same time,
gave us a way to measure the distances to these
big things and showed us that distant objects are actually
larger than some nearer objects in our sky. And so
it all sort of came together about the same time
(18:13):
when we had this revolution in our understanding about the
expansion history of the universe. It's really nice confirmation of
that idea.
Speaker 1 (18:20):
That's incredible. I mean, that must be so special as
a scientist to actually see these things that fit into
all of this theory and all all of these things
that you're discovering of like, oh yeah, there is visual
confirmation of this stuff at ward.
Speaker 2 (18:36):
It's amazing that we can look up the night sky
and we can piece the story together from again, just
these photons and the patterns within them. And for thousands
of years, our ancestors desperately wanted to do that. They
looked up at the stars and wondered what they meant,
and found patterns, but never really had an understanding anywhere
close to what we have until very very recently. So
many people lived in such deep ignorance. Unfortunately, real blessed
(19:00):
to live in an era when we know anything about
the universe. But you know, I wonder about the deep
future when people will know so much more about how
the universe works, and wonder what it was like to
be as ignorant as we are today.
Speaker 1 (19:12):
Uh, blissful is what I would say.
Speaker 2 (19:15):
All right, If you're listening to this podcast in the
year three thousand know that we were dumb, but we
were happy.
Speaker 1 (19:20):
We were dumb and happy. Take that future us all right.
Speaker 2 (19:24):
Thanks very much to Nick for your question, and thanks
to everybody who wonders about how the universe works and
wants to figure it out. Please don't be shy to
write to me to questions at Danielandhorgheay dot com. You'll
always hear back. Happy Summer solsis everybody, and you'll hear
from us again soon. For more science and curiosity, come
(19:44):
find us on social media, where we answer questions and
post videos. We're on Twitter, This org, Instant, and now TikTok.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of Iheartwriting. For more podcasts
from my Heart Radio, visit the iHeartRadio app, Apple Podcasts,
or wherever you listen to your favorite shows.
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