Up next in 10
3 discoveries that change the way we think of the Universe
598 views
Mar 29, 2025
"Asking the question of, where did the entire universe come from, is no longer a question for poets and theologians and philosophers. This is a question for scientists, and we have some amazing scientific answers to this question that have defied even the wildest of our expectations."
View Video Transcript
0:00
I like to think about what it was like when I was a small child and I first started to wonder about things
0:09
Maybe the most profound question that I knew how to ask was
0:14
what is all of this? You know, the planet and everything beyond the planet
0:20
And if I was being a little more detailed, I'd ask, and how does it get to be that way
0:26
Where did the universe come from? Where did our planet come from? Where did human beings come from
0:33
All of that was in the realm of poetry, of philosophy, of theology for millennia and 20th century began to change all of that
0:45
All of a sudden, we actually began to ask questions of the universe itself
0:51
and that's why talking about and asking the question of where did the entire universe come
1:01
from is no longer a question for poets and theologians and philosophers. This is a question
1:09
for scientists and we have some amazing scientific answers to this question that have totally in many
1:17
ways defied even the wildest of our expectations. Pause for dramatic effect
1:26
I'm Ethan Siegel, theoretical astrophysicist and science communicator, author of the James Webb
1:33
Space Telescope book Infinite Cosmos, and writer of the science blog Starts With a Bang
1:39
Let's get into some weeds. So we have this idea of where our universe came from that most people have heard of called the Big Bang
1:57
And the way we came upon this idea was we weren't actually looking for the beginning of the universe
2:04
We were looking at these faint, fuzzy objects that appeared through telescopes that looked like little spirals in the sky
2:14
And we were wondering, what are these things? And then some critical observations came in. It started in 1923. This astronomer named Edwin
2:26
Hubble had access to what was then the largest, newest, and most powerful telescope in history
2:35
And he was looking at the largest of these spirals that we see in the night sky, which today we know
2:43
as the Andromeda galaxy. And Hubble was saying, hey, I can see that there are these bright flares that go off
2:52
Something appears to brighten, and then it appears to fade away. Maybe these things that we see are novi
3:01
or, you know, the plural of nova, happening in Andromeda. And he said, well, goodness
3:08
A nova can't repeat itself in just a day or two. Novi take years, decades, centuries or more to recharge
3:19
So if this isn't a nova, what could it be? And he realized, oh, this is a variable star
3:29
And these classes of stars that do this brightening and faintening over time
3:35
had already been studied for decades by a woman named Henrietta Leavitt
3:40
who said, oh, these are what we call Cepheid variable stars. And there's a relationship between how quickly they brighten and faint in
3:51
and how intrinsically bright they are. Just from how bright you see it, you can know how far away it is
3:59
And that was what Hubble did, is he said, well, if I observe this star over time
4:04
I can figure out how distant it has to be. And it was by doing that that Edwin Hubble first said
4:13
oh my goodness, this nebula in Andromeda, it can't be within the Milky Way
4:20
Because it's not light years away or hundreds of light years away
4:24
It's something more like a million light years away. Today we know it's more like 2.5 million
4:30
What we would now call extra galactic objects. So Hubble took his same telescope and with his assistant, Milton Hummison, went out
4:42
And what he found was that, in fact, the Andromeda galaxy was one of the closest ones to us and that he was finding galaxies that were over 100 million light years away
4:56
And he was able to say, well, now I also have these observations from this older guy, Vesto Sleeper, who was measuring how fast these objects appear to be moving away
5:12
Light, just like sound, is a wave. If a light source like a galaxy is moving towards us the light that it makes is going to have a wavelength or what we call in astronomy blue But the opposite is also true If the object is moving away from you the wavelength lengthens
5:35
and the light becomes red-shifted. Hubble put this together in the late 1920s and early 1930s
5:44
But the first one to do it was in 1927, a Belgian Catholic priest named George Lemaitre
5:53
And he said, what does it mean that the farther away a galaxy is, the more its light looks redshifted
6:02
Well, the way I like to think about it is to think about baking
6:08
Have you ever had a ball of dough? And I don't want you to imagine plain, boring bread dough
6:15
Let's make it a little exciting and let's put some raisins in there
6:20
So what happens? As the dough starts to leaven, the raisins get carried by the dough
6:27
So that from the perspective of any raisin within the dough, it looks like the other raisins are moving away from it
6:35
In this ogy of the expanding universe, the raisins are galaxies. And the Do is this fabric of space itself that comes up in Einstein's general theory of relativity
6:53
In general relativity, space and time are woven together into a fabric
7:00
But this fabric is not constant. This fabric can evolve with time
7:06
And it wasn't actually Einstein who figured out how the fabric evolves
7:11
It was a Soviet scientist from 1922 named Alexander Friedman. Friedman said that the universe is going to either expand or contract
7:25
Otherwise, it won't be stable. So Lemaitre was the first to put all of this together
7:31
Einstein's equations, Friedman's solutions, Vesto Sleeper's redshift observations, and Hubble and Hummison's distance observations
7:43
He put them all together and said, oh my goodness, look what this means
7:47
The universe is expanding. And he said, well, hang on. If the universe today is expanding
7:57
then in the past it was smaller and everything was closer together
8:01
and it was more dense. All of space and time and all of the matter and energy within the universe
8:10
was once compressed into this tiny, indivisible point. And he called this point the cosmic egg
8:21
Something that today we might call a singularity. And this is where the big idea of the Big Bang first came from
8:34
All of these cornerstones of the Big Bang singularity are now in place, but people are still asking questions
8:45
One theory is before the Big Bang was a period of cosmic inflation
8:54
I want to start by going back to Alexander Friedman. Alexander Friedman was the first one to say, hey, if you have a universe that's uniformly filled with any type of matter or energy, it's going to either expand or contract
9:15
So for normal matter, what is its energy density? It is how many massive particles you have in a given region of space
9:25
As your universe gets bigger in all three dimensions, the density is going to go down
9:32
It's going to dilute as the volume of the universe increases. What about if your universe is made of radiation
9:41
The universe with radiation in it is going to dilute more rapidly
9:47
Its energy density is going to drop faster. And that means its expansion rate is going to drop at a different rate
9:55
But what if the universe wasn't filled with matter or radiation? Einstein didn't even know it, but he first put forth this possibility
10:07
way back with the general theory of relativity. This is what he called a cosmological constant
10:15
And in this case, we get a universe that expands not just rapidly
10:22
like a matter or radiation-filled universe will do early on and then decrease
10:28
it's going to expand at this rapid rate and it's going to keep doubling relentlessly
10:35
So the idea of cosmic inflation, thought up by Alan Guth and then since worked on by many, many others
10:43
was that there was a phase that preceded our universe being filled with matter and radiation where instead it was filled with a form of energy that was intrinsic to space itself And while it was in that phase space got stretched to be enormous in a minuscule
11:08
tiny amount of time. So we've got these two ideas now that are not compatible with each other
11:16
Either we had a singular hot Big Bang that gave rise to the hot, dense, rapidly expanding state, or we had cosmic inflation, the state where space has energy to it, becomes large, becomes uniform, spatially flat
11:37
And then inflation came to an end and gave rise to this hot, dense, rapidly expanding state
11:46
I like to think about it the way I think about a ball atop of a plateau that ends in a valley
11:56
As the ball stays on the plateau, you get inflation. But as the ball rolls off of the plateau into the valley, it loses energy
12:07
And all of that energy gets converted into matter and radiation. That's how we go from inflation, which is relentless and rapidly expanding, into the hot Big Bang
12:23
So how does inflation make predictions that differ from a Big Bang singularity
12:30
Well, if inflation only gives you a certain amount of energy inherent to space
12:37
and then it decays into matter and radiation, that's a different prediction right there because first off this tells you for the universe that
12:47
starts with the singularity we're not going to have a maximum temperature or a maximum energy
12:54
that it gets up to whereas with inflation no no no we were at the top of a plateau and we rolled
13:01
down and there's a limit to how hot the universe can get so if that's the case there should be a
13:09
difference that we see imprinted in all things of the cosmic microwave background in that
13:16
leftover primeval fireball from the Big Bang. Now, there's a second thing that comes about from
13:25
inflation that is remarkable, and that is due to the fact that right now we've been thinking about
13:32
inflation as being what we call a classical field. We've been picturing it like it's a ball on top of
13:38
a hill. But it's more than that. We live in a quantum universe, and inflation should be a
13:45
quantum field. And one thing that quantum fields do is fluctuate. So think about what happens if
13:54
you have a little fluctuation in space and your universe is inflating. This fluctuation gets
14:02
stretched. And then the universe keeps inflating. So it gets bigger and bigger and bigger. While on
14:09
tiny quantum scales, new fluctuations get created. So I should have a set of quantum fluctuations
14:18
that exist all throughout the universe. And they should be there on all scales. Now, the beautiful
14:26
thing about this is when inflation comes to an end, when the ball rolls down into the valley to
14:33
convert things to matter and radiation, that means those smallest scales, they get stretched just a
14:42
little bit less than the larger scales. So inflation gives us this specific prediction that when we
14:50
look at the fluctuations imprinted on the universe, they should be almost perfectly uniform on all
14:58
scales, except on smaller scales, we call this an almost perfectly scale invariant spectrum
15:08
And that's something we should be able to go out and look for. So one test is, do we see a maximum temperature imprinted in the cosmic microwave background
15:20
Second test is do we see an almost perfectly scale invariant spectrum, but with smaller fluctuations on small scale slightly than on large scale
15:32
A third prediction that should be made is if we have inflation going on for a long enough duration of time
15:41
it shouldn't just be stretching quantum fluctuations to a maximum scale of the observable universe
15:48
It should be stretching them continuously to all scales, including scales that are larger than the universe we can see, what we would call super horizon fluctuations
16:03
Those should be imprinted in the leftover glow from the Big Bang as well, whereas in a universe without inflation, you don't get those
16:13
So right there is three observable tests We now been able to go out and test those predictions of inflation against the singular Big Bang without inflation And what do we find
16:27
Well, we find that there is a maximum temperature that the universe got up to
16:34
And it's a high, high temperature. It could be as high as about 10 to the 16 giga electron volts
16:41
Wow, a lot of energy. Guess what? that's about a factor of a thousand below the Planck energy
16:48
there's a limit to how hot the universe got, and it wasn't arbitrarily high
16:56
We can look at the fluctuations we see, and guess what? They are almost perfectly scale-invariant
17:03
but the fluctuations on the largest cosmic scales are about 3% larger than the ones on smaller cosmic scales
17:12
consistent with what inflation tells you. We can look for, do we see evidence for the existence of
17:20
super horizon fluctuations? And the answer is yes. So all of these consistent with inflation
17:29
disagree with the singular hot big bang. You might ask what's left? What's left for inflation to do
17:38
And so far it's passed 100% of those tests. The big thing we'd like to know is, I drew you a potential where I said, imagine a plateau
17:50
and we fall into a hill and we oscillate in that valley at the bottom of the hill
17:57
We want to know, is that the right model for inflation? There are alternatives
18:03
What is the shape of that potential look like? There are two big frontiers that we're trying to push to gain answers to those questions today
18:15
One is to look at the spatial curvature. So far, we've only measured it down to about the 1% level or the one part in 100 level
18:27
And it is spatially flat down to that. But if we can get a few orders of magnitude more precise, we'll be able to see what is the precision to which our universe is flat
18:41
Is it consistent with what inflation predicts or does it not match
18:47
And finally, there's another type of fluctuation that should exist. There should be gravitational wave fluctuations imprinted by inflation on the universe, and there is an effort to look for these being conducted at the South Pole
19:06
So the more precisely we look for this signal, the better chance we have of not only finding these leftover signals, but of learning exactly what the properties of our cosmic origins, of cosmic inflation, were at those critical, very, very first moments that led up to the hot Big Bang
19:32
Since time immemorial, no creature came along on Earth that understood where we were in the universe, where we came from, and how we got to be here
19:48
All of this is new, like within many of our lifetimes new
19:53
We now know the universe began not from a Big Bang singularity, but from a state that was rapidly and relentlessly inflating, where space was empty, filled only with the energy that was inherent to space itself
20:14
An inflationary state came to an end that gave rise to the structure that came about in all the time since
20:25
Stars, galaxies, planets, and human beings. Here we are today, 13.8 billion years later, and we know our cosmic origins up until that moment
20:40
But big questions about the specifics of what inflation was and how and whether inflation began as well as what came before it are still mysteries waiting to be solved
20:57
Oh my goodness, about six or seven years ago, I had been wearing kilts for close to a decade
21:13
and I met an artist who specialized in fabric painting and loved doing space work. What she
21:21
brought me back was gorgeous, and I was very pleased with it. And in the time since, I have
21:26
had three additional kilts that I commissioned by her. And this is the most recent one featuring
21:33
the James Webb Space Telescope, NASA's newest flagship observatory for astronomy and astrophysics
#Astronomy
#Physics