In order to get the large scale structure of the Universe we see today, cosmologists
have proposed the idea of inflation, that the Universe expanded an enormous amount in
the earliest moments.
But if inflation really happened, then it has even stranger implications for the nature
of the Universe and the search for multiverses.
We've covered the topic of inflation a couple of times in the past, but I'll give you
the short version one more time.
The Big Bang exquisitely explains the expansion of the Universe we see today.
When we look out as far as we can, to the edge of the observable Universe we see the
afterglow of the Big Bang: the cosmic microwave background radiation.
This light was released the moment the Universe had cooled down a little, and has been traveling
for almost 13.8 billion years to reach us.
Thanks to the expansion of the Universe, it's been redshifted to just a few degrees above
absolute zero.
When astronomers measure the temperature of this background, it's incredibly consistent,
with only tiny fluctuations measurable with the most sensitive instruments.
This means that the entire Universe that we can see had time to transfer temperature to
each other before it expanded.
But the original Big Bang Theory suggests that the expansion of the Universe didn't
give the material time to even out its temperature.
In order to explain this, cosmologists developed the concept of inflation.
There was a period in the earliest Universe when the energy in matter was bound up in
the fabric of space itself.
The Universe expanded so quickly, that a region the size of a subatomic particle would have
been stretched to the size of the visible Universe in a fraction of a second.
Inflation also answered other challenges that the original Big Bang couldn't explain,
such as the flatness of the Universe, and total lack of monopoles.
Like I said, we've done a whole video about inflation.
But inflation has introduced its own set of strange ideas, including the concept of "eternal
inflation"; that inflation didn't end for the entire Universe like it did in our
local area.
There are regions undergoing inflation all over the place, creating multiple universes
within our Universe.
You know, a multiverse.
I'll be honest, though, the concept of eternal inflation is beyond my comprehension.
And so, in times like this, I like to bring in a ringer.
Today, I'm glad to bring you Dr. Ethan Siegel, an astrophysicist and science writer.
His most recent book is Treknology, all about the science of Star Trek.
Ethan tackles some of the most complex topics out there in an understandable way, and I
could really use his help.
Ethan, welcome to the Guide to Space
Before we get started, was there anything you wanted to add to my description of inflation?
Hi there Fraser, it's my pleasure to be here and I'm more than happy to tell you all about
the eternal inflation and why it lasts forever.
Lasts forever, it's going all the time?
I don't even know where to start.
Can you explain or add anything to the way I described inflation to set the stage for
how we're going to move into this idea of eternal inflation.
Sure, you did a great job.
You know that's something that people don't realize when they hear the Big Bang.
When people hear the Big Bang, they think oh, that's the origin of everythign, that's
where the whole Universe can from, and that's the birth of space and time.
And as far as we're concerned, the answers to that, the answers to that are yes, yes
and not quite.
The reason is that if you imagine the Universe today, you see it expanding, you see it cooling,
you see galaxies moving farther and farther apart from one another and you say to yourself,
oh, right, well if things are expanding now, and cooling now, the red shift you talk about,
because as space expands as the fabric of space stretches, you say oh, right, you have
this wavelength of light and that defines its energy.
So as space stretches, the wavelength gets longer, and that means the Universe gets cooler.
So if instead you look back in the past and ask what were things like in the distant past,
you say oh right, that means that the Universe was smaller and space was smaller, and things
were closer together.
So rather than being larger and getting larger and getting cooler, that means that in the
past it was hotter, it was denser, things were closer together.
And because it's had less time for things to clump together, the Universe was also more
uniform.
So you extrapolate back and say, well, if things were hotter and hotter and more uniform,
I should be able to go back to a time when there were no stars and no galaxies.
And we think we've seen that time, and we think that the James Webb Space Telescope
is going to reveal those first stars and galaxies.
And you go back even more and you say, well at some point it must have been so hot and
these wavelengths must have been so short that couldn't even have formed neutral atoms
at that time.
And absolutely, that's correct, there was a time when the Universe was just full of
an ionized plasma because all of the radiation in it was too powerful, that as soon as you
formed a neutral atom, an incoming photon would strike that atom, knock the electron
off, and you've have a plasma again.
When you talked about the cosmic microwave background, that refers to the time when atoms
finally became neutral that the scattering didn't occur any more.
And that's the leftover glow we see from the Big Bang, which happened when the Universe
was 380,000 years old.
You go back further and it becomes so hot that you can't even have atomic nuclei, that
they get blasted apart.
So we can do the calculations for the light elements for the abundence of the light elements
for how nuclear fusion happened in the first few minutes and seconds of the Universe.
And that's something that we make predictions for that we have observations that you learn
about the Universe.
But if you want to go all the way back to arbitrarily high densities, to arbitrarily
high temperatures, you run into a problem.
You start saying, well, if that's what I got, then what I should see for example in this
pattern of fluctuations in the microwave background is that different regions should have different
temperatures of a certain magnitude.
There should be big big massive temperature fluctuations that we don't see.
We don't see one part in one or one part in 10 or one point in 100 fluctuations.
We see like one part in 30,000 which tells us, no, there's a lower energy scale there.
We would expect that if you look 13.8 billion light years in one direction and 13.8 billion
light years in the opposite direction, there's no way for these two regions to have exchanged
information, to have exchanged photons, to have come to thermal equilibrium.
And yet, completely opposite regions of the sky started out with the same properties.
You also talked about spacial curvature, and we measure this spacial curvature of the Universe
to be zero, to be absolutely flat, even though we know that spacial curvature increases as
time goes on, so that means when the Universe was 10 to the minus some really large number
of seconds old, it would have had some kind of curvature that was super miniscule, like
10 to the -100 in order to give roughly zer0 curvature that we see now.
So all these things are motivations to say you know, either to get the Big Bang, we had
to start with these incredibly fine tuned initial conditions or you can appeal to physics
and say, you know, what kind of dynamics could have occurred to set this up.
This is the beginnings of where you get any scientific theory from.
You have a period, the Big Bang, that works really well in a certain regeme.
But when you go all the way back to the very beginning, you start to ask questions that
you don't have a good answer to.
You either have to say well, it was either like this and that's the story, or you have
to say that well, it started out with these sets of conditions, it was flat, it was the
same temperature everywhere, we don't have these high energy left over relics that these
extensions predict.
Either we start with those conditions and those were the conditions that the Universe
was born with, or something happened to set the Universe up like this.
And if that's the case, what are the other things that this theory would predict and
how can we go out and test them?
So that's what inflation is.
Right and so that sort of sets up what inflation is, it's this expansion that I mentioned and
what you were talking about, but in my mind I imagine it being this same, this thing that
happened across the entire Universe.
This stretching that happened at all places at once and then that phase is over.
So how does this play into the idea of eternal inflation.
Okay, so that's a great explanation, you've got this space and it's stretching exponentially.
And just so everyone explains what exponentionally means, imagine I've got this cube that's one
cm on a side and I let it expand for one time step.
So I take a time step and now it's two on a side and two on a side and two on a side,
so it's two by two by two.
And then I take another time step so now each of these has expanded again so it's 4 by 4
by 4.
And then I take another time step and each of those again doubles so it's 8 by 8 by 8,
you can see with inflation, that if you just take a small number of time steps, like 64
time steps or so, all the sudden you get something that's like 10 to the 30 times as large as
you started with.
Which is to say that if inflation goes on for just 10 to the minus 33 seconds, then
you can go from something the size of the plancke scale, the smallest conceivable scale
that makes sense to the size of the observable Universe today in just 10 to the minus 33
seconds.
So that's how rapidly space expands, that's what it means that it's expanding exponentially.
So then you say, okay, how does this happen, you say well, I imagine it's some kind of
quantum field, when you're up at the top of a hill, right, you get inflation going and
then you roll down the hill and inflation comes to an end.
And you say, okay, that's great and you told me this picture you have.
That inflation starts out here, you roll down the hill and it comes to an end and it does
that everywhere.
And that's fine if you're a ball rolling down a hill, but we know that inflation like everything
else in the Universe should be a quantum field.
It's not a ball rolling down the hill, it's a quantum field running down a quantum potential
so now let me ask you something about quantum mechanics.
If I give you an electron and I say, hey, hang onto this electron and you do, and you
hold the electron in your hand, as if that were something you could do to electrons.
And then I say, just take a chill pill, don't take a look at that electron for a while,
let that electron sit in your hand.
And come back after a couple of seconds.
Now where's that electron?
Is it in the same spot you left it.
And you can go and look.
And there's a probability that the electron will be in the same spot you left it, but
one of the properties of quantum wave functions is that they spread out over time.
This is just something inherent to the quantum nature of every particle and wave in the Universe
is that it has this inherent uncertainty to it and it has this inherent quantum spreading
to its wave function that happens with time.
So if you've got your inflationary potential, if you've got the field at the top of the
hill, right, the field's at the top of the potential, and it starts to roll, so it starts
to roll down the hill, you can say well, hang on, as it's starts to roll instead of having
this one by one by one box, I've got this let's say, 8 x 8 x 8 box, I've got 8 times
8 times 8, that's a lot, that's 512 independent regions that were the size of that original
region.
And then you say, okay, well, it's started to roll down the hill, that's on average.
So, on average, it's started to roll down the hill but because of this quantum nature
of things, the field spreads out.
What happens then, the field spreads out so that in some places you've rolled farther
down the hill and you're closer to inflation coming to an end.
In some places you're right where you expect to be, where inflation's rolled down the hill
the field has rolled down the end and inflation's coming to an end like you expect.
But in other places, inflation has caused the spreading to expand so much, that you're
closer to being back up the hill than you were initially.
In other words, you don't always roll down the hill, even if it's just a small percentage
of the time where you run up the hill, where you run against where you expected to be because
of this quantum spreading is faster and bigger than the rolling down the hill, well space
grows so fast from one thing to 512 things is such a small time that now you see oh no,
I have more regions that are inflating even more than they were when I started.
And so, yes, you're going to have regions where inflation rolls down that hill, where
it comes to an end, where it comes to an end, that's where the energy inherent to space
gets converted into matter, antimatter and radiation and you get that hot Big Bang.
And you get the birth of our Universe with all those properties that we talked about.
Where it's been stretched flat, where the temperature is the same in all directions
because things were connected during inflation.
And where you don't have monopoloes or other high energy relics, because you never got
up to that high temperature that you thought you would get if you extrapolated arbitrarily.
There's a maximum temperature that we can reach, and we've discovered with WMAP and
the Planck satellites, that that's somewhere around a factor of 1000 times or more below
the Planke energy.
So okay, with inflation you can reproduce those successes of the Big Bang, you can make
additional predictions of the fluctuations of the Universe, of superhorizon fluctuations,
about what types of structures you're going to get.
But you can also do this additional thing where you say, well, where does inflation
end?
How likely are we to have inflation end at any particular time?
And you can say, well, look, as we move forward in time, even if inflation comes to an end
with every time step in half or more than half of the regions, there were still an infinite
number and an increasingly infinite number of regions as time goes on where inflation
continues for eternity.
And that's where the idea of eternal inflation comes from.
That as this quantum field rolls down the field, it has a probability of spreading out.
And in some of those regions where it spreads down enough, you never roll down that hill,
you always stay at the top of the hill, which means that you're always inflating.
And so, if we looked across the Universe.
If we could somehow move into a God mode and actually look around and observe, we've got
our current observable Universe, which is only some fraction of what is the possible
actual Universe and possibly infinite.
But if we could see these other regions of inflation, what would we see?
Okay, so I'm going to ask you to visualize you were some higher dimensional creature,
that you could see all the 4-dimensions of our space and time, and you could see what's
going on in them from an outside view.
Inside, you would see what we call our observable Universe.
This is the stuff we can see from the moment of the Big Bang, the light that's reaching
us right now in all directions.
It would be this spherical Universe centered on us because that's where we happen to be.
If we were anywhere else it would be centered on wherever we were.
And you would see that's a part of our region where inflation ended.
It's getting bigger as time goes on, but we can't see all of it.
Now, if you look beyond that, beyond the part that's observable to us, you would also see,
oh wow, we're getting a big big spherical region that is where inflation ended.
And it may not be spherical, it may not be symmetrical, we just assume it's spherical.
You get this big region where inflation ended, and that's expanding outward at the speed
of light, and expanding with the expansion of space.
And anywhere in that region, if you put an observer, and said hey, you've been here since
the Big Bang, what do you see.
You could draw a sphere about 46 light years in radius, about the same as you can for us.
That's how far you can see in the expanding Universe.
Except if you're very close to the boundary, you would get there, but you would see a mysterious
end.
Like you would see a cut off in the structure of the Universe, of the microwave background.
There would just be empty space beyond that.
So if you asked what's going on beyond that empty space.
Well outside of the region where you had your hot Big Bang, that's inflating space, that
you would have this space where inflation continues and every so often you can say,
well if I look throughout this inflating space, what else am I seeing?
Well, the answer is that I would see a bunch of different pockets in the Universe that
looked like our part where inflation ended, except it may have ended at different times.
So, the Universe in some places, other universes within this multiverse within this eternally
expanding space, they may have not ended 13.8 billion years ago and said, that's when you
have the Big Bang and that's when you get your little pocket Universe.
Instead, you could have had something that ended more recently, you could have had something
that ended long before us.
You could have Universes much older than ours is, and you could also have universes where
inflation is just stopping right now and you're only having the start of that hot Big Bang.
But what's very important to recognize about inflation is this super rapidness at which
it causes space to expand insures that if you invision the Universe as okay, we have
this bubble that we form within this ocean of inflating space.
And you've got another little bubble.
Even though these bubbles are expanding at the speed of light.
The space in between them, the space is expanding exponentially and it always pushes these bubbles
away.
And that means that wherever you are in the Universe, your pocket universe should never
collide with another pocket universe.
The space in between them should always be expanding eternally.
And even though you're always producing a countless number of these universes, they'll
never interact with each other and you'll never be able to see them.
All that we can experience is our observable Universe within it.
So we could never reach these other universes.
We're stuck just doing the math and imagining them.
That's sad.
Right unless you turn on God mode, like you said.
If you turn on the God mode, then you can see all the things.
And this is an interesting thing, and a lot of people argue that is the multiverse and
is eternal inflation actually science.
And I would argue that it is, but you have to be very careful.
The reason that I would argue that it is, is you say, well, we've got this theory of
cosmic inflation and the evidence for it is overwhelming.
Right?
We've got some very good evidence for it, cosmic inflation has been validated, it's
very robust, we're very happy with it.
Then you come to the next thing, you say well, we also understand that the Universe and everything
in it is quantum in nature.
So you say okay, you've got cosmic inflation, you've got the quantum nature of the Universe.
We put these two things together and what are the consequences we get out of eternal
inflation is one of them.
This tells you no matter how long ago or how recently ago inflation began, once it begins,
it should continue eternally into the future.
This doesn't mean it continued eternally into the past.
Inflation may have had a beginning.
The Universe may have had a beginning, or it may have been eternal to the past.
The problem is that for us within our observable Universe, because of how rapidly inflation
causes an expansion, when we look to one end of the Universe and when we look to the other
end of the Universe in two opposite directions, we say okay, we're looking at the entire observable
Universe, we ask how much of inflation do we have access to, do we have information
about that exists in something observable to us.
The answer is unfortunately, we only have access to only about 10 -33 seconds.
The final 10 to the -33 seconds of inflation.
This is something that could have gone on for fractions of a second, or could have gone
on for many seconds, or many years, or billions of years, or googols of years, or forever,
or forever.
But we only have access to that tiny bit of information, so on one hand, it's incredible
how much we can learn about the Universe, just from this tiny bit we can access.
But it does make you wonder because we have this tremendous extrapolation that we've done
from it, is there some kind of physics somewhere.
Or is there some physics that we've gotten wrong along the path that means that this
conclusion is invalid.
And as a scientists, it's very important to keep an open mind about it.
We're doing the best science we can with all the information we have.
We've validated every part of this theory that we can physically validate.
And for the parts that we haven't validated yet, we're looking for ways to do that.
But as we come forward we hope to learn more and more things but at some point you're going
to hit a limit.
You're going to hit a limit, there's a finite number of particles in the universe, there's
a finite number of degrees of freedom that they have, there's a finite number of bits
of information encoded in it.
And even with everything we have, with 10 to the 90 particles or so, and how they're
correlated and how they interact with each other, that's finite.
So if you say, I want to know what happened before these last 10 to the -33 seconds of
inflation.
That information might not exist in the way that human beings or anything within our Universe
can ever gain access to.
Well, in a moment, Ethan and I are going to talk about his new book, but first I'd like
thank:
Lucas Húngaro Kane Doyle
Jackson van Deinsen
And the rest of our 812 patrons for their generous support.
If you love what we're doing and want to get in on the action, head over to patreon.com/universetoday.
Ethan, well thank you very much for blowing my mind with this bittersweet ending.
But tell me about your book.
Congratulations.
Yeah, so thank you, I have a new book out, it's called Treknology, it's a book about
Star Trek, the science of it in particular.
It's about the science of Star Trek, from tricorders to warp drive.
And what we've done in this book is that we've taken a look at 28 of the different technologies
featured in Star Trek, from warp drive to transporters to Georgie's visor to any of
the military or civilian communication advances you can think of to the ship's technology
to even Borg implants.
You take all these technologies and you want to ask yourself, Star Trek the original series
was 51 years ago.
Star Trek the Next Generation premiered 30 years ago.
How many of these technologies have already become reality?
You've got these things like flip communications and universal translator ear pieces, and touchscreen
computers like PADDs or electronic clipboards and sliding doors and you're like that's yesterday's
technology, and that's true.
In the mean time, we've made tremendous advances to some technologies you might not realize
are almost here, from a holodeck, we've got virtual reality with multi-sensory experiences,
not just sight and sounds, but using infrasonic sensors we've also got touch.
You might say what about things like synthehol and believe it or not, we've actually got
drugs and pharmacological compounds that have us well on our way to having this synthetic
version of alcohol that will give you all the positive effects without any of the negative
effects with the addition you can take an antidote pill that will sober yourself up
almost instantaneously.
You remember Geordi had his VISOR and later got ocular implants, well it turns out they've
developed an ocular implant they can put in your brain's visual cortex that can restore
sight to up to 85% of blind people including those that don't even have any optic nerves
to go through.
Fantastic, well Ethan, we're going to put a link to the book in the show notes of this
episode so people can go and get a copy.
You're bordering on spoilers and it sounds absolutely terrific and I can't wait to give
it a read.
Well Ethan, thanks for joining us on the Guide to Space today, I really appreciate it.
If people have questions, I'm going to encourage them to post them in the comments and maybe
you're going to show up and answer a couple of people's questions?
I'd love that, thank you.
That sounds great.
I think this episode deserves a playlist.
This time, you're going to get a series of really complex lectures and documentaries.
I think you can handle it.
First up, a lecture from Leonard Susskind about eternal inflation, next a short animated
video about Cosmic Inflation.
Another from PBS Spacetime about inflation.
A lecture from Lawrence Krauss about inflation.
And finally a video from the Perimeter Institute about the search for a multiverse.
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