In this series we explore competing models
for what happened at or even before the Big Bang. In many of the models we've explored
the universe is seen as having always existed. But one scientist who takes the opposite view
is Alex Vilenkin. In 1982, he published a paper showing how the universe might have
spontaneously created itself from nothing. And what he means by nothing is not the
quantum vacuum as some have alleged, but a state where there is not even any
space or time. What's more, this nucleation event wouldn't
just lead to one universe being created. Vilenkin was one of the first scientists to
argue that our universe is merely one of an infinite number of bubble universes. These are some of the most controversial
claims in Physics. So who better to explore them with,
but Alex Vilenkin himself. "Before the Big Bang" "Episode 9" "A Multiverse From Nothing" I had a good Math teacher, who encouraged me to study Mathematics
and gave me some challenging problems. So that was very helpful in
elementary school. And then in high school,
I had a good Physics teacher. I also had a friend who had
similar interests, and we decided to study together
the General Theory of Relativity. So that was a challenging project
and we had to learn a lot of math. We had some Calculus at school,
but we studied─ You know, Differential Geometry,
which was pretty advanced stuff. And we read a book which was Eddington's
"Mathematical Theory of Relativity". And at the end of the─ We met every week to discuss
what we learned, and at the end of the book there was some
discussion of Cosmology with, you know, discussion of the structure of the universe
and the beginning of the universe. And I was amazed that people can learn
anything about such matters. So, from then on,
I couldn't imagine doing anything else. In 1927, Werner Heisenberg published his
classic paper on the Uncertainty Principle, which implies pairs of particles and antiparticles
spontaneously appear from the vacuum. According to Quantum Mechanics vacuum is
actually a scene of a lot of activity. If you look at small microscopic distance scales,
particles pop in and out of existence, and they kind of live on borrowed energy. You can have electron and positron pop out,
but then they have to disappear, because energy conservation does not
allow particles simply to come into existence. So, they borrow a little energy from the vacuum
and then they have to disappear pretty promptly. More than a decade before Vilenkin's paper, Edward Tryon proposed that the universe
might be a vacuum fluctuation. Ed Tryon had what seemed to be
a weird idea that─ The whole universe could appear in that way,
as a vacuum fluctuation. You can picture it like you have an
empty space which you can, kind of, picture like a sheet of paper. And then you can imagine a bulge forming
on the sheet of paper, and taking the form of [a] balloon,
and then eventually pinching off. And this would be a new closed universe. So, the problem is that the universe is a lot
more massive than electron and positron. So, you would imagine that such as─ But you need the universe to exist for
billions of years. But Tryon realized that
there is no problem, because the energy of a closed universe
is equal to zero, actually. Because gravitational energy is negative
and energy of matter is positive. And in the case of a closed universe─ That is, the universe which closes on itself,
space closes on itself like the surface of a ball. For a closed universe it's a mathematical fact that
the total energy exactly adds up to zero. The gravitational energy compensates exactly
the positive energy of matter. And so there was no problem. No conservation law forbids creating a
closed universe from the vacuum. And Tryon told me actually how it happened
that he came up with this idea. He was sitting in a seminar, and I'm not sure that the topic of the
seminar was related to this, but he said that it came to him like a flash of light,
that he kind of had this sudden realization. And when the speaker stopped to collect
his thoughts, he just blurted out, maybe the universe is a quantum fluctuation. Everybody laughed because they thought
there was a funny joke, but he was serious. Tryon's proposal is that the universe came
from the vacuum fluctuation. But one can still ask
where did the vacuum come from? In 1982, Vilenkin decided to address this issue
in the context of inflationary cosmology, which implies a stupendous, exponential
growth spurt in the early universe. For more information on inflation,
watch Episode #4 of this series. 1982 was the year when the theory of inflation was,
kind of, more or less completed. Alan Guth originated this idea. He likes to say that inflationary expansion
can produce a big universe from almost nothing. All you need is a tiny piece of some
high energy vacuum, which can then expand and
produce a huge universe. You still need this initial piece,
and─ So, the picture to me seems incomplete. Where did that thing come from? So, that bothered me, and I kept thinking about what was
the possible beginning of inflation. What could trigger─ produce this initial thing? The trick to understanding the
Vilenkin's proposal is to think about something that is
impossible in Classical Physics, but it is permitted in Quantum Physics. It's a process that is essential
for the Sun to shine: Quantum tunneling. If you imagine, for example, that you want to get
a can of Coke out of [a] vending machine, you have to throw in a coin
and then the Coke comes out. It cannot come out otherwise because
there is a wall. There is a energy barrier that prevents it
from coming through. But according to quantum theory,
there is a small probability for the can of Coke actually to spontaneously
materialize outside of the veinding machine. Of course, if you wait there for this to happen, you'll have to wait much longer
than the age of the universe. But there is a small probability. Such quantum tunneling events happen
routinely on microscopic scales. For example, they are responsible for
most radioactive decays, where a nucleus is forbidden classically
to break up because there is an energy barrier, but quantum mechanically it happens
through quantum tunneling. See[ing] how what I call tunneling
from nothing is possible, let us imagine we have a closed universe
which has two ingredients: It has a high-energy vacuum,
of the kind that you need to drive inflation. Inflation, I should say, is a rapid
accelerated expansion of the universe, which is driven by this unusual stuff
which is called high energy vacuum. or sometimes false vacuum. And a remarkable thing about this vacuum
is that it has a repulsive gravity. So, when the universe is filled with this stuff─ [The] repulsive nature of gravity causes
the universe to expand with acceleration. Also, the other ingredient is just ordinary matter. So we have this universe with these
two ingredients. Now let us imagine varying the
radius of this universe. If we make the radius small
the density of matter will grow, and then the attractive gravity of matter
will dominate, and the universe will collapse. If you increase the radius,
the matter will be diluted, and the repulsive gravity of the
vacuum will dominate, and the universe will inflate,
expand with acceleration. Okay. Now, I wanted to start with a very small universe. So, suppose I have a very small universe─ Classically, it would collapse,. Because of gravity. However, there is an energy barrier
between that and the large size of the universe
that would make it inflate. But what I realized is that instead of
collapsing, the universe can do something more
interesting: It could tunnel to a larger radius. So, it would be a quantum tunneling process. So the universe will turn out to a larger radius
and will start expanding. And then I asked myself,
how small this initial universe can be. So, I looked at─ Mathematically, I discovered that when
I take the size of the initial universe to zero, the mathematical description of the
whole thing simplifies greatly, and what I had was a mathematical description
of a universe tunneling from a point, to a finite radius, and starting to inflate. So, a point is no space at all. So, basically this is no space,
it's no matter, and the universe in this picture
is created spontaneously from basically "nothing". I write "nothing" in quotation marks because it's not a philosophical nothing,
because─ We assume that the laws of
Quantum Mechanics are there. Somehow "there". There is no space or time,
and the universe tunnels from this timeless, spaceless state
into existence. As it appears the universe
has a very small size. It's filled with this high-energy vacuum,
and it starts to inflate very rapidly. The mathematical picture that I had gives the
probability for the universe to appear in different sizes and also filled with different kinds of
high-energy vacuum, and what I found was that the highest probability
is for the largest energy vacuum, and the smallest initial size. So, the universe appears extremely tiny. But then the high energy of the vacuum,
and its repulsive gravity, caused the universe to expand very fast. So, it doesn't stay small, it becomes huge
in [a] very tiny amount of time. So, how does for Vilenkin's
tunneling-from-nothing model differ from Tryon's vacuum fluctuation
model? It's different from Tryon's model
in two regards: First, Tryon had the disadvantage that
he didn't know about inflation. So, he wouldn't explain why─ I mean, if the universe appears
as a quantum fluctuation then a small quantum fluctuation is much
more probable than a large one. He assumed the pre-existing empty space,
pre-existing vacuum, and it wasn't clear where that came from. So, the main difference is,
in the picture of tunneling from nothing, there is no space before that and no time. When we say 'nothing' in this context,
tunneling from nothing, we don't mean quantum vacuum. It's actually what Tryon meant. And here we have a state without space,
completely. So there is no vacuum. There are─ The laws of physics I assumed to be there,
and that's a great mystery. Where they come from and what
determines which laws they should be? Most cosmologists accept that in order to
understand the origin of the universe we need to combine the General Theory of Relativity
with Quantum Mechanics into a theory of Quantum Gravity. But there is no agreement in the field
about how to do this. All Quantum Gravity theories are now still
at a pretty rudimentary level of development. So, you can use what is called
'semi-classical gravity', which is the approximation
where things are almost classical, but, for example, things like
quantum tunneling can still be described. And in that regime all these different theories
are pretty much more or less the same. The difference has come really at the
true quantum gravitational level, where the nature of space-time actually
may change like in String Theory, which says that space may have more
dimensions, or maybe even the space and time themselves are kind of
semi-classical concepts, and on a more microscopic level
we have some different structures, so that space and time emerge when
you go to sufficiently large scale[s]. And the same is true of
Loop Quantum Gravity. If the universe began from such a
quantum nucleation event, then what would be the cause? Many quantum mechanical processes
do not require a cause. For example, if you have a radioactive atom,
you know that it will decay. But you cannot tell when. So, there is a─ Half-life time, for example,
that you can tell that in a year the probability
for this atom to decay is 50%. Then the year has passed, it didn't decay. The probability for it to decay
the next year is still 50%. Eventually, it will decay. But if you ask why did it decay
at that particular moment? There is no reason. There is no cause. So, quantum mechanical processes
like these are uncaused, and the spontaneous creation
of the universe is of the same nature. It doesn't require any cause. While many physicists accept that a breakdown
of causality occurs at the quantum level, there are different interpretations of
Quantum Mechanics. So, how does this impact on the nature of
causality in Quantum Cosmology? The only interpretation of Quantum Mechanics
that appears to make sense in Cosmology is the Everett's interpretation
or many-worlds interpretation. Because the other─ For example, the so-called
Copenhagen interpretation─ This interpretation requires that there is
an observer outside of the universe with some measuring device,
measuring the universe. In the case of the universe,
we don't have such an observer. So, the universe is a
self-contained system, and I think many-worlds interpretation
is required here. In the Copenhagen interpretation
things are a-causal simply because it's kind of built in the nature of
[the] interpretation. You have a wavefunction
describing your atom, and then the wavefunction collapses
in the course of measurement, resulting in some of the outcome
probabilistically. And there is no cause
how you choose these things─ the outcomes. In the case of many-worlds,
there is no these collapses of wave function, and the wave function evolves
deterministically. So, in a sense, this is a deterministic
interpretation of Quantum Mechanics. However, this wave function describes
an ensemble of universes, and in different members of the ensemble,
in different universes, you get all possible outcomes
of your measurement. Simply, you don't know
which universe you are in. So, which universe you end up in
is also an a-causal kind of process. [Phil] I've heard some people claim
that's when─ Could the pilot wave theory,
or De Broglie-Bohm, that that is causal. Do you have any comment on that? Well, I─ I thought that this pilot theory
is a beautiful idea. I looked at it in my youth,
very which was very long time ago, and I didn't really follow it
afterwards. It was─ To my understanding,
it is not really a well developed theory. It applies to kind of simple settings,
a particle moves in some potential, but applying it to Quantum Field Theory,
or to Quantum Gravity, I don't think it is at that stage yet. If something could come from nothing,
then why doesn't this happen all the time? Why don't tigers just appear in our living room? In Quantum Mechanics many things are possible
that are not possible in Classical Physics. And, indeed you can have─ In principle, you can have very
strange things happening. Like objects coming out of thin air. However, there are some rules. And these rules are conservation laws. So, energy conservation is always enforced. So, for example, you cannot have a tiger
appear out of─ In the vacuum because
[the] tiger has a mass, some energy. But if you have a lump of matter,
in principle it can turn into [a] tiger. And Quantum Mechanics will not tell you
that this is absolutely impossible, but if you try to calculate the probability of
this happening, it will be pretty low. On the other hand, in [the] micro world, when you collide particles like they do
at the Large Hadron Collider, you collide two particles
and they turn into all sorts of things. They turn into other particles,
or you can collide two protons and they turn into a cascade of a huge
number of other particles. So, on the microscopic scale
such processes do occur, and─ If you think of the quantum
creation of the universe, it is a tiny microscopic universe
that has to pop out out of nothing. If you calculate the probability of this
happening─ I should say that, conceptually, interpreting
this probability is a little difficult. But still, if you do the calculation
you find that it is far more probable than having a tiger
materialized in front of you. Once the small universe nucleates,
it is thought to undergo inflation. But as Vilenkin pointed out in the early 1980s, this was a mind-blowing implication for
the large-scale structure of reality. It all has to do with how inflation ends. It happens through bubble nucleation. So, it is like boiling of water. A tiny bubble of our vacuum,
like the one we live in pops out in this expanding,
inflating universe, and it starts to grow. And this bubble nucleation is also
a random quantum process. It happens at different points randomly,
and so─ You will have, after a while,
this inflating space sprinkled with these different bubbles. The bubbles that formed earlier big,
the bubbles that are just forming are tiny. And as I said the bubbles grow,
but they very rarely collide, because the space between them
is expanding even faster. We cannot really travel to other bubbles because the boundaries of the bubbles
are expanding so fast. They expand at the speed approaching
the speed of light. So, no matter how fast we travel
we will not reach the boundaries. So, for all practical purposes,
we live in a self-contained bubble universe. And an unlimited number of such bubble universes
will be formed in the course of inflation So, that is why it is called "eternal inflation". Inflation never ends in the entire universe.
It ended in our part of the universe, and this is what we call our Big Bang,
when this energy of the vacuum went to ignite a fireball of particles,
and that's─ That was our local Big Bang in our bubble. But countless Big Bangs happened before it
in other bubbles and will happen after it. Many textbooks claim that inflation
happens after the Big Bang. But when we spoke to Alan Guth, the father of inflation,
in Episode #4 of this series, he claimed that it might be better to think of inflation
happening before the Big Bang. In the early interpretation,
Big Bang was kind of a singularity, where if you take the simplest cosmological models
and continue them back in time you find a point where the energy density
and temperature become infinite. It's simply the point where the
mathematics of the theory breaks down. You cannot go any further
and so, that's where you stop. But─ The meaning I use the term
Big Bang in is the beginning of the standard,
hot cosmological evolution. So, when the universe has a
very high temperature, very high density, is rapidly expanding─ That's the Big Bang. Before that, according to present views,
we have inflation. Now Big Bang, the term, is sometimes
applied to [the] initial singularity, if you want to consider one. But, in fact, I think [a] singularity is not a
useful thing to have in a physical theory, because you want your mathematics to work,
you don't want it to break down. "What happened before the Big Bang?": inflation.
"What happened before inflation?" No matter what you say you can keep
asking what happened before that. So, creation from nothing kind of seems to be
the only thing that stops this infinite regress. When I had the idea that inflation is eternal
I went to see Alan Guth and tell him about this. And he actually fell asleep. I should say that now he is a great
enthusiast of eternal inflation. When I got to know Alan better,
I discovered─ Well, first of all, I discovered that
he's a pretty sleepy fellow. He comes to seminars regularly, and he regularly falls asleep a few minutes
after the seminar begins in most cases. Sometimes actually [he] stays awake,
but these are exceptions. But then, no matter what, in the end Alan wakes up
and asks [the] most penetrating questions about─ About what was said in the seminar. If I knew his supernatural abilities,
I would continue telling him about my idea, but I quickly retired. Many have claimed that as other
bubble universes cannot be directly observed, the multiverse is not science. In Episode #4 we talked about the
possibility of detecting signatures, bubble collisions in the
Cosmic Microwave Background, But the Vilenkin and his collaborators have recently
worked on a new proposal for testing the multiverse. This multiverse picture,
there is not just one type of bubbles. String Theory, for example, predicts an
enormous number of possible types of vacua, and all these vac─ With this vacuum comes a corresponding
type of bubble which can be filled with that vacuum. And in the course of eternal inflation all these vacuum states will be populated
to have bubbles within bubbles, within bubbles. When inflation was going on
in our region of space, bubbles of different vacua
popped out and expanded. When we worked on this idea we thought, 'What is going to happen to these
bubbles when inflation ends?' The answer is that instead of expanding
they will start contracting and they will collapse. They will form black holes. And we've calculated the mass distribution
of these black holes. So, there are there is a very uniquely
defined distribution of masses. And, for one thing, these black holes are
interesting because they may explain, say, the origin of supermassive black holes
that we observe in galactic centers. But also if we really detect black holes with
this predicted mass distribution, that would be evidence for the multiverse, that we indeed had this period where
bubbles were nucleating. So, these are basically failed bubbles,
these big black holes. So, these are direct tests. If we are lucky enough,
we will be able to observe these things. But also there are indirect tests possible. The idea is that if you have indeed these
bubbles with variety of physical properties, some people noted that this will explain
fine-tuning, observed fine-tuning of the constants of nature. Because obviously we can live only in
those bubbles which are suitable for life. But you can turn this around
and make it a testable prediction. You can say, okay, if you have
a theory of this multiverse, Can we try to predict what kind of bubble
we are most likely to inhabit? In particular, what values the
constants of nature, like [the] gravitational constant,
or electron charge, or whatever
other parameters [it] will have. This prediction was actually successful
for one constant, which is the cosmological constant
of the vacuum energy density, or it is sometimes called dark energy. There was a great problem
related to this parameter, which is that particle physics models
naturally predict a huge value for this cosmological constant. And that would cause the universe
to inflate at tremendous rate, which we obviously don't observe. So, the the problem was why the
vacuum energy is so small. If you calculate the vacuum energy, this large value comes from
quantum fluctuations due to different fields. Like, for example, photons contribute
positively to vacuum energy and electrons being fermions
contribute negatively. So, in principle, you can imagine that
different contributions will cancel out, but that would require cancellation
up to 120 decimal points. So, that would be a tremendous fine tuning. However, if you have a huge multiverse with
a very large number of different vacuum types, in most of the bubbles you will have
[a] cosmological constant very large, and there will be no observers there. But in some very, kind of rare bubbles,
just by chance, you will have a small value. And that's where the observers will be. Now, you can try to figure out
what value we are likely to observe. Steven Weinberg was the first to find the bounds. He found the bounds if the─ He figured that if the cosmological constant
is bigger than some certain value, then the repulsive force due to it
is too large to allow galaxies to form. So that obviously will not be
a populated type of universe. But the next step was to figure out─ Okay, this is where we don't live, right? So, this is not really a useful prediction. But you can try to calculate
where we are most likely to live. And that's in those bubbles where
the cosmological constant does not start dominating before
galaxies are formed. So it allows galaxies to form. And then you have a large number of galaxies. After that, [the] cosmological constant
can dominate without damage. And that predicts a value,
or a range of values rather, which at the time when
the prediction was made people paid little attention to it, because
anthropic arguments were in disrepute─ Disresp─ Disrepute, I think.
─Yeah. And, then─ [A] value in the predicted range
was actually observed. It came as a shock to most physicists
when the antropically predicted value of the cosmological constant
was actually observed. And this changed many minds. So, no other possible explanations for the observed value of the
cosmological constant have been found. So, this may be our first indication that
there is indeed a huge multiverse out there. If the amount of dark energy in the universe
is delicately fine-tuned for life, the multiverse can explain why. We have to live in a part of the multiverse
that permits life. But a recent study has suggested that dark energy could be many times
larger than the observed value, without threatening life Furthermore, they claim that this puts pressure
on the multiverse as an explanation. I think they somewhat exaggerated the─ That─ So, the initial prediction when you calculate
the probability distribution. That calculation was actually a pretty rude. The─ Basically the probability of finding a
certain value of [the] cosmological constant was identified with the fraction of matter
that clusters in galaxies. So, if you have a
cosmological constant large enough to make, say, most of the matter
to avoid clustering in galaxies, and only a few galaxies are formed,
then the probability of such a universe is low. And if most matter is in galaxies,
then the probability is high. When you calculate the probability distribution
using this you find that it is pretty broad. And the observed value is on a low side of
the distribution but within the 95% range. So, what they did─ They did a more realistic calculation
using [a] numerical simulation of the universe, and they found that it is somewhat
broader than─ But, in my view, not dramatically broader
than analytic calculations did. But the main point is not that,
that I want to make. The main point is that this model is,
as I said, it is rather primitive. For example, it doesn't consider
differences between galaxies. So, if the cosmological constant is large,
for example, then it starts dominating early. And this means that galaxies
also must form early. Any galaxies that you form will form earlier
than galaxies in our version of the universe. Earlier means density is higher. So those galaxies will be denser. And stars will run into one another more often,
and what's more important, supernovae will explode closer to us, right? Because the density of stars will be higher. In the last few months
there appeared a paper where─ I don't remember the names,
there were four Japanese authors─ They did a simulation but now including
this effect of supernovae. And with some realistic assumptions
about how close you can afford to have a supernova to you without
causing a great extinction. And they found that as a result
the probability distribution changes in such a way that we happen to be
just in the middle of the distribution. So, I think there is no big problem there. While eternal inflation may be possible
to probe experimentally, is there any prospect for evidence of
tunneling from nothing? The mathematics of this proposal gives you
a probability distribution for initial states. So you can say what kind of state
the universe is most likely to appear. Inside it will be very small,
filled with this high-energy vacuum. What kind of fluctuations it will have,
and so forth. The problem is that after that, you have this eternal inflation
with bubbles, and so forth. So, the universe forgets
its initial state. So, you have tunneling from
one bubble to another. So, it's kind of a process where the
initial state is completely erased. And that's why it's very hard to test. So far nobody [has] really figured out
how it can be tested. The universe could be closed,
like a sphere, or negatively curved, like a saddle, or have no curvature, like a flat plane. One thing tunneling from nothing does suggest
is the shape of the universe. The universe in this picture
has to be closed. Because an open universe is infinite, and the probability for an infinite fluctuation
is exact exactly zero. [Phil] We observe the universe to be flat. Is that right? Geometrically, yes.
It's flat with a very high accuracy. [Phil] So, how do you make that? 'Cause, isn't a closed universe curved? Sure. The universe is closed,
and when it appears is like a three-dimensional sphere
of extremely small radius. But then it inflates,
and Inflation makes it huge. So, we see only [a] small portion of this
universe and it appears flat to us. The tunneling from nothing proposal requires that the laws of physics exist
platonically independent of the universe. One reaction has been that
this cannot be. As laws are just descriptions of
how objects behave and have no causal powers. People who say that was a mere
descriptions─ I don't know where
they get this knowledge. It seems to me that the laws
may well have some platonic existence. [Phil] Do you have any thoughts on
why the laws are what they are? Any ideas about that or it's just a given? I wish I had, but it's certainly the
question that suggests itself. And the only attempt to address it,
which I know, was made by Max Tegmark,
who suggested [an] even bigger multiverse. He said that, okay, maybe all possible
mathematical structures are somehow realized. I think this idea has some problems,
like, for example, there are many more complicated
mathematical structures than simple ones. The laws we observe have certain
simplicity to them. Einstein said beauty. So, this seems to be a different
selection criterion from just a random pick in the huge set of mathematical structures. But the bottom line is that we have no idea
where the laws of physics come from. A frequent problem that has been raised
in the context of a multiverse is that of a "Boltzmann brain". If universes can spontaneously appear,
why not just a brain? And if such brains dominated the multiverse,
then why aren't we one of them? In the multiverse you have these bubbles nucleating,
which are populated by observers like us, and you can also have these freak observers
that fluctuate out a vacuum, or isolated disembodied brains,
as people suggest, which have the same perceptions
as we have. Once you have a specific model of the multiverse,
you can figure out which are more probable. Like, you can compare their numbers, and if your model predicts that predominantly
the observers are Boltzmann brains, that the model is,
I would say, ruled out by observations. Maybe not ruled out by observations,
but I think this model is unsatisfactory. But there is a criterion which
you can figure out, and there are quite a few multiverse models
which do satisfy the criterion─ That ordinary observers dominate
over Boltzmann brains. So, I don't think it is an
insurmountable problem. It is just a condition that
needs to be satisfied. In one of our previous episodes, we discussed the No Boundary Proposal
of Hartle and Hawking. How do these proposals differ from
one another? The the two proposals are similar in spirit,
but mathematically, they are rather different. And the predicted initial conditions for
the universe are also rather different. In the tunneling proposal the prediction
is that the universe appears filled with the very high-energy vacuum
and it has initially a very small size. Because these things are related: The high-energy vacuum corresponds to
small size of the universe. The Hartle-Hawking proposal, on the contrary,
says that the universe should appear filled with very low energy vacuum
and have a very large size. The larger the initial size
the more probable it is. I find this rather counterintuitive,
but on the other hand, things do not have to be intuitive
with quantum gravity. While inflation is the mainstream
view of cosmologists, it does have its critics. And one of the most prominent of these
is Neil Turok. He and his colleagues recently took aim at
the Vilenkin's tunnelling from nothing proposal. They claimed that this thing doesn't work and,
basically, if you look at other particles, other than just gravity and this field
that is responsible for inflation, there are huge instabilities. That somehow these particles
are created in huge numbers, and─ So the model predicts various disasters
which we don't observe happening. And they claimed that this applies both
to my proposal of tunneling from nothing and to the ideas of Hartle and Hawking,
which were in a similar vein. So, there was now a stimulus
to reexamine these ideas, and I actually wrote a paper with
Masaki Yamada, here at Tufts, where we show that these things
don't really happen. But this required a better understanding
of mathematics of the model, and─ So, we felt that we made some progress. In 2003 Arvind Borde, Alan Guth and Alex Vilenkin
published a theorem, often known as the BGV theorem, which implies inflation must have a beginning. But does that mean that the universe
as a whole had a beginning? The theorem proves that inflation
must have a beginning, right? The universe as a whole─ It doesn't─
The theorem doesn't say that. It says that the expansion of the universe
must have a beginning, right? But it opens the door somewhat
for alternatives. One alternative that would
circumvent the BGV theorem is a universe that contracted
before it expanded. While many cosmologists we've interviewed
find this a plausible option, Alex Vilenkin takes the opposite view. Strictly speaking the theorem
allows the universe, which is contracting from infinite size,
for example, and then bounces and re-expands. It─ This excludes the model of
eternal inflation to the past. The question that the theorem
answers clearly is that inflation maybe eternal to the future,
but cannot be eternal to the past. There may be problems with
contracting universes, because basically contracting universes
are highly unstable. If you have, for example─ You know, the galaxy is formed by
gravitational instability. You have a small over density and
attracts matter and it grows. In flat space it grows faster
than an expanding universe. When the universe expands,
it slows down all these instabilities. But in contracting universes,
it grows catastrophically. So, if you have some inhomogeneities
in this contracting universe, they would grow out of hand. For example, if you have bubble[s] forming, all these bubbles instead of
being driven away from one another they would be driven towards one another. So, the whole thing will [be] filled up
with bubbles and inflation will end. But strictly speaking, it is allowed. One of the great mysteries of Cosmology is why did the Big Bang have
such a low entropy condition? Entropy is an extensive quantity,
it's proportional to the volume. And the very small universe
will necessarily have a very low entropy. But also it is filled with vacuum, and that is also the lowest entropy
that you can have, is the vacuum. While Alex Vilenkin's picture of
multiple Big Bangs is a radical one many of his critics propose
a cyclic model instead, which also has many Big Bangs. It seems there are very few,
if any cosmologists, proposing the standard picture
of a single Big Bang. If you call standard what is called
the Standard Big Bang Cosmology, which was the hot Big Bang.
You start with a very hot, dense universe which begins at [the] singularity. So that picture
I don't think anybody believes. In eternal inflation, there is thought to be
an infinite number of infinitely large bubble universes. But some have said that
infinity cannot exist in the real world. These bubble universes that form
in the course of eternal inflation─ These bubble universes, kind of,
if you look at them from outside, they're spherical bubbles
which expand. So, at any given time they are finite, but they grow indefinitely to
arbitrary large size. But then if you look at them from the inside,
if you're an inhabitant of these universes, the geometry of them is very interesting. Because in the interior these bubbles are
infinite spaces of negative curvature. So─ How an infinite space can fit into finite space? This is because the finite,
inflating total universe grows exponentially,
becomes exponentially large, and kind of infinity of space and time
mix together in an interesting way. So, it's hard to explain in words. But my point is that these
bubble universes are infinite from inside, and this is a mathematical fact. You could say, ok, maybe this─ The entire space of the bubble is not
in existence at any finite moment. But I don't think that you can really,
meaningfully make claims like that, simply because in these inflating
universes different points in the space-time
are not causally related. So, whether or not the universe is infinite
at this moment of time─ You have to define what you call time,
and there is no unique definition, because, you know, you cannot synchronize clocks
in [an] eternal inflating universe. Georg Cantor developed
a theory of infinite sets, and he defined sets of different
level of infinities, so you can have like one infinity
bigger than another infinity. Mathematicians, at least some of them─ Some well-known ones, deal with infinities without fear. Until a way to experimentally test the
tunneling from nothing proposal is found we may never know if it's right or wrong. But the fact there are still papers
being published about it, more than 30 years after
it was proposed, shows this idea has much
stayed in power. So where do we go from here? With the tunneling from nothing,
I think now we seem to have entered a very stimulating period stirred by
these papers by Turok and Lehners. I work with My collaborators here,
and Hartlel and his collaborators also Kinda were spirits to activity, So, at least there were some issues [with] the mathematical formulation
of these proposals which required clarification. And I think we are working on that now. I like the results that we get. We think that we clarified a great deal about
how [the] mathematics of the proposal works. It would be good to have progress
in Quantum Gravity. That, you know─ That would provide another
stimulus for the field. As for Early Universe Cosmology,
there are many things that one should be looking for,
including dark matter, and─ There are some anomalies seen in the
Cosmic Microwave Background which kind of call for for an explanation. They may be just flukes but they look
suspiciously, kind of, persistent So it will be good to explain those. And maybe I could add one of
my other favorite subjects, which is cosmic strings. It's, you know, the─ Progress in Early Universe Cosmology is closely related to progress in
Elementary Particle Physics at high energies. We are coming to a point where building bigger and bigger accelerators
becomes problematic. Because already we have an accelerator,
which is 30 kilometers in size. How much bigger can you get? So, it would be good to find some other
ways to investigate high-energy physics. One of the ideas is that in the early universe,
of course, at early times tremendous energies were reached,
tremendous temperatures, and the idea is that as the universe cools down
from this extremely high temperature, it can go through a series of
phase transitions, and as a result of these phase transitions,
it defects, like cosmic strings or monopoles. Other main walls can form. And cosmic strings appear to be the
most interesting of these defects. They are kind of lines of concentrated energy, and they can produce a variety of
observational effects. Observers are well aware of the
possibility of cosmic strings existing, For example,
they can produce gravitational waves and some electromagnetic phenomena. So, if cosmic strings are discovered we are going to learn a great deal about
high-energy physics that we cannot learn─ At energies that we cannot even
hope to get an accelerators. As astronomers look out into the cosmos, One can only hope that new phenomenon
like cosmic strings, or primordial gravitational waves
might be discovered. If they are they may be the keys that could
unlock the mystery of our cosmic origin. English subtitles by:
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