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.
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