Intro
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thank you very much greg for a kind introduction yeah so the question we want to ask today is whether
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it's time to take the big bang out of the big bang theory that might seem like a radical idea at
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first but it's not as radical as it seems it's true
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that there's an abundant amount of empirical evidence for the big bang theory any cosmologist
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would tell you that at the same time there's not a shred of empirical evidence for the big bang
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why is that possible well it's because the big bang theory is really emerging of two very
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different ideas ideas that are very different scientific footing
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idea one is that the observable universe was once smaller hotter denser
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and has been expanding and cooling for the last 13.8 billion years
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we have a lot of evidence for that beginning with edwin hubble's observations from
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the 1920s all the way up to the most recent observations from the most sophisticated
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telescopes and detectors on the ground in space and balloons
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and with that evidence we can trace the evolution of the universe from the present
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going back in time to when the first atoms formed around 400 000 years after the beginning
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of expansion and even further back to when uh
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the universe had a temperature that was at nuclear fusion temperatures and protons and neutrons could be fused
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together to form the first uh heavy nuclei in the history of the universe we can go
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beyond that using experiments in the laboratory from particle physics particle physicists which take us a few
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orders of magnitude earlier in time indirectly but id 2 is the idea 2 is the idea that
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the universe began in a big bang and to get to the big bang we have to extrapolate
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15 orders of magnitude higher in temperature than anything we've observed in a
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laboratory or 60 orders of magnitude higher in density and that's why we have
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no evidence for the big bang itself this dichotomy this idea this notion
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that there are these two ideas that make up the theory is something that cosmologists are well aware of
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i can do nothing better than to quote my colleague jim peebles who in 2019 won the nobel prize for his
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contributions to the big bang theory but was quick to explain in an interview afterwards that
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quote the first thing to understand about my field is that its name big bang theory is
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inappropriate it's very unfortunate that one thinks of beginning
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of the beginning whereas in fact we have no uh theory of such a thing as the
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beginning so he shares the same concern
The Big Bang
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in fact the big bang has always been a sketchy idea probably most of you when you first
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heard the idea thought it was rather strange mathematically it's a singularity
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that means it's a mistake or a problem when we solve the equations going backwards in time that
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we reach a point where temperatures and densities become infinite normally when we encounter a singularity
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and solving equations we know there's something wrong with the theory that we're extrapolating
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and that's true here most of us believe that before we get to that singularity
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an instant before we get to that singularity we have to take account of wild quantum
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gravity effects which produce wild undulations in space time as this
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picture is meant to figuratively represent and which excite all kinds of degrees of
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freedom exactly what this quantum gravity phase is and how it connects to the rest of
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the story isn't clear but there are various terms or euphemisms that
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fellow scientists use to describe it some describe it as a phase of quantum phone some as emergent
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space time some is tunneling from nothing and we can go all the way back to george
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lemaitre in the 1920s talked about a primeval atom
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but i'm not here today to try to explain to you what the big bang is or is not instead
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the bottom line of today's talk is going to be that the big bang has got to go all together
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and the reason isn't philosophical and isn't aesthetic it's actually purely practical
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pragmatic basic science the problem is that if we have a big
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bang and we follow it by expansion only then we can't explain the most apparent
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salient features of our universe and that's why the big bang has got to
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go and be replaced with something else
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now when i say the apparent salient features of the universe i can point to all the data that we've
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collected over the last century which supports this idea one that we talked about
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a few slides ago but a lot of this gets summarized in an image that we can
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create by collecting together the cosmic microwave background radiation
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that first began to stream through the universe when the first atoms formed and that we
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can use to provide a snapshot of what the universe looked like at that time before there were galaxies and
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stars when the first atoms were forming the iconic image has been
The Iconic Image
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is the one that's shown here it what it's supposed to represent is a surface around us at great
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distances far beyond the stars and galaxies so imagine that spherical surface
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and we're uh that's the image that we're that's what we're making an image of and then we're projecting it on an oval
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much like we do when we're projecting the surface of the earth on an oval in order to present it in a flat
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two-dimensional map so this represents the entire sky projected into this oval imagine us
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sitting in the middle the red and blue colorations in the map
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are false color added to the map to represent the variations in
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temperature and density at this very early stage this infinite
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infant stage of the universe they represent variations in temperature of less than
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0.01 percent very tiny variations for the purpose
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they're fascinating to uh to study and to study the implications of
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but for what we're going to what we're going to talk about today what we really want to do is focus
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on the same picture but with slightly lower resolution
The First Approximation
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and the striking thing that one gets if one reduces the resolution by just a little bit
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is the first approximation the universe is remarkably homogeneous
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remarkably universe remarkably uniform in any direction we look and wherever we
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look furthermore we can use the data from this image to infer what is the geometry
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of space in principle in general relativity that geometry could be curved or warped
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but in fact what we discover is that that geometry is flat euclidean the laws of euclidean
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geometry hold beautifully in this universe the sums of angles in the triangle add up to 180 degrees
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another interesting feature of this image is that it shows us that the matter and radiation were in thermal
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equilibrium which means that they were at maximal entropy
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but if you think about the total entropy in the universe at that time it's actually much smaller than it could
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be by many tens of orders of magnitude if you were to take the same matter and
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compress it into a black hole for example it would increase an entry entropy by 30
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orders of magnitude so there's something peculiar about this low entropy as well
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in fact it's partitioned in a strange way because the matter and radiation are in
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thermal equilibrium which means they're in at maximal entropy
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whereas the geometry the space time the gravity is nearly perfectly uniform so almost no
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entropy so how is it that we have a lot of entropy and matter radiation but almost no entropy in the
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gravitational degrees of freedom and then finally this uh phase is well
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described by classical physics that's not to say that we can ignore quantum physics but they make small
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quantum physics make small corrections to this picture it can be understand from a classical point of view now if you think
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about these five properties and you think about the big bang then you realize the big bang is about
The Quantum Phase
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as far away from these conditions as you can imagine it's very much a quantum phase of the
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universe where quantum physics reigns over the classical as you can see from the drawing it's
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anything from homogeneous it's certainly not flat in fact you can't even define the geometry of it in
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any sensible way in the quantum gravity phase gravity is acting
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very strongly on all degrees of freedom all degrees of freedom get excited gravitational and modern radiation so
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you would expect coming out of this phase the entropy to be very high and there'd be no particular
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reason why or be partitioned in the peculiar way we observe observe in the cosmic microwave
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background map realizing this the cosmologists
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have recognized realizing this problem cosmologists have thought hard about how we can connect
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the big bang to the what we see in the cosmic microwave background
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at later stages in the universe and what this has led to for the last few
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decades is adding a band-aid to the story a kind of patch to the story
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that we call inflation now the question we want to ask today is
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is this band-aid actually working is there an alternative
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and if so what is it but in order to address those questions i want to go
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back and say something more about how this inflation is supposed to do its
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work now i'm imagining that many of you have read or heard about inflation before and if i were to
How Inflation Works
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ask you or if i would ask a typical cosmologist in in my field how is it that inflation
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works most of them would describe it in a few words using what i would call a classical physics view
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so how do we smooth the universe well if we imagine space-time being analogous to a wrinkly
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rubber sheet and we imagine inflation which is a period of stretching very very fast
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you might think as we stretch it it's going to appear smoother and smoother
How We Flatten the Universe
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or if i ask you how we flatten the universe with inflation i think most of you who have heard of
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inflation and most cosmologists would drum up pictures like this which actually comes from a text
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in which you imagine space-time being analogous to
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an inflating sphere and as you inflate the sphere back the word inflation comes from that
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notion as you inflate the sphere the surface becomes flatter and flatter and flatter
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and it's the fact that people walk around with this very simple idea of how you smooth and flatten the universe
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that makes the idea of inflation so compelling to people it seems so simple it seems so obvious
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what could be simpler well the problems with the explanations that i just gave you
Misleading Explanations
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is that they're wrong they're not just wrong but they're
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misleading they mislead you into believing that expansion is the essential element to
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smoothing and flattening the universe and that is not true
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the truth is just the opposite as i'll try to convince you today
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the best hope we have the best idea we have for explaining the smoothness and
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flatness of the universe is not any kind of expansion but rather a kind of contraction
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a kind of slow contraction but to explain that i have to give you a
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little bit more background on what the correct explanation is as to how inflation works
Basic Concepts
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to do that i have to introduce some basic concepts so i've tried to redu reduce the number
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of concepts to as few as i can i'm going to be talking about three quantities during
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during the course of this talk the first is called the scale factor
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the scale factor is a dimensionless conformal factor which simply describes by what factor
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the universe has expanded or contracted compared to some initial time you get to choose the initial time let's
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say we use today the scale factor we could choose to be one today and then we could describe what how much
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it expanded or contracted going forward or backward in time it's not something we can observe it's
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just a factor of increase we can't observe its absolute value
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the second quantity is called the hubble parameter the hubble parameter is the logarithmic derivative
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of the scale factor or a dot over a the time that where a dot is the time
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derivative it measures the rate of expansion and it's something we do measure we can
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measure in fact hubble measured it for the first time back in the 1920s by observing the rates at which
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gallic distant galaxies were expanding away from us compared to their velocity and distance
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from us so that's a measurable quantity
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the third quantity is called the equation of states the one that will probably be least familiar to most of you it's
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saying something about the form of energy that occupies the universe at any given time the dominant form of energy
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the one that's determining the rate of expansion or contraction it's a measure of the ratio of the
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pressure to the energy density so more precisely it's three halves times the quantity
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one plus pressure over energy density
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in the universe which is dominated by radiation epsilon happens to be equal to two
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radiation is high pressure in a universe dominated by matter epsilon is equal to three halves
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in the universe that's dominated by dark energy epsilon approaches zero can be uh really
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much less than one and in thinking about our cosmological models how we're going to explain
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uh how we go from big bang to the microwave background or what we might use to replace the big
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bang we can imagine other forms of energy that come to dominate the universe
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these are often described in terms of scalar fields fields like the higgs field which have value everywhere in space and
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time and which evolve according to some potential some rules
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with time to produce different equations of state with such a scalar field one can imagine
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possibilities of having an equation of state epsilon that could be as small as zero
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or it could be arbitrarily big and we'll be thinking about that entire range during the rest of
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this talk now there are two two important things
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to keep in mind only two equation like equations that we need to think about or proportionalities
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we need to think about um the first is uh has to do with the energy density
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the energy density for any given type of energy depends on its equation of state and it's proportional to 1 upon a
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to the power 2 epsilon so if we have a universe which is expanding
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and a is growing therefore then the energy density is decreasing
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but note that in general it doesn't decrease as a cubed it doesn't decrease as the
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volume that would only be true if epsilon is equal to three halves only for the case of pressureless matter
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if the universe has high pressure epsilon greater than three halves then the energy density falls off faster
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and that's because energy is drawn from uh from
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from the energy sources whatever that energy is into gravity conversely if the universe is is
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contracting now a is shrinking now when epsilon is large
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now the energy density is growing and it's growing faster and faster the bigger epsilon is
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the second important fact is what is what we can derive from einstein's theory of general
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relativity if we assume a universe which is uh uniform for the moment
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then einstein's equations of general relativity relate those three quantities we had on
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the previous slide here on the right is the scale factor which is telling us something about geometry
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given a patch of space it's telling us the factor by which it grows or shrinks
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and the exponent is sitting epsilon a quantity which tells us the pressure to energy density ratio of whatever form
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of energy is dominating the universe at that time the quantity h inverse or one over h is
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a measure of what we call the hubble time it's roughly the a if we were talking
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about today's hubble parameter it's roughly the age or time since the universe began expanding
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and if we multiply it by c the speed of light it's roughly the distance that light can
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travel over that time in other words it's roughly speaking how far you can see for the purposes of
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this lecture i'll just use that phrase how far you can see or hubble radius to represent the same
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thing now let's return to this issue of how we
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flatten the universe for example so we go back to this picture that we see
Flattening the Universe
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in many books and texts and we notice there's something wrong with this set of pictures
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it's showing us the expansion it's telling us something about the scale factor but where's the epsilon
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where's the h inverse in this story all of them should be involved to really explain
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the smoothing or flattening of the universe so we need to
Smooth or Flat
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take a more close look closer look at what's going on so the first thing to realize is that
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when we talk about whether the universe is smooth or flat cosmology cosmologically smooth or flat
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we're not asking if it's actually smooth or flat we're asking about its apparent
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smoothness or apparent flatness by a parent i mean given how much of the universe we can see does
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it appear to be flat not whether it's actually flat and
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you'll see that's an important difference for example consider an expanding universe just like
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we did on the previous slide and a universe i'll represent here as a sphere and it's growing by a factor of
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two in radius as i go from the left to the right each at each step i'm thinking about the
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case where the pressure of the energy of the energy that occupies the universe is greater than zero where epsilon is
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greater than one like we have in a matter or radiation-dominated universe
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now we have to think about this relation we we show between the hubble radius and
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the scale factor if the universe is growing if a is
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increasing and if epsilon is greater than one then we see that the hubble radius
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is growing faster than a that means in terms of our spheres
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although the spheres are growing the hubble radius is growing faster so if you think of the hubble
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radius as how far you can see and you imagine you're standing at the north pole how far
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you could see would be determined by the hubble radius let's say at the beginning at a given time and then what's going to
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happen is as the as the ball grows in size as a increases the hubble radius is also
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going to grow but it grows faster because epsilon is bigger than one so that takes us now to a spherical cap
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which is bigger and bigger still the figure of merit of
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whether or not the universe is flattening or not flattening is a comparison of how much of it we can
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see or the radius of the spherical cap compared to the radius of the sphere so
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in this case we see that as we go from left to right the spherical crack cap is growing
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faster in radius than the radius of the sphere itself
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the curvature that you would observe if you could observe everything in that spherical cap on the right is becoming
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more and more apparent triangles are looking to have angles which are you know different than 180 degrees it
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becomes easier and easier to find such triangles the universe is not flattening
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the universe is unflattening the universe is becoming apparently curved
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so this is the problem with the universe that goes from a big bang straight to radiation and then matter
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domination their parent flatness is uh uh
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apparent flatness is not happening rather the universe is becoming apparently more and more curved as we as the
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universe expands and it's been expanding for a long time so doubling many many times more than 60 times
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so by now you'd expect the universe to be apparently highly curved that's not what we observe
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in the microwave background we observe the opposite it's flat and that was one of the reasons for introducing inflation
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how does inflation change the story well the crucial thing about inflation is that it's a state in which the
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dominant form of energy density has negative pressure or epsilon is much less than one and if
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epsilon is much less than one now the situation reverses itself the hubble radius is growing much slower
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than a although the universe is still growing in size a the hubble radius
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if epsilon is let's say close to zero is hardly changing at all so as the sphere grows
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if we're limited to only seeing what's within our hubble radius the apparent flattening of the universe
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is occurring that is to say the ratio of the hubble rate the hubble radius the ratio of the
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circles or the ratio of the spherical cap is becoming smaller and smaller compared to the radius of the space-time
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itself and that's how inflation is supposed to work
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but once you realize that you realize that there's a third possibility that we haven't mentioned yet and that's
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the idea of a contracting universe so at first you'd look at the contracting universe and you'd say it's
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hopeless because as my sphere contracts it's becoming more and more curved but we need to
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consider the hubble radius what's happening to that compared to the size of the sphere
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now the scale factor is decreasing because the universe is contracting in a slowly contracting universe we can
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have epsilon much greater than one it can be ten it could be fifty it could be a hundred
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and so that means that the hubble radius every time a increase decreases by a factor of two
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the hubble rate is decreases by an enormous factor an exponentially large factor so even if
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the universe were apparently curved to begin with if the universe just contracts a little bit if a just contracts a little
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bit the hubble radius contracts a lot and within that hubble radius the universe appears to be flat
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so this is this is an equally good way of flattening the universe
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in fact i'm being unfair it's actually a better way of contracting the universe because slow contraction is in a sense
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much faster than inflation consider the inflationary case let's imagine in going from the
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small sphere to the large sphere the radius of the sphere has increased by a factor of two
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during inflation the hubble radius is nearly constant so the ratio of the ratio of the circle
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radius of the circle to the radius of the sphere has decreased by a factor of two we flatten the
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universe but by only a factor of two now let's imagine slow contraction with
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epsilon say of a hundred during the same period
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during a period in which the space contracts by a factor of two the hubble radius
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contracts by a factor of two to the one hundredth power so although the radius of the sphere has
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shrunk the radius of the circle has shrunk by an exponentially large larger power so that means the degree of
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flattening has is much greater so slow contraction is an incredibly
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powerful flattener of the unit flattener of the universe and we could apply the same thing to our
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wrinkle sheet and conclude that it's an extremely powerful smoother of the universe
Inflation vs Slow Contraction
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but there's another difference between inflation and slow contraction inflation begins after
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a violent big bang and we have described the conditions at that big bang
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it's in homogeneous space is curved in warped it has very high entropy it has an
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entropy which is not partitioned at all equal in gravity gravitational degrees of freedom and matter degrees of freedom
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and it's a quantum phase and we said we have a problem linking up that condition to what we
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observe in the microwave background but we have exactly the same problem in
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linking it to the beginning of inflation because inflation also assumes that the universe at that time
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at the beginning of inflation can be described classically it also puts stringent requirements on
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the entropy of the universe the entropy of the universe must be incredibly low
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much lower than we needed to for the cosmic microwave background by you know nearly 100 orders of magnitude
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and if we don't come out the big bang very uniform or very smooth
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if we have significant inhomogeneities curves and warps then it will be hard to get inflation
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started or even if we get it started it may tend to end too soon and since it's more likely to have
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begun that way it's more likely that inflation will end up either not starting
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or starting but not smoothing enough to produce the kind of universe we
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observe or the way i like to summarize the story is it's hard to go from the big bang to
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inflation but if you do try to go no inflation is probably more likely than inflation
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and bad inflation producing inflate some inflation but not enough to explain what we observe
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is more likely than good inflation
Initial Conditions Problem
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this is called the initial conditions problem of inflation how do we manage to get it started
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now by contrast slow contraction begins under far gentler conditions we do the same thing as we
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did for uh inflation we extrapolate back in time but if because the universe is contracting
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going forward in time going backwards in time it's opening up it's becoming
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well less and less dense it's approaching minkowski space it's well defined
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classically it's nearly an empty minkowski space it has low energy density it has low en entropy
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density we can track such a universe in going from this initial state to slope contraction
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using ordinary classical equations of motion now that doesn't mean and we can also
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use the fact that um slow contraction is fast why is that important well because even
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though we have these gentler conditions they can in some sense still be wild it could be that the energy density or
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shear in the universe or curvature of the universe is still widely varying over space we still have
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to take care of that through the slow contraction so it's important that when we finally
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get to the slow contraction phase if the slow contraction is very fast
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to analyze what happens when we go from some wild initial state and go approach a period of slow contraction
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we can't use usual analytic paper and pencil tools that cosmologists use
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we need something more powerful we need to solve the full einstein equations in their full glory
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using the tools of numerical relativity the same tools that were developed for
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describing two black holes merging and coalescing can with some
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considerable mathematical and theoretical effort also be applied to describe the universe
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beginning from some wild initial state to approaching a phase of slow contraction
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i'm going to show you quickly some simulations from this kind of work the simulations are going to
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picture different contributions to the einstein equations
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what i called omega matter that's going to measure the energy density which is driving the
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slow contraction some scalar field which would be driving the slow contraction omega k the curvature of the universe
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the thing we're trying to get rid of and omega s the shear of the universe distortion shear distortions which you
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also need to get rid of for simplicity we're although we're solving the full
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three plus one einstein equations we're only going to imagine that we had spatial variations along
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two directions so i can represent that as a sheet so for example the green sheet
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represents the variations in the shear across the space and the fact that it's
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so high on this image means that the universe is completely dominated by this sheer contribution
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very far from the smooth universe that we need to get to explain the microwave background
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the curvature the red sheet similarly widely distorted is going to be is also distorted and
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that's there and then the matter is sort of in third place uh almost covered up entirely
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by these other contributions now using the tools of numerical relativity
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the combination of mathematics general relativity and numerical simulation i'm going to
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show you what happens as we go forward in time i should warn
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you that you should not blink because it's going to what's going to happen is going to happen
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very fast so ready set there it is
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the rest of the movie is very boring because nothing happens
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all the action happened at the beginning all the smoothing of the universe happened at the beginning and you could
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hardly notice it but let's go back and take a look at it
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so i'll show the movie sorry i'll show the movie again
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but this time i'll stop the clock and bring time backwards and notice that even though we started
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in some very wild state it only took a few clicks of the
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clock for the universe to become smooth after which that was
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the end of the story and by smooth we got just the smooth flat universe that we need to explain
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the cosmic microwave background the the the its average uniformity
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homogeneity and uniformity this is uh an indication of just
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how powerful and fast this slow contraction is i should mention that the time there is
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in units where it takes 10 clicks actually a dozen clicks of the clock a dozen
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a change of 12 in order for the universe to contract by just
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one e-fold so this entire smoothing process which in this clock uh which on this
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clock took only about what we'll see we'll call it when it's smooth it's smooth
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around this time it's about 10 clicks of the clock that means the universe is hardly contracted at all and already it's in the smooth state
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that you'd like it to be in and this isn't just one simulation this is one of a set of
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hundreds of simulations we've done changing conditions in all kinds of ways here's another example sorry here's
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another example we begin with even wilder conditions and we'll see how much time it takes this time see if you can follow it
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well almost the same amount of time it's very insensitive that's how fast and powerful this
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this kind of slow contraction is we can't do such a thing for inflation
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at the present time in fact it probably can't be done
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and that's the story about inflation versus slow contraction when we simply view
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things from a classical physics point of view slow contraction is a powerful powerful smoother
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classically but we turned off quantum physics let's turn back on quantum physics
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and ask what that change what that does to change the story well you think very little if you're
36:20
thinking about inflation which is supposed to be smoothing the universe classically through stretching we would think
36:27
quantum fluctuations would have rather small effects so if we began with let's say a patch of space
36:34
which was smooth and flat to begin with then we would expect that while it's inflating if it's smooth and flat to
36:41
begin with we'll start we're getting a give inflation to break we're going to start smooth and flat to begin with we'd expect that the hubble
36:47
parameter would be have the same value everywhere on that sheet and that's what i'm plotting along the
36:52
sheet it represents a patch of space i'm using a two-dimensional sheet rather
36:58
than three-space dimensions but just so we can visualize but think of the two-dimensional sheet
37:03
as representing a patch of space which is everywhere at high hubble paranormal high value of the
37:10
hubble parameter if inflation is really invulnerable to quantum fluctuations
37:16
if it's truly a smoothing kind of operation then it ought to be that even when we
37:22
add the quantum effects we ought to see that all that happens to this sheet is number one it stretches a lot
37:30
well i'm not going to show you the stretching because then it would be out of our field of view so we're going to divide out the
37:36
stretching of it just to keep that in mind we're dividing out the stretching but the other thing we should see is as
37:41
inflation comes to end comes to an end the value of the hubble parameter should everywhere
37:46
be drifting downwards like this and like this and that would be our litmus test
37:52
that inflation is invulnerable to quantum fluctuations but here's the reality here's what
37:59
happens when you actually perform the test
38:08
instead of remaining smooth and flat something very wild is happening
38:14
it's becoming weirder and weirder stalactites are going everywhere
38:20
universe is any anything but smooth and now we're going to loop it through several times you can watch it again and
38:25
again while i speak what's going on what's going on is that in order to inflict for inflation to end it
38:33
doesn't end purely through classical physics it's affected by quantum physics
38:38
quantum physics can change when one region of the space ends inflation
38:44
compared to another one one region of space dips down to small h compared to another
38:52
there can be rare fluctuations which actually keep patches of the universe in the
38:58
inflating phase long after you would have expected it to have remained there the fluctuations
39:05
keep it kicking the universe back into inflating more and more because that's happening all along you
39:12
can see that most of the manifold here is remaining high at large values of h that means it's still inflating
39:18
inflation in fact continues forever it's eternal and what about those stalactites that
39:24
are happening there those are the rare regions of the universe that manage to escape from inflation
39:29
and get to the bottom and end but you see those are
39:37
they form a space-time which is highly inhomogeneous and not smooth this is what inflation does to space
39:46
if you include quantum effects it's not a smoother it's an unsmoother and to make matters
39:52
worse if we were to paint this picture at the end we have these stalactites according to
39:59
what are the curvatures what are the inhomogeneities what are what is the microwave background
40:05
properties etc in each of those stalactites which are the few regions of space which is the set of measure
40:11
zero which space which is managing to get to the end inflation you'd find from as you can see from the
40:18
colors every conceivable possibility this is what cosmologists call a
40:25
multiverse a universe which in this case began perfectly smooth and uniform
40:30
but ended up anything but a universe with multi-multiplicity and infinite
40:35
multiplicity of possible endings where inflation ends and this is a major fail because our
40:42
goal was to explain smoothness and flatness and what's obvious from this picture is
40:47
that smoothness and flatness are not the general outcome of a multiverse
40:53
anything can happen that's not to say smoothness and flatness can't happen but
40:58
it's not the general outcome it's not what you have any good reason to expect
41:04
now what about if we play the same game as slow contraction with a slow contraction we begin at
41:10
small h so we try the same litmus test we begin at small h and as the universe contracts
41:17
well we should expect to see this sheet rise in some way and end up at large h at the end of slow
41:25
contraction in this case i have no further movie to show you this is the proper end point
41:31
for slow contraction and that's because the reason we got into the multiverse issue
41:36
we got into that multiverse picture was because of the rare fluctuations that were inflating that was to say
41:43
we're going to huge volumes compared to other regions which ended inflation but
41:50
here there is no inflation there's just contraction the smoothing is only in the form of contraction so there is no such quantum
41:56
runaway effect so a major difference one that we can never forget is that there is no quantum runaway so
42:03
when we say that slow contraction smooths and flattens the universe we've shown you now under
42:09
general conditions and wild initial conditions it does so and it does so in a predictive and
42:15
definite way now we could also ask about other ways
Quantum Gravity
42:22
of viewing what's going on this picture and what they tell us about cosmology here we get into a more
42:29
speculative area of physics that we call quantum gravity we don't have a precise theory of
42:36
quantum gravity at the present time there are some leading ideas sometimes called string theory
42:42
which dominate current thinking about what might be a good quantum theory of gravity but uh a
42:48
number of theorists uh particularly led by kumumvafa at harvard and
42:53
um hiroshi aguray at caltech have been thinking in a general way how
42:59
quantum gravity might have effects on cosmology and they've come up with a number of
43:05
conjectures arguments reasoned arguments based on all the examples that they studied of
43:11
string theory solutions for example or more general arguments based on quantum gravity
43:17
of any sort to come up with some conjectures that a sensible cosmology must satisfy
43:23
and one of those which i'll mention here there are several of them but i'll only talk about one of them
43:29
um is called the transplantian censorship conjecture or tcc
43:35
and what this conjecture says is that if you have a cosmological scenario that converts subplankin modes to super
43:43
plank super hubble radius modes it is going to be inconsistent with any theory of quantum gravity
43:50
so what what are they talking about well by subplanking modes they're talking about modes we're
43:57
talking about quantum fluctuations which are kind of wiggles of fields or wiggles of space-time whose
44:03
wavelength or typical size is smaller than the plank length the plank length is around 10 to the
44:09
minus 43 centimeters it's the scale below which we expect there to be large quantum gravity
44:15
effects what they're saying is as long as those fluctuations remain on those small
44:21
scales you're fine but if you imagine a scenario which takes those fluctuations and stretches
44:27
them so if their wavelength becomes bigger than the hubble radius now you've taken those quantum gravity
44:34
fluctuations and you've drawn them up to scales which are classicalized by being larger than
44:41
the hubble radius and since we don't have control of the quantum theory we don't know what the effects of those
44:48
large quantum effects are going to be on those fluctuations we can't base a cosmological scenario
44:54
on such a scenario in such a situation it would be inconsistent likely inconsistent with whatever
45:00
quantum gravity would dictate and that's important for our story because this is exactly what happens in
45:07
inflation in inflation we begin
45:13
we begin the universe expanding an accelerating rate and that is sufficient to take any kind
45:19
of fluctuation it will stretch in proportion to the scale factor a the scale factor grows by a huge
45:26
exponential amount and by the end of inflation those wavelengths would be greater than the hubble radius
45:32
and what they're saying is that would violate the tcc the trans-flanking censorship
45:38
conjecture you your theory that you used to describe this would not be consistent for the laws of quantum gravity now to
45:45
be honest it's a conjecture based on lots of theoretical evidence not proven but we don't know yet of a
45:52
counter-example either finally let me say a few words about
Observations
45:58
observations because it's also important to think about observations what observations support the idea of
46:04
slow contraction well i would point to homogeneity and flatness first
46:11
those are often properties that are credited to inflation or have been incredible
46:16
to inflation in the past but what we've seen is that we can't say we can't argue that inflation predicts
46:23
or produces or explains homogeneity and flatness since what it actually produces
46:28
the one thing it actually produces is a multiverse about outcomes most of which are not either homogeneous
46:36
or flat the proper way to ascribe homogeneity the observed homogeneity and flatness we
46:42
see in the microwave background based on ideas that we know are present the only one we can ascribe it to
46:48
i would say logically is slow contraction but you may not find that satisfying
46:54
because you may say but we already were thinking about homogeneity and flatness we already knew those properties of the universe
46:59
is there something you can tell us about that we haven't yet measured and the answer to that is also yes
47:06
another thing that happens while the universe is smoothing whether it's inflation or or um
47:13
or slow contraction is there will be quantum fluctuations that produce distortions in the tiniest
47:19
distortions in the distribution some in some cases tiny distortions in the distribution of matter or density
47:27
that will produce fluctuations that are supposed to be the fluctuations we see in the microwave background
47:34
they will also produce in most cases most ways we imagine producing the
47:39
fluctuations uh in density that we see in the microwave background they will also tend
47:45
to produce shears random shears in the universe what we call tensor modes or gravitational waves in the universe on
47:53
huge enormous scales there's one exception to this which is the case of slow contraction
48:01
which can produce fluctuations in density but doesn't produce lar detectable levels of these random
48:08
shears now these random shears produce an effect on the polarization pattern the
48:13
microwave background which is called b mode polarization and
48:19
um uh what we've what uh we have found so far uh and it attempts
48:25
to look for this b mode polarization is no such effect consistent with what you'd expect
48:30
for uh slow contraction what i'm showing you here's a picture from the atacama desert
48:35
showing the site of the forthcoming simon's observatory which shows which is where they plan to build a
48:41
detector that will greatly improve our limits or detection of this b mode
48:47
polarization effect and if they observe it that would be a big problem for what i've described for you today in
48:53
terms of slow contraction of course if they don't it would be perfectly consistent with what we expect
48:59
to deserve so let me just briefly sum i've tried i've discussed
Summary
49:05
uh the fact that we need to explain these salient features of the observable universe and i've discussed a number of
49:11
properties we need to do that we need gentle starting conditions we need a rapid smoother and flattener we
49:16
need highly robust smoother we need to make sure we have no quantum runaway no multiverse
49:22
if we believe the quantum gravity conjectures we better not have a long period of accelerated expansion
49:27
so that it's tcc satisfying and we ought to have a theory that explains in a natural way
49:34
why we're not seeing the b modes at least not so far and a big bang followed by expansion
49:40
inflation or otherwise doesn't satisfy all these conditions in fact a big bang followed by inflation does
49:48
not satisfy any of these conditions and that's the reason why
49:53
so far as we currently know a big bang followed by expansion cannot explain the salient features of
49:59
the universe that's why we need to get rid of the big
50:04
bang and on the other hand we've also recently learned most of the results i've been showing you in fact have been
50:10
just obtained in the last five years that slow contraction satisfies all
50:16
these conditions and so that's a real game changer that's a sign that
50:22
maybe we have to rethink our overall picture of the evolution of the universe so we can incorporate slow contraction
50:29
into the story now obviously one thing you need is a way to connect
50:35
the slow contraction to what we called idea one the evolution of the universe from the
50:41
one second on point onwards and we don't want it to be something that messes up the smoothness and flatness
50:49
and uniformity we produce during slow contraction we need a kind of bridge a bridge that will take us
50:55
from contraction and smoothly to expansion we need to replace the big bang with
51:02
something gentle a gentle bounce now as it turns out this is not
51:09
impossible in fact again in just the last five years the first examples of a gentle bounce that seem to
51:16
be stable and smooth that can connect a contracting universe to an expanding universe have been discovered
51:22
theoretically and so that means at least theoretically we have all the elements we now need
51:29
to put together a complete cosmology beginning from the far past where you might have some
51:34
wild beginning through a phase of slow contraction which smooths out that wild beginning
51:40
through a gentle bounce to the contraction and to the microwave background and beyond what we see
51:48
and if you can do all that through equations which are deterministic and predictable in this way
51:53
you've really accomplished an amazing feat it's really a kind of for forcing a kind of paradigm
52:00
shift in the way we view the history of the universe and once you do that you can also imagine there are many other
52:06
interesting implications from this kind of idea but that would be the subject of another talk
References
52:14
before i stop i want to say just a few quick words about some people who have played an
52:19
important role in what i've discussed here i have not given references and papers
52:25
it would just been too numerous to mention and cluttered the figures if i had done it but i have to mention three people whose
52:31
ideas are very much represented in what a talk i gave i learned a lot of these ideas from them
52:37
or through work with them uh one of them is ana aegis properly pronounced who
52:44
is the first it was the person who five years ago tried to convince me that a gentle bounce is possible
52:51
even though i was very skeptical and she proved her point a few months later by producing an actual working example she's also the
52:59
person who's leading our current efforts of using the tools of numerical relativity
53:04
to uh to describe what happens uh uh in the universe as we go from
53:10
beginning to slow contraction and eventually through a bounce into the press
53:16
so roger penrose of course is a giant in the field of general relativity and cosmology
53:21
and has influenced us in many ways and was the recent winner of the nobel prize but what's most relevant to the
53:26
discussion i gave today is he has been a fierce critic of inflation probably the first really fierce critic of inflation dating
53:34
back to the 1980s and some of the criticisms i discussed today date back to his
53:39
his ideas and then when it came to quantum gravity i benefited from discussion from a
53:45
number of people uh most recently and in terms of what i was talking about today from kumum vapha at harvard
53:52
and his student alec bajoya who introduced this idea of the transplantian censorship conjecture
54:00
and we've benefited from the generous support of the science foundation which has enabled us to build a group of
54:06
students postdocs um graduate students and others
54:12
who are joining in this effort to explore all the possibilities that have nourished from this idea because
54:18
what we're talking about here is really the tip of a very large iceberg if you want to learn
54:23
more about what we're talking about what we've been working on i point you to our website which is very easy to remember it's called
54:30
bouncingcosmology.com and with that i turn it over to gray