Alan H. Guth - Why is the Universe Expanding?

right so it's it doesn't start out hot it does not start out but it ends because at the end you have this enormous production now how much compared to what you had originally how much production of of energy does the the end of inflation produced okay uh the the numbers here are somewhat uncertain because we don't know how much inflation there was but if we take the absolute minimum needed to explain our units our universe right then that amounts to expanding by a factor of about 10 to the 24 uh or times 25 let's say uh and then the energy increases by the cube of that because it depends on the volume so the total energy in this region has to increase by at least a factor of about 10 to the 75. compared that's a ratio compared to what you started with which has to have been about a gram to get inflation going so you need the gram at beginning you have that period of incredibly rapid but very short very very short inflation in which you have a multiplicative effect of 10 to the 75 10 with 75 zeroes of of energy which then uh uh at the decay of the inflation that energy is produced at a huge hot whatever a spew of particles and stuff that begins and then from that point then the normal so-called normal physics begins to explain the so-called big bang is that right that is exactly right and once inflation ends it just joins on very smoothly to the pre-existing theory of the big bang so when people think of the big bang of this hot spewing that's really the end of inflation i mean if you think of it that way then that is the so-called popular big bang if that's the way you want to define it yes i think so okay all right well that's uh that that means now there were a whole set of problems with the pre-inflation um universe the big bang theory that inflation because it sounds terrific but but you know it's just uh uh to be theories you have to explain stuff that cannot be explained in a whole bunch of other more uh simple ways because what you propose is not so obvious and it's not so simple and it's rather incredible it's one of the you know arguably it's the most incredible thing that you know human beings have ever thought of so you've got to justify this mr guth of why we have this now you do it by being able to explain problems that couldn't be explained any other way and then you need to you or hopefully to have some predictive powers to do some predictions of things that we haven't even just dealt with yet and that would give us some confidence so what are the problems that inflation solves okay uh i i guess i would classify the kind of results that i talk about here in terms of uh two two problem solved and one prediction but the prediction's now been tested and so far it seems to work very very well so it's in a way that's also a problem solved um first problem is the uniformity of the observed universe the fact that when we look in different directions we see essentially the same thing everywhere the strongest evidence we have for that fact about our universe comes from measurements of the cosmic microwave background radiation the radiation that we view as the leftover heat of this initial explosion this initial distribution of very hot matter in the early universe that cosmic background radiation has now been measured very very accurately uh and to an accuracy of about one part and a hundred thousand it's the same temperature in all directions uh to be completely precise here i should maybe mention that one does see a variation of about one part in a thousand much bigger than what i just described uh but we attribute that to just the motion of the earth through the cosmic background radiation so once you surface once we subtract that out then the residual non-uniformity that what we have to attribute to the universe itself and not the earth uh is at the level of about one part and a hundred thousand unbelievably uniform okay now what's so strange about that why is that a problem okay the reason that's a problem uh is described in terms of the history of this cosmic background radiation uh for the first 400 000 years of the history of the universe this radiation was locked with the matter because the matter was in the form of a hot plasma which has very strong interactions with radiation uh at about four hundred thousand years after the big bang the universe cooled enough uh so that the matter became neutral atoms which are then very transparent to radiation uh so the particles of this radiation that we see the photons uh we believe have been traveling in straight lines since about 400 000 years after the big bang itself now that means that the universe must have had this uniform temperature at 400 000 years after the big bang and we can ask ourselves is that enough time for it to have come to a uniform temperature on its own things do tend to come to a uniform temperature if you just let them sit but here a simple calculation shows that in order for the universe to become uniform in temperature by 400 000 years the energy moving from the hot spots to the cold spots would have to be transferred at about 100 times the speed of light to smooth things out that quickly and that's impossible as far as we know physics just does not allow that it's like a cup of hot coffee into which you drop one drop of cold milk and it takes a certain amount of time to distribute the temperature so that it's all the same exactly and and if one tries to calculate for the early universe how long that takes uh what we find is that there's just not nearly enough time in fact what one finds is that to transfer the energy from the hot spots to the cold spots if they were there would require the transmission of energy at about 100 times the speed of light which according to all of our theories is just not possible one could of course have assumed that the universe just started out uniformly to begin with since we don't really understand the ultimate origin of the universe but there you know we don't have any explanation there this process called inflation does give us an explanation for how the universe came to be at the same temperature and how is that uh it turns out to be just very simple in the context of inflation uh inflation inserts into this scenario of the early universe this period of exponential expansion and because the exponential expansion is so fast it means that before the exponential expansion the region that's going to become our presently observed universe was unbelievably small about 10 to the minus 24 centimeters across while it was so small there was plenty of time for it to come to a uniform temperature by the same processes by which that drop of milk in the coffee uh causes the coffee to come to a uniform temperature if you let it sit long enough the coffee cup were very very small it happened very quickly and that's the way inflation in the early universe work so the uniform temperature is established before inflation then inflation takes this tiny speck and causes it to rapidly expand so that in a brief instant it suddenly becomes large enough to encompass everything that we see and and and keeps the uniformity of the temperature that's right the uniform temperature is preserved by the inflation because it's just a uniform process that happens the same way everywhere okay second problem that inflation solves uh second problem is usually called the flatness problem uh where flatness refers to the geometry of the universe uh now i should maybe clarify that flatness and this does not mean two-dimensional as i heard some people misunderstand flatness means euclidean as opposed to non-euclidean according to general relativity the generic universe the typical universe model would be curved either open or closed our observed universe is very close we know at least to being flat and it's very hard to understand why it would be flat given the conventional big bang theory uh to be reasonably flat today it has to have been extraordinary flat in the very early universe because flatness is an unstable property uh if the universe is just slightly closed it will become more and more closed as time goes on and the universe would have just against this collapse instantly that's correct exactly fly apart very very quickly and there would never be the option of forming uh any reason sometimes use the greek letter uh omega to to have a critical density of the ratio how does that work that's right uh according to general relativity this geometrical property called flatness uh or openness or closeness is linked to the mass density in the expansion rate so given an expansion rate one can calculate something called the critical mass density which would be the mass density that would be just right to make the universe flat euclidean which is just the borderline of being open or closed the ratio of the actual mass density whatever that turns out to be to this critical density is what's called omega and that's the magic number here omega equals one as a flat universe uh our universe is very close to flat today today we know that omega is equal to one to within a few percent uh but the amazing thing is not that few percent but what happens when you extrapolate that backwards uh to give you a number if you extrapolate backwards to one second after the big bang uh what one finds is that to be anywhere in the allowed range today omega then must have been equal to one to an accuracy of about one part and 10 to the 15 one part in a million billion which is absolutely extraordinary i'd like to sometimes kiddingly say that that implies that the mass density of the early universe at one second after the big bang is probably the most precisely known number in all of physics because we know at the 15 decimal places has to have been that for omega to be in the allowed range today because of this instability so that's a serious problem for the conventional big bang theory which in no way makes this critical mass density special in the conventional big bang theory without inflation the initial mass density uh could have been anything there's nothing that forces it to be equal to or near this critical density so how does inflation solve it uh inflation solves it because inflation actually creates the matter as the inflation takes place and it very rapidly drives the universe towards this critical density by creating just the right amount of matter to put you at the critical density one way to think about it is to remember the length that general relativity gives us between mass density and geometry that's in this case it's easier to understand the geometry than the mass density inflation just takes something and makes it bigger and bigger and bigger if you take something let's say is round if as an example if you make it bigger and bigger and bigger you look at a part of the surface it stays round but any particular part of the surface looks flatter and flatter so that's the reason why the earth looks flat to us even though we know it's really round and that really is the secret behind inflation it takes some curved space and stretches it so much that the piece of it that we see is inevitably flat okay great now for any good theory to be a theory we like some predictive powers as well something you didn't know before for sure but something that comes out of the theory that turns out to be experimental or observationally correct okay uh to some extent by the way omega is an example of that okay uh because in fact uh when inflation was invented we certainly did not know that omega today should be one uh i didn't mention it because this process of inflation because it drives the universe towards omega equals one exponentially fast it ends up predicting not merely that omega should be near one but it really predicts that omega today should be exactly one or equal to one to within a part ten to the four ten to the five uh which was certainly not known at the time inflation was invented in fact uh ten years ago we were pretty well convinced astronomers were pretty well convinced that omega was more like 0.2 or 0.3 uh only recently with the discovery of this dark energy uh which when added in makes omega 1 again was inflation an observational success but probably more important in terms of predictive predictivity are the non-uniformities that we see in the universe which we can measure most precisely in the cosmic background radiation which really show us what the early universe looked like before there was a lot of complicated evolution inflation as a process tends to smooth the universe out by stretching everything out and just making everything uniform and for a while we were very worried when we're formulating the theory of inflation that will produce a universe that would just be too smooth too uniform and if you don't have any non-uniformities in the early universe they don't develop later either you just end up with a uniform universe no galaxies no nothing interesting so it's important that there are non-uniformities in the early universe and the idea of how they come in inflation was i guess came from several sources not me but the idea was that they could originate as quantum fluctuations uh the real world is not described by classical physics but by quantum physics uh and quantum physics is fundamentally uh a random process uh so what one ends up with uh is the density being slightly hotter the temperature being slightly hotter in some places and slightly colder in others the density being slightly higher in some places and slightly colder than others and that's exactly the kind of non-uniformities that one wants and wants to pursue this idea mathematically and really predict a spectrum for these non-uniformities by spectrum i mean how the intensity of the fluctuations varies with their wavelengths and this could be compared directly with observations of the cosmic microwave background radiation observations that were not made until starting in 1992 long after inflation was first proposed and amazingly it worked spectacularly well there's just a wonderful agreement between the observations that people have brought back from satellite and earthbound experiments and what the theory of inflation uh predicts we should see so looking at the totality of your life with inflation since late 1979 what do you what can you say about it and bringing it all together well um i think it's been a fantastic experience um i was always you know amazed in my childhood about how powerful mathematics was of predicting things like how long it takes a pendulum to swing and all the simple things that we observe around us inflation is i think a wonderful example of how one can make really wild speculations it seems at the time based on mathematical ideas that could be pursued and at least in some cases they seemed to lead to predictions of wild kinds of experiments that weren't even envisioned at the time which turn out right

Channel: Closer To Truth

Views: 125,802

Keywords: closer to truth, robert lawrence kuhn

Id: wijFtCfDEK0

Length: 14min 58sec (898 seconds)

Published: Sat Jan 15 2022