roger the big bang has now entered common knowledge as the way the universe began and everybody always asked how did the big big bang begin and what happened before that and philosophers theologians physicists now begin to speculate on it i really come to you because you have some very unique ass uh insights into the big bang well there's something almost paradoxical about this microwave background it's telling us that there was something like the big bang there but it's also telling us because the observations of the microwave background you you can plot the uh frequency you
can pop the intensity against each frequency and you find this wonderful curve it's the the planck curve blackbody spectrum and it agrees with that to an extraordinary precision much better better than you could produce in the lab and uh this is telling us that the very early universe must have been in what's called thermal equilibrium now thermal equilibrium by definition is the maximum random state it's the state that the second law tells us we're going to get in the future if you like it's but there it is in the past so why is it that
the very special state that has to have been there at the beginning otherwise we don't have a second law the second law of thermodynamics tells us things get more and more random and that tells as you go back in time things get less and less random so it must be very very special in the initial stage but what we find is that the matter what we're seeing in this radiation is radiation that's light and that's been in equilibrium with matter so that's what you're looking at and that's at this maximum random state maximum entropy state
we say and the something special about that was in the gravity it's what you're not looking at is gravity and the thing is that the universe was very very uniform in the early days and we think of uniforms okay consistent with being random if you like but that's not true when is a principle ingredient because it tends to clump things together the sun is out there for instance because it's clumped together out of a previously uniform distribution of of of gas and that sun gets hot the background sky is cold the sun is hot and
it's the difference between the hot sun and the whole sky that we live off and that's where all life comes from that so uh it's it's that's the key thing now so the universe was very special but it was very special only in the gravity somehow gravity wasn't thermalized with everything else and that's something which needs to be explained to me it's it's the greatest puzzle about the big bang now i don't see most of these theories making any attempt to answer that question certainly the inflationary model doesn't it only in a certain sense makes
things worse and it's the argument is that it smooths out the universe and things like this but it doesn't do that unless you're already special or even more special in the early stages so if you follow the argument through you see that it really doesn't explain this initial specialness and it can't because it's all consistent with the second law of thermodynamics which says things get more and more random so how clearly they've got more and more special in in the early stages there has to be something else now for a long time if anybody would
ask me what happened before the big bang i would have given the conventional answer which is the big bang was this singular state when all our equations go haywire and time and space you know doesn't make any sense even the question before doesn't mean anything you see so that's the conventional answer you say you can't talk about it it's just meaningless questions there is no before there is no before now i now have changed my mind i'm not sure it's fair to have changed my mind but they have another idea which i'm pursuing which i
think has a reasonable chance of being right and this depends upon how you characterize the initial state of the universe so what i'm saying is that the gravity was special everything else seems to have been as random as it could be now can you characterize that in some geometrical way well a colleague of mine uh paul todd who the maths institute here in oxford um has uh had a particular way of characterizing this i'm worried about this for a long time and i've formulated a thing i call the vile curvature hypothesis let's not worry about
what that means but something about the particular type of space-time curvature that could have been present in the early universe now my colleague paul has a way of phrasing that in a nice geometrical way which is to say that you could extend the universe to before the big bang now this is just a mathematical statement you're not saying you believe any physics here just saying this mathematical statement you could extend it before to as long as you're somehow allowed to stretch the universe out so you i think what's the best way of explaining this i
think there are some nice pictures that escher has of everything a universe of angels and devils and they're all within this circular boundary and at the edge you see that's infinity and the whole universe is squashed into this this disc now if you forget about the size of those angels and devils and just worry about shapes then you can doesn't matter how big they are the little ones at the edge are the same sort of shape as the ones in the middle but but if you are prepared to stretch and squash in a uniform way
okay that you could stretch that boundary out to infinity or you could squash it back to this finite boundary now there is a universe and the universe in this picture goes stops at the edge that's infinity but you could imagine extending it to the other side and preserving this kind of geometry it's called conformal geometry that's a mathematical term which means that okay you you know about shapes small shapes but you don't know about sizes so small and big count is the same but different angles count as different or different small shapes kind of different
so if you don't mind stretching or squashing then you could step outside this universe to another side to it so i want you to imagine the same thing here that you've got the the big bang which is somehow stretched out to be a a surface as though that's a one-time surface but you could go before it now this mathematical trick is i'm not saying this is real i'm just thinking it's a mathematical trick this mathematical trick if you phrase the condition on your universe in this way you say you could extend to before then that
is a way of characterizing the initial state of the universe the universe seems to be like that it seems to be that the gravitational degrees of freedom are killed off which is what this picture is expressing it's hard to how to say this without being so technical but but i i hope this sort of idea gets across okay now that's one side of the picture and the if you like the physical justification for this is that in the very early universe the temperature was so high the energies of particles were so high that it didn't
matter a hoot what their masses were see mass is what you use if you want to build a clock you there there is a fundamental the two fundamental equations in physics that i'm referring to here one is the famous planck law which tells you energy equals e equals h nu so the energy is proportional to frequency the other is the famous even more famous einstein equation equals m c squared which tells you the energy is proportional to mass so if you put those two together it says mass and frequency are basically the same thing so
that means that there's a clock which is the frequency is a measure of mass now if you don't have any mass or if mass becomes irrelevant you can't build a clock so in the early universe the universe didn't know how to keep time you see it just it just it lost track of of how fast things were going you see and if you take that seriously you can imagine going to before it now it's a difficult idea to to grasp but that's it's it's mathematical it makes mathematical sense but it's hard to sort of think
this is real physics you see but okay that's one side of it now the other side is think of a very remote future what do we expect in the very we have the universe expanding the universe is expanding without limiting it's accelerating and it's expensive which is important for this whole picture it wouldn't work otherwise and this is this mysterious thing people worry about they call it dark energy it's maybe it's just the einstein's cosmological constant which is the way i would look at it but still we don't know why it should be there if
you like so but in this picture you need it so i want to say what's the remote future in this picture well the universe expands it exponentially expands um and okay there's black holes lying around which have got lots of mass in them according to stephen hawking these things will in a very remote future the universe will cool down to lower than the temperature of any given black hole even huge ones which have a very low temperature [Music] even huge ones which have a very low temperature and they will exp they will evaporate away get
smaller and smaller and smaller and eventually go off with a pop that's that's the pick now i'm accepting that i think that's probably right although there's some conjectural aspects to this um and eventually all the matter in the universe will disappear apart from radiation very dispersed that's right there's certain assumptions which go into this but let's accept that there's nothing left but radiation very dispersed now how do you talk about the very remote future in fact the picture i'm trying to describe is a sort of result of my worrying about how boring the universe is
you see it's going to get pretty boring you see and i can't think of anything more boring than waiting around for a black hole finally to disappear apart you see it sounds to me incredibly boring but then i thought well who's there to be bored not us you see the only things around will be these photons and things and it's pretty hard to bore a photon a photon doesn't experience any passage of time so eternity eternity is no big deal to a photon it just whoop there it is you see and this is there's a
sort of mathematical trick another mathematical trick which i've certainly been involved with for a long time how do you talk about infinity in relativity in einstein's theory united stands general relativity how do you talk about infinity well you'd use the same conformal trick but now instead of stretching out which is what i did for the big bang before i'm squashing it down so i'm saying that in the remote future there's nothing left which has any mass if that's the case somehow the universe doesn't know how to keep time in the remote view it doesn't know
how big it is you see so the universe forgets how big it is this sense and it might as well be a small new universe so the picture okay it's crazy i followed everything until that very last step where we have we have a complete expansion we have complete dispersion no mass photon radiations and and that and the similarity was that there's no clock because you have no mass that's right and that's the same characteristic of the early universe when it was very small but you have this very big universe that has no clock and
now how do you get to that small universe that has no clock and the point is that it doesn't know the difference between big and small because it has no cluster it has no clock you see okay you've got the speed of light which enables you to transform from time to space but since it's got no clocks it has no way of measuring distance either so spatial distance becomes irrelevant temporal distance becomes irrelevant or time so the universe forgets how big it is it forgets how big it is and so it sort of lost track
of that and it's and it becomes the next big bang now of course this needs to be filled out with with some honest mathematics and it also needs to be related to observation and the thing is it's you might think it's hopeless how would you ever know whether there was a previous universe you see but it's not so hopeless and one the first point is that i mentioned inflation inflationary universe is that in the early stages there was this exponential expansion but in this model the exponential expansion took place in the very remote future and
that is quite consistent with our present day view the universe will expand exponentially and so you will get same features that you get with inflation but without inflation and so then you the idea is that um that exponentially expanding universe you forgets how big it is and it's and it becomes the big bang of the next scale now how do you observe i mean as i say can you is the observation apart from agreeing with the well there are these things called scale invariants which is one of the observational supports for inflation so i'm saying
that that will i think will carry over to this particular model now the other thing is that there will actually be a little bit disturbances which will come about from i'll say they'd be in the remote future lots of black holes before they finally disappear with pops they'll be around for a long time and in the process okay in the middle of galaxies our own galaxy for instance has a black hole in the middle of it of something like three million times the mass of the sun okay that's quite sort of usual for a galaxy
in galaxies and clusters will you know run into each other their black holes will spiral around each other swallow each other up in that process they will emit gravitational waves ripples in space-time this sort of gravitational analog of light these ripples will make their mark on infinity it's a bit hard to explain that one too but although they get infinitely dispersed they still because you have to squash it down again to see what's going on they're still there and they will have an influence on the next stage of the universe and so in principle and
it will require delicate piece of analysis you should be able to see that i if i can use an analogy here think of a a pond and it's rains on the pond every time a drop of water hits the pond a ripple comes out now that's like these black holes colliding and the ripple goes out of disturbance and gravitational waves so you get these ripples after a while the rain stops that's when the black holes have all disappeared pop you see after a while the rain stops but you still see the ripples all messy it
looks like a just a mess you see but in principle you should be able to work out that these ripples are made up out of individual places where the raindrops have hit in the same way i'd say you can look at this background radiation and there's now a lot of information from these new satellites and so on which have been observing the very detailed structure of this background radiation you should be able to analyze it and see whether it's made up out of these individual events which is spread out in this way now that's something
for the future it could could completely destroy the whole idea on the other hand it might turn out that this is an obstacle i find it absolutely fascinating that it's even possible to consider what happened before the big bang well it's not so not so outrageous okay it is outrageous but not so uh it's it's conceivable that this would this would work yeah and it does explain the very special nature of the big bang the because the whole thing doesn't work without that so that's that's i think one positive feature that that other theories don't
seem to to give us
