Dark Energy & The Big Rip - Sixty Symbols

okay so we're going to talk about the universe as it is now not the early universe but the universe that's something like 13 billion years old and it's doing something unusual you look at distant galaxies by that I don't mean galaxies nearby our own Milky Way like the Andromeda galaxy so I'm talking about galaxies that are tens of millions of light years away from us and you look at how they're moving with respect to one another they're generally moving away now that's that's not unexpected because we know the universe is expanding so if the universe is expanding then it means the space between galaxies is stretching but what's going on that's unusual is that it's not just moving apart it's moving apart and moving apart faster and faster so the universe is actually accelerating so then you say okay well fair enough the universe is accelerating what's weird about that what's funny about it is that if you just think of what's in the universe we've got radiation and we've got matter that make up us and then we've got dark matter we wouldn't actually expect that the gravitational pull from those objects would slow down that movement so that the the universe is expanding because of the hot Big Bang the energy from the hot Big Bang but then gravity begins to pull things back and we should expect the galaxies to be slowing down but they're not they're moving away faster and so it implies that one of the things that implies is that there's some form of energy density being pumped in that's causing it to expand this overcoming the natural pull of gravity it's like an anti-gravity and this has been coined as dark energy so it's a bad name really but it's but it's and it not only is doing this it's dominating all the other energy contributions in the universe today in fact if you if you add up all the types of energy that you could have then you've basically got the radiation in the universe you've got the matter content in the universe and you've got this dark energy and you've got the possibility of the curvature of the universe acting like an energy source when you add all of these up they come to unity - in terms of these funny units the dark energy is about 70 percent so point seven of this contribution is from this weird stuff that we don't know we don't understand the dark matter and making up about 27 percent as well so together they're making up ninety seven ninety eight percent of the of the energy density of the universe that we just don't understand what it is but today we're going to talk about the dark energy so one of the things that I've been working on for the last few years is trying to understand the source of dark energy now it could be that the source has always been there and it's been there throughout the history of the universe and it's just that it's come to dominate everything today or in the recent past by recent past by the way I mean within the last seven billion years or so you know we're talking cosmology here it could be that it's been constant that would be a term that would be a quantity that has come known as a cosmological constant and we can come back to that another possibility is that it's something that has been evolving with time and that it has it was sub dominant early on but perhaps it mimicked the matter and radiation in the universe it pretended it it was like matter and radiation it followed the the the the density of matter and radiation dropping dropping dropping but for some reason about seven billion years ago it came to dominate everything and that those are called generally come to the name of quintessence theories where it's evolved with time then there's possibilities that it's not due to some energy source at all that what we're seeing here isn't either a cosmological constant isn't some new form of energy but what we're seeing is the first evidence the Einstein's theory of general relativity needs modifying and we're seeing that it needs modifying on these large scales there's size of the universe the observable universe and these are called modified theories of gravity and so these three possibilities are all being actively pursued as a way of understanding the observation that the universe is accelerating des and I've been involved in in kind of all three of them trying to come up with models which try and mimic these it doesn't have to be that's right if in fact so of course when we say in Stein's got it wrong it's a bit like he's got it wrong perhaps on these largest scales he's clearly brilliantly correct on solar system everything within the solar system is beautifully explained by Einstein's theory of gravity in fact to be honest if I say it quietly pretty much everything on even cosmological scale seems to be brilliantly explained by Einstein's theories of gravity so any modification would have to be kind of quite small but it could be that we just need these small modifications in which case we don't need these new exotic forms of energy to energy density to explain it as you say it's due to some change in the gravity of a the curvature of the universe that that's leading to the observations that the universe appears to be accelerating well I've just been thinking about that because I don't think dark energy is such a great name it's there's lots of things that are dark then it's not particularly that that's special lots of things have got energy and that's not really that that's special but the thing that makes dark energy the important quantity to it is is that the pressure it has is negative it's got a negative pressure and that means it actually causes things to push apart that's the way that you can imagine it so so it's kind of like you can think of it as a bit having a bit like a tension and associated with it and and it's smooth that's the other thing about it it's it's a very smooth components through it's pretty uniform throughout the whole universe so something like a smooth tension might be a be a better thing to have rather than there rather than dark energy just move tension yeah I guess the problem with that name is it doesn't it doesn't allude to the fact that it makes it sound like it like we might know what we're talking about all that we know is that when you write down the equations Einstein's equations and allow for something that can have a negative pressure it will do the job for you that's the clear so a cosmological constant will do that for you as will these concocted models that you have these quintessence models they will do it for you and then of course you can you can also use your modified gravity and you and the modifications will do it for you as well there is one other thing that that will that can effectively do it for you which is is less in favor but hasn't been completely ruled out and that is maybe the universe isn't as smooth and as uniform as we thought maybe and we knew our very structure in the universe right we know look at you and I we're quite different and even though you're down the gym and I'm down the gym we're still quite different so there are structures out there we know that there are that galaxies are kind of clustered together on in filaments and those filaments kind of surround voids or voids where there is lower density than on the average but the the thought is that as you get to bigger and bigger scales in the universe that that the distribution of those voice becomes a bit more uniform and and there that their effect drops off giving you this net homogeneity things are very smooth but maybe that's not the case you know maybe actually the universe is is a homage inhomogeneous it's not homogeneous nice and smooth on large scales and if it's in homogeneous then and we happen to be living in a void in a in a region that's got a lower density than on average then we would actually perceive the universe to be expanding at a different rate from outside and it could be that that's what we're seeing you know the effect of this acceleration is and that's what an acceleration is you're seeing a different rate here compared to here and the effect of the acceleration is just that but because there are so many that's probably not the case and the reason why it's not the case is is because of the the the fact that we can measure different things in the universe and from it infer something about the distribution of the matter so we can measure the microwave background radiation we can measure the distribution of the hot and cold spots in the microwave background we can measure the distribution of galaxies and when you do in particular the microwave background and look at the distribution of the of the hot and cold spots it becomes clear that in order for this in homogeneous model type of models to work then you need to be in a very special place in this void in fact you need to be very close to the center of the void and then you begin to ask well why is it I would be very close to the center of a void I could be anywhere in this void so there's some high degree of fine-tuning going on in order for you to explain the apparent acceleration of the universe from the idea of these in homogeneous distribution of matter so it's it's kind of being it's under under threat as a as a as a model but it hasn't been ruled out yet but it's it's certainly very fine-tuned if it is the cliche dark energy is this like some kind of particle or field that can't be measured is it is a thing is a thing with mass or no mass it has I've got energy that can be measured in joules or as this like is dark energy just a label and shall I get idea shall I get my cup of get my cup to show you see how much energy there is okay if you just hang on I'll go and get it what a shame Hannah's taken it yeah I think I understand it to university anyway she had a dark energy cut but his for this cup here it's about this size this cup has in it about it I think it's a Yocto gram of 10 to the minus 24 joules of energy of dark energy because that energy is smooth right it's throughout the whole universe so it says it has got an energy he's got and it's got an energy density and so you can work out how much end is using a cup and I think it's about 10 to the minus 24 joules which is not not a huge amount but it's it's it's there so what makes it spent you said what is it well we don't know what it is and that's why we we try lots of things that can mimic its effect the key ingredient isn't particularly that it's got an energy that's lots of things have got energy matter has got energy radiation has got an energy associated with it that doesn't make it that doesn't make the end of the universe accelerate it actually slows down the universe but what the things that have energy also often have pressure and now we usually think of pressures of as a positive quantity you think of particles banging against the surface of a container creating a pressure pushing out and that's that's a positive quantity so radiation has India in the early universe relativistic rate particles have a radiation pressure which is a third of the energy density dark matter particles and the particles that you and I are made up of they're called dust particles written as the general name given to them they've got basically no pressure because they're hardly moving they're moving nonrelativistic so you think of putting some dark matter or some baryons that we are made of in a in a in a tin and they'll hardly hit the side of the tin there because they're moving so they don't create a pressure so that's got zero pressure or very close to zero what makes dark energy special is that the pressure it has actually is negative and and so it's it's you need you need something that can that can produce have a positive energy density but have this negative pressure that's acting in the opposite direction to what you might imagine and there isn't any standard stuff does that the stuff that you and I are made of the particles that you name enough just does not do that you need something more a bit more exotic and the Thoth the the the two favorite ways of doing it are basically to use what the particle cosmologists lives to use which is a scale of field and a scalar field has both a it's an object that's got a value everywhere and it's its energy is made up basically of a kinetic energy so the kinetic energy of motion of that object and the potential energy that's sort of telling you at any given point in space and time and time what's its potential associated with it now if it turns out that potential dominates over the kinetic so the kinetic energy of the field it's oscillating around is small compared to its potential energy then it will have a negative pressure and it can cause a kind of a repulsive effect in terms of causing acting against gravity and and so that's what a quintessence scenario will do for you you work in a regime where the potential dominates over the kinetic energy and it will give me this negative pressure it's what a cosmological constant does in fact in a cosmological constant is a unique case where the the energy density is exactly minus the pressure so they're balanced that they're equal in magnitude but opposite in sign and that has an has the same effect the data suggests that the universe is perfectly consistent with the cosmological constant so that the acceleration of the universe has been driven by something that looks like a cosmological constant in actual fact just recently there's a set of death in coming out if you look at the plank and microwave background data if you look at people the data coming from people that have looked at some supernovae that by the way is the way in which we infer what the universe is doing you you I'll come back to that if you look at the their data they're actually finding that the pressure is actually less than and the pressure is less than the energy density the - okay what they're finding is that when I divide the pressure by the energy density for a cosmological constant that number would be minus one if I do the same for ordinary matter that number is positive so the pressure is a third of the energy density for radiation the pressure is zero for dust for a cosmological constant the pressure is minus the energy density so pressure divided by energy density is minus one for a quintessence scenario you find that the pressure evolves down towards minus one it it begins to look eventually like a cosmological constant but there seems to be some tentative suggestions that actually the pressure divided by the cup the energy dose is less than minus 1 so that's lower than a cosmological constant would give that's a weird scenario and it it's probably not going to last that I imagine the data will push everything back up towards a cosmological constant but at the moment there's a slight pressure to what in the data so yes I think it might be less than minus 1 that means that the universe is actually not just going to is going to end up with a big rip it's called a phantom scenario and in the Lyne there'll be a future singularity where it will just tear apart because it's expanding so rapidly that first of all the galaxies begin to move apart rapidly and then the matter within the galaxies begins to break up under the influence of the expansion of the universe so they all break up and then eventually the fabric of space-time itself will split that's the idea it's not good that's not gonna do much further no London or carry on I mean just around in London that'll be no problem gasps easy yeah it's that galaxy being pulled away from us or pushed away from us no that's a good question the there's the dark energy is driving the expansion of the universe so it's driving the relative size of the universe so this is so in that sense I would say it's it's it's pulling it's pulling the the galaxies apart you could also think of it as pushing them apart as well though but I I think I I think I would no actually I think I think of it as pushing the galaxies apart because to be pulling it apart you need something outside of it to be doing that and the the the dark energy is part of the overall system of space time and matter and energy density and so it's integral to it so it's affecting the space time it's not outside of it pulling the space-time so I'd say it's effectively pushing pushing them apart through its come through the effect it has on the overall size of the universe that's a good question I hadn't really thought about that yes it's everywhere it's it's there's simply very little variation in it in fact no noticeable variation today it's in it so a constant like a cosmological constant would do that it's just but the same everywhere that's a very good question so the the the key thing I think you need in cosmology really is not a single there's usually not a single smoking gun one of the benefits that you have in cosmology that you don't have necessarily at the Large Hadron Collider they back in some sense of complementary and the Large Hadron Collider it's huge asset is the fact that it can produce the same situation millions you know six hundred million times a second collisions are occurring so six hundred million times a second you're producing the conditions of the very early universe ten to the minus eleven seconds after the Big Bang you can't do that in cosmology it's gone but what you can do is you can probe the different epochs of the universe using different sets of data so for example the Cosmic Microwave Background is really probing the universe as it was three hundred thousand years after the Big Bang is telling you what the universe looked like as those photons decoupled from the matter but on the other hand if I look at say the distribution of galaxies are of clusters of galaxies that's telling me about the universe much later on a few billion years later if I look at the abundance of the primordial elements in the universe that's telling me about the universe much earlier than a few minutes after thee and Big Bang so that there are these different epochs that I can probe I can look for example the distribution of the supernovae in distant galaxies that is giving me a snapshot of how the universe is expanding at different moments and by each of these that the distribution of the galaxies the distribution of the hot and cold spots in the microwave background the relative distance between this distant supernovas they all depend upon how the universe is expanding they that means they all depend upon the source of dark energy that you've got whether it's a source that's constant over time or whether it's a source that's varying over time so what you really want to do is get as much data on these different epochs as you can and see what models they're all consistent with and that's the that'll be the key ingredient it won't be that the microwave background will pin it down for me all the clusters of galaxies will pin it down I will need all three and I'll have to be able to bring them together and find which of the if any of the models best fits the the combined total and that we'll be the way that we'll try and do it so currently one of the key things that's out there they're out there are specialist telescopes being built and now working there's the dark energy survey which you're knotting and we're part of and that it's a know that it says what it's going to do on the 10 right it's going to it this is trying to probe for evidence of you know trying to find out what the dark energy is and one of the ways it will is going to try and do it is by looking as deep as it can a distant supernovae in distant galaxies and why that could be really useful is that we've just said the universe is accelerating today and we've just said what's the consequence of that we've said it's going to rip everything apart okay ventually if it was to carry on but if I turn that about if that if the universe always accelerated then we would have never formed structures right the atoms in the universe would never have had a chance to pull together under gravitational attraction because the acceleration would have pushed pulled them apart push them apart too quickly for gravity to pull them back so we know the universe can't have always accelerated so that means there's an epoch there's a time in the in the universe where it went from a period of not accelerating to accelerating and this is the epoch which the dark energy survey is going to be trying to probe because it can look deep enough into the universe to look at the distribution of the supernovae in galaxies at that scale now if we can get enough information about that period then what we will be able to do hopefully you start discriminating between the various models that try and explain dark energy like the cosmological constant like a quintessence model like a modified gravity model because they should all have slightly different ways in which we leave the decelerating period that's called the matter dominated era and enter the accelerating period so that's one way for example that we will try and test these this dark energy is everywhere you're saying it's in this room it's it it's in that cup so yeah what is it about dark energy that makes you unable to detect it or measure it or be able to prove it's year because it's hardly interacts with anything in the sense of right on the Sun it's a bit like gravity it's not like gravity in the sense gravity sucks and this isn't but gravity in this room is you know very difficult to pin down and if you had very you know it's big influences on big scales with massive objects and dark energy is doing the same thing you know but just because you and have put on a bit of weight over the last few months and you know you've grown you you can't put it down to the accelerating universe right dark energy has absolutely no impact on us in our everyday lives because the forces that are binding us together I'm much stronger than the thought than the dark energy which is trying to pull us about the dark energy here is trying to pull us apart that's true but it's completely negligible compared to the forces within our bodies which are keeping us together and the force of gravity which is keeping us on the earth so it becomes very difficult to determine it in the lab but I am involved in an experiment a proposed experiment which is going to try and do that it's going to try and look for various forms of dark energy by which are called chameleon fields so this is one of these quintessence type models which where the dark energy can its contribution can change this particular model is I'll just explain it briefly it's very nice because the the effect of the chameleon field which is going to be my dark energy field depends upon its environment so in a you know in an environment where it's in the presence of lots and lots of matter then it it's a heavy field it doesn't move very much it doesn't do very much in the presence of very little matter like on these very large scales these cosmological scales the field is extremely light and then it can actually it has a negative pressure associated with it and it acts like a effective cosmological constant it drives the acceleration so we've got a difference right we've got if we had if I have a very light density of matter it it's a very low density of matter the field is very light if I have a high density of matter the field is very massive and it reacts to the density so what this experiment that is that we're proposing with a colleague of mine at its own uh team Clare borage and then with Ed Hines at Imperial is we're going to have an if it works out we're going to have a comp make a condensate so bose-einstein condensates you know in the lab in a in a chamber so you've got this gas of in the molecules in the in the in the chamber and then we're going to separate them you can move them apart and we're going to and remember it if the if this field is present we've said it's everywhere okay this chameleon field is every well at the moment it won't do anything special the two condensates are very similar if I have the same value in each of the condensate but imagine now I bring a really massive source something really massive close to one of the condensates so that means that this condensates is suddenly feeling a much bigger density it could be a heavy object okay it all be that would be ideal if it was some very massive heavy object then then this bit of the bo-zone remember I've split the bose-einstein condensate let me do it big I split it in a big way here so there's one bit here that is now feeling this big source so it feels more massive its density has increased but this one is still not experiencing that source but really what the chameleon field does it reacts okay it reacts to the local density so it will feel much heavier here than it does here and so it will change its profile and what you can then do is you can quickly bring the sources back and the two values of the chameleon field will kind of interact with one another and we'll see like in like in what's known as an interferometer you can see interference effects from this chameleon and and that would be a way of testing for the chameleon model of dark energy and that that is a lot that we can do in the we're going to try and do in the laboratory this matter-dominated period yes no it could have been present it just must not be dominant yes yes yes that's a good question yeah so when you've got em when you've got radiation and matter in a universe that's expanding its contribution to the energy density drops that's really straightforward to understand but let's just remember what we mean by energy density we mean the the energy stored in a given volume so and that's what a name that's what the density is it's per unit volume so if you imagine the volume has been our universe okay and I have a given number of particles these days are matter maybe I've got ten particles and initially the volume is maybe you know this big so I've got ten particles in this volume so the energy is is this number of particles divided by this volume so it's maybe quite high now I double the size of the box so the number of particles has remained the same I've still got ten the energy has not changed I've got ten but the box has doubled in size so it's volume has gone up by a factor of two cubed eight so the densities dropped by a factor of eight and if I double it again it goes down again by the cube and so you can see that that matter in an expanding universe its energy density inevitably drops okay right so dark energy because of its nature is its energy density can remain constant all the way through and the models that I've listed that we're putting forward in terms of these quintessence models they are such that they will act like radiation on matter so they they initially will drop but they'll have some feature about them in their potentials which eventually means they come to go constant and they eventually come to them in it so there will always be a tipping point as you said where the matter has to drop radiation actually has to drop even faster radiations probably worth just mentioning so I imagine I've got a photon of light and again I've put it in this box okay and I've got a photon of light in this box the universe expands so the Box doubles in size this photon it stretches because that's what the universe it will stretch though it's a its wavelength will double okay well we know that the energy goes as one over the wavelength energy in light goes as one over the wavelength so if you double the size that the wavelength its energy has halved and so now you've got the energy has gone down by a factor of two from the radiation but it's gone down by this factor of eight from the box so in fact it goes down by a factor of sixteen 2 to the power 4 so the energy in the radiation drops even faster than the energy in matter so the early universe is one where you're initially dominated by radiation and matters sub-dominant but the radiation density is dropping so rapidly that eventually there's a tipping point there that's a very famous epoch that's called matter radiation equality where they match radiation keeps dropping matters drops off less quickly but now eventually this there's another tipping point it's somewhere here where the dark energy comes to dominate and that's that's ER yeah that's right so unless this if it's if it's a pure cosmological constant that's will be the outcome the universe will just keep expanding it'll accelerating and goes into what's known as a dessert to sitter expansions and exponential expansion and yeah the first of all that what will happen is the galaxies will drift apart you know because they've been pushed apart so the first thing we will notice is that will all the distant galaxies just moving away from us and eventually we won't see any there won't be any in our vicinity so that's pretty sad but and then within our own galaxies the stars will begin to move apart and we'll see them drifting out and then within our own we won't be here by then of course but within our own solar system that will happen and then eventually they they act the molecules and the atoms will themselves break up but most people I think believe that this is probably a transient feature and that what we're going to see is that there'll be some decay of the if it's a field a scale of field responsible like a quintessence that will decay just like the decay and inflation which then allows the universe to reheat that there will be some decay and we'll move back into something like a matter or a radiation dominated universe again zero degrees you walk all the way around and you come back to 360 degrees with a straight string that's not quite right the string sort of cuts out a bit of space so when you walk around it makes your space conical

Channel: Sixty Symbols

Views: 988,884

Rating: 4.9229722 out of 5

Keywords: sixtysymbols, Dark Energy, Big Rip

Id: IJYDcXZMe9c

Length: 34min 7sec (2047 seconds)

Published: Tue Mar 25 2014