This video, presented by theoretical astrophysicist Katie Mack, explores various potential scenarios for the ultimate end of the universe. It delves into cosmological concepts such as the expanding universe, dark energy, the Big Rip, Heat Death, and vacuum decay, explaining how our current understanding of physics informs these possibilities and what future observations might reveal.
Here is the word-for-word transcript of the video's subtitles:
i am i am super thrilled to be here i i would be more thrilled to be there in person but uh at times being what they are this is uh this is still a wonderful opportunity to uh to work with the royal institution and to to give this this presentation so i'm um i'm super happy to to talk with you about the end of the universe uh the the topic of my of my book and uh and in this in this presentation i'll be i'll be sort of walking you through a little bit about what we know about the cosmos and a little bit about how we think uh things are going to progress in the future um so so yeah without without further ado i will uh i will start things off so we are here um we live on a planet inside the milky way galaxy uh the um sorry the uh when we look out into the night sky we can see this amazing um amazing uh band of stars across the sky and those are the stars of the milky way galaxy we have somewhere around a hundred billion uh stars within our galaxy um and if we were able to to look out uh to look at our galaxy from the outside it would look something like this um we would see uh spiral arms we would see a bulge of stars at the center this is actually our nearest neighbor large galaxy the andromeda galaxy um and we will that will that will come up again later in the talk but when we look out into the cosmos we see uh we see literally billions of other galaxies this is a little piece of the hubble ultra deep field um just sort of panning through uh panning through these uh this uh this uh this image where this entire image contains something like 10 000 galaxies in this in this picture right here and when we look out into the into these images we're seeing galaxies that are distant in the universe that are in some cases billions of light years away and because they're so far away they're showing us uh light from the universe when it was younger in in the verse when we look at something distant we're also looking back in time because light takes time to travel to us so if we are at an observatory on the earth and we look out at the moon we're looking at 1.3 seconds ago it's 1.3 light seconds away because it takes light 1.3 seconds to get to us the sun is about 8.3 light minutes away so it takes light 8.3 minutes to get to us so we're looking at 8.3 minutes ago when we look at the sun when we look at stars we might be looking at many years ago maybe thousands of years ago and we look at other galaxies we're often looking at billions of years ago so we're really seeing the universe as it was in the past when we look at something distant but we haven't always known what is out there how distant uh how distant things really are in the past we didn't even know that there were other galaxies it took uh it took a while to discover the existence of other galaxies so for a long time uh astronomers saw these things called spiral nebulae which were these collections of of bright something in in the sky and the question was are these parts of our own galaxy are they nebulae in our galaxy where there are you know little collections of stars in our galaxy or are these very distant things things like our own galaxy but much farther away and one of the ways that we were able to eventually figure out what how distant these things were was due to the work of this person henrietta swan levitt who was working at the harvard college observatory um and she was looking at a certain kind of variable star called the cepheid variable star which you can see there in the corner of your screen that's an example of these stars and these are stars that that brighten and dim in a particular pattern and they and because the brightening and dimming is is related to how bright they are intrinsically it was possible to use these as kind of mile markers in the universe and to look at some of these uh variable stars in nearby galaxies and see that that uh the these galaxies really were uh very very distant and outside our own galaxy so by seeing these variable stars in the spiral nebulae that gave us some impression of of the size of the universe of how far away these galaxies were and once we knew uh that these galaxies were very distant we were able to then learn something about how they were distributed in the universe and also how they were moving in the universe so um in uh in the 1920s uh the these two people uh were responsible for uh for figuring out that the universe is expanding um in indi they contributed in different ways so on the on the upper left there that's edwin hubble and this this diagram is a is a diagram that he put together in a paper where he was showing that as you go to farther and farther distances the the the speed that which galaxies appear to be moving away from us is increasing and the person in the in the corner and the bottom right corner on your screen there um that's uh georges lemaitre he was a theorist he was thinking about how um how the universe has been evolving over time and he worked out that in a universe where the universe is expanding it would look like all the galaxies are moving away from us it would look like every galaxy is moving away from every other galaxy and he used that reasoning to to decide well maybe this means that the universe was smaller in the past and has been increasing and maybe there maybe there was a time when the universe was very small and and that kind of led to the idea of of the big bang so once it was determined that distant galaxies were moving away from us um there was a uh there was a discussion about what that means for for the universe and one of the one of the people who um expressed that very well at a lecture at the royal institution in the 1930s was arthur eddington um he's probably most famous for uh taking some observations that helped confirm einstein's theory of relativity and you can see him here in this image sitting with is his colleague albert einstein um but i like the way that he talked about the way that galaxies are moving away from us he says the spiral and nebulae are almost unanimously running away from us moreover the greater the distance the greater the speed of recession the law of increase is found to be fairly regular speed simply being proportional to the distance at first sight this looks as though they had a rather pointed aversion to our society but a little consideration will show that the phenomenon is merely a uniform dilation of the system that is not specially directed at us if this room were suddenly to expand to twice its apparent its present size the seats separating in proportion you would notice that everyone had moved away from you the motion is not directed from any one center but is a general expansion such that each individual observes each every other individual to be receding and that's a really important point that the expansion of the universe is not the expansion from a center point it's the expansion of the entire universe in every direction at every point and so there's no center from which we're expanding the whole cosmos is getting bigger the whole cosmos may have always been infinite in size and it's just getting more infinite as it expands it's a strange concept to keep in your mind but really what we just what we see is just that things are getting farther apart so if we look at uh the distribution of galaxies in the universe and we kind of move forward in time in our um in our picture of this we see the galaxy are getting farther and farther away from each other as time as time goes on um and so when we look at some of these distant galaxies that are moving away from us very quickly and we're looking far into the distance we're also looking for into the past and we can look so far into the past so far into the distance we see these these rapidly receding galaxies we see them as they were when the universe was very young because they're so far away and so there are kind of two things that we can we can get from these kinds of these kinds of ideas that the universe is expanding galaxies are moving away from us we see very distant ones when the universe was young what that can tell us is that in the past the universe was much uh more compressed that the universe has been expanding over time so in the past it must have been smaller it must have been denser must have been hotter because everything was closer together because everything's compressed and when we look into the cosmos in the distance we should see parts of the universe that from our perspective are earlier in the cosmos in a cosmos that was smaller and hotter and denser and so when we can map out what that sort of would look like and we can say that you know our current universe is about 13.8 billion billion years old um and we can use uh telescopes like the hubble space telescope to see distant galaxies so in this in this image this purple band is the hubble ultra deep field that's that's that um the image i showed you before with the the ten thousand galaxies in it uh those galaxies are so far away that they're mostly living at a time when the universe was about half a billion years old so when we look at those galaxies we're looking at a half a billion year old universe rather than our own um 1.8 billion year old universe but in principle we can look beyond that and we can see times when the when their galaxies didn't exist yet when the universe was just hot and dense everywhere because in the beginning the universe was hot and dense everywhere the big bang was a time when the universe was hot and dense everywhere and and if you just dial back the expansion you get the idea that there was a time when the whole universe was um was sort of filled with this fiery plasma and it was everywhere because there's no center to this expansion every part of the universe was was filled with this hot plasma in the early days in the early i mean there weren't days yet in the earliest moments of the cosmos and so we should be able to see in every direction if we look far enough we should see that hot dense plasma we should see the light from that glow the glowing of that plasma before the universe cooled it cooled and expanded enough for the for that um fire to kind of calm down and go out and we do see that um some of the first people to observe that were these guys um pencils and wilson who used a microwave receiver to uh to examine the the sky and they had they saw this sort of background uh static in there in the receiver and they tried lots of ways to try to figure out what the static was and it turned out that this static that they were they were getting the receiver that was coming from every direction in the sky was actually this background light from the big bang this they were seeing parts of the universe that from from our perspective were still on fire and they were those parts of the universe those very very distant parts of the universe were emitting this radiation um that this microwave radiation that looks the same in every direction and so they were able to you know you can you can take that radiation you can map out what it looks like on the sky so just like you can take a map of of our whole sky invisible light this is a map of of the galaxy invisible light stretched out stretched out so you can see the whole sky in one image so this is a projection of the whole sphere around us as an oval you can do that with visible light and if you do that with microwave light um it's very boring actually it just looks like the same the same color of microwave light in every direction but if you if you crank up the contrast a little bit you can start to see variations on that uh background light and you can see parts of of the background light that were a little bit hotter or parts that were a little bit colder and this is a one part and a hundred thousand these are very small variations you have to crank up the contrast a lot to see them but what you're seeing is that background light that that light from when the universe was still on fire and and there are ways that we know that that's what we're seeing that it's not some other kind of weird signal in the universe because we can take the spectrum of that light we can analyze how that light is distributed along different colors of microwave light and we can see that the shape of the spectrum the the distribution of energy across different frequencies of microwave radiation is the kind of shape that you get when you see something that's just glowing with heat this is called a black black body spectrum it's it's a a way that that the energy distributes itself if you just heat something up and watch it glow so the same way that if you take a poker and put it in the fire and let it glow red hot that uh the spectrum of that light would make this this black body curve this uh this particular distribution of of radiation and when we look at the microwave background radiation this this background light from the early universe it's exactly that uh that shape i mean it's it's such a perfect fit to this spectrum that you get from things that are glowing because they're hot that uh you can't even see the errors in the measurement unless you unless you magnify them 400 times like in this in this diagram that i have here um where you can actually see uh the the uncertainties in the measurements because you you've we've magnified them it's it's the most perfect black body spectrum measure ever measured which means that that light we get from the background light of the universe really is just the universe glowing because it was hot it's an amazing thing um and we can can crank up the contrast some more on this image we can look at smaller variations and smaller variations and we can see this detail where we see little bits of the universe that were a little bit hotter or a little bit cooler a little bit more dense a little bit less dense in the early earliest days of the universe so this is somewhere around 380 000 years after the very beginning whatever the very beginning was um and uh 380 000 years might sound like a long time but compared to 13.8 billion years it really isn't it's a it was it took 380 000 years for that primordial fire to cool down to dissipate enough that that like could kind of travel freely through the universe and not just be you know in this this glowing space um and so we can use that that background light we can use the little variations in it to learn about what the universe is made of about how it's been evolving over time and we can start to learn something about the possible future of the cosmos through um through looking at at this light and examining um examining uh what we see in this background light and that brings me to discussing our the first possibility for the end of the universe uh which is the big crunch now the big crunch is an idea that's been around for a while and it's it's the one that was that was thought to be most likely around the time when the microwave background light was first discovered and the way the big crunch works is okay right now the universe is expanding galaxies are getting farther apart from each other that's the expansion that was kicked off by the big bang but as the galaxies are moving apart from each other um there's still gravity uh between each of these um each of these galaxies and all the gravity of all the stuff in the universe should be trying to kind of slow down that expansion all the gravity is trying to pull everything back and so there was a debate um you know in from the 1960s to the late 1990s about whether the expansion would keep going forever or whether at some point um the expansion would reverse and galaxies would start coming back and um and uh you know collapsing into a smaller and smaller space and sort of reversing the expansion of the big bang and bringing everything back uh together again um now we think it probably won't come back together again but let's just take a moment to talk about what it would look like if it did so the first thing that would happen is that um galaxies would start to collide with each other they would be interacting uh with each other and as when galaxies collide they um they sort of come together the stars within the galaxies get flung out in these long trails and we see um we see galaxy collisions all over the universe and this this is just a sample of some interacting galaxies that have been seen by by our telescopes and you can see these these amazing tendrils of stars and one of the things that happens when galaxies collide is that the the gas when it comes together uh in these these different galaxies the gas comes together and creates bursts of new star formation you get black holes colliding and the black holes are coming out putting out um you know jets of radiation and there's all sorts of amazing it's kind of an amazing light show that happens when galaxies collide um what doesn't happen generally is it doesn't happen that individual stars uh hit each other but uh but the the collisions of galaxies can be quite uh interesting and whether or not a big big crunch is coming we're actually going to get some very personal um experience with colliding galaxies in about 4 billion years because of this so this is the andromeda galaxy i showed a picture of before our nearest neighbor large galaxy and it is coming toward us it is coming for us at about 110 kilometers a second and in about four billion years uh the milky way and the andromeda galaxy will collide with each other um and that'll be really cool to see if uh if anything is left in the uh in our solar system to to watch it um we will see uh the andromeda galaxy starting to get larger on the sky as it as it approaches and oh no it will it will smash into the milky way move stars around in this image you can see little clumps of um of blue and red where stars are forming and going supernova um you can see these dust uh trails as as uh as stars are are going supernova and creating dust uh out um pushing dust out into the universe into the colliding collided galaxy um they'll it'll kind of go through more than one pass and stretch out the the milky way and the andromeda galaxy and over time it'll settle into a uh kind of as final state of this sort of elliptical collection of stars where uh the star formation will have settled down and and you'll just get these these uh old stars hanging out um and uh and slowly evolving over time so we're gonna see that up close or whatever's left of our solar system we'll see that up close of course by four billion years from now the sun will have expanded quite a bit we'll be moving toward his red red giant phase it will have swallowed up uh mercury and venus and might be uh might be destroying earth by then um so probably we won't be around to watch it but maybe we'll leave some little webcam going so we can see the uh so you see the light show as we as we've moved on to somewhere else in the universe but um the point is that that these kinds of collisions of galaxies don't necessarily disturb don't necessarily affect the stars or the solar systems per se when galaxies collide the chance of two stars hitting each other is minimal and so generally you know there's just so much space between stars that that you know more or less stars and solar systems will be okay when when the universe gets very dense in a collapsing universe the thing that kills the stars is not the matter but the radiation so i mentioned that there's this this radiation throughout the universe from the big bang this leftover light from the the afterglow of the big bang and that light is just kind of out there traveling across the universe and we're picking some of it up when the universe is collapsing that light will become more condensed and it'll be like moving back toward that time when the uni the whole universe was full of this this hot radiation but it's not just that um ever since stars first formed in the universe they've been putting out their own light their own radiation and when the universe is starting to collapse that radiation that light will also be condensed and it'll be it'll be moved to hotter parts of the spectrum it'll be more become more energetic light and as that light is compressed that's what kills the stars what happens is that that radiation that hot intense hard radiation is going to be so hot just through all of empty space that it will start to actually ignite the surfaces of stars and at that point um you know nothing can survive when when you have thermonuclear explosions happening across the surfaces of stars um everything's toast so uh so the big crunch would be a very um a very unpleasant thing to live through now uh we're pretty sure that's not going to happen the thing that's that's most likely if you ask us cosmologists is is called the heat death now the heat death starts the same way as the big crunch the universe is expanding galaxies are getting farther apart from each other and um i mentioned that there was a time when there was a debate about whether the expansion would keep going forever or turn around and come back and uh in the late 1990s uh astronomers were trying to work this out we're mapping the the expansion of the universe and uh trying to see how much the expansion was slowing down because the idea is that the gravity of everything in the universe should be slowing down the expansion that was kicked off by the big bang just like if you throw a ball up into the air the gravity of the earth slows down the ball and it's in its ascent at some point the ball will stop and fall down unless you can throw at 11.2 kilometers per second and then it might uh you know escape out into the cosmos and keep going forever and so the question was um will the the universe uh keep going forever like a ball that's thrown at the escape velocity of the earth where it it just barely sort of reaches uh you know gets to gets out of the atmosphere and then and then sort of close threat or will the the ball come back will the universe re-collapse and when that measurement was made uh the surprising thing was was that actually neither of those um neither of those answers were true the the the deceleration everyone was trying to measure turned out to be a negative number the universe was not decelerating and so it was actually speeding up in its expansion the universe was expanding faster and faster and that's very strange that's about as strange as if you throw the ball up in the air it slows down for a little while looks like it's going to stop and then it just shoots off into space it's really very similar physics there's something strange happening in the cosmos that's making the universe expand faster we don't know what that is we call it dark energy but something's making the universe expand faster and so because of that over time each galaxy or or a little clump of galaxies if they're colliding with each other is going to get more and more isolated so our little group of galaxies the local group of galaxies we will we will collide with each other you know the andromeda and milky way collisional half a little triangulum galaxy will come in and get mixed up but after that happens in about 100 billion years we will not be able to see other galaxies in the cosmos we will not be able to see the distant galaxies we won't be able to see the cosmic microwave background anymore we will not know that there is anything beyond our little sphere of darkness around our galaxy the universe will become this increasingly empty place as things are moved away from each other so far and so quickly that they can't that light can't travel between them anymore and over time the stars in our own galaxy will start to die out and fade away matter will decay and even black holes will evaporate and eventually we will be left with a cold dark empty lonely universe where all that's left is a tiny trace of the waste heat of all of the processes of the cosmos that's called the heat death and it's a very sad story it's it's a um it's a rather depressing way to go i think if if you're going to be a universe but it's also very gentle it happens over a very long time the time scale on these on this uh process is so long like you have to put exponents in the exponents to express how long this takes so you know we're talking about um you know 10 to the 10 to the something years in the future it's it's a very very very long time from now but uh you know it happens slowly it happens gradually and it's it's happening as we as we carry on i mean already star formation is slowing down uh we have the peak of star formation something around you know i don't know 10 billion nine billion years ago and um since then the stars have been forming less and less frequently there have been fewer of these collisions of galaxies and right now you can calculate how many stars will be formed in the future versus how many have been formed in the past and you can work out based on how galaxies are evolving coming together and you find that something like 90 or 95 of all of the stars that ever will have been or ever will be have already been formed so that from now until the end of time we're just working on those last five percent it's kind of a it's kind of an interesting thought that that from one perspective the universe is almost done from another perspective we have so much time and future and so you know who knows what will happen but um that's that's just one possibility uh there are still other ideas for how the universe might evolve and a lot of the uncertainty comes from the fact that we don't understand dark energy we don't understand what it is this stuff is that's making the universe expand faster and it could be weirder which leads us to uh the next scenario the big rip so the big rip is uh based on the idea that maybe dark energy is something stranger than than what we generally assume it is so the usual assumption about dark energy is that it's something called a cosmological constant a cosmological constant is something that einstein first came up with to solve a problem in cosmology the problem was that the universe should have destroyed itself already so if you look out into the universe if you don't know that the universe is expanding then you see there are galaxies out there and those galaxies should be pulling toward each other and they should just all come together and and be one super galaxy their gravity should pull them together and when einstein uh was first uh looking at this problem he didn't know the universe was expanding he also didn't know there were galaxies outside of our galaxy he was just looking at the stars and seeing that the stars weren't coming together um it works just as well with grav with galaxies as it as the argument at the time did with stars but um he thought like what is it you know how is this all staying out there why isn't the gravity pulling everything together and making it collapse he didn't know that the big bang set off the expansion so he had to put something into his equations to counteract the gravity of everything out there in the universe to keep it from falling in and just collapsing immediately or having already collapsed before so you put in a terminal equation it's called the cosmological constant and the cosmological constant was there to kind of balance the gravity of all the stuff in the universe so it was a property of space property of gravity just something where every part of space has a little bit of stretchiness in it that uh that kind of pushes against you know just kind of expands the universe and exactly counteracts the gravity of everything trying to pull the universe together and the idea was to keep the universe stable and steady when it turned out that the universe was not steady when it was turned out that the universe is actually evolving is expanding uh he took the cosmological constant away because you don't need it anymore if the universe is expanding then it's just this sort of sort of like the momentum from that expansion that keeps the universe from having already collapsed so there's the gravity that's slowing down the expansion um but it doesn't it doesn't immediately make it stop you know so the the big bang set off this expansion the gravity might be slowing it down um but you don't need a cosmological constant to keep the universe from having already been destroyed all in the past um but then when the universe was found to be accelerating in its expansion we needed something some uh something in the universe that could could uh be pushing out that could be stretching space to keep the expansion going even though there was gravity pulling everything in and so we put the cosmological constant back in and we said okay maybe there is something about every little part of space that makes it want to expand faster and so if you do that then you find that um that that leads to a universe that's just expanding that has is very little matter in it that leads you to a heat death because basically what it looks like is if you look at the density of things in the universe versus time then over time as the universe is expanding the density of matter is dropping because you have the same amount of matter but a larger space and so the density matter is dropping the density of radiation is dropping even faster because as the universe expands not only does the amount of radiation inside the cosmos go down but also it gets kind of stretched out to lower energies so as as the universe is expanding you're diffusing both matter and energy but dark energy if it's a cosmetic constant is just a property of space so when there's more space there's more dark energy and so the density of the cosmology becomes just totally the same and that means that eventually you're going to have a situation where the matter and radiation have been so diluted out that basically all that's left is cosmological constant that's how you get to heat death is you know the universe is basically empty other than this this whatever it is that's making the universe expand faster but the heat death is gentle in the sense that a cosmological constant can move galaxies apart from each other but it can't rip galaxies apart themselves like the amount of dark energy the amount of cosmological constant in a volume doesn't increase over time so you don't you don't get things that are already sort of orbiting each other being pulled apart from the expansion of space within them because it just doesn't work that way um but there could be a different kind of dark energy that's not a cosmological constant where the density of it actually increases over time where within a galaxy you can get more and more dark energy kind of popping up in that space and stretching apart the galaxy and that's called phantom dark energy it's a hypothetical possibility for how dark energy could work um it's not it's not the most accepted idea there are sort of theoretical reasons to think that maybe maybe it doesn't fit with our understanding of how energy works in the universe in quite that way but if it were true then it would lead to a total destruction of the cosmos because not only would galaxies be moved apart from each other they would also be torn apart themselves and and if dark energy is increasing in every point in the universe over time then you get to a stage where no matter how close together two things are the expansion of the universe is so violent that it moves them out to infinite distance which means that you're ripping apart fabric of space itself um and that leads to what we call the big rip so a few years ago there was a paper about this and nasa made a an animation to describe uh what the big grip would look like uh if it happened and um i'm i'm including this this animation because i think it's uh i think it's just such a cool um such a cool uh uh diagram so you have the expansion then oh no the galaxies get torn apart so let that play a couple times universe expands galaxies get farther apart and then they're they're disintegrated and the whole universe is destroyed um i think that's a um i think that's kind of a neat uh a neat possibility um uh for for an end of the universe it's a very violent one um so okay so do we have to worry about a big grip is this something that's that's going to happen to us well in order to know if a big rip is coming we have to know something about dark energy and what we specifically have to know about dark energy is something called the equation of state parameter this is uh written as w it's a ratio of the density and the pressure of of whatever the stuff is that's making the universe expand band faster the only thing you really need to know about the equation of state parameter right now is that for a cosmological constant the number is exactly negative one um and so we can do various ways we can measure the equation of state parameter to see if it is exactly negative one if w is anything less than minus one any little bit less than minus one then you get a a kind of dark energy that increases in density over time and that will eventually uh destroy the universe um so if dark if w is less than minus one then you can calculate uh exactly how long in the in the future universe will be destroyed you can work out um you can work out how when all sorts of various things will be destroyed you can get a date for the big rip um so okay so what what have we measured for w well this is the planck satellite um it's the one that gave us that picture of uh the early universe uh the the cosmic microwave background and uh it can measure it can measure w can it can uh determine what w would be um and it gets a number that's that's uh you know to a first approximation minus one but if you actually uh write down the whole um the whole number that's been measured you get uh negative one point zero two eight plus or minus point zero three two what that means is that the idea
