How It All Ends | Crash Course Pods: The Universe

[00:00:00]Dr Mack we're at the last episode of our podcast we are at the last and I am I am so sad I've been really enjoying these conversations it's meant so much to me I've never learned nearly This Much from a project and I just have really treasured our conversations and I'm looking forward to this one because this time we get to we get to go to the end yeah yeah yeah the the the real the the the real end the absolute end [Music] well we have finally arrived Katie has walked us through the entire history of the universe in just 10 delightful and sometimes dread inducing episodes she carried us through Cosmic inflation recombination the Dark Ages the formation of the first stars and she did all of this with great patience and camaraderie she even taught us about what the future holds for our universe but all things must end and that includes both this podcast and uh the universe while the podcast conclusion is simple and finite the universes is much less so we can't predict exactly how it will all end but there are a few strong possibilities some more disconcerting than [Music] others this is one of the things that people ask me about does the universe have to end and like as far as we know the laws of physics just are not okay with the idea of everything kind of staying as it is forever I mean one of the things that you run into is the second law of Thermodynamics and that's this statement that entropy which is sort of another way of thinking about disorder increases into the future that's why you can't have a perpetual motion machine or something like that if you make a you know an extremely efficient you know spinning wheel something at some point there's going to be a little bit of energy loss through heat to the atmosphere and it's going to slow down and it's going to stop right and that's the second law of Thermodynamics that everything kind of evolves toward Decay one way or another and so you can't really get away from that you know even if the universe were not expanding even if everything were kind of staying stable in some sense things would get eventually less livable you know Stars would use up their fuel and burn out and particles would decay and you would eventually get to something that looks like an end and we've seen the evolution of the universe so far has been changing the the livability of the cosmos has been changing over time you know the very beginning it wasn't livable because it was too hot and dense and plasma everywhere and then there was a stage of of this peak of star formation Cosmic noon there was lots of stuff going on galaxies and everything and since then it's been evolving toward a sort of cooling down phase right the amount of star formation is reduced you know galaxies are sort of drifting apart from each other except for Andromeda well yeah yeah the distant galaxies are drifting apart easy for you to say galaxies are drifting apart from each other we've got we're going to get hit by one yes yes yeah that really bummed me out when I found out about that but turns out that it's just one bummer among many I mean it'll be cool it'll be you know impressive when it happens like yeah well to be fair it won't be my issue and it likely won't be our issue as a species some some distance civilization will get to add one more you know cool interacting set of galaxies to their catalog I mean it'll it'll be neat anyway there's just isn't really a mechanism to like replenish the order in the universe right things will evolve somehow toward an end and when I say an end of the universe you know this this is the other thing that I want to emphasize about the end of the universe is that I don't necessarily mean existence of all kind comes to an end I mean that everything that's in the universe that you know has any kind of structure is destroyed right that is inevitable you know whichever way you go that happens there are some ends of the universe in which in some sense SpaceTime still exists you know and maybe some energy still exists but like everything that is now is destroyed that will happen as far as we know if we as in the way that we understand physics right now [Music] okay so walk me through our son is a is a white dwarf we are probably pollution inside that white dwarf and almost all the stars that have ever been born or will ever be born are already burning yes or or have already burned out and then what happens so I think last time we talked about how as The expansion continues distant galaxies get farther and farther away and move away more and more quickly and so we stop being able to see them right the jwst instead of seeing what it we see nothing yeah so you get to a point where we don't have any view of the distant universe and we don't have any view of like the past of the universe either because we can't see things in the distance so we can't see things in the past we can only see kind of see our immediate surroundings and everything else is just dark so you get to a point where there's not much to look at and then what happens from there I mean that's like a 100 billion years that's a reasonable extrapolation from what we know now but there are kind of different possibilities that can happen that depend partially on what dark energy is doing and those can change the story either after that stage or maybe somewhat before but the idea that we do get to a point where we're just looking out into the dark is is pretty likely based on our current understanding okay but because we don't know what dark energy is and we don't know how it's going to change over time potentially there are a few possibilities that are still kind of in play at least in terms of like matching our observations as we extrapolate into the future in my book I talked about five different endings of the universe so I can just kind of go through those three of them have to do with what dark energy is doing and then two are kind of wild cards um great for other things so okay so when when we've talked about Dark Energy before we've talked about the fact that our kind of best guess about dark energy is that it's a cosmological constant it's it's something that Einstein first wrote down and he wrote it down in the service of you know keeping the universe static so you know he didn't know that the Universe was expanding and there had to be some reason why all the stars didn't fall down on us basically was the way he was thinking because he knew that gravity should attract everything together and so he put in this term in his equations of gravity that kind of added a sort of pushing apart in addition to the pulling together of gravity and so it just it imbus every bit of SpaceTime with this little bit of kind of swelling or or or stretchiness so that in the picture that he was putting together all the gravity of all the stuff falling together would be perfectly balanced by this pushing apart that's like just inherent to SpaceTime and that would keep the universe static but it turns out it's not static because dark energy and gravity don't perfectly match each other well it's not static I mean the universe is expanding so when when it was found that the Universe was expanding that threw out the whole static and that naturally explained why everything hasn't fallen into a big Clump because it's been expanding since the beginning and so you know you can think of it in some sense as there a momentum of things moving apart so they threw out you know Einstein threw out the cosmological constant when it's when it was discovered the universe was expanding didn't need anything to hold the Stars up anymore but when we discovered that the expansion of the universe was speeding up then we had to have some reason why the expansion was speeding up because gravity wouldn't do that right okay in a universe with just stuff and gravity the gravity of everything would should be putting the brakes on the expansion right maybe the expansion is happening but everything is pulling together and it should slow down the expansion but when it was found that the expansion was not slowing down you know astronomers had to throw in something to make that the expansion accelerate and the cosmological constant was right there right it was this mechanism was already hypothesized you put it into the equations SpaceTime has a little bit of stretchiness in it and that means that when you get to a certain point as you know the expansion is set off by The Big Bang everything's moving away from everything else at some point you know and and it's it is slowing down at the beginning because the gravity is working to slow down the expansion things get far enough apart that that gravity is very weak then the stretchiness of space kind of takes over as the dominant effect and that makes everything speed up accelerate and that makes the expansion accelerate so the idea of a cosmotic constant fits the data pretty well there are very few sort of pieces of data that don't seem to favor just the idea that the expansion is this cosmical constant which is where you know every little bit of space has a little bit of stretchiness built in it's the same amount everywhere so you know it's called constant because the density of this stretchiness stuff is constant right the density of cosmal constant is constant there's more space there's more cosmological constant it becomes more important over time that's an idea for what this you know so-called Dark Energy could be is whatever it is that's making the universe expand faster we call that dark energy maybe it's a cosmotic constant but it could be something else right and if it's something else if it's like some kind of New dynamical Field energy field in the universe like we have hypotheses for energy fields that could affect SpaceTime expansion and you can set something up where you have some kind of an energy field in this in the universe that causes space time to expand faster and and we invoke that when we talk about Cosmic inflation so this very rapid expanish to the very early universe that we say was driven by an energy field we call it the inflaton field um and it's it's the worst yeah I know it's terrible um but um it's what we call a scaler field meaning that it's um a field with like a certain value associated with it everywhere which is I'm only getting into this because I'm going to come back to scaler Fields later but a scaler field is like if you look at the temperature in a room you can assign a temperature to every point in the room that's a scalar field you're just giving number everywhere okay okay and so so a scal field is is a field where there's just a number everywhere unlike you know an electric field or something where there's a number but there's also a direction of the field in every direction that's a that's a vector field okay so a scalar field is there's is the temperature the same in every part of the room um not necessarily but so the number could be different that's associated with it but there's a there's a number associated with every part of the room and that's the field yeah and but it's not heading in any particular Direction exactly exactly okay yeah whereas you can measure the electric field everywhere in a room but there would be it'd be pointing some direction right right right it'd be going somewhere yeah yeah yeah okay anyway so there could be a scalar field in the universe that is the dark energy right that is affecting the expansion rate and it could just happen to look like a cosmological constant in the way that it's acting on the universe right now that doesn't seem so likely but we can't rule it out I mean to me it doesn't seem so likely to others it seems like the most likely thing it varies like how what kind of how you how you evaluate this stuff so because we don't know what dark energy is there's a level of ambiguity in the the deep deep future of the universe but we can kind of put together a certain set of possibilities based on what Dark Energy might be and that's really what you do in your book The End Of Everything astrophysically speaking is look at these different versions of What the end of the universe might look like depending on what Dark Energy proves to be yes exactly exactly and so there are kind of there kind of three things the dark energy could do right it could become more powerful in its sort of expansion right it could become less powerful in its expansion maybe even like cause contraction instead if it's something that's just messing with the way SpaceTime moves or it could be constant the way that a cosmological constant [Music] is so does the universe have to end unfortunately the laws of physics say yes at least in the way we think of the universe as containing structure and order the question is not if but how and to answer that question it might help us to look at dark energy that mysterious Force causing the universe to to expand faster which Einstein accidentally took note of when he hypothesized the cosmological constant but Dark Energy could also take the form of a scalar field so because we don't know what dark energy is or how it might change we have a few different potential options to consider for how all this [Music] ends so right now the dark energy is causing the expansion to accelerate but it could Fade Away in some sense or it could it could be something that changes over time it could even be something that somehow turns around and causes compression so there's a little bit of a hint and this is very recent but there's a little bit of a hint from a recent Galaxy survey called Desi the dark energy spectroscopic instrument they did a Galaxy survey you guys love your acronyms I know love an acronym over there yeah yeah so they in their first data release they found sort of tentative indications that maybe dark energy is getting less powerful over time oh it's not clear cuz it's just it's the First Data release it's not a like the significance of the result is not high you know it involves bringing in data from other surveys as well to get this result but you know it's possible that dark energy is somehow getting less powerful over time if it got so much less powerful that it not only turned off but started to to reverse which and I don't know what kind of theories they'll do this but you can you can imagine a theory that could do this you could imagine a situation where the expansion of the universe stops accelerating stops happening where the expansion stops and everything starts falling together again and then you would end up with what's called a big crunch good name yes yeah and the Big Crunch even before dark energy was thought about the Big Crunch was an idea that was popular like in the 1960s when we knew the universe was expanding but we didn't know if it was going to continue expanding forever or stop or turn around and the data at the time which was not as you know accurate as the data we have now sort of hinted toward collapse right hinted toward the idea that the expansion was going to someday stop and turn around just basically the idea was that you know the Big Bang set off the expansion but there's so much matter in the universe that it's it's going to slow the expansion down so much that it'll stop and everything will fall together again that was the hypothesis and now that we know that the expansion is accelerating that doesn't seem to make sense there's not enough matter to do that but maybe Dark Energy could make that turn around so uh for to me so I'm going to talk about five different ends of the universe to me a big crunch is the scariest one really I don't think it's very likely it's not something where the theories are really pushing that direction in any way but to me it's the scariest one for a couple of reasons one like you would see it coming right so at some point we would start to measure the red shifts of distant galaxies and and clusters and things and we would start to see that things are not red shifted anymore they're not the light isn't being stretched out by the expansion anymore it's starting to be blue shifted so the light is being compressed to higher frequencies and then you know that these galaxies are coming for you yeah the way that I know Andromeda is coming for us thanks to you and so all of the galaxies are kind of coming for you yeah so you would see everything's coming for you and so the fact that you're going to get hit by a bunch of galaxies is disconcerting but it's actually worse than that because also space is going to get hotter oh so you know we talked about the cosmo microwave background back in the early days of the universe when everything was hot and dense like space itself was hot it was filled with hot plasma and we can see that when we look at the cosmic micro background we can look far enough away to see that hot glowing universe so if you compress the universe again then all of that energy gets compressed again and you start to get back toward that hot glowing plasma stage where just everything all the light from the cosmic microwave background that we see now has been red shifted a lot it's been stretched out by the expansion of the universe to microwave wavelengths but it would be compressed again till it came back to like visible you know ultraviolet wavelengths like hotter and hotter like higher and higher energy light like climate change but for the universe yeah but it's even worse because it's not just like the background light that would be compressed and hardened radiation it's also all of the light from all the stars that have ever shown right all of the Supernova explosions all the black hole outflows all these high energy photons that have been thrown out into the universe because of all of the astrophysics that's ever happened all of that light would also be compressed into a smaller space and also be shifted into higher frequencies harder radiation and so it's been calculated there's um Martin Ree who uh the astronomer Royal of the UK I guess or of England I'm not sure anyway he did a a paper in I don't know the early 70s or something maybe 69 where he calculated what would happen in a big crunch and found that you can get to a point where the ambient radiation in space is so hot and so high frequency and so so there's such high density of radiation that at some point it would start to ignite thermonuclear explosions on the surfaces of stars like there' just be so much energy in space oh God yeah yeah so like the idea of existing in a universe where you know that's coming that's terrifying to me I do not like that okay I agree I agree that thermonuclear explosions happening on the surface of all the stars sounds stressful agreed but counterargument is it possible that in a big crunch things go back to being a hot dense soup for a long time and then eventually the soup becomes hot and dense enough that there's another big bang an accordion style Recreation of the universe so in a generic big crunch you don't have a new beginning it just ends it goes to a singularity and you can't really do much with that that's a hard stop in an equation that's a full stop at the end of that sentence right it's just and we'll talk later about different ideas for cosmologies that do have a new beginning but a standard big crunch is not one of them you know what I just realized when you said that yeah I realized that in some like future Wikipedia some theoretical Wikipedia like I always think the most beautiful two words in English are from the Wikipedia article on small poox which begins small pox was nice yeah I just think this is the most impressive thing we've ever done right yeah yeah but given the fullness of time every Wikipedia article will be past tense yeah yeah so yeah here's to doing the best job that we can do to make the ones we want to be past tense in terms of Wikipedia articles past tense and the ones we want to be present tense Wikipedia articles present tense sounds good yeah [Music] so right now something like 70% of the energy density of the universe is in Dark Energy whatever dark energy is in the past it was less because the universe was more dense with matter and so most of the energy density of the universe was in matter at the very beginning it was mostly radiation you know most of the energy density of the universe was radiation so we had a radiation era we had a matter era now we're really in a dark energy era right so right now the bulk of the sort of evolution of the universe is governed by dark energy we call it dark energy domination okay um oh that's good that's really good that's I feel like you should be credited for that it's it's a good one I like that one yeah so you know if dark energy is a cosmological constant then we know what that's how that's going to go like we get to that stage where in 100 billion years we can't galaxies anymore and it just keeps everything just keeps diffusing the accelerated expansion becomes you know just that's what's happening and the matter in the universe becomes less and less and less and less important right now you know matter is something like 30% of the universe it'll become you know 5% of the universe it'll become 1% of the universe it'll become just very unimportant and will evolve toward a universe that's just got this accelerated expansion exponential expansion going and as that happens like you know all of the other galaxies will be out of contact with us uh in the sense of being too far away to see or moving away too quickly to see there's a certain distance out to which we will be able to get information and not beyond that and the stuff in our galaxy what's left of our galaxy you know the sort of milk dromeda the combination of Andromeda and US those Stars will be burning out right so the Stars will burn out um you know trillions of years you the Stars will be burning out even the little the small ones that burn real slow the red dwarfs those will start to burn out the white dwarfs will fade away they'll get cold and Fade Away black holes will start to evaporate so oh black holes uh we talked about this a little bit in the black hole episode but black holes have this Hawking evaporation process where they can lose Mass through radiation that'll happen to all the black holes the black holes will evaporate away into radiation everything's going to kind of Decay into radiation I talked about the second law of Thermodynamics it everything kind of decays into waste heat right and so the universe will be kind of very very diffuse and with just this little bit of background radiation from all the stuff that's decaying in it at some point probably particles will Decay I mean a lot of particles are already unstable but protons we've never seen a proton decay but we think that those will probably Decay eventually as well in some very large number of years 10 to the 40 or something um the protons will start to DEC oh wow and so at some point you get to the stage where there's there's just kind of nothing left the radiation from everything decaying will kind of diffuse away but there'll be a little radiation associated with the kind of horizon of the universe it's called a desitter horizon it's a kind of I don't know shell around us that's associated with the way that the SpaceTime is expanding but there'll be a tiny tiny bit of radiation from that but like the temperature is something like 10 Theus 40 kelv or something like very very close to absolute zero wow and so the universe will just be this very very cold dark empty place and because the second law of Thermodynamics says that entropy is always increasing over time the entropy will keep increasing until it gets to like this maximum point right where the disorder can't get any higher everything is already decayed and when you get to that maximum entropy Point that's called the heat death of the universe where heat here is kind of referring to something about sort of the way that the energy is is distributed and it's a death because once the entropy is maximized kind of nothing important can happen again in some sense like there could be random fluctuations like uh you can have it so that some of these photons that are floating around can occasionally randomly run into each other and you can have particles kind of fluctuating up from the vacuum and if you wait long enough then things will kind of appear and disappear through just random fluctuations but you can't can't have structure because that would be a decrease in entropy right so you can only have very temporary little fluctuations downward in entropy but you know everything is already fully entropies or I don't what the word is entropic yeah and so um in a sense you like not only does nothing important happen probably but also like you kind of lose the arrow of time like time stops having a direction because the way that we Define the future is the future is the direction in which entropy is increasing but if entropy can't increase anymore then you don't really have time yeah well there's I mean there's sort of time in a sense but it's not passing in any particular direction whoa whoa whoa whoa whoa whoa whoa whoa whoa time is just a measure of entropy increasing well the concept we have as the future is the direction in which entropy increases SpaceTime will still exist in this universe but isn't the past also just a less Copic Universe yeah yeah so so isn't the that's all that time is is just whether how how ordered things are it's it's a way of defining the arrow of time it's it's not a way of defining time itself I wouldn't say because time itself is kind of tied up in space time as as a concept but you know when people talk about the arrow of time like why is the future different from the past cuz in in a lot of equations it doesn't seem like it should make sense the only thing that really determines the difference between the future and the past is entropy okay I'm going to try not to let that freak me out okay yeah so so one of the cool things though about this is you get to this like steady state basically where the expansion is just exponential the entropy is maximized there have been some suggestions by some theorists that maybe maybe you could have these random fluctuations I mean if this goes on forever then then you have this situation where kind of anything that can happen will eventually right and just like if you had a if you had like a box of gas particles they're just moving around randomly every once in a while very rarely but every once in a while they're all going to gather in one corner for like a split second right it's not going to happen very often but there's a kind of a kind of recurrence time period you can calculate for when that'll happen and every you know trillion years or something this they'll all collect in that corner and then they'll you know on average right just from random fluctuations and so you can kind of do this with the universe you can kind of imagine the universe as this box of particles and you can work out like how often it'll happen or how long you'd have to wait for something to appear so uh some colleagues of mine um I think Sean Caroll was one of the one of them on this paper they calculated at some point in this sort of end stage heat death Universe how long it would take for a grand piano to assemble itself out of the vacuum just just randomly and they got some number I don't remember what the number is but it's was very large is this like trying to wait until the little DVD sign not to date myself when you would have a DVD in your television and you weren't watching television there would be like a little DVD sign it would go around the edges of the screen and then once every great while it would hit the absolute Corner instead of like bouncing off two walls it would hit the corner it's kind of like that but then you've got to wait until all these particles are hitting the corner at the same time to make a grand piano yeah yeah but like theoretically like if everything that could happen will happen a grand piano will be made will a monkey be made like I guess will biology happen again yeah so so there's a way of calculating this where you can work out like the whole universe will recur right so this this is this um this is this idea of um po recurrence you have this situation this hypothesis that in that sort of setup if you wait long enough every point in the past of the universe is contained in this kind of possibility set of things that could happen to the universe and so you know you're we're dealing with eternities here this moment right now could recur again you know in some vastly distant point in the heat death except you know in that point maybe it would be this moment exactly and then we and then we would just evolve into a new heat death over trillions and trillions and trillions of years but maybe this moment occur but in the future recurrence you know I'm wearing a green shirt and you're wearing a red one right whoa so like and that that's the power of just Quantum fluctuation yeah yeah or just statistical fluctuations statistical fluctuation and a tremendous amount of time yeah exactly exactly it's like a bunch of monkeys in a room with typewriters will eventually write Hamlet except the whole universe yeah yeah and so some people have suggested this as a way of having a new universe come out of the heat death is just you'd randomly fluctuate a big bang but you could also randomly fluctuate any point in time in the universe but then you get into you get into a problem it's called the boltzman brain problem this is one of my favorite things in physics so let's say that you want to randomly fluctuate a Big Bang from this heat death Universe well you're much more likely or much more often you're going to randomly fluctuate just like a single Galaxy cuz that's just that's less yep you know is more likely right less hard yeah yeah but then you know you're also kind of more likely to randomly fluctuate just one planet totally right with its atmosphere and then you're more likely to randomly fluctuate just one person right and you're more likely to randomly fluctuate just just a brain just a single human brain that thinks it has experienced the universe right and this is the boltzman brain problem because when you work out those probabilities more likely than we have fluctuated as a big bang out of the vacuum of space is that one human mind has fluctuated imagining that the whole universe exists vastly more likely yeah yeah which is not to say you know that it actually is true that we're Imagining the whole universe but it's kind of just bringing up this contradiction where these calculations no longer are really viable because you're going to hit that we're just imagining it thing before you're going to hit we've actually done it that makes my head hurt and I know right makes me afraid that I'm trapped in someone else's imagination yeah which I often feel right yeah it's it's like uh copsis is the the term right it's like this this ultimate victory for that for that concept um pic but uh but it it's really the the idea of the boltzman brain scenario was brought up just as a kind of way to point out that this is not a good way of doing these probabilities right right because you're going to run into that problem it wasn't ever a real suggestion right okay you know that we are um in a simulation yes yeah anyway that's the heat death so now we know what happens if Dark Energy reverses we know what happens if dark energy is a constant which is the heat death of the universe which is a little mind bendy but you know as endings go doesn't seem like the worst to me no it's very gentle it's very gentle it's that TS Elliott poem this is how the world ends not with a bang but with a whimper it's just the lights turn out it's almost like I was thinking that like if I left all the lights in my house on and just walked out of my house for the next 20 years I'd come back and all the lights would be out and that's the end of the universe yeah yeah and I know that a lot of times when I talk about the heat death as an ending people find it really sad one of my colleagues hanania Pius I interviewed her for the book and she said that when she gives talks on that people sometimes cry like the idea that the Universe Fades out and everything ends in cold and darkness but I think it's you know it's the best you can do and and things could be so much worse you know uh you could get you could get a big crunch you could have a big crunch and you could see the end coming at you yeah you'd see it coming yeah [Music] yeah things could be so much worse Let Us carry that hope with us even if it is the hope of a temporary species in a temporary Universe how will we ever find peace of mind in a life so fleeting well that is of course why there's life insurance that's right it's our last episode which also means it's our last policy genius ad God I love policy genius with policy genius you can find life insurance policies that start at just $292 per year for $1 million of coverage some options offer same day approval and avoid unnecessary medical exams planning ahead is crucial in life especially when it comes to what happens when you're gone and if we've learned anything from this podcast it's that we will all be gone in the fullness of time policy Geniuses the country's leading online insurance Marketplace and since they don't have an incentive to recommend one insurer over another their advice is unbiased so check out policy genius it's never too late to plan ahead head to policygenius.com crashcourse or click the link in the description to get your free life insurance quotes to see how much you could save that's policygenius.com crashcourse [Music] let's briefly recap the first two theories of how the universe ends first if dark energy is what's responsible for the expansion of the universe what would happen if it became less powerful it's possible that dark energy is decreasing over time so if it started to lose so much power that it not only stopped but began to reverse the universe would stop expanding and instead start falling together again space would become much hotter as energy is compressed and we would know the end of everything was coming because we would perceive light as blue shifting rather than red shifting we call this the Big Crunch for reasons I am uncomfortable with I'd much prefer to think of it as a brand of granola or like a breakfast cereal but what if Dark Energy didn't get more or less powerful what if it remained a cosmological constant well we call this outcome heat death which sounds like a heavy metal band but is in fact a gentle quiet way to end the universe it would continue to expand and everything would become increasingly more diffuse matter itself becoming essentially negligible Stars would burn out black holes would evaporate and eventually entropy would reach its maximum value and the universe would reach a thermodynamic equilibrium suffering a heat death meaning that nothing can really happen anymore time stops having a direction and everything that was going to happen already did despite how Bleak that sounds I I actually don't mind that one I'd take a heat death over a big crunch any day but those aren't the only potential outcomes so there's this idea that if the dark energy is some kind of dynamical field then there's a possibility and it's not a it's not a possibility theorist like because it messes with certain sort of energy conditions that seem important but maybe instead of just being constant in space it builds up in space so with a cosmological constant if you define like a sphere if you just IM Define an imaginary sphere in Space the amount of cosmological constant in that sphere is never going to increase because it's the same amount of space even as the universe expands you've defined you know a one Mega parex sphere around your Galaxy or something you're not going to get more Dark Energy in that sphere so in that case like if there's a galaxy in the universe that Galaxy is not going to be torn apart by the expansion of the universe because in that region the matter already won you know like the matter already is more important than the stretchiness of the Dark Energy in that sphere there's not going to be more Dark Energy in that sphere so it's a bound structure it's not going to expand it's like you know the universe is expanding but this room is not expanding because in this room The Binding energy of all the stuff is more important than the little bit of stretchiness of space right so you can imagine a kind of dark energy they call it Phantom Dark Energy in which the dark energy increases in every part of space the density of dark energy or the sort of expansion of dark energy increases in every part of space and if you work that out it turns out that that will eventually destroy the universe and the way it does that is that it builds up within already bound structures so let's say you have you know a cluster of galaxies they're Bound by gravity if the dark energy is increasing it's going to kind of pull the galaxies out of that Galaxy cluster it destroys the universe from the outside in in sense of like large scales to small so if the dark energy is building up in space first it's going to pull apart things that are like the most weakly bound so like clusters of galaxies then it'll start getting to galaxies so it'll start like pulling apart taking stars out of galaxies sort of unraveling galaxies then it'll go into solar systems it'll take the planets away from the Sun and then as it continues to build up and build up it can start breaking apart solid objects so you can break apart the Earth and then break apart you know Atomic molecular bonds and then eventually you get to a point and you can calculate for a given model you get to a point where the scale factor of the Universe goes infinite so the scale factor is a number that tells you about like the size of the universe you know how much is expanding so you know the scale factor today is one some number of you know billions of years ago it was.5 the universe was smaller that kind of just scales like how big SpaceTime is so in a finite time you can calculate the scale factor will go infinite which just means that the space is expanding so much that it just completely rips apart and that's called the Big Rip that's called the Big Rip which is a another really good name yeah yeah I just think we need to complement the far future cosmologists as opposed to the far past cosmologists for just in general doing a better job with naming yeah yeah no no inflaton right business just not no no removing a single letter from a word and thinking you've accomplished something so anyway and I'm sorry to the person who coined inflaton I'm sure they're a nice person but that's it's not a great name and I wish you'd consulted with me so in this case in this big rip case the size of the universe like the the Scale of the Universe becomes infinite in a finite period of time mhhm yeah that is really really hard to get into my head yeah yeah and there are reasons why theorists don't like this scenario because in order for that to happen you have to break some these kind of energy conditions that we think exist in the universe that prevent stuff like that happening but if you just put in Dark Energy with certain characteristics then you get something that looks like that in the simplest versions of the equations okay when we measure the properties of dark energy it seems to be completely consistent with a cosmological constant for the most part okay but you know you can't quite rule out a phantom dark energy this stuff that would cause a big rip in terms of just the observations just what you can measure about the equation of State parameter of dark energy which tells you about kind of how this stuff works and so you can work out like what the error bars are on the measurement of this like how uncertain we are about the measurement of this and in doing that you can get like a calculation of what's the soonest the Big Rip could happen if it were going to happen if Dark Energy really is not exactly a cosmological constant but this stuff that gets more powerful and when you calculate that we've got at least like 200 billion years so just based on the way that we can do our calculations so I mean 200 billion years like that's after we already can't see galaxies anymore so it's like you know there's not a lot to rip apart at that stage right I I don't know there's something like and I know that it won't happen on this scale but there's something terrifying about looking up into the sky seeing absolute darkness and then like slowly having Dark Energy rip apart your solar system and then your body that's true that's true that's true and you can't escape it because it's just space that's doing it right so no matter you know there's you can't like protect yourself from it like being in in a protective casing or something it'll rip the the casing apart it's just something that's going to happen that's a little terrifying it is a little bit terrifying and and again you would see it coming so that's not great so if I'm going to rank these first three I'm going to say big crunch is the worst yeah yeah heat death is the best big rip is the second worst yeah yeah I'd say that I'd say that I'd agree with you on those [Music] now there are two more ways that you wrote about that the world might well the world will be long long ended the universe might end walk me through those okay so the next two are vacuum Decay and cyclic cosmologies I'll start with vacuum Decay because that is my personal favorite okay and part of the reasons my personal favorite is because technically it could happen at any moment oh great I'm glad that's your personal favorite um now and when I say this I need to give a disclaimer which is that it almost certainly won't sure and please you know do not write us emails being sad about this and scared because this is not a thing to worry about but uh just to to the audience because I do get emails from people who are scared about vacuum Decay and it's really all explain why not to worry about it but let me just first walk through what the idea is so we talked a while back about the higs field so there was this event that happened in the early universe that altered the higs field so the higs field is this energy field throughout all of space it's a scalar field like we think caused inflation like dark energy could be it's it's this sort of energy field through space and it's connected to how particle physics Works in our universe and like this state of the higs field what kind of value it has what kind of number is associated with that field connects to what particles can exist in the universe whether they can have mass all of that stuff the setup of the standard model of particle physics and we think that we're pretty sure that in the beginning of the universe the higs field was in a different state than it is now there was a transition that happened and that transition allowed for particles to have mass it allowed for the particles of the universe in the standard model of particle physics to exist it allowed for us to have you know chemistry and bodies and all of that kind of thing right you know molecules could exist there is some suggestion Based on data that it's possible the higs field could change again and it's this transition would be like a Quantum tunneling event so Quantum tunneling is where if you have like a particle on one side of a barrier like an electron on one side of a barrier if you wait long enough that electron is just going to appear on the other side even though it didn't go through it didn't go around it's just that there's some uncertainty that's built into quantum mechanics about where that electron is and that means that some of the possibility for where it could be is on the other side of the barrier and so if you wait long enough that's where you'll see it that's Quantum tunneling and there's kind of a similar uncertainty that kind of works into the higs field where it might show up in this other part of its potential which is the potential is like the sort of physics governing what state it could be in so it could just kind of show up at a different value through this Quantum tunneling transition and the thing about Quantum tunneling is that it's inherently unpredictable you can calculate like a Decay rate like if you have a radioactive particle you can calculate the halflife of that particle like how long or substance you can you know how long it'll be before half of that substance is decayed that's a halflife and that's governed by the same kind of physics so we can calculate like a halflife for the universe for this transition of the higs field but we can't say exactly when it might happen or where if it is governed by this Quantum tunneling transition and the way it would work if that transition happened if you did have that transition happen somewhere in space is that you know somewhere in space where the field does that Quantum tunneling event then the higs field would go into this other state that's called a true vacuum so we have the current higs vacuum vacuum here just means like the state of the universe not nothingness but the state of the universe you talk about different kinds of vacuums so vacuum is just like the background state so potentially we're currently in a a false vacuum meaning that it's like not the sort of preferred state of the universe and we could transition to a true vacuum um it's possible the higs field is what is called metast stable which means that there could be a transition sometime in the future where the higs field could change to another state when we say the universe might be metastable we mean that it's kind of stable for now but it's stable in the way that like if you take like a cup and you balance it at the edge of a table so it's like really close to the edge it's still sitting there but like if it were nudged just a little bit it would fall over right and that's meta stable because that cup would really rather be on the floor in some sense right gravity says that it's it's better on the floor so there's like this gravitational potential which is like the shape of you know what the gravitational field is at different points and that just says that the cup would rather be on the floor in some sense and so it's possible that that's the way that the higs field is set up that it has some potential some kind of thing governing what state is more favorable energetically for the higs field and the the state that it's in right now is not the most favorable State and so that means that like it's there for now but if it could be nudged it would fall into that other state which is a true vacuum yeah which is the true vacuum so and we don't know any way to nudge it so well I'm glad we don't know any way to nudge it because I don't trust us not to nudge it yeah no I've I've worked on trying to figure out if there is a way and I don't think there is but um yeah yeah but because the universe is fundamentally quantum mechanical it's actually applies to this the cup balanced on the edge of the table too if you wait long enough for a cup balance on the edge of the table all the particles in that cup are going to Quantum tunnel through the table and it'll fall to the floor anyway like it's a ridiculous amount of time that you'd have to wait for that to happen and the cup would probably like Decay away anyway before that but in principle that could happen through Quantum tunneling and so the same kind of applies to the Hig field where even if it's in this sort of better false vacuum state if there is a true vacuum that's at a lower energy State basically then it'll eventually Quantum tunnel into the true vacuum and so when that happens what happens is that at some point in space a Quantum tunneling event occurs where the Hig field goes from being in the false vacuum to the true vacuum at that point and then it spreads because that's the more favorable configuration that's the way it wants to be yeah that's the better situ sitation for it how fast does that happen Katie uh so once that bubble of true vacuum nucleates once it starts that bubble expands at about the speed of light oh so fast yeah well depending on where it is right would we see it coming no oh fun because it's the speed of light yeah so the by the time the light from it gets to you it is on top of you also CU it's traveling at the same speed so in that case let's just say this happen s and I understand that it's very very unlikely it's it's like vanishingly small likelihood that it's going to happen anytime soon yes and so we shouldn't worry about it yes we have lots of real things to worry about so we shouldn't worry about these tiny tiny hypotheticals yes but let's say it happens in 500 million years which I I know is unlikely but let's say it happens at some point in the next 500 million years so that we're still we still have a chance of being around albe it a very small chance in that case we'd all go out together right yeah like we'd all go out within a fraction of a second of each other exactly and we just sort of be sucked into this true vacuum I like that one yeah yeah so the way it would happen is the bubble would have like a bubble wall that's like got some sort of high density like it's kind of high energy radiation or sort of on the edge so first that bubble wall would probably incinerate you but then you're inside the true vacuum and then in the true vacuum we don't have the same chemistry your particles don't hold together anymore you're fully disintegrated and then it's possible that the space inside the true vacuum also collapses to a black hole so it's like you're very you know coherently destroyed but um it happens very fast right because if this is traveling in the speed of light like you wouldn't see it coming you also wouldn't like feel it or no it because the speed of light is faster than your nerve impulses can travel right so you kind of don't know it h like you don't know it happened you don't feel it it would be like blinking your eyes and just never opening them again but like you don't know that that happened right so it would be very clean very painless and everybody you know the whole universe like within that bubble would be destroyed altogether so there's no like there's no tragic aftermath you know there's no fomo like it's just it's just done yeah hash no fomo yeah yeah yeah I think it's a very uh humane way way to end the universe honestly it seems pretty clean to me and that's a appeeling yeah yeah great uh let's do that one I does do we get to pick I don't I guess we don't get to pick but if we can let's do that one but so the the idea that we live in a false vacuum is based on measurements of things like the mass of the higs field or the mass of the higs bz on and its connection to the higs field the parameters of the standard model of particle physics so we don't know if there's other physics like we think probably the standard model of particle physics is not like the final word on you know what the universe is made of and so it's possible some other physics will come in that changes that picture but based on our current estimates you know if there's no other new physics that comes to change that picture the sort of Decay time of the universe is something like 10 to the power of 100 years maybe 500 so really really long time from now long after everything important has happened in the universe so it's very unlikely it would happen anytime soon and when people say that they're worried about it happening you know i' tell them like you're much more likely to like get struck by lightning while also being eaten by a shark and winning the lottery the probability is adding up here like it's very very unlikely to happen in our lifetime like to an astonishing degree even if we're right about all the physics and there's nothing beyond the standard model model so I definitely don't think we should worry about it okay but it's a it's a fun thing to think about as a physicist because it's this cool connection between like fundamental particle physics and like the existence of the universe and I I find that just really compelling to think about really fun to to think about yeah I hear [Music] that so maybe Dark Energy isn't a constant in space but instead builds up becoming the spookily named Phantom dark energy and if that was the case well that could also eventually destroy the universe if that dark energy increases it would destroy everything in the universe from the outside in unraveling galaxies ripping apart solar systems and eventually breaking up solid objects this theory is appropriately named the big rip and we would see it coming but what if the end of the universe has nothing to do with dark energy at all what if the culprit was vacuum Decay the higs field you'll remember is an energy field through space and is directly connected to how particle physics Works in our universe in the beginning of the universe we think that the higs field was in a different state than it is now and with that knowledge there's some suggestion that the higs field might change again and this change would likely be triggered by a Quantum tunneling event where a particle moves from one place to another another through apparent barriers upon this change the higs field would turn into a true vacuum the state it was leaning toward it all the time destroying the universe in the process now at least in this version Not only would we be blissfully unaware of that destruction happening it would happen to all of us practically in the very same moment a kind of United instantaneous farewell but we still have one Theory left so I'll let Katie take it from here all right so we've reached our last future this is the one that I want to be true because what I want and this is irrational and I don't even know why I want it really like I'd have to deeply inquire with myself but I want something of our universe to survive into the next Universe yeah I want something of us to be Eternal some memory some it doesn't I obviously it won't be us humans but I mean some memory of our universe to be preserved into the next universe and if there's some kind of cyclic cosmology as there is in some religious Traditions like in Hinduism other religious Traditions if there is some kind of cyclic cosmology where our universe is somehow in its ending births a new universe with some memory of us that's the most I guess like symbolically appealing one to me yeah yeah yeah so there are a few hypotheses about cyclic cosmologies and generally speaking their hypothesize not really to talk about the end of the universe but to talk about the beginning of ours so there are some weird technical problems with what we think might have been the initial conditions of our universe and we use the concept of cosmic inflation to explain some of the initial condition issues but there are others that have to do with like entropy that are a little bit more difficult to tackle and so some theorists have come up with ideas where a previous Universe ends and starts ours and something from the previous universe sets up the conditions for our current Universe okay yeah as a way of explaining how our current Universe got here the way it is and so you know one of the ones I talk about in my book is an idea from my my PhD adviser Paul steinhardt and and his colleagues who talked about a universe where there would be a contraction toward the end that would be not a full big crunch but a contraction to a smaller universe that would then re-expand to a new universe and there would be a cycle of those kinds of universes and some information like gravitational waves or something would be preserved through this and it would kind of set up the conditions for the next stage there's also Roger Penrose who recently won a Nobel Prize for his work on gravity and things he has this idea called conformal cyclic cosmology where the universe ends in heat death and then there's a a new Big Bang that comes out of that heat death and there's information that passes through that transition basically just sort of where the super massive black holes are but like some kind of uh information can get through there and there are other U models where a sort of previous cycle sets up ours it was really interesting when I was working on this book and talking to people about these kinds of models where you know I kind of get this feeling that like you some of the people working on these models like the idea that something persists of our universe that it's not fully forgotten you know because I think that that's something that you know I mean these models are I I don't have a whole lot to say about them because they're still kind of being developed and and they're not as um I don't know accepted as other models cuz you know for one thing they're still being pill and and also in some cases they don't fit the data as well or they're you know the data isn't there yet to distinguish but you definitely get the feeling that you know some of the people working on this are saying like you know it can't just be all over when it's over like there's something has to come through and I like this idea that there is some persistence yeah well this is something that I find really interesting about physics is that it's done by humans MH yeah there's no way to separate our humanness from our scientific inquiry on some level because we're the ones doing it and it's not just that I want gravitational waves from our universe to survive into another Universe it's also that like it just makes sense to me on a basic level that on some in some way they would it just tracks with other things I've observed in the universe so I understand that urge but of course like I guess when we're making scientific inquiry we try to push aside our biology as much as possible yeah it's interesting like you know there's another sort of setup that's not really about the end of the universe but about the beginning where inflation this Cosmic inflation process could have happened multiple times it could be happening in lots of different parts of this much larger universe yeah almost like a blanket of things and the little pieces of it become universes yeah little pocket universes come out of that we talked about that in the inflation episode and you know that's another situation where you know our universe will reach a heat death maybe but other universes might just be starting in other parts of this larger space larger Multiverse but whether or not there's any memory of our universe is kind of unclear in that kind of situation right one of the things that's really interesting to me about thinking about these different possibilities for the end of the universe is like like when people think about like I I don't like the idea of death right I I I find it very very uncomfortable to think about dying I don't want to die um I don't want anybody I care about to die like that idea of permanent ending is unsettling to me and and so you might think it odd that I wrote a book about the end of the universe I had to spend about two years thinking about the permanent ending of absolutely everything and that's not comfortable but it's kind of like the end of the universe is is not so personal as the idea of of me dying but there's a sense in which it's kind of worse for like when you think about dying which you don't want to think about but when you do like one of the ways that people become comfortable with the idea of of dying is that they think well you know something of me will persist right um my children my great works the marks that I made on society you know something will be left over for me I will have made the world better in some way I will have had an effect I will have had some kind of impact yeah like you're a little Pebble that gets thrown into a lake and then you Ripple out even after the pebble has sunk yeah yeah and even if it's like just the sort of butterfly effect version of like just having existed changes the universe in some way like you've mattered right and when you think about the end of the universe you kind of lose that because if the universe is really ending then at some point it will not have mattered that we existed any of us right there will be no memory of that no Persistence of that and so at some future point we will have all been erased it will not have mattered that we were ever here and that is much more unsettling of course yeah and that makes it hard to you know think about like something that kind of makes it all worthwhile retrospectively right where the ending kind of justifies what we all went through which is a very popular thing in fiction but also religion this idea that like yeah everything was hard or whatever but it turned out great in the end you know we got our just rewards whatever like it's meaningful in retrospect right that's how we tell stories there's also some truth to that on a micro level right like there's some truth to the idea that love survives death like my grandfather is dead and yet the love that we shared is still here yeah right but the end of the universe knowing that there will be an end to the universe which we've really only known for a few generations of human history right like this is a tiny fraction of human history if we think about the history of humans as being a calendar year begins 300,000 years ago and today is 12:59 p.m. at on December 31st like we've known that the Universe was going to end for like 3 minutes yeah yeah yeah yeah and so it's new and It's upsetting to me because we're we're this weird species that is finite but con conceive of infinity and so of course we want to be able to live in that Infinity somehow some way and this says you just won't well and and even not being able to live it like we're so used to creating meaning retrospectively right to assigning purpose after the fact and you can't do that if there is no after the fact there's no final Victory because the final is the lights turn off yeah yeah and I know that a lot of people get some sense of meaning through thinking about it that way through religion where they think like I'll get you know like I'll get my reward in the end or or you know God will sort it all out or something but if you're just thinking about like the physical existence of the universe you don't get it from that and so for me I found that thinking about it in these terms really made me think about the necessity to create meaning in the moment right to decide that this moment is Meaningful even if it is forgotten right that living life in the moment not with regard to what it might all turn out to be in the future which I obviously cannot predict it's important to have it meaningful right now so your feeling is that since there will not be a time at which we're able to look back on all of this and say well the suffering and the misery and the Injustice was justified because of X and Y and Z it matters more not less to try to decrease suffering and Injustice now yeah yeah and to try to appreciate Joy now yeah this is a little bit personal but when I was writing this book um my my grandmother passed away and I had been reasonably close with her but moved away and I hadn't seen her very often and and in the kind of the way everything worked out a lot of her sort of personal possessions went somewhere else and I didn't know where and there were things that she had had that I knew that she wanted me to have that I didn't get um in that in that process and and I was I was very sad that she had passed away and I was sad that I didn't have momentos from her that I had had hoped to but when I thought about that and I remembered her I thought like well everything is going to end eventually anyway we don't get to keep anything anyway and what really mattered about that relationship was that when I was with her we had Joy right and and we I'm going to die she died everything we have is going to be destroyed in some way in the future but it was still meaningful and the important thing was was was the time that we had in the moment and that kind of helped which which was a little surprising to me but it kind of helped to think well you know everything is going to end but that doesn't mean that that it's not meaningful in the in that time and and and it made me think about like really appreciating joy and love and connection when it's happen right I mean it was meaningful not only because you had that Joy but also because you shared that Joy between the two of you yeah and I think that's that's where I land as well it's that you you have to find meaning whether you are constructing it or or or trying to derive it from some higher source it doesn't really matter to me but you have to find meaning yeah and you have to find meaning together you have to find Joy together it's not just that you want to have it it's that that you need to have it you need to seek it you also need to share it yeah yeah definitely I've been going through a really hard time Katie I've been really struggling with um serious depression and it's interesting to think about it in that in that context because my my impulse has been to pull back to not share as much to not talk about it not you know just try to deal with it myself because I don't want to burden anybody I don't want to be a problem for anybody I don't want to make anybody's life worse I don't want to like increase the amount of misery in the world by talking about it right but maybe actually these things are better when they're shared yeah and and we find more meaning and connection in the moment when we're able to share what we're going through yeah yeah I think so I think that you know having other people having those connections is is a temporary thing you know someday the universe will be empty and cold and dark and we'll be alone in it and the uh I don't know I think that that making the most of human connection and building meaning together in the moment is really important yeah well I have to say that being able to have these conversations with you and learn from you has meant a lot to me not only because I didn't know anything about black holes or dark energy when we started but also because it's a way of sharing meaning and understanding and I really appreciate your generosity in sharing that with me and with everybody listening it's been it's been an extraordinary journey and we now know that it will end in perhaps one of five ways but not yet yeah not today yeah yeah and and you know I've I've really enjoyed this um I've uh I've it's just been great to talk with you about all this and uh to see it from a kind of different perspective and to to share it with people through through this podcast so I found this to be very joyful as well me too thank you thank you and thanks to everybody for listening [Music] [Music] so the beginnings of our universe may hold the answer to how a cyclic cosmology might occur bringing some parts of our universe into the next the universe could contract then re-expand another big bang could occur in that vast future there may be a mem Mor of us or there may not be we just don't know but we are here now we know the warmth of our sun and we know much about the warmth of other Suns we can see tiny particles that make us up and we can see ourselves as tiny particles in a vast Universe what a gift to be here for and with each other what a gift to begin to Fathom the universe in which we find ourselves our El maybe there won't be a memory of us in the long long Universe at all but to me there's something wildly beautiful about the fact that while we cannot live forever and while our universe probably won't be forever we can still engage with and imagine the infinite it's easy to grieve for the loss of us the loss of our planet our solar system our galaxy and I don't blame myself for feeling that grief it's hard not to feel sad knowing all of this will end but not yet not yet thanks for coming on this journey with us thanks for being a co-discoverer of the universe this show was hosted by me John Green and Dr Katie Mack this episode was produced by Hannah West edited by lonus openhouse with music and mix by Joseph tuna medish special thanks to the perimeter Institute for theoretical physics our associate script editor was Annie fillenworth our editorial directors were do Darcy Shapiro and Megan moery and our executive producers were Heather D Diego and Seth Radley the show was a production of complexly if you want to help keep crash course free for everyone forever you can join our community on patreon at patreon.com/crashcourse [Music] h [Music]