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00:01There's a mystery at the very heart of the universe. We don't know how old the
00:07cosmos is. Understanding the age of the universe is fundamental to understanding
00:12the universe at all. It's at the heart of everything. It's more than just
00:18celebrating a birthday. We want to know how much mass is in it, how much energy
00:21is in it, how it behaves. We have to have this number nailed down. The age of the
00:26universe enables us to not only understand where we came from but
00:31potentially the fate of the universe. What will happen millions and billions of
00:36years from now. But our quest to discover the age of the universe is starting a
00:41war. Usually nature just whispers to us, now nature is screaming in our ear that
00:48we're doing something wrong and that's exciting.
00:56We think the universe started with a bang.
01:05Everything that has ever existed is squashed up in the space smaller than a pinhead. And
01:12all of a sudden, space just starts expanding everywhere at once.
01:18The idea that the universe grew from a ball smaller than a pinhead is hard to
01:23understand. But figuring out when it happened sounds like it should be more
01:28straightforward.
01:30It seems like a simple question, right? But it turns out getting the age of the
01:34universe is pretty tricky.
01:36Scientists have just a single fact as their starting point.
01:40The universe is expanding.
01:42When people realized the universe was expanding, they thought they finally had a way to estimate
01:47the age of the universe. Take the universe now and run it backwards in time. Things get
01:52closer and closer till they come to a single point. That time, to that point, is the age
01:58of the universe.
02:00The expansion rate is so important, it's been given its own name, the Hubble Constant.
02:06The Hubble Constant is the present-day expansion rate of the universe. It is a key ingredient
02:14to understanding the entire expansion history of our universe and its age.
02:21Scientists discovered a strange radio signal permeating the cosmos. It's the remnants of ancient
02:28light from the early universe. We call it the Cosmic Microwave Background, or CMB for short.
02:37The Cosmic Microwave Background radiation is simply the afterglow of our Big Bang,
02:43the way the universe looked when it was 400,000 years old.
02:48The European Space Agency launched the Planck satellite. Using sensitive radio receivers,
02:54the orbiter studied the sky in every direction, measuring tiny changes in the temperature and
03:00polarization of the radiation signal.
03:02The CMB has all these variations in temperature, and they're not randomly generated. They are there
03:10because of physical processes that occurred when the universe was in its primordial fireball phase.
03:15The red blobs are where matter was hottest, and the blue areas are where matter was cooler. The smallest red
03:23blobs are where hot material was packed tightly together.
03:27That's where material in the universe would have been denser, and that's where galaxies would preferentially form.
03:32It's so cool to get to look at those blueprints and study them and see how that baby universe
03:39later grew up into the universe we see around us today.
03:44Although it doesn't look like much, hidden within this picture is almost everything we can know about the universe.
03:52In a complex process, using different mathematical models, cosmologists figured out how the ancient
04:00cosmos captured in the CMB became the universe we see today. They worked out how the universe got
04:08from small to big, and how fast that expansion happened.
04:12The data from the cosmic microwave background is absolutely the gold standard for cosmology. It's
04:20beautifully clean, we can understand it really well, and we have a lot of confidence that what we
04:26learn from it is pretty robust. By running the expansion backwards, we get an age. 13.82 billion years.
04:37Job finished. But it's not quite a slam dunk. The figure must be verified.
04:46We don't make a single measurement using a single technique. We make multiple measurements
04:51via multiple techniques. Another group of scientists use a totally different method
04:57to calculate the age of the cosmos. Measuring objects that we can see in our universe to determine
05:03how far away they are, and how fast they're moving away from us as the universe expands.
05:10The most direct and most accurate measurements are using what is known as parallax. Parallax
05:17is the apparent shift in an object relative to the background when it's viewed from two different
05:23locations. So if I look at my thumb with one eye, and then I close it and look at the other eye,
05:29it looks like my thumb moves. If I move my thumb closer to my face, then the distance it moves back
05:37and forth changes. It appears to move back and forth more. That parallax difference as we move the thumb
05:43closer and farther from the face is the way we measure distances to distant objects. Using parallax,
05:50we can measure the distance to bright stars called Cepheids in the Milky Way. Cepheids are stars that burn
05:58a hundred thousand times brighter than our sun. So they're extremely bright and they pulsate, meaning
06:04they get brighter and dimmer over a regular time period. Cepheids that pulsate at the same rate have the same
06:12brightness. They're known as a standard candle. A standard candle is something that is a standard,
06:19meaning we know how intrinsically bright it is. So all we have to do is measure the brightness that
06:24we appear to perceive on Earth, and then you solve for the distance. So imagine that you're on this
06:31street. By looking down the street, you'll see that the street lights get dimmer and dimmer the farther
06:37away they are. But that's not their intrinsic brightness. Their intrinsic brightness is the same.
06:42So by seeing how faint the farthest away ones are, you can understand how far away they are from
06:49you. We can use standard candles to measure the distance to stars farther away. But there's a big
06:57problem. Throughout the universe, there's a competition between the expansion pushing things apart
07:04and gravity pulling things together. In the Milky Way, there's so much matter that gravity wins.
07:12Even looking at galaxies in our neighborhood, the expansion is tiny. But at cosmic scales of very
07:20different galaxies, matter is more spread out and expansion wins. So we can only measure expansion
07:27over massive distances. The way we start to measure distances to things that are farther and farther away
07:34is to use something we call the distance ladder. Each category of object that we observe is on a
07:42separate rung of this ladder. Measuring the distance to one will then inform us how far away the second
07:50rung is and then the third rung. So each rung depends on the previous rung. And from stacking these together,
07:58we can start to measure things very, very far away from us. Using parallax to measure Cepheid stars in the
08:06Milky Way gives us a benchmark. We can then use their standard brightness to measure Cepheids in other
08:14galaxies. The next rung is a brighter standard candle called Type 1A supernovas. They can be seen in
08:22galaxies farther away. Finally, we can measure light from distant elliptical galaxies. And by looking at
08:30how red the light is, we can work out how fast they're moving away from us.
08:36So those three things give us the nearby universe, the somewhat far away universe, and the very distant
08:42universe rung by rung. March 2021. Scientists measure the light from 63 giant elliptical galaxies,
08:55the farthest rung of the distance ladder. They hope to get the most accurate measurement of the Hubble
09:01constant to date and a precise age for the universe. Their calculations make the universe 13.3 billion
09:12years old, not too far away from the figure of 13.82 billion years given by the cosmic microwave
09:19background, a difference of around 6%. That sounds trivial, but that equates to hundreds of millions
09:27of years of cosmic history that either happened or didn't happen. 50 years ago, when we weren't quite
09:34as good at measuring everything about the universe, we would have been thrilled to have our numbers
09:37agreeing to this level. But nowadays, having a difference like this, it's unacceptable.
09:43Clearly, the two techniques do not agree. Cosmologists split into two camps.
09:50We had hoped that these two methods were like building a bridge from either side and then meeting
09:55in the middle, but they're not. Now we know that something is going on we don't understand.
10:01Even though these measurements are roughly the same, it's really dangerous to just accept
10:06them and assume that everything's fine. Because in science, usually the initial really big
10:11discoveries start off as small differences that then you pull on that thread and something wonderful
10:17emerges. So does the simple question, how old is the universe, unravel everything?
10:25The universe is expanding outwards. The rate it's growing is called the Hubble constant,
10:41and it's the key to working out the age of the universe.
10:45So the Hubble constant might just seem like some, you know, academic number that doesn't mean anything,
10:51but that number contains information about the composition, the evolution, and the fate of the
10:59universe. It's an important number, but there's a problem. Our best measurement methods don't match.
11:08It's incredibly frustrating to not know how old the universe is. It's even more frustrating to know
11:14that there's two experiments, which are excellent experiments that we firmly believe in,
11:19that completely disagree with each other. My hair fell out a long time ago over this kind of stuff.
11:25This has been the number one question for over half a decade.
11:30There must be something wrong with one of the methods. There's a definite sense in the community
11:36that whichever camp you happen to fall into, the problems lie on the other side of the fence.
11:42So if you're mainly working with the cosmic microwave background, you probably think
11:46something is up with the distance ladder. If there's a problem with the distance ladder,
11:52there's a prime suspect. The ladder relies on stars that have a predictable brightness
11:58called standard candles. But there's evidence that these stars are not always the same brightness.
12:06So if you expect an object to have a particular brightness and it has a different brightness,
12:12then whatever conclusion you draw that relies on the brightness of that object is going to be off
12:18somewhat. Think of the stars like streetlights. If one light is broken and dimmer than the others,
12:25you might think it's farther away. The concern with the distance ladder is that if any of the single
12:31rungs is not perfect, then the entire ladder might be out of whack by the time you get to the top.
12:38What we need is a fresh approach to measuring the age of the universe.
12:43We're hoping we could bring in a tiebreaker, a referee, a brand new method that didn't care about
12:49any of this or any of that, and tell us what is the Hubble constant.
12:54We may have just found one. This observatory doesn't have a telescope. It's hunting for an
13:02invisible wave, a disturbance in space-time itself caused by massive objects accelerating
13:11or colliding. It's known as LIGO. LIGO stands for the Laser Interferometer
13:18Gravitational Wave Observatory, and it is a ground-based gravitational wave detector.
13:24A perfectly stabilized beam of laser light bounces in a five-mile-long L-shaped tunnel.
13:31As a gravitational wave passes through the detector, space stretches, forcing the light
13:38to travel a tiny bit farther. You're bouncing a laser over an incredible distance and trying to
13:45measure as space-time itself gets stretched and deformed whether that laser had to travel a tiny
13:52bit further or a tiny bit shorter. And a tiny bit here is the width of a single atom over miles and
13:58miles of distance. LIGO has already detected colliding black holes, but it's also received a signal from
14:09something less massive. Neutron stars are the densest thing in the universe other than black holes.
14:16They're the last stopping point before you would collapse all the way to form a black hole.
14:21They're the size of Washington DC, but they can have the mass of two suns.
14:28A collision between neutron stars is incredibly powerful.
14:31It's one of the most energetic events in the universe, and it distorts the fabric of space-time
14:38very strongly because their gravity is so strong.
14:40But unlike black hole mergers, neutron star collisions can also send out light.
14:46In 2017 LIGO sent out an alert. More than 70 telescopes on Earth and in space swung into action.
14:57This binary neutron star merger was the first time we'd witnessed gravitational waves and light
15:03waves coming from the same event. It was groundbreaking.
15:09This event is ideal for Hubble constant hunters. The light tells us how fast the colliding stars are
15:17moving away from us. Gravitational waves give us the distance.
15:23If we know how far away it is and how fast it's moving, that's the Hubble constant.
15:29Having neutron star mergers added to your arsenal of ways of measuring the universe's expansion
15:35is great because it's completely independent. It uses physics that's not related to either of the
15:41two competing methods we have so far. Sounds perfect. The result?
15:48So this brand new measurement that we're hoping would be a tiebreaker
15:53ended up coming right in between these two extremes. Thanks for the help.
15:59We shouldn't be at all disheartened by the fact that this hasn't actually decided the problem because
16:18there's a huge margin for error when you have just one object. We would like something like a hundred
16:24events like this neutron star merger. That might seem like a huge improvement we need but actually
16:30it's very feasible that in the next decade we'll get there. Gravitational waves may give us a precise
16:37age of the universe but there is a chance they'll tell us the problem isn't with our measurements but
16:43with our understanding of the cosmos. If we keep getting different answers for the Hubble constant,
16:48especially depending on the method we use, that's a big clue that we don't understand something
16:54fundamental about the universe's evolution, its makeup, something important. Our search for the age of
17:01the universe just might destroy our model of how we think the cosmos works, plunging physics into chaos.
17:10We don't know the age of the universe. We had hoped that the results from our experiments would be like
17:27building a bridge starting at opposite ends and meeting in the middle. As time goes on as the evidence
17:36accumulates, these two sides of the bridge are not going to meet. Something has to give.
17:44Some believe the problem lies in the way we've interpreted the picture of the early universe,
17:50the pattern hidden in the cosmic microwave background. We're really confident in the data
17:56that we have from the CMB but it's actually an indirect measurement of the universe's age. It depends on our
18:02model of the universe being right. It could be, it could very well be that our fundamental cosmological
18:10model that we've used to successfully describe the universe is coming up short, that there's something
18:17wrong in there, that that engine is broken. That engine is the standard cosmological model.
18:25Based on our knowledge of particle physics and general relativity, it's like an instruction manual for how the
18:32universe works. Rewriting it is a radical suggestion. For the most part, it matches what we see. But it
18:41does struggle with one thing. As the universe expands away from the Big Bang, the intuitive thing you would
18:48expect is for gravity to start pulling it back together again. So over time gravity would just reverse
18:55that and pull everything back in, back to a single point. But what we see in the data is completely
19:03opposite. What we see is that the universe is not only continuing to expand, but it's speeding up faster
19:09and faster all the time. To explain this weird phenomenon, the cosmological model relies on the
19:15existence of a strange unknown force, dark energy. Dark energy is the most perplexing and mysterious
19:24thing I've encountered in my research. Dark energy is a term that we slap on this idea that the universal
19:30expansion is accelerating. That's about all we know about it. We don't know what's causing it. We don't
19:37know how it behaves. We don't know what it was like in the past or what it's like in the future. So we just call
19:41it dark energy. It's invisible. It fills the whole universe and pushes galaxies apart. In some sense,
19:51it's like a spring, a contracted spring, and you let it go and it wants to push everything away. And things
19:59get stranger. Dark energy doesn't dilute as the universe expands. As empty space gets created or expands,
20:09the dark energy associated with that stays the same. It basically populates all this empty space.
20:15Imagine I'm draining a bucket of water and water just magically appears out of nowhere.
20:19That's like how dark energy behaves as the universe is expanding.
20:24Dark energy plays an important role in the standard cosmological model. If our understanding of it is
20:30wrong, then so too is the model, which means the age of the universe we get from the CMB is wrong too.
20:39Since nobody has a clue what dark energy is, there are a lot of different theories,
20:43but the biggest question of all is simply, is it constant?
20:48Our standard assumption about dark energy is that it's pushing apart the universe with the same strength
20:54throughout the history of the universe. Now, physicists are wondering if that idea is wrong.
21:02Maybe in the early universe, dark energy acted differently.
21:08Hey, you know the whole dark energy thing that's messing with the universe today?
21:12Maybe it messed with the universe back then.
21:15It could be that dark energy really has affected the rate of expansion a lot more than we thought.
21:22This is going to throw a big monkey wrench into our idea of how old the universe is
21:27and what it was like at different eras.
21:30The theory is called New Early Dark Energy.
21:34So the idea behind New Early Dark Energy is that dark energy was present during the very early periods
21:41of the universe, but in a very different state. Just like you can think of water being present in two
21:49states. It can be liquid water if the environment is quite hot, or it can be frozen water if the
21:57environment is colder. We call that a phase change. Maybe in the early universe, dark energy underwent a
22:05phase change as well. It was different before then and acts differently now. According to the theory,
22:11this more energetic state of early dark energy pushed apart the early universe much faster than we
22:18thought. So that speeds things up in the opening moments of our universe which starts to actually
22:25bring things back into agreement when you look at interpreting both the cosmic microwave background
22:31and the distance ladder measurements. One of the things that we see in the universe is that
22:37things change with time. Density changes, matter changes, energy changes. Why not dark energy?
22:43Adding New Early Dark Energy to the early universe changes the standard model. The CMB gives a higher
22:51figure for the expansion of the universe. And finally an age that matches the one given by the distance
22:58ladder method. If you think about that bridge analogy where the two parts just don't meet,
23:05the early dark energy adjusts the angle of the early universe part of the bridge and it just gets
23:12them to actually meet in the middle. It's still controversial, but new dark energy may be detected
23:20in detailed measurements of the cosmic microwave background. I mean, in one sense, like, do we really
23:27need to overcomplicate the universe here? But you know what? The universe is under no obligation to be simple.
23:33But there's one thing physicists can agree on. Dark energy truly is a can of worms we've just opened,
23:42and there may be some big changes coming up. There is a more radical possibility. Maybe we need to ditch
23:48dark energy altogether and question one of the most famous theories of all, general relativity.
23:56Is it possible? Did Einstein make a colossal mistake?
24:10In trying to work out the age of the universe, physicists have started a revolution. A revolution that
24:17could overturn everything we thought we knew about how the universe works, including the bedrock of modern
24:24physics, Einstein's theory of gravity, general relativity. Underlying everything, all of cosmology is
24:33general relativity. But maybe we need a completely new understanding of gravity.
24:42Gravity is a strange force. It's always attractive. The earth pulling on us gives us our weight.
24:49The force of gravity acts over huge distances. The sun tugs on objects throughout the solar system.
24:58The milky way pulls on other galaxies. On the one hand, gravity is incredibly familiar to us. You know,
25:05the apple falling from the tree and all of that stuff. And we also know that gravity behaves in a very
25:11predictable way throughout our solar system, from all the spacecraft and things we've sent out. But when it
25:18comes to how it behaves on incredibly tiny scales, and also on incredibly large scales covering the whole
25:25universe, it's possible that we just don't yet have the right picture of what's going on.
25:32Einstein's model of gravity has remained largely the same for a hundred years.
25:37So much of modern physics is really standing on Einstein's shoulders. But at the same time,
25:42we can't ever take anything for granted. Claudia de Romm works on a theory called massive gravity.
25:51It's based on a key part of Einstein's theory that says gravity doesn't have mass.
25:56Once you understand that general relativity is the theory of a massless particle,
26:02the immediate response should be, well, what if it was massive?
26:05The theoretical particle that carries gravity is called the graviton. If gravitons don't have any
26:13weight, then there's nothing to slow them down as they speed through the universe. They can act
26:19over infinite distances, just like photons of light. So one galaxy on this side of the universe
26:27can actually pull on a galaxy that's right on the other side of the universe.
26:31But if gravity has weight, things change. In some sense, we attach a little backpack to our graviton
26:39particle. Its effect is to slowly slow it down just enough so as to make its effect on very large
26:48distances being a tiny little bit weaker. And that's our way to switch off the effect of gravity on huge
26:57cosmological distances. If gravity is a little bit weaker, a galaxy on this side of the universe
27:05can't pull on one on the other side of the cosmos. It has a huge effect on the expansion of the universe.
27:14If the force of gravity actually just switches off at large distances, then you no longer have to counter
27:22the fact that everything's pulling everything else together because it isn't anymore. So that would
27:27quite naturally explain why the expansion of our universe would be speeding up.
27:33This acceleration is what we see in the universe today.
27:38Currently, we use dark energy to explain it.
27:43So if the graviton has mass, that means that we can get out of the universe what we see without the need
27:51for dark energy. What if actually what we are observing is simply the first sign of gravity
27:58switching off at very large distances? Maybe we're just observing the first effect of the graviton having
28:05a mass. Without dark energy to deal with, the universe is a lot easier to explain.
28:13Maybe we don't need these complicated physics. Maybe it's just all the normal ingredients of the universe,
28:19but operating under a different set of rules.
28:23Claudia hopes her theory will soon be put to the test.
28:29Around 2037, we'll have a new gravitational wave detector, the Laser Interferometer Space Antenna,
28:37or LISA. It'll be bigger than LIGO and will orbit the Earth.
28:42When LISA gets out there in space, we'll even have a bigger handle on gravitational waves evolving
28:49throughout the whole universe. And so it will allow us to go very deep in our understanding of gravity.
28:56LISA is a system of three satellites arranged in a giant triangular formation 1.5 million miles apart.
29:04It should pick up very low frequency gravitational waves from more ancient events, perhaps even shock waves from the birth of the universe.
29:19If the graviton has mass, then the waves will arrive more slowly than predicted. But until we receive those signals, all bets are off.
29:28It's a big deal to propose a difference in gravity, but then again, we don't know.
29:34I'm making no bets. The universe has proven itself to be so deceptive. So I'm going to wait until it tells me what it is.
29:45The question of the age of the universe opens Pandora's box. And the expansion rate of the universe holds another secret.
29:54Our ultimate fate. How the universe will end.
30:11We know exactly how the Earth will end.
30:13In around 5.4 billion years, the Sun will turn into a red giant, expanding to a thousand times its current size.
30:26The Earth will be destroyed.
30:30Humans, if we still exist, will have long deserted our home planet.
30:38But how will the universe end?
30:39The age of the universe enables us to not only understand where we came from, but potentially the fate of the universe, what will happen millions and billions of years from now.
30:53If scientists confirm the value of the Hubble constant, the elusive figure that tells us just how fast the universe is expanding, it will tell us the age of the universe and it will help us predict its end.
31:06Measuring the Hubble constant is measuring the expansion rate today, right now.
31:12It's like checking your speedometer at one moment.
31:15But just because it's your speed now, it doesn't mean it was the same speed when you left your home or the same speed when you'll be on the freeway.
31:23How the expansion changes over time will control the fate of the cosmos.
31:27So depending on the Hubble constant, the universe could continue to expand, it could accelerate its expansion rate, or it could be decelerating.
31:39At the moment, galaxies are racing apart.
31:43A continually expanding universe will cool down as it spreads out.
31:48Another name for this eternal expansion is the big freeze.
31:54Because as everything gets spread out, the density's lower and there's no more opportunities for temperature differences.
32:01Everything just gets colder and colder and colder and colder.
32:05Slowly, eternally approaching absolute zero.
32:10The more matter is spread out, the less chance there is for star formation.
32:14And so the universe's continued expansion means our night sky, and every night sky in the universe,
32:23will inevitably continue to get darker and darker and darker as things move further away and as stars die off.
32:31Eventually, all the stars will go out.
32:34And they'll just be the leftovers, which we call the degenerates.
32:39Black holes, white dwarfs, rogue planets.
32:43It's going to be a very, very sad place.
32:46The last refuge of any matter at all will be black holes.
32:50You've got a big black hole in the middle of each galaxy.
32:53Over trillions of years, everything in galaxies fall in.
32:57So finally, you're left with big black holes over vast distances, separated almost universes away.
33:05So getting towards the big freeze, black holes themselves start to evaporate.
33:11There won't even be black holes at the end of this accelerating universe.
33:17All that's left is very, very low energy photons and a little bit of matter dispersed throughout the universe.
33:24And there's nothing left. That's it.
33:26We call that the heat death of the universe.
33:29There's no longer any place that has more energy or more heat.
33:33It's all just thin, barely there photons.
33:36It's fascinating scientifically, but from a human standpoint, not a lot of fun to think about.
33:42But if the Hubble constant, the expansion rate of the universe, keeps increasing,
33:48then the end of the universe could be a lot scarier and come a lot sooner.
33:54One possibility is that the expansion of the universe will accelerate and continue to accelerate
34:03forever faster and faster and faster. And if that happens, we face a scenario that we call the big rip,
34:10where actually the whole of space essentially just gets ripped to shreds.
34:14So the solar system is going to get ripped apart. Then the sun and the planets themselves will start
34:21to get ripped apart. And finally, it works its way down to atoms. And atoms gets ripped apart.
34:27And we're starting to see effects on space and time. Space is ripped apart. Time comes to a stop.
34:35So in this scenario, time and space have no meaning. If everything is infinitely far apart, then
34:46space doesn't really exist. It's sort of beyond our comprehension.
34:53Working out the expansion rate will tell us which scenario we face. But for now,
34:59the lifespan of the universe is unknown.
35:01Maybe we need to investigate the other end of the timeline. But how can we get a fix on the age of
35:10the universe without understanding its origin? As you go back in time towards the big bang,
35:18our knowledge of physics really goes out the window. Temperatures off the scale, pressure off the scale,
35:25the way everything behaved is just so different that the rules we have now do not apply.
35:31The biggest problem of all? What came just before the big bang? Einstein's general relativity predicts
35:40that all the matter and energy in the universe was concentrated down to a single point, a singularity.
35:47The singularity is like the part of those old maps that says, here be dragons.
35:54Singularities are a problem. We don't like them. This is where basically you have a finite amount
36:00of matter in the universe, but it's squeezed down into zero volume, so it would be infinitely dense.
36:06Infinite densities don't actually happen in nature. This is a sign that our math is breaking down.
36:12This is a sign that we need to replace that with a new understanding.
36:17Many now believe Einstein was wrong. There was no singularity. Begging the question,
36:25could the age of the universe be infinite?
36:37Scientists investigating the age of the universe are struggling to understand its origins.
36:42Could that be because there was no beginning? Could the universe be infinite?
36:51Because we think we live and we die, we project that onto the universe. But that may not be the case.
36:55The idea of an infinite universe is no more strange than the idea of a singularity. And in fact,
37:02throughout most of history, astronomers thought that the universe was probably infinite.
37:09The foundation of our mathematical understanding of the universe,
37:12Einstein's general relativity, has a problem. It doesn't translate to the world of the very tiny,
37:19which is why its laws break down close to the big bang.
37:23General relativity does a great job at describing things on scales that you and I are familiar with,
37:30and things like how planets move and how galaxies evolve, all the big stuff.
37:35Quantum mechanics, on the other hand, describes the world of the very small, the world of the atoms.
37:41The problem is that these two theories don't fit well together at all.
37:45A new theory, known as loop quantum gravity, brings quantum theory and relativity together.
37:54And it makes a stunning prediction.
37:57So one possibility is that the end of the universe could kind of match onto the beginning of a new
38:04universe and create a cycle of universes, one after the other.
38:09Nicknamed the big bounce, it predicts a universe that stops expanding and switches into reverse.
38:16And the idea here is that the universe can expand for a time, stop expanding and then begin to contract
38:23again. And some have suggested that perhaps there's a cycle of expanding and compressing,
38:29it bounces back over again.
38:32One of the appeals of the bouncing model is that it allows us to get beyond the singularity.
38:37A bit like recycling on Earth, all the components get crushed down and then reused,
38:44giving the cosmos no beginning and no end.
38:48If the universe is cyclic, does the age even have a meaning?
38:53Age is a construct of humanity because we need to count time.
38:57But if the universe is infinite, maybe it doesn't matter in the big scheme of things.
39:01A contracting and expanding universe messes with the concept of age.
39:07But the very idea of an expanding universe provides another cosmic curve ball.
39:12It might not be alone. It might be just one ageless universe among many.
39:19It's an idea embedded in the math of the big bang.
39:23The most popular theory we have in astrophysics for what put the bang into our big bang is inflation.
39:30This idea that there was a kind of dark energy on steroids that made our universe double over and over,
39:35not every seven billion years, but every split second, creating out of almost nothing, a big bang.
39:44When the universe was just a hundredth of a billionth of a trillionth of a trillionth of a second old,
39:51it underwent a period of rapid expansion called inflation.
39:55It doubled in size at least 90 times, going from the size of a subatomic particle to that of a golf ball.
40:03The problem with this inflation is, but it doesn't really stop.
40:08It just makes this ever bigger space and says that, yeah, well, okay, there was one region of space where
40:13this crazy doubling stopped and galaxies formed and that's us.
40:17But there's this vast realm out there where inflation is still happening.
40:23In the spots where inflation stops, parallel universes form.
40:28This eternal inflation means that new universes are popping to existence all the time,
40:33but they're completely separated one from the other.
40:37Many of my colleagues hate parallel universes.
40:40They just don't like the idea that our universe is so big and most of it is off limits for us.
40:45If you are willing to be a bit more humble and accept that the reality might be much,
40:51much bigger than we will ever see, then parallel universes feel pretty natural.
40:59It's really interesting how everything in the universe is tied together.
41:02We can start with a simple question like, how old is the universe?
41:06And here we are questioning virtually everything about the universe.
41:10Cosmology's century-long search for the age of the universe
41:16forces us to question our cosmological model, the nature of gravity and even time itself.
41:24The age of the universe does bring up sort of profound philosophical questions
41:30about how a universe can even start. How can you create something from nothing?
41:36The vast majority of whatever the universe is, is eternally hidden to us.
41:44So we answer the questions, how big, how old?
41:47And those very answers show us that we don't even know if we've asked the right questions to begin.
41:53So
41:59we swap together.
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42:10
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