How the Universe Works - S10E05 - Hunt for the Universe's Origin WEB-DL x265 Garshasp)
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00:00There's a mystery at the very heart of the universe.
00:05We don't know how old the cosmos is.
00:09Understanding the age of the universe is fundamental to understanding the universe at all.
00:14It's at the heart of everything.
00:17It's more than just celebrating a birthday.
00:19We want to know how much mass is in it, how much energy is in it, how it behaves.
00:23We have to have this number nailed down.
00:26The age of the universe enables us to not only understand where we came from, but potentially
00:32the fate of the universe, what will happen millions and billions of years from now.
00:37But our quest to discover the age of the universe is starting a war.
00:42Usually nature just whispers to us.
00:44Now nature is screaming in our ear that we're doing something wrong, and that's exciting.
00:56We think the universe started with a bang.
01:07Everything that has ever existed is squashed up in the space smaller than a pinhead.
01:12And all of a sudden, space just starts expanding everywhere at once.
01:19The idea that the universe grew from a ball smaller than a pinhead is hard to understand.
01:25But figuring out when it happened sounds like it should be more straightforward.
01:30It seems like a simple question, right?
01:32But it turns out getting the age of the universe is pretty tricky.
01:37Scientists have just a single fact as their starting point.
01:40The universe is expanding.
01:44When people realized the universe was expanding, they thought they finally had a way to estimate
01:47the age of the universe.
01:49Take the universe now and run it backwards in time.
01:52Things get closer and closer until they come to a single point.
01:56That time, to that point, is the age of the universe.
02:00The expansion rate is so important, it's been given its own name, the Hubble Constant.
02:07The Hubble Constant is the present day expansion rate of the universe.
02:12It is a key ingredient to understanding the entire expansion history of our universe and
02:19its age.
02:21Scientists discovered a strange radio signal permeating the cosmos.
02:27It's the remnants of ancient light from the early universe.
02:31We 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.
02:53Using sensitive radio receivers, the orbiter studied the sky in every direction, measuring
02:58tiny changes in the temperature and polarization of the radiation signal.
03:04The CMB has all these variations in temperature, and they're not randomly generated.
03:09They are there because of physical processes that occurred when the universe was in its
03:13primordial fireball phase.
03:16The red blobs are where matter was hottest, and the blue areas are where matter was cooler.
03:23The smallest red blobs are where hot material was packed tightly together.
03:28That's where material in the universe would have been denser, and that's where galaxies
03:32would preferentially form.
03:35It's so cool to get to look at those blueprints and study them and see how that baby universe
03:42later grew up into the universe we see around us today.
03:46Although it doesn't look like much, hidden within this picture is almost everything we
03:50can know about the universe.
03:53In 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.
04:06They worked out how the universe got from small to big, and how fast that expansion
04:12happened.
04:14The data from the Cosmic Microwave Background is absolutely the gold standard for cosmology.
04:20It's beautifully clean.
04:22We can understand it really well, and we have a lot of confidence that what we learn from
04:27it is pretty robust.
04:29By running the expansion backwards, we get an age, 13.82 billion years.
04:39Job finished.
04:42But it's not quite a slam dunk.
04:44The figure must be verified.
04:47We don't make a single measurement using a single technique.
04:50We make multiple measurements via multiple techniques.
04:54Another group of scientists use a totally different method to calculate the age of the
04:58cosmos, measuring objects that we can see in our universe to determine how far away
05:04they 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.
05:17Parallax is the apparent shift in an object relative to the background when it's viewed
05:22from two different locations.
05:24So 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.
05:33If I move my thumb closer to my face, then the distance it moves back and forth changes.
05:38It appears to move back and forth more.
05:41That parallax difference as we move the thumb closer and farther from the face is the way
05:46we measure distances to distant objects.
05:49Using parallax, we can measure the distance to bright stars called Cepheids in the Milky
05:55Way.
05:56Cepheids are stars that burn 100,000 times brighter than our sun.
06:01So they're extremely bright, and they pulsate, meaning they get brighter and dimmer over
06:06a regular time period.
06:09Cepheids that pulsate at the same rate have the same brightness.
06:13They're known as a standard candle.
06:16A standard candle is something that is a standard, meaning we know how intrinsically bright it
06:21is.
06:22So all we have to do is measure the brightness that we appear to perceive on Earth, and then
06:27you solve for the distance.
06:30So imagine that you're on this street.
06:32By looking down the street, you'll see that the streetlights get dimmer and dimmer the
06:37farther away they are.
06:38But that's not their intrinsic brightness.
06:40Their intrinsic brightness is the same.
06:42So by seeing how faint the farthest away ones are, you can understand how far away they
06:48are from you.
06:51We can use standard candles to measure the distance to stars farther away.
06:56But there's a big problem.
06:59Throughout the universe, there's a competition between the expansion pushing things apart
07:04and gravity pulling things together.
07:08In the Milky Way, there's so much matter that gravity wins.
07:12Even looking at galaxies in our neighborhood, the expansion is tiny.
07:18But at cosmic scales of very different galaxies, matter is more spread out, and expansion wins.
07:25So we can only measure expansion over massive distances.
07:30The way we start to measure distances to things that are farther and farther away is to use
07:34something we call the distance ladder.
07:38Each category of object that we observe is on a separate rung of this ladder.
07:45Measuring the distance to one will then inform us how far away the second rung is, and then
07:51the third rung.
07:52So each rung depends on the previous rung, and from stacking these together, we can start
07:59to measure things very, very far away from us.
08:04Using parallax to measure Cepheid stars in the Milky Way gives us a benchmark.
08:10We can then use their standard brightness to measure Cepheids in other galaxies.
08:16The next rung is a brighter standard candle called type 1a supernovas.
08:21They can be seen in galaxies farther away.
08:24Finally, we can measure light from distant elliptical galaxies.
08:29And by looking at how red the light is, we can work out how fast they're moving away
08:34from us.
08:36So those three things give us the nearby universe, the somewhat faraway universe, and the very
08:41distant universe, rung by rung.
08:46March 2021.
08:50Scientists measure the light from 63 giant elliptical galaxies, the farthest rung of
08:56the distance ladder.
08:58They hope to get the most accurate measurement of the Hubble constant to date, and a precise
09:04age for the universe.
09:07Their calculations make the universe 13.3 billion years old, not too far away from the
09:14figure of 13.82 billion years given by the cosmic microwave background, a difference
09:21of around 6%.
09:23That sounds trivial, but that equates to hundreds of millions of years of cosmic history that
09:28either happened or didn't happen.
09:32Fifty years ago, when we weren't quite as good at measuring everything about the universe,
09:36we would have been thrilled to have our numbers agreeing to this level.
09:39But nowadays, having a difference like this, it's unacceptable.
09:43Clearly, the two techniques do not agree.
09:47Cosmologists split into two camps.
09:50We had hoped that these two methods were like building a bridge from either side and then
09:54meeting in the middle, but they're not.
09:58Now 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.
10:08Because in science, usually the initial really big discoveries start off as small differences
10:15that then you pull on that thread and something wonderful emerges.
10:19So does the simple question, how old is the universe, unravel everything?
10:37The universe is expanding outwards.
10:39The rate it's growing is called the Hubble constant, and it's the key to working out
10:44the age of the universe.
10:47So the Hubble constant might just seem like some, you know, academic number that doesn't
10:52mean anything, but that number contains information about the composition, the evolution, and
11:00the fate of the universe.
11:03It's an important number, but there's a problem.
11:07Our best measurement methods don't match.
11:11It's incredibly frustrating to not know how old the universe is.
11:14It's even more frustrating to know that there's two experiments, which are excellent experiments
11:19that we firmly believe in, that completely disagree with each other.
11:23My hair fell out a long time ago over this kind of stuff.
11:27This has been the number one question for over half a decade.
11:32There must be something wrong with one of the methods.
11:36There's a definite sense in the community that whichever camp you happen to fall into,
11:41the problems lie on the other side of the fence.
11:44So if you're mainly working with the cosmic microwave background, you probably think something
11:48is up with the distance ladder.
11:51If there's a problem with the distance ladder, there's a prime suspect.
11:56The ladder relies on stars that have a predictable brightness called standard candles.
12:02But there's evidence that these stars are not always the same brightness.
12:09So if you expect an object to have a particular brightness, and it has a different brightness
12:14than whatever conclusion you draw that relies on the brightness of that object, it's going
12:19to be off somewhat.
12:21Think of the stars like streetlights.
12:24If one light is broken and dimmer than the others, you might think it's farther away.
12:30The concern with the distance ladder is that if any of the single rungs is not perfect,
12:36then the entire ladder might be out of whack by the time you get to the top.
12:40What we need is a fresh approach to measuring the age of the universe.
12:45We're hoping we could bring in a tiebreaker, a referee, a brand new method that didn't
12:50care about any of this or any of that, and tell us what is the Hubble constant.
12:57We may have just found one.
12:59This observatory doesn't have a telescope.
13:03It's hunting for an invisible wave, a disturbance in space-time itself, caused by massive objects
13:11accelerating or colliding.
13:14It's known as LIGO.
13:17LIGO stands for the Laser Interferometer Gravitational-Wave Observatory, and it is a ground-based gravitational
13:24wave detector.
13:26A perfectly stabilized beam of laser light bounces in a five-mile-long L-shaped tunnel.
13:33As a gravitational wave passes through the detector, space stretches, forcing the light
13:40to travel a tiny bit farther.
13:42You're bouncing a laser over an incredible distance and trying to measure as space-time
13:49itself gets stretched and deformed, whether that laser had to travel a tiny bit further
13:54or a tiny bit shorter.
13:56And a tiny bit here is the width of a single atom over miles and miles of distance.
14:03LIGO has already detected colliding black holes, but it's also received a signal from
14:10something less massive.
14:14Neutron stars are the densest thing in the universe other than black holes.
14:19They're the last stopping point before you would collapse all the way to form a black
14:23hole.
14:24They're the size of Washington, D.C., but they can have the mass of two suns.
14:30A collision between neutron stars is incredibly powerful.
14:34It's one of the most energetic events in the universe, and it distorts the fabric of space-time
14:40very strongly because their gravity is so strong.
14:43But unlike black hole mergers, neutron star collisions can also send out light.
14:50In 2017, LIGO sent out an alert.
14:54More than 70 telescopes on Earth and in space swung into action.
14:59This binary neutron star merger was the first time we'd witnessed gravitational waves and
15:04light waves coming from the same event.
15:09It was groundbreaking.
15:12This event is ideal for Hubble constant hunters.
15:16The light tells us how fast the colliding stars are moving away from us.
15:21Gravitational waves give us the distance.
15:25If we know how far away it is and how fast it's moving, that's the Hubble constant.
15:31Having neutron star mergers added to your arsenal of ways of measuring the universe's
15:36expansion is great because it's completely independent.
15:40It uses physics that's not related to either of the two competing methods we have so far.
15:46Sounds perfect.
15:48The result?
15:50So this brand new measurement that we were hoping would be a tiebreaker ended up coming
15:56right in between these two extremes.
16:00Thanks for the help.
16:03But it might not be as bad as it sounds.
16:07The number of neutron star collisions where we've detected gravitational waves and light?
16:13One.
16:14We shouldn't be at all disheartened by the fact that this hasn't actually decided the
16:19problem because there's a huge margin for error when you have just one object.
16:24We would like something like a hundred events like this neutron star merger.
16:30That might seem like a huge improvement we need, but actually it's very feasible that
16:33in the next decade we'll get there.
16:36Gravitational waves may give us a precise age of the universe, but there is a chance
16:41they'll tell us the problem isn't with our measurements, but with our understanding of
16:46the cosmos.
16:48If we keep getting different answers for the Hubble constant, especially depending on the
16:51method we use, that's a big clue that we don't understand something fundamental about the
16:57universe's evolution, its makeup, something important.
17:01Our search for the age of the universe just might destroy our model of how we think the
17:07cosmos works, plunging physics into chaos.
17:24We don't know the age of the universe.
17:27We had hoped that the results from our experiments would be like building a bridge, starting
17:33at opposite ends and meeting in the middle.
17:37As time goes on, as the evidence accumulates, these two sides of the bridge are not going
17:43to meet.
17:45Something has to give.
17:48Some believe the problem lies in the way we've interpreted the picture of the early universe,
17:54the pattern hidden in the cosmic microwave background.
17:58We're really confident in the data that we have from the CMB, but it's actually an indirect
18:03measurement of the universe's age.
18:05It depends on our model of the universe being right.
18:09It could be, it could very well be that our fundamental cosmological model that we've
18:15used to successfully describe the universe is coming up short, that there's something
18:20wrong in there, that that engine is broken.
18:25That engine is the standard cosmological model.
18:29Based on our knowledge of particle physics and general relativity, it's like an instruction
18:34manual for how the universe works.
18:37Rewriting it is a radical suggestion.
18:40For the most part, it matches what we see.
18:44But it does struggle with one thing.
18:47As the universe expands away from the Big Bang, the intuitive thing you would expect
18:53is for gravity to start pulling it back together again.
18:57So over time, gravity would just reverse that and pull everything back in, back to a single
19:02point.
19:05But what we see in the data is completely opposite.
19:08What we see is that the universe is not only continuing to expand, but it's speeding up
19:12faster and faster all the time.
19:15To explain this weird phenomenon, the cosmological model relies on the existence of a strange,
19:21unknown force, dark energy.
19:25Dark energy is the most perplexing and mysterious thing I've encountered in my research.
19:30Dark energy is a term that we slap on this idea that the universal expansion is accelerating.
19:37That's about all we know about it.
19:39We don't know what's causing it.
19:40We don't know how it behaves.
19:42We don't know what it was like in the past or what it's like in the future.
19:45So we just call it dark energy.
19:48It's invisible.
19:50It fills the whole universe and pushes galaxies apart.
19:54In some sense, it's like a spring, a contracted spring, and you let it go and it wants to
19:59push everything away.
20:02And things get stranger.
20:04Dark energy doesn't dilute as the universe expands.
20:09As empty space gets created or expands, the dark energy associated with that stays the
20:15same.
20:16It basically populates all this empty space.
20:19Imagine I'm draining a bucket of water and water just magically appears out of nowhere.
20:23That's like how dark energy behaves as the universe is expanding.
20:28Dark energy plays an important role in the standard cosmological model.
20:32If our understanding of it is wrong, then so too is the model, which means the age of
20:39the universe we get from the CMB is wrong too.
20:43Since nobody has a clue what dark energy is, there are a lot of different theories.
20:47But the biggest question of all is simply, is it constant?
20:52Our standard assumption about dark energy is that it's pushing apart the universe with
20:57the same strength throughout the history of the universe.
21:02Now physicists are wondering if that idea is wrong.
21:06Maybe in the early universe, dark energy acted differently.
21:11Hey, you know the whole dark energy thing that's messing with the universe today?
21:16Maybe it messed with the universe back then.
21:20It could be that dark energy really has affected the rate of expansion a lot more than we thought.
21:26This is going to throw a big monkey wrench into our idea of how old the universe is and
21:31what it was like at different eras.
21:34The theory is called new early dark energy.
21:38So the idea behind new early dark energy is that dark energy was present during the very
21:44early periods of the universe, but in a very different state.
21:48Just like you can think of water being present in two states.
21:54It can be liquid water if the environment is quite hot, or it can be frozen water if
22:01the environment is colder.
22:03We call that a phase change.
22:05Maybe in the early universe, dark energy underwent a phase change as well.
22:09It was different before then and acts differently now.
22:13According to the theory, this more energetic state of early dark energy pushed apart the
22:19early universe much faster than we thought.
22:23So that speeds things up in the opening moments of our universe, which starts to actually
22:29bring things back into agreement when you look at interpreting both the cosmic microwave
22:34background and the distance ladder measurements.
22:38One of the things that we see in the universe is that things change with time.
22:42Density changes, matter changes, energy changes.
22:45Why not dark energy?
22:47Adding new early dark energy to the early universe changes the standard model.
22:53The CMB gives a higher figure for the expansion of the universe.
22:58And finally, an age that matches the one given by the distance ladder method.
23:03If you think about that bridge analogy where the two parts just don't meet, the early dark
23:10energy adjusts the angle of the early universe part of the bridge and it just gets them to
23:16actually meet in the middle.
23:19It's still controversial, but new dark energy may be detected in detailed measurements of
23:25the cosmic microwave background.
23:28I mean, in one sense, like, oh, do we really need to overcomplicate the universe here?
23:33But you know what?
23:34The universe is under no obligation to be simple.
23:39But there's one thing physicists can agree on.
23:42Dark energy truly is a can of worms we've just opened, and there may be some big changes
23:47coming up.
23:48There is a more radical possibility.
23:51Maybe we need to ditch dark energy altogether and question one of the most famous theories
23:57of all, general relativity.
24:00Is it possible?
24:01Did Einstein make a colossal mistake?
24:06In trying to work out the age of the universe, physicists have started a revolution, a revolution
24:22that could overturn everything we thought we knew about how the universe works, including
24:28the bedrock of modern physics, Einstein's theory of gravity, general relativity.
24:35Underlying everything, all of cosmology is general relativity.
24:40But maybe we need a completely new understanding of gravity.
24:48Gravity is a strange force.
24:50It's always attractive.
24:52The Earth pulling on us gives us our weight.
24:56The force of gravity acts over huge distances.
25:00The Sun tugs on objects throughout the solar system.
25:04The Milky Way pulls on other galaxies.
25:07On the one hand, gravity is incredibly familiar to us, you know, the apple falling from the
25:12tree and all of that stuff.
25:14And we also know that gravity behaves in a very predictable way throughout our solar
25:19system from all the spacecraft and things we've sent out.
25:23But when it comes to how it behaves on incredibly tiny scales and also on incredibly large scales
25:30covering the whole universe, it's possible that we just don't yet have the right picture
25:35of what's going on.
25:38Einstein's model of gravity has remained largely the same for 100 years.
25:43So much of modern physics is really standing on Einstein's shoulders.
25:47But at the same time, we can't ever take anything for granted.
25:52Claudia de Ram works on a theory called massive gravity.
25:56It's based on a key part of Einstein's theory that says gravity doesn't have mass.
26:03Once you understand that general relativity is the theory of a massless particle, the
26:08immediate response should be, well, what if it was massive?
26:12The theoretical particle that carries gravity is called the graviton.
26:17If gravitons don't have any weight, then there's nothing to slow them down as they speed through
26:22the universe.
26:24They can act over infinite distances, just like photons of light.
26:29So one galaxy on this side of the universe can actually pull on a galaxy that's right
26:35on the other side of the universe.
26:38But if gravity has weight, things change.
26:41In some sense, we attach a little backpack to our graviton particle.
26:45Its effect is to slowly slow it down just enough so as to make its effect on very large
26:54distances be a tiny little bit weaker.
26:58And that's a way to switch off the effect of gravity on huge cosmological distances.
27:06If gravity is a little bit weaker, a galaxy on this side of the universe can't pull on
27:11one on the other side of the cosmos.
27:15It has a huge effect on the expansion of the universe.
27:20If the force of gravity actually just switches off at large distances, then you no longer
27:26have to counter the fact that everything's pulling everything else together, because
27:30it isn't anymore.
27:32So that would quite naturally explain why the expansion of our universe would be speeding
27:37up.
27:38This acceleration is what we see in the universe today.
27:43Currently, we use dark energy to explain it.
27:48So if the graviton has mass, that means that we can get out of the universe what we see
27:56without the need for dark energy.
27:58What if actually what we were observing is simply the first sign of gravity switching
28:04off at very large distances?
28:07Maybe we're just observing the first effect of the graviton having a mass.
28:13Without dark energy to deal with, the universe is a lot easier to explain.
28:18Maybe we don't need these complicated physics.
28:21Maybe it's just all the normal ingredients of the universe, but operating under a different
28:27set of rules.
28:29Claudia hopes her theory will soon be put to the test.
28:35Around 2037, we'll have a new gravitational wave detector, the Laser Interferometer Space
28:42Antenna, or LISA.
28:45It'll be bigger than LIGO and will orbit the Earth.
28:49When LISA get out there in space, we'll even have a bigger handle on gravitational waves
28:54evolving throughout the whole universe.
28:57And so it will allow us to go very deep in our understanding of gravity.
29:01LISA is a system of three satellites arranged in a giant triangular formation, 1.5 million
29:09miles apart.
29:12It should pick up very low frequency gravitational waves from more ancient events, perhaps even
29:20shock waves from the birth of the universe.
29:25If the graviton has mass, then the waves will arrive more slowly than predicted.
29:30But until we receive those signals, all bets are off.
29:35It's a big deal to propose a difference in gravity, but then again, we don't know.
29:41I'm making no bets.
29:42The universe has proven itself to be so deceptive.
29:46So I'm going to wait until it tells me what it is.
29:51The question of the age of the universe opens Pandora's box.
29:56And the expansion rate of the universe holds another secret.
30:01Our ultimate fate.
30:03How the universe will end.
30:19We know exactly how the earth will end.
30:24In around 5.4 billion years, the sun will turn into a red giant, expanding to a thousand
30:30times its current size.
30:34The earth will be destroyed.
30:37Humans, if we still exist, will have long deserted our home planet.
30:45But how will the universe end?
30:49The age of the universe enables us to not only understand where we came from, but potentially
30:55the fate of the universe, what will happen millions and billions of years from now.
31:01If scientists confirm the value of the Hubble constant, the elusive figure that tells us
31:06just how fast the universe is expanding, it will tell us the age of the universe and it
31:11will help us predict its end.
31:15Measuring the Hubble constant is measuring the expansion rate today, right now.
31:19It's like checking your speedometer at one moment.
31:22But just because it's your speed now, it doesn't mean it was the same speed when you
31:26left your home, or the same speed when you'll be on the freeway.
31:30How the expansion changes over time will control the fate of the cosmos.
31:35So depending on the Hubble constant, the universe could continue to expand, it could accelerate
31:41its expansion rate, or it could be decelerating.
31:46At the moment, galaxies are racing apart.
31:51A continually expanding universe will cool down as it spreads out.
31:57Another name for this eternal expansion is the big freeze, because as everything gets
32:02spread out, the density is lower and there's no more opportunities for temperature differences.
32:09Everything just gets colder and colder and colder and colder, slowly, eternally approaching
32:15absolute zero.
32:18The more matter is spread out, the less chance there is for star formation.
32:22And so the universe's continued expansion means our night sky, and every night sky in
32:29the universe, will inevitably continue to get darker and darker and darker as things
32:34move further away and as stars die off.
32:38Eventually, all the stars will go out and there will just be the leftovers, which we
32:44call the degenerates, black holes, white dwarfs, rogue planets.
32:50It's going to be a very, very sad place.
32:54The last refuge of any matter at all will be black holes.
32:58You've got a big black hole in the middle of each galaxy.
33:01Over trillions of years, everything in galaxies fall in.
33:04So finally you're left with big black holes over vast distances, separated almost universes
33:10away.
33:12So getting towards the big freeze, black holes themselves start to evaporate.
33:19There won't even be black holes at the end of this accelerating universe.
33:24All that's left is very, very low energy photons and a little bit of matter dispersed throughout
33:30the universe, and there's nothing left.
33:33That's it.
33:34We call that the heat death of the universe.
33:37There's no longer any place that has more energy or more heat.
33:40It's all just thin, barely there photons.
33:44It's fascinating scientifically, but from a human standpoint, not a lot of fun to think
33:49about.
33:50But if the Hubble constant, the expansion rate of the universe, keeps increasing, then
33:56the end of the universe could be a lot scarier and come a lot sooner.
34:05One possibility is that the expansion of the universe will accelerate and continue
34:10to accelerate forever, faster and faster and faster.
34:13And if that happens, we face a scenario that we call the big rip, where actually the whole
34:18of space essentially just gets ripped to shreds.
34:23So the solar system is going to get ripped apart.
34:26Then the sun and the planets themselves will start to get ripped apart.
34:30And finally it works its way down to atoms, and atoms gets ripped apart.
34:35And we're starting to see effects on space and time.
34:39Space is ripped apart.
34:41Time comes to a stop.
34:44So in this scenario, time and space have no meaning.
34:49If everything is infinitely far apart, then space doesn't really exist.
34:56It's sort of beyond our comprehension.
35:00Figuring out the expansion rate will tell us which scenario we face.
35:05But for now, the lifespan of the universe is unknown.
35:11Maybe we need to investigate the other end of the timeline.
35:16But how can we get a fix on the age of the universe without understanding its origin?
35:22As you go back in time towards the Big Bang, our knowledge of physics really goes out the
35:28window.
35:30Numbers off the scale, pressure off the scale.
35:32The way everything behaved is just so different that the rules we have now do not apply.
35:39The biggest problem of all?
35:41What came just before the Big Bang?
35:45Einstein's general relativity predicts that all the matter and energy in the universe
35:50was concentrated down to a single point, a singularity.
35:55The singularity is like the part of those old maps that says, here be dragons.
36:02Singularities are a problem.
36:04We don't like them.
36:05This is where basically you have a finite amount of matter in the universe, but it's
36:09squeezed down into zero volume, so it would be infinitely dense.
36:14Infinite densities don't actually happen in nature.
36:17This is a sign that our math is breaking down.
36:20This is a sign that we need to replace that with a new understanding.
36:26Many now believe Einstein was wrong.
36:29There was no singularity.
36:31Begging the question, could the age of the universe be infinite?
36:46Scientists investigating the age of the universe are struggling to understand its origins.
36:53Could that be because there was no beginning?
36:56Could the universe be infinite?
37:00Because we think we live and we die, we project that onto the universe, but that may not be
37:04the case.
37:05The idea of an infinite universe is no more strange than the idea of a singularity.
37:11And in fact, throughout most of history, astronomers thought that the universe was probably infinite.
37:17The foundation of our mathematical understanding of the universe, Einstein's general relativity,
37:24has a problem.
37:25It doesn't translate to the world of the very tiny, which is why its laws break down
37:31close to the Big Bang.
37:33General relativity does a great job at describing things on scales that you and I are familiar
37:38with and things like how planets move and how galaxies evolve, all the big stuff.
37:44Quantum mechanics, on the other hand, describes the world of the very small, the world of
37:49the atoms.
37:50The problem is that these two theories don't fit well together at all.
37:56A new theory known as loop quantum gravity brings quantum theory and relativity together,
38:04and it makes a stunning prediction.
38:07So one possibility is that the end of the universe could kind of match onto the beginning
38:13of a new universe and create a cycle of universes one after the other.
38:19Nicknamed the Big Bounce, it predicts a universe that stops expanding and switches into reverse.
38:26And the idea here is that the universe can expand for a time, stop expanding, and then
38:31begin to contract again.
38:33And some have suggested that perhaps there's a cycle of expanding and compressing.
38:38It bounces back over again.
38:41One of the appeals of the bouncing model is that it allows us to get beyond the singularity.
38:46A bit like recycling on Earth.
38:49All the components get crushed down and then reused, giving the cosmos no beginning and
38:56no end.
38:57If the universe is cyclic, does the age even have a meaning?
39:02Age is a construct of humanity because we need to count time.
39:06But if the universe is infinite, maybe it doesn't matter in the big scheme of things.
39:11A contracting and expanding universe messes with the concept of age.
39:16But the very idea of an expanding universe provides another cosmic curveball.
39:22It might not be alone.
39:24It might be just one ageless universe among many.
39:28It's an idea embedded in the math of the Big Bang.
39:33The most popular theory we have in astrophysics for what put the bang in our Big Bang is inflation.
39:40This idea that there was a kind of dark energy on steroids that made our universe double
39:44over and over, not every seven billion years, but every split second, creating out of almost
39:50nothing a Big Bang.
39:54When the universe was just a hundredth of a billionth of a trillionth of a trillionth
39:59of a second old, it underwent a period of rapid expansion called inflation.
40:04It doubled in size at least 90 times, going from the size of a subatomic particle to that
40:11of a golf ball.
40:13That problem with this inflation is that it doesn't really stop.
40:17It just makes this ever bigger space and says that, yeah, well, okay, there was one region
40:22of space where this crazy doubling stopped and galaxies formed, and that's us.
40:27But there's this vast realm out there where inflation is still happening.
40:32In the spots where inflation stops, parallel universes form.
40:37This eternal inflation means that new universes are popping into existence all the time, but
40:43they're completely separated one from the other.
40:47Many of my colleagues hate parallel universes.
40:49They just don't like the idea that our universe is so big and most of it is off limits for
40:54us.
40:55If you are willing to be a bit more humble and accept that the reality might be much,
41:00much bigger than we will ever see, then parallel universes feel pretty natural.
41:07It's really interesting how everything in the universe is tied together.
41:11We can start with a simple question like, how old is the universe?
41:16And here we are questioning virtually everything about the universe.
41:21Cosmology's century-long search for the age of the universe forces us to question our
41:27cosmological model, the nature of gravity and even time itself.
41:34The age of the universe does bring up profound philosophical questions about how a universe
41:41can even start.
41:42How can you create something from nothing?
41:47The vast majority of whatever the universe is, is eternally hidden to us.
41:53So we answer the questions, how big, how old?
41:57And those very answers show us that we don't even know if we've asked the right questions
42:02to begin.