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00:00Our world, our solar system, our universe, none of it would exist without a ghostly particle
00:11called the neutrino.
00:13They can pass right through a wall, right through a planet, right through a star without
00:17even noticing.
00:20They are our early warning system.
00:23Whenever there's trouble in the universe, you can expect a flood of neutrinos.
00:29Neutrinos trigger star-killing explosions, supernovas.
00:36Neutrinos can answer so many questions, from why do we exist to how was the universe created.
00:44These tiny particles save the infant cosmos from annihilation.
00:50They cause destruction, sometimes they blow up a star, but at the end of the day, they
00:56could be the very reason that we exist at all.
01:01Neutrinos are the key to how the universe works.
01:19In the 1960s, our sun appeared to be dying.
01:25There was tantalizing evidence that our sun might be shutting down.
01:30This question was a biggie for astronomers.
01:32If the sun isn't undergoing nuclear fusion at the rate we thought it was, then that's
01:36a big deal.
01:40Was the sun's nuclear core shutting down?
01:45Stars including our own sun are giant nuclear fusion reactors.
01:51Inside these fusion reactors, hydrogen atoms smash together, producing heat and light in
01:59the form of photons.
02:06All the light and all the heat that we receive on earth comes from the sun.
02:10If the sun were to suddenly start cooling off, that would be seriously bad news for
02:15us.
02:19How do we check if the sun is shutting down?
02:23We have spacecraft monitoring the solar surface, but they can't see into the heart of the reactor,
02:33the sun's core.
02:35You can see the surface, and the sun is very bright.
02:38That makes it very easy to study.
02:40Sadly, the core of the sun is under 400,000 miles of sun, and that makes it pretty hard
02:46to look at.
02:49Studying the light made in the core doesn't help.
02:54By the time it gets to us, it's old news.
02:58Imagine a photon or this particle of light that's born in the center of a star, and now
03:03imagine that it wants to reach the surface of the star.
03:06It turns out that the star is so dense in the center and the star itself is so physically
03:12large that it will take it 30,000 years to escape the core.
03:19It's like being at a cocktail party where you're trying to leave, and every time that
03:23you make another step towards the door, another group of people want to talk to you, and you
03:28also want to talk to them, and then it just takes 30,000 years to leave your cocktail
03:32party.
03:34Any information we get from sunlight about what's going on in the core is tens of thousands
03:40of years old.
03:43If you want the current events, the news headlines of what's going on in the sun's core right
03:48now, photons are not the way to do it.
03:51You want neutrinos.
03:54So what are these mysterious particles?
03:58Neutrino literally means tiny neutral one, right?
04:01We think they carry no net electrical charge, and they're really, really small, so we call
04:06them neutrinos.
04:08Neutrinos don't like to interact with matter.
04:12They fly through almost everything.
04:15The sun itself is generating enough neutrinos to send 60 billion of them through your thumbnail
04:23every single second, and you will spend, this is the craziest thing, you will spend your
04:30entire life without feeling a single one.
04:37Neutrinos form during nuclear fusion reactions inside the core of stars.
04:44Hydrogen atoms collide, fuse into helium, and release photons of light, and neutrinos.
04:52In the core of the sun, nuclear bombs are going off, and all of these nuclear reactions
04:58release neutrinos.
05:00That's about 10 trillion, trillion, trillion neutrinos being created every second.
05:08The trillions of neutrinos shoot out of the core and up through 323,000 miles of the sun
05:16to the surface.
05:17A neutrino basically doesn't even notice the sun is there.
05:22It sails out at very close to the speed of light.
05:26If you imagine a gridlocked highway, the neutrinos would be the motorbikes that are
05:31just zooming through the traffic.
05:35The solar neutrinos race towards Earth.
05:39Most pass straight through.
05:43All the neutrinos, the trillions upon trillions of neutrinos passing through the Earth every
05:49single second, the entire Earth will only interact with one neutrino out of 10 billion.
06:00Because they pass through anything, they're hard to detect.
06:05I consider neutrino physicists to be the ghost hunters of the particle physics realm because
06:11we study something so elusive, and they're really, really hard to nail down and study.
06:21Hard but not impossible.
06:24While most neutrinos pass through Earth, a few collide with atoms in the planet, and
06:30we can detect those collisions.
06:33To spot these tiny impacts, we built underground neutrino detectors with giant sensors full
06:40of chlorine.
06:42When a neutrino strikes this chlorine atom, it transforms into argon, and then we can
06:49pick out the argon atoms from the detector and count them up to see how many neutrinos
06:55actually struck our atoms.
06:59The sensors detected neutrinos from the sun, but the numbers were lower than expected.
07:06We're only detecting about a third of the number of neutrinos that their models predicted.
07:12This is called the solar neutrino problem.
07:16That is a big deal.
07:17That either means we're doing something wrong, or our physics is wrong.
07:21Where were the missing two thirds of the solar neutrinos?
07:25They weren't AWOL.
07:27The detector had missed them because neutrinos can change identities.
07:33It turns out neutrinos can change what kind of neutrino they are as they're flying through
07:38space, and we call this flavor changing.
07:43Neutrinos come in three different flavors.
07:47Think of them as different types of playing cards.
07:52The king is the electron neutrino.
07:56The muon neutrino is the queen, and the jack is the tau neutrino.
08:02The sun produces electron neutrinos.
08:05But by the time they reach earth, they could be a different flavor.
08:10As they travel to the earth, they constantly wave back and forth, trading their identities.
08:15So you never know exactly what you're going to get until it arrives at the earth and we
08:21observe it.
08:22It could be anything.
08:25The detectors weren't seeing the different flavors.
08:29But when we fine-tuned the sensors, we saw all the solar neutrinos.
08:35So there were actually enough neutrinos coming from the sun, but we were only detecting a
08:39third of them.
08:41Flavor changing neutrinos showed the sun was healthy.
08:45The changing identities also answered an important question about neutrinos.
08:51Do they have mass?
08:54Einstein showed that only particles without mass can travel at the speed of light.
08:59And these particles don't experience time.
09:03But neutrinos can change their flavor, so that must happen over time.
09:08And that means neutrinos can't travel at the speed of light.
09:12And so, they must have mass.
09:15When scientists first started thinking about neutrinos, they thought that they were massless.
09:20And if a neutrino has no mass, then it's bound to be one flavor, or one type of neutrino
09:27forever.
09:30Experiments proved that neutrinos have mass.
09:34And if they have mass, they must produce gravity, which means they can influence other things
09:41around them.
09:43Neutrinos are also involved in moments of huge cosmic violence.
09:49Whenever there's trouble in the universe, you can expect a flood of neutrinos.
09:57These floods of neutrinos are the key to some of the biggest bangs in the cosmos.
10:03And new research suggests that without them, there would be no universe.
10:09And new research suggests that without them, there would be no solar system, no planets,
10:15and no us.
10:23Neutrinos are one of the smallest particles in the cosmos.
10:27However, new research suggests they play a role in some of the universe's biggest events.
10:35Exploding stars called supernovas.
10:41The deaths of giant stars.
10:46But there is a mystery surrounding their explosive ends.
10:51Why do these giant stars end their lives so violently?
10:56This is a major puzzle in astrophysics.
11:01We got a lead when we detected a huge flash of light in the Large Magellanic Cloud,
11:08a satellite galaxy of the Milky Way.
11:11The light was a supernova explosion.
11:16But three hours before the flash, astronomers spotted something else.
11:21A burst of neutrinos coming from the same region of the sky.
11:26This was the first time we have seen neutrinos coming from a source other than the sun.
11:31So there must be some sort of connection between neutrinos and supernovae.
11:36But what is that connection?
11:39To find the link, we need to travel deep inside a giant dying star where a battle is raging.
11:47A star owes its existence to the careful balance between the force of energy flowing out of the core of the star
11:53and the force of gravity pushing in toward the core of the star.
11:57So if these forces go out of balance, something is going to change.
12:02That balance changes as the giant star starts to age.
12:07When nuclear fuel runs out in the center of the star, fusion shuts down, gravity starts to take over.
12:16Gravity makes the star collapse.
12:20The extra pressure triggers a new burst of nuclear fusion,
12:24making heavier elements and more energy to support the star, for now.
12:31That process works fine as you build up heavier and heavier nuclei all the way to iron.
12:38And then it tries to fuse that iron into heavier elements to get some more energy out of it, but it can't.
12:44Creating iron doesn't release energy, it uses it up.
12:49That means that when a star gets iron in its core, it's like you've poisoned the nuclear process.
12:56You have this massive crushing weight of the star with a core of iron and nothing left to support it.
13:02So all that material crushes and squeezes that iron core down,
13:07squeezes it so tightly that all the electrons get shoved inside of the protons, turning them into neutrons.
13:16And very, very quickly you convert this massive ball of iron into a very small, very compact neutron star.
13:25When a star runs out of fuel, its core crushes down to a neutron star.
13:31Then the rest of the star collapses inwards.
13:34Hits the neutron star and bounces out, triggering a supernova.
13:40But computer models of supernovas reveal a problem.
13:44The star doesn't explode.
13:47When we run computer simulations of how a supernova might work,
13:52after this bounce, the explosion stalls.
13:57It peters out. The supernova isn't so super.
14:00It needs another source of energy to propel it to become an actual explosion.
14:07Could the neutrinos that appeared before the explosion be that energy source?
14:14First, we need to understand what created the burst of neutrinos.
14:21The core of the star collapses inward.
14:25The core of the star collapses inward.
14:29And eventually, the outer layers of the star fall in toward that star at an appreciable fraction of the speed of light.
14:38As the core rapidly collapses, the intense pressure squeezes atoms together.
14:44That core of iron gets squeezed down to become a neutron star.
14:48The electrons and the protons that are part of this core are under so much pressure
14:52that they fuse together to form neutrons and neutrinos in the process.
14:57The neutrinos shoot out from the newly formed neutron star core,
15:03carrying an enormous amount of energy.
15:0799% of the energy is carried by the neutrinos.
15:11Neutrinos are the main event.
15:13Trillions of neutrinos smash into the remains of the dying star.
15:18And when those neutrinos are flying out of that core region,
15:22a very tiny fraction of them interact with the gas.
15:26And that fraction heats the gas.
15:30Everything that's hanging around this newborn neutron star gets heated to an unimaginable degree.
15:38The heat creates pressures in the surrounding gas,
15:41it builds and builds until it triggers an enormous shockwave.
15:48And then the actual explosion, the actual fireworks show begins.
15:55The star explodes in one of the brightest events in the universe,
16:01powered by neutrinos.
16:04We think that if it weren't for neutrinos,
16:07we think that if it weren't for neutrinos,
16:10supernovas might not even exist.
16:13And we might not exist either.
16:16Our bodies contain heavy elements like calcium in our bones and iron in our blood.
16:23These elements form in supernovas and are scattered across the cosmos by the blast.
16:30Neutrinos are what kindle the fire in the forges of these elements.
16:38And without the neutrinos, you don't have the elements.
16:41And without the elements, you don't have planets like the Earth.
16:44And without planets like the Earth, you don't have life.
16:48There's this common phrase, you know, we are stardust, which is true,
16:52but I like to think we're more like neutrino dust.
16:55Neutrinos reveal how supernovas explode.
16:59And they also warn us when one is about to detonate.
17:03So neutrinos can even be these ghostly signposts for something very violent that's happened in the universe, right?
17:10If we detect a sudden burst of neutrinos, it could be that a star has gone supernova somewhere.
17:16Neutrinos are an early warning system because they barely interact with anything.
17:21They race out of a dying star ahead of the light.
17:25The neutrinos just slip right on through because remember, they're ghosts, they're ninjas.
17:30They get right through and they can arrive at the Earth before the light does.
17:37Supernovas happen suddenly, without warning.
17:41So we rely on neutrinos to give us a heads up.
17:44Neutrinos from supernova actually get to us faster than the light.
17:49Within tens of seconds, we're seeing neutrinos hit our detectors.
17:54If you have multiple detectors on the Earth,
17:59they will all receive the same neutrino blast as it washes over the Earth.
18:04But different detectors will receive that blast at different times.
18:09And you can use that to triangulate on the sky where that blast came from.
18:15And we can use that as a warning signal to alert our telescopes around the country
18:21to look in this direction, in this portion of the sky,
18:26because a firework display supernova is about to happen.
18:31This gives us the time to focus our telescopes on the sky.
18:36This gives us the time to focus our telescopes on the supernova
18:41and watch the light show that follows the initial neutrino burst.
18:50Neutrino bursts are cosmic watchdogs alerting us to danger.
18:56Neutrinos are definitely a sign that something troubling is happening.
19:02And in 2017, a single neutrino told us about something very troubling.
19:09One of the most intense sources of radiation in the universe.
19:14And it was pointing right at us.
19:21Spring 2017.
19:24Scientists at the South Pole are on the lookout for neutrinos.
19:28These ghostly particles are extremely hard to detect.
19:33Neutrinos are the biggest introverts in the universe.
19:37They just don't like interacting with anything.
19:40So if you want to detect one of these things, you need a lot of stuff, you need a lot of atoms in one spot.
19:45So scientists built a facility with lots of available atoms.
19:49It's called IceCube, with neutrino detectors buried deep beneath sheets of ice.
19:56It turns out that water is a very, very good detector of neutrinos.
20:03To catch neutrinos, you need to build a very large target for a reasonable cost.
20:09Large areas of ice checks both boxes.
20:13So you need a lot of water that's very, very clean.
20:17What's the cleanest source of water on the planet?
20:20The Antarctic Ice Sheet.
20:22The Antarctic detector IceCube measures 3,280 feet across.
20:29That's about the length of nine football fields.
20:33It contains 5,000 sensors, surrounded by more water atoms than there are stars in the universe.
20:41So if you want to catch neutrinos colliding with other particles, the thing you need to do is put a lot of targets in front of that gun, right?
20:48You need to pack a lot of particles in the path of the neutrino to give it a higher probability of interaction.
20:55The actual detectors are holes drilled in the ice.
21:00And these holes contain chains of detectors that are three times longer than the tallest building in the world.
21:10And these are sunk down into the ice and use all that incredible volume of ice to detect neutrinos.
21:19September 22nd, 2017.
21:24IceCube detects a neutrino colliding with a water atom.
21:29When a neutrino hits an ice atom inside of IceCube, a charged particle flies out.
21:35And it's this charged particle that makes a signal we can detect.
21:39The ejected particle appears to fly out faster than the speed of light.
21:43At first glance, this looks like it violates something very, very important about physics, that nothing can travel faster than light.
21:51But light slows down when traveling through a medium like air or water.
21:57And it is possible for other things, other particles to outrun light in a medium.
22:03As it hurdles through the ice, the particle generates a burst of blue light called Cherenkov radiation.
22:11It's almost like a sonic boom. If you travel faster than the speed of sound, there's a boom, right?
22:16When you hear that boom, you also see this cone of wind.
22:21It's the same thing with Cherenkov radiation. You get this cone of light.
22:27Neutrinos carry different amounts of energy.
22:30Some, like the 2017 neutrino, carry quite a punch.
22:35And the energy of the neutrino depends on its source.
22:40High-energy neutrinos come from high-energy events.
22:45So we're looking for stuff blowing up. We're looking for stuff colliding.
22:49We're looking for stuff colliding and blowing up. We're looking for awesome things.
22:53The blue burst of Cherenkov radiation gives us a clue about the fearsome origin of the neutrino.
23:01We can follow the path of that blue light, and we can look backwards to see where the neutrino came from.
23:11We track the neutrino to a galaxy nearly six billion light-years away.
23:15At its heart sits one of the most powerful objects in the universe.
23:23A blazar.
23:26A blazar is the biggest, baddest form of feeding active supermassive black hole out there,
23:35where material isn't just falling out of the sky.
23:38It's swirling around, creating a high-energy accretion disk.
23:43The blazar's accretion disk spins at millions of miles an hour,
23:48charging particles of gas and dust.
23:51The disk also generates magnetic fields that twist and tangle as they swirl around the black hole.
23:57Because you have magnetic fields that are twisted around, they also generate electric fields.
24:02The electric fields can then accelerate the charged particles along the magnetic fields
24:07and thus produce a lot of both particles and radiation coming out along jets.
24:14The Cherenkov black hole is the largest black hole in the universe.
24:18It's the largest black hole in the universe.
24:20And thus produce a lot of both particles and radiation coming out along jets.
24:26The jets blast out of the poles of the black hole.
24:32These are the most intense sources of radiation that the cosmos can ever produce,
24:40and they are pointed right at us from billions of light-years away.
24:45Do the jets create the powerful neutrinos?
24:49It's a bit of a mystery.
24:51For a while it was thought that neutrinos are produced directly by the jet.
24:55But now we think that matter, like protons, come in from the accretion disk
25:00and they slam into each other and that's what produces the neutrinos.
25:04Particles racing around the accretion disk crash into the base of the jet.
25:09The enormous energy there smashes the particles together, producing neutrinos.
25:14The jets focus the stream of neutrinos and fire them straight towards Earth.
25:20By just detecting one neutrino, we get to see a lot of information
25:25from the inner workings of an object outside of our galaxy.
25:29And that's what's really exciting about neutrinos, that it could peer into the unknown.
25:35Now we use neutrinos to probe even further into the universe.
25:40Back towards the first second of the Big Bang,
25:44to answer the biggest question of them all.
25:48How and why do we exist?
25:55The fact that our universe appears to be filled with matter is puzzling.
26:01There should have been equal amounts of matter and antimatter in the beginning,
26:05and they should have annihilated one another, producing just pure energy.
26:10So why do we exist?
26:13This is a fundamental question, because this is a question about
26:17why is there something rather than nothing?
26:22To answer that question, we have to rewind the clock back nearly 14 billion years
26:28to the birth of the universe.
26:30A speck of energy sparks into existence.
26:34This energy cools and forms tiny, primitive particles of matter, including neutrinos,
26:41the building blocks of everything we see today.
26:46The early universe appears chaotic, but it quickly establishes some ground rules,
26:52including symmetry.
26:55Our universe is full of symmetry.
26:58There are positive electric charges and negative electric charges.
27:02There's the yin and the yang.
27:05Well, there's also matter and antimatter.
27:08The Big Bang stuck to the rule of symmetry
27:12and made the same amount of both forms of matter.
27:16The mechanisms that we have for creating matter in the early universe
27:21create an equal amount of antimatter.
27:23That symmetry is baked into the laws of physics.
27:27The laws of physics also say that when matter and antimatter meet,
27:33sparks fly.
27:36So matter and antimatter, when they touch, they annihilate.
27:41They just disappear in a flash of energy.
27:44And as far as we understand the earliest moments of the universe,
27:48matter and antimatter were created in equal amounts,
27:50so they should have annihilated,
27:53leaving nothing but energy,
27:56which means no matter, no antimatter,
28:00no gas, no dust, no stars, no galaxies, no life, nothing.
28:04Somehow, matter won the battle over antimatter in the early universe.
28:12In some ways, the universe ignored the rule of symmetry.
28:16Something has to drive the universe off balance.
28:21There has to be a violation of this fundamental balance in our universe.
28:26That way, when the matter and antimatter met and annihilated,
28:31because there was more matter,
28:34there would be a residual of leftover matter,
28:37and there would be no antimatter.
28:40How did the Big Bang break the symmetry between matter and antimatter?
28:44So we're looking for any interaction, any process whatsoever,
28:49where matter behaves slightly differently than antimatter.
28:54We're trying to find a flaw in physics.
28:59We can't look for that flaw directly because we can't see the Big Bang,
29:04but we can recreate it, and we think neutrinos are involved.
29:09This is incredibly complicated.
29:12We are diving deep into the bowels of fundamental physics,
29:17and it is not a pretty sight.
29:21Japanese scientists conducted an experiment called TK-2.
29:26They recreated part of the Big Bang by studying neutrinos
29:31and their symmetrical twin, antineutrinos.
29:35The goal? To see if antineutrinos change their identity or flavor
29:40at the same rate as regular neutrinos.
29:44Matter and antimatter should behave exactly the same,
29:49but we found something very interesting with this experiment.
29:54The particles broke symmetry.
29:57Neutrinos and antineutrinos changed flavor at different rates.
30:01This was a clear-cut example of matter behaving differently than antimatter.
30:07And that has revolutionized our understanding
30:11of the formation of particles during the Big Bang.
30:15What could have happened in the early universe
30:18is that more of the neutrinos converted into matter
30:22than there were antineutrinos decaying into antimatter.
30:26And in this way, you end up with a very complicated
30:30And in this way, you end up with a surplus of matter over antimatter.
30:39Even though that surplus was just one particle in a billion,
30:43it was enough to build the cosmos.
30:47So neutrinos in the early universe could possibly solve
30:51the matter-antimatter asymmetry problem we have.
30:55Yes, they cause destruction.
30:58Sometimes they blow up a star.
31:01But at the end of the day, they did save the entire universe.
31:07Now, scientists hope that neutrinos
31:10may solve one of the biggest mysteries in the cosmos,
31:14the identity of dark matter.
31:25Neutrinos have been around since the birth of the universe.
31:29They may even be responsible for the formation of matter.
31:34Now, we investigate if they play an even larger role
31:38in the development of the universe,
31:41the formation of the cosmic web.
31:46At the very largest scales in our universe,
31:49galaxies are arranged in a very peculiar pattern.
31:53We see long, thin threads of galaxies,
31:57and at the intersections, we see dense clumps of galaxies called clusters.
32:01In between them, we have these vast, empty regions called the cosmic voids.
32:06For a long time, how the cosmic web formed and held together was a mystery.
32:11One of the real mysteries about our existence
32:15is why the universe was able to hold together at all.
32:19All the matter was simply spread apart too sparsely to ever form galaxies or stars.
32:25Instead, something helped to hold it together.
32:29We now think the glue binding the cosmic web
32:33is a mysterious substance known as dark matter.
32:37If it wasn't for dark matter in the very early universe,
32:41there might be no structure at all.
32:45But what is this architect of the universe,
32:49dark matter?
32:52Dark matter is invisible matter that we can't see.
32:56So you, me, all of the particles, everything that we see
33:00is actually only 5% of actual matter in the universe.
33:04The rest is dark matter.
33:07Dark matter is a fancy name for something we don't understand.
33:12What we do know is that there is much more stuff than we can see,
33:16but we have no idea what it is.
33:20It's one of the greatest open mysteries in science.
33:24Dark matter hardly interacts with anything.
33:28A bit like neutrinos.
33:31Also like neutrinos, dark matter was abundant and active in the infant universe.
33:36So could neutrinos and dark matter be the same thing?
33:41We don't know what dark matter is,
33:44but we kind of know how it behaves.
33:47And neutrinos sound like a pretty good candidate for it,
33:50because, hey, they are dark, they are everywhere in the universe,
33:53and they do have a little bit of mass.
33:56And by little, we do mean little.
34:00Neutrinos weigh around 10 billion, billion, billion times less
34:05than a grain of sand.
34:08But neutrinos are also exquisitely abundant.
34:11And so because they're so abundant,
34:15their individual tiny mass can actually add up
34:18to a large diffuse mass on very large scales.
34:29To investigate if neutrinos and dark matter are the same thing,
34:33we must return to the Big Bang.
34:36As the universe expands and cools,
34:38primitive matter forms,
34:41including dark matter and trillions of neutrinos.
34:45The dark matter clumps together,
34:48forming regions of higher gravity,
34:51which pulls in regular matter.
34:54It formed a structure, a scaffolding,
34:57that allowed regular matter to gravitationally begin to come together
35:01and collapse into galaxies, stars and planets.
35:05Could the combined mass of neutrinos in the early cosmos
35:09have produced the extra gravity to help structures form?
35:15Could it be possible that this really is dark matter,
35:19these tiny little particles, but in abundance across the universe?
35:22And we know more, not all,
35:25we know more about neutrinos than we do about dark matter.
35:29But there's still a question around
35:31whether or not neutrinos can be a specific type of dark matter.
35:38To answer this question,
35:41we have to work out what specific type of dark matter
35:44was around in the Big Bang.
35:47Hot or cold?
35:50People talk about hot dark matter and cold dark matter.
35:54And really what you're saying is the speed of the particles themselves.
35:58The cold dark matter is moving slowly
36:01and the hot dark matter is moving fast.
36:04This speed difference is an important clue
36:07to whether neutrinos make up dark matter.
36:10With hot and cold dark matter,
36:13the way they interact with regular matter
36:16has a lot to do with how fast they're going.
36:19So it's a good analogy to think about a river.
36:22With hot dark matter, you'd have a torrent.
36:25Basically it's going so fast it doesn't actually connect with anything.
36:28It just goes right on past.
36:31And then you have relatively slow-moving dark matter,
36:34cold dark matter. Think about a slow-moving river.
36:37A slow-moving river begins to deposit silt.
36:40Think of that silt as the billions of galaxies
36:43that make up the cosmic web.
36:46We observe that galaxies form very early in the universe.
36:49And this is good for cold dark matter,
36:52but it doesn't work for hot dark matter.
36:55So we think cold dark matter is really dominating structure formation
36:58in the early universe.
37:01Think of neutrinos.
37:04They move very fast, close to the speed of light.
37:07This is a problem with neutrinos
37:10because neutrinos would be hot dark matter.
37:13That rules out neutrinos as cold dark matter.
37:17The idea that neutrinos are dark matter
37:20hit another setback when we weighed the universe.
37:25If you add up the total mass of all the neutrinos in the universe,
37:28it would wind up being about a half a percent to 1.5 percent
37:32of the total mass of dark matter.
37:35Neutrinos were a good candidate for dark matter
37:38because they exist.
37:41And they're very shy, just like the dark matter particles are.
37:45But then we were able to measure more accurately
37:48how much dark matter there is and how much neutrinos there are.
37:52And there's just way less neutrinos than there's dark matter.
37:55Neutrinos do have mass, and there are a lot of them out there.
37:59So it might be some tiny, tiny fraction of dark matter
38:02is made up of neutrinos.
38:05But we know that these things do not make up the bulk of dark matter.
38:08It must be something else.
38:11So neutrino scientists hunt for a different contender for dark matter,
38:16a completely new kind of neutrino.
38:19We know about three fluid types of neutrinos.
38:22The electron neutrino, the muon neutrino, and the tau neutrino.
38:27But there could be a hidden fourth flavor of neutrino
38:32that could solve the riddle of dark matter.
38:39We call this a sterile neutrino.
38:42So-called because they interact even less than regular neutrinos.
38:47A particle so tiny, so hard to detect,
38:50could actually turn out to have lots of the secrets
38:53wrapped up inside it as to how the universe works.
38:58The first step to find out if sterile neutrinos are dark matter
39:03is to prove they exist.
39:06And that's tough.
39:09Even though sterile neutrinos are almost impossible to detect,
39:12we can still hunt for them.
39:15Back in the day, neutrinos were also said to be difficult to detect.
39:19Trying to find dark matter,
39:22trying to find these sterile neutrinos,
39:25it's almost like using one invisible, undetectable thing to find another,
39:28using a ghost to find a goblin.
39:31We are definitely pushing the limits of science.
39:35A team at Fermat is working on a new experiment
39:39A team at Fermilab has an ingenious idea.
39:43They can't spot sterile neutrinos directly
39:46because they don't interact with atoms in the detectors.
39:50So they're looking for neutrinos as they change flavor
39:54into sterile neutrinos.
39:57We know that normally neutrinos change type as they move through space.
40:01But they have to move far enough before that change happens.
40:04So tracking neutrinos over a short distance
40:08shouldn't show any flavor changing.
40:11In this experiment, they've constructed only a half-mile long path.
40:15It's not enough time for the neutrinos to change flavor in the normal way.
40:19If they do see something, if they see something change,
40:23this could be some interesting aspect,
40:26perhaps evidence for sterile neutrinos.
40:29So is it possible that over short distances,
40:31regular neutrinos can oscillate into this sterile neutrino?
40:40The team shoots beams of muon-flavored neutrinos along the detector.
40:47In theory, they won't have time to change flavor.
40:55We can see whether or not these muon neutrinos
40:58morphed into a different type of neutrino.
41:02They shouldn't change.
41:05But if they do, that points us towards sterile neutrinos.
41:11The team compare the number of muon neutrinos reaching the detectors
41:16to those fired along the beam.
41:20Fewer muon neutrinos hit the detectors.
41:24Some neutrinos had changed color.
41:26Some neutrinos had changed flavor.
41:31So we are seeing that oscillation of neutrinos
41:35changing from one type to another.
41:39We had an idea of how many we should have seen,
41:43but we're seeing more, and that could be sterile neutrinos.
41:49If sterile neutrinos do exist, would they be dark matter?
41:54Right now, we don't know the mass of the sterile neutrino.
42:00But if it's heavy enough, it could be a contender.
42:06If it exists, it's prevalent enough
42:09to account for all the dark matter in the universe.
42:12Fermilab's results haven't been verified by other scientists.
42:17So it's too soon to say definitively that sterile neutrinos are real.
42:24Or that they make up dark matter.
42:27Dark matter is probably one of the biggest questions of our time.
42:31And the fact that Fermilab may be one of the places to answer that question,
42:38and the fact that I am working here, is really fantastic.
42:42Because we're attempting the impossible.
42:46We have to wait to see if the impossible is possible.
42:53We know neutrinos have played a vital role in the history of our universe.
42:59And even now, they refresh it by powering supernovas.
43:07Without them, our sun,
43:10our world,
43:13and even our bodies would not have formed.
43:17Neutrinos are pesky little particles,
43:20super elusive, difficult to study,
43:23but when you can catch them, they offer secrets to the universe.
43:29The story of neutrinos has been really interesting.
43:32It's like reading a book and you think you're on the last page,
43:35and then you turn it and suddenly there's a hundred new pages.
43:38Neutrinos are teaching us that the universe is in many ways subtle,
43:42and hard to figure out.
43:44And the more we learn about these things,
43:46the more we learn about the universe.
43:48Neutrinos are the universe's great escape artists,
43:51the Houdini of particles.
43:53In fact, they may have helped us to escape the Big Bang and end up existing.
43:58At the end of the day, they're what saves us.
44:01The more we understand these elusive particles,
44:05the more we can gain insight into how the universe works.
44:10So, it's really cool.