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Documentary: Why Our Universe Is Expanding
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00:00As the 20th century came to a close, two independent teams of astronomers employed
00:12telescopes around the Earth and observatories in space to study supernovae in distant galaxies.
00:24Each team sought to measure how gravity was slowing the expansion of the universe.
00:30They made a shocking revelation. The expansion was speeding up and an unknown force was behind it.
00:41The 20th century birthed the physics revolution.
00:45We've worked for a century to test it. Einstein's relativity is holding up.
00:51Scientists are unraveling the Big Bang.
00:54It's an explosion of space itself.
00:57And asking new questions of our cosmos.
01:00We're still not entirely sure how supermassive black holes form.
01:03But we've been able to detect that they exist in pretty much every galaxy.
01:06New instruments are confirming old theories.
01:10It has taken about 50 years for us to build an instrument that's capable of registering gravitational waves.
01:16And shedding light on the ultimate fate of the universe.
01:21We may be sailing off on an infinite journey of expansion.
01:25With each piece of the puzzle, the mysteries deepen.
01:29We have defined the boundaries of our ignorance. And that's a very exciting place to be as a scientist.
01:34Of course.
01:36Before we leave the show, western
01:45We know continuer our next agenda.
01:50You will never visit our neoge-sì´ˆ before the ensang.
01:54Humankind has forever sought to understand the forces that govern the heavens and the
02:13world around us.
02:17Generation after generation, ideas were proposed, tested, revised.
02:24The shoulders of giants grew tall, building a vantage point with which to see reality.
02:39In the 17th century, an English polymath assigned mathematical theory to his observations.
02:47Newtonian physics was a revelation.
02:50It changed the separation between humanity and the cosmos and revealed not just that we
02:57were connected, but that we could understand it.
03:01Isaac Newton published the first volume of his Principia Mathematica in July 1687.
03:09Within its pages, he detailed an equation for gravitational attraction.
03:16Everything in the universe pulls on everything else.
03:21Particles of matter attract each other with a force directly proportional to their masses
03:27and inversely proportional to the square of the distance between them.
03:31We were able to predict accurately where planets were going to be in the future, where they
03:36were in the past.
03:37We were able to find planets through the perturbations of that planet on another one.
03:43After William Herschel discovered Uranus in 1781, astronomers noted its curious path across
03:51the sky.
03:53Employing Newton's version of gravity, the planet's motion seemed to be influenced by a distant body.
04:00European astronomers and mathematicians got together and they calculated that if there
04:06was an existing planet outside of Uranus, then this would explain these funny movements.
04:13Two astronomers working independently, each calculated the hypothetical planet's location
04:19in the night sky.
04:21A third trained his telescope toward that position.
04:25Collectively, they discovered Neptune.
04:32But there was one planet which refused to play by Newton's rules, Mercury.
04:42Tracing Mercury's orbits, its closest point to the Sun, its perihelion, moves ever so slightly
04:49every single year.
04:51The phenomenon is known as precession.
04:54Mercury's orbit never quite fit correctly with the classical model.
05:00It could not possibly be explained with Newton's law of gravity after we had observed it and
05:06seen the perihelion advance.
05:09According to the Sun hypothesis, there had to be another planet, even closer to the Sun,
05:14perturbing Mercury's orbit.
05:17The mystery planet was christened Vulcan, after the Roman god of the forge.
05:24While some hunted for Vulcan, others sought a new mathematical theory to explain Mercury's
05:30strange behavior.
05:34In 1915, a German physicist presented a new take on gravity.
05:46Einstein's relativity absolutely transformed the way we think about the universe.
05:52To step out of this classical world of gravitation and be able to just turn that completely upside
05:58down and be able to come up with a mathematical formula, which is actually quite simple.
06:05General relativity treats space as deformable.
06:08Mass, from a grain of sand to the entire Earth, bends space.
06:16And all natural bodies moving through the universe will follow those curves.
06:22The Sun doesn't pull on the Earth.
06:25The Sun sits at the bottom of a gravitational well.
06:29And the Earth falls around it, traveling just fast enough that it doesn't spiral inward.
06:35In relativity, we saw a sweeping away of the old and ushering in of a new paradigm.
06:43By applying the novel equations of general relativity, Mercury's quizzical orbit was finally explained,
06:50and its precession accounted for.
06:53Vulcan was no longer necessary.
06:59However, Einstein's theory would need a further demonstration to supplant Newton's work, which
07:06had reigned supreme for over 200 years.
07:11Einstein had predicted the presence of a massive object would bend the way light passed through
07:17space.
07:20An upcoming solar eclipse was set to darken the southern Atlantic on May 29th, 1919.
07:29A team of astronomers led by Arthur Eddington seized the opportunity to test Einstein's theory.
07:37The Eddington mission in 1919 was to measure the deflection of a star's light as it passed
07:45close to the Sun.
07:48If Einstein was correct, a group of stars would appear in a slightly different part of the sky,
07:55their light perturbed by the Sun's gravity as it passed in front.
08:00And in 1919 the eclipse was special, because this eclipse happened projected against the background
08:08of the so-called Hyades star cluster, a small concentration of stars that were the best studied
08:16stars to date.
08:20A first base of observation was established in Brazil, and Eddington positioned himself
08:26on an island off the coast of West Africa.
08:33The moon's shadow charged across the Atlantic.
08:37Each station only had minutes to capture the eclipse on photographic plates.
08:46Returning to England, both teams announced their findings.
08:52Einstein was right.
08:53This experiment was incredibly successful, and the deviations in the positions absolutely
08:59confirmed Einstein's predictions.
09:07Scientists have continued putting general relativity to the test.
09:11It keeps passing.
09:15For several centuries we've worked with Newton's gravity.
09:19Then came along Einstein with relativity and showed that in certain extreme situations
09:26Newton's gravity doesn't really work well.
09:28Einstein was able to explain all of everything we saw.
09:32Even to this day, we've still not been able to go against his theories of gravity.
09:37Now is Einstein's relativity the final answer?
09:42We've worked for a century to test it.
09:46Einstein's relativity is holding up, precisely.
09:49I find that astonishing.
09:51It all came back to a simple desire to understand and have a more complete picture of our universe.
10:03While we can still use Newtonian mechanics in our day-to-day lives, Einstein's revolutionary
10:12notion of gravity is required to understand the universe.
10:15We need to look out to places where we have extreme densities, temperatures, and things
10:23are very dynamic.
10:26That's where we will really be able to refine our ultimate theory of gravity.
10:34To study the heavens, astronomers capture light in telescopes.
10:50That light is composed of a spectrum of colors.
10:53We can understand the fundamental makeup and dynamics of something by taking its light and splitting
11:01it up into its component colors.
11:04Unique to each object, this combination of colors represents the chemical fingerprints of
11:11astronomical phenomena.
11:14But to understand how a star or a planet could have such a fingerprint, scientists needed
11:22to surpass the classical understanding of the universe.
11:26They needed to reconceptualize the very concept of matter.
11:32The ancient Greeks envisioned the atom as indivisible, the most fundamental building block of substance.
11:41In the 19th century, scientists discovered it was far more complex.
11:46As our ways to study materials advanced in the past 200 years, the structure of the atom
11:53is no longer just one homogeneous thing, which is the smallest thing in the world.
11:57The atom was instead demonstrated to be comprised of several components, a nucleus where most of
12:04the mass was contained with an electron that would orbit around.
12:11But according to classical Newtonian physics, the electrons in a planetary model would spiral
12:17down toward the nucleus, destroying every atom in existence.
12:25In 1913, the Danish physicist Niels Bohr introduced his model of the atom.
12:38The model relied on a new explanation of reality that had recently been developed, quantum physics.
12:48quantum physics is the idea that nature is divided into discrete but very much differentiated bits.
12:57It's trying to explain the very smallest scales of the universe.
13:01The tiny bits of atoms and how they behave, the weirdness and behavior of chemicals, and
13:07basically every tiny, tiny thing in our universe.
13:11In this re-envisioned universe, electrons are forced to orbit around their nucleus at precise
13:17distances, like rungs on an unevenly spaced ladder.
13:22The size of those rungs dictates the exact colors of light an atom can absorb or emit.
13:30Each element has its own arrangement.
13:33We have a very outstanding challenge in trying to visualize and explain what the quantum world
13:38actually is.
13:43For decades, astronomers had been cataloguing stars based on the missing colors in their
13:49rainbow of light, what they called spectra, and matching the missing colors to certain elements.
13:58If we look at a spectrum and we see certain wavelengths missing, we can tell which element in the star
14:07was responsible for absorbing that light.
14:12Scientists could utilize spectra to identify chemical compounds throughout the cosmos.
14:19The ghosts of unimaginably distant objects like quasars and pulsars can be made to tell
14:24their stories by manipulating the light that they emit.
14:30In the 1920s, quantum physics evolved.
14:36The particles within an atom were revealed to also act like waves.
14:41The electron is not in a position around the atom.
14:45It is rather spread out.
14:46There is a cloud of possible locations.
14:53It is not in a position around the universe became uncertain.
14:56And that uncertainty helped solve a long-standing mystery of how our sun was able to shine.
15:04Under classical physics, hydrogen nuclei in our sun's core can't get close enough for
15:10long enough to undergo fusion.
15:13It simply isn't hot enough.
15:16But under quantum physics, hydrogen clouds can.
15:20The chance of fusion is astronomically tiny, but it's enough.
15:28Combined, the theories of relativity and quantum physics have allowed us to explain the intricacies
15:34of the universe.
15:39Once employed in tandem, they failed to provide any answers.
15:48These are two theories of the way things behave, which are mutually exclusive.
15:52They don't sit together, and yet they both work very, very well.
15:56When we try and describe big things with quantum mechanics, it makes no sense.
16:00When we try and describe small things with general relativity, that doesn't work either.
16:06But gravity, as we currently understand it, cannot operate within a quantum realm.
16:12There must be a theory out there that we haven't grasped yet.
16:19Maybe it's a new law of physics, maybe it tweaks these laws, and maybe it's something else.
16:23Without a unified theory, we'll never be able to truly comprehend the beginnings of our
16:29universe.
16:3413.8 billion years ago, reality as we know it was born.
16:40All of the material that forms our universe at a single point, that's the Big Bang.
16:48An infinitesimal spark became an infinite cosmos.
16:54It's not so much an explosion into space, it's an explosion of space itself.
17:03As the universe grew to an unfathomable size, stars began to shine and galaxies coalesced.
17:16It's not so much an unfathomable size.
17:17It's not so much an unfathomable size.
17:18It's not so much an unfathomable size.
17:22But not the earliest points of our universe's history.
17:29When we turn back the clock, a cosmic barrier arises 380,000 years after the Big Bang.
17:38Before that moment, in the first few hundred thousand years, our universe was so hot and dense,
17:42atoms were ionized and they were a plasma.
17:44Any radiation a particle emitted would immediately be absorbed by another.
17:51And it acted like a fog.
17:53There was no way for the light to travel.
17:56It wasn't until the universe had expanded and cooled sufficiently that that radiation could start flowing freely.
18:02The atoms recombine, the fog clears and the light travels to us.
18:07And has traveled towards us essentially unimpeded since that moment.
18:12The oldest light is known as the cosmic microwave background.
18:18It wasn't discovered with our eyes, but rather our ears.
18:25In 1964, two astronomers at Bell Laboratories were testing their radio equipment.
18:37But something was interfering with the signal.
18:41Maybe it was caused by something in the Earth's atmosphere and they spent a long time trying to remove it from their data.
18:47They went to all sorts of lengths to account for and remove the effects of this static.
18:54They thought it was actually bird poop they were looking at and had to clean it out because pigeons nested in their telescope.
19:01They didn't call it bird poo in the paper that they published.
19:03They called it a white dielectric material.
19:09They had no idea what was causing it.
19:13It wasn't clearly coming from anything that we knew.
19:16And it was actually called Little Green Men for quite a while because, you know, what else could it be?
19:24They were able to talk to a physicist, Hans Beter, who had some understanding of what might be going on.
19:30And they were able ultimately to conclude that the static that they were hearing was actually the cosmic microwave background,
19:37the relic radiation from the Big Bang.
19:39Over the decades, space-based observatories have mapped this signal, which exists everywhere in the sky with eerie uniformity.
19:52One of the very earliest measurements was made by a satellite observatory called COBE, the Cosmic Background Explorer.
20:09It made an incredibly famous measurement of the temperature of the microwave background.
20:20It is one of the most precise, most pristine measurements in all of astronomy, maybe in all of science.
20:28The remnant heat of the Big Bang was predicted to be a few degrees above the coldest possible temperature,
20:35roughly minus 270 degrees Celsius.
20:40The COBE mission confirmed that prediction.
20:43Our universe had a beginning and had been expanding ever since.
20:48The cosmic microwave background is actually pretty constant all over the universe.
20:53It implies that it was all produced in the same place.
20:56And this is what enabled the cosmic microwave background experiments to confirm the Big Bang theory.
21:05Following COBE, further missions exposed hidden structures of an infant universe,
21:11where gravity could act to form stars, galaxies, and the Earth itself.
21:19However, all that we've learned from the cosmic microwave background only takes us so far back.
21:26The 380,000 year barrier. Any earlier, and we have to rely on scientific theories and models.
21:35We can use Einstein's relativity to work our way back in time.
21:41Quantum physics also comes in to describe how the particles would interact in this universe,
21:47which becomes amazingly small and amazingly concentrated.
21:51But at the earliest moments, when all cosmic energy was contained in a nearly infinitesimal point,
22:00theory fails us.
22:02A tiny fraction of a microsecond away from the Big Bang,
22:08our equations actually break down.
22:12We don't have a theory that works all the way into this extreme regime.
22:20Beyond our beginnings, there are other places throughout the universe,
22:25where physics appears to be broken.
22:28The most extreme objects in our universe are invisible.
22:38A black hole is a region of space-time which is so dense and has such strong curvature
22:47that nothing, including light, can escape from it.
22:52Fundamentally, it's what happens at the end of the lifetime of a very, very massive star.
23:00For the largest stars, life ultimately ends with a gravitational implosion.
23:07A supernova.
23:09The mass that's left behind won't have any fuel to sustain it.
23:13It all crumples together into an infinitesimal point.
23:17A black hole.
23:19There is no surface.
23:21Only an edge past which you could never retreat.
23:25Trapped for eternity.
23:27The idea of a hole is very apt.
23:31You would look up and the universe that you had just left
23:35would be an ever-narrowing well of light above you.
23:40The notion emerged from Einstein's equations.
23:52However, black holes spent decades as mere mathematical oddities,
23:58not real-world phenomena.
24:01In 1971, astronomers identified an intense X-ray source dubbed Cygnus X-1.
24:13It lined up almost perfectly with a known star,
24:17but the star was incapable of generating those X-rays.
24:21By 1973, a consensus had been reached.
24:26The star was orbiting a black hole.
24:32The very next year, a source of intense radio waves
24:36was identified in the heart of the Milky Way.
24:39The phenomenon, Sagittarius A star,
24:42was best explained by the presence of a black hole.
24:46It is not the black hole itself that emits the radio waves,
24:53but matter in a disk surrounding it, spiraling downward.
24:59As it gets ever closer to the black hole,
25:01its orbital speed increases and eventually becomes close to the speed of light.
25:05In doing so, it's constantly bumping and jostling against its neighbors.
25:09It is a tremendous amount of warmth, of heat, of light.
25:14While Sagittarius A star itself could not be resolved by our telescopes,
25:19astronomers could spy on its stellar neighbors.
25:23We've been able to measure its presence
25:25and we've been able to measure its mass,
25:27not because we can see it directly,
25:29but because we can measure its gravitational influence
25:31on the stars that orbit very close to it.
25:34The black hole at the center of our galaxy
25:37has a mass roughly 4 million times that of our Sun.
25:42There is a particular breed of black hole
25:47that we call the supermassive black holes.
25:51Most galaxies, if not all galaxies,
25:54contain at their very centers a supermassive black hole.
25:58So some of the biggest galaxies have black holes in them
26:01with 20 or 30 billion times the mass of the Sun,
26:06which is astonishing.
26:10We're still not entirely sure how supermassive black holes form,
26:14but we've been able to detect that they exist
26:16in pretty much every galaxy.
26:19While indirect data suggested black holes
26:24were a part of our reality,
26:26capturing visual evidence seemed nearly impossible.
26:30Because radio wavelengths are the longest amongst all types of light,
26:36they require much larger telescopes to capture.
26:41To image a supermassive black hole in another galaxy
26:45would require a radio telescope as big as our planet.
26:50We'd love to build a telescope the size of the Earth,
26:52but we can't do that.
26:54Nevertheless, humanity accepted the challenge.
26:58An international collaboration of organizations and observatories
27:03combined their efforts to birth the Event Horizon Telescope.
27:08We use several telescopes sighted around the world
27:17and we use them as a team to make a really good picture.
27:20You have large radio telescopes
27:23that are situated in varying places on the planet.
27:27You can combine those signals and you can simulate a radio dish
27:31which is the physical size of those separations.
27:35Beginning with three observatories in 2009
27:39and expanding to eight by 2017,
27:43the Event Horizon Telescope set its sights
27:46on the center of Messier 87,
27:49a galaxy 55 million light-years from Earth.
27:54The data collected by each telescope
27:57was fed into a supercomputer.
28:00Each individual telescope,
28:03its story is combined with the others in this computer
28:06to create one incredible, clear, revealing picture.
28:12When the images were finally processed,
28:15they did not expose the black hole itself,
28:18but rather its shadow.
28:21Measuring 100 billion kilometers in diameter,
28:25nestled within a supermassive black hole,
28:28six and a half billion times more massive than our sun.
28:32We see at a region where time stops.
28:35This is a very different part of the universe
28:38that we are seeing for the very first time.
28:41While scientists cannot explain the intricacies of black holes
28:45until we find a theory that unites physics,
28:48they have developed new methods to study them.
29:00Waves of light are not the only ripples that traverse the cosmos.
29:04According to general relativity,
29:07space itself can also ripple.
29:10Gravitational waves are caused by mass accelerating through space.
29:16Einstein first predicted gravitational waves in 1916.
29:22But the size of these waves was going to be so small
29:25that he never thought they would be detected.
29:31Only the most extreme events in the universe
29:34create measurable gravitational waves.
29:37When it got to the point that black holes were no longer ridiculed,
29:44the physicists started saying,
29:46maybe we can really seriously think now about building a detector.
29:53We would need a laboratory capable of isolating distortions of space
29:58smaller than an atom.
30:00It has taken about 50 years for us to build an instrument
30:04that's capable of registering gravitational waves,
30:07the shaking of space itself.
30:10In 2002, the Laser Interferometer Gravitational Wave Observatory,
30:16better known as LIGO,
30:18began its search for evidence to support Einstein's proposition.
30:23We detect gravitational waves using instruments called interferometers.
30:28They're shaped like an L.
30:31We shine a laser from the centre of the L down the two arms.
30:37After travelling four kilometres down either arm,
30:41the laser light strikes a mirror and returns.
30:47The principle is that if a gravitational wave
30:50is coming from out in space and passes into the detector,
30:55then it will alternately stretch one arm and then the other arm.
31:01And we'll see this in the interference pattern of the laser beams
31:05when they're recombined.
31:10To ensure a signal is genuine, LIGO built two detectors
31:14on opposite sides of the United States.
31:17Both stations were silent for years.
31:20But following an extensive refit, a signal was detected.
31:29When two black holes orbit around each other,
31:32they lose energy by the emission of gravitational waves.
31:36They enter a death spiral, orbiting faster until they collide.
31:41Ladies and gentlemen, we have detected gravitational waves.
31:52We did it.
31:56Einstein's theory dictated the event.
31:59And both LIGO signals matched it.
32:02We detected the merger of two black holes.
32:06A collision 1.3 billion years old.
32:11Once we had the first one,
32:12it became obvious that we were going to get more and more of them
32:16as we increased the sensitivity of the detectors.
32:22LIGO witnessed another type of collision two years later.
32:27The second really major detection for LIGO
32:31was a binary neutron star collision.
32:34When they collide, they give off all kinds of signals.
32:39Unlike a black hole merger,
32:41a neutron star collision could be witnessed
32:44not just with a gravitational wave detector,
32:47but optical telescopes as well.
32:50We were fortunate to be able to localize
32:54the area in the sky for that signal to a very small patch.
32:59We sent it out to the astronomers
33:01and this created an unprecedented avalanche
33:04of telescopes, satellites around the world
33:08redirecting their programs of observing.
33:11That had a flow-on effect.
33:12Everybody could image it in radio, x-ray and so on.
33:16And this gave us an enormous amount of physics
33:19that we didn't previously have.
33:23Humanity entered an era of multi-messenger astronomy.
33:26Studying the same object using a combination of information sources.
33:31And employing multiple phenomena to confirm, enhance or contradict
33:35our previous notions of the universe.
33:37Whether it's radio waves, light you can see with your eyes, x-rays,
33:46it requires a light particle that travels through space.
33:50If we are wrong about our view of light,
33:53everything in our universe is essentially wrong.
33:55So gravitational waves offer an independent way of measuring the universe.
34:01It gives you a holistic picture of what is actually going on out there.
34:06The more messengers you can use, the better informed you are about how the universe works.
34:13Messengers that come from traditional ways of viewing the sky,
34:17that come from particles, using all of the available information.
34:21That is how astronomy is progressing and big collaborations are being formed
34:26between specialists in the different disciplines.
34:30It's a bright future for astronomy.
34:33More powerful gravitational wave detectors could reveal the earliest moments in the cosmos.
34:40While the oldest light takes us no further than 380,000 years after the Big Bang,
34:46the oldest gravitational waves would reveal our history up until a tiny fraction of a second after the universe's creation.
34:57I look forward to that with great interest because it may turn out that we have got it all wrong.
35:04And when we look at the gravitational waves, perhaps they will tell us a different story from what we are expecting.
35:08In the 1920s, humanity realised that our Milky Way galaxy was just one of many, millions and even billions of light years away.
35:26Other galaxies nested together in clusters, slowly rotating around one another.
35:35A decade later, Swiss astronomer Fritz Zwicky had a mystery on his hands.
35:42He had been observing the Coma Cluster, a group of over a thousand galaxies stretching 10 million light years across.
35:49Given their collective mass and gravitational influence, some of the galaxies were moving so fast that they should have broken free.
36:00The gravity from the galaxies that we see is not enough.
36:04There must be more gravity to allow the cluster to hold together.
36:07Otherwise, the galaxies would have actually whizzed off into space and we wouldn't have clusters anymore.
36:12Several decades later, studies of individual galaxies pose the same problem.
36:19Just as all the bodies in our solar system orbit a central point, all the stellar systems, gas and dust in a galaxy, rotates around the galactic core.
36:31In the 1960s, Vera Rubin and Kent Ford began charting these rotation rates,
36:37measuring how fast material within a galaxy spun relative to its distance from the center.
36:44Spinning matter in our universe operates under a principle known as conservation of angular momentum.
36:53The total amount of mass and the distribution of that mass dictates how quickly an object will rotate.
37:00You could do this right now. You could spin around in your gesture and test it that as you move your arms out, you slow down and as you move your arms in, you speed up.
37:09Planets in our solar system are rotating exactly based on gravity, based on the sun's mass and the mass of the planets.
37:17But instead of slowing down at the outer edge of a galaxy, Rubin noticed that these stars move just as fast as those much closer in.
37:26Vera Rubin showed that the stars in a galaxy are not rotating like the planets in our solar system.
37:33They're rotating as if there's much more matter than we can see in the galaxy.
37:38And so either our laws of physics are wrong, but they kind of work in every other case, or there's another type of mass that we've never discovered.
37:45Zwicky called this invisible matter, dunkel materia. We know it today as dark matter.
37:57We call it dark matter because it doesn't emit light and it doesn't absorb light at all.
38:02So the only way it interacts with other material is via gravity.
38:07Dark matter forms huge structures across the universe, existing as gravitational scaffolding.
38:15We see its effects in photographs.
38:18Looking at galaxy clusters, the dark matter will distort the images of any galaxies behind it.
38:25This effect is known as gravitational lensing.
38:32The gravitational lensing is a really awesome effect in the universe where if you have a massive object, the light will get bent around.
38:40Just like a magnifying glass or like a refracting telescope.
38:44That magnifies the light of the background galaxy and distorts it.
38:47After accounting for the lensing caused by regular matter, astronomers can map the distribution and concentration of dark matter.
38:58At current measurements, it appears to be five times more abundant than all visible matter.
39:04There is a lot of mass out there in the universe that we just cannot see with our eyes that doesn't emit light, but interacts in every other way we measure it.
39:13Scientists continue to hunt for an explanation of dark matter.
39:19Dark matter could be made of particles that we haven't yet discovered.
39:24And we're looking in places like the Large Hadron Collider, smashing particles together with high energy to try and spew out these amazing particles that perhaps we haven't discovered.
39:34One hypothetical type of dark matter could be detected by studying neutrinos, the smallest and least interactive particles known to physics.
39:46So neutrinos are kind of the awkward cousin in the family.
39:51If dark matter is another type of particle that we haven't seen, as dark matter collides with itself, it should give off neutrinos that can travel through space, that we can see here on Earth, that we can measure and collect in a lab.
40:05Neutrinos are in the same family of subatomic particles as electrons, but they have no charge.
40:12They're cosmic ghosts, interacting with matter so irregularly that trillions of them are streaming through your body this very moment.
40:20There are some intriguing suggestions that come actually from deep underground experiments where you build particle physics detectors which are seeing things that can easily pass through the Earth.
40:36We go to the bottom of mines or underneath mountains to use the rock above as a shield so that we can have a better chance of seeing this rare collision with our detector.
40:49The internationally funded ice cube experiment at the South Pole is the largest neutrino detector in the world.
41:00A cubic kilometer in size, buried over two kilometers below the Antarctic ice, turning one billion tons of naturally frozen water into its own personal neutrino capture.
41:14Across two years, IceCube indirectly detected 28 neutrinos. However, none were attributed to dark matter.
41:24Experiments of that kind have given some weird hints that just maybe we might be getting close to finding whatever particle it is that makes up dark matter.
41:35No matter.
41:36Meanwhile, NASA's Fermi Space Telescope hunts for the signatures of dark matter from low Earth orbit.
41:46We think that a type of dark matter forms antimatter and matter, and it can form and then collide together and annihilate itself.
41:55Such an annihilation would create gamma radiation, which Fermi is designed to detect.
42:01So far hasn't detected them, but it's getting very close now.
42:04Until scientists find solid evidence, the lack of detections constrain the possibilities of what dark matter could be.
42:14We're narrowing the search field.
42:17Another possibility is that dark matter could not exist. It could be that we've just got the maths wrong.
42:24And that's perhaps even more intriguing and even more worrying.
42:29However, another search for the invisible leads to an even larger mystery.
42:34It was astronomer Edwin Hubble who revealed to the world that our galaxy was just one of billions in 1924.
42:50Less than a decade later, he announced another revelation.
42:55The universe was not constant in size. It was expanding.
42:59Over the decades, astronomers settled upon the Big Bang theory, and that the expansion set forth at our origins would slow under the power of gravity.
43:13The expansion of the universe originated in the Big Bang, but everybody thought that because the universe is full of matter, galaxies would, by their mutual gravitational pull, slow down the expansion.
43:25Eventually, the universe would begin to collapse. The race was on to calculate when.
43:32People were trying to set out how fast the universe was slowing down. When was the universe going to stop and collapse back on itself?
43:40In the 1990s, two international teams began measuring the expansion, utilizing some of the brightest phenomena in the known universe.
43:50The teams compared the age of particular supernovae with how quickly they were moving away from Earth.
44:06By 1997, the teams had collected a wealth of data, stretching back to half the age of the cosmos. The next year, they went public with their conclusions, with a result that defied the known laws of physics.
44:22The universe is not slowing down in its expansion. It is accelerating. And it seemed for a while that this must be a fluke of the measurements. Something wrong with the observations.
44:35Imagine the exact opposite answer that you are told to go and find. That is a big test of your scientific process and two independent teams found the exact same thing.
44:47We now know that the observations are absolutely secure. You can look at the universe in many different ways and you see the same phenomenon.
44:54In June 1998, the phenomenon was given a name. Dark energy. At the turn of the millennium, a supernova hiding in Hubble Space Telescope data added to the complexities of the universe.
45:12Ten billion light years from Earth. It was the oldest found up to that point. It revealed that billions of years ago, the universe's expansion was as physics had originally predicted.
45:27Dark energy was weaker than it is today. But before our sun and all its planets were born, dark energy began winning a cosmic tug of war.
45:42Pushing galaxies apart with more strength than gravity was pulling them in.
45:52The mystery of dark energy tantalizes cosmologists.
45:57I think the most exciting breakthrough in cosmology is the discovery that the universe is accelerating.
46:04So what's causing it? Particle physics says that there is an energy of the vacuum itself.
46:09In fact, if that had been the dark energy, the universe would have ripped itself apart, probably within the first second of its existence.
46:17It's often been described as the worst prediction or mismatch in history.
46:21The European Space Agency's Planck Telescope hunted for pieces of the dark energy puzzle, imaging the cosmic microwave background, the oldest obtainable light.
46:38This data in hand, astronomers calculated the exact composition of the universe.
46:56The exact composition of the universe.
46:59Dark energy comprises roughly 68% of all there is.
47:04Dark matter makes up another 27.
47:07Only 5% of the cosmos is matter we can touch or light we can see.
47:13But it's even worse than that.
47:18Most of the atoms in the universe will never even fall into a galaxy and reveal themselves.
47:23Bits that have made it into galaxies, that's basically 1% of all of the universe.
47:28But so far as we're aware, we're the only bit of the universe that has looked out and wondered about what a small bit of this universe we are.
47:36To understand dark energy is to know the ultimate fate of the universe.
47:47Our current understanding from dark energy is that we may be sailing off on an infinite journey of expansion and the universe would last forever.
47:56According to Einstein's relativity, that would be a runaway process where the expansion accelerates forever.
48:07More and more and more.
48:09Every star will at some point disappear, be infinitely far away from every other star.
48:15Every particle will decay eventually.
48:18It looks like the universe will come to an end as a cold, dark, lonely place.
48:28So far better to be alive now than maybe in 40 billion years time.
48:34Missions to probe the mysteries of dark energy are already in motion.
48:40There's different ways of measuring dark energy.
48:42We can see exploding stars.
48:43We can use clusters of galaxies.
48:45We can use how light bends around some galaxies.
48:50At the Vera Rubin Observatory in Chile, astronomers are embarking on a decade-long venture to map the sky deeper than ever before.
49:01Capturing 20 terabytes of data each night and cataloging 37 billion stars and galaxies.
49:09And we use those results as independent cross-checks of the other probes.
49:14And we're trying to pinpoint that intersection of what dark energy could be now, which tells us what dark energy actually is.
49:20Reality's greatest mystery is a test of patience.
49:23For something that didn't exist in 1997 to 20 years later, having hundreds of people around the world trying to discover it just shows how big of a question it is to understanding the nature of our universe, the beginning of our universe, and the future of our universe.
49:38We have defined the boundaries of our ignorance, and that's a very exciting place to be as a scientist.
49:53Humanity's perception of reality is ever-evolving.
49:56Our view of the universe was dramatically different 30 years ago, 50 years ago, and 70 and 100 years ago.
50:04And it's amazing to think what our view of the universe will be in 10 years, how much more we discover that we didn't know.
50:10The vast, unknown reaches of the cosmos are begging to be explored.
50:1595% of the universe is made up of completely mysterious dark matter and dark energy.
50:23We don't know what makes this up. We don't know if our picture of the universe is simply wrong.
50:28It's a never-ending series of amazing discoveries about what is possible.
50:34It challenges our notions of who we are, where we come from, where we'll go to, how precious our own planet is.
50:42The study of the universe, to me, it impacts humanity on practically every level.
50:48And that's why I find it endlessly inspirational.
50:51Our quest for answers is an essential part of what makes us human.
50:57The universe created planets, created life, and created people who can ask all of these questions.
51:05That's just marvellous.
51:07We hope to one day understand our place.
51:13We are just a vanishingly small fraction of the universe, but so far as we're aware, we're the only bit of the universe that has looked out and wondered about what a small bit of this universe we are.
51:24We are just
51:27A
51:40You know, oh, oh, oh, oh, oh.
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