State of the Universe
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00:00
Particle physicsSchmidt cameraAngeregter ZustandUniverseDark matterYearGentlemanLecture/Conference
00:40
Hull (watercraft)AtomBrightnessStarNoise figureRedshiftInertial navigation systemUniverseNeutrinoAngeregter ZustandPhotonicsRedshiftDose (biochemistry)Cosmic distance ladderYearYearCylinder headStarContinuous trackCogenerationPlanetBig BangStonewareRRS DiscoveryHubble's lawSpare partGalaxyVideoRadarmeteorologieVisible spectrumThrust reversalSurface acoustic waveLecture/ConferenceMeeting/InterviewEngineering drawing
03:30
Scale (map)SEEDOrbital periodUniverseMicrowaveBig BangQuantum fluctuationQuantumCosmic microwave background radiationSoundHyperbelnavigationGalaxyGround (electricity)Meeting/InterviewComputer animation
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Cosmic distance ladderGalaxyBig BangUniverseHubble's lawCosmic distance ladderYearThrust reversalNoise figure
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MinuteAstronomerHot workingClassical mechanicsAutumnPagerDaySunlightYearUniverseFACTS (newspaper)KopfstützeHubble's lawLightRoll formingField-effect transistorAstronomerBig BangGenerationMaterialNewtonsche FlüssigkeitRocketGround (electricity)Weather front
09:59
Model buildingStandard cellContinuous trackUniverseClothing sizesRedshiftHubble's lawTransmission lineDensityAbbe refractometerAtomNeutrinoPhotonDensityTiefdruckgebietKilogramSpare partRulerUniverseHubble's lawScale (map)RedshiftGas turbineBig CrunchKnifeMeasurementFlatcarAstronomerContinuous trackGround (electricity)Roll formingYearForgingDark matterWatercraft rowingNeutrinoSeries and parallel circuitsPhotonicsGenerationIceKopfstützeMetreLecture/Conference
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Big BangUniverseBig Crunch
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DensityMeasurementUniverseUniverseYearLightKnifeRoll formingHubble's lawForgingCableMeasurementRadio atmosphericMitsubishi A6M Zero
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UniverseMeasurementZwergsternMassUniverseTypesettingStarSunlightNuclear powerRoll formingMaterialSupernovaGasMassSizingBallpoint penGround (electricity)White dwarfPlanetary nebulaElectronLeistungsanpassungYearRailroad carLecture/Conference
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BauxitbergbauBrightnessYearGroup delay and phase delayBallpoint penMassRadioactive decaySolar energyGasCosmic distance ladderLeistungsanpassungDaySpare partSunlightRRS DiscoveryMeasurementHot workingComputer animation
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RRS DiscoveryFinger protocolKey (engineering)Spare partSupernovaPower (physics)Meeting/Interview
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Orbital periodSewing needleSpeckle imagingHourSpant
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AprilSpeckle imagingBuick CenturySkyComputer animation
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NightDudYearComputer animation
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MetreCosmic rayVisible spectrumTypesettingMeeting/InterviewComputer animation
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YearGauge blockSpare partTypesettingYearSupernovaCosmic distance ladderUniverseRedshift
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YearRedshiftSupernovaMeasurementUniverseKickstandYearHot workingGroup delay and phase delayBasis (linear algebra)Angeregter ZustandPaperLecture/Conference
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MaterialSupernovaUniverseSpare partYearUniverseModel buildingTrajectoryNegative pressureAstronomerQuantum fluctuationGalaxyHot isostatic pressingMeasurementSpace probeLecture/Conference
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Space probeGalaxyMeasurementAstronomerQuantumCrystal structureScale (map)UniverseModel buildingVisible spectrumPower (physics)YearHose couplingLecture/Conference
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Model buildingMeasurementHose couplingTypesettingYearPower (physics)FlatcarSpaceflight
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GalaxyFlatcarUniverseAtomGround (electricity)NeutrinoParticleMeasurementAtomismDark matterCluster (physics)UniverseGalaxyCosmic microwave background radiationSoundLecture/Conference
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MicrowaveGravel pitUniverseWavelengthWind waveGravel pitQuantum fluctuationCosmic microwave background radiationSoundUniverseYearBig BangFinger protocol
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Hull (watercraft)BaryonMultiple birthScale (map)AtomismWind waveDark matterModel buildingPresspassungFACTS (newspaper)UniverseSpare partAccelerationRelative datingRemotely operated underwater vehicle
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DensityUniverseFlatcarGalaxyWavelengthForceDark matterAtomTransmission lineSummer (George Winston album)AccelerationCrystal structureDark matterScale (map)Gravity waveHubble's lawUniversePhotonicsSupernovaCosmic microwave background radiationDensitySmoot, GeorgeTypesettingAtomismBuick CenturyNeutrinoMeasurementLecture/ConferenceDiagram
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NeutrinoUniverseCrystal structureScale (map)MeasurementNeutrinoMassFullingGalaxyHose couplingDigital electronicsNeutrinoQuantum fluctuationRailroad carModel buildingPhotonicsWeather frontBig BangComputer animation
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Model buildingMeasurementQuantum fluctuationDensityBaryonDark matterPhotonUniverseNeutrinoMassMeasurementHubble's lawDark matterCocktail party effectUniverseCosmic microwave background radiationBird vocalizationNeutrinoAtomismQuantum fluctuationMassPhotonicsModel buildingAstronomerSunriseLecture/Conference
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Right ascensionHall effectAngeregter ZustandUniverseYearAstronomerLastBuoy tenderDipolCosmic microwave background radiationAM-Herculis-SternNanotechnology
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Signal (electrical engineering)Transverse modeRight ascensionMeasurementKosmischer StaubAM-Herculis-SternHot isostatic pressingSignal (electrical engineering)ClockUniverseNanotechnologyAngeregter ZustandPlanck unitsConstraint (mathematics)Big Bang
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BauxitbergbauAnnulus (mycology)Big BangGround (electricity)UniverseConstraint (mathematics)
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VideoElectric power distributionNanotechnologyGround stationCommercial vehicleMaterialLecture/ConferenceComputer animation
Transcript: English(auto-generated)
00:15
Thank you. Excellent. So we're going to talk about the state of the universe, and
00:27
ladies and gentlemen, the state of the universe is good in 2016. The universe is expanding. That turns out to be useful. The universe is 13.8 billion years old. Not 13.9, not 13.7. The universe is very close to being
00:45
geometrically flat, and the universe is composed of dark energy, dark matter, atoms, neutrinos, photons, and perhaps some other things. So I want to talk about is how we know the state of the universe today. So let's start with history. Cosmology
01:04
really got started in about 1915. Both Einstein and the first observations happened in that year, but Hubble was the one who sort of put us on our current track when in 1929 he announced his discovery that the universe was expanding. And the way he did that is he took data, he looked at how bright stars
01:24
appeared in galaxies, and judged their distance by how far away, I mean how bright the stars were, and therefore inferred their distance. And he, or Vesto Slipher really, measured redshift, which they thought was Doppler shift. That is, they looked at the spectrum of galaxies, which have
01:42
nice little narrow lines, and then you can go through and you can judge very accurately the redshift of a galaxy, which we now know is the stretching of space between us and those objects. That is something we can actually measure to about one part in a million right now for objects across the galaxy or potentially across the universe, and that's what's going to allow us
02:03
eventually to hopefully find planets that are habitable and probably eventually planets that may harbor life. We do not know, but that's coming and that's a different talk. So from this diagram where there was a relationship that the further away, the more the redshift is what Hubble
02:24
proclaimed in 1929 meant that the universe was expanding. And so I'm going to expand the universe just to get this idea in your head, so I've expanded the universe there for you, and let's look what it seems when we look at that expansion. I overlay before and after, and what do I see? I see
02:44
nearby objects, well, they've moved a little bit, so their redshift is going to be small. The distant objects, well, they've moved a lot in that expansion. Their redshift will be large. In a universe that's expanding, you expect to see what Hubble saw. The further, the more the redshift.
03:02
Now, Hubble didn't really have his head around how this interacted with general relativity, and so we'll talk about that in a second, but I also want to just take the logical conclusion that if the universe is expanding, then we can run it in reverse. And when you run it in reverse, you sort of come to an almost inevitable conclusion that there will be something like the
03:23
Big Bang, when everything in the universe is piled up on top of everything else. So that is the thing that we thought happened 13.8 billion years ago. What is the Big Bang? That's one of the most common questions asked of me in talks. I have no idea. I only know what
03:40
happened after the Big Bang. We think the Big Bang, at the time right after the Big Bang, and I'm not going to even use that, but let's use that for a second. Right after the Big Bang, there was this period of inflation, where things at the quantum scales were magnified to the universal scales. Quantum fluctuations were expanded to the scale
04:04
of the universe, and those are the seeds of what we see in the cosmic microwave background, the sound waves, which I'll talk a bit later, and eventually have led to galaxies, led to the earth and everything that we know. Now that magnification of the universe from the subatomic to
04:22
macro scales seems kind of crazy, but it keeps on kind of predicting things that we see in the universe. From my perspective, that period, when the universe was exponentially expanding, really is the Big Bang. There may be something before it, I don't know, that may be an ill-formed
04:41
question, but if we were to understand that period, then I think we'd have a much better chance of understanding what the Big Bang really was. As I said, it may well be what the Big Bang is from our perspective. Now, the first thing that Hubble could do is he could do a very simple experiment, and he could say, how fast is the universe
05:02
expanding now? That's distance divided by red shift, and I could run the universe back in reverse, and therefore you get the age. You run the universe back in the age and back in reverse, and you figure out how old the universe is. We call that the Hubble constant, and that's exactly what I worked on for my PhD thesis, which I finished in 1993, and so here I am, as a much younger
05:23
person, I'm afraid, doing my first big experiment showing my PhD supervisor, Bob Kirschner, the age of the universe, and the answer that I got, which turned out to be about right, I wouldn't say I get credit for telling everyone this was the answer, but the age of the universe is about 14 billion years when you
05:43
linearly extrapolate. All right, general relativity, we haven't talked too much about that, but Einstein, of course, in 1907 came up with the idea of equivalence, that no matter where you are in the universe, you can't separate out the idea of whether or not being in a rocket, being accelerated, or
06:02
being on Earth, accelerated by gravity. Those things are equivalent, and he had that revelation while watching someone fall off a roof and thinking, wow, that guy doesn't feel gravity. I bet you under no circumstance would that person feel gravity. That is what makes Einstein different than the rest of us, because the rest of us would be thinking
06:21
something different when we see someone fall off the roof. But he thought about that for eight and a half years. Everyone told him he was crazy, you're never going to get there. He collaborated widely with the best mathematicians of the time, and he struggled. But in the end, he ended up in 1915 with the theory of general
06:43
relativity. His last person he was working with was Hilbert. Hilbert actually got there four days before he did, a little fact most of you probably won't realize. But I think Einstein did most of the hard work. But Hilbert was a better mathematician than was Einstein. So this is the thing that put Einstein on the world map.
07:06
He was, when it was shown to be correct via eclipses in 1920, all over the front pages of the newspapers. And he became a celebrity in the form that we know him now. He was famous amongst physicists. Before this, this is
07:22
what made him famous amongst the average people. Even though that's not necessarily what the average person remembers him for today. We always think of E equals MC squared, but the thing that made him famous back in the day was this. Alright, and so astronomers of course are useful very
07:41
occasionally for physicists, in that we can sometimes make experiments that can show things of physics to be correct. This is something that's become more true recently, but occurred back in 1919 when, by looking at eclipses, we could go through and verify that the deviation of light around body like the sun was indeed
08:03
predicted by Einstein's equation and not by something, for example, a hacked up version of Newtonian physics. Soon after that, as that was shown, relativity, general relativity became a very popular subject and people worked on it the world over.
08:21
Most notably, Alexander Friedman, who was in St. Petersburg, who was able to think very quickly about the idea of what happens if I have a universe that's full of some sort of material, what's the solution to general relativity? And he's gonna make a major approximation, which is the universe is homogeneous
08:42
and isotropic, but it's the same everywhere. That turns out, as I'll show you in just a second, is a major and very important approximation, which seems to be valid in our universe. The person up here, Georges Lamot, a Belgian monk
09:01
and a graduate of MIT in 1927, had a thesis, which was essentially, here's the expanding universe from Friedman's equations, which he derived himself, and with data from Hubble and Slipher, which showed that the universe was expanding. He showed that in the Solvay Conference in 1927
09:21
to Einstein, and Einstein, this was too far of a leap for Einstein, and he told poor old Lamot, your mathematics is fine, your physics is abominable. And so Lamot, who discovered and derived the expanding universe in 1927, is not as famous
09:41
as Hubble, even though he got there first. So Lamot went on to figure out that there should be something like a primeval atom, the Big Bang, and many other things. I think he was a bit of a crank in the days, and people didn't perhaps take him as seriously as they should have, but we should take him seriously. So Friedman is what we call the standard model,
10:03
Friedman equation from 1923, and essentially, you can take the coupled equations that are general relativity, and if you assume that the universe is isotropic and homogeneous, you can break it down to a single differential equation, which is quite easy to solve even as an astronomer.
10:22
And so it has a few pieces that are not, that may be unfamiliar, a scale factor. That's a ruler at any given time of the universe, and since it's kind of hard to define that in absolute terms, we define it in relative terms. We call it A right now, or A at any given time, T, over A naught, the time, the scale right now
10:40
of the universe. That turns out, we can measure extraordinarily accurately through the red shift. It is essentially directly related to the red shift, and so as I said, we can measure that to one part in a million over time. The other thing we have in here is geometry. So of course, general relativity says space is warped,
11:02
and we need to worry about that, and so there are actually three families of solutions, which I'll talk about in just a second, the plus one, the zero, and the minus one solutions, which are closed, flat, and open, respectively. And finally, you have rho, which I'll talk about in just a second, which is the density of what's in the universe.
11:21
So within that, you get a series of solutions, but there are things that are important in these solutions. One is the Hubble constant, so that's how fast the universe is expanding now, and of course, that does change over time, but it's the fractional derivative of the scale factor. So in other words, it just tells you in a unit of time
11:42
how much the scale factor's changing, so that's why one over that gives you the H of the universe as a linear extrapolation. We think that number in astronomers' units is around 70 kilometers per second per megaparsec, or a fractional change such that one over 13,
12:00
or one over 14 billion years, that is the value of that. We have something known as the critical density. That is the density where the universe switches from being closed to open geometrically, flat. So it's a knife edge, universe in practice cannot exactly be flat, although it can be very, very close
12:25
and this has a value when you put in that value of the Hubble constant of a very low density indeed, about nine times 10 to the minus 27 kilograms per meter cubed. Now you might wonder, we live here on Earth, where the density is 5,500 kilograms per meter cubed,
12:45
so clearly the universe is closed. Well, no, we live in a very special part of the universe. The Earth is nothing like the rest of space. Space is very empty, and so as we'll see, the density of the universe is very close
13:01
to the critical density. Finally, we keep track of how much stuff there is in the universe by this parameter omega, which is the density of the universe divided by that critical density. So for example, I'll show you that the density of matter that gravitates is about 30%, so omega is .3.
13:21
But you can have anything. You can have energy in the form of a cosmological constant, you can have atoms, you can have something we'll talk about called dark matter, photons, neutrinos, anything that has energy is expressed as part of that. All right, so the solutions, if you just have matter, which is of course a sensible universe that has normal gravitating matter,
13:42
then you can have an empty universe, the universe just coasts, gets bigger and bigger. You can have a universe that has some stuff in it so it slows down but keeps on getting bigger over time. You can have a universe that's heavy, has greater than the critical density, and it's finite in volume, and it's finite in time.
14:01
And so all universes start with this big bang, but only that one ends with the ganab gib, the big bang backwards. Related through geometry, if you have a universe that can have these different shapes, and if you think of the form of a triangle, they have in these hyperbolic geometries, the light universe, k equals minus one,
14:22
spherical geometry, k equals plus one, and then just normal Euclidean geometry and the k equals zero. All right, so how are we gonna go out and do this as a measurement? We're gonna go out and measure the universe's past and see how it changes. And of course, the universe is very big,
14:40
and so if I look further and further away, I'm looking farther and farther in the universe's past because light takes many years, billions of years to reach us if we look far enough away. So if I do an experiment to measure the Hubble constant back in time effectively, then I can see if the universe is coasting or whether or not it's on the other side
15:00
of this critical line where the universe is on the knife edge. So one side of that gravity wins, the other side of that gravity loses. That was what we were planning to do in 1994 when we started our experiment to measure the past history of the universe. So to do that, we needed something really bright
15:21
we could see on the other side of the universe, and nature gave us something in the form of a type 1a supernova. So imagine you have two stars, a little bit bigger than our own sun, stars start running out of nuclear energy in their cores, they puff up. If they're in a close binary, the bigger star will donate material to the smaller star.
15:41
Eventually, it will lose most of its material as it evolves, you get a white dwarf, which is an electron-degenerate ball of gas that weighs roughly .6 to one times the mass of our sun, all compacted in something about the size of the Earth. You get a planetary nebula, we think, which you can go out and see is a very pretty object in a telescope.
16:02
Meanwhile, inside the bigger star now that's got all this extra materials heated up and burning its material quickly, it'll start to expand and it can donate its material under certain circumstances to the white dwarf and make it grow in mass. And as Chandrasekhar showed, that when you reach 1.383 times the mass of our sun,
16:23
you become unstable. Turns out in practice just before that, the center gets dense enough that carbon starts burning and over about 1,000 years runs away and then bam, you get an exponential burn and the whole shooting match goes up in a period of about a second,
16:40
turning that 1.4 solar mass ball of gas into about 0.6 solar masses of nickel-56 and a mixture of other stuff. And that nickel-56 is highly radioactive and so over 21 days or so or 17 days, it's radioactive energy leaks out
17:02
and you get this incredibly bright ball of gas, five billion times brighter than the sun, that you can conveniently see up to probably 10, 11 or even 12 billion years in the past. A group in Chile that I worked with for my PhD thesis, well, they figured out how to measure distances
17:22
with these very accurately. And so that was a key ingredient that happened in the early 1990s and so all you have to do is go out and find them. Now, part of going out and making a discovery is starting a team and as I was telling people earlier today, when I was 27, I worked with a bunch of people, my mentors,
17:41
to start on this project and I had no power, I had no money, I had enthusiasm. That turns out to be enough to start a team in science. That's why science is so great. So we started our team and the key was to find supernovae. Now in 1994, we would take a thousand,
18:01
2,000 by 4,000 pixel images, that was a lot of data in 1994 and in a period of 24 hours, we had to find the needles in the haystack. There was like one object in every 250 frames and so let's see if we can find one. Here is an object. The reason we know it is we compare a before and after image
18:22
and so by comparing that, you can see what has come out and in that, we had to deal with essentially big data of 1994 and we used rudimentary artificial intelligence algorithms back then, which I can tell you weren't very good. They're much, much better now. But it was enough.
18:42
Well, enough with a lot of help. In practice, this is how we found them. We went off to a telescope in Chile. As the data came down from the sky, we had to be extremely well organized. So there's Nick Sunset who helped form this team with me in 1994 and you only get six nights a year, right?
19:03
And you need two pairs of observations. So you don't want to screw anything up because if you screw up once, they're not gonna give you any more telescope time. And so as the data comes in, we're checking it, checking it, checking it. The software's going through and trying to find the candidates but it finds lots of drunks, lots, not drunks, finds lots of duds.
19:22
It finds cosmic rays, it finds asteroids and occasionally finds supernovae. So we have literally a team of people that are pouring over the data in real time trying to find the things that are most interesting. We get together and then we have to send the data to Hawaii where the 10 meter telescope's
19:42
just been built, Keck, because it's the only telescope in the world big enough to take a spectrum. And so there we have this amazing telescope that Adam Ries and Alex Filippenko, they're gonna take a spectrum and so we can get a red shift and make sure that we're seeing really is one of these type 1a supernovae.
20:00
All right, so after three and a half years of doing this, you get to see what the data looks like and this is what our data looked like. You have red shift versus essentially distance where I've made it obvious what's going on here. Each supernova is a point on there and you compare nearby with the distant objects and what do we see?
20:21
Well, we see the distant objects are not in the bottom part of the diagram where the universe is slowing down over time. Rather, they're in the top part of the diagram where things are speeding up. So that was a surprise and you don't go eureka. You say, oh dear, what have we done for three and a half years
20:40
that we're getting a wrong answer that no one is ever gonna believe. So you spent a lot of time checking your work and trying to understand it. Then something interesting happens. There's a team that you're competing with and I should say Steve Chu is the boss of this team so I have to make sure that we announce it well or he'll give me a hard time later on. But you're in very strong competition with this group
21:03
and you suddenly realize both of us are both getting the same crazy answer at the same time. That's very highly motivating for publishing quickly but in the end, there were two papers that came out and we both got this crazy result. That doesn't mean it's correct.
21:21
We could have made the same mistake. We're using the same basic technique but at least it provides some assurance. And so it is on the basis of that paper that we were given the Nobel Prize. Here we are, our team. Now you will note there are 19 men on that group. That reflects my discipline in 1993.
21:42
The good news is right now half my group are women so we're changing but it takes time. Of course the other team, oops, sorry, I need to give Saul a little more time than that. So that is our competition. They came from physics and they actually did have some women which was great for them.
22:04
I mean that seriously. We should be ashamed of ourselves. They were better off than us in that. So what causes this? Well of course Einstein put the cosmological constant in his equations back in 1917, probably for a poor reason but he did put it there
22:22
and that energy which is part of space which is how you can think of the cosmological constant causes gravity, it has negative pressure and causes gravity to push rather than pull and so in practice, the stuff we now call dark energy, well we needed the universe to be about 70% that stuff
22:41
to explain our answer. The 30% is just normal stuff that we were used to. So if you think about it, what we're looking at is a model that looks like this, a universe which spent its first 13.8 billion years slowing down and has only recently started speeding up
23:01
and we expect to exponentially expand. That's what Einstein, the Friedman equation with the cosmological constant says the universe is going to do. It's a complicated trajectory. The universe should not be doing that. We live at a very specific place where things are changing. It's a funny place to be.
23:21
It has very conveniently, it gives almost exactly the same age as just a simple extrapolation within 3%. All right, so let's say you're gonna go out and make another measurement. So astronomers can go out and look at galaxies. There are probes of gravity and of course within a computer,
23:40
you can simulate different universes to see if I start with quantum fluctuations, expand it out, I can go through and I can see what the result would look like in terms of large scale structure. And so this is one such simulation done here in Germany and you can see that galaxies start to form
24:01
as gravity collapses the structures and then what you do is you say, let's make our own universe and see what things look like. So for example, here's the data that we took in Australia. There's a similar larger survey done a couple years later in the United States and you can say model one, two, three, or four,
24:21
which one looks right? Now of course we use power spectra in ways of statistically quantifying the data but the statistics show very clearly it's model three which interestingly enough essentially had a matter content 30% of flatness, omega equal 0.3. So that's what they can do.
24:42
Completely insensitive to the cosmological constant these measurements just like it doesn't exist for this type of measurement. So they get this measurement of omega matter equals about 0.3. There's a problem of course because there's about six times more matter than there is that we can account for from atoms and so that brings up the age old problem of dark matter.
25:03
Wherever we look in the universe, whether it be galaxies or in clusters of galaxies, there's always more gravity than there are atoms to explain that. So dark matter, how would we find that out? Well the cosmic microwave background is really the thing
25:20
that allows us to do precision experiments in the universe today. So these as I said are essentially sound waves of matter splashing around the universe after the Big Bang and so we can go through and we can learn from them because if you have, think of the universe as a pond, if you throw a rock in a pond, you get wave action
25:40
and it depends if the pond is made out of water or treacle, you get different waves and so you can actually use the wave action to figure out what the universe looks like because the Big Bang, those quantum fluctuations are like throwing gravel in the universe and then you let the waves go around for 380,000 years when suddenly the universe
26:00
turns transparent, we get a snapshot and you can look through and say what does the universe look like? Well in detail, you can go through and see this is the wave action for different amounts of atoms and different amounts of total matter including dark matter and you can deconstruct that because the data is so insanely good
26:21
and there's a model, fits through the data beautifully. That is a very complicated model so it's very precisely able to tell you what's the universe made out of? 6.5 parts dark matter, dark matter is stuff that only interacts by gravity. That's as simple as it gets to one part atoms. The other thing you can do is you can use the fact
26:42
that the universe's geometry magnifies or demagnifies the universe and so depending on which universe in, these things are gonna appear smaller or larger and so if you look at the curves, you can see that the geometry makes a big difference of magnifying, shifting things this way,
27:01
dark energy and the acceleration makes a small bit, enough to actually measure as well a bit. So what do you end up with? Well, you end up with the universe that looks exactly flat when we compare data. It's within 0.5% under almost any set of assumptions. The universe is very close to omega matter equal one. So you have the cosmic microwave background
27:22
that says the universe is flat, so omega is one. You have 30% from the large scale structure in gravity so that gives you essentially a universe that's 70% mystery matter. The same stuff the supernovae are seeing and so the latest numbers are this. We are in a universe that's only 5% atoms,
27:40
69% dark energy, 26% dark matter. It is a messy universe that should not be this way if you are a reductionist and you wanna bring things down to simplicity. So the density of the universe, well, we know the Hubble constant quite well. It's very close to 70, plus or minus about three right now and so we have a very,
28:00
a universe made up of these fractions of things but we can also see how many photons there are, for example. You get that from the black body so that's something that George Smoot measured but he talked about gravity waves so I'm gonna talk about George's Nobel Prize here. You can also go through and look at things like neutrinos. The best measurement of the mass of the neutrino,
28:23
at least the upper mass, now comes from astrophysics. Probably make a couple physicists mad there but if you have a universe full of neutrinos, you know how many there are compared to the photons, to the physics of the early universe, is they get heavier and heavier, the sum of them, then it affects the large scale structure
28:41
and right now our measurements of galaxies tell us that the mass of the neutrino, the sum of them must be below .25 eV. So our cosmological model, well, you start with these initial fluctuations from the big bang, inflation, you've got a universe made up of all this stuff and that agrees with essentially every measurement
29:02
we've made in the universe and that is what science is all about, is having a theory which predicts things. Now there's some problems which I'll talk about in a second. The only newly-arised issue is that Adam Rees, my co-Nobel Prize winner, has measured very accurately now the Hubble constant
29:21
and he is claiming that there may be a discrepancy between the local value of the Hubble constant and that which you get from the cosmic microwave background. I'm not sure yet, I'd call it 50-50, maybe, worthwhile looking at, it needs more attention and it's a very hard measurement. So dark matter, dark energy, inflation.
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Those are our three problems with that model. We don't know what inflation is, we don't know what dark matter is and we don't know what dark energy is. We know how they behave, we just don't know why they'd be there. So the big questions we have to look for is, what is dark matter, what is dark energy? Why do neutrinos have mass? That's an interesting one as well,
30:00
not exactly astrophysics. Why is the universe not just full of photons? The equations of the early universe say everything should have annihilated, we should only not have any atoms or dark matter. What is dark matter? And what is inflation? And just to give you a sense of how astronomers can answer what seems imponderable,
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how could you figure out what happened when the universe was 10 to the minus 35 seconds old? Well, down in South Pole, we had a bit of excitement last year where people measured what they saw was the imprint of gravitational waves in the cosmic microwave background through what we call B-moans, which are essentially the cross product of the polarization.
30:42
Very exciting. Unfortunately, it's a hard measurement because the universe is full of dust, which is also polarized. Here's the Planck measurement and its measurement of polarization. They were looking down in this area and it turns out the polarization there was stronger than their signal. So, shame on them?
31:01
No, probably shouldn't have done the press release, but it is good they're out there trying to make this measurement. This is profound because if and when we detect this, we are going to get a constraint when the universe was 10 to the minus 35 seconds old, our best chance to understand inflation and what the big bang is. And so, I will leave you with Einstein's,
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to my mind, his greatest quote, which is, if we knew what it was we were doing, it wouldn't be called research, would it? Excellent to use when your politician comes up to you and asks you what on earth you're doing. Tell him that. Thank you very much.