Dark matter in seven acts
In the beginning there was a big bang what do we mean by that. That means that the universe and everything in it started from a giant fireball of energy and space has been expanding out from that fireball continuously ever since then for almost 14 billion years. Everything that we know about was created in that big bang. If we take a freeze frame of that big bang though we'll notice that something that you didn't see was also created then. Dark matter particles were created shortly after the big bang.
So we say dark matter particles what do we mean by that. Dark matter is some form of matter that doesn't emit light doesn't reflect light doesn't absorb light doesn't do anything with light. So we take our light and our dark matter and our observer and we turn on the light. Nothing happens the light goes straight through. It's as if the dark matter wasn't there at all.
Normal matter doesn't act like that. Everything we know about might appear dark to our eyes but that's because you have your light source and your normal matter and your observer turn on the light and it either gets absorbed or reflected by the normal matter but it is affected by it. Whereas the dark matter doesn't have any effect on light or vice versa so in that sense a better name for dark matter might have been invisible matter but then we couldn't call this dark matter day could we.
Why do we believe this dark matter exists? The reason is because wherever we look in the universe with telescopes we see overwhelming evidence that there's something that although we can't see it by light we can see it by gravity so there's this dark matter out there. We don't see it but we have normal matter and normal matter is gravitationally attracted to it so something is out there and is affecting the motions of normal matter. Or is it possible that we don't understand gravity at all maybe the gravity that was predicted by newton and einstein with the apple falling on newton's head isn't quite the same as the gravity we see way out in the universe.
That's a possible scientific hypothesis but it doesn't seem to work uh all our theories of gravity have withstood every test up to now and nobody has figured out a way to modify them to explain these observations that i just mentioned when we look out into the universe. So moving on to Act Two: Let there be light, and dark! So Dan just showed you this picture showing the earliest moments of the universe and its subsequent expansion and evolution and i want to talk now about a period of time about 300 000 years after the beginning or about a little over 13 billion years ago when the first atoms formed. A period called recombination which is really the earliest that we can see directly with light today back in time and this period of time observations of this period of time provide some of the strongest evidence not only for dark matter but of the fact that dark matter is of a fundamentally different nature than the normal matter so what was the universe like at this early time.
Well it was kind of a soup of hot soup let's say of free protons electrons and photons that were buzzing around bouncing off of scattering off of these electrons and protons something we call plasma and as the universe expanded like kind of like a hot air balloon uh expanding uh and time went on uh it it slowly started to cool and uh eventually it cooled enough such that these protons and electrons lost enough energy that they could recombine into neutral hydrogen remember that a hydrogen atom is just one proton and one electron that are bound together and neutral hydrogen unlike this plasma at the hotter earlier times is transparent to light so all this light that was scattering around suddenly was able to freely stream that's what's shown or indicated by these yellow lines freely stream basically without ever interacting again throughout the entire history remaining history of the universe and we can go out and actually try to measure uh this light and it provides a snapshot a relatively pristine snapshot of what the universe looked like at this very very early time like 13 billion years ago if this is a little bit difficult to grasp an analogy that might help make a little more sense although it's not a perfect analogy so bear that in mind is fog a foggy day so when on a foggy day there are many water droplets in the atmosphere and those water droplets scatter light light scatters off of them and so it's difficult for you to see buildings such as these towers of the the bridge in london in the background but that water can condense and when it does so it falls as rain and there are many fewer droplets in the atmosphere for water to scatter off of and you can see much more clearly buildings in the background that's a little bit like what's going on at recombination in the before time before we combination it's like the fog after it's like the rain so you can go out and try to measure this light it should be coming from all directions on the sky relatively uniformly and it's what we call the cosmic microwave background so that's what these two gentlemen here did with this funny-looking antenna or really it's kind of a kind of telescope they they had a very very sensitive receiver like a radio receiver put it in the antenna and observed uh this very faint afterglow essentially from the big bang in all directions this is the first evidence first evidence actually for the the big bang model that there was a period of time long ago when the universe was much hotter and much denser today we still make these observations much better versions of them but using much more sophisticated uh techniques and and technology so just a couple of nice pictures of these experiments on the left there's a picture of the planck satellite which has uh many ultra cold detectors and was launched deep into space far away from the heat from the earth and then on the right is a telescope that we actually built a camera for recently at fermi lab called the south pole telescope and there are many experiments like this located at high dry and remote places like the south pole or atacama desert in chile so and what these experiments measure is is this really remarkable beautiful picture like this this is the all-sky map of what the cosmic microwave background looks like in all directions on the sky the the red points and blue points uh and are showing the fluctuations in temperature or the the brightness essentially in a way of of the light um and this may look like kind of a random pattern but the statistics of this pattern basically how big and how red or blue the dots look are intimately related to what types of matter are in the universe and provide us with really strong evidence for the existence of dark matter and the reason why it does this is a little a little technical but basically it's because um the these fluctuations that you see are the result of of sound waves propagating in the early universe so in the early universe much like a wind instrument that has a there's a fundamental mode a lowest frequency and then overtones or harmonics that are at somewhat higher frequencies at the integer the series of of higher frequencies and um what you can do the type of matter that these sound waves are propagating in results in slightly different types of waves different wavelengths or amplitudes and you can use measurements of these waves then to infer something about the types of matter that are producing waves so an example of this a cartoon example of this is imagine two different media for wave propagation depending on the nature of the medium you end up with differences in the wave propagation when you drop a pebble let's say into the pond you get different types of waves what we're seeing in the early universe is the result of many different pebbles that gives rise to this somewhat random pattern of fluctuations but the statistics of that allow us to infer something about the medium allow us to infer there's a certain amount of dark matter or a certain amount of normal matter because it would give rise to a characteristically different pattern in this somewhat random looking cosmic microwave background map that we see here and just to say that in a little more detail part of the reason for that is because the dark matter doesn't interact with photons as dan said so it tends to clump together but it doesn't feel the force that the photons feel unlike the normal matter and so you end up with these pressure waves essentially that are different depending on the ratio of normal to dark matter and that gives rise to a different distribution in the temperature of the the light that we see from the early universe and detailed measurements of that light and the sort of inference of of our measurements that you make from from the cosmic microwave background have led us to this really remarkable conclusion that something like 70 of the total matter and energy budget in the universe is actually in the form of what we call dark energy 25 a little over 25 is in the form of dark matter and only about five percent or so is in the form of normal matter and what these observations furthermore tell us is that the dark matter is fundamentally different than the normal matter because it doesn't interact with light in the same way as normal matter so this is this is the mystery uh that we're all trying to solve is what is this dark matter which seems so fundamentally different from the ordinary matter and is the majority of the matter that we we have in the universe moving along in our timeline act 3 darkness rules yeah so uh picking up on the story that uh where adam left us we have these uh primordial fluctuations in the density of the universe and because gravity is always attractive uh those over dense regions end up getting more and more dense with time and so the movie that's showing now is a computer simulation of what would happen if you start with the early fluctuations in the density of matter in the universe that are that we see imprinted on the cosmic microwave background and allow those fluctuations to evolve over cosmic history and what we're actually seeing here is what the structure of the universe would look like if we could actually see with dark matter eyes or see the dark matter itself so brighter regions in this simulation correspond to more dark matter and what we see is that as the universe evolves the universe as a whole is expanding but the gravity is is clumping and clustering the dark matter together and so as time goes on the dark matter gets more and more concentrated more and more clumpy and in fact in the center of what we call these these dark matter clouds which we call dark matter halos uh we end up seeing uh the visible galaxy galaxies form and so on this scale we're seeing a lot of dark matter clumping and clustering and in the center the brightest uh point which is the the highest density of dark matter uh would live a galaxy somewhat like our own and our own galaxy is is relatively small on this scale you can see it with this little inset picture at the bottom of your screen and so as we run forward in time we see this clumping and clustering of dark matter our simulations are predicting that we would uh see this this clustered filamentary structure sort of like uh soap bubbles the edges of soap bottle bubbles clustering together in dark matter space and this prediction has been borne out by some of our largest telescopes where we can go out and not measure the distribution of dark matter directly but measure the distribution of the galaxies that live in the centers of these dark matter clumps we see that the two distributions look very much alike and so this clustering and clumping of dark matter is occurring both on large scale scales that are larger than our galaxy but also on scales that are much smaller than our galaxy so if dark matter is slowly moving in the beginning of the universe it can clump uh into very small clusters of dark matter which would actually host small galaxies and so our picture again if we could see the universe in dark matter we would see this is a simulation in the background here as well where bright regions indicate higher dark matter density we would be able to see these clumps around our milky way galaxy however we can't see in dark matter we see an invisible light and so we can only see the galaxies that reside within these clumps of dark matter and so we know about these largest galaxies the large magellanic cloud but we can hypothesize that if our understanding of dark matter is correct we should also see very small galaxies living in very small clumps of dark matter so how do we find a really small galaxy well the first thing that we would do is we'd jump on an airplane and fly down to chile because that's where some of our best and largest telescopes are once you get down to an oceanside town called la serena you would jump in a pickup truck and drive out into the foothills of the andes where we have these observatories that are roughly 2 000 meters above sea level where it's very high and dry so you don't have much atmosphere to look through and you don't have much cloud cover you can build these really impressive telescopes to look for very faint objects in our universe so this is what the top of one of those mountains looks like this is ceratololo inter-american observatory in chile and inside this big silver dome we have a four meter large telescope called the blanco telescope that looks like this and on the uh on the front end of this telescope is a a huge digital camera that's almost the size of a small car that's sort of held uh 15 meters above the mirror of this telescope and it's a huge digital camera much much larger than the cameras you have in your cell phone and for scale you can see what a lowly cosmologist look stan looks like standing next to this telescope here and so with these really powerful instruments we can go out and look for the small galaxies that we expect to live in these small clumps of dark matter around our galaxy and this is what one of those galaxies looks like and it's not zoom that's playing tricks on you this is actually an extremely faint galaxy that we actually detect by its individual stars and so we can measure these very faint stars and then we can perform velocity measurements on them that show us that they are all moving through space together uh and so this is a gravitationally bound system uh the stars in this galaxy are held together by the gravity of dark matter and in fact this galaxy would not exist the stars would be unbound if there weren't the dark matter there to hold them together and so this is observational proof that our milky way is surrounded by these small clumps of dark matter that host these extremely small galaxies and this is actually uh something that allows us to test something about the fundamental nature of dark matter because if your dark matter particles were actually moving faster in the early universe they weren't what we call cold or slow moving moving they would have actually washed out these smaller structures and we wouldn't see any small galaxies around our milky way so the fact that we see these small galaxies means that dark matter must have been slow moving in the early universe and these warm dark matter or fast-moving dark matter models are not allowed of course we can't prove that we live in a completely cold dark matter universe but this is a hypothesis that we continue to test at smaller and smaller scales and so in our next act we're going to talk uh a bit about what what would happen if dark matter is not completely dark or invisible so as as you've heard to the extent that our current observations are sensitive we have not seen any indication that dark matter interacts with normal matter or with light but our telescopes and observations get more and more sensitive every year and so it could be that dark matter does interact just at a weak enough level a faint enough level that we haven't been able to see it in the past and if that's the case we might expect that the brightest signals from some kind of dark matter interaction would occur in the regions of space where the dark matter density is the highest these are the most concentrated regions of dark matter and so these are the places that we want to direct our telescopes and observations to try and see any possible signature of not completely dark dark matter so what we would see when looking or we hypothesize we could see when looking at one of these clumps of dark matter is some faint glow in particles of normal matter that could be produced when two dark matter particles come together and annihilate or if dark matter is not completely stable it could decay and produce uh normal matter particles these could be neutral particles like uh photons of light or neutrinos which are a very weakly interacting particle in which case those particles would travel straight to our detectors on earth and we can point directly back to the clumps of dark matter in our universe on the other hand these interactions that dark matter may have could produce charged particles and charged particles get caught up in electric and magnetic fields in the universe and get all scrambled up and so we may see the signature of these charged particles but we would have a much harder time pointing them back to where they originated so how do we make these detections of energetic particles from space well we have a few different detector technologies so one is to launch satellites into space and in space you can have one of these energetic particles potentially coming from a dark matter interaction and annihilation or decay come into your detector interact in your detector and produce photons or electrons either by scattering or annihilation of its own and you can then detect those standard those normal matter particles in your detector in observatories like the fermi large area telescope that's in orbit around earth on the ground you could also use our atmosphere itself to uh to be the conversion material so a photon or a proton or a gamma ray a highly energetic particle uh can enter our atmosphere interact in our atmosphere and lead to a shower of energetic particles that can be detected by telescopes that we actually have on the ground and one example of a detector array like that is the hess telescope which is in in africa uh finally for even less interacting partic interactive particles like neutrinos they can actually pass through our our atmosphere without interacting at all uh they can pass down into oceans or the ice sheets in the antarctic where they may then interact with uh with the molecules of water and shower to produce again energetic particles that can be detected by very sensitive detectors that we bury either in strings in the ice or or underwater in on the ocean attached to the ocean floor uh and one example of a of an instrument like this is the ice cube uh array that is in antarctica and so of course it it's challenging we can detect these particles and we do detect these normal matter particles showering down on earth all the time but it is challenging very hard to understand whether they were produced by normal astrophysical processes such as black holes shooting out energetic jets of particles or rapidly rotating neutron stars which could also give you jets of energetic particles or whether they were actually produced by some dark matter process and so the way we try to test this is by looking out observing our universe looking at regions where we expect high densities of dark matter like the center of our galaxy so this is again this is a map of our entire sky with our milky way being this bright band cutting across the middle of our picture and the center of our milky way being located in the center of the picture so we can go and look at the center of our milky way which lit which is located in the center of this very massive dark matter halo and we can see if we detect any excess uh emission coming from that region of the sky and we do see some extra emission that we can't explain with astrophysical processes at least to the level that we understand them today however this makes a prediction that when we look out at other places where we know there's a high density of dark matter such as these very small galaxies that we find located around our milky way we would if this if a signature from the center of the galaxy is coming from a dark matter interaction we would also expect to see some dark matter interaction signal coming from these small galaxies and right now to the sensitivity of our current telescopes we don't see any signature there and so this is sort of a conundrum it's a it's sort of one of these unanswered questions that we have in science today is uh what is making up this mysterious emission coming from the center of our galaxy and why aren't we seeing any corresponding emission in these small galaxies around the milky way moving on to act five scattering the dark we have seen in the previous acts that dark matter is everywhere in the universe and so we can see our solar system that is moving through the milky way alone and so we can imagine that our earth ourselves and everything is encountering like a wind of dark matter it's similar to the effect when we are traveling in a car and we are seeing a kind of relative wind hitting our car or if we put for example a hand out of the windshield uh sorry the window um so we have seen that we are saying that essentially uh for each seconds about 20 million of dark matter particle are crossing our hands but it's such a small interaction that we are not able to feel anything but at this point we can imagine to substitute our hand with the dark matter detector and try to capture one of these faint interactions these interactions uh um we can think about this kind of interaction like the billiard uh playing like billiard we can imagine that the dark matter particle is the white ball and our detector it's uh the remaining set of balls and so when the white balls uh hit the other balls it deposits some energy and this ball and then it scatters away and goes away and so we are able to measure the energy released in the detector we expect extremely rare signals so we expect a few events per detector per year and to give an idea to compare these um the quantity of the signal to events that could be close to our detector for example if we put our detector in the same room in which we are uh one person or 70 kilogram would generate uh 4 000 events per second in the detector or if we put a banana it would generate in the detector uh 13 events per second something like this even particle from the sky the cosmic ray radiation would create events in this detector for example we have that pair each cyclones we have at least one new one events passing through our hands and so we can imagine to give an idea of the number of these events to look at them and convert them to light and audio frequencies and the second one the second video is showing the radiation from the sky and the cosmic ray and nuances we were seeing before and the last one is going what is what could be the background after shielding the detector and so in order to detect that matter uh this gathering dark matter we need to protect from radioactivity our detectors and so first of all we go underground and we in order to shield from the cosmic ray radiation in the picture we have the edit a laboratory lab that is as an above ground laboratory in the underground laboratory which is two kilometer underground which is equivalent of in height to six eiffel towers um that are it's a six eiffel tower deep in the ground another way it's we are playing with lego well not exactly not with real lego but we are building really big shield with tiny bricks of lead in order to protect for ambient radioactivity to protect our detectors from the introductivity and then we care really a lot about cleanliness and radioactivity control and so we clean in we work in very clean suits okay so in the previous talk we heard that the dark matter particle is everywhere in the universe and i'm going to talk about how we can build experiments in our lab on the on the earth and if a dark matter particle is crossing through this uh space then we can probably detect it uh we have also heard from uh the previous acts that all the dark matter consists of about 25 of the of the of the of the mass in the universe um but we still don't know the mass or the frequency of this uh dark matter particle so if we build a radio type detector in the lab and we can listen to the dark matter signal if we dial the correct frequency however uh since this dark matter particle interacts very very weakly with our normal matter it's difficult to to to get a very good signal and we have to turn down the background noise on this radio so in order to do that uh we cool down our radio to a temperature which is almost 100 times colder than the outer space and see if we can spot the signal um so here is an experiment that is going on at fermilab on the left hand side i am showing an image of the actual experiment and on the right hand side uh i i'm showing how the inside of uh of of such a fridge looks like and this is a very special type of equipment where we use a mixture of helium isotopes to cool down the temperature of this entire instrument to as i mentioned earlier 100 times lower than the than the temperature of the outer space and on the bottoms you can see a microwave cavity or a fancy radio which is hanging from the from the fridge where we cool down the temperature to get rid of any thermal motions inside the inside this cavity and in we can use uh very novel uh detectors that we have developed here uh at fermilab to count the number of photons uh which are deposited by the dark matter particle and these counters help us lower the background noise to a very low level such that we can start seeing the dark matter signal and one way to think about this this counter is we just make artificial atoms and we know that artificial we know that atoms absorb light and based on the number of photons that that the atoms absorb we can infer how many photons were deposited by the dark pattern particle and so on act six cooking up dark matter okay so moving up to our next ad we've explored that dark matter maybe the hypothesis attack matter maybe come to us as a wind of dark matter um but if it doesn't come to us we can explore something opposite where we come to that matter where we originate or produce dark matter this is an assumption that dark matter may behave as everything else in our universe fundamentally in the form of particles so now we start thinking about particles and we start thinking also about how we usually produce particles in here at earth and one particular setup that we as particle physicists like are particle accelerators so particle accelerators so it was useful to produce a ton of particles and also produce particles with very high energies we can consider two types of accelerators that i'm going to talk about here one is a collider setup where you have two beams colliding against each other two beams of particles and in the point of the collision you build a detector that tries to identify everything that comes out of that collision the other setup is called a fixed target setup this only needs one beams one beam of particles that goes in one direction hits a target that is usually made of some sort of nuclei and then you place a detector upstream of that target or downstream that target and you can try to detect the products of that collision there are two examples that i want to give here of actual accelerators that live here in our earth the first one is called the large hadron collider mainly referred to it as the lhc this is a big collider that is in the border between switzerland and france there is geneva the city is also underground and equalized beams of protons are very high energies but they achieve very large velocities there are four collision points and around each of these collision points there are again builds and detectors if you're curious of what a particle collision looks like we have actual image here at the end what you end up seeing after these two beams of protons collide is really a bunch of particles that come out of that very high energy collision um these particles that come out of that collision can be actually tracked this is why you see this each of these lines that come out of this the center point and depending on either the curvature of that of those lines or how far those lines reach we can tell what kind of particles have been produced in that collision finally there's also another experiment which i can also give you an example of uh of a fixed target setup where you actually have an electron beam that is insiding in top of one nuclei target is a very thin target this is called the light dark matter experiment uh ldmx and after that target you actually place again some material either on some detectors that can trace what comes out of the of that electron target collision you would still expect to see the electron uh that was incident but also maybe something else that gets produced with electron i want to emphasize though that one thing about these experiments is that also the scale is usually quite different there are two pictures one of the detectors in the large hadron collider in the in the left and one of the magnets that is used in this proton is the electron fixed target experiment on the right and there are two people kind of standing next to uh each of these images and the scale really is very different especially for these very large accelerate very powerful accelerators or colliders you usually end up building very large detectors whereas for these fixed target setups things are a little different and you need a much smaller scale so now actually coming back to dark matter how can we actually produce dark matter with these experiments there are two hypotheses that we can use both of them use the hypothesis that um that when particles collide they may achieve either very um very large intensities so there's lots of particles or very large energies and they can produce dark matter in these collisions so really energetic beings which are here just uh simulated by this by these two needles that are colliding with each other can produce very heavy dark matter particles but it's quite hard to collide them exactly next to each other that's why we need a collider setup that can accelerate them to very high energies if we don't have the capability of accelerating them to very high energies we can use these fixed stereo experiments which can actually use very intense beams so lots of particles lots of needles going into this target and producing dark matter so actually what do these experiments see in contrary of any of the experiments that we saw in the last act all of these detectors are not built to detect dark matter they're actually built to detect ordinary matter particles like photons muons electrons quadrants quarks and this means that if if dark matter is made in this in these collisions we won't be able to detect it with using these detectors so what is our approach we can usually use two questions that we that we try to answer to see if that matari was producing these collisions the first one is was energy and mass conserved in these collisions we know the energy of the initial particles that energy and mass should be conserved by looking at the outgoing particles if there's an imbalance we know that there's something missing and that points us to the existence of that matter the second is also related to a hypothesis where dark matter can actually interact with ordinary mighty particles or some other kind of particles that can produce normal particles that can produce meows can produce electrons and therefore if we detect these muons and electrons and see if something fishy is going on with them that can point us again back to the hypothesis of dark matter being produced finally act seven how will it all end my exceptional young colleagues have convinced you i hope that the universe is made of a little bit of ordinary matter a lot of dark matter and some mysterious thing called dark energy we don't know for sure what the balance is between those and how that evolves with time what if dark energy dominates what if it's this huge ugly cloud in the room what will happen to the universe that scenario is called the big rip where dark energy wins and vanquishes dark matter and ordinary matter and visualize that with a simulation what happens is that not only do the galaxies spread apart from each other but ultimately they get ripped apart the stars get ripped apart all ordinary matter dissolves not a pretty picture not not a scenario we'd like to see happen but don't worry too much about it it's billions and billions of years in the future but that doesn't have to be the way it ends there's another scenario where dark energy isn't quite this huge ugly cloud it's a little bit less dominant over dark matter and normal matter if that's the case then the universe is still expanding the galaxies are still moving away from each other from our vantage point we see fewer and fewer of them so the universe looks darker and darker and becomes colder and colder but normal matter does not get disrupted so in the end we're alone but we're not torn apart still still a better scenario than the big rip finally what happens if dark matter wins dark energy isn't the the gorilla in the room then the universe collapses back on itself and perhaps restarts another universe with another big bang and we play out the scenario all over although we don't uh our our people who inherit the new universe will uh we'll see how it turns out in their universe that's our play we would like to if you would like to learn more about dark matter there are plenty of resources on darkmatterday.com or these other websites here we encourage you to look at those you