By the time I finish this sentence, up to a billion billion dark matter particles may have streamed through your body like ghosts. The particle or particles of the dark sector
make up the vast majority of the mass in the universe - so to them, you are the ghostly one. Today we're going to try to make contact. We see the influence of dark matter in the
orbits of stars and galaxies, in way light bends around galaxies and clusters, in the
clumpiness of the cosmic background radiation, and more. It’s become disturbingly clear that we can’t
see around 80% of the matter in the universe. Even more disturbing is that there doesn’t
even seem to be a candidate for dark matter in the known family of particles. We’re faced with the eerie reality of the
dark sector - perhaps there’s an entire family of particles that exists in parallel
to those we can see - a dark universe that overlaps our own, but so far is hidden from
even our most ingenious experiments. Today we’re going to open the gateway to
the dark sector and see what we can find. When we talk about the “dark sector” we
typically mean a particle or family of particles that contribute to dark matter. Now it’s possible that dark matter is not
particles - it could be black holes or failed stars or even weirder so-called “compact
objects”. It might even be that what we perceive as
dark matter is really a glitch in the laws we use to describe gravity. But those possibilities are for another time
- today we’re focusing on the bizarre physics of the dark sector. So let’s begin with what we do know. Our best understanding of the subatomic world is given by the Standard Model - which describes the behavior of the known family of particles with incredible success. The visible universe is made of these particles, interacting with each other through the standard model forces - the strong and weak nuclear
forces and electromagnetism. Plus gravity. In general, the behavior of a particle is
determined by the forces it interacts with. We can think of forces as the languages that particles use to communicate. Any electrically charged particle experiences the electromagnetic force and can communicate with other charged particles by exchanging
photons. But for an electrically neutral particle like
a neutrino, electromagnetism is a language it doesn’t speak. Neutrinos are unaffected by that force, and
so they are quite literally invisible to photons. A more technical way to think about this stuff is in terms of quantum fields - where each particle and force is a vibration in its own
field. These fields fill the universe, overlapping
each other - and if a particle field is connected to - coupled with a force field then it can
speak the language of that force. The force of gravity is a sort of lingua franca,
a common language that every particle with mass can speak. But gravity is a little different to the other
forces - it’s not part of the Standard Model, and we don’t even know if it has a quantum
field. The main requirement for a dark matter particle is that it doesn’t “speak electromagnetism”. It doesn’t produce light - hence the “dark”. But it also doesn’t absorb light - otherwise
we’d be able to detect it when it blocked light from the more distant universe - in
the same way we “see” the black lanes of dust that block the light from the center
of our galaxy. No, dark matter is both perfectly dark AND
perfectly transparent. Good - so it must be electrically neutral
like the neutrino. Dark matter can’t have charge but it must
have mass because the only thing we’ve ever actually seen dark matter do is to exert its
gravitational influence. Dark matter “speaks gravity”. And we can learn an awful lot from HOW it
exerts its gravity. We can map where dark matter is found by how it affects the rotation of galaxies, and how it drives the orbits of galaxies inside galaxy
clusters, and by the way it bends light around galaxies and clusters. These tell us something really important:
dark matter is far more spread out - more diffuse - than almost all of the visible matter. And that tells us a lot about any prospective
dark matter particle. For one thing, dark matter doesn’t tend
to interact with itself - at least not very much. If it did, then giant regions of dark matter
would lose energy in those collisions and contract. They might collapse into dark matter galaxies or dark matter stars or dark matter people. But no - dark matter seems to stay puffed
up in gigantic halos surrounding the much more concentrated clumps of visible matter. In fact, galaxies are really just shiny dustings of stars, sprinkled deep in the gravitational wells of massive reservoirs of dark matter. But the fact that dark matter forms those
giant halos at all tells us something very important. It gives dark matter a temperature. More accurately, it tells us how far dark
matter particles were able to travel in the early universe. This “free-streaming length” of dark matter
is how far a dark matter particle could travel before interacting with something - typically
another such particle. In the early universe, that distance influenced the size of the seed structures which galaxies would later form from. We’ve talked about that structure formation
before. Now, based on how that structure did end up forming, it seems likely that dark matter was moving pretty slowly. We refer to such dark matter as “cold”. So let’s review - if dark matter is a particle,
it’s electrically neutral and doesn’t interact much with itself, and it’s relatively
slow-moving, and also insanely abundant. For a long time people thought the neutrino
might be dark matter - being neutral and the most abundant known particle in the universe. But the neutrinos of the standard model move too fast - they are “hot” - and there just isn’t enough mass in neutrinos to do
the job, due to them being ridiculously light. There’s really nothing else in the standard
model that works - which sounds annoying, but actually gets physicists very excited
- because discovering a dark matter particle may be our best for finding a bigger, deeper
theory than the standard model. It would also be a no-brainer Nobel prize
- and many researchers have devoted their lives to hunting down this particle. Dark matter hunters come in two breeds. One type searches for new evidence out there in the universe or in our particle experiments here on Earth for evidence of particles that
don’t fit the standard model. The other delves deep into theory - in speculative mathematics beyond the standard model for signs of new particles. Today we’re going to focus on the theoretical prospects - because we might as well have some fun before those pesky “observations”
ruin everything with their “facts”. Actually, we don’t have to go too far beyond
the standard model to find our first dark matter candidate. Completely independently of our quest for
dark matter, physicists have hypothesized a new type of neutrino - the so-called sterile
neutrino. I won’t describe these in detail now because we’ve been over them before - but in short: as ghostly as neutrinos are, sterile neutrinos are far ghostlier. They don’t even interact by the weak force,
which means they’re almost impossible to detect. There are some exceedingly clever experiments to do so - like we saw that time we visited FermiLab. If sterile neutrinos exist AND are massive
and slow-moving enough, they’re a great dark matter candidate. Another candidate we’ve discussed is the
axion. This is a weird little particle that popped
up in the math when physicists were trying to solve another mystery of physics - the
so-called CP problem. Axions, if they exist, would be incredibly
light - maybe 1% or less the mass of the already-puny neutrino. So to account for dark matter they’d need
to exist in prodigious numbers … but according to pro-axion physicists, that may well be
the case. OK, enough with the things we’ve already
talked about. Explorations of the theoretical landscape
have led physicists to multiple possibilities for dark matter particles. One of the most promising ideas comes from supersymmetry. We’ve also talked about supersymmetry, but
not about how it could give us dark matter. Supersymmetry is an extension of the standard model which proposes that all the regular particles - both matter and force-carrying
- have twins - counterparts on the opposite side of the table. Every matter particle or fermion has a supersymmetric force-carrier, or boson. And every boson has its fermion twin. It’s expected that these supersymmetric
particles are much heavier than their standard model counterparts - and that may explain
why we haven’t seen them in our particle accelerators - perhaps we just haven’t produced enough energy to make one yet. But they may have been produced in the insanely
energetic early universe, and the leftovers from that time could still be throwing their
weight around, so to speak. The simplest kind of dark matter we get from supersymmetry is called a ‘neutralino.’ It’s a sort of ‘three in one particle’
where the electrically neutral superpartners of the Z boson, photon, and Higgs particle,
all mix together. In some models these are the lightest supersymmetric particles possible - ”LSPs” - but they’re still incredibly heavy. And while normally heavy things tend to decay to lighter things, if these can’t decay into Standard Model particles then they’d be stable and long lived- an almost perfect dark matter particle. There are other dark matter candidates in
different flavours of supersymmetry - all of them “LSPs” - for example the counterparts of the neutrino or the graviton. The expected mass of these particles is eerily
close to the mass expected for a certain type of dark matter - which some would say is a
point in favor of supersymmetry. This seeming coincidence is sometimes called “the WIMP miracle”. But for that to make sense I should probably explain what a WIMP is. Supersymmetric dark matter particles like
the neutralino are examples of a general dark matter particle type called the WIMP, or “weakly
interacting massive particle”. The idea of the WIMP was proposed independently of any actual WIMP candidates. It’s a description of what some physicists
thought dark matter particles had to be like- which is to say, weakly interacting and massive. The massive part is obvious enough - it helps if you want to make up 80% of the mass in the universe, and also slows the particle
down - helps make it “cold”. We also covered weakly interacting - it helps dark matter halos stay puffed up. But it also turns out that the interaction strength of dark matter is extremely important - it may have governed how every interesting thing in our universe first formed. The idea is this: In the first fractions of
a second after the Big Bang, particles and their antimatter counterparts would have been popping into existence constantly, borrowing energy from the crazy radiation of that time. And then when the particle bumps into its
antiparticle they both annihilate, releasing that energy again. As the universe cooled and energy dropped,
that process ceased. We were left with a universe full of particle-antiparticle pairs that would then just annihilate over time. But its possible some particles may not have been able to find an antiparticle counterpart before the expanding universe pulled them
too far apart. Things like electrons and antielectrons, or
positrons, interact very strongly via the electromagnetic force - which means they find each other too easily. The universe didn’t expand fast enough to
throw these particles apart, and so almost all annihilated. But a WIMP, with its extremely weak interaction, would more easily dodge its antimatter buddy - and so countless may have survived to this day. So it turns out you can do a calculation of
what interaction strength such a relic particle would need to have in order to survive in sufficient numbers to give us dark matter. And that interaction strength is about the
same as the weak interaction. Ergo, WIMPs interact by the weak force only, or something weaker. OK, so we have multiple possible members of the dark sector - and we didn’t even cover all of them. Perhaps none exist, but perhaps several do. It’s possible that an entire ecosystem of
particles are going about their dark business across the universe - interacting by dark
forces, all of them oscillations in their own dark quantum fields - perhaps with their own complexity and diversity. Because dark matter is weakly-interacting,
our light sector is probably more complex - probably. But to know one way or another one of the
many brilliant experiments currently in progress or planned will need to bear fruit. We’ll talk more about those experiments
another time. For now, we’ll just have to enjoy knowing
that our light-weight light sector exists in parallel to this completely invisible and
vastly more massive sector of dark spacetime. Before we get to comments go, we just wanted to let you know that while we love talking about Space, if you have more Earthly concerns, you should check out PBS Terra on YouTube, and their show Weathered. Here is a quick trailer: Are you prepared for the next disaster? The flood's right there. Keep going. Are you ready to help your family and neighbors? Welcome to Weathered. In this show we'll explore the most pressing natural disasters in the US How they're changing and how you can prepare. This is literally a life saver. Like this saves peoples lives. In the last episode we asked the question I know all of you had on your mind. Would a spaceship traveling near the speed of light cause a closed universe to contract so much that it would smash into its own ass? In the process of answering that we explored the ladder paradox and the twin paradox, and that's what our questions are about today. Cezar Catalin asks what if the ladder traveling through the barn stops when the outside observer sees both doors closed? Does the ladder instantly expand, exploding both doors outwards? Well that's awesome question - the answer looks different depending on your reference frame. From the frame of the barn, the ladder is
fully inside and the exit door remains closed. Let’s say it’s an adamantium barn door. The ladder collides with the door at a high fraction of the speed of light and a shockwave blasts down the length of the ladder totally destroying it and embedding the walls of the barn with splinters. And all those splinters have to be inside
the barn. From the ladder’s perspective, its butt
was still outside the barn when its front end hit the door. There’s an absolute limit to how fast that
impact could communicate itself down the ladder. The shockwave can go no faster than the speed of light. So by the time the base of the ladder even
knows that anything at all has happened to the front of the ladder, it’s already inside
the barn. The shockwave hits the base of the ladder
at the same position in the barn from either the ladder or the barn’s point of view. Etu Suku points out that the difference between the traveling and stay-at-home twin is actually acceleration, so shouldn’t acceleration
be used to account for the time difference? Well I’m actually glad you mentioned that. Yes, the traveler needs to decelerate and
then accelerate again to turn around, and according to Einstein’s equivalence principle, acceleration is fundamentally the same as gravitation as far as the laws of physics are concerned. So can we apply the gravitational time dilation to the traveling twin, as though they were standing on the surface of a planet with the
same gravitational acceleration? Yes we can, and we’d get exactly the same
answer as if we just have the traveling twin instantly flip its velocity around with no
slowing down or speeding up. It turns out that there’s no distinction
between all the types of time dilation as far as the universe is concerned. And that’s something we really do need to
explore more carefully - in an episode coming very very soon. From Brandon Treu: This isn’t related to
this episode, however having listened to this show over the last year and being challenged intellectually regularly has inspired me to go to college. I had my first class today! Well Brandon, thank YOU for sharing this! This is exactly the kind of feedback that
keeps us doing the show. If we played a small part to inspire you to
embark on this new adventure then it’s all been worth it. But really, it sounds like you’re the kind
of guy who seeks inspiration and runs with it - the the credit’s yours my friend. And a final thought from Sebastian Elytron,
who expresses surprise that apparently Einstein was a real person. He always thought he was a theoretical physicist. *theoretical mic drop*
