[MUSIC PLAYING] Dark matter literally
binds the galaxy together. But there's a problem. Nobody knows what
dark matter is. My name is Matt and
this is "SpaceTime." Physics has a problem. The Milky Way galaxy
is spinning so fast that it should be scattering
its stars into the void. Based on the amount
of binding gravity that we calculate from
everything we can see, we can only account for
10% of the mass needed to hold its stars in orbit. So what's wrong? Either we're missing and frankly
don't understand at least 80% of all the matter
in the universe or our current understanding
of gravity is wrong. This is the mystery
of dark matter. Now, before we get into figuring
out exactly what dark matter is or isn't, I want to give you
completely independent evidence for its existence,
gravitational lensing. Thanks to general
relativity, we know that light fall is
the curved geodesics of a gravitational field. Place a strong
gravitational field on an axis between a light
source and an observer and voila, you
basically have a lens. And galaxy clusters
do this all the time, turning the background
universe into a funhouse mirror of stretched out and
duplicated galaxies. From this, we can figure out
exactly how much mass is needed to cause the observed lensing. But again, we find
the clusters appear to have way more mass
than we see in the stars alone, that is if we
understand gravity. So knowing this, let's summarize
the actual possibilities for dark matter. One, best case scenario,
it comes from particles that we've already discovered,
just in a form that's very difficult to detect. Two, not so great, dark
matter is a type of particle that's beyond our current
understanding of particle physics. Or three, even worse, we're
actually not missing any mass. Gravity just behaves
differently on the vast scales of galaxies and clusters. So general relativity, wrong. OK, let's start with
the first possibility. The standard model
of particle physics is basically the periodic table
of known fundamental particles and fields. It underpins everything we know
about the subatomic universe. If dark matter
exists in this model, its mass probably needs to
come from protons and neutrons. But they can't be
interacting with light. If this is dark
matter, the galaxy would need to be swarming
with baryonic things as massive as stars,
but that are so compacted that they're
basically invisible. Is this even possible? Actually, it is. They're called MACHOs,
massive compact halo objects. And they're basically
crunched down, compact, dead or failed stars,
black holes, neutron stars, brown dwarfs, Macaulay
Culkin, et cetera. And they are very hard to see. But we can see these
guys, at least sort of, with gravitational lensing. The alignment has to be perfect. But when, say, a black hole
passes between us and a more distant star, we sometimes see
a brightening of that star. Astronomers spent years
counting MACHOs this way. And they found plenty. But not nearly enough to account
for all of the dark matter. So option one is out,
which means we're left with two bad choices. Either particle physics
is wrong, or at least horribly incomplete, in that
we're missing 80% to 90% of the mass in the universe,
or Einstein is wrong. Sacrilege, right? Remember when I said that the
Milky Way spinning too fast? Well, the problem is that the
stars on the edge of the galaxy are moving just as fast as
the stars near the center. But they should be moving slower
because the gravity out there should be weaker. According to Newton,
gravity weakens proportional to distance
from its source squared. This relationship is
definitely true on the scale of the solar system. But what about
the entire galaxy? Could it be that what we
see as dark matter just comes from gravity
behaving differently on truly gigantic scales? Well, it turns out that if
you make a simple change to Newton's gravity,
things work out. Give gravity a bit more
staying power, make it drop off proportional to distance
instead of distance squared, and then you don't
even need dark matter. The stars alone give
you plenty of gravity. The original modified
Newtonian dynamics hypothesis, and its general relativity
extensions, tries to give us this basic relationship
for gravity. 1 over R squared at small
scales, 1 over R at large. But you can't just break general
relativity and start over. Any replacement theory
has to reproduce all, and I mean all, of the
verified predictions of Einstein's theory and be
able to explain dark matter. Modified versions
of GR can actually do pretty well, especially
predicting orbits within galaxies. But they ultimately
have a hard time getting all of the observed effects. They either need some
serious fine-tuning or you have to add back
in some actual dark matter particles, which kind
of defeats the purpose. But there's an even
bigger nail in the coffin of modified gravity. Say hello to the Bullet Cluster. It's actually two clusters
that smashed right through each other. The gas was ripped
away from the stars and now lives
between the clusters. In the Bullet Cluster,
most of the mass actually is in the gas. So if dark matter really comes
from weirdly behaving gravity, then the cluster's gravity
should stay concentrated on the gas. But if dark matter is
an unseen particle, and it's the type of particle
we think it might be, then that dark matter should
pass right on through, just like the stars. How do we test this? Again, gravitational lensing. Map the mass based on
the warping of light from more distant galaxies. And we see that in
the Bullet Cluster, the dark matter
is with the stars. This tells us that matter
is a real particle, not just broken gravity. Once again, Einstein prevails. Dark matter exists and it
represents, if not broken, at least incomplete
particle physics. But what do we know about it? Well, it's slow and it's heavy. And those two go together. It has to be pretty
slow moving, or cold, because we know that dark matter
clumps together gravitationally to build galaxies and clusters. Remember the hot,
smooth plasma way back in the early universe
that produced the CMB? And the last guy
talks about it here. Well, in order to go from
that highly smooth ocean of orange plasma to today's
highly structured universe of clusters and
galaxies, something had to act with enough gravity
to pull stuff together. There's no way there's enough
regular matter to do that. Dark matter, as well as
binding the galaxy together, is also the main force
in forming galaxies in the first place. No dark matter, no galaxies. And even then,
galaxies could only have formed if dark matter
particles are cold, massive, and weakly interacting. Weakly interacting
massive particles, WIMPs, actually refers to a specific
and popular contender for dark matter. WIMPs are a family
of particles that may arise out of supersymmetry. This is a funky extension
to the standard model of particle physics. Now, there's a lot
to supersymmetry. But, in short,
versions of this theory predict the existence
of a set of counterparts to the familiar standard
model particles, but that are hundreds
of times more massive. Some of them fit the
bill for dark matter. Sinking down into the depths
of quantum field and string theory, you can find all sorts
of strange fish, WIMPs, axions, neutralinos. Some of which may actually
exist and some of them may be dark matter. But it's all
mathematical fantasy until we detect the particle. We have detectors
here on Earth designed to catch the fall-out between
the unthinkably rare collisions between a dark matter particle
and an atomic nucleus. We also watch the heavens
for the equally elusive gamma radiation produced when dark
matter particles annihilate each other out in space. There's a big fat
Nobel Prize waiting for the scientists who
figure this one out. So get cracking in
the comments below. And I'll report any previously
undiscovered dark matter particles on the next
episode of "SpaceTime." Last time on "SpaceTime,"
we talked about black holes. And you guys asked some
seriously challenging questions. Let's see what you had to say. SafetySkull and others asked
whether a monkey falling through a black
hole's event horizon should see the entire future
history of the universe happen in the instant
before it crosses over? The answer, no. But it would see some of it. If the monkey were to
calculate the clock time of an external
observer as it fell, then that calculated time
would approach infinity as the monkey drew extremely
close to the event horizon. That lasts an infinitesimal
fraction of a second before it crosses. It would encompass
all future time. But would the monkey witness it? No. The time interval
that encompasses all future everything
approaches zero, or at least the Planck time. So it's not really happening
over a meaningful portion of the monkey's in-fall. And anyway, the photons
from the future universe will never catch
up to the monkey because that light
has to contend with the same
crazy-curved space-time that the monkey does. Although signals from the
monkey to the outside universe can be received at arbitrarily
distance times in the future, only signals within its past
light cone can catch up to it. The monkey may see some
time dilation effects from the local part
of its universe. But it's limited. Agen0000, and
others, pointed out that Hawking radiation will
eventually cause a black hole to evaporate. Given that a free-falling
monkey is eternally frozen on the event horizon with
respect to a distant observer, shouldn't the black hole
evaporate beneath it? And would this save the monkey;s
life or fry it in an eternity of Hawking radiation? Again, the answer is no. The simple and unsatisfying
way to put this is that the free-falling
monkey doesn't see anything weird about
space at the moment it crosses the event horizon. The space itself is in
free fall with the monkey. And so in its reference frame,
the rate of Hawking radiation is not time dilated. The idea of the event horizon as
this boiling hot sea of Hawking radiation isn't right. In fact, in the vicinity
of the black hole, this radiation is
poorly localized, having a wavelength of order
of the Schwarzschild radius. So at the instant of
crossing the event horizon, the monkey is not actually
bathing in this stuff. But is the monkey saved at the
last moment as the black hole evaporates way beneath it? Sorry, no. By falling through
the event horizon, the monkey's clock,
its universe, now contains events that happen
at the horizon, including the horizon's existence. The monkey's horizon crossing
corresponds to a time when the black hole exists. The distant observer
does witness the instance that the
black hole evaporates, with a huge burst of
Hawking radiation. And with that radiation comes
all of the remaining photons that the monkey emitted
before crossing the horizon. But the moment of horizon
crossing is never seen. It never even happens in the
distant observer's universe, either before or after the
black hole's evaporation. These were really
good questions. Keep them coming. And I'll see you next time. [MUSIC PLAYING]
Talks slower AND gestures more with his hands/body. He's like a Super Gabe!
Great work Matt! Looking forward to new episodes.
Yaaay! They're back! Can't wait for more!