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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.
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