Whenever I mention dark matter in anyway in the Guide to Space, or in a questions show,
I get a bunch of responses that have essentially the same point.
Astronomers are just speculating, why do they even think dark matter is a thing?
They’re sure going to be embarrassed in the future when they realize they were wasting
all this time.
Oh, astronomers.
Foolish, gullible astronomers.
The reality, of course, is that many astronomers have dedicated their lives to the mystery
of dark matter.
More than a decade of school, working with incredibly complicated math, and then many
more years of observations, using some of the most powerful and sensitive instruments
ever designed by human beings.
And obviously I know that people can spend their lives dedicated to nonsense.
So in this video I want to do two things.
First, I’m going to spend some time explaining how astronomers realized that dark matter
is something real.
In fact, the evidence is overwhelming, and I’m going to get into it.
And then I’m going to talk about the fascinating work going on around the world to search for
dark matter.
What are the individual experiments, observatories and projects which are trying to chip away
at this mystery.
Before I go further into this dark matter.
I want to give you an analogy that comes from my Astronomy Cast co-host, Dr. Pamela Gay.
Because I don’t think that most people really understand the state of the search for dark
matter.
Let’s imagine you’re driving your car and it starts to make a knocking sound.
You take it into the mechanic and they can’t figure out what’s causing it.
They ask you to drive some more in different conditions and maybe you can help located
the problem.
You realize that it only makes the sound when you’re going up a hill and turning left.
You bring back this new information to the mechanic and this gives them a better place
to search for the source of the problem.
If some friend ridiculed you because of your “dark knocking sound”, all they’d have
to do is spend a little time in your car and they’d hear the sound too.
The problem is definitely there, it’s just that you and the mechanic haven’t figured
out what’s causing it yet.
But you will, oh… you will.
You have a mystery, and you haven’t solved it yet.
That’s dark matter.
And dark energy is an stranger mystery, but that’s a topic for another video.
I think the name “dark matter” is probably the source of the confusion.
It should have been called something like invisible matter, or mystery matter, or crazy
gravity, or… something.
Okay, back to dark matter, and let’s start with a brief history.
I’d like to thank Dr. Brian Koberlein for his comprehensive history of dark matter on
his blog.
I’ll put a link in the shownotes so you can learn more about it.
The effect of dark matter was first discovered by the astronomers Fritz Zwicky, who was studying
the motion of galaxies in the Coma Cluster.
Located about 321 million light-years from Earth, this cluster contains more than 1,000
separate galaxies.
During his study of the cluster in 1933, Zwicky calculated that the motion of all the galaxies
was too fast for the gravitational interactions of the galaxies themselves.
There had to be some kind of missing mass that was contributing to their movement.
Of course, it’s possible that the individual galaxies happened to be flying past each other,
but the same result was found in all the galaxy clusters that astronomers could locate.
The next key piece of evidence came with the way that galaxies themselves rotate.
Think about the way that planets orbit the Sun.
Each planet goes at a different speed depending on its distance from the Sun.
Mercury completes an orbit every 88 days, while Earth takes 365 days and Pluto takes
248 years.
You would expect the stars within a galaxy to do the same thing.
Stars close to the center of the galaxy whip around quickly, while the ones in the outskirts
take their time.
Through her pioneering work of measuring the rotation rates of individual stars in distant
galaxies, Vera Rubin figured out that spiral galaxies rotated like disks.
All the stars moved the same speed around the galactic center.
One idea, of course, was that there was some kind of hidden matter, like the dark nebula
we can see here in the Milky Way.
These block the light from a more distant object, hiding it from our point of view.
But astronomers developed techniques to measure the radio signals coming from these dark clouds
of matter, and the amounts in galaxies didn’t account for the amount of mass it would take
to make galaxies and galaxy clusters behave the way they do.
Astronomers were left with two possibilities.
Either their understanding of gravity at the largest scales was wrong.
This idea was known as Modified Newtonian Dynamics, or MOND.
As long as you were willing to put in new equations for gravity, you could predict the
kind of motions observed in nature.
The other idea was that there was some kind of invisible particle.
A particle that accounts for the vast majority of the mass in the Universe, but it doesn’t
interact with regular matter in any way we can detect, apart from gravity.
These were known as Weakly Interacting Massive Particles.
In order to better map out the dark matter in the Universe, astronomers used a technique
called gravitational lensing.
This is where the gravity from a foreground object, like a galaxy cluster, distorts the
light from a more distant object, like another galaxy cluster.
Astronomers have done incredibly comprehensive surveys of the sky, and mapped out where the
blobs of dark matter are, and how they surround galaxy clusters.
One famous example of this is the Bullet Cluster, where astronomers could observe clusters of
galaxies colliding with each other.
They could see the stars in the galaxies, they could measure the locations of giant
clouds of hot gas colliding because of the X-rays they emit, and they could measure the
dark matter through its gravitational lensing.
And what they found was amazing.
The stars are so far apart, they just pass by one another without colliding.
The gas does collide, and bunched up into regions that glowed bright in X-rays.
But surprisingly, the dark matter didn’t collide with anything, not with the gas, stars
or even itself.
If dark matter is a particle, it must be tiny - astronomers say it has a small cross section.
And yet, it has to be massive, since it dominates the area with its gravity.
Better observations across the large scale structure of the Universe show how dark matter
must have been necessary to get these galaxy clusters collapsing in the ways they do, and
the gravitational lensing observations are now so precise, they can see the exact distributions
that match these predictions.
Another survey of dark matter was to search for it in the Cosmic Microwave Background
Radiation, of course.
This is the afterglow from the Big Bang.
A time when the Universe was about 380,000 years old, and light was finally able to escape
into space.
The European Space Agency’s Planck satellite performed an all sky survey of this cosmic
microwave background, mapping out the distribution of dark matter compared to regular matter
in the sky.
When you look at the CMB, the temperature fluctuations tell you how much regular matter
and energy there is compared to dark matter.
When that early Universe was so hot and dense, the radiation pushed against regular matter,
while it didn’t push against the dark matter.
Astronomers have built models with different ratios of dark matter to regular matter, to
match up the one they see in the CMB.
Based on this survey, astronomers were able to calculate that the Universe is made of
4.9% regular matter and 26.8% dark matter.
Oh, and another 68.3% dark energy, but again, that’ll have to be another episode.
Astronomers are certain that dark matter is there, but they still don’t know what it
is.
As my friend Dr. Ethan Siegel says, “When someone puts forth the hypothesis that dark
matter doesn’t exist, the onus is on them to answer the implicit question, okay then,
what replaces General Relativity as your theory of gravity to explain the entire Universe?”
What’s your general theory of sound that replaces my idea that my car is making a strange
knocking noise?
Now, I hope, I’ve convinced you that astronomers aren’t arrogant, they’ve got a genuine
mystery they’re trying to chase down through observation and experiment.
And I’ll get to them in a second, but first I’d like to thank:
Hadi Zolfaghaari Dany Noacco
Gaute Moon Incrediwebbs
Joseph Matheny Bruce Jividen
And the rest of our 837 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
In the last few decades, astronomers have continued to search for dark matter.
Narrowing down what it might be: invisible particles or gravity behaving strangely at
large distances.
When it comes to particles, there are three possibilities: hot, warm and cold.
In this case, hot dark matter would be a particle that’s moving close to the speed of light,
while cold would indicate that it’s moving very slowly.
An example of hot particles are neutrinos.
These are the nearly massless particles streaming from the Sun and other stars.
At any point you’ve got about 100 trillion of these tiny particles passing through your
body, moving at nearly the speed of light.
They rarely interact with anything out there in the Universe.
In fact, a neutrino will, on average, be able to fly through a light-year’s worth of lead
without getting stopped.
Physicists detect neutrinos in enormous underground reservoirs of water surrounded by incredibly
sensitive detectors.
When the occasional neutrino does interact with a molecule of water, it releases a cascade
of particles which can be detected.
That sounds like a good candidate for hot dark matter, right?
Well, the problem is that neutrinos are moving close to the speed of light.
This means that won’t ever clump up in the way that astronomers observe dark matter doing,
through gravitational lensing and the cosmic microwave background radiation.
Since dark matter doesn’t seem to clump at all, hot, fast moving particles are ruled
out.
Sorry neutrinos.
Instead, slower moving, cold dark matter particles seem like the most likely culprit.
There are literally dozens of experiments searching for cold dark matter particles right
now.
They’re all based on the idea that even if dark matter barely interacts with matter,
it can happen from time to time and you can observe it.
Experiments are running to detect every possible particle theorized.
Let me give you just one example: the Super Cryogenic Dark Matter Search, or SuperCDMS.
The experiment is located 700 meters underground in an old mine in Minnesota.
Assuming that dark matter is this cold, slow moving particle that comprises the vast majority
of matter in the Universe, and assuming that it doesn’t really interact with regular
matter, you’d expect many of these particles to be passing through any spot on the Earth
at all times.
Every now and then, one of these dark matter particles would interact with regular matter
and release a cascade of particles that could be detected.
This old mine is deep underground, shielded away from cosmic rays and human pollution,
so only particles that can pass through hundreds of meters of rock will be detected.
It gives scientists a clean signal.
The detector is equipped with silicon and germanium crystals cooled down just above
absolute zero.
This is going to sound totally new age, so bear with me.
If dark matter particles pass through the detectors, they’ll set off vibrations in
the crystals that will be detectable.
An even more sensitive version is under construction at a deeper facility in Sudbury, Canada.
Once it’s fully operational in the 2020s, SuperCDMS SNOLAB will be able to detect cold
dark matter particles with a mass between 1 and 10 protons.
Another way scientists are searching for dark matter is using particle accelerators, like
the Large Hadron Collider.
Instead of waiting for dark matter particles to drift into their detectors, they’ve tried
to create them.
Particle accelerators work by pushing particles to immense speeds, creating an enormous amount
of kinetic energy.
When the particles are slammed into each other, that kinetic energy freezes out into matter,
which can then be studied.
Different models for dark matter have been proposed, and the right combination of energy
and particle collisions could generate a particle that matches the properties of dark matter.
Another experiment at CERN is called OSQAR, or the Optical Search for QED Vacuum Bifringence,
Axions and Photon Regeneration.
It’s searching for particles known as axions, which could be a candidate for dark matter.
It involves firing a laser down a vacuum chamber which is exposed to an incredibly powerful
magnetic field.
As the photons travel down this chamber, some of them could turn into axions.
At the end of the chamber there’s a barrier.
The visible light is blocked by the barrier, but the axions should be able to pass through
this wall and then turn into photons on the other side again.
At this point, there’s no concrete evidence for axions, but there are several experiments
searching for them.
A much longer vacuum chamber is in the works, and there’s a counterpart to OSQAR called
the CERN Axion Solar Telescope, which is looking for axions coming from the Sun.
There’s a detector on board the International Space Station called the Alpha Magnetic Spectrometer
which could be the one to discover dark matter.
In its first 5 years of operation, the instrument detected over 90 billion cosmic ray events:
protons and other particles moving at close to the speed of light.
These particles contain vastly higher energy than anything that could be produced in the
Large Hadron Collider, so they’re like a natural particle accelerator.
One indication for dark matter could be the hundreds of thousands of particles of antimatter
which have already been detected by the AMS.
The source of this antimatter is still a mystery, but one idea is that it’s a side effect
from particles of dark matter occasionally colliding with itself.
Perhaps the most epic particle detector is the IceCube neutrino lab, located in Antarctica.
This giant telescope is a series of detectors embedded into a glacier - it’s one cubic
kilometer of ice.
When neutrinos and other particles pass through a vast volume of water, they can occasionally
interact and release a cascade of particles.
IceCube has been one of the most important instruments for physicists, setting limits
on the mass of particles that dark matter could be.
At this point, scientists still don’t know what dark matter is.
But with dozens of experiments, they’re continuing to search, and better narrowing
down what it can’t be.
At some point in the future, we can look back at this search with a definitive answer.
I really enjoy a mystery, and being a journalist gives me a chance to watch the search for
dark matter unfold, day after day, as ideas are tested, falsified, rejected, and replaced.
This is science.
This is how it works, and the journey is as important as the destination.
What do you think?
Let me know your thoughts in the comments.
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