December 27, 2019 0

Dark Matter: Crash Course Astronomy #41


A lot of people have noted that astronomy
is a humbling enterprise to pursue. After all, every time we make a new discovery, we
find ourselves further removed from importance. The Earth is but one planet among many, orbiting
a Sun that is one star among hundreds of billions, out in the suburbs of a galaxy that is one
among hundreds of billions more. It’s easy to feel pretty small when you
see all that magnificence out there. And we astronomers keep making it worse! Because
now we know that what we can see isn’t even everything there is. Normal matter, the stuff
that makes up you and me and all we observe in the Universe? That’s only a small fraction
of what’s actually out there. It’s time we talk about some very, very
dark matters. In the 1960s and 1970s, astronomer Vera Rubin
was observing spiral galaxies. She was interested in how they rotate, because you can learn
a lot about a galaxy that way. Think about the solar system: back in the 1600s, Johannes
Kepler figured out that the farther a planet is from the Sun, the slower it orbits. Isaac
Newton put numbers to that, calculating the strength of the Sun’s gravity, which means
we could, in turn, get the Sun’s mass. Same with galaxies. If you can measure how
they rotate — how rapidly gas clouds move in their orbits near the edge of the galaxy
for example — you can calculate the mass of the entire galaxy. Galaxies are so big
that you can’t physically see the nebulae move, but you can measure their Doppler shift,
which gives you their velocity. What Rubin expected to see was that the farther
out from the center of the galaxy the gas cloud was, the slower it would be moving,
just like more distant planets from the Sun move more slowly in their orbits. What she got though was the opposite. For
many galaxies, the farther out from the center you went the faster the clouds were moving!
Even at best, the velocities flattened out with distance, when they should have declined. That meant the gravity of the galaxies was
constant throughout the disk, not dropping from the center as you’d expect. But that’s
bizarre! Images of the galaxies showed that the number of stars and other massive objects
clearly got lower the farther from the center you went. There simply isn’t enough mass
far out from the center to account for the rapid rotation rates. Or — not enough mass from things we can
see. The only explanation is that there must be
dark material contributing to the gravity, something besides stars, gas, and dust. Not
only that, the galaxy must be embedded in a halo of this material to get the shapes
of the rotation graphs right. And there must be a lot of it! Rubin found there must be
five or times as much of this invisible material than the visible matter in galaxies. Back in the 1930s, astronomer Fritz Zwicky
had drawn a similar conclusion measuring the speeds of galaxies in galaxy clusters. The
member galaxies were moving too quickly to stay in the cluster; at the measured speeds
they should have been flung off. Therefore, he concluded, there must be far more gravity
in the clusters than just from the visible material. It turns out Zwicky’s observations had way
too much uncertainty in them to make any solid claims. He hugely overestimated the amount
of invisible material. Rubin’s observations, were far, far better and more accurate. However,
the term Zwicky used to dub this mysterious material stuck, and we still use it: dark
matter. Over the years, more observations have only
confirmed Rubin’s measurements. We see similar behavior in elliptical galaxies, for example.
Ironically, better measurements made of galaxy cluster member velocities show they do in
fact move too quickly, and clusters must have dark matter in them too. Zwicky was right
for the wrong reason — and in the end, Rubin is credited for making the discovery. Of course, the idea that so much of the material
in the Universe must be dark was met with skepticism by astronomers. Everything gives
off some kind of light. But more observations just kept supporting the existence of dark
matter. So, what IS dark matter? That was the big
question. Astronomers were methodical. They listed every single thing they could think
of that dark matter could possibly be: cold gas, dust, dead stars, rogue planets, everything.
Even weird subatomic particles that were predicted to exist in quantum mechanics theories, but
never seen before. Then they thought of ways they could detect
these objects. Cold gas would emit radio waves, for example. But everything they tried came
up empty. One by one they crossed objects off the list, and eventually everything made
of normal matter – atoms and molecules, protons, electrons, and neutrons – was eliminated. All that was left on the list was that truly
bizarre stuff: those screwy subatomic particles no one had ever seen before. One such particle
is called an axion. They’ve never been detected, but their properties match what we see of
dark matter: axions have mass, so if you have a huge cloud of them they’ll have enough
gravity to affect galaxies. They don’t tend to emit much light, so even a huge cloud of
them would be dark. And they another weird property: they don’t
interact with normal matter terribly well. An axion would pass right through you like
you weren’t there. If dark matter were made of axions, then clouds
of it could be enveloping clusters of galaxies and we’d never see them. If that’s the case,
how could we ever know if they’re there or not? It turns out there is a way. But before I
talk about that, we have to go over something pretty weird. Actually several somethings
weird. As I mentioned in our black hole episode,
one of Albert Einstein’s big ideas was that space wasn’t just emptiness between stars.
In a sense it was an actual thing, with all of matter and energy embedded in it. Although you have to be careful not to take
the analogy too literally, in many ways it acts like a fabric with everything stuck to
it. This is more than just a theoretical construct; it has real implications. For one, what we perceive as gravity – the
force pulling two objects together – was actually just a bending of this fabric of
space, a warp. It’s like a bowling ball sitting on a soft mattress; the surface of
the mattress bends, and if you roll a marble past it, the path of the marble will curve. This is true for light, too! It’s like having
a bend in the road; cars follow the bend as they move, and trucks do too. Everything does.
With light, it doesn’t bend nearly as much as matter does, but it does curve if it moves
through space distorted by gravity. The more massive an object is, the more gravity it
has, the more it warps space, and the more it can warp the path of a light beam. You know what else bends light? A lens! So
we call this effect “gravitational lensing.” Now picture a cluster of galaxies. It has
a lot of mass in a relatively small space – well, in cosmic terms. If there’s a
galaxy on the other side of the cluster from us, much farther away, the light that more
distant galaxy sends out gets bent on its way to us. The image of the galaxy can smeared
out, distorted, forming fantastic and weird shapes. Einstein’s equations tell us that the amount
of bending depends on the mass of the cluster, so we can, in theory, measure the mass of
the cluster by the distortion of objects behind it. Not only that, but it gives us a map of
where that mass is! Astronomers used this method on a cluster
of galaxies located about 3.5 billion light years away called the Bullet Cluster. It’s
a very special object; it’s actually not just a cluster, but a collision of two clusters.
That’s right, two huge groups of galaxies are physically colliding, and may eventually
merge to form one huger cluster. When galaxies collide they tend to pass through
each other like ghosts. But in clusters, between the galaxies, there are vast amounts of gas.
When clusters collide, the gas in the two clusters does indeed smack into each other,
and gets incredibly hot. So hot, in fact, the gas will emit X-rays. This provides an interesting opportunity.
Optical light images show the two clusters next to each other. They’ve already one
pass, in fact. The galaxies moved through each other as expected. The gas in the clusters
can’t do that, though, so you’d expect most of it to be between the galaxies, having
slowed down as the clouds collided with each other more or less head on. Using the Chandra X-ray observatory, astronomers
could map out where that hot gas was. And, as expected it lies mostly between the galaxies,
having slowed down after the collision. You can even see how the collision has shaped
the gas, forming a bow shock in one cluster like the waves of water created by a rapidly
moving boat. But there’s more. Even though the Bullet
Cluster is very far away, there are actually hundreds of galaxies even farther away that
can be seen in the optical images. The gravity of the matter in the Bullet Cluster distorted
those background galaxy images subtly, and by very carefully measuring that distortion,
a map of all the mass in the Bullet Cluster was made. Including dark matter. If dark matter is made
of axions, then you’d expect it to mostly be surrounding the subclusters themselves,
because, like the galaxies, the clouds of dark matter axions would pass right through
each other. And when you do make the map that’s exactly
what you see! The background galaxies show that there’s a lot of matter, shown here
in violet, centered on the two clusters, but it’s clearly not the hot gas seen by Chandra,
and is giving off no light. It looks very much like dark matter. Since the Bullet Cluster observations were
made, several other clusters have been observed showing the same sort of behavior. Attempts
have been made to explain these clusters without using dark matter, but in the end the simplest
explanation looks to be the best one. The stuff we see isn’t all the stuff there
is. To be honest, we still don’t know what dark
matter is. Axions are one possibility, but others exist. Lots of experiments have been
set up to try to detect the various flavors of subatomic particles, but the very nature
of dark matter — it doesn’t give off light and doesn’t interact with normal matter
well — makes it really hard to find. That’s why it took so long to even know it existed
in the first place! But even though it’s incredibly elusive, it turns out that
dark matter has had a profound effect on the Universe. As we’ll see in upcoming episodes, we’re
getting a pretty good idea of how the Universe got its start, and how it’s evolved over
the eons. We think smaller objects formed first, clumping together into larger and larger
structures. So stars formed first, then galaxies, then clusters. It turns out that larger structures
would have had a hard time forming in the early Universe as energy was blasted out by
the newborn stars and galaxies; bigger stuff couldn’t aggregate due to all that heat. That is, without dark matter. When you include
dark matter in the physics, the structures we see in the Universe CAN form. How about that? Something like 85% of the
matter in the Universe is stuff we can’t see, can barely detect, and is made of something
we know not what. But the largest structures in the cosmos owe their existence to it. We humans can get a little arrogant, thinking
we occupy a special place in the Universe. In a sense, we do, because most of the Universe
is cold, empty space, and we live in a relatively warm and dense part of it. But the stuff that
makes us up, the protons, electrons, and neutrons of normal matter – that’s in a serious minority
when it comes to all the matter there is. In a way, Obi-wan Kenobi was right; there
may not be an actual Force, but there IS dark matter. It surrounds us and penetrates us;
it binds the galaxy together. Today you learned that the kind of matter
we see – what we call normal matter – is only one kind of matter. There is also dark
matter, which we cannot directly see, and which interacts with normal matter only through
gravity. It affects how galaxies rotate, how galaxies move in clusters, and how large structures
form in the Universe. It can be detected in many ways, one of which is by seeing how its
mass affects the path of light coming from distant galaxies as it passes through dark
matter in galaxy clusters. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.

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