“I soon became convinced… that all the theorizing would be empty brain exercise and therefore a waste of time unless one first ascertained what the population of the Universe really consists of.” –Fritz Zwicky
You very likely know that there are four fundamental forces in the Universe: gravity, electromagnetism, and the weak and strong nuclear forces. While only some particles experience the nuclear and electromagnetic forces, anything with mass or energy — which is everything we know of — is subject to gravity.
The strong nuclear force binds all the nuclei heavier than hydrogen together, the weak force is responsible for radioactive decays and the (incredibly rare) neutrino interactions, but the two you’re most familiar with are gravity and electromagnetism. Operating practically everywhere in the Universe, the two of them are the reason you can sit where you are right now as you read this.
The reason that you are safely anchored down to the ground is the force of gravity, accelerating everything at Earth’s surface downwards at 9.8 m/s2. But you are (most likely) not accelerating downwards, and that’s because your body is composed of these electromagnetically-interacting particles — atoms — that the floor/ground/chair pushes back up on you at with an equal and opposite force to gravity.
The net result is that the electromagnetic force and the gravitational force cancel out, and so you remain where you are. When you toss a ball, it makes a parabola, as it gets accelerated down due to gravity. But once the ball hits the ground, it interacts electromagnetically, preventing it from falling through the floor. But imagine, hypothetically, that you instead tossed a neutrino, an object which doesn’t interact electromagnetically? What happens then?
Because the neutrino doesn’t interact electromagnetically or via the strong force, but does interact gravitationally, it would follow the same parabolic path that the ball did, until it ran into the surface of the Earth. At that moment, while the atoms in the Earth would collide with the atoms in the ball (and that’s an electromagnetic interaction), because neutrinos don’t interact electromagnetically, the neutrino would simply pass through the Earth’s atoms, as though they weren’t even there. It would plummet in an orbit — a nearly perfect ellipse (nearly, because the Earth isn’t a point mass concentrated at its very core) — and would eventually return to its starting point. This is the same way a comet has an elongated, elliptical orbit with the Sun at one focus of the ellipse: due to the force of gravity alone.
The lack of strong and electromagnetic interactions, a vital characteristic of neutrinos, is also a characteristic of dark matter! What would happen if we took two giant blobs of matter — a mix of dark and normal matter — and let them go in space? What would happen?
Shown above is the results from such a simulation. In pink is the normal matter, made up of atoms, while in blue is dark matter. Initially, both the dark and normal matter move together, accelerating towards the other blob. When they first collide, the atoms smash into one another, slowing down and heating up! But collisions are electromagnetic interactions, so dark matter doesn’t do it. Instead, the dark matter passes right through everything: through the normal matter in its own blob, through the other blob’s dark matter, through the other blob’s normal matter. So what we should see, shortly after a collision like this between, say two galaxy clusters in space, should be a separation in space between the dark matter (observable through the effect of weak gravitational lensing) and the normal matter (observable through the hot X-ray emissions that should come from the colliding gas).
These colliding clusters — known as the Bullet Cluster — strongly support a picture where galaxy clusters are made up predominantly of a large, diffuse halo of dark matter with a much smaller amount of normal matter in the form of collapsed, star-containing structures and gas.
We can then go back to the beginning of the Universe and simulate how galaxies should form in a Universe filled with dark matter, dark energy, and normal matter in the proportions we think are present.
And we can compare it to the galaxies that are actually there in the observed Universe, and see how well the simulations agree with the data!
The answer is extremely well, of course, but we don’t merely rely on a visual inspection. Rather, we do our analysis quantitatively, and see what the best fit cosmological model is to this data. What do we find?
A Universe, dominated by dark energy, where 17% of the matter is normal matter and 83% is dark matter. The “wiggles” you see in the power spectrum, above, come from normal (baryonic / atomic) matter, which would go all the way down to the bottom of the graph were there no dark matter. This is one of the strongest arguments against a Universe without dark matter, and has been made even stronger in recent years by better observations by the Sloan Digital Sky Survey.
But the Bullet Cluster is not the only cosmic smash-up between galaxy clusters in the Universe, although it might be the simplest, earliest-stage one we’ve observed. Things are much less easy to decipher in, say, cluster Abell 520.
With the X-ray and lensing data clearly more complicated than in the Bullet Cluster, it is difficult to piece together exactly what’s going on here. Perhaps this is an intermediate stage in a cluster merger? Perhaps something unusually violent is happening here? Or, spectacularly, perhaps dark matter is not behaving the way all other indicators are pointing?
The above image was released in 2007, and what you want to make sure to do is check that your observational data is solid. So they went back, got to use the Hubble Space Telescope to improve their observations, and got even better data, which was just released a few days ago. What — in a new set of false-colors — did they find?
What are we looking at here? From the NASA site itself:
Starlight from galaxies, derived from observations by the Canada-France-Hawaii Telescope, is colored orange. The green-tinted regions show hot gas, as detected by NASA’s Chandra X-ray Observatory. The gas is evidence that a collision took place. The blue-colored areas pinpoint the location of most of the mass in the cluster, which is dominated by dark matter.
So in this case, the optical presence of the galaxies (orange), so well-aligned with the dark matter in the Bullet cluster, are independent of both the gas, in green, and the dark / overall matter, dominated by the blue color. Perhaps this is easier to see if we look at each of these components separately.
There’s some overlap of the dark matter with the luminous galaxies, which ought to move together with the dark matter, but there’s also substantial overlap of the dark matter with the hot gas, which is an unexpected surprise!
Does this mean that dark matter’s doing something weird? My first instinct — mostly because the evidence for the standard theory dark matter is so overwhelming from so many different sources — is to wonder whether this isn’t either a later-stage collision, and/or if there isn’t enough cool gas mixed in with the central hot gas to obscure a group of galaxies at the center that are actually there, but behind the gas? (This cosmic catastrophe, after all, is about 2.4 billion light-years away, and in three dimensions.) The jury is still out:
“This result is a puzzle,” said astronomer James Jee of the University of California, Davis, leader of the Hubble study. “Dark matter is not behaving as predicted, and it’s not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing.”
If I make a composite of the three separate image components — luminosity, mass, and X-ray emissions — without those distracting background galaxies, what do we find?
Something very puzzling is going on here. If this is as young a collision as the Bullet Cluster, it’s conceivable that dark matter is doing something very weird. We have very few examples of high-speed galaxy cluster collisions in the Universe, with Abell 520 and the Bullet Cluster being the two best measured ones, and yet they appear — at first glance — to be inconsistent with one another!
I’ll definitely be following this story to see if there’s a resolution, but if it turns out that this is as young a collision as the Bullet Cluster, there are no hiding galaxies, and the current picture of dark matter cannot explain what these galaxy clusters are doing, we may be learning an awful lot more than we bargained for awfully soon. Some are betting that’s what will happen; I am by far more cautious, and would still happily bet on the standard picture of dark matter, with perhaps some complex kinematics — maybe involving multiple mergers — for the collision. For the time being, I’m content to agree with Ray Sanders, and say that the dark matter core of Abell 520 is mysterious. What’s the solution to the mystery? I’ve made my wager; what’s yours?