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Rumors abuzz about Dark Matter

September 8, 2008 on 9:10 pm | In Astronomy, Dark Matter | 8 Comments

Our galaxy, like just about every galaxy we’ve ever observed, contains a very massive black hole at its center. Take a look!

We know some stuff about it, like its mass (3.6 million solar masses), its near-exact location (see the animation below), the orbits of many stars close to it, and possibly the black hole’s spin, too.

But one thing we don’t know about the galactic center is why there are the number of positrons that there are coming from it. We know there are positrons because like any type of anti-matter, they annihilate with their normal-matter counterparts, which is electrons, in this case. When matter and antimatter annihilate, they produce a special signal: two photons of the exact energy of that particle, as given by E=mc2. For electrons and positrons, this means you’ll see gamma-rays of exactly 511 keV in energy. Do we see them?

Oh, oh yes we do. So something is going on at the galactic center making this anti-matter!

And there have been rumors all over the internet that PAMELA, a detector, may have detected a signature of dark matter in these gamma-ray lines. I’m not on the PAMELA team, and I haven’t seen the raw data (just the preliminary stuff below), but I’m telling you now, don’t believe it so easily.

We don’t know what’s causing these positrons to be created, we don’t understand the galactic center environment well at all, and there are a myriad of candidates, astrophysical candidates for explaining this, without needing to invent a new particle. So it could be dark matter, but I wouldn’t bet on it. And I don’t think you should, either. People have been calling this gamma-ray excess “indirect proof of dark matter” for a long time, most recently with the EGRET team. When you scrutinize the results, however, you see that dark matter cannot explain this. Why not? Because annihilating dark matter should make anti-protons too, and we just don’t see ‘em.

Seriously, even physicists are writing about this like it’s already a great discovery. They don’t see the anti-protons, folks! That’s not evidence!

So when you hear it from somewhere else, take a moment and think about it. We’re scientists, and our standards for accepting things are simply higher than this.

Dark Matter: More Irrefutable Evidence

August 27, 2008 on 5:29 pm | In Dark Matter, cosmology | 24 Comments

A lot of people don’t like dark matter. It’s a crazy idea, after all. Think about everything in your experience: the skies, the sea, you and me, along with the Earth, the Moon, the Sun, and everything we see.

It’s really beautiful, isn’t it? And yet dark matter tells us that for every atom of normal matter in the entire Universe, there is five times as much dark matter, or matter that isn’t made up of protons, neutrons, and electrons.

How do we possibly know this? Well, all matter has something in common: gravity. No matter what you’re made of, if you have mass, you exert a gravitational force on everything else in the Universe. But if you’re made up of normal matter (remember: protons, neutrons, and electrons), you can also emit light under the right conditions. The Sun does this, for example.

So how can we find dark matter? Let’s take the biggest things in the Universe: clusters of galaxies. These are regions of space that have hundreds or even thousands of galaxies the size of our Milky Way in them, and all told they weigh over a quadrillion times as much as our Sun. It would make a great experiment if we could smash two of them together. Because the normal matter should stick together and heat up, and emit X-rays. But if there’s some other type of matter, something different from normal matter, it should just pass right through everything, right on through the other galaxy cluster, like an object in motion remaining in motion. So we look for two clusters of galaxies colliding:

Ladies and gentlemen — BEHOLD! I give you exhibit A, known as the Bullet Cluster. This is two clusters of galaxies colliding, and you can see the individual galaxies here. The pink is the X-ray gas, and you can see that’s where the normal matter collided, stuck together, and emits powerful light in the form of X-rays. But the blue, that’s where all the mass is. From a phenomenon called gravitational lensing, we can measure how much mass there is in a certain region of space. And as you can see, we find that most of the mass is not where the normal matter is.

But we are responsible people, us scientists, and we like to have more than one example before we draw a conclusion. And so I offer you exhibit B, cluster Abell 520:

BEHOLD! This is a cluster in the later stages of merger, so that some of the dark matter has had a chance to come back around to their mutual center of mass. Still, the light coming from the normal matter doesn’t trace where the mass is.

So we’re getting close, but can we find an example of this far away? Can we find a very distant cluster that has these same properties? Let’s take a look at a very special cluster: MACS J0025.

Looks like a big honking cluster, and it contains over 1,000 galaxies and weighs in at over a quadrillion solar masses. A big one, for sure. But where is all the mass? Let’s take a look at the gravitational lensing data and see (in blue false color) where it is:

Ahh, it looks like just two lobes, one towards the left of the image and one towards the right. Well, if these were two clusters that just collided, that would be all the matter that just passed through the center. Except normal matter doesn’t do that! It collides, sticks together, and heats up. What if we took our big X-ray observatory, Chandra, and looked at this cluster. Would we see X-rays coming from the middle? Where there’s very little mass?

ha-HA, we do! So when we put all these together, what do we find? Could the normal matter explain all the gravity we see, or do we need something else; some new type of matter? Ladies and gentlemen:

BEHOLD!!! It’s dark matter, different from normal matter, not made up of protons, neutrons, and electrons, but full of mass and exerting a gravitational force nonetheless. There are some awesome animations up at the Chandra Website, which I highly recommend to you.

Dark Matter in Our Solar System

June 25, 2008 on 2:05 am | In Dark Matter, Solar System | 70 Comments

Disclaimer: cutting-edge research ahead.

Sometimes I spend so much time on education and communication that I lose sight of an important fact: I’m a really good astrophysicist! So, for the last six months, I’ve been working on a project with a graduate student (hi, Xiaoying!) that I’ve been mentoring, and our work has finally come to fruition. Today, our paper entitled Dark Matter in the Solar System becomes publicly available. In it, we figure out how much dark matter there is at every place in our Solar System, and it’s pretty awesome!

Let’s give you the understandable version. First, in every big galaxy that we know of, including ours, there’s a big, diffuse halo of dark matter that surrounds and permeates all the stars and gas, looking something like this:

So we can model the halo, and figure out how much dark matter there should be where our Solar System is. And that’s what everyone else has done up until now when they’ve tried to figure out how much dark matter is in the Solar System. But our Sun and planets have been around for 4.5 billion years, moving around the galaxy for a long time:

Well, all this dark matter flies by the Sun, and flies off again. But here’s where our planets are actually important: occasionally, dark matter will fly by a planet, and the planet will steal some energy from the dark matter. We’ve done this for spacecrafts, using planets to both steal energy from them (which is how we get to the inner planets) and to give extra energy to them (which is how we get to the outer planets). The only thing we need to do this is gravity and a good initial aim. Here’s the Galileo spacecraft as an example:

So, how much dark matter experiences this lucky kind of interaction to bind it to the Solar System? Not most of it, that’s for sure. (In fact, we had to simulate over one trillion dark matter particles to find the answer!) But the final answer turns out to be a lot; about 1020 kilograms of dark matter is bound to the Solar System, or about 0.0018% of the mass of the Earth.

Not impressed? How about this little fact: the density of dark matter present at Earth is a factor of 16,000 times larger than the old, naive prediction that doesn’t take this into account! I can even show you, from our paper, where the dark matter is, relative to the old, naive expectation:

You see that first big spike on the left? That’s Mercury. The next two? Venus and Earth. (Mars is too little and too far away to show up.) Then the next big one is Jupiter, and if you look closely, you can see a little bump (that’s Saturn) and then another one (that’s Uranus and Neptune combined). Think people doing experiments looking for dark matter will pay attention to this? I hope so!

Will the LHC Create Dark Matter?

May 5, 2008 on 9:53 am | In Dark Matter | 28 Comments

Hector writes in and asks about someone from Sheffield in the UK who says that the Large Hadron Collider (LHC) will create Dark Matter:

The massive ATLAS detector will measure the debris from collisions occurring in the Large Hadron Collider (LHC) which recreates the conditions found in the early universe during the Big Bang when Dark Matter was first created. If the LHC does indeed create such particles then it will be the first time that the amount of Dark Matter in the universe has increased since the Big Bang - the LHC will effectively be a Dark Matter ‘factory’.

Well, Hector basically wants to know if this is true, or if this is about as likely as your car safely tunneling through a brick wall? Well, the first part is true. The LHC accelerates protons in a big circle. Some go clockwise, some go counterclockwise, and they smash them together at two separate locations, where they have detectors to see what comes out:

The collisions are energetic enough that they can make massive particles from that energy (since E=mc2). They expect to be able to make particles that are up to about 100-500 times heavier than the protons that they started with. This includes the Higgs Boson that particle physicists are looking for, but it also includes other possibilities that might exist at those high energies, including Dark Matter.

And they’re right that if we put enough energy into the accelerator, we can make anything that has a mass! But they’re wrong that that’s the only way to do it in the Universe. Ever hear of a…. black hole? They spin, and they accelerate particles very quickly. Much more quickly than our dinky accelerators on Earth:

In fact, the amount of energy released when particles near a black hole smack into other ones are thousands of times higher than the LHC will ever produce, and so if the LHC can make dark matter, the “natural accelerators” that we see have been making it for billions of years. Also, the LHC, if we’re lucky, will make “thousands” of these dark matter particles, totaling up to a whole 10-20 grams of dark matter! OooOOOooohhh! (That was sarcastic.)

Warning: details ahead! But even if the LHC makes dark matter, I don’t think we’ll be able to know it. Why not? Because dark matter has a mass, but no charge. It also is stable: it doesn’t decay. So if you make dark matter in an accelerator, you don’t see anything! Because we can measure things very well, we will be able to say, “Hey, there’s energy missing from this!” But we produce things very commonly that just show up as missing energy. We call them neutrinos. Will the LHC be precise enough to distinguish between, say, the production of 2 neutrinos (a very common event; it happens 20% of the time whenever you make a Z boson) and the production of a dark matter particle? My sources say no, but it’s possible. In any case, that’s more information than you asked for, but look at me all motivated on a Monday!

Ethan takes on Johannes Kepler

May 2, 2008 on 7:48 am | In Dark Matter, Solar System | 12 Comments

Can you believe that I had a fight today with someone who’s been dead for over 350 years, and I’m losing? — Ethan, yesterday

Of course you can believe it, when the man I’m fighting with is Johannes Kepler. I don’t get a chance to tell you about my research very often, mostly because it’s still a work in progress. But my latest paper was just submitted and is now out of the way, and so I’d like to tell you what I’m working on at the moment.

Well, there we are in the galaxy. We look up at the night sky, and we see our planets as well as all the stars that surround us. But you know what we don’t see? Dark Matter. We know it’s there, in a giant halo around our galaxy, the same way we find evidence for it in all galaxies. So I can pretty easily figure out how much dark matter is in the galaxy where our solar system is:

My problem is, if I just did that calculation, I’d get the wrong answer! Why? Because we’re not just lazing around in our galaxy; our Sun, with our four inner, rocky planets and our four gas giant outer planets have been zipping around through the galaxy for over four billion years. In that time, they’ve had plenty of opportunity to scoop up dark matter, like a vacuum cleaner. And that’s why I’m having a fight with this guy:

Turns out this problem of figuring out how much dark matter is in our Solar System now is actually pretty hard. But if I can figure out how the dark matter is moving, how the solar system is moving, and how gravity works to capture some of that dark matter, I win. How? Because I’ll be able to predict how much dark matter we should have surrounding our Sun, and I can compare it with observations to see if it matches!

But first, I have to figure out a way to beat this Kepler guy. My strategy?


Well, that’s not going to work. I’ll have to buckle down and solve it with a combination of physics and astronomy, using calculations and writing simulations. But that’s what I’m working on, and if I can solve this, then for the very first time, we’ll know how much dark matter we have in our own solar system! And that’s pretty damned cool!

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