Starts With A Bang! » Dark Matter Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en Rumors abuzz about Dark Matter Tue, 09 Sep 2008 04:10:59 +0000 ethan 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 Thu, 28 Aug 2008 00:29:29 +0000 ethan 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 Wed, 25 Jun 2008 09:05:19 +0000 ethan 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? Mon, 05 May 2008 16:53:50 +0000 ethan 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 Fri, 02 May 2008 14:48:13 +0000 ethan

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!

DAMA is at it Again! Fri, 18 Apr 2008 19:22:49 +0000 ethan Remember how I told you earlier this week that DAMA was going to announce that they found dark matter, even though the signal that they found is not consistent with other experiments?

Looks like my powers of predicting the future are pretty damned good. They have a new plot with more data showing the continued modulation at a certain energy range:

and also one showing the fact that they see a bunch of extra events happening in that energy range:

So here’s the stuff that DAMA has seen: a nuclear recoil that has many events in a certain energy range, and has a 2% annual modulation in that energy range. Do we know what causes it? No. Do we know that it isn’t WIMPs of a certain type (from CDMS/Edelweiss)? Yes. But does this necessarily mean that it’s dark matter? No, although it’s possible. What we need is a way to identify what’s actually causing the signal that they see, and they don’t actually have that yet.

So in conclusion, they see more of the same signal they saw before, we still know that it isn’t the dark matter that CDMS and Edelweiss are looking for, and we’re still not sure what’s causing their signal. It could be a lot of things, and although certain types of dark matter can be ruled out, not all of them can. Dark matter? Maybe. But if I had a farm, I wouldn’t bet it on this.

Dark Matter: Anything Practical About It? Tue, 15 Apr 2008 09:05:00 +0000 ethan Yesterday, my good friend (and SWAB reader) Brian wrote a great comment about the practical reasons to explore space, where he talked about the overall economic impact that Space Exploration has had on the economy, as well as the impact it has had on our knowledge and understanding of the Earth, its environment, and how to manage/mitigate the threats to it. And that’s wonderful for exploring our Solar System and others.

But what do I do in the meantime? After all, this isn’t what I study or explore. So I asked this:

The practical arguments as to why exploration of space is worthwhile certainly hold a lot of water! But by that argument, the stuff that I do — looking for dark matter, trying to figure out dark energy, galaxy formation, the fate and birth and evolution of the Universe, etc. — is completely worthless. I agree that understanding the Universe helps us to understand our place/role in the Universe, but is there then a practical argument for understanding the stuff that is unrelated to us?

And so I thought about it: long-term, is there anything practical about studying dark matter? Well, the most abundant and efficient source of energy in the Universe is nuclear fusion, such as what goes on in our Sun: 4 Hydrogen nuclei fuse into one Helium nucleus, emitting about 25 MeV of energy per Helium nucleus. That’s about 0.7% efficient: for every kilogram of hydrogen that you fuse, 0.7% of that mass becomes pure energy. Is there anything more efficient than that? Sure: if you collide a hydrogen nucleus (a.k.a. a proton) with its anti-matter counterpart (an antiproton), that is 100% efficient!

Well, this happens to all particles and antiparticles: you run them into one another, and what you get out is 100% pure energy. There’s very little anti-matter in the Universe, and most of it would be very very detrimental to a spaceship, as it would annihilate with whatever it came into contact with first!

But dark matter, which we know doesn’t interact much with (and certainly doesn’t annihilate) normal matter, is very special. Because all realistic models of dark matter that we have consist of a very special property: Dark Matter is its own antiparticle! The Universe is also full of dark matter. So, if we could figure out how to collect and collide dark matter particles, we would have a 100% efficient source of energy that would virtually be unlimited. Because finding dark matter is 5 times easier than finding normal matter in the Universe.

Is this a long way off? You bet. But is this, long-term, the most efficient source of energy imaginable? Well, let’s see, at 100% efficient? You bet. And that’s the most practical thing I can think of about dark matter. All it’ll take, I’m sure, are some good sci-fi writers to put this in the public’s imagination!

Future News: DAMA discovers Dark Matter?! Mon, 14 Apr 2008 20:06:57 +0000 ethan The DAMA collaboration, an experimental team searching for dark matter via direct detection, is poised to report this week that they have discovered Dark Matter. And I’m here to pre-empt that by bringing you the truth: no, they haven’t.

They take some very cold (cryogenic) atoms, look for nuclear recoils resulting from dark matter collisions, subtract the background, and draw conclusions based on whatever’s left over. Their expectations are based on the following:

  1. Neutrons, neutrinos, and other standard model particles from the Earth, the Sun, and the Milky Way galaxy will collide with the detector, producing a background.
  2. Dark matter in a halo around our galaxy will also collide with the detector, producing a signal.
  3. The dark matter will be modulated annually; when the Earth moves faster with respect to the galactic center, we’ll see a greater signal; this signal will be periodic over the course of a year, every year.
  4. The problem with this reasoning is that a whole number of things happen over the course of a year, with a one-year period, including:

    1. The Earth gets closer and farther from the Sun during the year, with aphelion occurring around January 4th every year at a distance of 147.5 million km and perihelion occurring around July 4th at a distance of 152.6 million km.
    2. The amount of solar radiation striking Gran Sasso, Italy, where the experiment is performed peaks during the summer solstice every year, June 22, and is minimized during the winter solstice, December 21st.
    3. The Earth has a motion towards the center of the galaxy and away from the center of the galaxy that is also periodic over the course of the year; any background that is modulated annually will look just like the dark matter signal you expect.

    So the DAMA experiment ran for a number of years, and saw this annual modulation in signal, and declared that they had found Dark Matter, and released this graph:

    And it’s plain as day: whatever they’re seeing is changing in amplitude from about 102% of the average signal from sometime in June/July to about 98% of average in December/January. The problem is that you don’t know that you’re seeing any dark matter! This could all be normal stuff that’s just more abundant in the summer than the winter.

    So other dark matter searches looked for dark matter where DAMA predicted it. CDMS and Edelweiss were the two major ones. Here’s what they found when they did their experiments, where they could discriminate between background events and dark matter events. The CDMS results exclude everything above the red line, the Edelweiss results exclude everything above the dark green line, and the DAMA results predict a detection in the shaded red zone:

    Oh! Guess what, DAMA? You didn’t see any Dark Matter! So when the DAMA-LIBRA collaboration announces at the APS meeting this month that they found Dark Matter, or, perhaps, on Wednesday at the NO-VE international workshop in Italy (at R. Bernabei’s talk: first results from DAMA-LIBRA), you’ll be among the first to know that they saw noise that gets louder and softer over the course of a year, and nothing more.

    ]]> Somethin’ Neat about our Galaxy Thu, 27 Mar 2008 03:42:04 +0000 ethan So last week I was up at Pacific University in Portland, OR for a job interview. As part of a faculty interview, you have to lecture on a topic for undergraduates, but they give you the topic just a couple of days before. My topic was Gauss’ Law, which talks about the relationship between an Electric Field and an Electric Charge. Well, the same law holds for Newton’s theory of Gravitation with a gravitational field instead of an electric field and a gravitational charge (i.e., mass) instead of an electric charge.

    So I’m at work today thinking about this, doing the thing I do messing around with Dark Matter, and I asked the question:

    If I applied Gauss’ Law to our solar system within the galaxy, how much mass would it “see,” and how much of that is normal matter vs. how much is dark matter?

    So, I took the best measurements and simulations that were available, and calculated it! Well, the part of the galaxy we “see” is all the mass within a sphere of radius ~8 kpc (the distance from the center of the galaxy to us) centered on the galactic center. And that turns out to be about 90,000,000,000 Solar Masses. Guess what? That’s less than 10% of the mass of our galaxy! So over 90% of the mass in our galaxy is invisible to us gravitationally. What’s more, of the stuff we can see gravitationally, about 3/4 of that, locally, is dark matter. So even where we are, in the inner regions of our galaxy, the normal matter visible to us gravitationally is only about 2% of the mass of the galaxy! (And a large fraction of that is concentrated at the central bulge.)

    Just a quick note to share with you about my day!

    Afraid of the Dark? Tue, 11 Mar 2008 19:03:29 +0000 ethan So I gave a public lecture last (Monday) night called, “Afraid of the Dark: How We Know What We Can’t See” and videotaped it. Now, I’m pretty good at what I’m doing right now (research in theoretical cosmology), but I’m really good at public speaking and teaching, and here is me telling a public audience all about dark matter, how we know it exists, what makes it different from normal matter, and what I’m trying to do to find it/discover its nature for a good 40 minutes. (The intro and question/answers are cut out).

    It was a lot of fun; the audience was wonderful, and actually kept me for more than a half hour after my talk ended, asking questions that they were curious about! The video and audio qualities are not the best, but if you have 40 minutes and a pair of headphones at work, check it out. We’ve got a (60 Meg) video of the event right here… enjoy!

    Alternatively, you can see it in 4 separate youtube parts (below):