Tim Linden

The Galactic Center Gamma-Ray Excess

The Milky Way Galactic Center

High Mass X-Ray Binaries

Dark Matter


The modern periodic table, which now contains 118 different elements.

This is the period table according to Mendelev, in 1871. Click to see how the period table has expanded over the past 140 years.

Over the last few hundred years, physicists have been busy characterizing all the forms of matter that exist, and breaking it down into its most elementary components. In 1871 Mendelev took an important step, realizing that all the elements that we interact with today (all of the Oxygen, Hydrogen, Lead, and Carbon etc.) are transmutations of the same basic structure. Moreover, it was possible to understand how an unknown element would behave, by placing it into the periodic table, and comparing it with nearby elements.

While Mendelev's original table had many wholes (elements that weren't yet discovered), the table told us where to look for this missing matter. In addition to finding (or producing) all the missing components, we now have a physically motivated interpretation of this table. We understand that all elements are formed from three components (protons, neutrons and electrons), and that the chemical characteristics of the elements is due to the interaction of the electrons on their surface. Moreover, we now have a physical model that describes the interactions of these electrons (Quantum Mechanics), and explains the periodic nature of this table.

Over the last 100 years, physicists have dug deeper, trying to condense all of the crazy ddynamics in our universe down to only a few basic parts. And to a large degree, they've succeeded, finding that nearly all of the interactions in the universe - the gravity that causes us to fall to the Earth, the electrical interactions that allow us to surf the internet, and the weak nuclear forces that govern radioactive decay, can all be explained by the interaction of six quarks, six leptons, and five bosons.

However, something is missing. Observations taken by Fritz Zwicky in the 1920s, Vera Rubin in the 1970s, and many others indicated that galaxies and clusters of galaxies rotate faster than can be expected based on all of the matter that could be seen with telescopes. There must be some other type of matter, invisible to us, that produces the gravity that causes these massive galaxies to spin. You can think of this like spinning on a merry-go-round. The faster you spin, the harder you have to hold on. In the same way, if you look at a galaxy and see how fast all the stars are spinning, then you know how strong the gravitational force must be to hold all the stars together. Because physicists are creative, they named this invisible matter dark matter


The Andromeda Galaxy, still at low-resolution compared what modern telescopes observe!

The Andromeda Galaxy, the target of observations by Vera Rubin in the 1970s. Click to see in full detail!

Obviously, the first explanation wasn't to invent a new particle! Physicists first tried to apply the standard model that we have to explain this interaction. Maybe there was additional gas in these galaxies which was difficult to see with telescopes? However, this idea was ruled out because gas tends to absorb visible light that moves through it, and then when the gas gets heated up it emits infrared light that we can see with low-frequency telescopes. If there was enough gas to explain all the dark matter, then we wouldn't be able to see all the stars in the image of Andromeda above, instead we would see a "foggy" image with very few individual stars, and instead a dense cloud of low-energy infrared light. Similarly, this same analysis technique was used to rule out other explanations using the standard model particles we know of, including: dust, small planets, and even black holes (using a slightly trickier method). It seems that this new dark matter has an interesting property - not only is it not itself visible, it doesn't appear to interact with light at all!

In addition to the observation of spinning Galaxies, our observations of the Cosmic Microwave Background, the earliest light that can be observed in our universe, indicates that 13.4 billion years ago there was about 6x as much dark matter in the universe than visible Matter. This is the same ratio that is observed in galaxies today -- which gives us confidence that the dark matter is a stable particle in our universe.

In 2006, astronomers announced a shocking discovery, the "Bullet Cluster". This system is actually two clusters of galaxies (each with thousand of galaxies like our own Milky Way inside), which collided at nearly 1000 miles per second! Interestingly, an offset was observed between the location of the gas in these galaxies, and the location of the dark matter. While the colliding gas appears to have gotten stuck in the middle of the collision, the dark matter (and stars) from each galaxy cluster appear to have slipped right past each other. This indicates that the dark matter particle does not interact strongly with light (or itself).

However, these results seems to post a puzzle of their own. The standard model above contains four forces (gravity, electromagnetism, the weak nuclear force and the strong nuclear force). The dark matter particle does seem to interact with the standard model via gravity, but doesn't interact with electromagnetism (light). But we know that there is about as much dark matter in the universe as there is normal matter (there is 6x as much dark matter, but not a million times as much dark matter, or one one-billionth as much dark matter). This implies that these particles must have had some process by which they could interact in the early universe, so that they could sort out how much regular matter, and how much dark matter there was supposed to be. Think about a large jar filled with blue marbles and red marbles -- if the number of each type of marble were completely random, you might be surprised to find about the same number of blue marbles and red marbles in the jar. However, now lets set up some rules, or ``interactions" for the marbles -- every time two marbles crash into each other, they switch colors. So every time two blue marbles crash, they make two red marbles, and vice versa -- but when a red marble hits a blue marble, you still get a blue marble and a red marble at the end. Now, if we shake the jar enough -- at the end you will get 50% blue marbles and 50% red marbles, no matter what we started with.

We think that the universe works the same way -- in the early universe, there was some interaction between the dark matter particles, and the standard model particles. Dark matter could turn into standard particles, and vice versa. Since we know of the four forces for standard model particles, and we know that dark matter already interacts with one of these forces (gravity), it makes sense that dark matter might also interact via another force. It turns out (for technical reasons) that if dark matter interacts with standard model particles via the weak nuclear force -- that you expect the universe today to contain about 6x as much dark matter as regular matter!

This is exciting, because once we have an interaction to look for - we can build detectors to try and find the dark matter particle by searching for these very rare weak nuclear force interactions. There are three main search strategies. The first is to look for dark matter particles bumping (having weak-force interactions with) standard model particles, after the particles interact, the standard model particle might get knocked away, and we can search for it moving through our detector. Because these inteactions happen very rarely (and standard model particles bump against each other all the time) we have to devise sensitive experiments to remove standard model interactions. Thus, we go deep underground where there are not many cosmic rays that can collide with our detectors.

The LUX dark matter detector, located nearly 1 mile underground at Homestake Mine in Lead, South Dakota. This detector is expected to observe only a few dark matter interactions per year (perhaps as low as one) -- if the detector was placed near Earth, thousands of interactions would occur every minute due to cosmic-rays pelting the reactor from outer space. Even you are far too radioactive to be placed near this detector, the radioactive decays happening inside your body would produce thousands of noise events in this sensitive machine.

The LHC is the most powerful particle accelerator on Earth, capable of accelerating protons to 0.999999991 the speed of light! This is the ATLAS detector, one of the two chambers where these protons collide with each other to produce all sorts of matter. There is a person in this image for scale, can you find them?

Another method is to look for two standard model particles to crash into each other, and make dark matter particles in a detector. Of course, because any dark matter that we make is invisible, it is tricky to observe this process. Instead, we look for interactions where two protons crash into each other, and make some observable particle plus two dark matter particles. We can distingish this from interactions where only a visible particle is produced in the following way. Pretend to you are playing pool, but with an invisible 6 ball. You take a shot at the 7 ball, and you expect it to go in the same direction as the white ball that hit it - but instead it flies off to the side. Even though you can't see the 6 ball, you can guess that it must have been touching the 7 -- so that the momentum in this collision was conserved. Another way to say this, is that if two things crash, we expect one to go to the right, and one to go to the left -- if we have an interaction in our accelerator and everything goes to the right, we can guess that an unseen particle must have flown to the left, and that particle might be dark matter.

Finally, we can look for dark matter particles colliding with each other, and annihilating to produce standard model particles that we can observe. To see this, we want to look in regions where there is lots and lots of dark matter, adn that means that we need to use telescopes to look in space. One of the best ways to detect the dark matter particle is to use Gamma-Ray telescopes, and this is another major topic of my research interests. To find out more visit here!

This is a "map" of what the dark matter annihilations (that produce standard model particles) should look like in space. By far the brightest region is the center of our own Milky Way galaxy -- which has become an intriguing target for searches for the dark matter particle.

We have only covered some of the exciting history of dark matter here. There are a number of alternative particles that might explain the missing mass of our universe, low-mass Axions which interact with our standard model particles even more feebly than the "Weakly Interacting Dark Matter" particles described here, or more massive cousins of the neutrinos called "Sterile Neutrinos". The answer to the dark matter mystery might even require amending Einstein's thoery of relativity. The universe is full of surprises, and the mystery of dark matter tells us that there is still much left to discover.

Latest Results

TeV Halos

TeV electrons accelerated by pulsars may explain the diffuse TeV excess observed by Milagro.

Dark Kinetic Heating

Dark Matter collisions with neutron stars set a minimum neutron star temperature. This may be observable with next-generation instruments.

Star-Forming Galaxies

An analysis of 584 SFGs finds significant dispersion in their far-IR to gamma-ray correlation. SFGs significantly contribute to the IGRB.