Dark Matter

Both the rotation of spiral galaxies and the motion of galaxies in clusters suggest that, if our theories of gravity are correct, there is a mysterious dark matter pervading the Universe. For a while, it was thought that the dark matter could be large objects, such as stars that failed to ignite (brown dwarfs), the remains of dead stars (white dwarfs, neutron stars, or black holes), or even black holes that formed in the chaos of the Big Bang. Collectively, these objects were referred to as "Massive Compact Halo Objects," or MACHOS. However, these large objects would occasionally pass in front of other stars, and their gravity would bend the light from distant stars, focusing it towards us like a lens. Therefore, astronomers spent many years observing tens of thousands of stars to see how many became brighter over the course of several days, as the hypothetical large, dark objects passed in front of them. They did find the "microlensing" events they were looking for, but the numbers of them were too few to explain dark matter.

At the same time, particle physicists suggested that dark matter could be a new particle, one that never had been seen in laboratory experiments on Earth. Physicists can identify particles because they interact with other ones through one of the fundamental forces. For instance, electrons and protons, for instance, interact through the electromagnetic force. Likewise, the neutrons and protons within atoms, and the quarks within protons and neutrons, interact through the strong nuclear force. Physicists calculated that if the hypothetical new particle interacted via either of these forces, we would already have seen in in laboratory experiments.

That left two forces to look at. The hypothetical dark matter particle had to have a mass, and therefore interact gravitationally, because we saw evidence for it in the gravitational interactions within and between galaxies. It might also interact via the weak nuclear force, which was known to physicists as determining how radioactive element decayed, and in the interactions between neutrinos and other forms of matter. If the dark matter particle interacted through the weak interaction, it would be very rare for an experiment on Earth to see the interaction. Nonetheless, with enough patience and care, it might be possible to identify these interactions, and determine what the dark matter particles were. Theoretical physicists developed a class of candidate particles that interacted with other matter through only the weak nuclear and gravitational forces, and called them collectively "Weakly Interacting Massive Particles," or WIMPS. (The pun was most definitely intended!) There are several ongoing laboratory experiments to identify these particles.

However, until these particles are detected in the laboratory, one might justifiably wonder, "Is it possible that we simply don't understand gravity?" Indeed, we know that our theory for gravity, General Relativity, must be incorrect on the smallest scales, because the mathematics of it cannot be reconciled with quantum mechanics (in General Relativity, space is smooth and infinitesmally small objects can exist; in quantum mechanics, space has a smallest meaningful length, and is therefore made of discrete bits). Could it be possible that we also don't have a good theory for gravity for very large distances, the sizes of galaxies, clusters of galaxies, or even the universe? Are we really justified in inventing a new particle?

Astronomers believe so, in large part because of the above image. In the image are the yellow-colored galaxies of two clusters that have recently collided, as seen with the Hubble Space Telescope. The diffuse red glow is hot gas that is shining in X-rays, as seen by the Chandra X-ray Observatory. There is actually more mass in this gas than in the two clusters of galaxies. The blue color is a map of where the force of gravity comes from in the two galaxy clusters. This was computed by modeling the way in which the light from more distant was lensed by the closer clusters. In our current theory for gravity, the origin of the force is matter. The matter represented by the blue glow is many times larger than either that of the red gas or the yellow galaxies.

In the aftermath of the collision, the galaxies appear to have passed right by each other, and are moving toward the right and left of the image. The galaxies did not interact much, because the space between them was so large. In contrast, the hot gas did collide, slowing it down and causing it to remain between the two clusters of galaxies. The most important thing, however, is the blue model for the source of gravity, which we believe is dark matter. That mass seems to have followed the galaxies, and is moving away from the site of the collision. If there was no dark matter, there is no reason that the gravity should come from the galaxies, because they contain much less mass than the red gas. Instead, this image implies that dark matter is real, and consists of massive particles that, unlike ordinary matter, only interact very weakly with each other.

Simply put, the above image provides the most solid evidence yet that dark matter exists.