13Apr2012

Fermi Space Telescope Searches For Dark Matter.

Most of the Universe is made up of ‘stuff’ we can’t see

Scientists have further narrowed the search for a hypothetical particle that could be dark matter, the mysterious stuff that makes up 80 percent of all the mass in the universe. Caltech postdoc Jennifer Siegal-Gaskins presented the researchers’ results, compiled from two years’ worth of data from NASA’s Fermi Gamma-ray Space Telescope, at a meeting of the American Physical Society in Atlanta earlier this week.

Gamma rays are very energetic light, and the telescope looks for faint gamma-ray signals that are generated by a variety of sources, such as gas and dust spiraling into supermassive black holes or exploding stars. But another potential source of gamma rays is dark matter. Although no one is sure what dark matter is, one of the leading candidates is a yet-to-be-discovered particle called a weakly interacting massive particle (WIMP). When two of these WIMPs meet, the theory goes, they can annihilate one another  and generate gamma rays.

There are many possible versions of WIMPs, and they’re expected to span a wide range of masses, producing a range of gamma rays with different energies. Using Fermi, the scientists focused on 10 small galaxies that orbit the Milky Way, searching for gamma-ray signals within a specific range of energies. They found no signs of annihilating WIMPs, which rules out certain kinds of WIMPs as dark-matter candidates. Source: Caltech Press Release

What is Dark Matter..?

It’s everywhere… but where is it?

The first time that dark matter was suggested to exist, it probably sounded like a cop out. In the early part of the 20th century, physicists were having a difficult time explaining the rotation curves of other galaxies. In other words, galaxies didn’t appear to have enough mass to explain their observed rotation.

Then in 1933 physicist Fritz Zwicky proposed that perhaps the mass was there, but was non-luminous (not visible to the naked eye.

Subsequent studies on everything from galactic rotation curves, gravitational lensing, star cluster movements and measurements of the cosmic microwave background all indicate the presence of more mass than what we can directly measure in the electromagnetic spectrum.

In other words, in just about every way that we measure the Universe there appears to be more there than what we can see. So what is this “dark matter”?

Dark “Normal” Matter

Normal, luminous matter is made up of baryons – particles such as protons and neutrons. At first, dark matter was believed to also be made up of such material, but simply emitted little to no electromagnetic radiation.

While it is likely that at least some dark matter is composed of baryonic dark matter, it is likely only a small part of all dark matter.

Observations of the cosmic microwave background coupled with our understanding of the big bang theory, lead physicists to believe that only a small amount of baryonic matter would continue to survive today that is not incorporated in a solar system or stellar remnant.

Non-Baryonic Dark Matter:

It seems unlikely that the missing matter of the Universe is to be found in the form of normal, baryonic matter. Therefore, researchers believe that a more exotic particle is likely to provide the missing mass.

Exactly what this matter is, and how it came to be is still a mystery. However physicists have identified the three most likely types of dark matter and the candidate particles associated with each type.

Cold Dark Matter (CDM):

The most likely candidate for dark matter is Cold Dark Matter (CDM). However, there isn’t a strong candidate particle known to exist. The leading candidate for CDM is known as a Weakly Interacting Massive Particle (WIMP). However, there is a general lack of justification for existence of such particles; namely we are not certain how they were arise under natural circumstance. To investigate, researchers are conducting particle physics experiments hopping that collisions would produce a candidate particle.

Other possibilities for CDM include Axions – theoretical particles needed to explain certain phenomenon in Quantum Chromodynamics (QCD). Though these particles also have never been detected. And, finally, MACHOs (MAssive Compact Halo Objects) could explain the mass, but the specific dynamics remain a reach. These objects would include black holes, ancient neutron stars and planetary objects which are all non-luminous (or nearly so) and contain a significant amount of mass.

The problem here is that there would have to be a lot of them (more than would be expected given the age of certain galaxies) and their distribution would have to be surprisingly (impossibly?) uniform.

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