Knocking on Heaven's Door - Lisa Randall [187]
The instruments that search for these Standard Model products of dark matter annihilation weren’t initially designed with this goal. They were conceived as telescopes or detectors out in space or on the ground to detect light or particles in order to better understand what is in the sky. By looking at what types of stuff gets emitted by stars and galaxies and exotic objects that lie within them, astronomers can learn about the chemical composition of astronomical objects and deduce the properties and nature of stars.
The philosopher Auguste Comte in 1835 mistakenly said about stars, “We can never by any means investigate their chemical composition,” which he thought beyond the boundary of attainable knowledge. Yet not too long after he said those words, the discovery and interpretation of the spectra of the Sun—the light that was emitted or absorbed—taught us about the composition of the Sun and proved him decidedly wrong.
Experiments today continue this mission when they try to deduce the composition of other celestial objects. Today’s telescopes are very sensitive, and every few months we learn more about what is out there.
Fortunately for dark matter searches, the observations of light and particles that these experiments are already engaged in might also illuminate the nature of dark matter. Since antiparticles are relatively rare in the universe and the distribution of photon energies could exhibit distinctive and identifiable properties, such detection could eventually be associated with dark matter. The spatial distribution of these particles might also help distinguish such annihilation products from more common astrophysical backgrounds
HESS, the High Energy Stereoscopic System located in Namibia, and VERITAS, the Very Energetic Radiation Imaging Telescopic Array System in Arizona, are large arrays of telescopes on Earth that look for high-energy photons from the center of the galaxy. And the next generation of very high-energy gamma-ray observatory, the Cherenkov Telescope Array (CTA), promises to be even more sensitive. The Fermi Gamma-ray Space Telescope, on the other hand, orbits the sky 550 kilometers above the Earth every 95 minutes on a satellite that was launched at the beginning of 2008. Photon detectors on Earth have the advantage of having enormous collecting areas, whereas the very precise instruments on the Fermi satellite have better energy resolution and directional information, are sensitive to photons with lower energies, and have about 200 times the field of view.
Either of these types of experiments could see photons from annihilating dark matter, or from radiation produced by electrons and positrons resulting from dark matter annihilation. If we see either, we stand to learn a lot about the identity and properties of dark matter.
Other detectors look primarily for positrons, the antiparticles of electrons. Physicists working on an Italian-led satellite experiment called PAMELA have already reported their findings, and they look nothing like what was predicted. (See Figure 80 for PAMELA results.) The acronym in this case stands for the mouthful “Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics,” which is somewhat mitigated by the nice way PAMELA sounds when spoken with an Italian accent. We don’t yet know if the PAMELA excess events are due to dark matter or to misestimations of astronomical objects such as pulsars. But either way, the results have absorbed the attention of