Questions About Dark Matter
Lately I’ve been researching dark matter and dark energy, and I’ve been wondering, out of all the candidates that could be dark matter (brown dwarfs, neutrinos, black holes, etc.), which is most likely? I was also wondering the same about dark energy.
Thank you for your inquiry! Here is what I can tell you about dark matter and dark energy:
Actually, developments since the 1970s and 1980s have helped rule out all of the dark matter candidates you listed below (planets, brown dwarfs, black holes, neutrinos) as the primary dark matter candidate. Planets, brown dwarfs, and black holes belong to a class of dark matter candidates called ‘MACHOS’, which stands for MAssive Compact Halo Objects, and the idea is that these objects are all quite massive compared to the amount of light they emit. Microlensing surveys of the Galactic bulge, which detect massive bodies (i.e. MACHOs) bending the light of background stars, have found far too few of these MACHOs for MACHOs to be the primary dark matter candidate. Meanwhile, neutrinos, which are “hot” (a jargon term meaning that they travel at almost the speed of light that has *nothing* to do with their temperature). Because neutrinos move so fast, they never would have slowed down enough in the early universe to coalesce via gravity into the galactic dark matter halos we observe today.
The current theory, called Lambda-cold dark matter (or Lambda-CDM), favors a new type of particle for dark matter that, unlike neutrinos, moves slowly enough to coalesce and form galactic halos. This particle is often called a ‘WIMP’, for Weakly Interacting Massive Particle. Several theories of particle physics, including super-symmetry, predict the existence of the WIMP, although it has not been found yet. Many different physics experiments are looking for the WIMP. These all involve detectors made of heavy nuclei located up to a mile underground! WIMPS should interact with other matter very rarely, so keeping the detector underground protects it from other particles like cosmic rays that would introduce noise to the experiment, while still allowing the WIMPs, which can pass through the entire Earth without interacting with a single atom, to (sometimes) interact with the detector. There is also the possibility that the Large Hadron Collider ( http://lhc.web.cern.ch/lhc/ ) at CERN could produce dark matter particles; some scientists are actively looking for a dark matter signal in the LHC data.
The currently favored cosmology, Lambda-CDM, involves a component called dark energy that exerts a negative pressure. Unlike matter, which attracts via gravity, dark energy is a term we use to mean that the fabric of space time does work on the universe to accelerate its expansion. Lambda-CDM predicts that the acceleration of the expansion has always had the same value (i.e. the pressure has always been the same).
I think you would be interested in a fantastic website, Particle Adventure, in which you can explore the science and history of particle physics through interactive displays and quizzes. The website has won several awards, and it is where I learned much of the particle physics I know.