Dark Matter: What is it?

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1 Dark Matter: What is it? Dark Matter: What is it? Key Concepts 1) Some dark matter consists of MACHOs (MAssive Compact Halo Objects). 2) Some dark matter may consist of WIMPs (Weakly Interacting Massive Particles). 3) The universe contains five times as much dark matter as ordinary luminous matter. Dark matter could also be called invisible matter. The properties of invisible objects are rather difficult to determine. We know dark matter exists because of its gravitational pull on luminous matter; otherwise, information is lacking. Some of the dark matter in galaxy halos consists of Massive Compact Halo Objects (MACHOs, for short). MACHOs can be failed stars ; balls of gas smaller than a star but bigger than Jupiter. Strictly speaking, brown dwarfs are dim matter, not dark matter. A cold brown dwarf (T = 300 K) These failed stars are called brown dwarfs. Brown dwarfs are warm enough to glow feebly at infrared wavelengths. 1

2 MACHOs can be ex-stars ; very dense compact objects that once were stars. These ex-stars can be white dwarfs (dense), neutron stars (denser), or black holes (densest). White Dwarfs Remnants of low-mass stars: M * < 4 M sun Mass < 1.4 M sun Radius ~ 0.01R sun (size of the Earth) White dwarfs are initially bright, but eventually they cool and fade away. Neutron Stars Remnants of massive stars: 8 < M * <18 M sun (??) Mass ~ M sun (???) Radius ~ 10 km (size of a small city) Density ~ g/cc With all reserve, we advance the view that supernovae represent the transition from ordinary stars into neutron stars, which in their final stages consist of extremely closely packed neutrons -F. Zwicky & W. Baade 1930s A sugar-cube of neutron star stuff weighs as much as all of humanity! 2

3 Black Holes Close to a black hole: Gravity is so strong that nothing, not even light, can escape. Infalling matter is shredded by powerful tides and crushed to infinite density. Becomes a Black Hole Black : neither emits nor reflects light Hole : nothing entering can ever escape Seeing what can t be seen Q: If no light gets out of a black hole, how can we ever hope to find one? A: Look for the effects of their gravity on their surroundings: A star orbiting around an unseen massive companion. Less than 20% of the dark matter is MACHOs: Most of the dark matter in galaxy halos consists of exotic matter. Suppose there existed a type of massive elementary particle that didn t absorb, emit, or scatter photons. Particle Physics for Dummies Astronomers Electron: low mass, negative charge Proton: higher mass, positive charge Neutron: proton mass, no charge We d detect such a particle only by its gravitational pull on luminous matter. ordinary exotic Neutrino: VERY low mass, no charge What are Neutrinos? Weakly interacting neutral subatomic particles. Mass-less (or very nearly mass-less). Travel at (or very near) the speed of light. Interact with matter via the weak nuclear force. Can pass through lead 1 parsec thick! Cosmic Gall (John Updike) Neutrinos, they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a pane of glass. 3

4 Neutrinos: Observed! Neutrinos make up part of the exotic dark matter. Detection of neutrinos is very difficult: Need massive amounts of detector materials. Work deep underground to shield out other radiation. We detect neutrinos from the Sun and from other sources. Although detecting neutrinos is difficult, it has been done! Although we don t know the mass of neutrinos exactly, we know it s tiny Most of the dark matter must be particles other than neutrinos. neutrinos electron One candidate for the position of dark matter : the WIMP. WIMP = Weakly Interacting Massive Particle Neutrinos provide < 10% of the dark matter. According to particle physics theory, WIMPs should be much like neutrinos only more massive. Dark matter is also found on larger scales, in clusters of galaxies. Neutrinos have already been detected: particle physicists are still trying to detect WIMPs. I predict a Nobel Prize for the 1 st to succeed! 4

5 Consider the Coma Cluster of galaxies; in the constellation Coma Berenices, about 100 million parsecs away. The Coma Cluster contains an intracluster medium (ICM) of very hot gas (T > 10 million Kelvin). It contains at least 10,000 galaxies, moving relative to each other at roughly 1000 km/sec. Visible light from stars in galaxies X-ray light from the very hot ICM Mass of stars in the Coma Cluster solar masses Mass of ICM (hot gas) in the Coma Cluster solar masses In the universe, there must be roughly five times as much dark matter as ordinary luminous matter (made from protons, neutrons, and electrons). 1/6 luminous Mass of dark matter in the Coma Cluster solar masses 5/6 dark 5