Hot dark matter is a hypothetical form of dark matter which consists of particles that travel with ultrarelativistic velocities. The best candidate for the identity of hot dark matter is the neutrino. Neutrinos have very small masses, and do not take part in two of the four fundamental forces, the electromagnetic interaction and the strong interaction. They do interact by the weak interaction, and gravity, but due to the feeble strength of these forces, they are difficult to detect. A number of projects, such as the Super-Kamiokande neutrino observatory, in Gifu, Japan are currently studying these neutrinos.
Dark matter is matter that cannot be detected by electromagnetic radiation, hence dark. It is postulated to exist to explain how clusters and superclusters of galaxies formed after the big bang. Data from galaxy rotation curves indicate that around 90% of the mass of a galaxy cannot be seen. It can only be detected by its gravitational effect.
Hot dark matter cannot explain how individual galaxies formed from the big bang. The microwave background radiation as measured by the COBE satellite is very smooth and fast moving particles cannot clump together on this small scale from such a smooth initial clumping. To explain small scale structure in the Universe it is necessary to invoke cold or warm dark matter. Hot dark matter therefore is nowadays always discussed as part of a mixed dark matter theory.
Famous quotes containing the words hot, dark and/or matter:
“Away went the messenger’s bicycle,
His serpent’s track went up the hill forever.
And all the time she stood there hot as fever
And cold as any icicle.”
—John Crowe Ransom (1888–1974)
“A shadow
not of a bird, not of a cloud,
draws a dark stroke over
the hills, the mind.
And another, another.
Our fears keep pace with us.”
—Denise Levertov (b. 1923)
“As deaths have accumulated I have begun to think of life and death as a set of balance scales. When one is young, the scale is heavily tipped toward the living. With the first death, the first consciousness of death, the counter scale begins to fall. Death by death, the scales shift weight until what was unthinkable becomes merely a matter of gravity and the fall into death becomes an easy step.”
—Alison Hawthorne Deming (b. 1946)