Saturday, December 4, 2010

Dark Matter

The existence of dark matter in the universe can only be inferred from its gravitational effects on ordinary matter and on electromagnetic radiation. This is so because dark matter can neither emit nor scatter electromagnetic radiation. Dark matter constitutes 80 percent of the matter in the universe while ordinary matter makes up the remaining 20 percent. Ordinary matter is basically everything which makes up the Earth, the planets, the stars and the vast quantities of gas and dust across interstellar and intergalactic space.

Although ordinary matter can account for a tiny proportion of dark matter, the vast majority of dark matter is made up of something else entirely. While the properties of dark matter can be somewhat constrained, the particle constituents of dark matter continue to elude detection. Dark matter is not made up of atoms and it does not interact with ordinary matter via electromagnetic forces. Hence, the study of dark matter has so far been largely based on the observable gravitational effects that dark matter imposes on ordinary matter and on electromagnetic radiation.

There are a number of independent sources of evidence for the existence of dark matter. Stars are known to orbit around the centre of galaxies and their orbital speeds do not decrease with increasing distance from the galactic centre. This is rather unexpected because the galaxy must have much more mass than can be attributed to ordinary matter alone; otherwise the orbital speeds of stars should decrease with increasing distance from the galactic centre. Thus, dark matter is responsible for the additional mass that can’t be attributed to ordinary matter alone.

Gravitational lensing is another independent piece of evidence for the existence of dark matter and this phenomenon occurs when light from a background object gets deflected by the gravitational field of a foreground object. This can distort the image of the background object and also change its observed brightness. A more massive foreground object will create a more pronounced gravitational lensing effect. Observations of gravitational lensing by foreground galaxies have shown that the amount of mass required by the galaxy to generate the observed lensing far exceeds the combined mass of all its stars and ordinary matter.

A paper entitled “Planet-Bound Dark Matter and the Internal Heat of Uranus, Neptune, and Hot-Jupiter Exoplanets” by Stephen L. Adler from the Institute for Advanced Study at Princeton explores the possibility that the accretion of planet-bound dark matter by gas giant planets could significantly contribute to their internal heat.

The Milky Way galaxy is situated at the centre of a vast halo of dark matter. The dark matter in the vicinity of the solar system is believed to be distributed in a way such that a cubic volume of space measuring 10000 by 10000 by 10000 kilometres contains around 500 grams of dark matter. Although this may seem sparse, scaling up the volume of space to one cubic light year will give a mass of dark matter this is around 80 times the mass of the Earth. One light year is the distance light travels in a period of one year. The dark matter in the vicinity of the solar system orbits around the galactic centre of mass along with the solar system.

It is not known if there is dark matter that is gravitationally bound to the Sun or to the planets in the solar system. Gravitational conglomerations such as stars and planets can accrete the ambient galactic dark matter over time such that Sun-bound and planet-bound dark matter can have densities that are many orders of magnitude greater than the ambient density of dark matter.

Planet-bound dark matter can contribute to internal heating of the planet by depositing energy inside the planet as the dark matter particles lose orbital energy by interacting with the particles of ordinary matter that make up the planet. Planet-bound dark matter can also contribute to internal heating if the particles that comprise them are self-annihilating. When self-annihilating dark matter particles meet, they annihilate each other and convert their mass into energy which can be deposited within the planet as internal heating.

Interestingly, the contribution to internal heating of a planet by the accretion of dark matter can explain the anomalously low rate of internal heat production for Uranus as compared to Neptune. Uranus has an almost identical internal structure and composition as Neptune and it should be producing the same amount of internal heating as Neptune. However, one key difference between Uranus and Neptune is that Uranus is tiled 98 degrees with respect to the plane of the solar system, whereas Neptune is only tiled 28 degrees. For comparison, the Earth has an axial tilt of 23.4 degrees.

The large axial tilt of Uranus is believed to be caused by a massive impact event, whereby an object around the mass of the Earth slammed into Uranus. This impact event could have pushed Uranus out of its accreted planet-bound cloud of dark matter and leave it with a much lower rate of internal heat production than Neptune. Before the impact event, Uranus would have a similar rate of internal heat production as Neptune.