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Dark Matter and its implications


Dark matter is non-luminous matter that cannot be directly detected by observing any form of electromagnetic radiation (light), but whose existence is suggested because of the effects of its gravity on the rotation rate of galaxies and the presence of clusters of galaxies.

The Density of the Universe

The universe as we understand it now started with the 'Big Bang' when it began a period of very rapid expansion. Astronomers and cosmologists alike have been trying to figure out whether the Universe will continue to expand forever or recollapse in a 'Big Crunch'.

The fate of the Universe all depends on the ratio of the actual density of the Universe to its critical density. Cosmologists denote this ratio with the Greek letter Omega. If Omega is greater than one, then gravity will cause the universe to recollapse, if less than one it will perpetually expand, and if equal to one it will expand at a decelerating rate perpetually.

The critical density of the Universe has been worked out to be about 5 atoms per cubic metre, which is very little considering that it is far closer to a perfect vacuum than experimenters on Earth could ever hope to achieve.

The actual density of the Universe is not known, though if one were to take only the visible material you would be left with a density of 0.2 atoms per cubic metre. Many cosmologists feel that there is much more matter in the Universe than is visible through our telecopes, and they have observational and theoretical proof for their beliefs, as I will describe in the next few paragraphs.

Rotation Rate of Galaxies

Galaxies near the Milky Way appear to be rotating faster than would be expected based on the amount of visible matter that appears to be in these galaxies. Based on their rates of rotation, many astronomers think that up to 90 percent of the matter in a typical galaxy is invisible.

Clusters of Galaxies

In the universe stars are grouped into galaxies and the galaxies themselves are grouped into clusters. Some astronomers argue that if some reasonable assumptions are accepted - specifically, that the clustered galaxies are bound together by gravity, and that the clusters formed billions of years ago - then it follows that more than 90 percent of the matter in a given cluster is made up of dark matter. Otherwise, the proponents of this interpretation argue, clusters would lack enough mass to keep them together, and the galaxies would have moved apart by now.

What Is Dark Matter?

Astronomers and cosmologists know that dark matter exists but as yet do not know what it is composed of or how much of it there actually is.

There are many candidates for dark matter, including undetected brown dwarf stars, white dwarf stars, black holes, or neutrinos with mass (neutrino, fundamental nuclear particle that is electrically neutral and of much smaller mass, if any at all, than an electron), or indeed exotic subatomic particles, such as WIMPs (Weakly Interacting Massive Particles) or MACHOs (MAssive Compact Halo Objects). Physicists are currently searching for such particles in underground laboratories (to prevent interference) and ways to detect them.

So, Will the Universe Recollapse Then?

Despite the fact that up to 90 percent of the mass of the universe may still be made up of dark matter, cosmologists don't think that it would be enough to have gravity halt the expansion and cause a recollapse. This thinking is due in part to observation of supernovae in 1997, which indicated that the expansion is still accelerating and not slowing down at all.

A longer lasting universe won't do much for us though as it is thought that in the very distant (like, 1030 years distant - 1,000,000,000,000,000,000,000,000,000,000 years!) the proton will decay, all the stars will run out of fuel and be engulfed by black holes, which in turn will radiate all their mass, (as described in the article on black holes), leaving the Universe a vast, cold, sterile and lifeless place.

However, some cosmologists do think that there will be no Big Crunch!

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Author: Astronomy Today Staff

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