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Introduction Main Topics Important Dates and Discoveries Important Scientists Cosmological Theories Through History The Universe By Numbers Glossary of Terms A Few Random Facts Sources E-mail: lukem@lukemastin.com Web-site designed by: Luke Mastin |
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DARK MATTER
The simple Big Bang theory is, however, not without its potential problems, and some aspects require further investigation and explanation. One such problem is the rather unfortunate fact that about 90% of the matter which is predicted to exist in the universe appears to be invisible or otherwise unaccounted for! The evenness of the cosmic microwave background radiation (the afterglow of the initial Big Bang) suggests that the matter emitted from the Big Bang should have been spread around very smoothly. But we know that the universe is in fact clumpy, with clusters of galaxies and great voids of empty space in between. Actually, in 1992, NASA’s Cosmic Background Explorer (COBE) satellite did discover some variations or ripples in the brightness of the afterglow, which probably resulted from a period about 450,000 years after the Big Bang, when some parts of the universe became just a few thousandths of a per cent denser than others. These barely noticeable clumps of matter grew to become bigger clumps due to the cumulative effects of gravity, and the denser regions (the “seeds” of structure) became ever denser over time, leading to the great clusters of galaxies we see today. However, the modelling of this theory revealed that the 13.7 billion years which has elapsed since the Big Bang is actually nowhere near long enough for the huge structures of today’s universe to have developed, by the gradual process of gravity and increasing density, out of the tiny imperfections and clumps indicated by the COBE satellite. This could only have happened if there was, and/or is, much more matter in the universe than our estimates of the matter tied up in visible stars. This has led to speculation about so-called "dark matter", an unknown substance which emits no light, heat, radio waves, nor any other kind of radiation (thus making extremely hard to detect).
There is corroborating evidence for the existence of dark matter from other sources. The stars in spiral galaxies like our own Milky Way whirl about the galactic centre, prevented from flying off into intergalactic space by gravity. However, calculations of the speed of the whirling (dating back to work by Fritz Zwicky and Vera Rubin in the 1930s) suggest that the galaxy is actually spinning much faster than it theoretically should be in order to maintain its current equilibrium, and the only way this can occur is if galaxies (ours and all the others) actually contain about ten times as much matter as is visible in stars. The same applies on a larger scale to entire clusters of galaxies, millions of light years across, which would also need to contain about ten times more material than we can see in order to hold together. By coincidence, this is exactly the factor of additional matter required by the models to allow the structures we see in today’s universe to have developed from the ripples in the cosmic microwave background radiation discovered by the COBE satellite. The problem is that dark matter, whatever it may be, is extremely hard to detect. It is affected by gravity, but not by any of the other fundamental forces; it has no electrical charge; it does not seem to stick or clump together but floats freely; and it passes through atoms of normal matter without any kind of interference we can detect. So, despite its apparent ubiquity, no-one really knows what dark matter is. Among the possible candidates are so-called MACHOs (short for MAssive Compact Halo Objects), such as small brown and black dwarf stars, cold unattached planets, comet-like lumps of frozen hydrogen, tiny black holes, possibly even mini dark galaxies. Astronomers are using a technique known as gravitational lensing to try to spot where such matter might lie.
Neutrinos and other so-called “exotic particles” are another possibility. Neutrinos are elementary particles which have no electric charge and hardly interact at all with ordinary atoms. It is hypothesized that they could have come into existence during the first second after the Big Bang as part of the reaction with the photons that were created at that time, and it is calculated that there could be hundreds of millions of them for every atom in the universe. So, even if each neutrino weighed a hundred-millionth as much as an atom, they could theoretically still be the dominant matter in the universe. Scientists are also investigating another kind of exotic particle, known as WIMPs (short for Weakly Interacting Massive Particles), hypothetical particles which may be all around us but which pass through normal matter without stopping. Experiments to look for WIMPs are being carried out deep down in rocky mines where other cosmic rays cannot penetrate.
Back to Top of Page Introduction | Main Topics | Important Dates and Discoveries | Important Scientists | Cosmological Theories | The Universe By Numbers | Glossary of Terms | A Few Random Facts | Sources |
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