Mystery of Matter is Behind the Great Darkness of Our Universe
This article first appeared in today's Irish Times and has been reposted with the kind permission of the author, William Reville.
There is very good evidence that the universe began about 13.7 billion years ago in a huge explosion (Big Bang) at a point. The universe has been expanding outwards like a balloon from this point of origin ever since. It was long thought that this expansion is gradually slowing down under the braking influence of gravity. However in the 1990s, to the great surprise of astronomers, it was discovered that, far from slowing down, the expansion of the universe is currently accelerating, propelled by a mysterious dark energy that acts like negative gravity.
The evidence for accelerating expansion came from studying distant Type 1a supernovae. A supernova is the death of a star in a catastrophic explosion when, for a short time, it becomes millions of times brighter than our sun. A type 1A supernova is produced in a binary star system – two stars, one a white dwarf and the other an ordinary star, revolving around a common centre of gravity. A star like the sun becomes a white dwarf after it exhausts its nuclear fuel and expels most of its outer material, leaving a very hot, extremely dense and gravitationally strong core.
|A White Dwarf Star|
We know dark matter exists because of its gravitational effects on visible matter (as discussed in my column of April 29th) and physicists have reasonable ideas about its fundamental nature. But dark energy remains a total mystery. All we know is that it fills the vast “empty” spaces in the universe and accelerates the expansion of this space.
Some astronomers think that dark energy is equivalent to the cosmological constant introduced by Albert Einstein (1879–1955) into his general theory of relativity equations in order to keep the universe constant in size – everybody then believed the universe was static. When astronomer Edwin Hubble (1889–1953) discovered that the universe is expanding, Einstein discarded the cosmological constant, describing it as his “greatest mistake”, but recent developments may revive the cosmological constant. Einstein realised that “empty” space is not empty and his equations allow for more space to come into existence. Also, the cosmological constant version of relativity theory predicts that “empty space” has its own energy. Therefore, as the universe expands, more space and its associated space-energy appears and, so, space energy is not diluted as space expands. Therefore, this constantly growing space energy could cause the universe to expand at an accelerating rate. But, nobody understands why the cosmological constant should exist.
This is very exciting stuff. Anyone who doesn’t think so must be made of dark matter.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC – http://understandingscience.ucc.ie/