The nature of the universe remains a mystery because we don't understand dark energy, writes Dr William Reville
THE CURRENTLY accepted cosmological model tells us that the universe was born about 11 billion years ago in a massive explosion at a point and has been expanding outwards from this point origin ever since - the Big Bang model. Until recently it was generally believed that the rate of expansion is gradually slowing down under the influence of gravitational attraction, but in 1998 the shocking discovery was announced that our universe is accelerating its expansion. The acceleration is driven by a mysterious form of energy called "dark energy". If the universe continues to expand like this it will eventually thin out into "nothingness".
The evidence for accelerating expansion comes from studying a special type of distant exploding star (supernova) called "la supernova". This type of supernova occurs when a white dwarf star in a binary star system explodes. Matter moves from the normal star in the binary system to the white dwarf and when the white dwarf reaches a critical mass, called the Chandrasekhar limit, it erupts in a thermonuclear explosion. Since all white dwarfs achieve the same mass before exploding, they all achieve the same luminosity on explosion, and the brightness fades predictably with time. They can be used as "standard candles" by astronomers and their distance from us can be calculated, using the inverse square law, by observing their brightness. The distance tells us how long ago the explosion occurred.
Also, because the universe is expanding, the light that reaches us from these supernovae gets stretched out to longer wavelengths (redshift). Measurement of the redshift tells how much the universe has expanded since the explosion. By studying supernovae at different distances, astronomers constructed a history of the universe's expansion.
Two teams of astronomers carried out measurements in the 1990s and, to their great surprise, found that the expansion of the universe is accelerating. There are three possibilities for the large-scale geometry of the universe - it could either be "closed", "flat", or "open", corresponding to positive, zero, or negative curvature respectively. Other measurements of the cosmic microwave background radiation in the universe (a faint echo that remains from the Big Bang explosion), indicates that the universe is spatially flat. Because there is too little matter in the universe to produce this flatness on its own, the result is attributed to a "dark energy". This dark energy also drives the accelerating expansion of the universe. The effect of dark energy seems to vary at different times, slowing down or speeding up the expansion of the universe.
Amazingly, physics doesn't understand the nature of the vast majority of the "stuff" that makes up our universe. The ordinary matter with which we are familiar, found in the stars, planets and gas of the universe, and in life on Earth, accounts for only 5 per cent of the universe. Seventy-two per cent of the universe is composed of dark energy and 23 per cent of dark matter. While physicists have some notions as to the nature of dark matter, the nature of dark energy remains a mystery. We seem to be closer to the beginning than to the end of discovering all there is to be known about our universe.
When Einstein formulated his general theory of relatively in 1916, he introduced a constant into his equations called the cosmological constant. This constant ensured that the universe was static. In the absence of this constant, Einstein's equations described either an expanding or a collapsing universe, but such a universe contradicted the evidence for a static universe generally accepted in the early 20th century. However, when Edwin Hubble discovered the expanding universe in 1929, Einstein disowned his cosmological constant and is reputed to have said it was the biggest mistake he ever made.But some astronomers now identify dark energy with Einstein's cosmological constant. It acts as an anti-gravitational repulsive force and drives the accelerated expansion of the universe.
This whole field of dark energy is just opening up and there is much speculation as to its exact nature. But it is all only speculation at present. More data is needed to provide more information about the expansion of the universe. A Joint Dark Energy Mission is being planned in America for possible launch in 2014. It is hoped that this probe will find thousands of la supernovae a year (only about 60 supernovae have been analysed so far).
The discovery of the cosmic acceleration will very probably win a Nobel Prize. The two groups primarily involved in the discovery are the Supernova Cosmology Project, led by Saul Perlmutter at the University of California at Berkeley and the High-Z Supernova Search Team led by Brian Schmidt of the Australian National University. An interview with Saul Perlmutter is posted on the Scientific American website www.sciam.com
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - understandingscience.ucc.ie