Mystery of matter is behind the great darkness of our universe
ONE OF THE biggest mysteries in science is the fundamental nature of the largest component of the fabric of the universe – dark energy. Identified for sure only in 1998, dark energy constitutes 74 per cent of the total mass-energy of the universe and is responsible for the current accelerating expansion of the universe, writes 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.
The massive gravity of the white dwarf in a binary system sucks matter to itself off the ordinary star until the white dwarf grows to a critical mass (known as the Chandrasekhar limit) when it erupts in a thermonunclear explosion. Because all white dwarfs explode having reached the same mass, they all emit the same luminosity (astronomers refer to them as standard candles). By observing this brightness astronomers can calculate the distance the supernova is from Earth by using the inverse square law (the luminosity decreases in inverse proportion to the square of the distance from the source). The distance also indicates how long ago the supernova occurred.
As light from the supernova travels to Earth, the space through which it travels is expanding, thereby stretching the wavelength of the light wave to a longer wavelength. This stretching is called “red-shifting”. Astronomers can calculate, from the extent of the red shift, how much the universe has expanded since the supernova explosion. And, by studying many supernovae at different distances away, the history of the expansion of the universe can be calculated. And so it was discovered that the universe is now expanding at an accelerating rate.
The universe is flat on a large scale, ie its geometry is planar (Euclidean) rather than elliptical or hyperbolic, eg parallel straight lines remain the same distance apart along their lengths; in an elliptical universe, straight lines would eventually meet. A flat universe contains just enough matter (the critical mass) for the gravitational force to halt the expansion associated with the Big Bang in an infinity of time. However, there is too little ordinary matter plus dark matter in the universe to account for this flatness. Dark energy is the missing ingredient that makes flatness possible and it also causes the accelerating expansion of the universe. Astronomers calculate that the total mass-energy (fabric) of the universe is partitioned as follows: dark energy 72 per cent, dark matter 24 per cent, ordinary matter (made of atoms) 4 per cent.
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 – understandingscience.ucc.ie