Astronomers at the Sloan Digital Sky Survey have achieved an amazing feat of mapping the galaxies in the universe in a three-dimensional plot. They have used this to get a handle on how fast the universe is expanding.
Five years of observing the sky has yielded this enormous map which comprises 1.2 million galaxies spread out over 650 cubic billion light years.
Using this map, the astronomers have obtained an estimate of the rate of expansion of the universe, the best so far. Not just that, knowing the expansion rate better also gives a better handle on the so-called dark energy.
Observations of distant galaxies, made in 1998, revealed that the rate of expansion of the universe was increasing. This was completely unexpected, because while the universe had to expand as a result of the big bang, cosmologists thought this expansion would slow down due to the effect of gravitational attraction. Eventually, the acceleration of the expansion was attributed to dark energy — a mysterious and hitherto unexplained factor. Some other observations pointed out that the attractive force was provided by not just the matter in the universe but a large amount of “dark matter,” too. It was this interplay between the attractive (retarding) effects of matter and dark matter on the one hand and the tearing apart (accelerating) effect of dark energy on the other hand that resulted in the speeding up of the universe’s expansion rate.
It is believed now, judging by the way these effects play out, that the amount of matter makes up just about 5 per cent of the universe. The remaining, nearly 95 per cent consists of some amount of dark matter and a huge amount of dark energy.
While neither dark matter nor dark energy has been observed or understood through experiments, major efforts are on to detect or measure these around the world. It is in this context that the present measurement by BOSS (Baryon Oscillation Spectrographic Survey) assumes significance.
The study made observations on the so-called Baryon Acoustic Oscillations — sound waves generated in the early universe from the big bang which happened about 13.8 billion years ago. These waves got frozen about 400,000 years after the big bang, and set up a length scale for measuring distances. This resulted in there being a slight preference in distance between two galaxies. This distance is known as the acoustic scale and has been measured before by observing the cosmic microwave background. What the astronomers at SDSS have done now is to calculate the way this acoustic scale has increased since then, thereby getting a handle on the expansion rate of the universe and thereby an estimate of dark energy, based on popular theories.
It must be pointed out, however, that while there is a large body of scientists who believe in the concept of dark energy, there is a small section that thinks that there must be an alternative explanation.
More experiments to verify or dispute the existence of dark energy are in the pipeline across the world.
