The universe,which has been expanding ever since the Big Bang,might have slowed down its rate of acceleration states city-based professor Varun Sahni of Inter-University Center for Astronomy and Astrophysics (IUCAA) in a recent paper published in collaboration with Arman Shafieloo of department of Physics,University of Oxford and Alexei Starobinsky of the Landau Institute of Theoretical Physics,Moscow. The trio poses the question- Is cosmic acceleration slowing down? In the year 1998,two international teams of scientists discovered that the universe was not only expanding,but that its expansion appeared to be accelerating. These claims were made after detailed observations of stellar explosions called Supernovae. When a star lights up as a supernova it can outshine an entire galaxy. Supernovae exploding in galaxies at vastly different distances from us allow astronomers to determine the rate which the universe was expanding in the past. Supernovae observations tenaciously carried out over the previous decade have shown that two thirds of the density in the universe is of a new form called- Dark Energy. Dark Energy has the curious property that,unlike other forms of matter,its pressure is negative. The expansion rate of Universe dominated by Dark Energy should ideally accelerate instead of decelerating. Although the presence of dark energy has received growing support,its nature remains a mystery, says Sahni. In its simplest variant,Dark Energy could be the cosmological constant,a non-evolving term introduced by Einstein in 1917 and later described by him as his 'greatest blunder'. As its name suggests,the energy density associated with cosmological constant does not change even as all other forms of matter dilute during cosmic expansion, adds Sahni. In January this year,observations of about a hundred new supernovae were published by an international team led by the Harvard-Smithsonian Center for Astrophysics (CfA). The distinguishing feature of these new supernovae was that they were much closer to our own galaxy,the Milky Way. If proved correct,the research paper published by the trio could change a number of fundamental assumptions about dark energy.
