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doi:10.1038/nindia.2013.45 Published online 10 April 2013
Three Nobel laureates, joint winners of the 2011 physics Nobel Prize, have challenged the claim by a group of Indian astrophysicists, who say that the very foundation of modern relativistic cosmology (MRC) proposed by Albert Einstein is "vacuous".
The Indian theoretical astrophysicists at the Bhabha Atomic research Centre (BARC) in Mumbai, led by Abhas Mitra, recently published two rather strongly worded and widely-read papers^{1,2} also claiming that the supposed acceleration of the Universe — a topic for which the 2011 Nobel Prize in physics was awarded — is an "illusion."
The Nobel winners — Brian Schmidt, Adam Riess and Saul Perlmutter — say that their results are observational in nature and boil down to the observed "Luminosity Distance Relationship", which shows that the acceleration of the Universe is "homogeneous and consistent in all respects".
"(The Universe is homogeneous) with a Cosmological Constant dominated cosmos in a flat homogenous Universe. So I think this result stands independent of the (BARC astrophysicists') claim," Brian Schmidt, speaking for the trio, wrote in an e-mail to Nature India.
Mitra, head of BARC's theoretical astrophysics section, on the other hand, said Einstein introduced the concept of spherical Universe — an abstract mathematical space which has finite volume and yet has no edge. "It is something like the surface of a balloon; a bug crawling on it will find no edge even if the surface area is finite."
Later observations, Mitra said, showed that the observed Universe appeared to be expanding, and the 'Big Bang' model was invented. This model of Universe envisages Einstein's balloon to be inflating so that dots (galaxies) painted on its surface find themselves receding with respect to each other.
Further, from 1998 onwards, mainstream cosmology believes that the Universe is undergoing an 'accelerated' expansion — a discovery that won the Nobel Prize in 2011. This extra push causing the acceleration was ascribed to an unseen, undetected 'Dark Energy' having repulsive gravity in contrast to normal matter for which gravity is attractive. It is surmised that the most reasonable form of 'Dark Energy' is the Cosmological Constant (represented by the Greek letter Lambda Λ) that Einstein hypothesized in 1917 in connection with his spherical Universe.
The two recent papers by the BARC physicists claim that Cosmological Constant is actually zero and Einstein's spherical Universe flattens to an infinite plane. "In simpler terms, this means Einstein's universe is just the three dimensional infinite vacuum which has neither any matter nor any energy," Mitra said.
These papers point out that matter in the universe, whether galaxies or their clusters, are distributed in a lumpy manner known as fractal distribution —far from the perfect smoothness and homogeneity assumed by both Einstein and the modern 'Big Bang' cosmology.
"Our result is a blow to the very idea of relativistic cosmology where matter is assumed to be spread in the overall cosmos in a smooth uniform manner," Mitra said. "Further, non-existence of Lambda puts a serious question mark on the concept of Dark Energy."
The BARC physicists conclude that Dark Energy and 'acceleration' of the observed universe are artifacts of departure of the lumpy and fractal universe from a smooth universe assumed by cosmologists. These papers also point out that some of the astrophysical effects can be better explained in the framework of a universe which is static rather than expanding.
Schmidt said while he was always in favour of theorists and observers to challenge this paradigm, "I believe the assertions of these authors that the Universe is fractal are not widely held. Indeed, I believe they are counter to the latest observations made from Large Scale Structure^{3}."
"We have made the largest volume measurement to date of the transition to large-scale homogeneity in the distribution of galaxies. We use the WiggleZ survey, a spectroscopic survey of over 200,000 blue galaxies," Schmidt added.
The team presented a new method of defining the 'homogeneity scale', which Schmidt says, is more robust than methods previously used. "Due to the large cosmic depth of WiggleZ, we are able to make the first measurement of the transition to homogeneity over a range of cosmic epochs," he added. They have demonstrated the robustness of their results against selection function effects using "a Λ cold dark matter (ΛCDM) N-body simulation and a suite of inhomogeneous fractal distributions".
"Our results are in excellent agreement with both the ΛCDM N-body simulation and an analytical ΛCDM prediction. We can exclude a fractal distribution at 99.99 per cent confidence," he said.
It is to be seen how the BARC theorists' view is received by mainstream cosmologists. "One needs considerable effort to read and appreciate Mitra's papers", says eminent Indian cosmologist Jayant Narlikar, proponent of steady-state cosmology which also assumes smooth matter distribution. "My general impression is that despite the rather aggressive tone of his papers he has a point to make," he told Nature India.
But Mitra insists that though their result could be embarrassing and unpleasant to many relativistic cosmologists, "it is nonetheless an exact one and obtained from two independent considerations."
Additional reporting by K. S. Jayaraman.
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