Ashoke Sen

Indian Scientist Ashoke Sen Wins World’s Richest Academic Prize

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The prize, which is almost three times that of the Nobel Prize – which is frequently shared by two or three winners, has been introduced by Yuri Milner, a Russian student of physics who dropped out of graduate school in 1989 and later made billions as an investor in companies like Facebook.

The reward is aimed at recognizing contributions of younger researchers to fundamental physics. The nine winners of 2012 are expected to form the committee to decide on the awardees of next year.

Ashoke had received the Padma Shree in 2001 and the SS Bhatnagar Award in 1994. He was also elected Fellow of the Royal Society of London in 1998, and to the Indian National Science Academy in 1995.

Further,Yuri Milner, 50, became an overnight sensation in California’s Silicon Valley. In the past three years, he has invested greatly in social-media companies including Twitter, Facebook and Spotify and today his various investment funds are worth approximately $12 billion, and his private worth is set at $1 billion.

He created the award out of a love of theoretical physics, which he studied at Moscow State University and the Russian Academy of Sciences during the 1980s and early 1990s. The early prize winners were chosen by Milner himself.

Unlike other awards, such as the Nobel Prize, this award can be given to theorists whose ideas have not yet been supported by data. The objective is to reward innovative concepts that are driving theoretical thinking forward.

Hidden clue

Astronomers had known for many decades that the Universe, after its birth in a cataclysmic explosion dubbed the ?Big Bang? 13.7 billion years ago, has been expanding, with the galaxy clusters moving away from one another. The expansion, experts believed, would slow down with the ageing of the cosmos, because gravitational attraction would pull the clusters closer, eventually shrinking the size of the Universe. That notion received a jolt in 1998 when astronomers discovered that the expansion rate, instead of slowing down, was speeding up.

So some kind of an anti-gravity force must have been at work. Astronomers called it the ?dark energy.? It was a chance discovery; astronomers studying distant supernovae, or the explosive moments of dying stars, came upon it. The dark energy remains mysterious because while subsequent observations have confirmed that it exists, conventional theories of the Universe can?t explain such an anti-gravity force.

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Attempts so far to explain this accelerating expansion of the Universe by tweaking the existing theories dealing with the birth and evolution of the cosmos have either run into trouble or thrown up hard-to-swallow results. Now, Sami, at the Inter University Centre for Astronomy and Astrophysics (IUCAA) in Pune, and his colleagues from three countries have shown that the dark energy may be explained through the complex mathematics of the string theory, originally invented to account for the myriad particles and forces comprising the cosmos.

In a paper just published in the journal Physics Review D, the researchers have shown that the acceleration may spring from an exotic all-pervasive field of exotic entities called tachyons pervades the entire Universe. Besides Sami, the three other researchers are Edmund Copeland from the Nottingham University in the UK, Shinji Tsujikawa at the Gunma National College of Technology in Japan, Mohammad Garousi from the Institute of Theoretical Physics and Mathematics in Iran. They have established a set of mathematical rules that can give rise to the puzzling dark energy.

The word ?dark? in dark energy is actually a glorified adjective to camouflage our ignorance,? says Prof. Naresh Dadhich, director, IUCAA. ?No one knows what dark energy is. Sami and the others have now tried to build a model for dark energy directly from the mathematics of string theory.?

The calculations by the four researchers are part of an effort to find ways to connect string theory with the observed features of the Universe. String theory emerged as an attempt to close the gap between two extremely powerful theories of physics ? quantum field theory and Albert Einstein?s general relativity. The first describes matter at the smallest level, while general relativity provides the foundations for gravity, explaining the movements all big objects in the cosmos.

The dream of physicists absorbed in the string theory is that it will explain all the particles and forces in the Universe. The theory tries to account for all the particles as vibrations of a single fundamental entity called the string. Just as a single guitar string can be plucked to create different notes, each particle is a different note on the cosmic string.

The string theory is far from perfect. It has not made any predictions that can be verified. ?And strings themselves can?t be seen directly,? says Sami. Physicists accelerate fundamental particles such as protons and electrons to ultra-high energies in accelerators to probe their structures. But the direct verification of the predictions of the string theory will require tremendously high energies, impossible to achieve even in the largest possible accelerators.

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Yet, some researchers harbour hopes that it may be possible to find indirect evidence for the strings through cosmological observations. ?The Universe is a natural accelerator ? tremendously high energies were available in the Universe during its early moments,? says Sami. ?Strings may have left some imprints on cosmic evolution that may be observed in the future. And cosmology can benefit if features such as dark energy can be explained with the string theory.?

The first model to account for the dark energy involved the so-called the cosmological constant, an idea discovered and later discarded by Einstein, and again revived by physicists in the late-1990s after they noticed the faster-than-expected cosmic expansion.

Two years ago, a group of researchers from Stanford and Mumbai showed that string theory could be used to build Universes that could have a fast-accelerating expansion. It was the first substantial attempt to use ideas from the string theory to explain the dark energy

The good thing was that we got an acceleration,? said Sandip Trivedi, a physicist at the Tata Institute of Fundamental Research in Mumbai and member of the Stanford-Mumbai team. What some physicists find unpalatable is that the calculations predict a hideously large number of possible Universes, each with different values of the accelerating expansion.

In applying the mathematics of strings to the cosmos, Trivedi and his colleagues Shamit Kachru, Renata Kallosh, and Andre Linde from Stanford had discovered a way to generate a Universe with an accelerating expansion. But the solutions to their equations also raised the possibility of 10 raised to the power of 100 Universes ? a number far larger than all the stars in the Milky Way galaxy.

For many physicists, that?s a source of huge discomfort. ?It?s almost a philosophical decision whether to be content with such a result or not,? said Trivedi. The result would mean that there is nothing ?special? about the Universe we live in. It is just a single Universe among countless others where the acceleration in expansion has a range of values. Some of those Universes won?t be able to support life. The findings have prompted some researchers to speculate about the specific conditions in the Universes that would sustain life.

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Three years ago, string theorist Ashoke Sen at the Harishchandra Research Institute, Allahabad, independently put forward another idea that bolstered the attempts to bridge the string theory and cosmology. Sen published a paper titled ?Rolling Tachyons?, ushering into mainstream physics the tachyons, a set of all-but-forgotten particles.

Physicists had first proposed the existence of tachyons in the 1960s, dubbing them as renegade particles that break the cardinal law of the Universe: nothing can travel faster than light. Tachyons pay a price for their faster-than-light status ? they can have only ?imaginary mass?. Because they can?t be observed, physicists lost interest in them, classifying them as theoretical concept not of much relevance to the real world.

But there is a berth for the freakish tachyons in the string theory. In it they are not the traditional faster-than-light particles with ?negative mass.? According to Sen, the tachyons in the string theory is a technical word for an instability. ?Imagine a ball finely balanced on top of a hill. Any small disturbance can start the ball rolling down the hill,? said Sen. His paper in 2002 discussed such instabilities, or ?rolling tachyons,? in the string theory.

Gary Gibbons at the Cambridge University and Thanu Padmanabhan at the IUCAA followed up Sen?s work in attempts to use ideas from the string theory to describe the cosmology of the early Universe.

The work by Sami and his colleagues is the latest in this effort to link the string theory and dark energy. Their calculations show that certain energy conditions of the tachyon field give rise to dark energy. ?The tachyons are exotic entities and it is not surprising that they can account for dark energy which it itself exotic,? says Sami.

Dadhich cautioned that it?s another attempt at model buil-ding that looks promising, but needs to be examined and refined further. Sami thinks there is a need for string theorists and cosmologists in India to start working together. Toward that end, he had organised a workshop last October at IUCAA for a bit of brainstorming among experts from both the domains. The next one is expected to be held in Calcutta in 2006. ?The time is now ripe for such interactions,?

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