Hidden Clue

This Article is Received from Dr. M. Sami, Professor, Centre for Theoretical Physics, Jamia Millia Islamia, New Delhi, India

The string theory solves a puzzle — faster-than-expected expansion of the cosmos. G.S.

Mudur reports Physicist Mohammad Sami has joined a league of matchmakers trying to wed the hardest mathematics ever invented with one of the most baffling features of the Universe. The stakes are high. It’s an attempt to bring together a theory that seeks to

describe nature at the smallest level and the cosmos at the largest scale. If the matchmaking attempt succeeds, the mathematics, called the string theory, may establish its first tangible connections with reality. And it may also explain a mysterious acceleration of the Universe discovered seven years ago.

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.

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.

Fine balance: Prof. Ashoke Sen also invokes tachyons to explain cosmic phenomena

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. Yet, some researchers harbor 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.

Three years ago, string theorist Ashoke Sen at theHarishchandra 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,” says Trivedi.

Copyright © 2005 The Telegraph. All rights reserved.

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Personal Communication from Professor Mohammad Sami of Jamia Millia Islamia, Ctr for  Theoretical  Physics, New Delhi 110025, India.

 "M. Sami" <sami@iucaa.ernet.in>