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Dark energy: Einstein's blunder or flawed gravity?

Quamrul Haider
THE Universe is more mysterious than we previously' thought. As if grappling with the impact of the inexplicable "unknown," the dark matter, is not enough, another unknown even more inexplicable than dark matter appeared on the stage of cosmology' in die late 1990s. It is a smoothly distributed pervasive energy' called the "dark energy."
The idea of dark energy is a classic case of science confounding expectations. We know that expansion of the cosmos is a legacy of its violent birth — the "big bang" that occurred about 13.7 billion years ago. We also know that the attractive force of gravity' will put the brakes on the expansion and ultimately bring the Universe to a halt. It will then reverse direction and gravity will draw all the mass together until the Universe collapses into its point of origin. Our notion of an expanding Universe got a big jolt in 1998 when a group of astronomers while studying dozens of supernovae (exploding stars) discovered that the Universe is not only expanding, it is doing so at an ever-fasterrate.
Astronomers and astrophysicists went back to the chalkboard and suited pondering: What could be the cause of this accelerated expansion? Why isn't gravity inherent in luminous and dark matter strong enough to prevent such an expansion? Could the expansion be propelled by a repulsive force much stronger titan the attractive gravity? Whatever it is, the mysterious cosmic force causing the Universe to accelerate is neither matter nor radiation.
Many different theories have been advanced to answer the above questions. Most of them agree that there is some sort of a mysterious matter with anti-gravitational properties that overtook the force of gravity in the last few billion years. This gravity-defying matter, termed dark energy, is pushing the galaxies apart at an accelerated pace. While the exact nature of the dark energy is still unknown, it is nevertheless believed to be the dominant constituent of the Universe and could account for three-fourth of its total mass.
One of the theories invokes the prediction of quantum mechanics that vacuum, devoid of all matter and radiation, has a residual energy called "vacuum energy'." On a cosmic scale, this energy gives rise to a repulsive force that grows in strength as the Universe expands,
thereby causing the expansion to accelerate.
Some astronomers identify dark energy' as a manifestation of Einstein's "biggest blunder" -the cosmological constant he originally introduced in the equations of the
general relativity to make the Universe static (TDS 10-23-12). The constant, interpreted to represent a reservoir of energy, scales as the Universe expands and can distinguish the effects due to ordinary matter and dark energy.
There's a school of cosmologists who view the theory of gravity as flawed. According to them, it needs to be modified by adding an extra dimension to space, as suggested by the string theory'. Others who don't want the law of gravity to be modified espouses that our galaxy and the ones in its neighborhood possibly lie in a giant void about 8 billion light years across. The amount of matter in this void is not enough to slow down the expansion. Hence, it is the void which is expanding at a faster rate than the rest of the Universe. The "void theory'" does not require the existence of dark energy.
It has also been suggested that the accelerated expansion could be due to a new fluid-like negative gravitational mass called "quintessence." An alternative theory proposes that the acceleration could be caused by an energy that doesn't scale uniformly with the expansion because of topological defects of the Universe. Another theory proffers that dark energy is an illusion created by the curvature of space-time caused by extremely massive objects, like the galaxies.
While we are not quite in the dark about what dark matter could be, we are still in the dark about the source of the dark energy'. But we do know that it opposes gravity, and despite its invisibility it is presumed to fill the void in the Universe. The quest to solve the riddle of dark energy' is, therefore, one of the topmost priorities in the field of cosmology.
The writer is a Professor of Physics at Fordham University, New York.