The day the Universe froze

Washington, May 9 (ANI): A new model for dark energy has suggested that the Universe froze about 11.5 billion years ago, when it was a quarter of the size it is today.

The model was developed by Research Associate Sourish Dutta and Professor of Physics Robert Scherrer at Vanderbilt University, working with Professor of Physics Stephen Hsu and graduate student David Reeb at the University of Oregon.

A cosmological phase transition - similar to freezing - is one of the distinctive aspects of this latest effort to account for dark energy.

Dark energy is the mysterious negative force that cosmologists now think makes up more than 70 percent of all the energy and matter in the universe and is pushing it apart at an ever-faster rate.

Another feature that distinguishes the new formulation is that it makes a testable prediction regarding the expansion rate of the universe.

"One of the things that is very unsatisfying about many of the existing explanations for dark energy is that they are difficult to test," said Scherrer. "We designed a model that can interact with normal matter and so has observable consequences," he added.

The model associates dark energy with something called vacuum energy.

Like a number of existing theories, it proposes that space itself is the source of the repulsive energy that is pushing the universe apart.

For many years, scientists thought that the energy of empty space averaged zero.

But, according to quantum theory, empty space is filled with pairs of "virtual" particles that spontaneously pop into and out of existence too quickly to be detected.

This sub-atomic activity is a logical source for dark energy because both are spread uniformly throughout space.

This distribution is consistent with evidence that the average density of dark energy has remained constant as the universe has expanded.

The theory is one of those that attribute dark energy to an entirely new field dubbed quintessence that plays the role of the cosmological constant, a term that Albert Einstein added to the theory of general relativity to keep the universe from contracting under the force of gravity.

One of the consequences of allowing quintessence to interact with ordinary matter is the likelihood that the field went through a phase transition - froze out - when the universe cooled down to a temperature that it reached 2.2 billion years after the Big Bang.

As a result, the energy density of the quintessence field would have remained at a relatively high level until the phase transition when it abruptly dropped to a significantly lower level where it has remained ever since.

This transition would have released a fraction of the dark energy held in the field in the form of dark radiation. (ANI)