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Simulation Of Universe Expansion Could Shed Light On Dark Energy

A human being is a speck of dust compared to the vastness of the known universe, which is itself constantly expanding and changing. In fact, as the universe continues to "stretch," new cosmological structures and objects are merging.

About 5 percent of the universe is composed of ordinary matter, including quarks, leptons, atoms, stars and galaxies. Approximately 25 percent of the universe is comprised of an invisible, theoretical substance called dark matter, which can only be detected from its gravitational effects.

The other 70 percent is attributed to a cosmological constant called dark energy, a type of energy density that seems to exist in completely empty space. It permeates all of space and is responsible for the accelerated expansion of the universe.

Dark energy remains a mystery for scientists, but a new study could possibly shed light on its nature.

The Universe In Motion

Physicists in the past have studied the formation of large-scale cosmological structures by relying on simulations of gravity according to Newton's law. These Newtonian numerical simulations or codes say that space itself is static and does not change while time moves forward.

The simulations are very precise if the matter in the universe moves slowly at about 300 km (186.4 miles) per second.

But when the particles of matter move at high speed, the codes only permit approximate calculations and does not describe the fluctuations of dark energy. It was indeed necessary to develop a new method to simulate the formation of structures in the universe.

As a result, a team of physicists in Switzerland has developed numerical simulations based on Albert Einstein's equations to offer us a better glimpse of the complex formation of structures in the universe.

Led by Professor Ruth Durrer, scientists at the University of Geneva (UNIGE) were able to incorporate the rotation of space-time into their calculations and even determine the amplitude of gravitational waves.

The new code, which is called Gevolution, is highly based on Einstein's theory of general relativity. Unlike Newton's static space theory, Einstein's theory considers space as dynamic, and space-time as constantly changing.

In order to predict the amplitude and impact of gravitational waves, as well as the rotation of space-time induced by the formation of cosmological structures, the research team at UNIGE analyzed a cubic portion in space. The space consists of 60 billion zones with each containing a portion of the galaxy.

UNIGE scientists were able to study the motion of particles and calculate the distance and time between two galaxies in the universe through Einstein's equations, with the help of the LATfield2 library and the Swiss Supercomputer in Lugano.

The resulting spectra of the team's calculations allowed them to see the difference between the findings of Gevolution and Newtonian codes. For the first time ever, they measured the effect of gravitational waves and rotation of space-time induced by cosmological structure-formation.

An Invaluable Tool

Because this has never been done before, scientists believe they will be able to test the theory of general relativity on much larger scales. It may soon shed light on the mechanisms of dark energy.

"I think it is an important step forward," said University of Oxford Professor Jo Dunkley, who was not involved in the study. "It's something that people have been trying to work towards for a while."

However, some experts believe Einstein's equations are too complicated and expensive to solve because it takes too much computer time.

"To study growth of structure on cosmological scales we can usually make do with the older Newtonian theory of gravity," said University of Sussex's David Seery, Ph.D., adding that although Gevolution can offer advantages, it has yet to reveal new revelations.

Still, Dunkley believes the simulations will prove themselves valuable.

"This is really timely because we are just about to embark on this whole wealth of new data," said Dunkley. "[W]e will need these computer simulations available if we want to learn new physics from the new data that is coming."

In the meantime, the findings are featured in the journal Nature Physics.

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