Is Universe Driven By Gravity or Dark Energy?

Until now, we have believed that gravity is the force that is driving our universe. But now, astronomers have found substantial evidence that dark energy, not gravity, is driving our universe apart at accelerating speeds.

Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe's expansion to accelerate.

A five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, have even questioned Albert Einstein's concept of gravity. According to this alternate theory, Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances.

New results confirm that dark energy is a smooth, uniform force that now dominates over the effects of gravity.

The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into the sky faster and faster, said Chris Blake of the Swinburne University of Technology in Melbourne, Australia. The results tell us that dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we wouldn't be seeing these constant effects of dark energy throughout time.

Dark energy is thought to dominate our universe, making up about 74 percent of it. Dark matter, a slightly less mysterious substance, accounts for 22 percent. So-called normal matter, anything with atoms, or the stuff that makes up living creatures, planets and stars, is only approximately four percent of the cosmos.

The idea of dark energy was proposed during the previous decade, based on studies of distant exploding stars called supernovae. Supernovae emit constant, measurable light, making them so-called standard candles, allowing calculation of their distance from Earth. Observations revealed dark energy was tossing the objects out at accelerating speeds.

Meanwhile, dark energy is in a tug-of-war contest with gravity. In the early universe, gravity took the lead, dominating dark energy. At about 8 billion years after the Big Bang, as space expanded and matter became diluted, gravitational attractions weakened and dark energy gained the upper hand.

Astronomers, who say billions of years from now dark energy will be even more dominant, predict our universe will be a cosmic wasteland, with galaxies spread apart so far that any intelligent beings living inside them wouldn't be able to see other galaxies.

The latest survey provides two separate methods for independently checking the supernovae results. Astronomers began assembling the largest three-dimensional map of galaxies in the distant universe and acquired detailed information about the light for each galaxy and studied the pattern of distance between them.

Sound waves from the very early universe left imprints in the patterns of galaxies, causing pairs of galaxies to be separated by approximately 500 million light-years.

This standard ruler was used to determine the distance from the galaxy pairs to Earth -- the closer a galaxy pair is to us, the farther apart the galaxies will appear from each other on the sky. As with the supernovae studies, this distance data were combined with information about the speeds at which the pairs are moving away from us, revealing, yet again, the fabric of space is stretching apart faster and faster.

The team also used the galaxy map to study how clusters of galaxies grow over time like cities, eventually containing many thousands of galaxies. The clusters attract new galaxies through gravity, but dark energy tugs the clusters apart. It slows down the process, allowing scientists to measure dark energy's repulsive force.

Observations by astronomers over the last 15 years have produced one of the most startling discoveries in physical science; the expansion of the universe, triggered by the Big Bang, is speeding up, said Jon Morse, astrophysics division director at NASA Headquarters in Washington. Using entirely independent methods, data from the NASA's Galaxy Evolution Explorer have helped increase our confidence in the existence of dark energy.