NASA photo

After a five-year study of 200,000 galaxies, scientists are more certain than ever that dark energy acts as a repulsive force, tearing the universe apart at an accelerating rate. The research confirms the idea that dark energy dominates gravity throughout the cosmos. But no one has any idea what dark energy actually is or how it works.

The WiggleZ project used NASA’s Galaxy Evolution Explorer space telescope and a massive Australian observatory to peer back 7 billion years in time, equivalent to the cosmic time span that has been dominated by dark energy. It’s the first time a single study has looked at such a lengthy period of cosmic history.

Dark energy was first proposed in the 1990s as astronomers discovered supernovae were moving away from us at accelerating speeds. This did not fit with prevailing theories of gravity, so scientists determined a new force called dark energy was to blame. The new survey independently verifies those earlier cosmological expansion observations, according to researchers at NASA and the Swinburne University of Technology in Melbourne, Australia.

The universe is about 13.7 billion years old, according to the best estimates, and for a little more than half that life it was dominated by the influence of gravity. All the baryonic matter, meaning matter with atoms and their constituent parts, was close enough together for gravity to have an influence. It helped form galaxies and galaxy clusters, for instance. But roughly 8 billion years after the Big Bang first flung everything apart, as the universe grew more and more diffuse, gravity’s power apparently succumbed to the increasing influence of dark energy. Galaxy cluster formation slowed down. Things started to fall apart.

To measure this, the researchers used a 3-D map created by NASA’s Galaxy Evolution Explorer, which identified bright, young galaxies in the distant universe. Then the team used the Anglo-Australian Telescope to gather detailed light information about each galaxy. They examined the patterns of distance between pairs of galaxies, which tend to wind up about 490 million light years apart. (This has to do with sound waves left over from the very early universe that resulted in areas of higher or lower pressure.) As the universe has expanded because of dark energy, this pattern has shifted.

The team also measured the rate at which galaxy clusters have been growing, and were able to show that something is counteracting gravity, slowing down the clusters’ formation.

This information was combined with data about how quickly the galaxy pairs are moving away from us, and together that verifies the earlier supernova findings: Yes, they are moving away, and yes, it is happening faster and faster.

Dark energy accounts for about 73 percent of the mass-energy of the universe. Dark matter, which is only slightly better understood, makes up about 23 percent of the universe. The remaining 4 percent is baryonic matter — galaxies, stars, the solar system, and you.

Dark energy will continue to speed up this cosmic expansion, and someday, everything will be so far apart that we won’t be able to see other galaxies or even other stars in the Milky Way. Let’s hope we find ETs before then.