A few weeks ago, a team of astronomers shocked the world when they announced that they had detected evidence of cosmic inflation—an event that occurred 10-34 seconds after the Big Bang. But what has the Universe been up to since then? Observations and measurements tell us that it is expanding, but at what rate? How has that rate changed over time? The researchers operating the Sloan Digital Sky Survey (SDSS) announced yesterday at a conference of the American Physical Society that they have made the best measurements yet of the expansion of the Universe.
The team investigated the universe as it was 10.8 billion years ago, at about 25% of its current age. They found that the Universe was expanding at a rate of one percent every 44 million years.
This conclusion was made by studying hydrogen gas clouds and 140,000 quasars. Quasars are formed when high-speed gas gets too close to a black hole and is then bounced off due to electromagnetic forces. The friction heats up the gas which causes it to glow in beautiful colors, revealing the quasar as a massive jet of bright light. By measuring how much light from the quasar is obstructed by clouds of intergalactic hydrogen gas over time, the team was able to determine the rate at which the Universe was expanding.
This technique expands upon prior research the team completed just over a year ago. “A little over a year ago we tried this for the first time and demonstrated that it really works”, Dr Matthew Pieri, from the University of Portsmouth said in a press release. “Now we’re back with twice as much data and with remarkable precision of 2 per cent. We are measuring the expansion of the universe with exquisite detail. Like the rings of a tree trunk that tell is its age, each quasar spectrum becomes an archive of the universe’s history.”
About 6 billion years ago, dark energy began to dominate the Universe. Dark energy, which now comprises an estimated 70% of the Universe, is believed to be the culprit behind the acceleration. There is still much we don’t know about dark energy, but scientists believe that the density of dark energy remains constant, even in the face of an expanding Universe. It doesn’t “dilute” the way a salty solution would if you kept increasing the volume of water. This persistence of density likely plays a role in the acceleration of expansion. How or why does dark energy do this? That remains to be determined.
Pieri cryptically states that there is a bit of tension between the recent results and what was expected based on Cosmic Microwave Background. “It’s odd,” he notes, “but nothing you want to hang your hat on just yet—it’s going to be fun finding out where the truth lies".