November 13, 2012 | 19
A new method for investigating dark energy has allowed astronomers to peer further back into the past than ever before, revealing a universe that was very different to the one we live in today.
Today’s universe is expanding. Not just that, its expansion is accelerating. Galaxies are not only getting further away from us all of the time, but they’re doing so faster and faster with each passing day.
It has been this way for as long as we’ve been around and a long, long time before then too. We didn’t always know it, though. In fact, until 1998 we thought the expansion was slowing down. But since the universe was around six billion years old, it has been dominated by an unknown force we call dark energy that is pushing everything further and further apart.
It’s difficult to express to a non-physicist just how weird dark energy is, because most people are used to encountering things that they don’t understand in physics, and they generally assume that someone else is on top of the situation. That’s not the case with dark energy.
We’re pretty sure there was a time before dark energy. In the first six billions years or so after the universe came into existence, its expansion was controlled by matter. This meant that the expansion of the universe actually slowed down with time, because gravity brought everything together.
But today dark energy acts like a kind of anti-gravity, pushing galaxies apart faster than gravity can pull them together.
To try to work out what dark energy actually is, some astronomers make maps. By measuring distances in the universe, they can work out exactly how fast the universe is expanding and measure the effects of dark energy. Measuring what dark energy does can help us to narrow down what it is.
So far, all of these maps have been of times after dark energy started to take over. One reason for that is that it is difficult to study the early universe because there aren’t many bright objects, says David Schelgel, principal investigator on an experiment called BOSS, the Baryon Oscillation Spectroscopic Survey.
So, to get a glimpse of that early period, what BOSS has now done (and detailed in a paper the collaboration published on arXiv today) is look at some of the brightest objects in the early universe: quasars.
Quasars are active galaxies (the name is a contraction of “quasi-stellar radio source”) that exist in the early universe. And they’re bright. Really bright. Schlegel and the BOSS team use the quasars as “backlights” to the hydrogen gas in between us and them.
They have made a map of the line of sight to each quasar. “It’s like skewers through the universe,” he says. From the skewers they reconstruct what they think is a true 3D map of the universe. “One of the cutest descriptions of this is that it’s like you paint a map on the back of a porcupine.”
In these skewers of light astronomers see markers, or absorption lines, from when the light passed through an intervening hydrogen gas cloud on its way from the quasar. Often the light will pass through hundreds of hydrogen clouds. The markers reveal the distribution of the gas along the line of sight to each quasar, and then the astronomers look at all of the lines and use them to work out the whole map just from the skewers.
To do this, you need enough quasars. The BOSS team observed 48,129 quasars to build their 3D map. The quasars they’ve studied reach as far back as 11.5 billion years, into the matter dominated era. In other words, before dark energy took over.
“It’s the first measurement when the universe was slowing down,” says Schlegel. “Which is pretty neat.”
In this map they looked for a structure known as baryon acoustic oscillations, which are essentially sound waves frozen into the early universe. Astronomers know how big these sound waves were when they formed. By measuring them later on in cosmic history they can tell how much the universe has expanded in the mean time. Because the expansion is governed by dark energy, measuring baryon acoustic oscillations provides a way to test whether dark energy is behaving as we think it should.
The BOSS team saw the pattern of deceleration of expansion, before dark energy kicked in, and later acceleration that they expected.
But this study only mapped a third of the volume that BOSS will eventually map. In the next few years BOSS will map a million and a half galaxies and 160,000 quasars.
Watch this space.
Update 03/12/12: Peter Edmonds points out that some work has in fact measured the time when cosmic deceleration turned into acceleration before, using supernovae rather than BAOs. Read more on his blog.