Science & Environment

Cosmos speed-check probes dark energy

SDSS telescope Image copyright Sdss
Image caption BOSS data is acquired by the 2.5m Sloan telescope at Apache Point Observatory

Scientists have produced their most precise measurement yet of the rate at which the early Universe was expanding.

They find that some three billion years after the Big Bang, the cosmos was pushing itself apart by another 1% every 44 million years.

It is the latest result to come from the Baryon Oscillation Spectroscopic Survey (BOSS).

The international group clocks the expansion by making detailed maps of the distribution of matter in space.

The hope is such studies can provide further insights on "dark energy".

This is the mysterious force that appears today to be driving the cosmos apart ever more quickly.

What is interesting about the new result is that the BOSS-measured expansion rate 10 billion years ago is quite a bit slower than that expected from the standard model of cosmology.

"This is the most precise measurement that's ever been done, and all I'll say at the moment is that there is a tension there," explained Dr Matthew Pieri, a BOSS team-member from Portsmouth University, UK.

"We expected to see the Universe expanding faster than what we found.

"The disagreement could still be a statistical fluke, or it could be that the Universe was different to how we thought it was, but we'll have to explore this further to find out."

Theory holds that the Universe has been on a rollercoaster-like ride.

From the Big Bang 13.8 billion years ago, it expanded rapidly - but that expansion decelerated in the first few billion years under the constraining influence of gravity.

Then, some six billion years ago, the cosmos started to speed up again under the influence, scientists suspect, of dark energy.

The BOSS group has determined both the near and far expansion rates.

And while the near values fit very well with expected numbers, it is the new result for the far Universe that is the cause of some head-scratching.

Two largely independent measurements have been made.

One is to map the distribution of quasars, which are extremely luminous, distant galaxies.

The second measure involves using the light from those quasars to pinpoint the positions of clouds of hydrogen gas along the line of sight to Earth.

Image copyright BOSS/SDSS
Image caption The BOSS team used 140,000 distant quasars to "back-light" and map the distribution of clouds of hydrogen gas in the early Universe

In both instances, the BOSS team is probing so-called baryon acoustic oscillations (BAOs).

These refer to the pressure-driven waves that passed through the post-Big-Bang Universe and which subsequently became frozen into the distribution of matter once it had cooled to a sufficient level.

These oscillations show themselves as a "preferred scale" in the distribution of matter.

For example, in the near Universe, there is a slight excess in the numbers of galaxies with separations of about 500 million light-years.

BAOs can be used as a kind of "standard ruler" to measure separations and distances through the entire history of the cosmos.

The new BOSS information does this for a sample of quasars and gas clouds over a quarter of the sky.

Because it uses a 2.5m telescope in New Mexico, this sample is predominantly in the northern sky above the plane of the Milky Way.

To be able to measure the Universe in its deceleration phase - which BOSS has done - is a remarkable enough result.

But to get a number lower than expected is potentially very exciting.

It may be that the apparent divergence will simply disappear with further measurements that make use of bigger samples.

However, if it does not, it could hint for the first time that dark energy is not a cosmological constant, as has been the assumption for more than a decade.

Perhaps, rather than having a fixed value, it varies with time or space or some other physical parameter.

"We're not jumping up and down at this stage and saying it's time to re-write the whole of cosmology," observed Prof Bob Nichol at Portsmouth. "But this is the kind of thing that can balloon into new thinking.

"We've assumed that dark energy is a constant and this approach has been very successful so far.

"But as we take more data and push down into the noise, it is possible something new will emerge.

"It is also possible, of course, that as we take more data, the tension simply goes away. But we have to go where the data takes us and today the data is tantalising."

Image copyright BBC/ESA
  • Before Nobel Prize-winning research in 1998, it was assumed gravity was slowing the post-Big Bang expansion of the Universe
  • Now scientists say the expansion - far from slowing - is accelerating, pushing galaxies apart at a faster and faster rate
  • The BOSS team has traced dark energy's part in the expansion over more than 10 billion years of cosmic history and follow me on Twitter: @BBCAmos

More on this story

Related Internet links

The BBC is not responsible for the content of external Internet sites