Astronomers study ‘roller coaster effect’ of dark energy

British astronomers working with colleagues around the world have mapped the universe for the first time as it was 11 billion years ago.

Astronomers at the University of Portsmouth, the only UK institution to be involved, have  been part of an international effort to measure the rate the universe was

How SDSS-III was able to measure the distant Universe

An illustration showing how SDSS-III was able to measure the distant Universe.
Light rays from distant quasars (dots at left) are partially absorbed as they pass through clouds of intergalactic hydrogen gas (centre). When the light arrives at the spectrograph of the Sloan Foundation 2.5-Meter Telescope (square at right), some has been absorbed, leaving behind a record in the form of a “forest” of small absorption lines in the observed spectrum.
These lines can be interpreted to make a map of the gas along the line of sight between us and the quasar. By examining light from thousands of quasars all over the sky, astronomers can make a detailed three-dimensional map of the distant universe.
In this illustration, the dots at the far left are quasars, and the thin lines show light rays that left those quasars more than 10 billion years ago. Yellow dots are quasars that had been measured by prior projects of the Sloan Digital Sky Survey. By measuring the spectra from ten times as many quasars in this range (red dots), BOSS can reveal the large-scale structure of the early universe in much greater detail.
Illustration credit: Zosia Rostomian, LBNL; Nic Ross, BOSS Lyman-alpha team, LBNL; and Springel et al, Virgo Consortium and the Max Planck Institute for Astrophysics

expanding in its youth, just three billion years after the Big Bang.

The Big Bang is estimated to have happened 13.75 billion years ago, so this study is going back to when the universe was very young. This is the first time scientists have been able to map so far into the past.

Dr Mat Pieri, Marie Curie research fellow at University of Portsmouth and co-author of the study said: “We already know about the universe in its infancy using the afterglow of the Big Bang.

“We have seen the universe reach maturity by looking at the distribution of distant galaxies in the second half of its history.

“Only now are we finally seeing its adolescence by exploring the distribution of gas on the largest scales in the first half of its history, just before it underwent a growth spurt.”

In the last five billion years the universe has started to rapidly expand, due to a mysterious repulsive force that scientists have named ‘dark energy’. This study, undertaken by astronomers from the Sloan Digital Sky Survey (SDSS-III), looks at the universe when it was young, and its growth was being slowed by the effects of gravity.

Dr Pieri said: “If we think of the universe as a roller coaster, then today we are rushing downhill, gaining speed as we go.

“Our new measurement tells us about the time when the universe was climbing the hill — still being slowed by gravity.

“It looks like the roller coaster crested the hill just about seven billion years ago, and we’re still going.”

The results were presented in a paper submitted to the journal ‘Astronomy and Astrophysics’ and posted today on the preprint site.

A graph showing how the Universe's expansion rate has changed over the last ten billion years.

A graph showing how the Universe’s expansion rate has changed over the last ten billion years.
Until recently, three-dimensional maps by BOSS and other surveys were able to measure the regular distribution of galaxies back to only about five and a half billion years ago, a time when the expansion of the Universe was already accelerating.
The numbers along the bottom of the graph show the time in the Universe’s past, in billions of years. The vertical scale (y-axis) shows the expansion rate of the Universe; higher means the Universe was expanding faster. These older measurements appear as data points toward the right of the graph.
The new SDSS-III measurements, shown as the data point to the far left, have now probed the structure of the early Universe at a time when expansion was still slowing down.
Credit: Zosia Rostomian, LBNL, and Nic Ross, BOSS Lyman-alpha team, LBNL

The new measurement makes use of the clustering of intergalactic hydrogen gas in the distant universe. We can see this gas because it absorbs some light from quasars lying behind. When we measure the spectrum of a quasar, we see not only the light emitted by the quasar, but also what happened to that light in its long journey to Earth. When we look at a quasar’s spectrum, we can see how the intervening gas absorbs some of the quasar’s light. Measuring this absorption — a phenomenon known as the Lyman-alpha forest — yields a detailed picture of the gas between the quasar and us.

Professor Will Percival, Professor of Cosmology at University of Portsmouth said:  “It’s a very useful technique: we’re essentially measuring the shadows cast by gas along a series of lines, each billions of light-years long.

“The tricky part is combining all those one-dimensional maps. The problem is like trying to recognise an object from a picture that’s been painted on the quills of a porcupine.”

This technique uses so called “baryon acoustic oscillations (BAO)”: echoes frozen into matter soon after the Big Bang, as a “standard ruler” to compare the size of the universe at various points in its history. This was pioneered by SDSS in 2005 using the locations of galaxies, but using that ruler comes with its own difficulties because galaxies that are far away are also very faint.

Last year, astronomers used the first 10,000 quasars from SDSS-III’s Baryon Oscillation Spectroscopic Survey (BOSS) to make the first large-scale map of the structure of the faraway “Lyman-alpha forest” gas. As enormous as that map was, it was still not large enough to detect the subtle variations of BAOs. But the new map is big enough — it measures the Lyman-alpha forest using light from 50,000 quasars all over the sky.

“No place in the universe is really empty. There is gas in even the most remote parts of the universe and we have used it to measure how the universe expands. “ said Dr Pieri.

“This expansion is telling us that there is more than just gas out there – it seems that space itself comes with its own energy and the more space you have, the more of this ‘dark energy’ there is.”

The team’s new measurement of the BAOs, combined with measurements at other points in the universe’s history, paints a picture of how the universe has evolved over its history. The picture that emerges is consistent with our current understanding of the universe — that dark energy is a constant part of space throughout the cosmos. What is fascinating about the new result is that, for the first time, we see how dark energy worked at a time before the universe’s current acceleration started.

The Lyman alpha forest technique is itself in its youth. Professor Bob Nichol, said: “Our goal for BOSS was to measure the expansion of the Universe. We planned to make that measurement in two ways — one a sure thing and one a risky new idea.

“It’s really exciting that, thanks to the dedicated work of so many people, we know that both methods work. We have shown that the Lyman-alpha forest can accurately measure the expansion of the Universe when it was only one-fifth its current age.”

SDSS-III will continue to learn more about dark energy as it collects more than a million and half galaxies and more than 160,000 quasars by the end of the survey.  Now that the Lyman-alpha technique is no longer just a risky idea, SDSS-III will make it a standard method by which astronomers explore the nature of the faraway Universe

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