̽��ֱ�� of Cambridge - Girish Kulkarni

Variations in the ‘fogginess’ of the universe identify a milestone in cosmic history

sc604 — Mon, 15 Apr 2019 23:00:25 +0000

̽��ֱ��results, reported in the Monthly Notices of the Royal Astronomical Society, have enabled astronomers to zero in on the time when reionisation ended and the universe emerged from a cold and dark state to become what it is today: full of hot and ionised hydrogen gas permeating the space between luminous galaxies.

Hydrogen gas dims light from distant galaxies much like streetlights are dimmed by fog on a winter morning. By observing this dimming in the spectra of a special type of bright galaxies, called quasars, astronomers can study conditions in the early universe.

In the last few years, observations of this specific dimming pattern (called the Lyman-alpha Forest) suggested that the fogginess of the universe varies significantly from one part of the universe to another, but the reason behind these variations was unknown.

“We expected the light from quasars to vary from place to place at most by a factor of two at this time, but it is seen to vary by a factor of about 500,” said lead author Girish Kulkarni, who completed the research while a postdoctoral researcher at the ̽��ֱ�� of Cambridge. “Some hypotheses were put forward for why this is so, but none were satisfactory.”

̽��ֱ��new study concludes that these variations result from large regions full of cold hydrogen gas present in the universe when it was just one billion years old, a result which enables researchers to pinpoint when reionisation ended.

During reionisation, when the universe transitioned out of the cosmic ‘dark ages’, the space between galaxies was filled with a plasma of ionised hydrogen with a temperature of about 10,000˚C. This is puzzling because fifty million years after the big bang, the universe was cold and dark. It contained gas with temperatures only a few degrees above absolute zero, and no luminous stars and galaxies. How is it then that today, about 13.6 billion years later, the universe is bathed in light from stars in a variety of galaxies, and the gas is a thousand times hotter?

Answering this question has been an important goal of cosmological research over the last two decades. ̽��ֱ��conclusions of the new study suggest that reionisation occurred 1.1 billion years after the big bang (or 12.7 billion years ago), quite a bit later than previously thought.

̽��ֱ��team of researchers from India, the UK, Canada, Germany, and France drew their conclusions with the help of state-of-the-art computer simulations performed on supercomputers based at the Universities of Cambridge, Durham, and Paris, funded by the UK Science and Technology Facilities Council (STFC) and the Partnership for Advanced Computing in Europe (PRACE).

“When the universe was 1.1 billion years old there were still large pockets of the cosmos where the gas between galaxies was still cold and it is these neutral islands of cold gas that explain the puzzling observations,” said Martin Haehnelt of the ̽��ֱ�� of Cambridge, who led the group that conducted this research, supported by funding from the European Research Council (ERC).

“This finally allows us to pinpoint the end of reionisation much more accurately than before,” said Laura Keating of the Canadian Institute of Theoretical Astrophysics.

̽��ֱ��new study suggests that the universe was reionised by light from young stars in the first galaxies to form.

“Late reionisation is also good news for future experiments that aim to detect the neutral hydrogen from the early universe,” said Kulkarni, who is now based at the Tata Institute of Fundamental Research in India. “ ̽��ֱ��later the reionisation, the easier it will be for these experiments to succeed.”

One such experiment is the ten-nation Square Kilometre Array (SKA) of which Canada, France, India, and the UK are members.

Reference:
Girish Kulkarni et al. ‘Large Ly α opacity fluctuations and low CMB τ in models of late reionisation with large islands of neutral hydrogen extending to z < 5:5.’ Monthly Notices of the Royal Astronomical Society (2019). DOI: 10.1093/mnrasl/slz025

Large differences in the ‘fogginess’ of the early universe were caused by islands of cold gas left behind when the universe heated up after the big bang, according to an international team of astronomers.

These neutral islands of cold gas explain the puzzling observations

Martin Haehnelt

Amanda Smith, Institute of Astronomy

Artist's impression of reionisation period

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Measuring ripples in the cosmic web

ps748 — Sat, 29 Apr 2017 13:18:49 +0000

̽��ֱ��most barren regions of the Universe are the far-flung corners of intergalactic space. In these vast expanses between the galaxies there are only a few atoms per cubic metre – a diffuse haze of hydrogen gas left over from the Big Bang. Viewed on the largest scales, this diffuse material nevertheless accounts for the majority of atoms in the Universe. It fills the cosmic web, with its tangled strands spanning billions of light years.

Now a team of astronomers, including Alberto Rorai and Girish Kulkarni, from the ̽��ֱ�� of Cambridge’s Institute of Astronomy and Kavli Institute, have made the first measurements of small-scale ripples in this primeval hydrogen gas. Although the regions of cosmic web they studied lie nearly 11 billion light years away, they were able to measure variations in its structure on scales a hundred thousand times smaller, comparable to the size of a single galaxy. Their results appear in the journal Science.

Intergalactic gas is so tenuous that it emits no light of its own. Instead astronomers study it indirectly by observing how it selectively absorbs the light coming from faraway sources known as quasars. Quasars constitute a brief hyperluminous phase of the galactic life-cycle, powered by the infall of matter onto a galaxy's central supermassive black hole.

Quasars act like cosmic lighthouses --- bright, distant beacons that allow astronomers to study intergalactic atoms residing between the quasars’ location and Earth. But because these hyperluminous episodes last only a tiny fraction of a galaxy’s lifetime, quasars are correspondingly rare in the sky, and are typically separated by hundreds of millions of light years from each other.

To probe the cosmic web on much smaller length scales, the astronomers exploited a fortuitous cosmic coincidence: they identified exceedingly rare pairs of quasars, right next to each other in the sky, and measured subtle differences in the absorption of intergalactic atoms measured along the two sightlines.

Schematic representation of the technique used to probe the small-scale structure of the cosmic web using light from a rare quasar pair Credit: Springel at al/J. Neidel MPIA

Rorai, lead author of the study, says “One of the biggest challenges was developing the mathematical and statistical tools to quantify the tiny differences we measure in this new kind of data”. Rorai developed these tools as part of the research for his doctoral degree, and applied his tools to spectra of quasars obtained with the largest telescopes in the world. These included the 10m diameter Keck telescopes at the summit of Mauna Kea in Hawaii, as well as ESO's 8m diameter Very Large Telescope on Cerro Paranal, and the 6.5m diameter Magellan telescope at Las Campanas Observatory, both located in the Chilean Atacama Desert.

̽��ֱ��astronomers compared their measurements to supercomputer models that simulate the formation of cosmic structures from the Big Bang to the present. “ ̽��ֱ��input to our simulations are the laws of Physics and the output is an artificial Universe which can be directly compared to astronomical data. I was delighted to see that these new measurements agree with the well-established paradigm for how cosmic structures form.” says Jose Oñorbe, from the Max Planck Institute for Astronomy in Heidelberg, who led the supercomputer simulation effort. On a single laptop, these complex calculations would have required almost a thousand years to complete, but modern supercomputers enabled the researchers to carry them out in just a few weeks.

Joseph Hennawi, professor of physics at UC Santa Barbara who led the search for these rare quasar pairs, explains: “One reason why these small-scale fluctuations are so interesting is that they encode information about the temperature of gas in the cosmic web just a few billion years after the Big Bang.”

Astronomers believe that the matter in the Universe went through phase transitions billions of years ago, which dramatically changed its temperature. These phase transitions, known as cosmic reionization, occurred when the collective ultraviolet glow of all stars and quasars in the Universe became intense enough to strip electrons off the atoms in intergalactic space. How and when reionization occurred is one of the biggest open questions in the field of cosmology, and these new measurements provide important clues that will help narrate this chapter of cosmic history.

Reference
Rorai, A et al. Measurement of the small-scale structure of the intergalactic medium using close quasar pairs. Science; 28 Apr 2017; DOI: 10.1126/science.aaf9346

Astronomers have made the first measurements of small-scale fluctuations in the cosmic web 2 billion years after the Big Bang. These measurements were conducted using a novel technique which relies on the light of quasars crossing the cosmic web along adjacent lines of sight.

One of the biggest challenges was developing the mathematical and statistical tools to quantify the tiny differences we measure in this new kind of data

Alberto Rorai

J. Onorbe/MPIA

Volume rendering of the output from a supercomputer simulation showing part of the cosmic web 11.5 billion years ago

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