Measuring ripples in the cosmic web

The 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 University 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.

The astronomers compared their measurements to supercomputer models that simulate the formation of cosmic structures from the Big Bang to the present. “The 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