Zooming in on Clouds of Hydrogen when the Universe was Young

SDSS-III scientists in the Baryon Oscillation Spectroscopy Survey (BOSS) have just completed the second phase of a project to provide a map of the observable universe showing the positions of giant clouds of hydrogen gas. BOSS is mapping the Universe by measuring the positions of galaxies, clusters of galaxies, and clouds of hydrogen gas. These maps allow astronomers to understand the history of the universe, starting with the Big Bang all the way through to the formation of the galaxies, stars, and planets that provide the site for the development of life.

Hydrogen is the most abundant element in the Universe. Thus measuring the position of hydrogen gas clouds reveals important details on the distribution of normal matter in the Universe. To detect these hydrogen clouds, BOSS uses super-luminous quasars as backlights. Quasars can be billions of times brighter than the Sun and are currently thought to be powered by matter falling into super-massive black holes. The quasars that BOSS uses as backlights are located billions of light-years from us, when the Universe was one fifth its current age of 13.7 billion years.

Because light travels at a finite speed (186,000 miles per second) this extreme distance means that we are also looking further back in time, back to when things in the Universe were closer together. As the light travels from the quasar to us, the Universe has expanded and so the light from the quasar is stretched out – a phenomenon known as redshift.

The light that the BOSS detector receives is partially absorbed by the intervening gas clouds and the pattern of absorption gives a map of the gas. Each gas cloud absorbs at a wavelength particular to a given energy transition in hydrogen.
The redshifting of the quasar light means that each cloud leaves an imprint at a different wavelength in the light we eventually observe from the quasar. The amount of absorption in the quasar spectrum at any given wavelength reveals the position of a hydrogen gas cloud between us and the quasar.

A map of the entire Earth at first glances reveals large features like continents, but reveals mountains, hills, and valleys when you zoom in. In the first phase of BOSS, scientists used a very coarse-grained map of the absorption due to hydrogen to study the “large-scale structure” of the universe at scales of hundreds of million light years, analogous to studying the size and disposition of the continents on the surface of the Earth. BOSS’s “continents” were clouds typically many millions of light-years in size. BOSS found that the gas was arranged in a pattern that reflected sound waves that existed during the first 380,000 years after the big bang. The pattern gave information on the conditions in the universe at this time and confirmed that the Universe consists of a strange cocktail of normal matter, photons, neutrinos, dark matter (matter that doesn’t interact with light and that forms the bulk of galaxies), and the completely mysterious dark energy which is currently driving the increasing expansion rate of the Universe.

SDSS-III scientists are producing detailed numerical simulations of the universe to compare to BOSS observations. This image shows a simulated map of the gas clouds (the red blobs are clusters of galaxies and the blue filaments are lower-density gas regions), in a cube of the universe 65 million light-years on a side.

In the second phase of their effort, BOSS scientists have been able to zoom in using more sensitive techniques to study the “small-scale” structures, down to scales of a few million light years, analogous to the mapping the mountains on a continent. This increased resolution reveals the characteristics of the gas clouds that are on the brink of forming galaxies.

The new BOSS observations so far appear consistent with current theories of the composition of the Universe. But this new data is sufficiently precise that BOSS hopes to use it to glean information on one of the least understood ingredients of the cocktail: neutrinos. These (ghostly) objects move through the universe nearly at the speed of light and cannot condense to form galaxies. However, their presence has an effect on the map of the gas clouds. The speeding neutrinos dilute the pattern that seeded the gas clouds. Current measurements indicate that neutrinos have a tiny mass (about one billionth of the mass of an electron). The cosmological maps from BOSS reflect its effects on the “small-scale” structures of the universe. Further analysis will reveal new constraints on the mass density of neutrinos in the cosmos.

The paper “The one-dimensional Ly-α forest power spectrum from BOSS” by N. Palanque-Delabrouille et al. was recently submitted to Astronomy & Astrophysics and is available at http://arxiv.org/abs/1306.5896

A simulation of the interaction of dark matter, gas, stars, and neutrinos in forming the structure of galaxies we observe. The overall expansion of the Universe has been scaled out of this simulation to more clearly illustrate how the large scale structure forms. Previous work mapped hydrogen clouds on the scale of this box. The current work can map down to the scale of the red spheres shown here that are turning into galaxies.

This second image is a map showing the ghostly neutrinos, here drawn in yellow, in the same cube of universe. The only hint of condensation appears about the largest density region, in the center of the image. With further analysis BOSS scientists plan to explore this faint imprint on the structure of hydrogen clouds from neutrinos in the early Universe.

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