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A new test for dark energy
Posted: 21 July 2010

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An effort to detect more intergalactic hydrogen than ever before is creating a new opportunity to test the strength of dark energy – that is, the force that is causing the expansion of the Universe to accelerate – say astronomers using the world’s largest steerable radio telescope, the Robert C Byrd Green Bank Telescope in West Virginia, USA.

The National Radio Astronomy Observatory’s Robert C Byrd Green Bank Radio Telescope. Image: NRAO/AUI.

A survey for intergalactic hydrogen – the so-called ‘cosmic web’ out of which the galaxies formed – has turned up vast fields of neutral hydrogen gas amounting to 200 trillion times the mass of the Sun (the Sun has a mass of 2 x 1030 kilograms), out to a redshift of 1.12 (a look-back time of approximately 6.5 billion light years), which is ten times further than any intergalactic hydrogen previously detected.

“Our project mapped hydrogen gas to greater cosmic distances than ever before,” says Tzu-Ching Chang, of the Academia Sinica in Taiwan and the University of Toronto.

Tzu-Ching Chang’s team developed a new technique called intensity mapping that allows them to study how dark energy has influenced changes in the structure of these filaments of hydrogen over the course of billions of years, and possibly revealing changes in the strength of dark energy over time. Determining the strength of dark energy, and whether it varies over time, is crucial for the task of nailing down which theory of dark energy – be it the cosmological constant, quintessence or something else – is correct.

The large scale structure of the Universe, in the form of these filaments of hydrogen gas, dark matter and chains of galaxy clusters, is a relic of conditions in the Universe shortly after the big bang. For 380,000 years after the big bang, the Universe was so hot that matter could only exist as a plasma of atomic nuclei and free electrons. Sound waves rippled though this universal plasma ‘ocean’, creating peaks and troughs of greater and lesser densities of matter. When we observe the cosmic microwave background (CMB) radiation using the WMAP and Planck spacecraft we can see this pattern as variations in temperature, and because we know how large the biggest waves in the plasma could have been, we can use them as a ‘standard ruler’. Finally, after 380,000 years when the Universe cooled to below 3,000 degrees Celsius, the atomic nuclei soaked up the electrons and the plasma disappeared to be replaced by a fog of neutral hydrogen. However, the pattern of the waves in the plasma remained superimposed on the distribution of the hydrogen. Over time the denser filaments formed chains of galaxies, and by comparing them to the standard ruler in the CMB, we can discover how dark energy has affected their size.

“Since the early part of the twentieth century, astronomers have traced the expansion of the Universe by observing galaxies,” says team-member Jeffrey Peterson of Carnegie Mellon University. “Our new technique allows us to skip the galaxy-detection step and gather radio emissions from a thousand galaxies at a time, as well as all the dimly-glowing material between them.” Their results are published in the 22 July issue of the journal Nature, and you can read about the evidence for dark energy in the August 2010 issue of Astronomy Now, on sale now.