Cosmic motion offers \’new window\’ on evolving Universe



The first observation of a cosmic effect could give astronomers a new tool for understanding the forces behind the Universe\’s formation and growth, including the enigmatic phenomena of dark energy and dark matter.

An international team led by Princeton University, and including Oxford University scientists, has detected the movement of distant galaxy clusters via the kinematic Sunyaev-Zel\’dovich (kSZ) effect, which has never before been seen.

The team report their findings in a paper submitted to Physical Review Letters. Almost sixty collaborators from the Atacama Cosmology Telescope (ACT) and the Baryon Oscillation Spectroscopic Survey (BOSS) projects contributed to the research.

Proposed in 1972 by Russian physicists Rashid Sunyaev and Yakov Zel\’dovich, the kSZ effect results when the hot gas in galaxy clusters distorts the cosmic microwave background radiation (the glow of the heat left over from the Big Bang) that fills our universe. Radiation passing through a galaxy cluster moving toward Earth appears hotter by a few millionths of a degree, whilst radiation passing through a cluster moving away appears slightly cooler.

The kSZ effect could prove to be an exceptional tool for measuring the velocity of objects in the distant Universe, providing insight into the strength of the gravitational forces pulling on galaxy clusters and other bodies. Chief among these forces are the still-hypothetical dark energy and dark matter, which are thought to drive the universe\’s expansion and the motions of galaxies.

‘The benefits of the kSZ effect stem from a unique ability to pinpoint velocity,’ said Nick Hand, a PhD student at the University of California-Berkeley, who led the paper. The researchers detected the motion of galaxy clusters that are several billion light years away moving at speeds of up to 400 miles per second. ‘One of the main advantages of the kSZ effect is that its magnitude is independent of a galaxy cluster\’s distance from us, so we can measure its velocity at much larger distances,’ said Hand.

To find the kSZ effect, the researchers combined data from the ACT and BOSS projects. ACT is a 6-metre telescope in Chile built to produce a detailed map of the cosmic microwave background radiation using microwave frequencies. BOSS is a visible-light survey in New Mexico, part of the Sloan Digital Sky Survey-III project, which has observed thousands of galaxies and quasars.

The kSZ effect is so small that it is not visible from the interaction of the cosmic microwave background with an individual galaxy cluster, but can be detected by combining signals from many clusters. The BOSS survey was used to find the locations of these clusters, and the combined signal was then found in the ACT maps. “The overlap of data from the two projects was essential because the amplitude of the signal from the kSZ effect is so small,” said ACT collaborator David Spergel, professor of Astrophysical Sciences at Princeton.

Dr Joanna Dunkley, lecturer and RCUK fellow in Astrophysics at Oxford University, leads the Oxford researchers working on data analysis for the ACT project, funded by the European Research Council. She said: ‘It is very exciting to be able to find this tiny signal in the ACT data. We have been expecting it to be there, but finally detecting it has relied on a combination of better quality data than available before, and also clever techniques to find it.’

To extract the signal, the group used a particular mathematical average that was devised by Professor Pedro Ferreira of Oxford University’s Department of Physics more than ten years ago, that reflects the slight tendency for pairs of galaxy clusters to move toward each other due to their mutual gravitational attraction, which made the kSZ effect more apparent in the data.

Professor Ferreira called the paper a ‘beautiful piece of work’ that neatly demonstrates an accurate method for studying the evolution of the Universe and the distribution of matter in it: ‘This is the first time the kSZ effect has been unambiguously detected, which in and of itself is a really important result.’

Professor Ferreira and Dr Dunkley now lead the Oxford Dark Universe research group, bringing together theorists and observers to understand the puzzle of the ‘missing’ 95% of the Universe. ‘By probing how galaxies and clusters of galaxies move around in the Universe, the kSZ effect is directly probing how objects gather and evolve in the Universe,’ Professor Ferreira said. ‘Therefore it is hugely dependent on dark matter and dark energy. You can then think of the kSZ effect as a completely new window on the large-scale structure of the Universe.’