An international team of astronomers and physicists has, for the first time, detected the large-scale motion of galaxy clusters, using an effect that was proposed almost 40 years ago. This is the first direct measurement of the motion of objects at cosmological distances and such observations could lead to a better understanding of how the universe formed and evolved and also help astronomers study dark matter and dark energy.

In 1972 Russian physicists Rashid Sunyaev and Yakov Zel'dovich argued that a moving cluster of galaxies should, in theory, cause a slight temperature shift in the cosmic-microwave-background (CMB) radiation – the leftover thermal radiation from the Big Bang – as it passes through it. This Sunyaev–Zel'dovich (SZ) effect is caused by high-energy electrons distorting the CMB through inverse Compton scattering and can be divided into three categories, or "effects" – thermal, kinematic and polarized. It is the second variety – the kinematic Sunyaev–Zel'dovich (kSZ) – that was used in the new work to detect the cosmic-scale motion. The kSZ is a second-order effect where the CMB photons interact with high-energy electrons in the galaxy clusters, as a result of the electron's bulk motion. Radiation passing through a galaxy cluster moving toward Earth appears hotter by a few millionths of a degree, while radiation passing through a cluster moving away appears slightly cooler. Although proposed 40 years ago, this is the first time the kSZ effect has been observed.

Come together

To get around the difficulties of detecting such a small temperature change, lead author Nick Hand from University of California, Berkeley in the US, along with 58 collaborators from the Atacama Cosmology Telescope (ACT) in Chile and the Baryon Oscillation Spectroscopic Survey (BOSS) project in New Mexico, compiled signals from several clusters to detect the temperature shift. Data from a catalogue of 27,291 luminous galaxies from BOSS were laid over maps of the same region of sky observed by the ACT between 2008 and 2010. As each galaxy likely resides in a galaxy cluster, their positions were used to determine the locations of clusters that would distort the CMB radiation.

In a galaxy far far away

The teams detected the motion of galaxy clusters that are several billion light-years away and moving at velocities of up to 600 km/s. The velocities of these distant objects are extremely difficult to detect as they require very precise distance measurements.

"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 the velocity of an object's motion toward or away from Earth at much larger distances than was possible," explains Hand. He also says that the method could serve as an additional statistical check, independent of currently used measuring methods, for future large-scale measurements.

Of the 27,291 galaxies in the BOSS data, the team used 7500 of the brightest galaxies to uncover the kSZ signal. As two galaxy clusters move toward each other as a result of their mutual gravitational attraction, the team found that the kSZ effect becomes more pronounced – a slight cold spot in the CMB data would suggest that a cluster was moving away from us, whereas a slight hot spot would mean the cluster was moving towards us, similar to the Doppler effect. As the temperature shift data is averaged over thousands of the BOSS objects, a clear kSZ signal was seen.

"The kSZ signal is small because the odds of a microwave hitting an electron while passing through a galaxy cluster are low, and the change in the microwave's energy from this collision is slight," says ACT collaborator and physicist David Spergel of Princeton University, US. "Including several thousand galaxies in the dataset reduced distortion and we were left with a strong signal."

Two collaborations are better than one

The researchers point out that if the data from just the ACT or the BOSS project was analysed by itself, the signal would not have been apparent, as neither was originally built to look for it specifically. Both the ACT and the BOSS projects differ in the objects they study, their method of data collection and even the wavelengths in which they operate – microwaves for the ACT, visible-light waves for BOSS. This work highlights the importance of large collaborations, which might even be fundamentally different in their missions, sharing and combining their data to study subtle physical effects that no single survey could detect – says the team.

According to Hand "The [kSZ] signal agrees remarkably well with the typical CMB cosmology that we have developed over the past decade…so well, in fact, that it was not expected! So it was quite exciting to confirm something predicted 40 years ago, when neither the ACT or BOSS teams were planning on it."

The strength of the kSZ effect's signal depends on the distribution of electrons in and around galaxies. So the signal could be used to trace the location of atoms in the nearby universe, revealing how galaxies form. In the near future, Hand hopes that increased sensitivity for the ACT, which is due an upgrade, and larger data sets will mean even further improvements of the kSZ signal, which in turn will mean better velocity measurements.

The research is to be published in Physical Review Letters. A preprint of the work is available on arXiv.