SZ effects

In Measuring the Universe I talked about the Sunyaev-Zel’dovich effect (or the SZ effect, for short). It’s named after Rashid Sunyaev and Yakov Zel’dovich, who studied the concept in the late 1960s and early 1970s.

The SZ effect is a distortion in the observed cosmic microwave background radiation caused by high-energy electrons scattering of low-energy CMB photons. The collisions give the photons an energy boost – it’s the familiar inverse Compton scattering effect – and this in turn generates a slightly hotter patch in the microwave background. (‘Slightly’ is the operative word here: a microwave photon passing through a cloud of hot electrons on its journey towards Earth will appear hotter by just a few millionths of a degree.)

The high-energy electrons that can cause the SZ effect are to be found in the extremely hot gas clouds that are found at the centre of galaxy clusters. And, because the SZ effect is caused by scattering, its size doesn’t depend on redshift. In other words, the SZ effect in a high-redshift cluster can be detected just as easily (or, more truthfully, with just as much difficulty!) as in a cluster at low redshift. The SZ effect provides what is in essence a standard ruler – see Measuring the Universe for details – and so it can be used as a distance indicator over quite large reaches of the cosmos.

But there’s another type of SZ effect – the so-called kinematic SZ effect. I didn’t bother discussing it in the book because it is about 20 times fainter than the main (or thermal) SZ effect. Since the thermal SZ effect is hard enough to measure I didn’t think that anyone would be measuring the kinematic SZ effect anytime soon. Well, I was wrong. Cosmologists have now measured it.

The kinematic SZ effect arises because of the motion of galaxy clusters. Imagine a CMB photon passing through a cluster that’s moving away from us: when we observe the photon it will be slightly cooler (redder) than it otherwise would be due to the kinematic SZ effect. And if the photon moves through a cluster that’s approaching us then it will be slightly hotter (bluer). Sunyaev and Zel’dovich considered this from a theoretical point of view four decades ago, in 1972; but it’s taken until 2012 for researchers to measure it, such is the difficulty of teasing out the signal.

Kinematical SZ effect

If a CMB photon passes through a galaxy cluster that’s moving away from Earth it becomes slightly redder and cooler (left part of the diagram). If it passes through a galaxy cluster that’s moving towards Earth then it becomes bluer and hotter (right part of the diagram). These wavelength shifts are extremely tiny, so this effect has only just been observed.
(Credit: Sudeep Das, University of California-Berkeley)

A paper by Nick Hand (and about 60 other scientists, who were part of the Atacama Cosmology Telescope and the Baryon Oscillation Spectroscopic Survey projects), called Detection of Galaxy Cluster Motions with the Kinematic Sunyaev-Zel’dovich Effect, has identified the local velocity of galaxy clusters at a distance of up to several billion light years. Because the kinematic SZ effect is independent of redshift (in the same way that the thermal SZ effect is independent of redshift) cosmologists now have a tool for measuring velocities as well as distances way out into the cosmos.

The SZ effects can probe how clusters form and move around – something that depends critically on dark matter and dark energy. The SZ effects thus have the potential to deepen our understanding of the most mysterious elements of our Universe.