Astronomers have confirmed a 40-year-old prediction that there should be tiny variations in the cosmic microwave background radiation (CMB) as a result of moving clusters of galaxies.
Back in 1972, an astrophysicist in Moscow, Russia, named Rashid Sunyaev found himself pondering the “afterglow” of the birth of our universe, a.k.a., the CMB, which offers a snapshot of those early days when the cosmos was a mere 380,000 years old.
Sunyaev and a colleague, Yakov Zel’dovich, didn’t have the technology to make precise measurements of the CMB, but they figured that the light had to pass through lots of galaxy clusters as it traveled across the vast expanse of space — and there should be tiny effects as a result of that interaction. The light should become just a wee bit cooler (redder) if that cluster is moving away from Earth, and very slightly hotter (bluer) if the cluster is moving toward Earth.
This should happen because there is a lot of very hot, ionized gas lurking between those galaxies, and quite a few so-called free electrons floating around as a result. (A gas becomes ionized when the temperature rises sufficiently to rip electrons away from atoms.)
Statistically, microwaves from the CMB would eventually collide with one of those electrons — a rare event, to be sure, but those collisions should leave a detectable trace in the form of slight variations in temperature as a microwave moves through a galaxy cluster, traveling away from Earth.
We’re talking a very slight variation, on the order of a few millionths of a degree, because a microwave doesn’t hit an electron very often. This became known as the kSZ (“kinematic Sunyaev-Zel’dovich”) effect. It was an interesting prediction, but physicists had no way of experimentally testing it for over 40 years.
Princeton astronomer David Spergal knew that two different kinds of data would be needed to detect the kSZ effect.
As a collaborator on the Atacama Cosmology Telescope (ACT), he figured it should be possible to strengthen the effect’s signal, and decrease the noise, by averaging all of the ACTs temperature maps of the cosmic microwave background for thousands of galaxy clusters.
But one would also need accurate maps to pinpoint the exact locations of the clusters. That’s where the Sloan Digital Sky Survey (SDSS) came in. The SDSS dataset includes the location of the 7500 brightest galaxies clusters.
The ACT operates in the microwave regime; the SDSS’s Baryonic Oscillation Spectroscopic Survey (BOSS) operates in the visible light regime. It wouldn’t have been possible to detect the kSZ effect by analyzing them separately. The power of the technique lies in combining the two.
Spergal assigned the painstaking and time-consuming task of combining these two datasets to Nick Hand, then a senior at Princeton, who was looking for a senior thesis project. (Hand is currently a graduate student at UC-Berkeley.)
First, Hand carefully marked where the the clusters were located. Then he used the ACT data to measure the CMB temperatures at those locations. When the analysis was completed, Hand confirmed that, indeed, the CMB temperatures were every so slightly higher in those spots, just as the kSZ effect predicted.
The kSZ effect should be particularly useful as a means of tracking how galaxy clusters move through the cosmos. And what about the gravitational forces that caused them to clump together in the first place? The most obvious culprits would be dark matter and dark energy, and because the kSZ effect is a direct means of probing how large-scale structures like galaxy clusters evolve in the universe, it could be used one day to evaluate competing theories.
The announcement of the discovery had to be a pretty exciting day for Sunyaev, who pronounced the work “extremely elegant.” He was in his 20s — like Hand is today — when he first predicted the effect. He’s now 69 and director of the Max Planck Institute for Astrophysics in Garching Germany. That’s a long time to wait for confirmation.
Images: (top) Map of the cosmic microwave background radiation. Public domain. (bottom) Figure: Sudeep Das, UC-Berkeley.