ALMA Observations Of Sunyaev-Zel’dovich Effect Reveal A Dark ‘Hole’ In Center Of A Massive Galaxy Cluster

The universe, as Douglas Adams once astutely observed, is mind-bogglingly big. Understandably, the sheer size of the cosmos makes it extremely difficult for us — a species inhabiting a pale blue dot in an insignificant little galaxy — to discern what space-time looks like at sufficiently large scales.

Be that as it may, we are not entirely clueless. The Cosmic Microwave Background — the radiation created shortly after the Big Bang — is one of the few windows that allow us to see what the cosmos we live in is actually like. Analysis of the omnipresent CMB has not only given us incontrovertible evidence for the Big Bang and the universe’s expansion, it has also provided us vital insights into the composition of the observable universe as a whole.

Now, a team of researchers has used a unique property of the CMB to create a map of one of the most massive galaxy clusters in the known universe — RX J1347.5–1145. The map, created through observations made using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, shows the first measurements of what’s known as the Sunyaev-Zel’dovich effect.

The Sunyaev-Zel’dovich effect, which was first observed over 30 years ago, appears when high-energy electrons in galaxy clusters disturb the constituent microwave photons in the CMB. These electrons give the photons a tiny boost of energy.

“Detecting these boosted photons through our telescopes is challenging but important — they can help astronomers to understand some of the fundamental properties of the Universe, such as the location and distribution of dense galaxy clusters,” the European Southern Observatory (ESO), which built and operates ALMA, said in a statement released Monday.

The ALMA image of the galaxy cluster shows the first measurements of the thermal Sunyaev-Zel’dovich effect in blue. The center of the cluster, on the other hand, shows up here in the dark “hole” in the observations.

“The energy distribution of the CMB photons shifts and appears as a temperature decrease at the wavelength observed by ALMA, hence a dark patch is observed in this image at the location of the cluster,” the ESO explained in the statement.

This is not the first time astronomers have parsed the CMB for secrets to the universe. Previous analysis has shown that the cosmos, at the largest scales, is homogenous and isotropic. What this means is that unlike the obviously clumpy universe we see all around us, where galaxies, stars and interstellar dust are unevenly distributed, a bird’s eye view of the universe would reveal a structure that doesn’t just look the same no matter where it's viewed from, but one that also possesses properties that are the same no matter what direction you measure them in.