Window to the Universe: The ACT measures the temperature of the microwave sky. Photograph from ACT Collaboration.

The early universe was not a fun place to be. The logical consequence of an expanding universe, first proposed around ninety years ago, is that the universe started in a hot, dense state – the big bang.  This fiery early universe was a soupy mix of protons and electrons. Radiation interacted with the electrons – bouncing around and making the universe appear opaque, for much the same reason that we can’t see into the centre of the sun.

As the universe expanded, however, it cooled, until it was cool enough for atoms like hydrogen to form – the building block from which all other atoms formed.

In this slightly cooler universe, the photons no longer interacted with the electrons and were free to start their long journey through space, eventually reaching us billions of years later as Cosmic Microwave Background (CMB) radiation. The existence of the CMB is a direct prediction of the big bang model – the afterglow of a fiery birth.

Visible to Microwave

While this light was incredibly hot when it started its journey towards us, the expansion of space has stretched the light to long, microwave wavelengths, and cooled it dramatically: the temperature of this radiation is less than three degrees above absolute zero. And yet this cold, microwave light unlocks the secrets of the universe.

In order to measure the distribution of the temperature of this microwave light on the sky, an extremely dry site is needed. The Atacama Desert in northern Chile proved to be ideal: at 5,190m above sea level and with less than a few millimeters of rainfall per year, the site is atmospherically stable and provides an excellent view of the sky. It was therefore chosen as the home for the Atacama Cosmology Telescope (ACT), which began its survey of the sky in 2007, and continued until 2010. Unlike telescopes that ‘point’ at certain known objects on the sky, ACT is a survey telescope, scanning the sky repeatedly and making a map of the radiation above it. In doing so, it takes a picture of the baby universe.

By looking at the photons that have been travelling to us from the early universe, we are basically looking back in time to the infancy of the universe, which started 13.7 billion years ago. The detection of this radiation earned Penzias and Wilson the 1978 Nobel prize. After its detection, scientists began to measure and analyse the CMB radiation, and in particular to measure its energy spectrum. The Cosmic Microwave Background Explorer, which turned 20 in April, was the first probe to measure the spectrum of the CMB. The pioneering work of the scientists who led that search was again given the nod of the Nobel committee in 2006, when George Smoot and John Mather shared the prize. Measuring the temperature of the CMB, and how that temperature changes as a function of position on the sky, tells us how uniform the universe is.

While the universe started out as an incredible uniform ‘soup’, it was the tiny ripples in the density of the universe that would grow to form the structures we see today. Just as a small snowfall can start an avalanche, so tiny over-dense regions in the universe grew under gravity, becoming more and more dense over time and collapsing to form large structures. This growth is extremely sensitive to how much of the various components of the universe, such as dark matter and dark energy, are present. The CMB, light from the nascent universe, allows us to understand the ‘birth conditions’ under which our universe began.

Massive Cluster Shadows

While we use the CMB light to illuminate early times, the photons that have been travelling towards us for over 13 billion years have also been interacting with all the stars, galaxies, and clusters of galaxies along the way. Hot gas in the clusters of galaxies far away cause photons to scatter, with large clusters effectively casting a ‘shadow’ compared to the light normally seen from CMB.

Cosmic Juggernaut. Dubbed El Gordo, this cluster was detected with by looking for ‘shadows’. Photo credit: ESO/SOAR/NASA

ACT is specifically designed to detect large clusters, and El Gordo (or “fat one” in Spanish) scooped the top spot recently for the largest cluster in the universe. El Gordo is composed of two separate clusters that have collided with each other at several million kilometers per hour. This collision can be seen in the X-Ray image, where the blue colour traces the hot gas in the cluster, which clearly shows a ‘wake’ as the two clusters merge and combine. Felipe Menanteau from the University of Rutgers in New Jersey led the ACT search for this goliath, and was delighted to be able to find “the most massive, the hottest [cluster], which gives off the most X-rays of any known cluster at this distance or beyond”.

ACT has studied the sky for three years and is now in the process of being upgraded to have even more sensitive detectors capable of measuring not only the temperature of the CMB, but also the amount of polarisation in the radiation. The next few years will provide more data, more exciting discoveries, and more challenges as we strive to push the limits of our understanding to greater and greater depths, and to smaller and smaller scales.