University research fellow Christopher Mullis and his international team of astronomers are cosmic archaeologists. Just as conventional archaeologists dig through the accumulated sediment of hundreds and thousands of years to uncover relics from long ago, his group searches the skies for relics of a much younger universe.
Rather than shovels and picks, they travel back in time using telescopes, and they dig much deeper. They recently discovered an ancient object that pushes the limits of the observational universe to only five billion years after the universe began.
The object that they found is a massive galaxy cluster nine-billion light years away, the farthest ever observed. A light year is the distance light travels in one year, meaning that light from the cluster has been traveling across the vacuum of space for two-thirds the lifetime of the universe. In making the discovery, Mullis worked with astronomers from the European Southern Observatory, the Max Planck Institute for Extraterrestrial Physics and the Astronomical Institute at Potsdam.
A galaxy cluster is the name astronomers give to groups of galaxies that are trapped together by their mutual gravitation. Each galaxy is made of hundreds of billions of stars like the sun, and there can be thousands of galaxies in a cluster. Galaxy clusters take billions of years to coalesce from an unstable assortment of partially formed galaxies into a mature cluster. Clusters interact with one another to form the largest structures in the universe.
“Clusters are often difficult to find based on visual observations,” Mullis said. “However, they glow brightly in X-ray light due to the hundred-million-degree gas that fills the space between the galaxies.”
This gas, heated during cluster formation, is what allowed the team to make their record-breaking find.
The team used a complex computer program to search through archived images from the XMM Newton X-ray satellite and identify possible distant clusters. XMM-Newton was launched in 1999 by the European Space Agency. Its wide field of view and excellent light-gathering ability make it well suited for detecting faint, distant objects. The cluster was discovered in the background of an image of a black hole.
The cluster’s X-ray signal is extraordinarily faint, Mullis said. The entire signal from the galaxy cluster is made up of a scant 280 photons, gathered during an exposure lasting 12.5 hours. As a comparison, on a sunny day, your eye receives 10 quadrillion photons every second.
To measure the distance to the cluster, the team made optical observations from the ESO Very Large Telescope in South America. By passing light through a spectrograph — an instrument that separates the light from a source according to wavelength, much like a prism — astronomers can tell what chemical elements are present in the object they are observing.
Because the universe is constantly expanding, light that leaves an object long ago gets stretched out during its journey. Astronomers call this increase in wavelength “redshift.” The more redshifted an object is, the farther away it is. Using the unique spectral fingerprints of certain elements to measure this change, Mullis said the “eureka” moment came in late 2004, when he analyzed the spectra from the cluster and determined an astonishing distance of nine billion light years.
“We see an evolved cluster at five billion years (since the Big Bang). That means it was forming at something like two or three billion years,” Mullis said. “The general expectation is that at higher distances clusters get more youthful, so (this discovery) is particularly exciting.”
Gus Evrard, a University professor of astronomy and physics, said fully developed clusters at this distance are expected from current theoretical models of the universe. “In that respect, theory is actually ahead of observation,” Evrard said.
There have been protoclusters found at distances still greater than the cluster found by Mullis and his team. Those objects, however, are not well-formed and lack the multi-million-degree gas that characterizes a fully evolved cluster.
Evrard compared the situation to population density in a sparsely populated state. There are bound to be counties with a higher population than the surrounding area, but those are not the same as a full-fledged city. Mullis and his team have discovered the farthest and therefore most ancient “city” of galaxies known.
“The finding of such a large cosmic structure at this age … is expected in a universe dominated by dark energy but not in a universe dominated by dark matter,” Evrard said. Dark matter is the mysterious “missing mass” that cosmologists believe makes up almost a quarter of the total mass/energy budget of our universe. The amount of dark matter in the universe determines the structures that will form and how large they become.
By searching for the earliest formed clusters, astronomers hope to get an estimate of the universe’s dark matter content. The current estimate is that dark matter makes up about one quarter of the total mass/energy in the universe, Mullis said.
Dark energy is the energy contained in the vacuum of space itself. It governs the rate of expansion of the universe. Measuring the redshift of distant clusters helps cosmologists measure that expansion and get an estimate of the dark energy content. Current estimates place dark energy at three quarters of the total, Mullis said.
This amount of dark energy leads to the surprising conclusion that the universe is not only expanding, but accelerating. Normal matter makes up only a small percent of the total energy density. The vast majority of the universe is made of matter and energy that we do not understand.
The discovery of such a distant cluster by analyzing archived images from XMM-Newton was relatively easy, Mullis said. This bodes well for the discovery of many more super-distant galaxy clusters in the future, he added.
With a whole range of distant clusters to study, astronomers will be able to infer the fundamental parameters of the universe, and conclude how it evolved from the Big Bang to the structures that we observe today.