Never-before-seen galaxy clusters make shadowy debut.

You can run, but you can't hide from the Atacama Cosmology Telescope... especially if you're a massive galaxy cluster. Located high in the mountains of Chile, this telescope takes measurements of the CMB, the radiation that has filled the entire universe since the time of the big bang. Earlier this year, the Plank telescope mapped the CMB with unprecedented accuracy. Now, the ACT is providing astronomers with a way of using this radiation to discover galaxy clusters long before observing them visually.


The Atacama Cosmology Telescope, located in the Andes Mountains or northern Chile.
Image courtesy of Till Niermann.
About 300,000 years after the big bang, the universe had cooled and expanded to such an extent that photons could finally travel freely across long stretches of space without being absorbed by atoms. The background radiation that astronomers observe today is made up of those same primordial photons, whose wavelengths have been stretched by the expanding universe. The CMB now reveals itself as a faint 2.7K glow in the microwave range of the electromagnetic spectrum. Small anisotropies in the CMB sky denote regions of the universe that are either slightly more dense or slightly less dense than average.


WMAP provided one of the first maps of the CMB sky.
Image courtesy of NASA.
Thanks to the Atacama Cosmology Telescope, a team of astronomers from Rutgers University was able to predict the locations of several massive galaxy clusters from these anisotropies in the CMB. "The hot gases within the galaxy clusters cause a tiny fraction of the cosmic background radiation to shift to higher energies, which then makes them appear as shadows in one of ACT's observing bands," explained Jack Hughes, a senior member of the team. This phenomenon, called the Sunyaev-Zel'dovich (S-Z) effect, was predicted back in the 1970s, and has been experimentally verified a number of times since its conception. Astronomers hope that the unparalleled sensitivity of the ACT will provide them with more extensive results than ever before. In the game of galaxy detection, new technologies like the ACT are changing all the rules.