Other Galaxies Are Giving You a Tan

The Sun deserves most of the credit for your lovely tan, but not all of it. Ten-trillionths of your suntan comes from the light of other galaxies, according to a paper published today in Astrophysical Journal. The study was conducted by scientists at the International Centre for Radio Astronomy Research (ICRAR), Arizona State University, and Cardiff University.

Using data collected by multiple observatories, they accurately measured the amount of "extragalactic background light" hitting the Earth. Their findings further suggest that not only do other galaxies give you a tan, but they also give you sunblock. Cosmic dust attenuates about half of the extragalactic background light near their radiation sources, reradiating it at a less-harmful mid- and far-infrared wavelength. In short, says Roger Windhorst, a professor at Arizona State University, "The galaxies themselves provide us with a natural suntan lotion with an SPF of about two."

The study has implications beyond your appearance in a swimsuit. It is one part of the work being conducted by the ICRAR to help scientists understand the evolution of the universe, from the "smooth distribution of atoms" in the beginning, to the cosmos we see today.

Scientists used data from many sources: The NASA Galaxy Evolution Explorer, an orbital ultraviolet telescope that was decommissioned in 2013; the NASA Wide-field Infrared Survey Explorer, an infrared telescope still in orbit; the NASA Spitzer Space Telescope, an orbital infrared observatory; the Hubble Space Telescope, which last year celebrated its 25th anniversary in orbit; the ESA Herschel Space Observatory, a decommissioned space observatory in heliocentric orbit; and the Galaxy And Mass Assembly project, a network of ground-based observatories.

To further refine their measurements, scientists look forward to the launch of two spacecraft in coming years. ESA's Euclid spacecraft will host a science payload aimed at accurately mapping the "large-scale structure" of the universe by focusing on its "dark side." That is, Euclid is intended to answer longstanding questions about the nature of dark energy and how it affects the cosmos. The spacecraft is scheduled to launch in 2019.

In the mid 2020s, NASA intends to launch the Wide Field InfraRed Survey Telescope (WFIRST), a successor to Hubble and the long-delayed James Webb Space Telescope. WFIRST is capable of measuring light from a billion galaxies, and will help us study the expansion of the universe. It will also be used to study exoplanets. You might recall the mysterious donation to NASA by the National Reconnaissance Office, the U.S. space intelligence agency, of two spy telescopes similar in design to Hubble. WFIRST uses one of those telescope mirrors.

So the next time you're lying on your sandy towel, you can think about these upcoming missions to peer deep into the universe as the Sun plies you with 1,000,000,000,000,000,000,000 photons per square meter per second, and you are bathed in reflections of light from cosmic dust, leftover light from the Big Bang, and this newly measured extragalactic background light.

The upshot is that if you're ever asked whether you just got back from the beach, you can respond with the words of Simon Driver of the ICRAR, who led the study. Tell everyone your tan was "minted in the cores of stars in distant galaxies and from matter as it spirals into supermassive black holes."