Light from the Depths of Time

All Your Light Are Belong to Us

Here’s an interesting thing to ponder as you’re out watching the reflected light from the planets gleam at you over the next few nights:  almost all the photos of light (and this includes everything from ultraviolet to far infrared) ever emitted by all the galaxies that ever existed in the history of the universe is still traveling through the universe.

Light contains information about the universe in all its phases throughout history. If we could carefully measure the number and energy (the wavelength) of all the photons of light throughout history, we’d know some pretty cosmic things. In a very cosmic sense. We might find out how galaxies of long ago differed from those of today, for example.  This “extragalactic background light” is tremendously difficult to measure, though. We’re inside a very bright galaxy, and that drowns out this bath of ancient and young photons.  So, astronomers are looking for other ways to measure this bath of light from across time and space.  They found one, by measuring this background light by looking at the absorption of very high-energy gamma rays from distant blazars. Those are supermassive black holes in the centers of galaxies that are pointing their jets right at us across space.  Not all the gamma rays reach us — so scientists figured out how much gamma radiation is missing by studying the gamma rays that DO reach us.

This technique yielded a HUGE result. Astronomers have been able for the first time to measure the evolution (changes in) the extragalactic background light over the past five billion years — essentially since about the time the Sun and planets began to form. And, they found out that the kinds of galaxies we observe today are responsible for all the EBL over all time. There are more distant, earlier galaxies emitting gamma rays, but they are beyond what’s called the Cosmic Gamma Ray Horizon, and that poses more challenges to astronomers wanting to measure light from even earlier times.

Still, it’s an interesting way to study the universe. And, on a late spring night, when you’re out looking at the planets in the west after sunset, it’s interesting to ponder what else there lies out there to be discovered.

Want to know more about this study?  Check out the University of California’s High-Performance Astrocomputer Center’s announcement about this discovery.