Total emissions: Adding up all the light

Wave after wave

How much light has been emitted by all the galaxies across the universe since the Big Bang? It is a fundamental question, poignant given that almost every photon ever emitted whether in the ultraviolet to far infrared wavelengths in the electromagnetic spectrum are still wending their way across the cosmos to this day with relatively few stopped in their tracks by matter.

If we could carefully measure the number and energy of all those cosmic photons - not only at the present time, but also back in time - we might be able to unlock some of the secrets of the universe, its evolution and how ancient and modern galaxies compare. Ultimately, it is detection of the extragalactic background light (EBL) that astronomers seek.

An accurate measurement of the EBL is one of the fundamental properties of the universe and measuring it is of the same importance as the more well known measurements of the heat radiation left over from the Big Bang, the so-called cosmic microwave background, at radio wavelengths. Alberto Dominguez and six co-authors have now published results in the Astrophysical Journal based on observations of blazars that span wavelengths from the radio end of the electromagnetic spectrum to highly energetic gamma rays obtained from several NASA spacecraft and several ground-based telescopes. These are the best measurements yet of the most recent 5 billion years of the evolution of the cosmos.

The photonic tide

The work overcomes one of the biggest problems facing modern astronomers. Anyone who has looked up at the night sky in the centre of a major city, will know only too well that is generally not possible to see the dim trail of the Milky Way. Moreover, Earth itself sits within the dim band of the Milky Way, which despite our city-bound observations is a myriad of billions of shining stars and glowing gas. Moreover, cosmic dust within the solar system itself scatters sunlight like motes seen through the chink of the curtains from one's place of repose on a sunny morning. There is, to say the least, a lot of photons out there that preclude ground-based and even space-based telescopes from making reliable measurements of the EBL directly.

The researchers have devised a workaround for this problem. They by focusing on blazars, supermassive black holes in the centres of galaxies with brilliant jets pointed directly at us like the beam of a torch, they could measure the EBL indirectly by measuring the attenuation of the gamma rays from the blazar beams. The attenuation is due to the collision of gamma photons from the blazars colliding with EBL photons en route, which annihilates both photons and generates an electron and a positron, which rapidly disperse. The team reasoned that because different energies of the highest-energy gamma rays are annihilated by different energies of EBL photons, they could determine how many EBL photons of different wavelengths exist along the line of sight from a blazar to Earth based on the discrepancy in the measurements of the blazar beams.

Blazar beams

The team used observations of blazars made by NASA's Fermi Gamma Ray Telescope spacecraft and demonstrated for the first time that gamma rays from distant blazars are indeed attenuated more than gamma rays from nearby blazars, as predicted by theory. "We use X-rays and low-energy gamma rays detections from the Fermi satellite, which are not attenuated by the EBL given the energy threshold of the pair production interaction, to figure out using models of photon emission in blazars which would be the unattenuated higher energy gamma rays. Then, this guess is compared with the actual detections of these higher energy gamma-ray photons by Cherenkov telescopes on the ground," explains Domínguez. "To be clear, neither the X-rays nor the low energy gamma rays are attenuated by the EBL, only the higher energy gamma rays are due to energy conditions of the pair production." Domínguez and colleagues have thus quantified the evolution of the EBL stretching back to about 5 billion years ago (redshift of about z = 0.5).

"Five billion years ago is the maximum distance we are able to probe with our current technology," Domínguez explains. "There are blazars farther away, but we are not able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us, so weakened that our instruments are not sensitive enough to detect them." Nevertheless, these measurements represent the first statistically significant observation of the "Cosmic Gamma Ray Horizon" as a function of gamma-ray energy and confirm that the types of galaxy seen today are the source of most of the EBL over all time.