Scientists using data from NASA’s Fermi Gamma-ray Space Telescope have measured all the starlight produced over 90 percent of the universe’s history. The analysis, which examines the gamma-ray output of distant galaxies, estimates the formation rate of stars and provides a reference for future missions that will explore the still-murky early days of stellar evolution. ...
One of the main goals of the Fermi mission, which celebrated its 10th anniversary in orbit this year, was to assess the extragalactic background light (EBL), a cosmic fog composed of all the ultraviolet, visible and infrared light stars have created over the universe’s history. Because starlight continues to travel across the cosmos long after its sources have burned out, measuring the EBL allows astronomers to study stellar formation and evolution separately from the stars themselves. ...
The collision between a high-energy gamma ray and infrared light, for example, transforms the energy into a pair of particles, an electron and its antimatter counterpart, a positron. The same process occurs when medium-energy gamma rays interact with visible light, and low-energy gamma rays interact with ultraviolet light. Fermi’s ability to detect gamma rays across a wide range of energies makes it uniquely suited for mapping the EBL spectrum. Enough of these interactions occur over cosmic distances that the farther back scientists look, the more evident their effects become on gamma-ray sources, enabling a deep probe of the universe’s stellar content. ...
<<The diffuse extragalactic background light (EBL) is all the accumulated radiation in the universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers almost all wavelengths of the electromagnetic spectrum except the microwave covered by primordial Cosmic Microwave Background. The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the overall electromagnetic spectrum. After the cosmic microwave background, the EBL produces the second-most energetic diffuse background.
The direct measurement of the EBL is a difficult task mainly due to the contribution of zodiacal light that is orders of magnitude higher than the EBL. Different groups have claimed the detection of the EBL in the optical and near-infrared. However, it has been proposed that these analyses have been contaminated by zodiacal light. Recently, two independent groups using different technique have claimed the detection of the EBL in the optical with no contamination from zodiacal light.
The understanding of the EBL is fundamental for extragalactic very-high-energy (VHE, 30 GeV-30 TeV) astronomy. VHE photons coming from cosmological distances are attenuated by pair production with EBL photons. This interaction is dependent on the spectral energy distribution (SED) of the EBL. Therefore, it is necessary to know the SED of the EBL in order to study intrinsic properties of the emission in the VHE sources.>>
<<High-energy (from 80 GeV to ~10 TeV) gamma rays arriving from far-distant quasars are used to estimate the extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus the density of the background light may be estimated by analyzing the incoming gamma ray spectra.>>
<<Breit–Wheeler process or Breit–Wheeler pair production is a physical process in which a positron–electron pair is created in the collision of two photons. It is the simplest mechanism by which pure light can be potentially transformed into matter. Although this mechanism is still one of the most difficult to be observed experimentally on Earth, it is of considerable importance for the absorption of high-energy photons traveling cosmic distances. The process can take the form of γ γ′ → e+ e− where γ and γ′ are two light quanta. The inverse process, e+ e− → γ γ′, in which an electron and a positron collide and annihilate to generate a pair of gamma photons is known as electron-positron pair annihilation or Dirac process.
Although the pure photon-photon Breit–Wheeler process was one of the first source of pair to be described, its experimental validation has yet to be performed. Indeed, this mechanism is theoretically characterized by a very weak probability and requires an extremely bright and collimated source of photons, having photon energy close or above the electron and positron rest mass energy, to obtain a significant number of pairs. The generation of such a source – in essence, a graser – is still a technological challenge. In many experimental configurations, pure Breit-Wheeler is dominated by other more efficient pair creation processes that screen pairs produced via this mechanism. The Dirac process (pair annihilation) has nonetheless been by far verified experimentally.>>