Long before astronomers proposed the Big Bang theory, they understood that the universe is infinite, remains constant over time, and that there are no dark patches in the night sky if it is indeed filled with stars. If stars populate space uniformly, then starlight would illuminate every point in the sky. Consequently, if this light persisted over time, it would illuminate Earth equally, making the sky uniformly bright.
This insight is known as Olbers’ Paradox, which suggests that the universe is neither infinite nor static, as it contains gaps of darkness between stars. Instead, astronomers agree that the universe has evolved over time, originating from the Big Bang. These transformations prevent the sky from being completely filled with starlight because, even if the universe were infinite and abundant with stars, only some would have had enough time to reach Earth.
The sky is not merely filled with starlight; it is awash with various light types. The most prominent is the long-wavelength radiation remaining from the Big Bang, known as the Cosmic Microwave Background, or CMB. Additionally, short-wavelength radiation emanates from beyond our galaxy, termed the Extragalactic Gamma-Ray Background, or EGB. The origins of EGB are more elusive compared to CMB, with possible sources ranging from black holes in distant galaxies to reactions of subatomic particles and even dark matter.
Since the discovery of EGB in the 1970s, scientists have pinpointed specific large, high-energy objects such as active galactic nuclei, which comprise nearly half of the EGB. These entities produce bright spots of resolved EGB that are observable from Earth.
This accounts for only part of the EGB, leaving the other half unresolved. The unresolved EGB is distributed across the sky, with sources too distant to be identified by telescopes. To investigate unresolved EGB sources, a scientific team explored the hypothesis that galaxies are significant, if not the primary, contributors. They deduced that nearby galaxies may account for resolved EGBs, implying that distant galaxies could be sources of the diffuse, unresolved EGBs.
It is believed that galaxies generate gamma rays through a series of events; firstly, a star is formed, which may either explode or undergo a supernova event. Supernovae accelerate particles, such as protons and electrons, to high velocities, creating cosmic rays. These cosmic rays then collide, releasing energy and initiating a chain reaction that leads to the formation of high-energy gamma rays, existing within a specific energy range of approximately 0.01–1,000 Giga-electronvolts (GEVs).
Researchers have developed a model to estimate the gamma radiation that galaxies are capable of producing and the energy levels of that radiation. This model derives two equations to calculate how frequently a galaxy generates cosmic rays based on its star production rate. Physical attributes of the galaxy, including mass, radius, and star formation rate, were necessary to finalize the model.
Data was collected from 22,087 galaxies in the Goods-S Catalog using the Hubble Space Telescope. They utilized the model to estimate the gamma radiation contributions these galaxies could provide to small regions of the sky, comparing the model’s estimates with actual gamma radiation observations from each galaxy obtained through the Fermi Gamma-Ray Telescope.
The results indicated that within the 1-10 GEV energy range of gamma radiation, these galaxies might account for 50-60% of the unresolved EGB across all observed areas. Furthermore, the contribution from galaxies diminishes sharply below 1 GEV and above 10 GEV. Their findings suggest that earlier research into the star-forming galaxy hypothesis may have underestimated their contributions to the unresolved EGB.
The team concluded that beyond star-forming galaxies, other sources must account for the unresolved EGB found in deep space. They propose that future research focus on active galactic nuclei and millisecond pulsars as potentially fruitful candidates. This observation indicates the possibility of other unexplored phenomena, suggesting that galaxies may not be the primary sources of unresolved EGBs. The disappearance of dark matter could be a piece of this cosmic puzzle.
Where does Deep Space Gamma Ray come from? It first appeared in Sciworthy.
Source: sciworthy.com
