Chapter 1: The Enigmatic Extragalactic Background Light

The cosmos is enveloped by a faint luminosity known as the Extragalactic Background Light (EBL), a residual glow resulting from all the stars that have existed since the Universe's inception. Researchers have analyzed this ancient radiation, using gamma-ray emissions from 150 blazars to derive an astonishingly low average stellar density: merely 1.4 stars for every 100 billion cubic light-years.

This diffuse background, spanning from infrared to ultraviolet wavelengths, is a remnant of the light emitted by stars and active galactic nuclei throughout cosmic history. Observing the EBL directly poses challenges, as its subtle glow is often overshadowed by the intense light from nearby galaxies, particularly the zodiacal light, which is sunlight scattered by tiny dust particles in the solar system.

The EBL can be likened to a delicate mist, yet its measurement is crucial for deciphering the Universe's evolutionary narrative, especially in terms of star formation rates and the distribution of stars across vast expanses. A comprehensive study published on November 1, 2012, in Science tackled the complexities of quantifying the EBL.

Researchers employed an indirect approach to gauge EBL's intensity. They constructed a model depicting how gamma rays emitted by 150 distant blazars were attenuated by the EBL, based on their energy levels. By comparing the gamma-ray counts detected by the Fermi space telescope with the redshift distances of these blazars, they were able to extrapolate the EBL's average density.

The Fermi Paradox: Into the Void delves into the mysteries of cosmic voids and the implications of the EBL in understanding our universe's structure.

Continuing our exploration, we turn our attention to supermassive black holes located at the centers of galaxies. When these colossal entities consume substantial amounts of matter, they generate powerful plasma jets that are ejected at incredible speeds. When one of these jets is directed towards Earth, it manifests as a bright point source known as a blazar. Notably, these blazars are formidable sources of gamma rays. Between 2008 and 2012, Fermi's detectors identified over a thousand blazars, from which 150, classified as BL Lacertae types with gamma-ray emissions exceeding 3 GeV, were selected for further analysis.

A gamma photon emitted by a blazar travels for billions of years before being captured by the Fermi Large Area Telescope (LAT). However, not all of these gamma rays reach us; some are transformed into particles—an electron and its antimatter counterpart, a positron—upon colliding with an ultraviolet or visible light photon from the EBL.

This interaction results in fewer gamma rays being detected from distant blazars compared to those that are nearer, particularly at higher energies (over 25 GeV). By analyzing the gamma-ray energy spectra from both nearby and distant blazars, researchers developed gamma-ray attenuation curves, revealing connections to the distances of the emitting blazars. From these curves, they calculated the average density of the EBL “fog,” through which the gamma rays had to traverse.

The findings shed light on the Universe's evolutionary timeline. The first stars, massive and luminous, emerged about 400 million years post-Big Bang, potentially a bit slower than previously thought. Star formation peaked approximately three billion years after the Big Bang, followed by a steady decline.

The analysis of the EBL's density provided a revelation: the average stellar density in the Universe stands at just 1.4 stars per 100 billion cubic light-years, translating to an average separation of 4,150 light-years between stars. Within galaxies, however, star distances are much closer, as seen in the Alpha Centauri system, which is approximately one-thousandth the distance from the Sun.

To grasp the scale of this cosmic void, consider that 4,150 light-years equals nearly 40 million billion kilometers (3.926 × 10¹? km). This vast distance becomes even more staggering when one reflects on the billions of stars that have existed throughout cosmic history. Despite the countless stars in each galaxy and the numerous galaxies throughout the Universe, their distribution results in an astonishing emptiness—averaging only 1.4 stars per 100 billion cubic light-years.

The measurement of EBL density has enabled scientists to approximate the number of stars within a volume of 100 billion cubic light-years, showcasing the vastness and sparsity of the cosmos.