October 19, 2021
The observable universe is a spherical region of the universe that comprises all the matter that can be observed from Earth or its space telescopes and exploratory probes today, because the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There may be 2 trillion galaxies in the observable universe, although that number has recently been estimated to be only several hundred billions based on new data from New Horizons. Assuming the universe is isotropic, the distance to the edge of the observable universe is approximately the same in all directions. That is, the observable universe has a spherical volume (a ball) centered on the observer. Each location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth. The word "observable" in this sense does not refer to the ability of modern technology to detect light or other information from an object, or whether there is something to be detected. It refers to the physical limit created by the speed of light itself. No signal can travel faster than light, so there is a maximum distance (called the particle horizon) beyond which nothing can be detected, as the signals could not have reached us yet. Astrophysicists sometimes distinguish between the visible universe, which includes only the signals emitted since recombination (when hydrogen atoms were formed from protons, electrons, and photons were emitted)—and the observable universe, which includes signals since beginning of cosmological expansion (the Big Bang in traditional physical cosmology, the end of the inflationary epoch in modern cosmology). According to the calculations, the current comoving distance—the distance that takes into account that the universe has expanded since light was emitted—to the particles from which cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, it is about 14.0 billion parsecs (about 45.7 billion [[light-years]), while the moving distance to the edge of the observable universe is about 14.3 billion of parsecs (about 46.6 billion light years), about 2% larger. The radius of the observable universe is therefore estimated at about 46.5 billion light years and its diameter at about 28.5 gigaparsecs (or 93 billion light years, or 8.8 × 1026 meters). Using the critical density and diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic backlight (LFE) totaled 4 × 1084 photons. As the universe's expansion is accelerating, all currently observable objects outside our local supercluster will eventually appear to freeze in time , while emitting progressively redder and weaker light. For example, objects with the current redshift z of 5 to 10 will remain observable for no longer than 4-6 billion years. Furthermore, light emitted by objects currently situated beyond a certain moving distance (currently about 19 billion parsecs) will never reach Earth.