deetmari addofo maddie
Rosalia Kemme
The observable universe is a ball-shaped region of the universe consisting of all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time; the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. Initially, it was estimated that there may be 2 trillion galaxies in the observable universe.[7][8] That number was reduced in 2021 to several hundred billion based on data from New Horizons.[9][10][11] Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical region centered on the observer. Every 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 capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself. No signal can travel faster than light, hence there is a maximum distance, called the particle horizon, beyond which nothing can be detected, as the signals could not have reached us yet. Sometimes astrophysicists distinguish between the observable universe and the visible universe. The former includes signals since the end of the inflationary epoch, while the latter includes only signals emitted since recombination.[note 2]
According to calculations, the current comoving distance to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years). The comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years),[12] about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years.[13][14] Using the critical density and the diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about 1.51053 kg.[15] In November 2018, astronomers reported that extragalactic background light (EBL) amounted to 41084 photons.[16][17]
The universe's size is unknown, and it may be infinite in extent.[19] Some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable. Regions distant from observers (such as us) are expanding away faster than the speed of light, at rates estimated by Hubble's law.[note 3] The expansion rate appears to be accelerating, which dark energy was proposed to explain.
Assuming dark energy remains constant (an unchanging cosmological constant) so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter the observable universe at any time in the future because light emitted by objects outside that limit could never reach the Earth. Note that, because the Hubble parameter is decreasing with time, there can be cases where a galaxy that is receding from Earth only slightly faster than light emits a signal that eventually reaches Earth.[14][20] This future visibility limit is calculated at a comoving distance of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that can ever be theoretically observed in the infinite future is only larger than the number currently observable by a factor of 2.36 (ignoring redshift effects).[note 4]
In principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.[21][22][23] A galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its history, say, a signal sent from the galaxy only 500 million years after the Big Bang. Because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future, so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang,[24] even though it remains at the same comoving distance less than that of the observable universe.
This can be used to define a type of cosmic event horizon whose distance from the Earth changes over time. For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away.[14]
The space before this cosmic event horizon can be called "reachable universe", that is all galaxies closer than that could be reached if we left for them today, at the speed of light; all galaxies beyond that are unreachable.[25][26] Simple observation will show the future visibility limit (62 billion light-years) is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years).[27][12]
Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe".[citation needed] This can be justified on the grounds that we can never know anything by direct observation about any part of the universe that is causally disconnected from the Earth, although many credible theories require a total universe much larger than the observable universe.[citation needed] No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place. However, some models propose it could be finite but unbounded,[note 5] like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.
If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. Bielewicz et al.[30] claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface. This value is based on matching-circle analysis of the WMAP 7-year data. This approach has been disputed.[31]
The comoving distance from Earth to the edge of the observable universe is about 14.26 gigaparsecs (46.5 billion light-years or 4.401026 m) in any direction. The observable universe is thus a sphere with a diameter of about 28.5 gigaparsecs[32] (93 billion light-years or 8.81026 m).[33] Assuming that space is roughly flat (in the sense of being a Euclidean space), this size corresponds to a comoving volume of about 1.22104 Gpc3 (4.22105 Gly3 or 3.571080 m3).[34]
The light-travel distance to the edge of the observable universe is the age of the universe times the speed of light, 13.8 billion light years. This is the distance that a photon emitted shortly after the Big Bang, such as one from the cosmic microwave background, has traveled to reach observers on Earth. Because spacetime is curved, corresponding to the expansion of space, this distance does not correspond to the true distance at any moment in time.[40]
Assuming the mass of ordinary matter is about 1.451053 kg as discussed above, and assuming all atoms are hydrogen atoms (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom. The result is approximately 1080 hydrogen atoms, also known as the Eddington number.
The mass of the observable universe is often quoted as 1053 kg.[50] In this context, mass refers to ordinary (baryonic) matter and includes the interstellar medium (ISM) and the intergalactic medium (IGM). However, it excludes dark matter and dark energy. This quoted value for the mass of ordinary matter in the universe can be estimated based on critical density. The calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.
Critical density is the energy density for which the universe is flat.[51] If there is no dark energy, it is also the density for which the expansion of the universe is poised between continued expansion and collapse.[52] From the Friedmann equations, the value for ρ c \displaystyle \rho _\textc critical density, is:[53]
Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure. The organization of structure appears to follow a hierarchical model with organization up to the scale of superclusters and filaments. Larger than this (at scales between 30 and 200 megaparsecs),[57] there seems to be no continued structure, a phenomenon that has been referred to as the End of Greatness.[58]
The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale. Stars are organized into galaxies, which in turn form galaxy groups, galaxy clusters, superclusters, sheets, walls and filaments, which are separated by immense voids, creating a vast foam-like structure[60] sometimes called the "cosmic web". Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, since the early 1980s, more and more structures have been discovered. In 1983, Adrian Webster identified the Webster LQG, a large quasar group consisting of 5 quasars. The discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.
c01484d022