#RAHASYATvIndia Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density. Most dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.[a] Its presence is implied in a variety of astrophysical observations, including gravitational effects which cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think dark matter to be abundant in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it undetectable using existing astronomical instruments.[1]
Primary evidence for dark matter comes from calculations showing many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.[2] Other lines of evidence include observations in gravitational lensing,[3] from the cosmic microwave background, also astronomical observations of the observable universe's current structure, the formation and evolution of galaxies, mass location during galactic collisions,[4] and the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy.[5][6][7][8] Thus, dark matter constitutes 85%[b] of total mass, while dark energy plus dark matter constitute 95% of total mass–energy content.
n physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.[1][2] Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.[3]
Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount.[4][5][6][7] The density of dark energy is very low (~ 7 × 10−30 g/cm3), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.[8][9][10]
Two proposed forms of dark energy are the cosmological constant,[11][12] representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy.[13] Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.On 15 May 2015, a brief, single radio signal at 11 GHz (2.7 cm wavelength)[10] was observed in the direction of HD 164595 by a team led by N. N. Bursov[11] involving Claudio Maccone at the RATAN-600 radio observatory. The signal may have been caused by terrestrial radio-frequency interference or gravitational lensing from a more distant source.[12][13] It was observed only once (for two seconds), by a single team, at a single telescope, giving it a Rio Scale[14] score of 1 (insignificant) or 2 (low). Discussions in the media from 29 August 2016 onwards featured speculation that the signal could be caused by an isotropic beacon from a Type II civilization.[15][16]
The senior astronomer of the SETI Institute, Seth Shostak, stated that confirmation by another telescope is required.[17] Astronomer Nicholas Suntzeff of Texas A&M University stated that the signal is in a military frequency band, and that it could have been a satellite downlink, implying that some such systems may be kept secret and therefore would be unknown to SETI scientists.[10]
SETI and METI studies followed with the Allen Telescope Array and the Boquete Optical SETI Observatory.[18][17] Also, scientists at Berkeley SETI Research Center at the University of California, Berkeley observed HD 164595 using the Green Bank Telescope as part of the Breakthrough Listen program. No signal was detected at the position and frequency of the transient reported by the RATAN group.
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