Exotic star

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An exotic star is a hypothetical compact star composed of exotic matter (something not made of electrons, protons, neutrons, or muons), and balanced against gravitational collapse by degeneracy pressure or other quantum properties.

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Types of exotic stars include

Of the various types of exotic star proposed, the most well evidenced and understood is the quark star, although its existence is not confirmed.

In Newtonian mechanics, objects dense enough to trap any emitted light are called dark stars , [1] [2] [lower-alpha 1] , as opposed to black holes in general relativity. However, the same name is used for hypothetical ancient "stars" which derived energy from dark matter.

Exotic stars are largely hypothetical – partly because it is difficult to test in detail how such forms of matter may behave, and partly because prior to the fledgling technology of gravitational-wave astronomy, there was no satisfactory means of detecting compact astrophysical objects that do not radiate either electromagnetically or through known particles. While candidate objects are occasionally identified based on indirect evidence, it is not yet possible to distinguish their observational signatures from those of known objects.

Quark stars and strange stars

A quark star is a hypothesized object that results from the decomposition of neutrons into their constituent up and down quarks under gravitational pressure. It is expected to be smaller and denser than a neutron star, and may survive in this new state indefinitely, if no extra mass is added. Effectively, it is a single, very large hadron. Quark stars that contain strange matter are called strange stars.

Based on observations released by the Chandra X-Ray Observatory on 10 April 2002, two objects, named RX J1856.5−3754 and 3C 58, were suggested as quark star candidates. The former appeared to be much smaller and the latter much colder than expected for a neutron star, suggesting that they were composed of material denser than neutronium . However, these observations were met with skepticism by researchers who said the results were not conclusive.[ who? ] After further analysis, RX J1856.5−3754 was excluded from the list of quark star candidates. [3]

Electroweak stars

An electroweak star is a hypothetical type of exotic star in which the gravitational collapse of the star is prevented by radiation pressure resulting from electroweak burning; that is, the energy released by the conversion of quarks into leptons through the electroweak force. This process occurs in a volume at the star's core approximately the size of an apple and containing about two Earth masses. [4]

The stage of life of a star that produces an electroweak star is theorized to occur after a supernova collapse. Electroweak stars are denser than quark stars, and may form when gravitational attraction can no longer be withstood by quark degeneracy pressure, but can still be withstood by electroweak-burning radiation pressure. [5] This phase of a star's life may last upwards of 10 million years. [4] [5] [6] [7]

Preon stars

A preon star is a proposed type of compact star made of preons, a group of hypothetical subatomic particles. Preon stars would be expected to have huge densities, exceeding 1023 kg/m3. They may have greater densities than quark stars, and they would be heavier but smaller than white dwarfs and neutron stars. [8] Preon stars could originate from supernova explosions or the Big Bang. Such objects could be detected in principle through gravitational lensing of gamma rays. Preon stars are a potential candidate for dark matter. However, current observations [9] from particle accelerators speak against the existence of preons, or at least do not prioritize their investigation, since the only particle detector presently able to explore very high energies (the Large Hadron Collider) is not designed specifically for this and its research program is directed towards other areas, such as studying the Higgs boson, quark–gluon plasma and evidence related to physics beyond the Standard Model.[ clarification needed ]

Boson stars

A boson star is a hypothetical astronomical object formed out of particles called bosons (conventional stars are formed from mostly protons and electrons, which are fermions, but also contain a large proportion of helium-4 nuclei, which are bosons, and smaller amounts of various heavier nuclei, which can be either). For this type of star to exist, there must be a stable type of boson with self-repulsive interaction; one possible candidate particle [10] is the still-hypothetical "axion" (which is also a candidate for the not-yet-detected "non-baryonic dark matter" particles, which appear to compose roughly 25% of the mass of the Universe). It is theorized [11] that unlike normal stars (which emit radiation due to gravitational pressure and nuclear fusion), boson stars would be transparent and invisible. The immense gravity of a compact boson star would bend light around the object, creating an empty region resembling the shadow of a black hole's event horizon. Like a black hole, a boson star would absorb ordinary matter from its surroundings, but because of the transparency, matter (which would probably heat up and emit radiation) would be visible at its center. Simulations suggest that rotating boson stars would be torus, or "doughnut-shaped", as centrifugal forces would give the bosonic matter that form.

As of 2024, there is no significant evidence that such stars exist. However, it may become possible to detect them by the gravitational radiation emitted by a pair of co-orbiting boson stars, [12] [13] and GW190521, thought to be the most energetic black hole merging, may be the head-on collision of two boson stars. [14]

Boson stars may have formed through gravitational collapse during the primordial stages of the Big Bang. [15] At least in theory, a supermassive boson star could exist at the core of a galaxy, which may explain many of the observed properties of active galactic cores. [16]

Boson stars have also been proposed as candidate dark matter objects, [17] and it has been hypothesized that the dark matter haloes surrounding most galaxies might be viewed as enormous "boson stars." [18]

The compact boson stars and boson shells are often studied involving fields like the massive (or massless) complex scalar fields, the U(1) gauge field and gravity with conical potential. The presence of a positive or negative cosmological constant in the theory facilitates a study of these objects in de Sitter and anti-de Sitter spaces. [19] [20] [21] [22] [23]

Boson stars composed of elementary particles with spin-1 have been labelled Proca stars. [24]

Braaten, Mohapatra, and Zhang (2016) have theorized that a new type dense axion star may exist in which gravity is balanced by the mean-field pressure of the axion Bose–Einstein condensate. [25] The possibility that dense axion stars exist has been challenged by other work that does not support this claim. [26]

Planck stars

In loop quantum gravity, a Planck star is a hypothetically possible astronomical object that is created when the energy density of a collapsing star reaches the Planck energy density. Under these conditions, assuming gravity and spacetime are quantized, there arises a repulsive "force" derived from Heisenberg's uncertainty principle. In other words, if gravity and spacetime are quantized, the accumulation of mass-energy inside the Planck star cannot collapse beyond this limit to form a gravitational singularity because it would violate the uncertainty principle for spacetime itself. [27]

Q-stars

Q-stars are hypothetical objects that originate from supernovae or the big bang. They are theorized to be massive enough to bend space-time to a degree such that some, but not all light could escape from its surface. These are predicted to be denser than neutron stars or even quark stars. [28]

See also

Footnotes

  1. ↑ Quantum effects may prevent true black holes from forming and give rise instead to dense entities called black stars . [2]

Related Research Articles

<span class="mw-page-title-main">Black hole</span> Object that has a no-return boundary

A black hole is a region of spacetime where gravity is so strong that nothing, including light and other electromagnetic waves, is capable of possessing enough energy to escape it. Einstein's theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. A black hole has a great effect on the fate and circumstances of an object crossing it, but it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly.

<span class="mw-page-title-main">Elementary particle</span> Subatomic particle having no known substructure

In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model presently recognizes seventeen distinct particles—twelve fermions and five bosons. As a consequence of flavor and color combinations and antimatter, the fermions and bosons are known to have 48 and 13 variations, respectively. Among the 61 elementary particles embraced by the Standard Model number: electrons and other leptons, quarks, and the fundamental bosons. Subatomic particles such as protons or neutrons, which contain two or more elementary particles, are known as composite particles.

<span class="mw-page-title-main">Neutron star</span> Collapsed core of a massive star

A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses (M☉), possibly more if the star was especially metal-rich. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

A quark star is a hypothetical type of compact, exotic star, where extremely high core temperature and pressure have forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks.

In astronomy, the term compact object refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.

A strange star is a hypothetical compact astronomical object, a quark star made of strange quark matter.

In physical cosmology, baryogenesis is the physical process that is hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e. the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe.

In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to ordinary matter.

<span class="mw-page-title-main">Gravitational collapse</span> Contraction of an astronomical object due to the influence of its gravity

Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter, after sufficient accretion, may collapse to form pockets of higher density, such as stars or black holes.

<span class="mw-page-title-main">False vacuum</span> Hypothetical vacuum, less stable than true vacuum

In quantum field theory, a false vacuum is a hypothetical vacuum that is relatively stable, but not in the most stable state possible. In this condition it is called metastable. It may last for a very long time in this state, but could eventually decay to the more stable one, an event known as false vacuum decay. The most common suggestion of how such a decay might happen in our universe is called bubble nucleation – if a small region of the universe by chance reached a more stable vacuum, this "bubble" would spread.

In particle physics, preons are hypothetical point particles, conceived of as sub-components of quarks and leptons. The word was coined by Jogesh Pati and Abdus Salam, in 1974. Interest in preon models peaked in the 1980s but has slowed, as the Standard Model of particle physics continues to describe physics mostly successfully, and no direct experimental evidence for lepton and quark compositeness has been found. Preons come in four varieties: plus, anti-plus, zero, and anti-zero. W bosons have six preons, and quarks and leptons have only three.

Alternative models to the Standard Higgs Model are models which are considered by many particle physicists to solve some of the Higgs boson's existing problems. Two of the most currently researched models are quantum triviality, and Higgs hierarchy problem.

A Q-star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. Such a star can be smaller than the progenitor star's Schwarzschild radius and have a gravitational pull so strong that some light, but not all light, cannot escape. The Q stands for a conserved particle number. A Q-star may be mistaken for a stellar black hole.

In particle physics, hexaquarks, alternatively known as sexaquarks, are a large family of hypothetical particles, each particle consisting of six quarks or antiquarks of any flavours. Six constituent quarks in any of several combinations could yield a colour charge of zero; for example a hexaquark might contain either six quarks, resembling two baryons bound together, or three quarks and three antiquarks. Once formed, dibaryons are predicted to be fairly stable by the standards of particle physics.

Strange matter is quark matter containing strange quarks. In extreme environments, strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers, as in the hypothetical strange stars. At high enough density, strange matter is expected to be color superconducting.

A strangelet is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. An equivalent description is that a strangelet is a small fragment of strange matter, small enough to be considered a particle. The size of an object composed of strange matter could, theoretically, range from a few femtometers across to arbitrarily large. Once the size becomes macroscopic, such an object is usually called a strange star. The term "strangelet" originates with Edward Farhi and Robert Jaffe in 1984. Strangelets can convert matter to strange matter on contact. Strangelets have been suggested as a dark matter candidate.

In particle physics, W′ and Z′ bosons refer to hypothetical gauge bosons that arise from extensions of the electroweak symmetry of the Standard Model. They are named in analogy with the Standard Model W and Z bosons.

In theoretical physics, a mass generation mechanism is a theory that describes the origin of mass from the most fundamental laws of physics. Physicists have proposed a number of models that advocate different views of the origin of mass. The problem is complicated because the primary role of mass is to mediate gravitational interaction between bodies, and no theory of gravitational interaction reconciles with the currently popular Standard Model of particle physics.

A cosmological phase transition is a physical process, whereby the overall state of matter changes together across the whole universe. The success of the Big Bang model led researchers to conjecture possible cosmological phase transitions taking place in the very early universe, at a time when it was much hotter and denser than today.

References

  1. ↑ Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (February 2009). "Small, dark, and heavy: But is it a black hole?". arXiv: 0902.0346v2 [gr-qc].Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (2009). "Small, dark, and heavy: But is it a black hole?". arXiv: 0902.0346v2 [gr-qc].
  2. 1 2 Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (30 September 2009). "How quantum effects could create black stars, not holes". Scientific American . No. October 2009. Archived from the original on 15 November 2013. Retrieved 25 December 2022. Originally published with title "Black Stars, Not Holes".
  3. ↑ Truemper, J.E.; Burwitz, V.; Haberl, F.; Zavlin, V.E. (June 2004). "The puzzles of RX J1856.5-3754: neutron star or quark star?". Nuclear Physics B: Proceedings Supplements. 132: 560–565. arXiv: astro-ph/0312600 . Bibcode:2004NuPhS.132..560T. doi:10.1016/j.nuclphysbps.2004.04.094. S2CID   425112.
  4. 1 2 Shiga, D. (4 January 2010). "Exotic stars may mimic Big Bang". New Scientist . Archived from the original on 18 January 2010. Retrieved 18 February 2010.
  5. 1 2 "Theorists propose a new way to shine – and a new kind of star: 'Electroweak'" (Press release). Case Western Reserve University. 15 December 2009. Archived from the original on 21 February 2020. Retrieved 16 December 2009– via ScienceDaily.
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  24. ↑ Brito, Richard; Cardoso, Vitor; Herdeiro, Carlos A.R.; Radu, Eugen (January 2016). "Proca stars: Gravitating Bose–Einstein condensates of massive spin 1 particles". Physics Letters B . 752: 291–295. arXiv: 1508.05395 . Bibcode:2016PhLB..752..291B. doi:10.1016/j.physletb.2015.11.051. hdl: 11573/1284757 . S2CID   119110645. Archived from the original on 25 November 2021. Retrieved 25 July 2021.
  25. ↑ Braaten, Eric; Mohapatra, Abhishek; Zhang, Hong (2016). "Dense axion stars". Physical Review Letters . 117 (12): 121801. arXiv: 1512.00108 . Bibcode:2016PhRvL.117l1801B. doi:10.1103/PhysRevLett.117.121801. PMID   27689265. S2CID   34997021. Archived from the original on 28 April 2020. Retrieved 26 September 2018.
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