In loop quantum gravity theory, a Planck star is a hypothetical 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. The accumulation of mass-energy inside the Planck star cannot collapse beyond this limit because it violates the uncertainty principle for spacetime itself.
The key feature of this theoretical object is that this repulsion arises from the energy density, not the Planck length, and starts taking effect far earlier than might be expected. This repulsive 'force' is strong enough to stop the collapse of the star well before a singularity is formed, and indeed, well before the Planck scale for distance. Since a Planck star is calculated to be considerably larger than the Planck scale, this means there is adequate room for all the information captured inside of a black hole to be encoded in the star, thus avoiding information loss.
While it might be expected that such a repulsion would act very quickly to reverse the collapse of a star, it turns out that the relativistic effects of the extreme gravity such an object generates slow down time for the Planck star to a similarly extreme degree. Seen from outside the star's Schwartzschild radius, the rebound from a Planck star takes approximately fourteen billion years, such that even primordial black holes are only now starting to rebound from an outside perspective. Furthermore, the emission of Hawking radiation can be calculated to correspond to the timescale of gravitational effects on time, such that the event horizon that 'forms' a black hole evaporates as the rebound proceeds.
The existence of Planck stars was first proposed by Carlo Rovelli and Francesca Vidotto, who theorized in 2014 that Planck stars form inside black holes as a solution to the black hole firewall and black hole information paradox. Confirmation of emissions from rebounding black holes could possibly provide evidence for loop quantum gravity. Recent work demonstrates that Planck stars may exist inside of black holes as part of a cycle between black hole to white hole.
An exotic star is a hypothetical compact star composed of something other than electrons, protons, neutrons, or muons, and balanced against gravitational collapse by degeneracy pressure or other quantum properties. Exotic stars include quark stars (composed of quarks) and perhaps strange stars (composed of strange quark matter, a condensate of up, down and strange quarks), as well as speculative preon stars (composed of preons, which are hypothetical particles and "building blocks" of quarks, should quarks be decomposable into component sub-particles). Of the various types of exotic star proposed, the most well evidenced and understood is the quark star.
Exotic stars are largely theoretical – 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 cosmic objects that do not radiate electromagnetically or through known particles. So it is not yet possible to verify novel cosmic objects of this nature by distinguishing them from known objects. Candidates for such objects are occasionally identified based on indirect evidence gained from observable properties.Hayward metric
The Hayward metric is the simplest description of a black hole which is non-singular. The metric was written down by Sean Hayward as the minimal model which is regular, static, spherically symmetric and asymptotically flat. The metric is not derived from any particular alternative theory of gravity, but provides a framework to test the formation and evaporation of non-singular black holes both within general relativity and beyond. Hayward first published his metric in 2005 and numerous papers have studied it since.Hypothetical star
A hypothetical star is a star, or type of star, that is speculated to exist but has yet to be definitively observed. Hypothetical types of stars have been conjectured to exist, have existed or will exist in the future universe.Loop quantum gravity
Loop quantum gravity (LQG) is a theory of quantum gravity, attempting to merge quantum mechanics and general relativity, while incorporating the standard model particles. It takes seriously the key insight from general relativity that space-time is a dynamic entity, not a fixed framework. It competes with string theory, another candidate for a theory of quantum gravity. However, unlike string theory, LQG is not a candidate for a theory of everything the goal of which is to explain all of particle physics, unifying gravity with the other forces at the same time. In contrast to LQG, string theory (for the most part) is background-dependent (built on a fixed framework) and doesn’t account for the dynamic nature of space-time at the heart of relativity.
From the point of view of Einstein's theory, all attempts to treat gravity as another quantum force equal in importance to electromagnetism and the nuclear forces have failed. According to Einstein, gravity is not a force – it is a property of spacetime itself. Loop quantum gravity is an attempt to develop a quantum theory of gravity based directly on Einstein's geometric formulation.
To do this, in LQG theory space and time are quantized, analogously to the way quantities like energy and momentum are quantized in quantum mechanics. The theory gives a physical picture of spacetime where space and time are granular and discrete directly because of quantization just like photons in the quantum theory of electromagnetism and the discrete energy levels of atoms. A minimum distance exists.
Space's structure prefers an extremely fine fabric or network woven of finite loops. These networks of loops are called spin networks. The evolution of a spin network, or spin foam, has a scale on the order of a Planck length, approximately 10−35 metres, and smaller scales do not exist. Consequently, not just matter, but space itself, prefers an atomic structure.
The vast areas of research developed in several directions that involve about 30 research groups worldwide. They all share the basic physical assumptions and the mathematical description of quantum space. Research follows two directions: the more traditional canonical loop quantum gravity, and the newer covariant loop quantum gravity, called spin foam theory.
Physical consequences of the theory proceed in several directions. The most well-developed applies to cosmology, called loop quantum cosmology (LQC), the study of the early universe and the physics of the Big Bang. Its greatest consequence sees the evolution of the universe continuing beyond the Big Bang called the Big Bounce.Magnetospheric eternally collapsing object
The Magnetospheric eternally collapsing object (MECO) is an alternative model for black holes proposed initially by Indian scientist Abhas Mitra in 1998, later on proposed by Darryl Leiter and Stanley Robertson a generalization of the eternally collapsing object (ECO) proposed by Abhas Mitra in 1998. A proposed observable difference between MECOs and black holes is that a MECO can produce its own intrinsic magnetic field. An uncharged black hole cannot produce its own magnetic field, though its accretion disc can.White hole
In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it. In this sense, it is the reverse of a black hole which can only be entered from the outside and from which matter and light cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. However, this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented occurrence of a white hole.
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