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In string theory, believed by some to be a consistent theory of quantum gravity, the graviton is a massless state of a fundamental string. If it exists, the graviton is expected to be massless because the gravitational force has a very long range, and appears to propagate at the speed of light.
The graviton is a hypothetical particle that has been included in some extensions to the standard model to mediate the gravitational force. It is in a peculiar category between known and hypothetical particles: As an unobserved particle that is not predicted by, nor required for the Standard Model , it belongs in the table of hypothetical ...
In astrophysics, the gravastar (a portmanteau of "gravitational vacuum star") is an object hypothesized in a 2006 paper by Pawel O. Mazur and Emil Mottola as an alternative to the black hole theory. It has the usual black hole metric outside of the horizon , but de Sitter metric inside. on the horizon there is a thin shell of matter.
The theory is based on the reformulation of general relativity known as Ashtekar variables, which represent geometric gravity using mathematical analogues of electric and magnetic fields. [47] [48] In the quantum theory, space is represented by a network structure called a spin network, evolving over time in discrete steps. [49] [50] [51] [52]
The model does not explain gravitation, although physical confirmation of a theoretical particle known as a graviton would account for it to a degree. Though it addresses strong and electroweak interactions, the Standard Model does not consistently explain the canonical theory of gravitation, general relativity , in terms of quantum field theory .
In what is called the second superstring revolution, it was conjectured that both string theory and a unification of general relativity and supersymmetry known as supergravity [199] form part of a hypothesized eleven-dimensional model known as M-theory, which would constitute a uniquely defined and consistent theory of quantum gravity.
The first example of this was the star MACS J1149 Lensed Star 1 (also known as Icarus), thanks to the boost in flux due to the microlensing effect. Gravitational lenses act equally on all kinds of electromagnetic radiation , not just visible light, and also in non-electromagnetic radiation, like gravitational waves.
The model is a braneworld theory developed while trying to solve the hierarchy problem of the Standard Model.It involves a finite five-dimensional bulk that is extremely warped and contains two branes: the Planckbrane (where gravity is a relatively strong force; also called "Gravitybrane") and the Tevbrane (our home with the Standard Model particles; also called "Weakbrane").