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As has been pointed out by some scientists, that black holes aren't stable bodies but are eternally collapsing objects. How does one come to such a conclusion? Is hawking radiation related to this in some way?

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    $\begingroup$ Wrt "eternally collapsing", they are probably referrring to the fact that in the reference farme of an external observer, time dilation prohibits matter from ever reaching the event horizon (the "surface") of the black hole. However, this is only for the external observer; an observer falling with the collapsing matter would cross the horizon and reach the center in a finite time. Wrt "not being stable", you're probably right that they are referring to Hawking radiation which presumably makes the black hole slowly "evaporate". $\endgroup$
    – pela
    Jun 2, 2015 at 7:42
  • $\begingroup$ As to your first question, it's very similar to this one: physics.stackexchange.com/questions/5031/… As for your 2nd question, that's a no. The eternal collapse which may not be true (except from the point of view of an observer - see Pela's answer) and the Hawking radiation are entirely separate things. $\endgroup$
    – userLTK
    Sep 1, 2015 at 10:25

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With respect to "eternally collapsing", they are probably referrring to the fact that in the reference frame of an external observer, gravitational time dilation prohibits matter from ever reaching the event horizon (the "surface") of the black hole.

Denoting the radius of the event horizon $r_\mathrm{S}$ (for "Schwarzschild radius"), time runs slower by a factor of $(1 - r_\mathrm{S}/r)^{-1/2}$ for an observer at a distance of $r$. As $r$ approaches $r_\mathrm{S}$, this factor goes to infinity, i.e. the observer will never reach $r_\mathrm{S}$

However, this is only for the external observer; the falling observer would cross the horizon and reach the center in a finite time.

With respect to "not being stable", you're probably right that they are referring to Hawking radiation which presumably makes the black hole slowly "evaporate". Near $r_\mathrm{S}$, pairs of virtual particles are being created and can be turned into real particles by the gravitational field. If they avoid falling into the black hole, energy is taken away from the hole, reducing its mass.

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"The magnetospheric eternally collapsing object (MECO) is an alternative model for black holes initially proposed by Indian scientist Abhas Mitra in 1998 and later generalized by American researchers Darryl J. Leiter and Stanley L. Robertson." -- Magnetospheric eternally collapsing object @ Wikipedia

In a nutshell, general relativity prevents a black hole from forming: it would take an infinite amount of time for it to form.

Radiation pressure supported stars in Einstein gravity: eternally collapsing objects

Final State of Spherical Gravitational Collapse and Likely Source of Gamma Ray Bursts

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  • $\begingroup$ MECO is a fringe theory, inconsistent with general relativity, and believed by almost no one. $\endgroup$
    – benrg
    Aug 21, 2022 at 18:37
  • $\begingroup$ @benrg So? The OP asked what it was, not how popular it was. $\endgroup$ Aug 21, 2022 at 22:18
  • $\begingroup$ The OP only said "eternally collapsing", which could refer to the so-called river model, or to time dilation as pela mentioned in a comment. In any case, your summary of MECO ("in a nutshell, GR prevents a black hole from forming") is incorrect since MECO is not GR. $\endgroup$
    – benrg
    Aug 21, 2022 at 22:31
  • $\begingroup$ @benrg I believe this answer is fine, as long as it clearly specifies that it talks about an alternative model that is incompatible with general relativity. After all, this might be the theory that the OP reported being pointed out by some scientists. $\endgroup$
    – Prallax
    Aug 22, 2022 at 8:05

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