Yes, black holes exist in gravitational theories with unbounded speeds of propagation

Yes, black holes exist in gravitational theories with unbounded speeds of propagation!
The foliation of the timelike hypersurfaces on which the khronon phi becomes a constant, and the location of the universal horizon xi = xi_{UH}. The khronon defines globally an absolute time, and the trajectory of a particle is always along the increasing direction of phi. Thus, once it cross the horizon, the particle move toward the singularity r = 0 and reaches it within a finite proper time. Credit: Anzhong Wang

Lorentz invariance (LI) is a cornerstone of modern physics, and strongly supported by observations.

In fact, all the experiments carried out so far are consistent with it, and no evidence to show that such a symmetry needs to be broken at a certain energy scale. Nevertheless, there are various reasons to construct gravitational theories with broken LI. In particular, our understanding of space-times at Plank scale is still highly limited, and the renomalizability and unitarity of gravity often lead to the violation of LI.

One concrete example is the Horava theory of quantum gravity, in which the LI is broken in the ultraviolet (UV), and the theory can include higher-dimensional spatial derivative operators, so that the UV behavior is dramatically improved and can be made (power-counting) renormalizable.

On the other hand, the exclusion of high-dimensional time derivative operators prevents the ghost instability, whereby the unitarity of the theory—a problem that has been faced since 1977 [ K.S. Stelle, Phys. Rev. D 16, 953 (1977)]—is assured. In the infrared (IR) the lower dimensional operators take over, whereby a healthy low-energy limit is presumably resulted.

However, once LI is broken different species of particles can travel with different velocities, and in certain theories , such as the Horava theory mentioned above, they can be even arbitrarily large. This suggests that black holes may not exist at all in such theories, as any signal initially trapped inside a horizon can penetrate it and propagate to infinity, as long as the signal has sufficiently large velocity (or energy). This seems in a sharp conflict with current observations, which strongly suggest that exist in our universe [R. Narayan and J.E. MacClintock, Mon. Not. R. Astron. Soc., 419, L69 (2012)].

A potential breakthrough was made recently by Blas and Sibiryakov [D. Blas and S. Sibiryakov, Phys. Rev. D84, 124043 (2011)], who found that there still exist absolute causal boundaries, the so-called universal horizons, and particles even with infinitely large velocities would just move around on these boundaries and cannot escape to infinity.

This has immediately attracted lot of attention. In particular, it was shown that the universal horizon radiates like a blackbody at a fixed temperature, and obeys the first law of black hole mechanics [P. Berglund, J. Bhattacharyya, and D. Mattingly, Phys. Rev. D85, 124019 (2012); Phys. Rev. Lett. 110, 071301 (2013)]. The main idea is as follows: In a given space-time, a globally timelike foliation parametrized by a scalar field, the so-called khronon, might exist.

Then, there is a surface at which the khronon diverges, while physically nothing singular happens there, including the metric and the space-time. Given that the khronon defines an absolute time, any object crossing this surface from the interior would necessarily also move back in absolute time, which is something forbidden by the definition of the causality of the theory. Thus, even particles with superluminal velocities cannot penetrate this surface, once they are trapped inside it.

In all studies of universal horizons carried out so far the khronon is part of the gravitational theory involved. To generalize the conception of the universal horizons to any gravitational theory with broken LI, recently Lin, Abdalla, Cai and Wang promoted the khronon to a test field, a similar role played by a Killing vector, so its existence does not affect the given space-time, but defines the properties of it.

By this way, such a field is no longer part of the underlaid gravitational theory and it may or may not exist in a given space-time, depending on the properties of the space-time considered. Then, they showed that the universal horizons indeed exist, by constructing concrete static charged solutions of the Horava gravity. More important, they showed that such horizons exist not only in the IR limit of the theory, as has been considered so far in the literature, but also in the full Horava theory of gravity, that is, when high-order operators are not negligible.


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More information: The corresponding author of this paper is Anzhong Wang, anzhong_wang@baylor.edu, and the paper can be found in the International Journal of Modern Physics D, via the following link, www.worldscientific.com/doi/ab … 42/S0218271814430044 . Access the PDF directly from www.worldscientific.com/doi/pd … 42/S0218271814430044 .
Journal information: Physical Review Letters

Citation: Yes, black holes exist in gravitational theories with unbounded speeds of propagation (2015, January 23) retrieved 23 July 2019 from https://phys.org/news/2015-01-black-holes-gravitational-theories-unbounded.html
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Jan 23, 2015
This article is too jargon-intensive - sans explanations - for my tastes.

Admittedly, Phys.org readers are not exactly typical of the general public. Still, lots of us are not physicists, and Phys.org has tackled the task of bringing physics to the public eye and mind, not merely to jargon-fluent insiders (there are journals for that).

So please, explain your terms, and make it accessible to the interested layperson.

Jan 24, 2015
Urgie, only if higher order partial derivatives scare you, and the implication of infinite V, which may violate the religion of some.

rgw
Jan 24, 2015
Benedicamus te, Renfield.

Jan 24, 2015
This comment has been removed by a moderator.

Jan 24, 2015
This comment has been removed by a moderator.

Jan 24, 2015
This comment has been removed by a moderator.

Jan 24, 2015
Yes, black holes exist in gravitational theories


That is the only "place" they exist, just as unicorns only exist in fairy tales.

Jan 24, 2015
This comment has been removed by a moderator.

Jan 24, 2015
Urgelt stop my praise for the clarity and well written Physorg article by commenting first.

I felt the use of jargon as tailored to a typical above average interested layperson.

Journals are for the numerical or analytic treatment:

Excluded here are title, abstract, pacs, msc, introduction, methods, technics, backround, main body, results, discussion, conclusion, summary, references, figures (except one above), and equations.

My only redemption is Urgelt's comment is about taste
and a true interest will take one right to the source of originality after seeing this 'trailer'..


Jan 24, 2015
To know what an electron is like, one has to think like an electron and what it would be like without you. All fields, i.e. spectral analysis, is only based upon a definition of temporality. In other words do not ignore the infinitesimally small stuff. How is relative motion expressed? Near collisions, close encounters, stability, expressed fields and controls ... These fast motions in space and time produce an event with a given shape if untethered.

Jan 24, 2015
The singularity may exist, but not with the present limitations.

Jan 25, 2015

In this strictly defined by laws universe there is no place for physical uncertainties as singularities.

Silly rabbit, the main law is - no law is perfectly exact (except this one). Thusly, LEAVING some wiggle room. Think Pi or Phi...
They are only mathematical artifacts, serving mainly as evidence of imperfect mathematical apparatus or theoretical building of given theory.

The math is perfect, but interpretable. Thusly. following the main law stated above.

Jan 25, 2015
Urgelt stop my praise for the clarity and well written Physorg article by commenting first.

I felt the use of jargon as tailored to a typical above average interested layperson.
...
My only redemption is Urgelt's comment is about taste
and a true interest will take one right to the source of originality after seeing this 'trailer'..

That makes clarity a function of the mental process of the beholder...:-)
It was a rather dry trailer, tho...
Not sure I'd wanna see the whole movie...

Jan 25, 2015
Urgie, only if higher order partial derivatives scare you, and the implication of infinite V, which may violate the religion of some.

V= velocity? If so, I smell another event horizon on the horizon.

Jan 25, 2015
So that the real physics doesn't favor the idealized general relativity model of pin point singularity at one side, but from event horizon perspective it goes even further than the general relativists even dared to think.

Thanks for the links, Losik-Zeph, interesting formulation... But I seriously doubt there are any extensions so steeped in relativity and conformal field theory that "relativists" dare not go.

Jan 25, 2015
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Jan 26, 2015
WG, one less drink perhaps. Pi is exactly the ratio of a circle's circumference to its diameter.

However, your imperfect math is unable to express this exactly except by a symbol representing the ratio. Fractions and decimals approximate the ratio. So no .. Math is not perfect.


Jan 26, 2015
All extensions of relativity have the ugly property...

Ugly? These violations are exactly what we need to know in order to test the various predictions of each tenable extension. We currently don't even have an equation of state for a neutron star. "The strong-field dynamics of general relativity are entirely untested." The theorists have produced beautiful work—what we lack is data from instruments like Advanced LIGO (e.g., see Coalescing binaries) and the SKA (see Challenging Einstein – SKA).

What's ugly is killing—which is a crime in every country on the face of the planet. Yet "leaders" at the national level somehow think killing is desirable in certain circumstances so tons of research dollars are used for wars... it's beyond ugly.

Jan 26, 2015
WG, one less drink perhaps. Pi is exactly the ratio of a circle's circumference to its diameter.

Notice that it's not an exact number?

However, your imperfect math is unable to express this exactly except by a symbol representing the ratio. Fractions and decimals approximate the ratio. So no .. Math is not perfect.

Perhaps I said this wrong. The math is perfect, the numbers we subject it to - aren't...

Jan 26, 2015
The up-shot of all this?
Nothing's perfect.
And THAT is the perfection.

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