Cosmic censorship hypothesis

The weak and the strong cosmic censorship hypotheses are two mathematical conjectures about the structure of singularities arising in general relativity.

Singularities that arise in the solutions of Einstein's equations are typically hidden within event horizons, and therefore cannot be seen from the rest of spacetime. Singularities which are not so hidden are called naked. The weak cosmic censorship hypothesis conjectures that no naked singularities other than the Big Bang singularity exist in the universe. The hypothesis was conceived by Roger Penrose in 1969. Cosmic censorship hypotheses should be distinguished from chronological censorship under which every closed timelike curve passes through an event horizon, which might prevent an observer from detecting the causal violation.[1]

Contents

Basics

Since the physical behavior of singularities is unknown, if singularities can be observed from the rest of spacetime, causality may break down, and physics may lose its predictive power. The issue cannot be avoided, since according to the Penrose-Hawking singularity theorems, singularities are inevitable in physically reasonable situations. Still, in the absence of naked singularities, the universe is deterministic — it's possible to predict the entire evolution of the universe (possibly excluding some finite regions of space hidden inside event horizons of singularities), knowing only its condition at a certain moment of time (more precisely, everywhere on a spacelike 3-dimensional hypersurface, called the Cauchy surface). Failure of the cosmic censorship hypothesis leads to the failure of determinism, because it is yet impossible to predict the behavior of space-time in the causal future of a singularity. Cosmic censorship is not merely a problem of formal interest; some form of it is assumed whenever black hole event horizons are mentioned.

The hypothesis was first formulated by Roger Penrose in 1969, and it is not stated in a completely formal way. In a sense it is more of a research program proposal: part of the research is to find a proper formal statement that is physically reasonable and that can be proved to be true or false (and that is sufficiently general to be interesting).[1]

Weak and strong cosmic censorship hypothesis

The weak and the strong cosmic censorship hypothesis are two conjectures concerned with the global geometry of spacetimes.

Mathematically, the conjecture states that, for generic initial data, the maximal Cauchy development possesses a complete future null infinity.

The two conjectures are mathematically independent, as there exist spacetimes for which the weak cosmic censorship is valid but the strong cosmic censorship is violated and conversely, there exist spacetimes for which the weak cosmic censorship is violated but the strong cosmic censorship is valid.

Example

The Kerr Metric, corresponding to a black hole of mass M and angular momentum J, can be used to derive the effective potential for particle orbits restricted to the equator (as defined by rotation). This potential looks like:[2]

V_{\rm{eff}}(r,e,l)=-\frac{M}{r}%2B\frac{l^2-a^2(e^2-1)}{2r^2}-\frac{M(l-ae^2)}{r^3},~~~ a\equiv \frac{J}{M}

Where r is the coordinate radius, e and l are the test-particle's conserved energy and angular momentum respectively (constructed from the killing vectors).

To preserve cosmic censorship, the black hole is restricted to the case of a < 1. For there to exist an event horizon around the singularity, the requirement: a < 1 must be satisfied:.[2] This amounts to the angular momentum of the black hole being constrained to below a critical value, outside of which the horizon would disappear.

The following thought experiment is reproduced from Hartle's Gravity:

Imagine specifically trying to violate the censorship conjecture. This could be done by somehow imparting an angular momentum upon the black hole, making it exceed the critical value (assume it starts infinitesimally below it). This could be done by sending a particle of angular momentum l = 2Me. Because this particle has angular momentum, it can only be captured by the black hole if the maximum potential of the black hole is less than (e^2-1)/2.

Solving the above effective potential equation for the maximum under the given conditions results in a maximum potential of exactly (e^2-1)/2! Testing other values shows that no particle with enough angular momentum to violate the censorship conjecture would be able to enter the black hole, because they have too much angular momentum to fall in.

Problems with the concept

There are a number of difficulties in formalizing the hypothesis:

In 1991, John Preskill and Kip Thorne bet against Stephen Hawking that the hypothesis was false. Hawking conceded the bet in 1997, due to the discovery of the special situations just mentioned, which he characterized as "technicalities". Hawking later reformulated the bet to exclude those technicalities. The revised bet is still open, the prize being "clothing to cover the winner's nakedness".[2]

Counter-example

An exact solution to the scalar-Einstein equations R_{ab}=2\phi_a\phi_b which forms a counter example to many formulations of the cosmic censorship hypothesis was found by Mark D. Roberts in 1985:

ds^2=-(1%2B2\sigma)dv^2%2B2dv\,dr%2Br(r-2\sigma v)\left(d\theta^2 %2B \sin^2 \theta \,d\phi^2\right),\quad \phi = \frac{1}{2} \ln\left(1 - \frac{2\sigma v}{r}\right),

where \sigma is a constant.

See also

References

Footnotes:

  1. ^ A Bet on a Cosmic Scale, And a Concession, Sort Of (New York Times, February 12, 1997)
  2. ^ a b James B Hartle, Gravity in chapter 15: Rotating Black Holes. (2003. ISBN 0-8053-8662-9)

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