What is Expanding Horizon of a black hole?

In summary, the causal boundary of a black hole is now affected by the indefinite future of the black hole.
  • #1
yicong2011
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What is Expanding Horizon of a black hole?

Recently, I want to read something general on Expanding Horizon...
 
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  • #2
See sections 5.1.7, 5.1.8, and 5.4.1 from Poisson's notes,

http://www.physics.uoguelph.ca/poisson/research/agr.pdf,

which evolved into the excellent book, A Relativist's Toolkit: The Mathematics of black hole Mechanics.
 
  • #3
George Jones said:
See sections 5.1.7, 5.1.8, and 5.4.1 from Poisson's notes,

http://www.physics.uoguelph.ca/poisson/research/agr.pdf,

which evolved into the excellent book, A Relativist's Toolkit: The Mathematics of black hole Mechanics.

Thanks George! Really cool link! I never thought about this before, but after reading the details, the most surprising conclusion seems 'obvious' in hindsight. That is, the causal boundary now must be affected by the indefinite future of the black hole. I propose the following simple conceptual argument:

Imagine at some future time, a small chunk of matter will fall into the black hole. However small, and however far in the future, there is some point sufficiently close the apparent horizon that whether its outgoing light ever makes it to the 'distance' depends on that matter falling in, in the future.
 
  • #4
PAllen said:
Imagine at some future time, a small chunk of matter will fall into the black hole. However small, and however far in the future, there is some point sufficiently close the apparent horizon that whether its outgoing light ever makes it to the 'distance' depends on that matter falling in, in the future.

Hi PAllen,

I was hoping to catch you while you were still online so I have not had time to read the linked material posted by George. Could you elaborate a bit more on the above quoted text and what exactly you mean by the "distance"?
 
  • #5
yuiop said:
Hi PAllen,

I was hoping to catch you while you were still online so I have not had time to read the linked material posted by George. Could you elaborate a bit more on the above quoted text and what exactly you mean by the "distance"?

Normally, light gets to null infinity (in asymptotically flat spacetime). For events within a BH event horizon, they don't. I was just describing this in a non-technical way as 'getting to distant observers'.

And my argument is simply that for some event whose light is 'almost trapped' by the current curvature, whether it ultimately gets to null infinity or not depends on the future infall of matter. No matter how small or far in the future such an infall is, there is, in principle, some event now whose light's failure to escape depends on that infall.

Think, simplistically, of a light ray 'almost orbiting' the black hole, that would take a thousand years (distant observer time) to escape. If, instead, matter falls in after 999 years, the light fails to escape because of the increased mass of the BH. Thus the true causal boundary now is affected by the the arbitrarily distant future of the BH.
 
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  • #7
thanks...
 

FAQ: What is Expanding Horizon of a black hole?

What is the Expanding Horizon of a black hole?

The Expanding Horizon of a black hole refers to the boundary or point of no return around a black hole, where the gravitational pull is so strong that nothing, not even light, can escape. This boundary is also known as the event horizon.

How does the Expanding Horizon of a black hole form?

The Expanding Horizon of a black hole is formed when a massive star collapses under its own gravity. As the star collapses, it becomes infinitely dense and all its mass is concentrated into a single point, known as the singularity. The event horizon is formed around this singularity.

What happens at the Expanding Horizon of a black hole?

At the Expanding Horizon of a black hole, the gravitational pull is so strong that it warps the fabric of space and time. This means that time and space are distorted, and objects passing through the event horizon will appear to slow down and eventually cease to exist. Anything that crosses the event horizon is pulled into the black hole and cannot escape.

Does the Expanding Horizon of a black hole keep expanding?

Yes, the Expanding Horizon of a black hole will continue to grow over time. As matter falls into the black hole, it adds to its mass and increases the size of the event horizon. However, the rate of expansion is limited by the speed of light, so it does not expand infinitely fast.

Can anything escape the Expanding Horizon of a black hole?

No, once an object crosses the Expanding Horizon of a black hole, it cannot escape. This is because the escape velocity at the event horizon is greater than the speed of light. Therefore, not even light can escape from the black hole, which is why it appears completely black to outside observers.

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