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Natural Science Forum / Physics / Research / September 2007Black Holes in the Big Rip
If the expansion of the Universe is accelerating, as recent observations indicate, and this continues, then eventually space will be expanding at faster than light speed in a vacuum (the so-called "Big Rip"). Presumably in that case, event horizons of black holes would start shrinking, and I would be interested to know whether any work has been done by black hole cosmologists on these extreme conditions. In particular, what would the effective temperature be at the event horizon? (Zero one might naively think, as the exterior would be in effect a perfect heat sink.) Also, is it possible or likely that features inside black holes, such as mass-inflated horizons, would be uncovered in their futures and perhaps even locally counteract the expansion, or does GR prohibit local variations in the cosmological "constant" (even if it is changing overall)? Any insights and informed and/or idle speculation welcome! Regards John R Ramsden (jhnrmsdn@yahooo.co.uk) <-- remove one 'o' from Yahoo > If the expansion of the Universe is accelerating, as recent > observations indicate, and this continues, then eventually [quoted text clipped - 17 lines] > > Any insights and informed and/or idle speculation welcome! Isn't a wormhole essentially a black hole with negative mass stabalizing it? Could a black hole become a wormhole? Another perhaps silly question. The "rip" seems to me to have a certain symmetry about it. If we ripped a quark apart would we have Inflation and a new Universe. Just a thought. Good SF story. Plunging though a black hole in a dying Universe. -Ian Parker > If the expansion of the Universe is accelerating, as recent > observations indicate, and this continues, then eventually > space will be expanding at faster than light speed in a > vacuum (the so-called "Big Rip"). This is wrong in a couple of ways. First, the rate of expansion of space doesn't have units of speed and hence can't exceed c. Space is already expanding "faster than light" in some sense: the comoving relative speed of opposite ends of the visible universe is around 7c. Second, the Big Rip doesn't refer to exponential expansion but to a different scenario in which the scale factor goes to infinity in a finite time. This only happens if the dark energy density increases with time. If the dark energy behaves like a cosmological constant (constant density everywhere in spacetime), then eventually it dominates the expansion and you get what's effectively a de Sitter vacuum. The universe is still perfectly inhabitable in this phase; it becomes impossible to see or travel to other galaxies, but nothing gets ripped apart. -- Ben > If the expansion of the Universe is accelerating, as recent > observations indicate, and this continues, then eventually > space will be expanding at faster than light speed in a > vacuum (the so-called "Big Rip"). Faster relative to what? Check out http://en.wikipedia.org/wiki/Big_Rip. It suggests all structures would be ripped apart. I couldn't understand the wikipedia article, so I've tried an attempt at explanation here: A galaxy in a region of space expanding at the speed of light relative to us will disappear because in the next second dark energy expansion will take it beyond our ken. Eventually every galaxy will go that way, then every star, then everything! Idle speculation: >From the viewpoint of a black hole, the only things in the universe would be the singularity, the event horizon, whatever is between the singularity and event horizon. The event horizon would be ripped off the black hole at each moment of expansion to be replaced by another event horizon. The black hole is losing energy to the expanding universe so, by conservation of energy, it must cool down and keep on cooling. Would it keep on cooling for ever, getting exponentially closer to absolute zero but never reaching it? What about from the viewpoint of ..er... a point in empty space? It would be all empty space until (at a distant time in the future) a vacuum fluctuation initiates another big bang? |
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