Starship Asterisk*

MarsFKA wrote:that a black hole is nothing more than the collapsed core of a massive star

wormholes, white holes, bridges named after famous physicists and now, our universe *inside* a black hole in a larger universe

A new study explains why astronomers have never seen one of these weird objects.

Black holes are among the most exotic of astrophysical objects and consequently one of the most deeply studied. White holes, on the other hand, are largely ignored by astrophysicists. So it's time, therefore, to change the balance with some deeper theoretical development of the properties of these objects, says Stephen Hsu at the University of Oregon in Eugene.

White holes are closely linked with black holes, being their time-reversed equivalent. The thinking is that whatever black holes can do, white holes also do in reverse.

That leads to an odd conclusion. In the 1970s, Stephen Hawking showed that in certain circumstances black and white holes become identical. When they are in thermal equilibrium with their surroundings, he said, they ought to absorb and emit the same amount of radiation and therefore be indistinguishable.

But what of black and white holes in other circumstances? Nobody has been quite sure, until now. Hsu tackles this question by examining how white holes would behave in isolation, surrounded by empty vacuum.

He points out that the time reversal symmetry of black and white holes only works when they are in equilibrium with their surroundings. But when they are in isolation, they are not in equilibrium. In this case, a black hole emits radiation in the form of hawking radiation. However, the white hole does not behave in a time reversed fashion; it does not absorb any radiation because it is isolated in space.

So the black hole gradually evaporates as it emits Hawking radiation, but the white hole cannot perform the time reversed operation which would be to absorb radiation because there is none to absorb. Instead, it is forced to explode, releasing large amounts of thermal energy, concludes Hsu. "Isolated white holes explode into quasithermal radiation," he says.

He also says that while it is possible to construct an eternal black hole that lasts forever (as long as it isn't disturbed), a similar state does not appear to be possible to white holes.

• arXiv.org > gr-qc > arXiv:1007.2934 > 17 Jul 2010

Science fiction fans love the possibility of other universes, even more so contemplating the possibility of being able to travel between them through exotic configurations of spacetime, notably wormholes, which are pretty much just black holes with an opening poking through the singularity.

Less well known is the equally exotic (and purely hypothetical) possibility of "white holes:" the opposite of black holes. Whereas matter and light can fall into a black hole and never escape, white holes would emit light and matter but wouldn't take anything in, for example.

But while we see evidence for black holes in space, thus far there hasn't been any observational evidence of white holes. Now a physicist at the University of Oregon in Eugene thinks he might be able to explain why.

Here's the standard analogy for the formation of a wormhole: Picture a bed sheet stretched taut. Place a large bowling ball in the center of the sheet, and the sheet will bend inward in response, creating a gravitational pull.

Now imagine that the bowling ball is being squeezed, so that the same amount of mass must fit into a smaller and smaller space. The ball will become denser and denser as it becomes smaller and smaller. This causes the sheet to dip lower and lower, until finally the ball has been squeezed down to the size of a pinhead.

At that point, its density becomes so great and the gravitational force so strong that it pokes a small hole in the center of the sheet. That’s what would happen if a wormhole formed at the center of a black hole.

But what lies on the other side?

Always a stickler for symmetry in his equations, Einstein hypothesized that a “mirror universe” must exist on the other side: a "white hole."

If you think of a black hole as a large funnel with a long throat and then “cut” the throat and merge it with a second black hole that has been flipped over (a “white hole”), you end up with something that looks like an hourglass or a funnel, with the two ends connected by a thin filament. This so-called Einstein-Rosen bridge (named for Einstein and his collaborator, Nathan Rosen) is an early theoretical incarnation of a wormhole.

Back in 1971, an astrophysicist named Robert Hjellming of the National Radio Astronomy Observatory published a paper in Nature proposing that white holes could be more than mirror images of their black counterparts. Matter could actually fall into a black hole and re-emerge elsewhere in space -- or even in a completely different universe, a notion proposed by British physicist Roger Penrose a few years earlier -- via a white hole.

Hjellming even speculated that white holes might account for the huge amount of energy being emitted from distant quasars and the centers of galaxies -- far more than scientists could account for at the time by known physical processes.

That was 1971; this is now. Scientists know quite a bit more about our vast universe than they did 40 years ago. That excess energy coming from quasars? It's probably coming from supermassive black holes as matter falls in and emits telltale radiation in the process.

• Nature Physical Science 231 (03 May 1971) DOI: 10.1038/physci231020a0