Chewed up and spat out

Seems like I've featured a lot of research about astrophysics here at Skeptophilia lately, and that's not only because I'm really interested in it, but because the astrophysicists keep discovering stuff that is downright amazing.

Consider two papers last week highlighting different bizarre behaviors of one of the weirdest beasts in the cosmic zoo -- black holes. 

Since the first serious proposal of their existence, by German physicist Karl Schwarzschild in 1916, they've captivated the imagination.  Not only are they created in supernovas -- surely the most spectacular events in the universe -- their intense gravitational warping of space makes it impossible for anything, even light, to escape.  If you were falling into one (not recommended), time would slow down, at least as perceived by someone watching you from a safe distance.  From your perspective, though, your own watch would continue to run normally, until it (and you) succumbed to spaghettification -- yes, that's actually what the astrophysicists call it -- the point where the tidal forces across even such a short distance as the one between your head and your feet became sufficient to stretch you into the universe's most horrifying pasta.

As strange and terrifying as they are, they were thought for a long time to be physically quite simple; physicist John Archibald Wheeler said that "black holes have no hair," by which he meant that they have no arbitrary differences between each other that cannot be accounted for by three externally-observable parameters: their mass, angular momentum, and electric charge.  It took no less a luminary than Stephen Hawking to demonstrate that this wasn't true.  In 1974 he showed that (contrary to the picture of a black hole as a one-way-only object) they slowly evaporate through a phenomenon now called Hawking radiation in his honor.  The general idea here is that the extremely warped space near the event horizon generates sufficient energy to facilitate significant pair production -- creation of particle/antiparticle pairs.  Almost always, those pairs recombine and mutually annihilate in a fraction of a second after creation, so they're called "virtual particles" that have a measurable effect on ordinary matter but no long-term reality.  However, in the vicinity of a black hole, things are different.  Because of the extraordinary gravitational field at the event horizon, sometimes there's enough time for the two particles in the pair to separate sufficiently that one of them crosses the event horizon and the other doesn't.  At that point, the one that's fallen in is doomed; the other one just keeps moving away -- and that's the Hawking radiation.  

But what this does is robs a small bit of the mass/energy from the black hole, so its volume decreases.  What Hawking showed is that black holes actually evaporate.  It's on a huge time scale; a massive black hole has a life span many times longer than the current age of the universe.  But it suggests that everything -- even something as seemingly permanent as a black hole -- has a finite life span.

[Image is in the Public Domain courtesy of NASA/JPL]

Even that, though, doesn't begin to plumb the depths of the weirdness of these things.  Take for example the two papers I referenced earlier, each of which shows an only partially-explained behavior of black holes.

In the first, that appeared in The Astrophysical Journal, researchers looked at the odd behavior of an object called X-7 that is close to Sagittarius A*, the massive black hole at the center of the Milky Way galaxy.  X-7 is a cloud of gas and dust about fifty times the mass of the Earth, and is so close to Sagittarius A* that it orbits it once every 170 years.  The tidal forces are spaghettifying X-7 -- fast enough to observe in real time.

"No other object in this region has shown such an extreme evolution," said Anna Ciurlo of UCLA, who is the paper’s lead author.  "It started off comet-shaped and people thought maybe it got that shape from stellar winds or jets of particles from the black hole.  But as we followed it for twenty years we saw it becoming more elongated.  Something must have put this cloud on its particular path with its particular orientation."

From its current trajectory, the researchers think that it will get close enough to the black hole by 2036 that it will be torn apart completely.

If X-7 is being chewed up, there's another place in the universe where a black hole has been spat out.  The galaxy RCP 28, 7.5 billion light years from Earth, appears to be undergoing something cataclysmic; its central black hole, with an estimated mass of twenty million times that of the Sun, has been ejected from the middle and is moving away at a speed of 5.6 million kilometers per hour, pulling along a streamer of stars behind it like the tail of a comet.

What could possibly slingshot an object that massive at such high velocities remains to be seen; the researchers think it was in some kind of unstable orbit with two or more massive bodies.  (As I described in a post a couple of years ago, the three-body problem -- the mathematics of three or more objects of similar masses orbiting a common center of gravity -- is one of the most famous unsolved problems in classical mechanics, and models show that most of the time, these sorts of configurations are unstable.)  But the authors are clear that more study is needed to confirm the analysis, and then, to come up with an explanation for what exactly is going on.

In any case, what's obvious is that we've only scratched the surface of these strange objects.  Every time we look up into the star-spangled sky, we find new and amazing things to wonder at.  The astrophysicists, I think, are in for a long and exciting ride.

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