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Mystery deepens over origin of biggest black holes

Sunday, June 15, 2008

Mysteries are popular, so here's a good one, about another popular subject, black holes.

Mystery deepens over origin of biggest black holes (5/19/08)
Where did the universe's biggest black holes come from? One idea suggests the behemoths began as smaller "seed" black holes that gobbled up surrounding gas. But new computer simulations suggest these seeds were born with practically nothing around them to eat, deepening the puzzle over how the biggest black holes came to be. ...

How these supermassive black holes grew so big so fast has been a major puzzle. Some astronomers have suggested that they grew from smaller black holes of about 100 times the Sun's mass, left behind when the universe's first stars collapsed at the end of their lives.

Note that this isn't the only way that black holes could have formed, not even black holes of about this size. That's good, because what this study seems to show is that it would be difficult for a black hole formed from a collapsed early star to accrete enough matter to grow into a supermassive black hole (SBH).
But the universe's first stars were not born until a few hundred million years after the big bang. Even though they lived only a few million years before collapsing to form black holes, this does not leave much time for these seeds to grow into the monstrous black holes powering quasars.

The puzzle now appears to have deepened, with new computer simulations suggesting that these seed black holes were born with little food around them from which to gain weight.

Tom Abel of Stanford University in California, US, and his colleagues made computer models simulating the first generation of stars. These first stars are thought to have been very massive and luminous, weighing about 300 times as much as the Sun. The simulations reveal that the stars' prodigious radiation would have blown away the gas around them.
Early famine

As a result, the black holes that formed when the stars died a few million years later would have had very little to eat. In the simulations, it took about 100 million years for the gas to fall back towards the first black holes and provide them with something to eat. The time lost due to the early famine makes it even harder to imagine how these black holes could have swelled to billions of times the Sun's mass soon thereafter.

Let's just suppose this simulation result makes the formation of SBHs from such seeds very unlikely. Are there other reasonable possibilities?

Sure there are. One is that black holes don't exist at all, so neither do SBHs. Never mind that most galaxies seem to contain very massive objects, whose mass is 105 solar masses or more. (Normally written as 105M.) The Milky Way itself has a black hole at its center, whose mass is estimated at 3.7×106M.

Such massive objects need an explanation too, which would have at least the same difficulties as for SBHs. But we've already noted that the evidence for black holes is very solid (see here), so let's rule this out.

Here's another recent hypothesis:

Biggest black holes may grow inside 'quasistars' (11/29/07)
The biggest black holes in the universe might have grown within the bellies of giant stars, a new study suggests. If these hole-bearing "quasistars" exist, then they might be bright enough to see from across the universe.

Quasistars are one attempt to explain the existence of supermassive black holes, which astronomers have detected at the hearts of most large galaxies, and whose origin is still unknown.

It is calculated that this could account for objects up to 104M in size. One problem here is that possible quasistars have never actually been observed, and observation is expected to be difficult at best, since they would have formed so soon after the big bang, and consequently now be very distant (like more than 1010 light-years).

Another problem is that SBHs can be far larger than this. The largest SBH known so far is about 1.8×1010M. (See here, here.) SBHs that formed in quasistars would still need to accrete something like 106 times their initial mass to grow into the largest known SBH, let alone any larger ones that might be out there. (For comparison, the mass of the Milky Way, a medium-size galaxy, is about 1012M. See here, here.)

So a quasistar origin for SBHs doesn't seem too likely either, but if you want to pursue it further, the relevant paper is here.

We haven't yet even mentioned one obvious possibility: primordial black holes (PBHs). That is, black holes that formed directly very soon after the big bang itself. Exactly at what time this might have been depends very much on how PBHs might have formed, and there's plenty of uncertainty about that.

In fact, we have no evidence yet that PBHs exist at all. Since we don't know exactly how they might have formed, we don't know how big they might be, which strongly affects what we should look for. If PBHs were small enough, they should eventually "evaporate", as Stephen Hawking suggested, by the process of Hawking radiation.

There's much debate as to what happens when a sufficiently small PBH evaporates completely (see here, here, here, here, here). But such an event is generally supposed to include emission of gamma-rays, and these might be detected by the newly-launched GLAST mission. (See here.)

The most likely scenario for the formation of a PBH is as a result of gravitational collapse of overdense regions existing because of density fluctuations dating from the earliest instants after the big bang. Since fluctuations are the key, formation of PBHs is governed by probability, and the larger the PBH, the lower its probability, and hence the fewer that form altogether. While there's no obvious upper limit to the size of a PBH, the formation of one around 104M would be very improbable. Too improbable? No one knows.

Other mechanisms hypothesized for formation of PBHs tend to feature rather exotic things like topological defects (cosmic strings or domain walls) or certain kinds of phase transitions.

But this is all so speculative that it's not much help for evaluating whether PBHs might make a good source of SBHs. A great reference for stuff about PBHs is this: Primordial Black Holes: Do They Exist and Are They Useful?.

The bottom line is that closer study of SBHs, which almost certainly do exist, should eventually lead us to some concrete evidence for things that are presently much more speculative.

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