GAM 2012 Blog

April 16

By Ian O'Neill

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Supermassive black holes are thought to live in the cores of most galaxies, including our own. But there's a problem; they shouldn't exist at all. Supermassive black holes are simply too massive to be explained by current theories of black hole formation.

We're not talking about stellar-mass black holes that form after the collapse of a massive star, however. These stellar-mass black holes may still be massive, but they are nothing when compared to the mass of a supermassive black hole. What's more, their formation processes may be completely different.

The most massive black holes -- recently discovered lurking in the cores of the galaxies NGC 3842 and NGC 4889 -- weigh-in at 9 billion times the mass of our sun. These two behemoths are 2,000 times the mass of the 'puny' supermassive black hole living in the core of the Milky Way -- known as Sagittarius A*.

So, they're big. Really, really big. But how did they get so big?

One theory of supermassive black hole formation says that shortly after the Big Bang, primordial black holes were created from the soup of energy that filled the young universe. Through their gravitational dominance, these singularities gathered matter to form the first galaxies. (Whether the galaxies or the black holes came first is a matter for debate -- a cosmic "chicken and egg" conundrum.)

These first black holes were voracious eaters, consuming anything that came close. Orbiting the black holes, accretion disks of swirling hot matter formed, generating powerful jets of radiation. These powerful entities are known as quasars and supermassive black holes are thought to be the engines behind them.

So the earliest black holes pulled in matter, created accretion disks, which in-turn supplied material to the feeding black holes (generating quasars along the way). Today we see the aftermath of the black hole feeding frenzy -- there are no more quasars in today's universe, but the overfed supermassive black holes remain, hidden deep inside their host galaxies.

But how did they get so big? Sure, they ate a lot of matter, but the "accretion disk model" cannot account for supermassive black holes that possess masses of millions to billions times the mass of our sun. The accretion disks that supplied the young black holes with matter couldn't have supplied mass fast enough, even if they did so from the Big Bang to now -- some 13.75 billion years later.

But say if the universe's first black holes were even greedier than this theory predicts? What if these black holes feasted on more than just one accretion disk "dinner"?

Researchers from the University of Leicester (UK) and Monash University(Australia) recently took a look at the problem and ran a computer simulation of a black hole with two accretion disks.

Although this situation may seem unlikely in the centers of today's galaxies, galaxies in the young universe may have had very chaotic flows of matter in their cores. This could have created a situation where two accretion disks formed, one close to the black hole and one further away.

The researchers modeled the flow of material around the two accretion disks at varying angles to one another. As the simulation progressed, matter from the two accretion disks spread out and began to interact. Like two lanes of oncoming traffic crossing one another, the accretion disk gas slammed into one another. During the accretion disk collision, the gas experienced a rapid slow-down, allowing the black hole's gravity to suck it in very quickly.

As the researchers point out, this mechanism would have fed the black hole very fast, perhaps accounting for the black hole growth spurts in the early universe.

But this theory is just the most recent in a string of possible supermassive black hole creation mechanisms.

Another theory calls on the catastrophic collapse of a vast cloud of gas shortly after the Big Bang. Gravity pulled all the matter into a point, kick starting a "relativistic star" which promptly collapsed, instantly creating a supermassive black hole. No accretion disk required.

But a personal favorite "black hole spawning mechanism" is a little more exotic. Whatif we live in a "cyclical universe" where a Big Bang is followed by a "Big Crunch,"followed by another Big Bang? Between each consecutive Big Bang and Big Crunch, a new universe forms.

Though each Big Crunch should destroy all information (that means all stars, galaxies, gas, dust, people, aliens, subatomic particles and time), a couple of theoretical physicists reckon there may be a condition that allows black holes from the previous universe to survive the Big Crunch and re-emerge from the Big Bang.

Those primordial black holes from the previous universe could then go on to help form the supermassive black holes in our universe.

These theories may sound a little crazy, but when considering the exotic and extreme nature of black holes, anything seems possible.


Dr. Ian O'Neill, a Space Producer and science writer for Discovery News. Before working for Discovery News, he was a solar physics researcher at the University of Wales, Aberystwyth, UK where O'Neill carried out numerical simulations of coronal loop heating mechanisms.