November 7, 2013
Two Black Holes Form, Fuse Together After Star Collapses
[ Watch the video: Supermassive Star Collapses, Forms Two Black Holes ]
April Flowers for redOrbit.com - Your Universe Online
Black holes are massive objects in space that have gravitational forces so strong that not even light can escape them. These objects, swirling at the centers of galaxies, also come in a variety of sizes.
On the small end of the spectrum are stellar-mass black holes, which are formed during the deaths of stars. On the largest end are supermassive black holes. These behemoths contain up to one billion times the mass of our sun. Small black holes, over billions of years, can slowly grow into the supermassive variety by acquiring mass from their surroundings and by merging with other black holes. This is a slow process, however, that doesn't explain the problem of supermassive black holes existing in the early universe, as those black holes would have formed less than one billion years after the Big Bang.
A new study from the California Institute of Technology (CalTech) provides findings that may assist in testing a model that solves this problem. The findings are published in a recent issue of Physical Review Letters.
Some theoretical models of supermassive black hole growth suggest the presence of "seed" black holes. These seed black holes, which result from the deaths of very early stars, gain mass and increase in size by picking up the materials around them -- a process called accretion -- or by merging with other black holes.
"But in these previous models, there was simply not enough time for any black hole to reach a supermassive scale so soon after the birth of the universe," says Christian Reisswig, NASA Einstein Postdoctoral Fellow in Astrophysics at Caltech and the lead author of the study. "The growth of black holes to supermassive scales in the young universe seems only possible if the 'seed' mass of the collapsing object was already sufficiently large," he says.
Reisswig collaborated with Christian Ott, assistant professor of theoretical astrophysics, to investigate the origins of young supermassive black holes. They, and their team, turned to a model involving supermassive stars. Scientists believe these giant, rather exotic stars only existed for a short time in the early universe. Supermassive stars, unlike ordinary stars, are stabilized against gravity, in the most part, by their own photon radiation. Photon radiation, which is the outward flux of photons generated due to the star's very high interior temperatures, pushes gas outward from the star in opposition to the gravitational force that pulls gas back in.
When the two forces are equal, a balance called hydrostatic equilibrium is achieved.
A supermassive star cools slowly during its lifetime due to energy loss through the emission of photon radiation. During this cooling, the star becomes more compact, and its central density slowly increases -- a process that lasts for a couple million years. At some point, the star reaches sufficient compactness for gravitational instability to set in and for the star to start collapsing gravitationally.
Previous research has predicted that when supermassive stars collapse, they maintain a spherical shape. This shape possibly becomes flattened due to rapid rotation into a shape called axisymmetric configuration. The current study incorporated the fact that very rapidly spinning stars are prone to tiny perturbations. The team predicted that these disturbances could cause the stars to deviate into non-axisymmetric shapes during their collapse. The tiny perturbations would increase rapidly, causing the gas inside the collapsing star to ultimately clump and form high density fragments.
The fragments would become increasingly dense as they picked up matter orbiting the center of the star during the collapse. This would also increase the temperature of the fragments.
According to Reisswig, "an interesting effect kicks in."
When the fragments reach sufficiently high temperatures, there would be enough energy to match up electrons and their antiparticles, or positrons, into what are known as electron-positron pairs. The creation of these pairs would cause a loss of pressure, further accelerating the collapse. The two orbiting fragments would ultimately become so dense, as a result, that a black hole could form at each clump. These black holes might then spiral around one another before merging to become one large black hole.
"This is a new finding," Reisswig says. "Nobody has ever predicted that a single collapsing star could produce a pair of black holes that then merge."
Using supercomputers, the team simulated a supermassive star on the verge of collapse. They created a video to visualize this collapse, combining millions of points representing numerical data about density, gravitational fields, and other properties of the gases that make up the collapsing stars.
This study is purely theoretical, involving only computer simulations and not empirical data. In practice, however, Reisswig says the formation and merger of pairs of black holes can give rise to tremendously powerful gravitational radiation -- ripples in the fabric of space and time, traveling at the speed of light -- that is likely to be visible at the edge of our universe.
Albert Einstein first predicted this gravitational radiation in his general theory of relativity. Today, ground-based observatories such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) are searching for signs of the radiation. Reisswig says that future space-borne gravitational-wave observatories will be necessary to detect the types of gravitational waves that would confirm these recent findings.
The findings of this study have important implications for cosmology, according to Ott. "The emitted gravitational-wave signal and its potential detection will inform researchers about the formation process of the first supermassive black holes in the still very young universe, and may settle some—and raise new—important questions on the history of our universe," he says.