Trade.Information

Topic Name: Did the big bang spawn trillions of black holes?

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Massimo Ricotti, Rachel Bean

Location: University of Maryland College Park, MD 20742-2421, United States

Details

Were vast numbers of black holes spawned during our universe's earliest moments? It is an intriguing idea, made possible by the extreme densities associated with the big bang.
So far, there is no hard evidence that such primordial black holes (PBHs) ever existed, but new observations just around the corner could change that.

Detecting them would be a tremendous boon, because they could be used to probe the very early universe a mere fraction of a second after it all began, when the conditions were so extreme that our best physics theories have trouble describing them. Primordial black holes might also make up part of the mysterious invisible substance called dark matter that seems to make up most of the matter in the universe.

There are a variety of ways that PBHs might form in the inferno of the early universe. For example, concentrations of energy associated with exotic energy fields could collapse under their own gravity – according to Einstein's relativity, energy exerts gravity just as matter does – to make black holes. One such energy field is thought to be responsible for the rapid expansion of the early universe, a phenomenon called inflation.

A wide variety of masses for PBHs are possible, depending on the formation scenario. The least massive ones, with less than about the mass of a comet, or 1 trillion kilograms, would quickly evaporate through a quantum process known as Hawking radiation.

Detonating black holes
There have been unconfirmed reports of radiation from slightly more massive PBHs, the last traces of which would just be evaporating now (see Black holes detonating all over our galaxy).
More massive PBHs, which may be born with up to 100,000 times the mass of the Sun, could survive to put an imprint on the cosmic microwave background (CMB), radiation emitted by warm matter roughly 400,000 years after the big bang.

That's because the black holes emit X-rays as they swallow matter from their surroundings, and these X-rays can escape the vicinity of the black holes to break apart, or ionise, hydrogen atoms. This would subtly affect how matter distributes itself into regions of high and low density - a distribution reflected in the cosmic microwave background radiation.
This effect might explain a puzzling discrepancy between results of the Wilkinson Microwave Anisotropy Probe (WMAP), which measures the CMB, and studies of how galaxies are clustered.

The two disagree on a parameter called sigma8, which describes how matter clumped together in the early universe. But according to a recent study led by Massimo Ricotti of the University of Maryland in College Park, US, the two measurements agree if PBHs are included in the models.

But Ricotti himself says it is too soon to claim there is evidence for primordial black holes. It is still possible that refining the measurements will bring them into agreement without invoking these exotic objects, he says.

First stars
The study also suggests that the ionising effect of PBHs would have helped spark the formation of the first stars in the universe. The presence of free electrons helps pairs of hydrogen atoms to join together to form molecular hydrogen. "You form a lot of molecular hydrogen – about 10 to 100 times more than you would form if you didn't have primordial black holes," Ricotti told New Scientist.

Molecular hydrogen helps to cool gas clouds by emitting radiation, allowing the clouds to contract enough to condense into stars. Ricotti says the James Webb Space Telescope, scheduled to launch in 2013, just may be able to detect this enhancement of star formation.

Perhaps most intriguingly, if primordial black holes survive in great enough numbers today, then clouds of them could account for some or even all of the mysterious dark matter that seems to make up most of the matter in the universe.
The main problem with this possibility is that it is not clear whether the conditions needed to form PBHs in large numbers ever occurred in our universe.

In the formation scenario involving the inflation field, for example, the number of PBHs formed depends on unknowns such as the size of fluctuations in the inflation field. "In some inflationary models, you can form a lot of PBHs; in others you form very few of them," Ricotti says. "It's not obvious if they form in sufficient numbers to be interesting."

Window to the past
It is possible that unusually large amounts of ionisation in the early universe - possibly due to the X-rays emitted by PBHs - could be detected by Europe's Planck satellite, scheduled to launch in mid-2008, says WMAP team member Rachel Bean of Cornell University in Ithaca, New York, US. "It's conceivable that such effects could be measured by Planck," she told New Scientist.

If convincing evidence of primordial black holes ever emerges, it would give scientists an extremely important window into the universe at very early times. "Proving that even very few primordial black holes exist would teach us so much about the early universe," Ricotti says. "We don't know much about those times."

The mass of the black holes would reveal the time at which they formed, since the different scenarios for their formation occur at different times and give different masses. If they formed at the end of inflation, then their existence would reveal important information about the murky physics of this period of rapid expansion.
"You could rule out models of inflation that don't produce these black holes," says physicist James Chisholm of Southern Utah University. "Someone would probably get a Nobel prize."
 

About Researchers:

Name: Massimo Ricotti
Title: Assistant Professor
Room: CSS 0213
Phone: (301) 405-5097
E-mail: ricotti
Department of Astronomy, University of Maryland College
Park, MD 20742-2421
Phone: (301) 405-3001
FAX: (301) 314-9067

Massimo Ricotti's main interest is in theoretical cosmology. Currently I am studying the first epochs of galaxy and star formation in the Universe. A part of my workis numerical and involves the use of supercomputers. Using large simulations I try to understand which physical processes and feedbacks are important for the formation of the first galaxies and how it proceeds.
The formation of the first galaxies is tightly linked to the evolution of the intergalactic medium (IGM) from which they form. For this reason my work includes studies of the thermal and reionization history of the IGM and of the Lyman-alpha forest.
I am also interested in the physics of the interstellar medium, focusing on galaxies with nearly primordial composition and damped Lyman-alpha systems. So far my investigations have lead me to use a variety of observations from the nearest galaxies in the Local Group to the most distant objects in the Universe.

Visit Massimo Ricotti's Personal Home Page
ADS Listing for Massimo Ricotti
Astro-PH Listing for Massimo Ricotti

Rachel Bean

612 Space Sciences Bldg Department of Astronomy
Cornell University Ithaca, NY 14853 607 254 4920.

Related Online Links:

http://www.murky.org/blg/category/geeky/science/physics/
http://en.wikipedia.org/wiki/Primordial_black_hole
http://www.rdrop.com/users/green/school/primordi.htm
http://www.physicsforums.com/showthread.php?t=154391&page=3