A black hole walks into a bar and says, "Hey, where'd everybody go?"
That old joke captures the essence of what most people believe about black holes -- places in the universe so dense that they suck in all matter and light, letting nothing escape.
In the past 10 years, though, scientists have turned that concept upside down, and Carnegie Mellon University's Tiziana Di Matteo has helped rewrite the book on the subject.
Position: Astrophysics professor, Carnegie Mellon University.
Education: Bachelor's in astrophysics, University College London, 1995; Ph.D., astrophysics, University of Cambridge, 1998.
Previous positions: Professor of astrophysics and research associate, Max Planck Institute, Munich, Germany, 2003-05; postdoctoral fellow, Harvard University, 1998-2001
Professional awards: Carnegie Science Award of Excellence, emerging female scientist, 2008; Chandra Fellowship, Harvard University, 1998-2001; Royal Astronomical Society Michael Penston Prize for best doctoral thesis in the UK, 1999.
Publications: More than 70 papers in refereed scientific journals.
Using complex computer simulations, she and others have shown that rather than being exotic, black holes are abundant in the universe. They number in the millions, and in the center of each galaxy are supermassive black holes that are millions or billions of times heavier than the sun.
These supermassive black holes also throw out energy at the same time they pull matter in, said Dr. Di Matteo, an astrophysics professor at Carnegie Mellon, and are responsible for the intense bursts of light known as quasars.
Each galaxy rotates, she said, and the black hole at its hub pulls in massive clouds of gas in the process. "As this material comes in, it swirls in like water when you open a drain in the sink, and as it does so, the center is moving faster than the stuff rotating on the sides," creating friction that heats up the gas and emits intense energy.
That energy doesn't just put on a light show but heats up the entire galaxy enough to keep new stars from forming.
The computer simulations Dr. Di Matteo and others carried out have been critical for understanding the growth of the massive black holes that sit in the center of galaxies.
The researchers started by plugging in the math that represented conditions in the universe after the Big Bang. The trick to that, she said, is that "you want to start off as simply as possible -- you don't want to put any preconception into it. You want to create the universe the way it looks from as little input as you can put in."
When they watched that simulated universe evolve to the point where galaxies began to form, Dr. Di Matteo and her fellow cosmologists discovered that there should have been many more stars in each galaxy than there actually were.
The only explanation for the difference was an outpouring of energy that would have kept new stars from forming, and the only object capable of producing that kind of energy was a huge black hole that could convert up to 40 percent of the gas it was vacuuming up into heat and light, she said.
Many of the universe's largest black holes formed when galaxies slammed into each other, and their black holes then merged, she said. That eventually will happen with our own galaxy, the Milky Way, and its neighbor Andromeda, but unless you are planning to live for billions of years, it's nothing you need to worry about in your lifetime.
The peak period of galaxy formation was about 8 1/2 billion years ago, she said. The universe is now estimated to be about 13 1/2 billion years old, and "now that we've used up most of the gas we once had, we're stuck with a universe that is much quieter."
And thanks to an anti-gravity force known as dark energy, the galaxies are moving away from each other at an increasing rate, so the chance of new galaxies forming is more remote.
A native of Bologna, Italy, Dr. Di Matteo is one of the few women in her field, but that is rapidly changing, she said. "At the most recent conference I attended," she said, "I was stunned by the number of young female Ph.D. students, and it was obvious it was approaching a 50-50" gender split.
Her unusual position has never felt like a hindrance, she said, and she went through school being largely blind to the issue because in Mediterranean countries, the number of women studying the "hard sciences" is nearly equal to the number of men.
She is typical of cosmologists in one other way, though -- she has moved around a lot. She attended an international high school in Vancouver, British Columbia, got her Ph.D. at Cambridge University in the United Kingdom, did postdoctoral work at Harvard University, started her research at the Max Planck Institute in Munich, and then joined her husband, Rupert Croft, at Carnegie Mellon's physics department.
She is now a leading light at the school's new McWilliams Center for Cosmology, established with a grant from alumnus Bruce McWilliams, CEO of Tessera Technologies, an electronics miniaturization firm.
As she sits here on a planet orbiting one of hundreds of billions of stars in the Milky Way, she knows that the dark energy force that is pushing the galaxies apart eventually may cause the universe to disintegrate in a "big rip, so everything will be ripped apart."
But she is more focused on the past, and how the universe began. The cosmological future, she said, is "something to talk and think about, but it's not something you want to spend too much time on."
Correction/Clarification: (Published Sept. 1, 2009)
There are hundreds of billions of stars in the Milky Way. This story on black holes as originally published Aug. 31, 2009 gave an incorrect figure.
Mark Roth can be reached at firstname.lastname@example.org or at 412-263-1130. First Published August 31, 2009 4:00 AM