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Frontiers Community Lecture: Black Holes and the Fate of the Universe

NGC 6240

In his talk about black holes, Günther Hasinger described the galaxy NGC 6240, which consists of two smaller galaxies in the process of merging. Each galaxy is home to its own supermassive black hole, which will eventually merge and become one. It is the first such double black hole ever discovered.

A black hole is an object that is so dense that even light cannot escape from it, in accordance with Einstein's general theory of relativity. If the Sun were squeezed down to the size of Honolulu, it would be a black hole, Hasinger said. The first, relatively small, black holes came into being in the early Universe when the first generation of massive stars in the middle of the early Universe's primal collapsing hydrogen cloud used up their fuel and collapsed at the end of their lives. As the hydrogen cloud continued to collapse, the black holes grew larger and larger until the cloud became galaxies with supermassive black holes at the center. The most distant objects in the Universe, and therefore the oldest, that astronomers have seen are likely gamma-ray bursts resulting from the creation of these early black holes.

There is a black hole with a mass of 300 million times that of our Sun in center of the Milky Way. Astronomers see flares in the vicinity of the galactic center almost daily. These flares happen because the black hole is "snacking" on comets and asteroids. Angular momentum—the momentum a body has because of its rotation—prevents stars orbiting the black hole from being easily sucked into it. But every hundred thousand years or so, a star is captured by the black hole. This means that the black hole "eats" about 10 percent of the star. The rest of the star's matter is belched back out into the galaxy, where it may become part of new stars. There are galaxies that are much more active than our Milky Way. Their black holes consume stars at a much higher rate.

The other way black holes grow is through galaxy mergers. When galaxies merge, they produce binary black holes. When the binary black holes merge, they produce the very bright and powerful quasars. These quasars are powerful enough to blow out all the gas from their galaxies. What remains are elliptical galaxies with no spiral arms and no gas with which to form new stars. They are "red and dead," as Hasinger put it. But then, after a long time, these galaxies start to grow a new disk, get spiral arms again, and the cycle starts over. This is the cycle of black hole evolution.

Astronomers who have studied black holes with a succession of X-ray satellites—ROSAT, XMM Newton, and Chandra—have concluded that the most massive and brightest black holes lived when the Universe was still rather young, but that most black holes have grown relatively recently.

As dark energy accelerates the expansion of the Universe, the largest black holes will continue to grow to hundreds of billions of solar masses and live as long as 10100 years. They will outlive all other forms of matter in the Universe until they too return to the "nothingness" with which the Universe began.

If you missed this talk, it will eventually be on our website: www.ifa.hawaii.edu/videos/