As long as stellar black holes were the only kind of black holes for which science could find even indirect evidence, the universe seemed a far less scary place than it does today. After all, stellar black holes did not appear to pose any major short- or long-term danger to the universe as a whole or to the existence of life within it. True, when a giant star collapses to form a black hole, any living things inhabiting the planets or moons of that solar system will first be fried and then frozen. No life of any kind will be able to survive for very long. However, these lethal effects would remain localized to that system. This is because the distances separating most stars are immense—about four to seven light-years, or 24 trillion to 42 trillion miles. The gravitational effects and radiation of even the most massive stellar black hole could be felt over only a small fraction of such distances. Therefore, this kind of black hole would pose no credible threat to neighboring stars, their planets, and any life forms they might harbor.
When one considers the larger scheme of things, however, such safety zones become illusory and ultimately useless. Scientists now know that the danger posed by black holes increases significantly in areas of space where many large stars lie very close together (on the order of only a few light-weeks, light-days, or even light-hours apart). In such an environment, several neighboring giant stars can collapse into black holes over time. As these superdense objects drift and meander, some will merge, producing more massive bodies with stronger gravities.
Finally, one very massive black hole will dominate the scene. It will continue to draw in clouds of gas, stars, planets, smaller black holes, and other materials floating in its cluttered cosmic neighborhood; and over the course of millions and billions of years, it will grow still more massive. Indeed, it will become a sort of cosmic monster with an insatiable appetite. Only recently have astronomers come to the unsettling realization that such giant, or supermassive, black holes not only exist, they may well play a major role in the ongoing evolution and ultimate fate of the universe and everything in it.
First, it is important to determine just how massive a black hole must be to qualify as a giant. The standard stellar black holes that scientists believe exist in some binary star systems are mostly in the range of about eight to twenty, and occasionally up to about fifty, solar masses. By earthly and human standards, these are very massive objects to be sure. But in the last few years, evidence has been found for the existence of much more substantial black holes.
These larger black holes fall into two broad categories—intermediate, or midsized, holes, and supermassive, or giant, holes. Since the early 1970s, astronomers had speculated about the possibility of midsized black holes, which they theorized would contain from a few hundred to several tens of thousands of solar masses. It was clear that such objects would most likely form in regions of densely packed stars and gas clouds; after all, the holes would have to have a lot of matter to feed on to grow so large. One such crowded region is a globular cluster, of which the Milky Way contains several hundred. Isaac Asimov describes globular clusters as stellar groups in which
some tens of thousands or even hundreds of thousands of stars are clustered together in a well-packed sphere. Here in our own neighborhood of the universe, stars are separated by an average distance of about 5 light-years. At the center of a globular cluster, they may be separated by an average distance of½ light-year. A given volume of space in a globular cluster might include 1,000 times as many stars as that same volume in our own neighborhood. 36
Astronomers examined several globular clusters in the 1970s and found that they did emit high doses of X rays, as the likely black hole candidate Cyg X-1 did. However, no concrete evidence for midsized black holes in these star groups surfaced until 2002. Late that year, a team led by Roeland Van Der Marel at the Space Telescope Institute found two midsized black holes. One, possessing about four thousand solar masses, is in M15, a globular cluster in the Milky Way. The other resides in G1, a globular cluster in the neighboring Andromeda galaxy, and has roughly twenty thousand solar masses. In an interview following the
discovery, Luis Ho, one of the team members, exclaimed: "It's very exciting to finally find compelling evidence that nature knows how to make these strange beasts." 37
Early in 2003, another research team, this one led by Jon Miller at the Harvard-Smithsonian Center for Astrophysics, discovered two more midsized black holes. Situated in a spiral galaxy designated NGC 1313, lying at a distance of 10 million light-years from Earth, they each contain several hundred solar masses.
Proof for the existence of members of the other broad category of larger-than-stellar black holes—the supermassive ones—has also begun to emerge in recent years. These giants always appear to inhabit the centers, or cores, of galaxies, so it has become common to refer to them as "galactic black holes." The reasons that it took so long to verify their existence are fairly simple. First, the cores of galaxies are extremely far away; even the center of our own Milky Way lies at the considerable distance of about twenty-six thousand light-years. Second, the galactic cores are also generally blocked from easy viewing by dense layers of gases, dust, and other cosmic debris.
In spite of these obstacles, astronomers persevered. Over the years, new and larger telescopes, along with more sophisticated detection equipment, revealed more and more information about the Milky Way's core. There, it became clear, many huge stars lie very close together. Some of them are as large as 120 or more times the size of the Sun, and many of them float among the expanding and often overlapping gaseous remnants of many prior supernovas. "Like silken drapes blown in the wind," writes noted science writer Robert Zimmerman,
the erupting waves of gas from scores of supernovas sweep through an inner region approximately 350 light-years across, filling space like froth and geysers. Here supergiant stars—many times more massive than the sun and rare elsewhere in the galaxy—number in the hundreds.
And within those 350 light-years are three of the galaxy's densest and most massive star clusters, surrounded by millions of additional stars. So packed is this core that if the solar system were located there, a handful of stars [in addition to the sun] would float among the planets. 38
More ominously, astronomers also discovered something dark, monstrous, and frightening in the crowded galactic core. Almost all stars and other matter there are sweeping very rapidly around an extremely massive object. The first hints that something unusual lay in the center of our galaxy came in the 1950s. Radio telescopes, huge bowl-shaped antennas that gather and record radio waves from outer space, showed that a powerful source of these waves lies in the galactic core. These early images were crude and inconclusive. And thanks to the masses of gases and dust obscuring the core, visual images showed nothing.
It took the development of more advanced radio telescopes in ensuing decades to begin to unravel the mystery of the Milky Way's core. In the mid-1970s, radio images revealed three distinct nonstellar objects in the core. Two, which looked like hazy, cloudlike patches, were dubbed Sagittarius East and Sagittarius West (after Sagittarius, the archer, the constellation in which the core is situated in Earth's night sky). The third object, a pointlike, very powerful radio-wave source lying in the galaxy's very center, received the name Sagittarius A* (pronounced A-star).
For a long time, astronomers were puzzled by Sagittarius A*. It is clearly too energetic and hot to be an ordinary star. Indeed, studies reveal that it is hotter than any other object in the Milky Way. In the 1980s and early 1990s, more sophisticated images of the core were taken using infrared telescopes, which can see through most of the layers of gases and dust. These showed huge filaments of gases swirling around Sagittarius A*. Even more detail was revealed in 1997 by German astronomers Andrea Eckart and Reinhard Genzel, who announced that they had mapped the frenzied motions of the seventy stars closest to the core's central object. According to Zimmerman:
They found that many of the stars were streaking across the sky at tremendous speeds, and that the closer to Sagittarius A* the stars were, the faster they moved. Stars at distances of more than half a light-year traveled at less than 100 miles per second. Closer in, the speeds increased to more than 500 miles per second, and the closest star to Sagittarius A*, dubbed S1, also had the fastest velocity, estimated at almost 900 miles per second. Furthermore, Eckart and Genzel found that the 100 nearest stars seemed to be moving in a generally clockwise direction, opposite to the rotation of the rest of the galaxy. This suggests that they were part of a large torus [doughnut-shaped structure] of stars orbiting a single invisible point. At the center of this whirling collection of stars was the radio source Sagittarius A*, which unlike any other star in the sky has no apparent proper [visible] motion. 39
Members of the scientific community are now nearly unanimous in their belief that Sagittarius A* is a supermassive black hole. As for just how massive it is, numerous estimates appeared in the 1990s, the most common being 2.6 million solar masses. In October 2002, however, the results of a study by Rainer Schödel, of Germany's Max Planck Institute for Extraterrestrial Physics, showed a larger mass for the giant black hole—3.7 million times that of the Sun.
To measure the mass of Sagittarius A*, the scientists observed the speeds at which matter is orbiting it and determined how massive the central object would have to be to produce these movements. "In the same way that Master Yoda and his disciples [in the Star Wars series] saw through an attempt to wipe a planet from the Jedi archives [by detecting the telltale signs of the planet's gravity]," William Keel quips, "astronomers can discern the existence of this object." 40
Having already drawn in and consumed more than 3 million stars, Sagittarius A* is certainly far more massive than stellar and midsized black holes (not to mention mini–black holes). Yet mounting evidence suggests that this giant's growth cycle is far from finished. As Keel points out, "Even at a mass of 3 million suns, this black hole proves quite modest by the standards of other galaxies." 41
Indeed, astronomers have intensified their studies of galactic cores and continue to discover truly enormous supermassive black holes in many distant galaxies. The nearby Andromeda galaxy, for instance, harbors a 30-million-solar-mass black hole in its core. A galaxy named NGC 4486B has a central black hole measuring about 500 million solar masses, and the core of a galaxy designated NGC 4261 features a stupendous object of some 1.2 billion solar masses. This suggests that there may be no physical limit to the size of a supermassive black hole.
Also, the fact that these giants seem to be integral features of galaxies and that they are eating their way through the galactic cores is surely significant. It now appears certain that supermassive galactic black holes must strongly affect the structure, evolution, and ultimate fate of galaxies. Says science writer Steve Nadis, "New evidence strongly suggests a much more intimate connection than astronomers ever thought possible between galaxies and the supermassive black holes that dominate their cores." 42
But the nature of this grand cosmic connection is for the moment problematic for scientists. Central to
In this excerpt from an article in the October 2001 issue of Astronomy magazine, science writer Robert Zimmerman describes the possible origins of Sagittarius East. It is now believed to be the remnants of an unusual supernova created by the immense gravitational effects of the black hole Sagittarius A*.
Sagittarius East is now believed to be a large bubble, possibly one of the largest supernova remnants known, that formed fewer than 100,000 years ago and maybe as recently as 10,000 years ago. Although it engulfs Sagittarius West [a cloudlike region nearby] and Sagittarius A*, it lies mostly behind both. Astronomers think that the energy required to punch out this shell of gas in such a dense region would have to be as much as 50 times greater than the most powerful supernova explosion. What could have produced this much energy still puzzles astronomers. Some theorize that Sagittarius East was created when a star approached within 50 million miles of the central black hole and was torn apart by the strong gravity.
the present debate on the topic is a variation of the old "chicken or the egg" question, in this case, Which came first, galaxies or giant black holes? Some astronomers think that galaxies and their central black holes form from the "outside in." In other words, swirling masses of gases and dust condense to form spinning galaxies of stars, and over time some of the giant stars in the core collapse into black holes, which in turn merge to become one really massive black hole.
In contrast, others argue for the "inside out" hypothesis. In this version, as Asimov says, "The black hole may have come first and then served as a 'seed,' gathering stars about itself as super-accretion disks that become clusters and galaxies." 43 As for where these initial seed black holes came from, no one knows. They may have been created somehow in the Big Bang along with mini–black holes.
Whichever came first—galaxies or large black holes—the two seem to grow and develop together in step, so to speak. Late in 2000, astronomer Michael Merrifield and his colleagues at the University of Nottingham, in England, found a telling correlation between the age of galaxies and the masses of the supermassive black holes at their cores. Simply put, the older the galaxy, the more massive its central hole. "We're measuring the time scale over which black holes grow," Merrifield explains, "and it appears to be comparable to the age of the host galaxies. So they really are developing together." 44
This new finding raises an important question. If giant black holes continue to grow at the expense of their host galaxies, why do astronomers not see some galaxies in their death throes, almost totally absorbed by the cosmic monsters within? The most obvious answer is that the universe is not yet old enough. Indeed, present-day humans probably exist at a time in the life cycle of the universe when most galactic black holes are still relative youngsters possessing
from a few million to a few billion solar masses. According to this view, if humans could somehow travel far ahead in time, they would see many galactic black holes with tens and hundreds of billions of solar masses devouring the last remains of their parent galaxies.
If this scenario is correct, what does the awesome process of giant black holes consuming entire galaxies mean for the future of the universe and for humans and any other intelligent beings that may exist in the vast reaches of space? First, the process will take a long time, perhaps thirty, fifty, or even hundreds of billions of years or more. So most galaxies and intelligent civilizations are not in any immediate danger. Eventually, though, Sagittarius A* will likely swallow up all the normal matter surrounding it, including the Sun and its planets. After its humongous meal, this bloated black hole may then float through space until it encounters other giant holes that have already devoured their own former galaxies. And relentless gravity will inevitably cause these phenomenally massive objects to move ever closer to one another and merge in an embrace of self-annihilation.
In this excerpt from his book Black Holes , scientist John Taylor points out that the concept that the present universe developed from a black hole containing the remnants of a prior universe is difficult for humans to comprehend because it does not define the beginning of the process.
The hardest question of all to answer is where did our universe come from? If we reply that it came from somewhere else, brought to its present state by the laws of physics, we need only add that somewhere else, filled with whatever was in it before it formed us and our material surroundings, to our present world. We then ask again, where did that new totality come from? Any definite answer to our first question is the wrong one, since it would lead us to an infinite chain of similar questions…. But we could try to find from what our world, as we know it today, arose. We might do so by conjecturing that our present universe sprang into being from the final stage of collapse in a spinning black hole in a different universe, bubbling out of the black hole's center…. That might or might not fit with experimental facts if we looked for them carefully enough, but it would still beg the question, since we would then have to explain where the previous universe came from.
Carrying this possible sequence of future events even further, after unknown numbers of eons all of the galactic black holes—together containing all the former matter in the universe—might merge into one huge monster of a black hole. The fate of this bizarre cosmic creature can only be guessed. But some astronomers postulate that this ultimate end of the present universe will somehow give rise to the birth of a new one. Perhaps there will be a new Big Bang, in which immense quantities of matter rush outward from a central point and slowly coalesce into stars, galaxies, planets, and so forth. Logically, in this new universe new black holes will form. And in time, these will slowly but steadily begin a new cycle of growth and merger. Astronomers call this theoretical situation in which all matter repeatedly contracts and rebounds the "oscillating universe."
It is only natural to wonder about what will happen to humanity in the ultimate cosmic crunch, when all matter in the present universe is incorporated into one or more titanic black holes. However, it is highly unlikely that human beings, at least in their present form, will exist billions of years from now. If our species is not long since extinct by that time, it will have undergone profound physical and mental changes, enough to be totally unrecognizable to people alive today. Still, it is at least possible that our descendants, in whatever form, will be around to witness the climactic ending of what will become essentially an all-black-hole universe. Could they survive the final crunch? John Taylor gives this thought-provoking answer:
The fate of the physical universe is catastrophic…. It is either to be crushed into its fundamental constituents, as far as possible, to make a universal black hole, or it is to be slowly absorbed by local black holes, again to be crushed out of existence as we know it…. At such an end, [we would surely face physical death, so] we could only appeal to our souls, if they exist, to preserve us…. It could only be if the universe bounces back again after its collapse that these separated souls have any chance of returning…. There is very little evidence of such a bounce being able to occur, but if it does, only then can one expect any form of immortality. 45