Black Holes and Astrobiology

Overview

For centuries we have gazed at the skies and wondered what they might contain. While some speculations have been filled with the fantasy of science fiction, such as little green aliens and portals to other worlds, science has moved beyond conjecture to expand our understanding of the reality of the universe. Significant lore surrounds black holes, but it is now known that these entities, once a matter of mere speculation, not only exist but are fundamental components of all galaxies. The newly emerging field of astrobiology explores the possibility of life beyond Earth and the conditions, on an astronomical level, that make life possible. To explain these recent advances, Caleb Scharf presented a seminar titled Black Holes and Astrobiology at the Academy on October 23, 2012. Scharf is the director of Columbia University's Astrobiology Center and the author of Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos. He described the nature of black holes, which are much more than massive voids that simply suck all matter and light into their depths; black holes shoot energy and clouds of matter out into space, altering the landscape of galaxies. In closing, Scharf speculated about the possible relationship between black holes and life in the universe.

The term "black hole" was not coined until late in the twentieth century, but the concept originated much earlier. In 1783, Reverend John Mitchell, a scientist in northern England, became interested in how gravity affects light. In the classical, Newtonian view, gravity was thought to slow light down, leading Mitchell to posit that objects may exist in the universe that are so massive—with such large gravitational fields—that they could prevent light from escaping. As a consequence, these objects and others within their gravitational field would be invisible. Although the reasoning behind his theory was incorrect (Einstein's physics re-framed the properties of gravity and light), Mitchell had inadvertently stumbled upon a fascinating reality. Nevertheless, the idea vanished for over a century, likely due to an inability to prove the existence of such objects.

The concept of black holes was not revisited until 1905 when Einstein developed his special theory of relativity. As Scharf briefly explained, Einstein theorized that the "outstanding problems in physics could be solved [if] the speed of light never changes"—if light moves at a defined, constant speed. A consequence of this theory is that everything else in the universe is flexible, including space and time. This contradicts the Newtonian view of gravity, in which the force between two objects is dependent on the distance between them. Under Einstein's theory, distance is flexible; therefore gravity is also variable. Einstein developed a new model of gravity, based on the fixed speed of light, which describes gravity as simply the distortion of space by mass. To illustrate this, Scharf used the analogy of a rubber sheet with a grid printed on it to represent three-dimensional space (see slide). When a weight symbolizing a mass in the universe is dropped onto the sheet, it stretches and deforms the sheet—the space and time—around it. As two objects move past one another their paths are altered and curved by this distortion of space; the changed path we observe is what is we call gravity.

The effect of a mass on space and time is best illustrated by the analogy of an object placed on a rubber sheet. The object deforms the sheet just as a mass distorts space and time around itself (left). A supermassive object, such as a black hole (a highly dense and compact mass), stretches the sheet so far that it becomes a funnel from within which nothing can escape (right). (Image courtesy of Caleb Scharf)

In one of the first applications of Einstein's newly developed theories, Karl Schwarzschild produced the mathematical equations describing how a sphere of matter distorts space and time around itself. His solution suggests that if matter becomes compact and dense enough, it can create a region from which nothing can escape: a black hole. Using the rubber sheet analogy, Scharf illustrated how a highly massive object could create a funnel that would trap matter inside. In a black hole, time slows and the wavelength of light is stretched to almost nothingness, making anything inside the black hole impossible to see from the outside. The region beyond which nothing is visible is known as the event horizon.

How dense must matter be to form an event horizon? Scharf provided a few examples. The mass of Earth would need to be compacted to a diameter of 9 millimeters to create a black hole. A modest black hole, about ten times the mass of our sun, would span approximately 37 miles, roughly the diameter of London. Even a black hole one billion times the mass of our sun could fit easily within the orbit of Neptune, a miniscule space in contrast to the size of a galaxy.

Scientists at first found it difficult to believe that such compact, massive bodies could exist in the universe, but over time it became evident that black holes do exist; in fact, they are present in nearly every galaxy. One way to observe a black hole is to look for an unexpected and enormous gravitational influence on nearby objects. As an example, Scharf pointed out a star at the center of our Milky Way galaxy that is orbiting around something at 3,000 miles per second, indicating that the object it is moving around must be about 4 million times the mass of our sun: a supermassive black hole at the center of our galaxy. Black holes ranging from millions to billons times more massive than our sun can be found at the center of most galaxies.

In addition to observing their gravitational influence, scientists can also detect black holes by the enormous amount of energy they produce. The energy is generated by collisions around the event horizon, as matter falling toward a black hole accelerates to nearly the speed of light and crashes into other rapidly moving matter, producing light and subatomic particles. Many black holes also carry electric charge and spin, dragging space around and shearing and tearing matter apart. As the charged mass spins, it creates currents and generates incredible amounts energy. Radio and space telescopes can image the hot subatomic particles that stream in jets outward from supermassive black holes. As they rocket through space, these jets collide with other particles and create clouds of energy spanning hundreds of thousands of light-years. A black hole—which can convert matter into energy with 50 times more efficiency than nuclear fusion—can generate a trillion, trillion, trillion watts of energy.

Black holes are commonly thought of as voids of darkness from which nothing can escape. In reality, outside the event horizon, black holes generate massive amounts of energy spanning hundred of thousands of light-years. The small square in this image is the galaxy Cygnus A, which contains a supermassive black hole at its center. Jets of light, energy, and subatomic particles are emitted from high-energy collisions near the event horizon to form clouds of energy spanning about 500 thousand light-years of space. (Image courtesy of Caleb Scharf)

The energy generated by a supermassive black hole alters the landscape of space over a vast region. This is best illustrated in galaxy clusters where the space between neighboring galaxies is filled with extremely hot gas: an intergalactic 'atmosphere.' Electrons and energy streaming away from a supermassive black hole heat the gaseous atmosphere and form hot bubbles and sound waves that radiate from the center of the black hole through the galaxy.

Building on the idea that black holes can affect vast regions of space, Scharf proposed that the energy they generate may regulate the growth of galaxies. As the hot gas in galaxies cools, it condenses and matter falls towards the center to form new stars and planets. But a black hole disrupts that process. It stands in opposition to the growth of the galaxy by heating the atmosphere and preventing gas from cooling enough to form matter; thus, "black holes actually impact the number of stars a galaxy can make and impact the size that the galaxy can ever become," according to Scharf. In this way, he suggested, the balance between cooling particles and the energy emitted from black holes might govern the size and nature of galaxies. In support of a link between galaxy formation and black holes, he noted that "galaxies and supermassive black holes seem to coexist," that they "seem to have coevolved." There even seems to be a common ratio in many galaxies between the mass of the supermassive black hole and the mass of the stars—1:1000. Interestingly, not all galaxies, including the Milky Way, display this relationship, although they do contain supermassive black holes.

Scharf ended with a discussion of astrobiology and the possible relationship between black holes and life in the universe. Because "black holes influence the grand environment of galaxies," and the matter existing in them, it is possible to question how the activity of a black hole may impact life—or the conditions that make life possible. Scharf pointed out that life, at least on Earth, requires the formation of complex, pre-biotic molecules made up of heavy elements that are formed in large, high-energy stars. Only large stars have sufficient energy to form the heavy elements; smaller stars burn more faintly, with lower energy. The type of stars in a galaxy also influences the planets it contains. Most stars in our galaxy are smaller than our sun and are surrounded by "small and rocky" planets; these are less likely to contain water and appear to be less hospitable to life, at least as we know it.

Scharf noted that some galaxies contain a higher proportion of smaller, lower energy stars, and smaller planets as a consequence, while others, like our Milky Way, have larger planets and stars. It seems that galaxies containing a very active black hole have a higher proportion of smaller stars. The reason for this—the reason why a black hole can affect the proportion of small and large stars in a galaxy—is unknown. Scharf hypothesized that it may be due to the relationship between the matter in the galaxy and the activity of the black hole. He posited that a black hole influences the formation of a galaxy, its chemical environment, and its potential for life. Scharf ended by noting that although we cannot deem galaxies with more active black holes and smaller stars "bad" for life, they are at least "a very different place" for it. Thus, perhaps surprisingly, life as we know it may have been "strongly influenced" by the activity of black holes.

Use the tab above to find multimedia from this event.

Presentation available from Caleb Scharf, PhD (Columbia University)