“[T]he glory and greatness of Almighty God are marvelously discerned in all his works and divinely read in the open book of heaven.... Within its pages are couched mysteries so profound and concepts so sublime that the vigils, labors, and studies of hundreds upon hundreds of the most acute minds have still not pierced them, even after continual investigations for thousands of years.” – Galileo GalileiGalileo, “Letter to Madame Christina of Lorraine, Grand Duchess of Tuscany,” 1615
Astronomy is a humbling enterprise. When we study the stars, we are confronted by two undeniable facts about ourselves: we are very small, and we know very little! The entirety of human history is like a fleeting wisp of vapor, and we have explored only a mote of the magnificent tapestry of creation. Astronomy and cosmology may be the oldest sciences, yet we have barely begun to understand the cosmos. Indeed, as new phenomena are discovered, the universe appears more mysterious than ever. Let’s conclude our introduction to astronomy and cosmology by examining some of the greatest unsolved mysteries in these sciences.
Perhaps the most profound and intriguing mystery is the cosmic “fine-tuning” problem, which we’ll consider at the end of this chapter. The problem, simply put, is that the fundamental physical laws and constants that govern the behavior of the universe seem to have been precisely tuned, or carefully chosen, to make the existence of biological life possible! (This may not be surprising to those of us who believe that the universe was designed by a Creator, but it is a serious problem for anyone who hopes to find a purely naturalistic explanation of the origin of the universe.) In order to understand the fine-tuning problem, however, it will be helpful first to consider several other mysterious discoveries of astronomy and cosmology. We’ll start with the most famous: black holes.
Einstein’s theory of gravity, the general theory of relativity, predicts that strange things happen when a lot of mass is compressed into a tiny region of space. If the density of an object is great enough, the curvature of spacetime around it becomes so extreme that all the nearby timelike and lightlike geodesics point inward. In other words, anything that comes within a certain distance of the dense object will inevitably fall toward it. That crucial distance is called the Schwarzschild radius.
The Schwarzschild radius is equal to 2mG/c 2, where m is the object’s mass, G is the gravitational constant, and c is the speed of light. Since G and c are constants, the length of the Schwarzschild radius depends only on the mass of the dense object: the heavier the object, the longer its Schwarzschild radius (and the larger the black hole it creates).
Within the Schwarzschild radius, nothing can escape the object’s gravity—not even light! If a ray of light (or any other thing) wanders inside the Schwarzschild radius, it can never get out again, because all possible worldlines lead inward. The region of space within the Schwarzschild radius, a spherical region surrounding the dense object, is called a black hole.
It may not be entirely true that nothing ever escapes a black hole. Physicist Stephen Hawking has hypothesized that energy may slowly leak out of a black hole via quantum mechanical processes. This leakage, called Hawking radiation, could cause a black hole to “evaporate” over many trillions of years.There is no direct observational evidence for Hawking radiation, but something analogous to an event horizon has been created in the lab, using the speed of sound rather than the speed of light as a boundary between the “inside” and “outside” of the simulated event horizon. These experiments suggest that Hawking radiation does indeed occur. See here for more details.
Just how dense would an object have to be to create a black hole? Well, consider the density of the densest macroscopic objects in the universe: neutron stars. As explained earlier in this chapter, a neutron star is a ball of neutrons, typically around ten miles in diameter, left over from the collapsed iron core of a huge star. Its density is almost unimaginable. The mass of a single teaspoon of neutron star material (neutron-degenerate matter) is around 3 billion metric tons!I calculated this based on the overall density of a large neutron star, which is about 5.9×1017 kg/m3 on average. Near the center of the neutron star, the density is greater still. To understand how this is possible, recall from chapter 4 that almost all of an atom’s mass is in the nucleus, which is about 100,000 times smaller than the diameter of the atom as a whole. Imagine crushing a solid ball of iron down to one hundred-thousandth of its original diameter, and you’ll have some idea why neutron stars are so dense! In fact, the density of a neutron star is even greater than the density of an atomic nucleus, because the powerful compression of gravity squeezes the neutrons closer together. That’s pretty dense, but it’s still not dense enough to create a black hole.
A neutron star is supported by neutron degeneracy pressure, similar to the electron degeneracy pressure that supports white dwarf stars: the Pauli exclusion principle prevents gravity from compressing the neutrons any further. Neutron degeneracy pressure is incredibly strong—strong enough to support the immense weight of a neutron star, which is no small feat, because the extreme density of a neutron star makes its surface unimaginably heavy. (Recall that weight is the force of gravity on an object. It’s not the same as the object’s mass.) The heaviest neutron stars have a surface gravity 100 billion times stronger than Earth’s. On the surface of a neutron star, a 1-gram feather would weigh more than the largest ship that ever sailed the Earth!The weight of an object is equal to its mass times the gravitational acceleration, as explained on this page of chapter 2. Gravitational acceleration near the surface of a neutron star can be as high as 7 × 1012 m/s2, so 1 gram of mass would weigh 7 × 109 newtons. On Earth, the gravitational acceleration is a little under 10 m/s2, so the mass of an object would have to be over 7 × 108 kg in order to weigh that same amount on Earth. The largest and heaviest ship ever built was the oil tanker Seawise Giant. At 458 meters in length, it was over three times the length of Noah’s ark and nearly twice the length of the Titanic. When fully loaded, it had a mass up to 657,019 metric tons and weighed about 6.4 billion newtons (in Earth’s gravity). On the surface of a neutron star, a feather would weigh more than that! Despite this absurdly strong gravity, neutron degeneracy pressure is tough enough to support up to three solar masses of neutrons. However, if the collapsing core of an enormous star is greater than three solar masses, or if additional matter falls into a neutron star and raises its mass above that critical threshold, the power of gravity finally overcomes neutron degeneracy pressure and compresses the star further. At last, the density is great enough to create a black hole.
The Pauli exclusion principle is never violated, even when gravity overcomes neutron degeneracy pressure. The Pauli exclusion principle forbids two neutrons from occupying the same quantum state at the same time, but it doesn’t forbid them from occupying the same physical location if their other properties differ—for example, if they differ in energy. So, gravity can squeeze a neutron into a space already occupied by other neutrons whenever doing so raises its energy higher than any of the other neutrons already present there. If the mass of a neutron star exceeds three solar masses, gravity can keep doing this indefinitely, so the star collapses into a black hole. See this PBS Spacetime episode for further explanation.
The gravitational singularity at the center of a black hole differs from the spacetime singularity at the initial moment of the Big Bang. The singularity of a black hole exists within spacetime. The singularity of the Big Bang, on the other hand, was not a singular point inside some surrounding space (so far as we know). Rather, all of space was contained within that singularity: space and time came into being at that moment. Moreover, the singularity of the Big Bang may not have been infinitely small. As mentioned earlier in this chapter, the size of the whole universe (not just the “observable” part) is unknown. It may even be infinite in size, in which case it would have been infinite in size even when it was infinitely dense at the initial moment of the Big Bang.
What happens inside a black hole? There is no way to examine the interior of a black hole, of course, since nothing that falls in ever comes out again.As mentioned in the Fine Print section above, Hawking radiation may allow energy to leak out of a black hole. However, this leakage has never been directly observed; and even if we could detect it, it probably wouldn’t enable us to figure out what’s going on inside the hole. However, general relativity implies that everything inside a black hole must collapse all the way down to an infinitely dense, infinitesimally tiny point! Inside the Schwarzschild radius, spacetime is curved so much that an object would have to move faster than the speed of light to avoid falling inward. Since nothing can move faster than light, everything must fall inward until it reaches the very center point. The infinitely dense point at the center of a black hole is called a gravitational singularity, because the equations of general relativity yield a mathematical singularity (i.e., they are mathematically undefined) at that point. In other words, Einstein’s field equations yield mathematical nonsense, like dividing by zero, when we try to understand what happens at the center of a black hole.
The singularity is not the only puzzling feature of a black hole. General relativity also gives strange predications about a black hole’s outer boundary, called its event horizon. The event horizon is the boundary between the inside and outside of the black hole: the sphere-shaped boundary located at the Schwarzschild radius. Anything that crosses the event horizon—including light—must fall into the singularity; there is no turning back. But can anything cross the event horizon in the first place? Recall from chapter 6 that time itself slows down near massive objects. From the perspective of an outside observer, gravitational time dilation becomes infinite at the event horizon of a black hole. In other words, time just stops there—or so it seems, when judged from the reference frame of someone outside the black hole.
For example, imagine that you are flying through the galaxy in a spacecraft, pursued by a hostile alien battleship. As you weave and dodge to avoid the battleship’s death rays, you adroitly maneuver around a black hole; but your alien pursuer doesn’t notice the hole until it is too late to change course. His battleship tumbles out of control toward the event horizon. You won’t see him fall in, because light from his battleship will be gravitationally red-shifted out of the visible spectrum before he even arrives at the event horizon. So, as his battleship turns a deep red color and then fades out of view, you decide to use Einstein’s equations to calculate how long it will take before he is inside the black hole. You do the math and discover that he will never get there! According to your frame of reference, all physical processes—including the motion of the alien battleship—slow to a halt near the event horizon.
From the alien’s perspective, on the other hand, things happen all too quickly. According to his reference frame, his battleship tumbles past the event horizon in a fraction of a second. A moment later, he reaches the singularity and is crushed to oblivion. How can both of these perspectives be correct? The answer, according to general relativity, is that we cannot regard events inside a black hole as simultaneous with events outside (or vice versa) no matter what reference frame we choose. Reference frames cannot be extended across the event horizon. The alien does fall into the black hole, but this event isn’t simultaneous with any events in your reference frame. In a sense, he crosses the event horizon at a time infinitely far in your future!For further discussion of this weird prediction, see Paul Davies, About Time: Einstein’s Unfinished Revolution. New York: Simon & Schuster, 2005, p. 119.
Given the paradoxes and enigmas associated with black holes, it is tempting to suspect that such strange objects couldn’t really exist. However, black holes are not merely theoretical constructs. Though their nature is profoundly mysterious, these bizarre entities do exist. We cannot see them directly, but their presence is manifested in numerous ways. For example, when an unlucky star or nebula falls into a black hole, it usually forms an accretion disk—a thin disk of matter orbiting just outside the event horizon—before being consumed. The material orbits at varying speeds depending on the distance from the black hole, and this creates friction that heats the accretion disk to millions of kelvins, causing it to glow brightly in wavelengths all across the electromagnetic spectrum. Although the black hole itself emits no light, the accretion disk is easy to detect. Numerous accretion disks have been found orbiting black holes in our galaxy. The first to be discovered was Cygnus X-1, detected in 1964 from the powerful x-rays it emits.Located 6,000 light-years away in the constellation Cygnus, this black hole weighs about 15 solar masses. The black hole and a nearby star orbit around each other, and the accretion disk consists of gas torn from the star by the black hole’s powerful gravity. (Don’t panic. It’s about 6,000 light-years away, so we’re in no imminent danger.)
Furthermore, there is compelling evidence that most large galaxies have a supermassive black hole at the center—a gigantic black hole millions or even billions of times the mass of a star. At the center of our own Milky Way galaxy, for example, stars orbit rapidly around an invisible object. There is no accretion disk in this case, but we can calculate the object’s mass by observing the orbits of stars around it. Its mass is more than 4 million times the mass of the sun! Astronomers have concluded that this invisible object must be a supermassive black hole, because no other type of object could pack that much mass into such a small region of space. The creation of a supermassive black hole is not fully understood, but it probably involves the mergers of many smaller black holes, which also grow by gobbling down stars and other matter while a galaxy is forming.
Black holes are also responsible for several of the most powerful phenomena in nature. The most distant objects we can detect with telescopes, called quasars (or quasi-stellar objects), are the intensely bright cores of young galaxies, some of which are estimated to be over 13 billion light-years away.As of December 2017, the most distant quasar is estimated to be 13.1 billion light-years away. (See this news article from NASA’s Jet Propulsion Laboratory for more details.) Most of a quasar’s light comes from the accretion disk around a supermassive black hole, which rapidly devours stars and nebulae during the early stages of galaxy formation. The brightest quasars emit thousands of times more light than our entire galaxy!
On rare occasions, the birth of a black hole can be observed even when the associated GRB isn’t aimed toward us. In 2009, a large star—estimated to be around 25 times the mass of the sun—became momentarily brighter and then suddenly faded away. The best explanation for its disappearance is that its core collapsed to form a black hole.See here for more details. For further discussion of evidence for the reality of black holes, check out this PBS Space Time episode on “How to See Black Holes.”
A similarly powerful phenomenon occurs with the birth of a new black hole. When an extremely large star explodes in a supernova, its collapsing core may have too much mass to become a neutron star, so it forms a black hole instead. The formation of the black hole produces powerful magnetic fields that focus most of the supernova’s energy into two narrow beams of radiation: one beam from each of the magnetic poles. The beam of high-energy particles, light, and gamma rays is called a gamma ray burst (GRB). A gamma ray burst is so intensely powerful that if the beam happens to be aimed at Earth, we can see it from billions of light-years away, sometimes even without a telescope. The brightest GRB on record was detected on March 19, 2008. For about 30 seconds, the beam was bright enough to be seen with the naked eye, even though it came from a galaxy 7.5 billion light-years away!For more details, check out this NASA press release and feature story.
An even more powerful release of energy occurs when two black holes merge to form a larger black hole. The merger of two black holes produces a burst of gravitational waves, which shake the very fabric of space and time, even billions of light-years away. This burst is so powerful that—for a brief moment—the merging black holes release energy at a faster rate than all the stars and galaxies in the observable universe combined.See this LIGO press release for more details.