L. Ferrarese (Johns Hopkins University) and NASA

This is a Hubble Space Telescope image of an 800-light-year-wide spiral-shaped disk of dust fueling a massive black hole in the center of a galaxy, NGC 4261, located 100 million light-years away in the direction of the constellation Virgo. By measuring the speed of gas swirling around the black hole, astronomers calculate that the object at the center of the disk is 1.2 billion times the mass of our sun, yet concentrated into a region of space not much larger than our solar system.

The only objects that survive unscathed through that epoch of proton decay will be black holes. Black holes may or may not be composed of protons, but the beauty of black holes is that you cannot even really see whether they’re made of protons or not. As we leave behind the Degenerate Era, we enter into the fourth era, called the Black Hole Era. Black holes are objects that are so dense and so massive that their escape velocity is greater than the speed of light. The original definition of a black hole is that nothing can escape its surface because it would require a speed faster than light. And that would be true if it weren't for quantum mechanics. Stephen Hawking realized that if you add quantum mechanics to our understanding of a black hole, then it is possible for radiation to leak out every once in a while. The process is so slow that you don't need to worry about this today. But in the future, when all of the other stellar objects are gone, black holes shining through this Hawking radiation are the brightest objects in the universe. They will fill the role now played by stars in our universe today.

Before we actually talk about the implications of Hawking radiation, let me justify to you that black holes can exist. One way to create a black hole is through a super nova. When the most massive stars explode at the end of their lifetimes, they do so in a super nova explosion. Left behind is either a neutron star, which is a very dense remnant, or a black hole. If a neutron star remnant gets too massive, and here massive means more than about twice the mass of the sun, the object will collapse to a black hole. We now have observational evidence that these stellar-sized black holes exist. It is hard to project exactly how many there will be in this universe of the future, but there should be about a million such black holes in a galaxy the size of our Milky Way.

There's another kind of black hole that we also "see." The middle of every galaxy has a super-massive black hole. Not surprisingly, a super-massive black hole means one that has a lot of mass, but here, "a lot of mass" means millions to billions times the mass of the sun. Our own galaxy has a rather modest such specimen containing about 3 million solar masses of material. The central black hole of other galaxies can be up to billions of solar masses. If you look at the central region of a galaxy and watch how fast the stars are moving around the very center, you find that the stars are orbiting very quickly around something that you can’t see. That something you can't see is compressed to an extraordinarily tiny volume. About the only thing it can be is a black hole, and the mass of these black holes is large. If you do an inventory of the universe in the Black Hole Era, you will only find black holes. Each galaxy that is about the size of ours contributes about one super-massive black hole and about a million stellar black holes to this inventory. And each black hole is shining brightly, or as bright as it can be, through Hawking radiation.

But black holes have their own lifetime, which depends very sensitively on their mass. Not only do the bigger black holes have more mass to radiate away, but they also radiate that mass away at a lower temperature. So the super-massive black holes live much longer than stellar black holes. The numbers work out so that if you have a one solar mass black hole, a black hole with the mass of the sun, it will live for 65 cosmological decades (10⁶⁵ years). The typical stellar black hole that is produced by a super nova explosion is expected to be about 10 solar masses. This kind of black hole would live a thousand times longer, or about 68 cosmological decades (10⁶⁸ years). A million-solar mass-black hole--like the one that lives in the middle of our galaxy--would live for 83 cosmological decades (10⁸³ years).

The largest black holes you can envision would swallow nearly the whole mass of a galaxy, but even such an enormous black hole as that would radiate itself away in only 98 cosmological decades (10⁹⁸ years). And finally, if you took every bit of mass in our observable universe today, and you balled that up into a tremendously massive black hole, that object, too, would radiate itself away in only 131 cosmological decades (10¹³¹ years). That is a very long time, but compared to forever, it’s really quite short. Eventually, black holes are going to make their explosive exits from the universe, and we will enter a new era--the fifth and final era, called the Dark Era.