As we go this far into the future, our powers of prediction begin to lose focus. One thing we can say is that all the processes that we have talked about so far leave behind backgrounds of radiation. As we saw, red dwarfs continually pump radiation into the universe during the Stelliferous Era, and the stellar background light will overwhelm the microwave background light. So the stellar contribution to the radiation fields will rise, but only until the time when stars die. At that point, that radiation will decay in prominence. In the Degenerate Era, the dark matter annihilation in white dwarfs then becomes the most important process, followed by the proton decay process that produces the most important source of radiation. Finally, the black hole radiation process--Hawking radiation--becomes the most important radiation background in the universe. That brings us to the beginning of the Dark Era, where we have no more stellar bodies, only an inventory of leftover particles: positrons, electrons, neutrinos and lots of long-wavelength photons.

What is surprising about the Dark Era is that the universe is not dead yet, and interesting things still continue to happen. For example, protons often leave behind positrons when they're gone, so for every electron that is still around in this dark future, there will be a corresponding positron. Electrons and positrons can get together and form what’s called a positronium atom, which is like a hydrogen atom but where the positron plays the role of the proton. We can make these positronium atoms today, but they are microscopic. They live for a tiny fraction of a second and then annihilate when the electron and the positron converge.

In this far future, if you form a positronium atom, it will be bigger than our galaxy in size. It can be even as big as the whole universe today in size. The universe is so empty that these things can live as tremendously large atoms, and the electron and positron will circle around each other for perhaps 145 cosmological decades (10¹⁴⁵ years) while they cascade down through various energy levels until they annihilate, emitting very long wavelength photons as they go.

There are other possibilities. One of the possibilities is that we now live in a universe that has a non-zero cosmological constant. A recent set of experiments, done in the last couple years, suggests that our universe may be accelerating. If this is true, everything just explained still holds. But it also opens up another possibility, namely that our universe lives in a false vacuum. In order for the universe to accelerate, empty space needs to have an energy level. But as soon as you allow the universe to have a vacuum with an energy level, then automatically the universe is theoretically allowed to have two energy levels. There is the possibility that a lower energy level exists, which means there is a possibility that a transition can occur, from the high-energy state of today to a low-energy state of tomorrow.

What's surprising is that, if you knew the theory, you could actually calculate the time scale over which this happens. However, the theory and the parameters are unknown, and the result is extremely sensitive to the unknown parameters. But if such a thing is happening or does happen in the future, what you get is something like this: A nucleation picture in which tiny regions make a transition from the old vacuum state to the new vacuum state. These regions grow with time, eventually percolate and merge, and transform the whole universe into a new phase.

If the universe undergoes such a phase transition, then the transformation can actually change the laws of physics. Because the strength of the forces and the masses of the particles can be wrapped up in the properties of this phase transition, the properties of physics can be different before and after the phase transition. Even though this dead universe, 10¹⁰⁰-plus years old, has essentially done all it is going to do, there is a chance for a new start.

Now, we're really starting to get a little bit speculative, so what I want to do is bring us back to earth and remind you of two things. One, that as the universe continues to age, it goes through a whole host of interesting processes which last from now until the universe is at least 100 cosmological decades (10¹⁰⁰ years) old. Second, with this grander perspective, we can now look back to where we are now. Although we don't occupy a particularly important place in space or a particularly important place in time, in cosmic terms, we actually do occupy a pretty good planet for the qualities that we have. We are pretty lucky to be here on Earth.