Martin Rees in Prospect:
In cosmological or Darwinian terms, a millennium is but an instant. So let us fast forward not for a few centuries or millennia, but for an astronomical timescale millions of times longer than that. The stellar births and deaths in our galaxy will gradually proceed more slowly, until jolted by the environmental shock of an impact with the Andromeda Galaxy, maybe four billion years hence. The debris of our galaxy, Andromeda and their smaller companions—which now make up what is called the Local Group—will thereafter aggregate into one amorphous swarm of stars. Many billions of years after that, gravitational attraction will be overwhelmed by a mysterious force latent in empty space that pushes galaxies apart from each other. Galaxies accelerate away and disappear over a horizon. All that will be left in view, after 100bn years, will be the dead and dying stars of our Local Group, which could continue for trillions of years. Against the darkening background, sub-atomic particles such as protons may decay, dark matter particles annihilate and black holes evaporate—and then silence.
As we attempt to grapple with this bleak post-human future, we must also confront the question of what humans can hope to understand. Parts of the physical world are understood. They can be observed and described by theories—but much of it cannot. Human observation bumps up against stark limits. Human reasoning is not limitless either, but it does allow us to think through what might in principle be “over the horizon.”