Back around the early 1900s, the universe was a fairly simple place. It was static, had always been there, and largely consisted of our own galaxy and a few neighboring bits of matter. Over the course of the 20th century, that view collapsed. Many sources of light were revealed not be stars, but rather galaxies like (and, in many cases, unlike) our own. Distant galaxies were found to be rocketing away from us, propelled by the unfolding of the universe itself, which has accelerated since the big bang. Modern cosmology has revealed a universe teeming with dark matter and unseen energy, entering a new stage of inflation.
According to a paper that will appear in October (arXiv link), we're lucky to be able to reach this understanding—literally. The authors of the paper run the clock forward 100 billion years and reveal that it's going back to the future, a conclusion clear in the paper's title: The Return of a Static universe and the End of Cosmology.
The 100-billion-year figure was chosen because that's expected to be the lifespan of the longest-lived stars. By that time, only clusters of galaxies will be bound together strongly enough to resist the Hubble expansion. In our case, that means the Milky Way, Andromeda, and a number of smaller globular clusters in our neighborhood. By that time, we'll have collided and merged with Andromeda, making the local group one big galaxy. By then, however, everything else we can see will have been pushed so far away by the universe's expansion that all other sources of light will have been redshifted beyond our ability to detect them. All matter other than that in our galaxy will be invisible, and our view of the universe will look suspiciously like it did in the pre-Hubble days.
The cosmic microwave background, which has provided our most detailed understanding of the Big Bang, will also be gone. Its wavelength will have been shifted to a full meter, and its intensity will drop by 12 orders of magnitude. Even before then, however, the frequency will reach that of the interstellar plasma and be buried in the noise—the stuff of the universe itself will mask the evidence of its origin.
Other evidence for the Big Bang comes from the amount of deuterium and helium isotopes in the universe. By 100 billion years from now, however, much of the deuterium will have been burned in stars, with lots of helium produced in the process, erasing this evidence of our history. Worse still, we currently measure early deuterium levels by checking its absorbence of light from distant quasars. In the future, those quasars will have vanished.
Observers would still be able to piece together the age of our local cluster, based on a combination of knowledge about how elements are produced in stars and measuring their current abundance. But, in the end, the rest of the universe will vanish, taking with it any evidence of the Big Bang and some of its fundamental properties, such as dark energy.
The authors go on to ponder what this means in terms of the anthropic principle: the idea that we exist in a universe that's got conditions favorable to life largely because anything else would preclude any life arising that could ponder the universe. They suggest that there's another layer of complexity on top of that, namely that we only recognize that there is an anthropic principle because we came along at the right time. Too much earlier, and we wouldn't be able to detect that the universe is in a new inflationary era, which tells us that it's dominated by dark energy. Too much later, and we wouldn't be able to know that there's a universe at all. As the authors put it, "we live in a very special time in the evolution of the universe: the time at which we can observationally verify that we live in a very special time in the evolution of the universe!"