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.