As galaxies in our universe expand away from one another at the speed of light, and as the cosmic glow from the Big Bang fades, what clues about the Big Bang and the birth of our universe will be left for astronomers to study a trillion years from now? How will our distant descendants know the universe is expanding, when galaxies have gotten so far from each other that, from our Milky Way vantage point, we cannot see other galaxies at all?
Big thoughts. But not too big for Harvard theorist Avi Loeb, who directs the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. He considered this question in a paper available on-line in the Journal of Cosmology and Astroparticle Physics.
In a trillion years, when the universe is 100 times older than it is now, our home – the Milky Way galaxy – will have merged with the Andromeda galaxy to create what some astronomers call Milkomeda. Our sun will have burned out, along with many other stars, and all the galaxies visible to us now will have rushed beyond the cosmic horizon, forever out of view. The glow left over from the Big Bang, detectable as cosmic microwave background (CMB), is also expanding at the speed of light and will fade as its wavelengths stretch out to an invisible spectrum. Dr. Loeb said:
We used to think that observational cosmology wouldn’t be feasible a trillion years from now. Now we know this won’t be the case. Hypervelocity stars will allow Milkomeda residents to learn about the cosmic expansion and reconstruct the past. Astronomers of the future won’t have to take the Big Bang on faith. With careful measurements and clever analysis, they can find the subtle evidence outlining the history of the universe.
Hypervelocity stars are extremely rare – occurring about once every 100,000 years. This type of star gets flung from the black hole at the center of a galaxy when a binary-star system is pulled into the black hole and torn apart. One star disappears into the black hole, and the other is ejected as a hypervelocity star at over a million miles per hour – fast enough to escape black hole gravity. The light from a hypervelocity star would be the most distant light source available to an astronomer from Milkomeda.
Loeb explains that future astronomers would have the technology to measure not only the hypervelocity star speed but the extra speed exerted by the expanding universe. This would be their evidence for an expanding universe; it would be akin to Edwin Hubble’s discovery but based on smaller effects. Stars within Milkomeda would reveal when the galaxy formed. Combining that evidence with the hypervelocity star measurements would give the age of the universe and key cosmological parameters.
They could figure it out, but the evidence just wouldn’t be as spectacular as what we can see before us now, for example, the farthest and therefore youngest galaxies in our universe seen through the extended vision of the Hubble Space Telescope. In a trillion years, even for powerful telescopes like Hubble, that distant view of our universe’s own past will be gone forever.
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