Section 7: From Deceleration to Acceleration
We have learned that the universe—far from being small and static as was thought in Einstein's day—is large and dynamic. At present, it is expanding at an ever-faster rate. As it does so, the relative amounts of matter and dark energy change. If this version of cosmology is correct, we can use it to reconstruct the past and predict the future. Whenever possible, we test these predictions against observation.
Figure 17: High-redshift supernovae, together with their host galaxies.
Source: © NASA, ESA, A. Riess (STScI). More info
SN Ia provide a proven method for tracing the history of cosmic expansion. One direction that has proven fruitful has been to press the search for supernovae to fainter fluxes and hence larger distances. That allows us to probe the accumulated effect of cosmic expansion over ever-longer stretches of time. The initial measurements from 1998 had their strongest information at distances that correspond to about 5 billion light-years. But it is possible, with great effort, to push these measurements to earlier epochs. Astrophysicists have done this most effectively using the Hubble Space Telescope (HST), which is no larger than the telescope Hubble himself used at Mount Wilson, but observes from a much better site, above the blurring caused by the Earth's atmosphere.
Looking back at the past
If the universe consists of a combination of dark energy and dark matter, adding up to a of 1, the equations of cosmic expansion guarantee that at earlier times (and later) the will remain 1. But the balance of dark energy and dark matter is expected to change with cosmic epoch. As we look into the past, we see that the density of dark matter must have been higher than it is now. After all, every cubic meter of the universe we see today has stretched out from a smaller region. Compared to redshift 1, the length of any piece of the universe has doubled; so the volume of space that occupies one cubic meter today took up only 1/2 x 1/2 x 1/2, or 1/8, of a cubic meter back then. If you had the same matter in that volume at redshift 1 as we do today, then the density would have been higher in the past, by a factor of 8.
How this affects cosmic acceleration depends on the difference between dark matter and dark energy. If dark energy is identical to the cosmological constant, it lives up to its name by having a constant value of energy density. In that case, we expect that, when we look back to redshift 1, we would find dark matter to be eight times more important than it is today, while dark energy would show no change. This means that dark matter, the gravitating material, would have had the upper hand, and the universe should have been decelerating.
This is not just a matter for speculation. Using the HST, we can find and measure supernovae at this epoch, halfway back to the Big Bang. Measurements of 23 supernovae with large redshifts discovered and measured with HST were reported in 2004 and 2007 by Adam Riess and his colleagues. The data show that we live in a universe that was slowing down about 7 billion years ago. The balance shifted somewhere around 5 billion years ago, and we now live in an era of acceleration. In more detail, the data now in hand from 400 supernovae near and far allow us to trace the history of cosmic expansion with some precision. A dark energy that acts like the cosmological constant would be adequate to fit all the facts we have today. That doesn't mean this is the final answer, but a better sample will be needed to find out if the universe has a more complicated form of dark energy.
The prospect of a lonely future
A telescope lets us collect light emitted in the distant past. Looking into the future is more difficult, since we have only human minds as our tools, and our present understanding of dark energy is incomplete. But if dark energy acts just like the cosmological constant, the future of cosmic expansion is quite dramatic.
A constant dark energy produces exponential expansion. The larger the universe becomes, the faster it will expand. The expansion can become so fast that light from distant galaxies will never reach us. Even galaxies we see now will be redshifted right out of our view; so as the universe ages, an observer at our location will see fewer galaxies than we can see today.
Figure 18: Hubble Space Telescope panoramic view of thousands of galaxies in various stages of evolution.
Source: © NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (ASU, Tempe), R. O'Connell (UVA), P. McCarthy (Carnegie Observatories), N. Hathi (UC, Riverside), R. Ryan (UC, Davis), H. Yan (OSU), and A. Koekemoer (STScI). More info
If we follow the logic (and assume that our present understanding is perfect), eventually our Milky Way Galaxy and nearby Andromeda will be separated from this outwardly accelerating scene, and Andromeda and our other local neighbors will be the only galaxies we can see. Worse, we will eventually collide with Andromeda, leaving just one big galaxy in an otherwise empty universe. In a strange way, if this prophecy for the long-term future of our galaxy comes true, it will produce a situation much like the picture of the Milky Way as the entire universe that prevailed in Einstein's time.