Acceleration |
![]() |
So we have estimates for H0 and OmegaM. Is that the whole story? Suprisingly not. Evidence is mounting that the universe may not only be expanding, but that its expansion may be accelerating with time. How can that be? Right now, astrophysicists and particle physicists are working hard to figure out that answer. One current theory is that empty space is filled with "virtual particles" and that these particles have energy -- in other words, there is a sort of "vacuum energy" associated with empty space, and that this energy may drive the expansion. These theories are in their infancy, though, and remain largely qualitative. Nonetheless, if the Universe is being accelerated, it changes its expansion history and future evolution. For example, an OmegaM > 1 universe can keep expanding forever, if the acceleration can overcome the gravitational collapse. An accelerating universe also implies an older universe, since it was expanding more slowly in the past. The acceleration factor is termed Lambda, or OmegaL, and is defined in such a way that it can be compared to the density parameter OmegaM. Remember that current cosmological models want Omega = 1, but that current estimaes of OmegaM are < 1. We can reconcile these two numbers if OmegaM + OmegaL = 1, i.e., OmegaTOTAL = 1. So many astronomers are studying cosmological models for the universe where the combination of OmegaM and OmegaL equals one. Again, that not be the way the universe is, but its what the current models would like... The best observational estimates of OmegaL come from the study of distant supernovae in galaxies in order to determine the relationship between redshift and distance. These estimates place OmegaL ~ 0.7, which nicely give us OmegaM + OmegaL ~ 1, but there are considerable uncertainties here... |