If we take the WMAP value for critical density at ρ c,0 = 9.47 x 10 -27 kg/m 3and presume that dark energy makes up about 73% of that, then the effective density of the dark energy would amount to just over 4 hydrogen atoms (m = 1.67 x 10 -27 kg) in a cubic meter of space. This is an energy density which we have not directly detected observationally - hence "dark energy". That acceleration implies an energy density that acts in opposition to gravity which would cause the expansion to accelerate. Measurement at these great distances provided the first data to suggest that the expansion rate of the universe is actually accelerating. Since the 1990s it has become apparent that type Ia supernovae offer a unique opportunity for the consistent measurement of distance out to perhaps 1000 Mpc. One of the great challenges of astronomy and astrophysics is distance measurement over the vast distances of the universe. So we are left having to account for the remaining 73% of the effective density, and the name chosen is "dark energy". The WMAP projection of the ordinary baryonic matter is only 4.4% of critical density, and only 27% even when the projected " dark matter" is included. A big conceptual problem has been that we haven't been able to observe more than a fraction of that density in the form of ordinary matter. That is, the universe is very close to the critical density, above which it would slow down and collapse inward toward a future "big crunch". Measurement of the expansion rate is a critical part of the study, and it has been found that the expansion rate is very nearly "flat". One of the observational foundations for the big bang model of cosmology was the observed expansion of the universe.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |