Observations in the 1990s, however, proved that the universe was not only flying apart, it was doing so faster and faster. This seemed to point to a dark energy filling space that actually repelled ordinary matter with its gravity, in contrast to all other known stuff, including dark matter. A number of theories have been developed to explain what this dark energy might be, including Einstein's long discarded cosmological constant.
Now new observations from an international team of astronomers seem to show that dark energy is like the cosmological constant, unvarying throughout space and time. By measuring the distances to 71 far-off supernovae, the scientists were able to ascertain with a high degree of confidence that the effect dark energy exerts on supernovae light does not vary with distance. The researchers also plugged this data into a so-called equation of state, which measures the relationship between pressure and density, and found that dark energy must be less than -0.85--awfully close to Einstein's cosmological constant at -1. "Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands and, as far as we can see, it doesn't," says team member Ray Carlberg of the University of Toronto. The results will be published in a future issue of Astronomy & Astrophysics.
The 71 supernovae observations are the results of just one year of an ongoing Supernova Legacy Survey being carried out using telescopes throughout the world, such as the Canada-France-Hawaii Telescope in Hawaii and the Very Large Telescope in Chile. Data collected over the next four years should improve the precision of the finding and help researchers determine more about the enigmatic nature of dark energy that seems to constitute at least 70 percent of the universe.
But the finding brings to the fore another question: the so-called cosmological coincidence. Observations like this one seem to prove that regular matter and dark energy have similar densities at precisely this moment in time, even though the density of matter has been declining steadily since the big bang. Even Einstein couldn't answer why that would be.