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Background information useful for exploring this news release:

http://hubblesite.org/newscenter/archive/releases/2006/52/astrofile/

What Are HST's New Results on Dark Energy Telling Us?

1. Astronomers have greatly improved the accuracy in the measurements of the acceleration in the cosmic expansion. In 1998, astronomers discovered that the expansion of our universe is speeding up, propelled by the repulsive force of "dark energy." The nature of this dark energy remains a mystery.

2. Astronomers have strengthened the evidence that the early universe was decelerating, but that it gave way to acceleration by around 4 to 5 billion years ago.

3. Astronomers have obtained the first meaningful measurement of the strength of dark energy in the distant past. It appears to have roughly the same strength that it does today, with a value consistent with Einstein's cosmological constant but does not prove Einstein was right. Astronomers are trying in particular to determine how much pressure this dark energy exerts for a given energy density, and if the relation between pressure and density remains constant or changes with time.

4. The "pressure" exerted by dark energy far back in time was negative, as it remains today, resulting in a repulsive gravitational force.

5. The new results rule out any rapid changes of in the "strength" of the dark energy's pressure, and in so doing, they rule out certain models for the dark energy. By observing a larger sample of supernovae, the researches have been able to place tighter constraints both on this "strength" of the dark energy and on its constancy. One possibility is that the dark energy represents the energy of empty space (the physical vacuum). The physical vacuum has a peculiar property that its pressure is negative, resulting in a repulsive force of gravity. Other models for the nature of dark energy involve fields (a bit like the electromagnetic field) that decay with time.

6. There is strong evidence that the Supernovae Type Ia, the "standard candles" used to measure the rate of cosmic expansion, have not changed over the past 10 billions years, i.e., supernova evolution is not fooling astronomers into drawing false conclusions about dark energy. The new results yield the tightest constraints to date on both the "strength" of the dark energy pressure and on its constancy. The results are consistent with Einstein's cosmological constant. This means that at least some models that involve varying fields can be ruled out.