Re: Was Einstein's 'biggest blunder' a stellar success? (Forwarded)

>>>>> "JR" == Jeff Root <jeff5@xxxxxxxxxxxx> writes:

>> "The significance is huge," said Professor Ray Carlberg of the
>> Department of Astronomy and Astrophysics at U of T. "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."

JR> Does that mean that the expansion did NOT first decelerate for a
JR> billion years or two before it began to accelerate?

Others have responded already, let me try to add a few specifics.

Currently the Universe is expanding, so we expect that the average
matter density in the Universe should be decreasing (by the factor
[1+z]^3, for a redshift z). Does dark energy's density also change
with redshift (or time)? Some models predict that it does, others
(including Einstein's cosmological constant) predict that it does
not.

This is what leads to your statement above. If matter dominates the
energy of the Universe, then we expect the expansion to decelerate
from the mutual gravitational attraction of the matter. However,
because the matter density decreases with time, if dark energy's
density does not, there will come a time when dark energy begins to
dominate and the acceleration increases. That's what appeared to
happen at a redshift of about 0.7.

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