The whole business of dark energy or cosmological constant or quintessence is novel, unfamiliar, and nebulous. Different models or mechanisms have been offered to explain apparent acceleration in expansion of U during past one-to-four billion years.(adsbygoogle = window.adsbygoogle || []).push({});

If you go back more than four or five billion years ago, the expansion appears to have been slowing down, but then somehow it changed over and began accelerating.

This is tracked by observing supernovae (used as standard candles) and comparing distances with redshifts-----this data is then converted to a history of the scale-factor a(t) over time.

Lineweaver has a plot of a(t) over the entire age of the universe. It is an important curve to understand and its hard to measure and its shape contains information about what could have caused the acceleration to start.

All this is well-known. what's new? Well Eric Linder is a reputable cosmologist and he just posted a short article describing how the different explanations may eventually be tested and the right one selected---or at least how the range of choice could be narrowed down.

It depends on getting increasingly accurate measurements of a(t) the history of the scale-factor.

Some people might like to look at Eric Linder's article

"Probing Gravitation, Dark Energy, and Acceleration"

http://arxiv.org./astro-ph/0402503 [Broken]

the "equation of state" number called "w" always plays an important role in these discussions. Nereid has described this in other posts. Linder talks about w and how getting a grip on it can help rule out one or another model. Comparing, for instance:

cosmological constant (always has w = -1)

some braneworld picture (Linder says tends to imply

that w > - 0.7 under realistic assumptions about the density of matter)

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# Comparing different kinds of dark energy

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