the version of GR used in cosmology is the Friedmann equations(adsbygoogle = window.adsbygoogle || []).push({});

which are a radically simplified version of the 1915 Einstein GR equation (simplified by assuming large-scale uniformity "looks same in all directions" kind of thing)

if you put a constant energy density into the friedmann equations you get that the scale-factor of the universe increases in a ramp that is convex for a while and then at some point turns concave (begins to look like accelerating exponential growth)

here's a picture

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg

so expansion first slows for a while and then (the models usually put it at a billion or so years ago, fairly recent IOW) begins to speed up

the constant energy density that they put into the friedmann equation (typically about half a joule per cubic km) is called various things like Lambda and "dark energy" and "cosmological constant"----main thing is it is just some energy density constant thru space and time, its effect on expansion derives mathematically in a simple way from its constancy.

Back in "Archive" someone asked about this.

There is a simple explanation why in General Relativity if you put in a constant Lamda then (as long as it is not unreasonably large) you get a slowing first, while Lambda is still small compared to the MATTER density, and then when matter has thinned out enough for the effect of Lambda to take over you get a speeding up.

Lineweaver "Inflation and the CMB" goes into this

http://arxiv.org/astro-ph/0305179 [Broken]

Lineweaver's is still the clearest introductory explanation of mainstream cosmology, clearest diagrams, plainest talk

His article is mirrored at the CalTech Knowledgebase site, I will get the link and edit it in:

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

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# Expansion slows down and then speeds up

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