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the version of GR used in cosmology is the Friedmann equations
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
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
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
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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