Talk:Scale factor (cosmology)

Title
72.196.123.2 (talk) 13:12, 28 December 2008 (UTC) Could somebody change the title to "Cosmic Scale Factor"? I don't know how to change titles.

please clarify
"where t is counted from the birth of the universe and t0 is the present age of the universe: "

so at a(13 billion years) is the scale factor 13 billion years ago? I thought thats what t=0 was. — Preceding unsigned comment added by IIAOPSW (talk • contribs) 03:59, 17 October 2011 (UTC)

Requested move

 * The following discussion is an archived discussion of the proposal. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section. 

No consensus to move. Vegaswikian (talk) 08:26, 26 April 2010 (UTC)

Scale factor (cosmology) → Cosmic scale factor — Originally I moved the page from Scale factor (Universe) to Scale factor (cosmology), but I think Cosmic scale factor would be a better title, since it is a more specific term and wouldn't require parentheses. "Cosmic scale factor" currently redirects to "Scale factor (Universe)", so I can't make the move myself. Stebbins (talk) 03:47, 19 April 2010 (UTC)
 * Against the move. "Scale factor" is what is it usually called by cosmologists.  "cosmic inflation" was moved to "inflation (cosmology)" for a similar reason. --Michael C. Price talk 07:03, 19 April 2010 (UTC)
 * The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

Scale factor units
If the universe is expanding, then an object at 1m away from an observer is moving at a velocity v, at 2 m a velocity 2v and so in. The dimensions of this are velocity/distance, which reduces down to simply 1/time. So it would make sense to measure rate of expansion in units of seconds, as in "number of seconds for a distance to double".

I have a different question:

if the universe is expanding, then there is a horizon beyond which it's expanding faster than the speed of light relative to us. That's an event horizon, and so it must be giving off hawking radiation. Is this related to cosmic background radiation? — Preceding unsigned comment added by Paul Murray (talk • contribs) 04:49, 14 June 2011 (UTC)
 * No. And note that by the Friedmann equations the scale factor has units of distance. -- cheers, Michael C. Price talk 07:15, 14 June 2011 (UTC)

historical values
Is there any place where we have historical values for a, a' and a\'\' the scale and its first two derivatives during the intervening timescales from Big Bang -to- now? I can find no graph for this, no papers on it. 67.188.202.139 (talk) 00:02, 25 December 2011 (UTC)

External links modified
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How can $$\ddot{a}(t)$$ be positive and yet H decreases?
Is there an error in the article? How can $$\ddot{a}(t)$$ be positive and yet "H parameter decreases"? Reasoning and the math is not telling me that. As an approximation, I thought double cosmological time from now will have non-local galaxies twice as far apart, if H were constant as was previously assumed. Ywaz (talk) 16:29, 4 April 2016 (UTC)


 * $$\ddot{a}(t)>0$$ implies that $$\dot{a}(t)$$ increases, but not that $$\dot{a}(t)>0$$. Something can increase and be negative. - DVdm (talk) 09:06, 7 April 2016 (UTC)

This needs graphics and animations
That ^ --TiagoTiago (talk) 07:28, 9 October 2016 (UTC)

How well established are the time periods?
This article cites the transition from the radiation-dominated era to the matter-dominated era as occurring 47,000 years after the Big Bang, with a citation. [Chronology of the universe#Early universe] puts the time at 70,000 years ago, with a different citation. I've googled around several physics pages and found still other estimates. So how firm are these numbers? Would it make more sense to give a general time, or to at least note that there are a range of proposed times for the shift?

131.96.47.18 (talk) 16:33, 8 November 2017 (UTC)

Please clarify the exponential growth
Please, clarify the exponential growth / source the derivation of the Dark Energy dominated universe. --77.215.232.29 (talk) 18:04, 2 December 2019 (UTC)