Talk:Radiation-dominated era

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Astronomy: Cosmology

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Length of the Radiation-Dominated Era

The article says that the radiation-dominated era lasted 380,000 years (i.e. until the formation of the CMB).

But Timeline of the Big Bang places the end of the radiation-dominated era at 70,000 years.[1] Eroica (talk) 15:11, 28 June 2008 (UTC)[reply]

The value should be about 47,000 years, the text notes that the time of decoupling is "often mistaken" for this. — Preceding unsigned comment added by GeorgeDishman (talk • contribs) 00:08, 25 February 2013 (UTC)[reply]

Scale factor equation

This article gives an equation for the scale factor of a radiation-dominated universe, but it doesn't state any of the assumptions used to derive this simple equation. In its current form, that equation can be incorrectly interpreted. I don't have time to fix it right now, but chapter 19 of the book "Theoretical Concepts in Physics" by M.S. Longair (most of it is on google books) explains cosmological parameters, such as the scale factor, in different cosmological models.--Kris1284x (talk) 23:50, 9 May 2009 (UTC)[reply]

Cosmological redshift

"The Radiation-Dominated Era occurred from the universe's creation until a cosmological redshift of approximately 3400". Could someone with knowledge on the subject put that into layman's terms? Or, well, give some point of reference? 'Cosmological redshift' redirects to Hubble's law, which is a huge article, full of formulas for redshift, but it's not very accessible. -pinkgothic (talk) 16:57, 18 May 2010 (UTC)[reply]

From my User page, from a friend:

It means "until the universe was about 51 thousand years old". It can be calculated online: http://www.astro.ucla.edu/~wright/CosmoCalc.html (the redshift is named "z"). A redshift of zero would be "now" and it grows as one looks back in time. A bit more concrete: the apparent color of an object (for very loose definitions of "object" ;-) from that time, if we were to see it here & now, would be a lot redder than its real color. This is somehow determined by the distance and therefore the age [of the event that emitted the light], though I do not know why/how. Normally one would think that the color would be the result of the relative speed (as in the doppler-effect, better known in the context of soundwaves) but in cosmology it is instead related to "the expansion of spacetime" (http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/expand.html). HTH, rob --89.0.81.171 (talk) 05:08, 19 February 2012 (UTC)[reply]

Could someone who won't screw it up work that into the article? Or just use layman language ('until the universe was approximately 51 thousand years old (observable as a cosmological redshift 3400)')?

Basically, from my perspective, 'redshift' (providing it's understood at all) tends to be understood as the 'Doppler effect' of light, and so using 'until' outside the context of speed strikes me as very confusing without any explanation attached. -pinkgothic (talk) 13:10, 19 February 2012 (UTC)[reply]

I've included the approximate age with a reference to a modern textbook. Since redshift can be measured unambiguously but changing that into an age depends on the other cosmological parameters, astronomers generally use 'z' to avoid presuming a model. That convention is used here even though we can't receive light from that era, the universe was still opaque. Cosmological redshift is similar to Doppler for small speeds (v<<c) but is best understood as the wavelength being stretched by the same factor as the expansion of the universe. It's a complicated subject in its own right. George Dishman (talk) 00:14, 25 February 2013 (UTC)[reply]