2

What caused the transition, and how to we know that this did not warp our calculations for the age of the early universe ?

Gödel
  • 1,064

1 Answers1

4

What caused the transition,[...]

As the universe expands, the number of particles per unit volume goes down. But in addition to this, photons suffer a cosmological red-shift. So the density of mass-energy due to nonrelativistic particles goes down, and the density due to photons goes down, the latter goes down faster. By extrapolation, we predict that photons dominated in the early universe.

and how to we know that this did not warp our calculations for the age of the early universe ?

Because the models have the correct physics built in. The transition from radiation-dominated to matter-dominated is a prediction of the models, not an assumption. We also have good reason to believe that this aspect of the models is correct, since we can empirically verify predictions of the models, such as big-bang nucleosynthesis, that are sensitive to the early-universe part.

  • So, if it happened so gradually, when do we stop using scale factor equations for the radiation dominated universe, and start using the scale factor equations for the matter dominated universe? – Gödel Oct 24 '14 at 01:43
  • @Gödel: By scale factor equations, do you mean the Friedmann equations? The Friedmann equations are simpler when there is only one form of matter present, so textbooks often present those simpler cases. That doesn't mean that realistic cosmological models only use the simpler versions. –  Oct 24 '14 at 05:15
  • @Gödel: see How does the Hubble parameter change with the age of the universe?. This shows how the scale factor is calculated taking into account radiation as well as matter. – John Rennie Oct 24 '14 at 10:06
  • @BenCrowell Yes, I was speaking of the Friedmann equations. What you said does make sense though. Are there analogous equations that take statistical mechanics into account ? – Gödel Oct 24 '14 at 17:33
  • @JohnRennie That was a pretty interesting a(t) fit. But it still seems to rely on the simplistic axiom, that only one type of state can exist at a time (matter or radiation). – Gödel Oct 24 '14 at 17:44
  • @Gödel: no, all four contributions to the density are treated together. – John Rennie Oct 24 '14 at 19:18
  • But, you just did a "changeover," it doesn't seem like you really synthesized them . If you use the standard Friedmann solutions (which only look at of state at a time) to analyze the doubling time, isn't that using some form of circular reasoning? – Gödel Oct 25 '14 at 03:13
  • @Gödel: the scale factor was calculated using equation (2) in the linked answer. This contains all four parameters. Are you asking about the equation I used for the fit? If so, this equation has no physical significance - it just happens to fit the curve well. – John Rennie Oct 25 '14 at 17:54
  • Okay, I'm sorry, I got confused, because the equation you obtained had c1 proportional to t^2/3, just like one of the original simplifications. Thank you for being patient with me, you are a great help as always. – Gödel Oct 26 '14 at 00:11