Question about Recombination Redshift values

  • Context: Graduate 
  • Thread starter Thread starter Dr. Strange
  • Start date Start date
  • Tags Tags
    Recombination Redshift
Click For Summary

Discussion Overview

The discussion revolves around the redshift values associated with recombination in cosmology, specifically comparing the values of z = 1500, z = 1210, and z = 1090. Participants explore the implications of these values in relation to ionization fractions and the optical depth parameter in the context of the ΛCDM model.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that Wikipedia cites a redshift of z = 1500 for recombination, while the Planck report lists a value of z = 1090 for the optical length equaling unity, questioning the difference between these values.
  • Another participant explains that z = 1500 is based on a 50% ionization fraction using the Saha equation, which assumes thermal equilibrium, while the Peebles equation, which does not assume equilibrium, yields a 50% ionization fraction at z = 1210.
  • A participant seeks clarification on which redshift value corresponds to the point where electrons traveled freely through the universe and requests a layman's explanation of optical depth, τ.
  • One participant references a previous post that may help explain optical depth, suggesting it is a parameter relevant to the ΛCDM model.
  • Another participant inquires about the relationship between the first peak of the BAO spectrum and the redshift values discussed, particularly the z = 1090 value from the Planck results.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate redshift values for recombination and the conditions under which they are derived. The discussion remains unresolved regarding the relationship between the BAO spectrum and the redshift values.

Contextual Notes

There are assumptions regarding thermal equilibrium in the calculations of ionization fractions, and the implications of these assumptions on the derived redshift values are not fully explored. The discussion also highlights the complexity of the optical depth parameter without reaching a consensus on its physical interpretation.

Dr. Strange
Messages
22
Reaction score
1
Wikipedia gives the redshift of recombination at z = 1500 (roughly 4,000 K). The Plank report list a 'redshift for which the optical length equals unity' and gives a value of 1090. What is the difference between the two?
 
Space news on Phys.org
z=1500 is approximated based on achieving a 50% ionization fraction under the Saha equation. This calculation, however, assumes the universe was in a state of thermal equilibrium, which it was not. Under the Peebles equation, which does not assume equilibrium, the 50% ionization fraction is achieved around z=1210. The value of z=1090 merely represents the ratio between the reionization temperature of hydrogen and the current measured temperature of the CMB.
 
Sorry, I didn't follow all of that. In layman's terms, which value represents the red-shift where the electrons traveled freely through the universe. Also, if it isn't asking too much, could you also give a layman's description of optical depth, τ. I see it's a parameter to ΛCDM and if you Google it, you get tons of information about various attempts to measure and constrain it, but I can't find a conceptual guide to tell me what this parameter is physically.
 
Dr. Strange said:
Is there any relation between the first peak of the BAO spectrum and one of these z values
Hi Dr. Strange:

You question requires expertise way over my head. The two people who have been particularly helpful in answering my questions about recombination are
@Chronos and @Chalnoth. Perhaps they can answer your question,

Regards,
Buzz
 

Similar threads

Undergrad Microwave background radiation - temperature at recombination
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Seeking reference for math related to the age of Recombination
  • · Replies 3 ·
Replies
3
Views
2K
Graduate Could Re-Analyzing Redshifts Reveal New Physics?
  • · Replies 0 ·
Replies
0
Views
2K
Graduate Angular power spectrum dependence in redshift z and k
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Have I got the right picture for cosmological redshift?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Some questions about Cosmic Microwave Background radiation
  • · Replies 13 ·
Replies
13
Views
7K
Graduate Is there a stretching of the light curves of supernovae 1a?
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Calculation of mean energy value of photons in recombination
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Is high redshift data a problem for ΛCDM?
  • · Replies 7 ·
Replies
7
Views
3K
High School Questions about the Eddington Limit and Super - Eddington black holes
  • · Replies 0 ·
Replies
0
Views
3K