# Question about Recombination Redshift values

1. Dec 13, 2015

### Dr. Strange

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?

2. Dec 13, 2015

### Chronos

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.

3. Dec 14, 2015

### Dr. Strange

Sorry, I didn't follow all of that. In layman's terms, which value represents the red-shift where the electrons travelled 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.

4. Dec 14, 2015

### Buzz Bloom

5. Dec 14, 2015

### Dr. Strange

6. Dec 14, 2015

### Buzz Bloom

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