However, I get the error as written in the title. This is probably a basic question - I looked at http://cosmocoffee.info/viewtopic.php?t=2593 which brought up the idea of finding the starting value of theta that commensurates with w -

**how can I derive an 'intelligent' start value for theta?**

In the radiation-dominated universe, I can write

[tex]D_A = \frac{1}{H_0(1+z)} \int_0^z \frac{dz}{\sqrt{\Omega_r(1+z)^4}}[/tex], where

[tex]a = \frac{1}{1+z} = \sqrt{2H_0\sqrt{\Omega_r}}[/tex]; I'm not sure where to integrate to though. Additionally, I'm wondering where the equation of state parameter [tex]w[/tex] comes in. Knowing D_A, I was wondering if there was something I need to consider for getting the sound horizon distance [tex]r_s[/tex]. From there, I want to get [tex]\theta[/tex]. Thanks in advance.

For now, I have (conservatively)

Code: Select all

```
H0_min = 1
H0_max = 150
param[theta] = 1.0411 1.00 1.08 0.0004 0.0004
param[w] = 0.33333 (but plan to put a prior on it).
```