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Matthew Lewandowski
Joined: 10 May 2016 Posts: 2 Affiliation: Stanford University

Posted: May 10 2016 


I have tried to perform the following simple sanity check in CAMB, so as to better understand what is included in the output. Basically, I am comparing the total matter power spectrum (adiabatic mode) outputted by CAMB at two different times with the linear evolution (D^{2}) from one time to the other. I think that these two things should exactly match, since CAMB is solving the linear equations.
In equations, I think that
should be equal to 1 for all k that are inside the horizon, and as long as z_{1} and z_{2} are within matter domination. The D(z) here is the linear growth factor which is a solution for linearized continuity and Euler equations. The result was that there is a 4% difference from z = 100 to z = 0, and a 0.5% difference from z = 10 to z = 0, in the range k / h from 0.1 to 100
Does anyone know what's going on here? 

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Antony Lewis
Joined: 23 Sep 2004 Posts: 1239 Affiliation: University of Sussex

Posted: May 10 2016 


At high redshift there are effects from radiation and decaying modes. You may have a massive neutrino contributions (massive neutrinos also affect the background evolution, so may depend on how you defined D(z)). And as k gets large you can start to see linear effects of baryons (pressure of baryons becomes large at k~500). 

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Matthew Lewandowski
Joined: 10 May 2016 Posts: 2 Affiliation: Stanford University

Posted: May 19 2016 


Hi Anthony, thank you for your quick reply. I am now pretty sure that the mismatch was indeed due to radiation effects. At z=100, these can have percent level effects, and I think that is what I was seeing. Thanks!
Matt 

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