CosmoCoffee

[Angel Torres Rodriguez]

Hello

I am using camb to obtain the ISW cross-correlation spectra of the CMB with a certain mass tracer (use do_lensing=true and a modified tracer).

When I run the code for different values of w around a fiducial model, say: 1%, 2%, 5%, etc., I get a noticeable scatter that hinders me from deriving the slope around the fiducial (see plot). Increasing the accuracy_boost improve things a bit but I have to go higher than 3 and things get very, very slow.

Can anyone suggest how to tackle this problem?
thank you

http://roger.phy.unp.ac.za/~angel/cross.eps

[Ben Gold]

It looks to me like you're trying to determine changes in C_l which are less than one percent. I don't know as much about CAMB's accuracy, but I'd get very suspicious of any result out of cmbfast that relied on things which changed the C_l by such a small amount. I suspect you'll have to do at least one of the following:

1. Look at much larger changes in your parameter (w)
2. Turn the accuracy way up and cope with the slowdown (and hope that there aren't unknown errors in the code at the sub-percent level)
3. Come up with some kind of analytic or semi-analytic technique to quickly calculate dC_l / dw to the precision you need (looks like 0.1% or better)

[Angel Torres Rodriguez]

Thanks Ben

I was thinking about your first solution. I agree, it might be better to look at larger variations of w, then fit a more general curve and then get the derivatives at the point of interest.


Angel