CAMB - numerical precision for different w

Use of Cobaya. camb, CLASS, cosmomc, compilers, etc.
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Angel Torres Rodriguez
Posts: 2
Joined: February 22 2006
Affiliation: Univeristy of KwaZulu-Natal - South Africa

CAMB - numerical precision for different w

Post by Angel Torres Rodriguez » December 13 2006

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
Posts: 81
Joined: September 25 2004
Affiliation: University of Minnesota
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CAMB - numerical precision for different w

Post by Ben Gold » December 14 2006

It looks to me like you're trying to determine changes in [tex]C_l[/tex] 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 [tex]C_l[/tex] 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 ([tex]w[/tex])
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 [tex]dC_l/dw[/tex] to the precision you need (looks like 0.1% or better)

Angel Torres Rodriguez
Posts: 2
Joined: February 22 2006
Affiliation: Univeristy of KwaZulu-Natal - South Africa

CAMB - numerical precision for different w

Post by Angel Torres Rodriguez » December 14 2006

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

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