# Temperature of the CMB, from WMAP

1. Aug 12, 2008

### Barnak

Maybe someone could help me on a technical detail and the WMAP (the latest "five years" results). On this web site (and several others too), you can find some very nice textures of the CMB from the WMAP study :

http://lambda.gsfc.nasa.gov/product/map/current/sos/

Then, they say (read the WMAP 5-Year CMB Map paragraph) that the CMB temperature is 2.725 Kelvins, with a fluctuation (blue and red spots) of 0.0002 Kelvins around the mean value. Well, I'm just wondering about the number of digits here :

T = (2.725 ± 0.0002) Kelvins ??

Is it really (2.7250 ± 0.0002) ? If it's really 2.725x, then what about the "x" value ?

Here is the complete list of 5 years legacy papers
http://lambda.gsfc.nasa.gov/outreach/recent_papers.cfm

I can read that the thermal fluctuations are about 200 microKelvins, so +/- 0.0002 K is okay. But what about the mean value ?

I want consistency !

Last edited by a moderator: Apr 23, 2017
2. Aug 19, 2008

### Quasar2287

Wouldn't the mean value of the 2.725 $$\pm$$ 0.0002 degrees K be 2.7250 degrees K since its the mean value of the values given since 2.7252 and 2.7248 are the upper and lower bounds? I don't think I understand your question 100%. Just out of curiosity what exactly is it that you're looking for?

3. Aug 19, 2008

### marcus

The figure I have is from Mather et al
http://arxiv.org/abs/astro-ph/9810373
"...Improvements in understanding the temperature measurements of the calibrator allow an improved CMBR temperature determination of 2.725 +/- 0.002 K. "

So I believe 2.725 +/- 0.002 kelvin for the aggregate whole sky black body temperature and I ALSO believe 200 microKelvin for the regional fluctuations in the map.

I think that they measure percentage fluctuations in various sky directions and definite frequency bands with pretty fine accuracy, but lumping it all together and trying to extract a single temperature for the whole sky destroys most of the accuracy. For an aggregate temperature I expect the best they can do is 2.725 +/- 0.002. Maybe that is all the accuracy one needs anyway. It amounts to curve fitting----fitting Planck black body over a whole lot of frequency bands. Inherently approximate.

When I look at the data I see it all broken down into frequency or wavelength bands. Or into sky directions. I can believe that they can tell me very accurately what the percentage range of variation is in a given band around the central value for that band.

I believe they can tell me +/- 0.0002 K regional variation even though they can't do better than 0.002 K when naming an aggregate whole sky black body value. So I believe they can tell the variation with direction in terms of microKelvin, even with only a crude central value.

I don't know where that figure of 2.725 +/- 0.0002 that you quote comes from. misprint maybe, or someone in the outreach or public relations department??? If you find a more reliable source please let me know! All I know about is Mather's 2.725 +/- 0.002.

maybe someone more knowledgeable will help us out

Last edited: Aug 19, 2008