On the WMAP image of the cosmic microwave background radiation, are the higher density irregularities the brighter bits or the darker bits?

1 Answer
Mar 2, 2016

The colour code shows fluctuations of CMBR temperature about its mean value of \bar{T}=2.725\quadK. The pixels coloured red / deep blue represent points that are 200\quad\muK above/below the average value. Higher temperature indicates higher concentration of matter .

Explanation:

The averaged CMBR temperature is \bar{T} = 2.725\quadK

Given below is a map of temperature fluctuations about this average value, created from the 9 year data of WMAP. In this map the temperature fluctuations about the average value are shown in a scale of \pm 200 \muK.

Colour Code:

The pixels in red represent temperatures that are 200\muK above the average. i.e T-\bar{T} = +200 \quad\muK

The pixels in dark blue represent temperature that are 200\muK below the average. 1.e. T-\bar{T} = -200 \quad \muK

What do they mean?: Higher temperature indicates higher matter density.

Density Contrast Field: Fluctuations in mass density are quantified by the density contrast field (\delta\rho)/\rho.
(\delta\rho)/\rho \equiv \frac{\rho-\bar{\rho}}{\bar{\rho}}, where \rho is the density at a point and \bar{\rho} is the average matter density.

Temperature Contrast Field: Fluctuations in CMBR temperature are quantified by the temperature contrast field (\deltaT)/T
(\deltaT)/T\equiv\frac{T-\bar{T}}{\bar{T}}, where \bar{T} is the average temperature and \deltaT is the fluctuations about this average.

There is direct relation connecting the two which can be deduced from the fundamental principles:

(\delta\rho)/\rho \propto (\delta T)/T

http://map.gsfc.nasa.gov/media/121238/index.htmlhttp://map.gsfc.nasa.gov/media/121238/index.html