Question

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

Answer 1

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`