Cosmic Microwave Background

The Cosmic Microwave Background (CMB) radiation is a very faint but observable form of radiation that is coming to us (and to other places too) from all directions. (By ‘radiation’ here is meant photons of light, or electromagnetic waves, from a wide range of possible frequencies or energies.) In today’s standard model of cosmology, this radiation is believed to emanate from about a time 200,000 to 400,000 years after the Big Bang – a timeframe known as ‘last scattering’ because that was when superheavy collisions between photons of light and other particles (electrons, protons, neutrons, etc) eased off to a degree that photons can ‘escape’ into the expanding space. At the time of last scattering, this radiation was very hot, around {3000}^{\circ}K (in the Kelvin scale). And in time, as space expanded, radiation cooled to its currently observed value of 2.726^\circ K.

One of the amazing facts about this radiation is that it almost perfectly matches Planck’s radiation formula (discovered in 1900) for a black body:

\displaystyle I(\nu) = \frac{8\pi h}{c^3} \cdot \frac{\nu^3}{e^{h\nu/kT} - 1}.

In this formula, \nu is the frequency variable (a positive real number that gives the number of cycles per second of a wave) and I(\nu) is the energy density as a function of frequency \nu.

The other variables are: T is the temperature of the black body which is assumed to be in equilibrium (so the temperature is uniformly constant throughout the body of radiation), c is the speed of light in vacuum, h is Planck’s constant, and k is Boltzmann’s constant from statistical mechanics.

If you plot the graph of this energy density function (against \nu) you get a curve that looks like a skewed ‘normal distribution’. Here are some examples I hijacked from the internet:

Various Planck radiation density graphs depending on temperature T.

Here you see various plots of Planck’s function for different temperatures T. The horizontal axis labels the frequency \nu, and the vertical gives the energy density I(\nu) per frequency. (Please ignore the rising black dotted curve.)

You’ll notice that the graphs have a maximum peak point. And that the lower the temperature, the smaller the frequency where the maximum occurs. Well, that’s what happened as the CMB radiation cooled from a long time ago till today: as the temperature T cooled (decreased) so did the frequency where the peak occurs.

To those of us who know calculus, we can actually compute what frequency \nu gives maximum density and give our answer in terms of the temperature T. All we do is compute the derivative of I(\nu) with respect to \nu and set it to zero and solve the resulting equation for \nu. You will get an equation whose solution isn’t so trivial to solve, so we’ll need some software or a calculator to approximate it. Anyway, I worked it out (and you can check my answer) and obtained the following:

\displaystyle\nu_{\max} = 2.82 \frac{kT}{h}.

(The equal sign here is an approximation!)

The \nu_{\max} is the frequency that gives maximum density and as you can see it is a straight linear function of temperature. The greater the temperature, the proportionately greater the max frequency. The colder the temperature gets the smaller the max-frequency \nu_{\max} gets, and from Planck’s energy equation E_{\max} = h\nu_{\max}, so also does the energy of the radiation drop.

Now plug in the observed value for the temperature of the background radiation, which is T = 2.726 (degrees Kelvin), and working it out we get (approximately)

\displaystyle\nu_{\max} = 160.2 \text{ GHz}

This frequency lies inside the microwave band which is why we call it the microwave radiation! (Even though it does also radiate in other higher and lower frequencies too but at much less intensity!)

Far back in time, when photons were released from their collision `trap’ (and the temperature of the radiation was much hotter) this max frequency was not in the microwave band.

Homework Question: what was the max-frequency \nu_{\max} at the time of last scattering? What frequency band does it belong to? In the visible range? Infrared? Ultraviolet? Higher still? (I’m dying to know! 😉 )

(It isn’t hard as it can be figured from the data above.)

Anyway, I thought working these out was fun.

The CMB radiation was first discovered by Penzias and Wilson in 1965. According to their measurements and calculations (and polite disposal of the pigeons nesting in their antenna!), they measured the temperature as being 3.5^\circ K plus or minus 1 Kelvin. (So the actual value that was confirmed later, namely 2.726, fits within their range.) The frequency of radiation that they detected, however, was not the maximum yielding one but was (as they had it in the title of their paper) 1080 Mc/s — which is ‘mega cycles per second’ and is the same as MHz (megahertz). The wavelength value corresponding to this is 7.35 cm. To do the conversion between wavelength \lambda and frequency \nu for electromagnetic waves we use the simple formula

\displaystyle\lambda\nu = c

where c is the speed of light (in vacuum).

And that’s the end of our little story for today!

Cheers, Sam Postscript.

The sacred physical constants:

Planck’s constant h = 6.6254 \times 10^{-27} \text{ erg sec}

Boltzmann’s constant k = 1.38049 \times 10^{-16} \text{ erg/K}

Speed of light c = 2.9979 \times 10^{10} \text{ cm/sec}.  

  1. June 1, 2015 at 10:05 am

    For more on the CMB, see Stephen Weinberg’s book “Cosmology.”

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