The Cosmic Microwave Background Radiation

Cosmology is the study of the physics of the universe from its birth to its ultimate fate. As in any science, there is a relationship between theory and experiment in cosmology. Though there are several theories of how the universe began, the most widely accepted is the Big Bang Theory. It postulates that the universe began at an infinitesimally small point and then expanded outward, eventually cooling and forming galaxies, stars and planets.

Why is the big bang theory so thoroughly accepted? This is where experiment comes into play. If the big bang theory were true, we would expect to see certain characteristics in the sky. First of all, the big bang predicts that the universe is expanding. If this is true, then we should see all other galaxies moving away from us with a speed that depends on how far away the galaxy is from us. In fact, Edwin Hubble saw this in the 1920's with his telescopes, and the interpretation was that things were indeed moving farther apart. This revelation led Einstein to admit that he had made a great mistake.

The second major thing that the big bang should produce is a characteristic radiation spectrum to be seen in the sky. If the universe were once much smaller and hotter than it is now, and if it has been expanding ever since the beginning, then it must also have been cooling down this whole time. (Imagine a gas that expands adiabatically, if the volume increases, the temperature must decrease). Thus, if we know roughly how old the universe is and what it's expansion rate is, we can form a pretty good guess of what it's current thermodynamic temperature must be. Measurements of these sorts first occured in the 1960's. In 1989, a satellite named COBE measured the spectrum of the left-over radiation from the big bang and the result is below:

The solid line above is actually the theoretical prediction for the radiation's spectrum. It has a special name: Blackbody radiation. The data from COBE has data points that all lie so close to the theoretical prediction that the points have error bars that lie within the solid curve above! The blackbody spectrum above is that of a radiation field at a thermodynamic temperature of 2.728 K. (in other words, the radiation from the big bang has cooled to -429 degrees fahrenheit!)

With blackbody radiation, there is a correspondance between thermodynamic temperature and radiation frequency. For example, when you turn on the burners on your stove top, they glow red. This is because they are so hot they are emitting blackbody radiation that is visible red light. A radiation field at 2.728 K is really just microwaves. The universe initially had radiation of an infinitely small wavelength, but the expansion has "stretched" the radiation out and we now see microwaves. This is another type of redshift. Thus, the remnant light from the big bang is called the cosmic microwave background radiation (CMB).

Theory predicts that the big bang would also have produced some simple elements; hydrogen, helium and deuterium being the most common, and these elements would have been produced in very specific ratios. These ratios are also seen in various sky studies. Thus, the observed recession of the galaxies, the CMB and the abundance of certain light elements are the three so called pillars of big bang cosmology. No other theory can explain these three observed phenomena as well as the big bang theory can, and this is why it is so widely accepted.

Of the three pillars, the CMB encodes the most information about the nature of the universe that we live in, and hence it has become widely studied by many experimentalists, including us. The CMB is remarkable in that no matter which direction in the sky you look, it appears to be almost the same temperature. This is called isotropy.

The above is a map of the entire sky shown on a scale of 0 to 4 K. The color of each pixel in the map above corresponds to the pixel temperature. It seems to be a perfectly uniform temperature distribution in the radiation field. However, the fact that we see galaxies and stars in the sky today tells us that there must have been some "clumpiness" in the matter distribution in very early times. This uneven matter distribution should have left an imprint on the radiation distribution of the time. Thus, we should see tiny irregularities in the CMB today which represent these ancient matter inhomogeneities.

Another set of instruments on the COBE satellite were designed to look for these irregularities in the CMB; they were called the Differential Microwave Radiometers. If there were to be irregularities in the CMB, they could be seen as tiny hot and cold variations on the sky. In 1992, the COBE research team announced that it had evidence that these hot and cold spots did exist, and they released the map below.

The temperature fluctuations are extremely small, their amplitude has an rms value of 1 part in 100,000 on angular scales of 10 degrees on the sky. Only within the last few years has receiver technology progressed to the point that such tiny variations were even detectable. Since COBE, a number of other groups have also reported detecting anisotropies in the CMB at various angular scales.

Polarization in the CMB

Even though the universe is now a chilly 2.728 K, it was once much hotter. In fact, from the moment of the initial big bang singularity until nearly 100,000 years later, the universe was hot enough so that electrons and protons had too much energy to come together and form neutral atoms. What existed was a "primordial soup" full of free electrons, protons and neutrons.

When radiation sees free electrons, it bounces around off the charges like a pinball. We can't see radiation from this period, because it was so tightly coupled to the free electrons, and thus the early universe is said to be opaque. However, when the universe had cooled down to just the right temperature, the electrons started to bind to the protons to form atoms. This epoch is called "recombination." When the free electrons became bound in atoms, they were no longer able to collide with the background radiation, and hence this background radiation was able to travel unimpeded to us and our sophisticated detectors. Therefore, the recombination era is referred to as the Surface of Last Scattering, and the CMB radiation that we see today encodes the information of the universe from exactly this time.

Recall that the background radiation was anisotropic, even in those early days. When radiation that has an anisotropy is incident on free electrons at the time of last scattering, the resultant radiation becomes polarized at a faint level.

In this picture, radiation that is "hot" is blue and "cold" radiation is red. When this kind of anisotropic radiation is incident on an electron (green), you get a scattered wave that is linearly polarized.

This polarization depends on the anisotropy amplitude and also electron density at last scattering among other things, and theory predicts that the CMB radiation will be polarized at a level between 1% and 10% of the anisotropy amplitude. In other words, we are looking for signals where our antenna temperature must be sensitive to variations of only a few micro kelvin at most. This is no easy task.

DMR and FIRAS images courtesy of the NASA Godard Space Flight Center and the COBE Science Group.

Diagram of Thomson Scattering by Wayne Hu.

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