The Cosmic Background Radiation

In the document “The Beginning Of Time” we learned that a universe with a hot beginning would be filled with radiation today, with a temperature above 1K, more probably not too far from 5K. In “The First Three Minutes” we learned something about element building. Here we explore the radiation.

The early, hot, dense universe was opaque. The radiation trapped within it was constantly scattering, primarily from electrons. As the universe expanded, the radiation cooled, but always had a black body spectrum, a thermal spectrum.

At cosmic time t=380 thousand years the universe became transparent. This was caused by the ionized H & He finally becoming cool enough to combine with the electrons to become a transparent gas of neutral atoms. The temperature was about 3,300K, a few percent hotter than an incandescent light bulb filament. Looking far back in time, anywhere on the sky, we can see this horizon - the surface of last scattering.

Spectrum:

The 3,300K temperature is red-shifted to 2.73K. At that temperature the black-body radiation peak is at a longer wavelength than infrared, about 2 mm. The spectrum is shown below. Notice that the error-bars are 400 sigma. Normal 3 sigma error-bars would be too small to see! It is the only spectrum to ever receive a standing ovation from a room-full of astrophysicists.

Image Of The Radiation-A Baby Picture Of The Universe

The surface of last scattering is very uniform, but can’t be completely so. Several major satellite studies have characterized temperature variations. Shown below are results from the Wilkinson Microwave Anisotropy Probe (WMAP) Every point on the sky is shown as a microwave brightness. The tiny temperature differences are shown as different colors. Darker regions are cooler. The temperature differences are about 40 parts per million. These are correlated with density at small angular scales, and anti-correlated at larger scales. They contain an incredible wealth of information about the very early universe.

The density differences are necessary to provide for the gravitational clumping of the gas that became galaxies a little later. The cause of the density variations goes back to almost the beginning, even before the element formation.

WMAP Results

The large-scale patterns, including patterns of polarization shown in white above, prove that the geometry of the universe is flat, answering a long-standing cosmological question.

The knowledge that the universe is flat allows the calculation of its age: 13.7 billion years.

Its mass-energy density must therefore be exactly the critical density.

The lack of even greater clustering limits the amount of matter, including dark matter, to 27% of the critical density. Therefore there is a huge energy density of some sort. Its name at present reflects our ignorance--“dark energy”.

The percentage of primordial deuterium is very sensitive to the cosmic density of baryons. Deuterium can only be made in the Big Bang, and gradually is destroyed by stars. The deuterium abundance in early quasar spectra shows 4.4% of the critical density in baryons (H & He). Therefore, about 5 times that amount must be non-baryonic, exotic matter.

Even that 4.4% in about 3 times the matter in stars. It is outside the galaxies, and has been detected.

The uniformity of cosmic temperature, the significant variations in density, and the flatness of geometry all raised fundamental questions, even before WMAP. See the document “Cosmic Inflation”.