The Glow From the Past

Almost all of our knowledge about the Universe relies heavily on the understanding of the Cosmic Microwave Background Radiation (CMBR). CMBR has now become an important source of information for the nature, evolution and constituents of the Universe. Its discovery was one of triumphs of the classical Big Bang theory, and the discovery of fluctuations in its isotropy was among the most convincing evidences for the Inflationary Big Bang model. Naturally, any discussion on cosmology doesn’t start without referring to the CMBR, for it’s the most ancient light that we can see.

Background about Background Radiation

To understand the origin of the CMBR we have to go back a little to the expansion of Universe. In 1924, Edwin Hubble discovered that the Universe as a whole is expanding. Based on these observations he established a law, Hubble’s law, according to which the farther a galaxy is, the faster is it receding from us. So, as one rewinds the clock, all the matter (or energy) that we see in the Universe, must have been localized at a tiny place. It is the extrapolation of the Hubble’s law back in time, which gives the age of the Universe [i.e. from the time of the Big Bang] to be 13.75 billion years.

The farther we look into the space, we peer further back in time. The light from the distant galaxies are hugely red-shifted. Computer simulations conclude that the stars with the highest redshifts, emerged when the universe was about 100 million years old (Turner, 2009). Before that time, the Universe went through a time where there was no light. There were no stars, no galaxies - just a featureless gruel of hydrogen, helium and dark matter. Fluctuations in the mass densities, caused initial clumping of matter, which became larger and larger, finally culminating into stars and galaxies that we see (This description is oversimplified. Formation of the early Universe is in itself an interesting topic). Radiation from these stars and galaxies are greatly red-shifted and are detectable only in radio frequency range.

After-glow of a Hot Past

Beyond these dark ages is the glow of the hot Big Bang. This has a redshift of about 1,100. That redshift pushes the frequency of this radiation, right into the Microwave region. What one sees, is a wall of microwave radiation which fills the sky almost uniformly. And this is the Cosmic Microwave Background Radiation (CMBR). CMBR was discovered in 1964 by Arno Penzias and Robert Wilson. Using Hubble’s Law, one concludes that this time corresponds to about 380,000 years after the Big Bang.

CMBR thus gives us a glimpse of how the Universe looked in the early epochs, and the physical processes which were predominant during that time. Any model of the early Universe should explain the features of the CMBR, and any predictions from the model should agree with CMBR observations. It thus gives us the way to model the Universe and understand its evolution.

Features of CMBR

The intensity of CMBR is same in all direction. Penzias and Wilson measured the wavelength of this radiation to be around 7.3 cm. If one assumes that the radiation spectrum is from a blackbody, this corresponds to a temperature of 3.5 ± 1.0 K. These predictions have been refined and the currently accepted temperature is 2.73 K.

There are broadly three different features of CMBR which provide direct and indirect evidence about the origin of the Universe

Angular Dependence– Intensity of radiation is almost isotropic, but has a small dependence on the direction.

Spectrum– The radiation is almost Planckian [i.e. obeys Planck’s law of radiation], but has slight deviation.

Polarization– The degree of polarization is slightly different in different directions.

In next post, we shall discuss about the origin of CMBR, and each of its features, especially the isotropy, in detail.

Citations

Turner, M. S. (2009, September). The Origin of the Universe. Scientific American.