COSMIC MICROWAVE BACKGROUND RADIATION

Cosmic microwave background (CMB) radiation is a type of electromagnetic radiation that pervades the universe. If you observe stars and galaxies using a traditional optical telescope, the space between them (the background) is pitch black. But observe the same with a radio telescope and there is a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the Electromagnetic Spectrum, hence the name cosmic microwave background radiation.

Discovery of the CMBR

In 1965, Arno Penzias and Robert Woodrow Wilson of Bell Telephone Laboratories had built a radiometer that they intended to use for radio astronomy and satellite communication experiments. Their instrument had an excess 3.5 K antenna temperature which they could not account for. Later on, experts determined that the antenna temperature was due to the microwave background. Wilson and Penzias were honored with the 1978 Nobel Prize in Physics for their pioneering discovery.

Why the CMBR exists?

The CMBR is well explained as residual radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, much before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the radiation and the plasma filling it grew cooler. When the universe cooled enough, stable atoms could form. These atoms could no longer absorb the thermal radiation (the CMBR), and the universe became transparent instead of being an opaque fog i.e. when viewed through a radio telescope, the radiation can be seen. The photons of the CMBR that existed at that time have been propagating ever since, though growing fainter and less energetic, since the exact same photons fill a larger and larger universe (as the Universe is continuously expanding).

Significance

The CMBR is like the echo left by the Big Bang. It has existed ever since the formation of the Universe and this means that, by studying it’s distribution, we can get a snapshot of the entire Universe. That is exactly what they did in the late 1980's. NASA launched the Cosmic Background Explorer spacecraft (COBE). The main scientists were John Mather and George Smoot. (Nobel Prize Winners in 2006). Their goal was to take photos of the leftovers of the Big Bang, to measure the remnants of the energy that still exists from that explosion 13 billion years or so ago. The Mission was a success and the Scientists were able to obtain the first picture of the Universe.

 Questions to Ponder

• Why is the CMB not absorbed by interstellar hydrogen?
• Why is the WMAP (Wilkinson Microwave Anisotropy Probe) picture of the CMB an ellipse?