Monthly Archives: December 2000

Cosmic Background Radiation

Before we start todays VERITAS here is a little something I read somewhere :


  “Beware of the man who works hard to learn something, learns it, and finds himself no wiser than before. Bokonon tells us that he is full  of murderous resentment for people who are ignorant and have not come by their ignorance the hard way” –Kurt Vonnegut.


      The topic of today’s veritas is cosmic background radiation.

As you know when we look at stars we do not see what is going on now .

We see what happened when light left their surfaces . The nearest star to earth is Alpha Centauri and it is 4 light years away . This means that when we look at it we see it  as it was 4 years ago .

So as we look deeper into space we also look further back in time .

Makes us wonder ….. if we look really really far can we see the time of creation itself …. the big bang ????


      In 1965 two young radio astronomers Arno Penzias and Robert wilson got some disturbance in their horn antenna . They could not identify the source of the disturbance . The disturbance seemed to be the same in all directions of the sky and had a temperature of 2.7 kelvin . They tried to find the source but failed ( They even considered the possibility that it was due to “little white dielectric substance”(pigeon droppings)) .

Later when they talked to Robert Dickie and Jim Pebbles , they realized that they had discovered what the Physicist George Gamow had predicted years ago : The cooled relic of something that had occurred billions of years ago : The Big Bang .


This radiation is called the cosmic background radiation . It is background because it is “visible” no matter where you “look” . It is present in all directions of the sky . It does not come from any nearby objects like galaxies . Imagine the whole Universe filled with a background of microwave photons .


      About 20 billion years ago , just after the big bang the Universe was very hot and it expanded very rapidly . All of nuclear physics happened in just the first three minutes . Within that time the constituents of matter had formed ( photons and other elementary particles ) . For the next 300,000 years nothing much happened . The Universe was a sea of ions ,electrons and photons . There was so much radiation(light ) that ions could not combine with electrons to form atoms . Each time an ion tried to grab an electron it would be zapped by a photon and they would saparate again .

But the Universe was getting cooler . After 300,000 years or so the radiation got weak enough and ions started combining with electrons to form atoms which later combined together to form lumps of matter . This is called the era of recombination . AFter this time the ability of matter to interact with the sea of background radiation kept decreasing . At that time the background radiation existed at 3000 Kelvin . Today it exists at 2.7 Kelvin.

So the cosmic background radiation is what is left of the last interaction of matter with the sea of photons .

      SO the cosmic background radiation is the collection of photons that hit us every second and are from a time billions of years ago .

So when you look at these photons you look back in time .


      Another property of the cosmic background radiation is that it

comes closer to perfect black body radiation than anything scientists

have ever been able to create in laboratories .


      In the early seventies it bacame clear that there is a small

difference in the temperature of the cosmic background radiation in

one direction . The difference is just a few milli Kelvin . This can

be explained in this way : Our local group of galaxies is moving

through the cosmic radiation in one direction at a rate of 600 Kms/sec.

The radiation towards which we are moving shows a doppler effect and

the frequency of light is shifted towards the blue end of the

spectrum ( blue shift ) and thus appears hotter . The radiation

we are moving away from shows a red shift and appears cooler .


      A satellite to explore the cosmic background radiation was launched

in 1989 . It is called COBE( cosmic background explorer ) . In 1992 it

found fluctuations(ripples) in the cosmic background radiation at the edge

of the universe . The radiation coming from this part is of the time 100,000

( or so ) after big bang . The radiation fluctuations support the theory

 that galaxies started forming at about that time .


      FOr a very interesting account of what happened in the first three

 minutes after big bang see a book by Steven Weinberg . It is called :

 “The first three minutes” . Wow book !