Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I've understood that the Cosmic Background Radiation(CBR) is an electromagnetic wave that originated from the big bang. However, we now live on a planet which that is also originating from the big bang.

Why does that Cosmic Background Radiation reaches us now? Why does CBR reaches us now, and not a couple of billion years earlier?

share|cite|improve this question
up vote 1 down vote accepted

The cosmic background radiation was always with us, it is not reaching us now. It just became cooler and cooler as time went on.

One has to understand that when we are talking big bang and general relativity we are talking of the universe starting from one (x,y,z,t) point and as time goes on, expanding. This means that all (x,y,z) points of our universe trace back to that one point singularity that went "bang".

Envision the two dimensional surface of a balloon, as shown in the wiki link. At time=0 the balloon is one point, call it r=0. As it expands the points on its surface start receding from each other, and all points on that surface were at r=0 at t=0. Their neighborhood expands as the balloon blows up, and this means the electromagnetic radiation that started in the earth's neighborhood hot, cools due to the expansion and becomes the Cosmic Microwave Background. That is what I mean it is not coming from anywhere, it just is.

share|cite|improve this answer
Thanks, I think I'm grasping it. – Erik Jun 23 '12 at 14:00

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.