Science Questions

How is it possible for the microwave background radiation to be "left over" from the Big Bang?

Sun, 13th Mar 2011

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Question

Jim Morris asked:

Hello!!

 

I've recently become an avid listener to both The Naked Scientist and The Naked Astronomy podcasts. I find both very interesting and entertaining. Keep up the good work.

 

Here's my question: Since microwaves travel at the speed of light, how is it possible for the microwave background radiation to be "left over" from the Big Bang?

 

It seems that the microwaves would have traveled out in front of all of the matter in the universe as it expands. Did it "bounce" off of "something" or am I just missing an obvious point?

 

Thank you for your time

 

Jim Morris

 

Tampa, Florida USA

 

Answer

Dave -   That's a really interesting question.  As far as we know, the universe is effectively infinite.  The Big Bang wasn’t an explosion in one place in space and stuff moving out in the rest of the space.  It was the fact that all of space was just a lot smaller.  So, everything was a lot closer together.  Because the universe was infinite, however far back you go, there’ll always be some more universe further away, so the light could’ve travelled from somewhere just slightly further away - so it can travel through more universe and get to us.

The other thing with the cosmic microwave background radiation was that it didn’t date from the very beginning of the Big Bang.   It’s actually light given off when electrons joined up with protons to form hydrogen atoms and joined up with helium nuclei to form helium atoms, and that released a lot of x-ray or ultraviolet light, and that happened about 380,000 years after that very, very violent beginning of the Big Bang.  So by that point, the universe was actually, already quite big.  We just happen to see that out 13 or 14 billion lightyears away.  So, that's the edge where light is travelling towards us from. As we wait in a billion year’s time, we will see that light coming from another billion lightyears further away.  And so, as far as we know, if the universe is infinite, we’ll keep on seeing it forever because the light will have just travelled further, so, for a longer time.

Chris -   I mean the point is that as you say, the universe was created everywhere all at once in those early days.  So every bit of it is emitting radiation, and therefore, as it expands and grows, and then 13 billion years later, here we are.  We are seeing light which is coming from one side of it, the opposite side to which we are.  And so, as a result, there are still stuff coming our way be it was coming from everywhere all at once.

Dave -   And as far as we know, it will keep on coming.

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Jim Morris asked the Naked Scientists: Hello!! I've recently become an avid listener to both The Naked Scientist and The Naked Astronomy podcasts. I find both very interesting and entertaining. Keep up the good work. Here's my question: Since microwaves travel at the speed of light, how is it possible for the microwave background radiation to be "left over" from the Big Bang? It seems that the microwaves would have traveled out in front of all of the matter in the universe as it expands. Did it "bounce" off of "something" or am I just missing an obvious point? Thank you for your time Jim Morris Tampa, Florida USA What do you think? Jim Morris , Tue, 15th Mar 2011

Your answer to Jim in Tampa about the Cosmic Background Radiation was a bit incomplete. Something that I still have a hard time wrapping my mind around is that while nothing in the universe can move faster than light, the universe itself can and does expand faster than light. So, since the places from which the CBR that we now see emanated are currently moving away from us faster than the light itself, we can view the light. The other result of this fact is that while the CBR won't "run out" per se, there will come a time that all part of the universe from which CBR might reach us are all moving away from us faster than the light is moving, so we _will_ stop seeing it (contrary to what was said on the show). engineer27, Mon, 4th Apr 2011

The ''universe'' in nothing more than the set of objects that interact between temselves. If we see the distance of a far away object increasing with time faster than the velocity of light, and as all velocity, save that of light, is relative to the velocities of other objects, what does it mean that the -set- of objects interacting has a velocity in the saparation of these objects? Antonio, Mon, 14th Sep 2015

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