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

13 March 2011

Question

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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