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Why can we see the cosmic microwave background (CMB)?

We all have seen evidence of radiation left from the Big Bang, but how is it still detectable? Why didn't it disperse early on?

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Related: physics.stackexchange.com/q/25070/2451 –  Qmechanic May 8 '12 at 6:43
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marked as duplicate by Qmechanic, Manishearth Dec 11 '12 at 11:40

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Disperse into what? All of space was completely filled with hot matter and radiation. Space has expanded enough that the incredibly high temperatures that once were the equilibrium could cool down to the current temperature of the Cosmic Microwave Background, 2.725 Kelvin. That is very cold, but certainly not absolute zero! Moreover, it is certainly detectable.

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The CMB is not itself from the Big Bang but rather the residual glow from the early intense radiation stage of the universe after the Big Bang.

The radiation has been and still is dispersing in every direction as the universe expands. Eventually in the distant future as the universe gets old and dark it will have dispersed and red shifted so far as to be undetectable by any would-be astronomers that might still be around.

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The thing is that it ought to still be detectable.

According to the Big Bang theory, one of it's predictions is that the universe should still be laced with these remnants, this cosmic background radiation - should it not be, it would be troubling for the theory - so the agreeable factors aid us in asserting our understanding (well, disagreement would improve our knowledge, too, but might end up leaving us more perplexed to start with) of our universe, how it came about and how it continues to evolve.

If you imagine the primeval fireball exploding, the fallout of shortwave radiation being emitted with an immensely high temperature (of up to trillions of degrees), like cosmic bombs in each and every direction within the expanding space, but with no place to escape, then our understanding of nature dictates it would continue to populate the available space, cooling down and populating the universe in a uniform manner; in our current time we expect the radiation to be in the form of microwave radiation with a much cooler temperature (a few degrees above absolute zero, and characteristic of radiation by a black body) as it impacts on the Earth, and it does.

Eventually it will become undetectable, but that's a long time coming insofar as humanity is concerned.

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Well, it's been dispersing ever since the Universe became transparent to most radiation 380000 years after the Big Bang, 13.8 billion years ago. It disperses as the Universe expands. Right now, we see the background as very cold, just 2.7K (degrees Kelvin) above absolute zero. Eight billion years ago, long before the Sun was formed, it was twice as strong, around 5.4K. Further back in time, it was even stronger and back when our Milky Way galaxy formed, it was over 20K, in the infrared, and would have impeded star formation.

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