Why is cosmic background radiation a microwave?

Answer 1

The cosmic background radiation has cooled down to microwaves.

Soon after the big bang, the universe was very hot and filled with high energy gamma rays. The universe was much smaller then.

As the universe expanded, the background radiation got progressively red shifted. This has resulted in all of the background gamma rays from the early universe getting longer wavelengths and reduced energy.

Now, the universe has expanded so much that the background radiation is in the microwave region of the spectrum.

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Answer 2

Christopher Ales

Because of the universe's expansion, high-energy radiation that was first released shortly after the Big Bang has been redshifted into the microwave region of the electromagnetic spectrum, where it is now observed as microwaves.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.