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Chapter 39: Problem 18

The radiation that we observe as the cosmic microwave background started out largely as infrared. Why is it now the microwave background?

Short Answer

Expert verified
The radiation of the Cosmic Microwave Background (CMB) started largely as infrared. However, due to the expansion of the Universe and the associated cosmological redshift, the energy and frequency of the radiation decreased, stretching it into the microwave part of the electromagnetic spectrum. That's why today, we observe it as microwave radiation, hence the name Cosmic Microwave Background.

Step by step solution

01

Cosmic Microwave Background (CMB)

The CMB is the leftover radiation from the Big Bang. This radiation filled the entire Universe and since the Universe was very dense, it was primarily treated as 'opaque' to this radiation.
02

Expansion of The Universe

As the Universe started to expand, it cooled down and the density decreased over time. This cooling and expansion caused the energy of this radiation, and thus its frequency, to decrease.
03

Cosmological Redshift

The decrease in frequency and energy due to the expansion of the Universe is termed as 'cosmological redshift'. Back when the Universe was dense and hot, the radiation was primarily in the form of infrared light. But the expansion over billions of years has 'stretched' this radiation to longer wavelengths, pushing it into the microwave part of the electromagnetic spectrum.
04

Current Observation

Today, when we observe this radiation, we see that it's predominantly in the microwave region of the electromagnetic spectrum. Hence, we refer to it as the Cosmic Microwave Background, even though it started out as largely infrared.

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