The cosmic microwave background (CMB) is a faint glow of microwave radiation that fills the entire universe. It is one of the strongest pieces of evidence for the Big Bang theory.
What is the CMB?
The CMB is electromagnetic radiation in the microwave region of the spectrum. It was first detected in 1965 by Arno Penzias and Robert Wilson.
It has a temperature of approximately 2.7 K ( C) and is remarkably uniform in all directions (isotropic), with tiny variations of about 1 part in 100 000.
Why is the CMB evidence for the Big Bang?
According to the Big Bang theory:
- The early universe was extremely hot and dense, filled with high-energy radiation.
- As the universe expanded, this radiation was stretched to longer wavelengths (redshifted).
- After about 13.8 billion years of expansion, the radiation has been stretched from its original high-energy form to the microwave region.
- The 2.7 K temperature is consistent with the predicted cooling of the universe.
The CMB is essentially the “afterglow” of the Big Bang.
Why this supports the Big Bang over alternative theories
- The CMB comes from every direction equally, which is consistent with the entire universe being very hot at an early time.
- The steady-state model (which proposed no beginning and continuous creation) could not explain why this radiation exists or why it has this specific temperature.
- The detailed pattern of tiny temperature variations in the CMB matches predictions from Big Bang cosmology.
Common errors and how to correct them
- Saying the CMB is visible light. It is microwave radiation (much lower frequency than visible light).
- Confusing the CMB with background radiation from radioactive sources on Earth. The CMB is cosmic in origin; background radiation on Earth is from radon, rocks and medical procedures.
How to apply this in an exam
Explain that the CMB is microwave radiation detected in all directions. State that it is the cooled remnant of the very hot early universe. Explain how the expansion of the universe stretched the original high-energy radiation to microwaves.
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