Explain that the Cosmic Microwave Background Radiation (CMBR) was produced shortly after the Universe was formed and that this radiation has been stretched into the microwave region of the electromagnetic spectrum as the Universe expanded.
The CMBR is the faint glow of light that fills the Universe. It was created when the Universe was only about 380,000 years old – a time called the recombination epoch – when electrons and protons finally combined to form neutral hydrogen, allowing photons to travel freely for the first time.
Think of the Universe as a giant balloon that is slowly inflating. The photons that were released at recombination are like tiny dots on the balloon’s surface. As the balloon inflates, the dots move apart, and the distance between them (their wavelength) gets longer.
Initially, the photons had a temperature of about 3,000 K, which corresponds to the visible light region of the spectrum. As the Universe expanded, the wavelength of each photon stretched by a factor of about 1,000, cooling the radiation down to just 2.725 K. This is now observed as microwave radiation.
Mathematically, the relationship between the temperature T and the wavelength λ of a black‑body spectrum is given by the Wien’s displacement law:
\$ \lambda_{\text{max}} = \frac{b}{T} \$
where b ≈ 2.9 × 10⁻³ m·K. When T drops from 3,000 K to 2.725 K, λ_max shifts from the visible to the microwave region.