the CMB power spectrum tells us

The CMB is a faint glow detected by radio telescopes, indicating a nearly uniform background radiation across the universe. This uniformity is a key piece of evidence supporting the Big Bang theory, as it suggests a common origin for all matter in the universe.

The CMB provides crucial information about the age, composition, and geometry of the universe. By studying the CMB power spectrum, scientists can infer the universe's age, as the peak frequencies correspond to the time when the universe was about 380,000 years old. Additionally, the spectrum reveals the universe's composition, showing the relative amounts of normal matter, dark matter, and dark energy.

Understanding the CMB power spectrum also helps determine the universe's geometry. The angular size of the first peak in the power spectrum indicates whether the universe is flat, open, or closed. A flat universe corresponds to a total density equal to the critical density, which is consistent with current observations.