The cosmic microwave background, or CMB, is faint microwave light that fills the universe in every direction. It is often called the afterglow of the Big Bang because it comes from a time when the early universe first became transparent. This radiation matters because it gives astronomers a snapshot of the universe when it was about 380,000 years old.
By studying it, scientists can test ideas about the origin, age, shape, and contents of the cosmos.
In the early universe, hot plasma scattered light constantly, so photons could not travel freely. As the universe expanded and cooled, electrons joined with protons to form neutral hydrogen, allowing light to stream through space. That ancient light has been stretched by cosmic expansion from visible or infrared wavelengths into microwaves, giving it a nearly perfect blackbody temperature of about 2.725 K today.
Tiny temperature differences in the CMB, only about 1 part in 100,000, reveal the seeds of galaxies and large scale structure.
Key Facts
- The CMB was released about 380,000 years after the Big Bang, during the era called recombination.
- Today the CMB has an average temperature of T = 2.725 K.
- Cosmic expansion stretches light, so wavelength increases by a factor of 1 + z, written λobserved = λemitted(1 + z).
- The CMB redshift is about z = 1100, meaning the universe has expanded by about 1100 times since the light was released.
- The peak wavelength of a blackbody is given by Wien's law: λmax = 2.90 × 10^-3 m K / T.
- CMB temperature variations are tiny, with typical differences of about ΔT/T ≈ 10^-5.
Vocabulary
- Cosmic microwave background
- The cosmic microwave background is ancient radiation from the early universe that now appears as microwaves coming from all directions in space.
- Recombination
- Recombination is the time when electrons and protons joined to form neutral atoms, allowing light to travel freely through the universe.
- Redshift
- Redshift is the stretching of light to longer wavelengths as space expands or as a source moves away.
- Blackbody radiation
- Blackbody radiation is the spectrum of light emitted by an ideal object whose radiation depends only on its temperature.
- Anisotropy
- Anisotropy means a small difference in a measured property depending on direction, such as tiny temperature variations across the CMB sky.
Common Mistakes to Avoid
- Thinking the CMB comes from stars or galaxies is wrong because it was released before the first stars formed and fills all directions almost uniformly.
- Calling the CMB visible light today is wrong because cosmic expansion has stretched its wavelengths into the microwave part of the spectrum.
- Assuming the CMB is perfectly smooth is wrong because its tiny temperature variations are real and contain information about early density differences.
- Using Celsius for CMB temperature calculations is wrong because blackbody and cosmology formulas require absolute temperature in kelvin.
Practice Questions
- 1 Use Wien's law, λmax = 2.90 × 10^-3 m K / T, to estimate the peak wavelength of the CMB if T = 2.725 K. Give your answer in millimeters.
- 2 A photon was released when the universe was about 1100 times smaller than it is today. If its original wavelength was 1.0 micrometer, what is its observed wavelength today?
- 3 Explain why tiny hot and cold spots in the CMB are important evidence for the formation of galaxies, even though the CMB looks almost uniform overall.