Cosmic microwave background

The cosmic microwave background (CMB) is the afterglow of the Big Bang, a fossil radiation that fills the entire observable universe and reveals the primordial structure of the cosmos.

About this subject

The cosmic microwave background (CMB) is a form of electromagnetic radiation that fills the entire universe. Accidentally discovered in 1965 by radio astronomers Arno Penzias and Robert Wilson, the CMB is considered the strongest evidence for the Big Bang theory. It consists of photons emitted about 380,000 years after the Big Bang, when the universe cooled enough for protons and electrons to combine into neutral atoms, allowing radiation to travel freely.

The current temperature of the CMB is about 2.725 Kelvin (minus 270.4 degrees Celsius), with tiny variations of only 0.0001 Kelvin. These temperature fluctuations, mapped by satellites such as COBE, WMAP, and Planck, reveal the seeds of the first galaxies and galaxy clusters. The CMB is nearly uniform in all directions, but the dipole anisotropy indicates Earth's motion relative to the cosmic background.

Studying the CMB has allowed precise determination of cosmological parameters: the universe's age (13.8 billion years), composition (5% ordinary matter, 27% dark matter, 68% dark energy), and the flat geometry of spacetime. Additionally, the polarization of the CMB provides clues about cosmic inflation, a period of exponential expansion just after the Big Bang.

The CMB is observed at millimeter and sub-millimeter wavelengths, requiring sensitive telescopes in space or at high altitudes to avoid atmospheric interference. Future projects like the Simons Observatory and LiteBIRD aim to map primordial polarization in search of primordial gravitational wave signals, testing theories about the earliest moments of the universe.

Frequently Asked Questions

What is the cosmic microwave background?

It is the afterglow of the Big Bang, emitted when the universe was only 380,000 years old. It fills the entire cosmos and has an average temperature of 2.725 K.

How was the CMB discovered?

It was discovered accidentally in 1965 by Arno Penzias and Robert Wilson while testing a radio antenna. They detected a persistent noise coming from all directions.

Why is the CMB important for cosmology?

It provides a snapshot of the early universe and allows precise calculations of its age, composition, and geometry. The fluctuations in the CMB are the seeds of all cosmic structures.