Cosmic Microwave Background

The Cosmic Microwave Background (CMB) is the faint, uniform radiation that fills the entire universe, considered the afterglow of the Big Bang. It represents the earliest observable light — a relic from the time when the universe was about 380,000 years old, providing a snapshot of the cosmos at its infancy. The discovery and study of the CMB stand among the most important achievements in cosmology, offering crucial evidence for the Big Bang theory and helping scientists understand the structure, composition, and evolution of the universe.

Origin and Formation

In the moments following the Big Bang, about 13.8 billion years ago, the universe was extremely hot and dense — a plasma composed of photons, electrons, and protons. Light could not travel freely because photons constantly scattered off free electrons, making the universe opaque.
As the universe expanded, it cooled gradually. When the temperature dropped to around 3000 Kelvin, electrons combined with protons to form neutral hydrogen atoms — a process known as recombination. This allowed photons to decouple from matter and travel freely through space for the first time.
These freely moving photons form what we now observe as the Cosmic Microwave Background radiation. Over billions of years, the expansion of the universe stretched their wavelengths, reducing their energy from visible/infrared light to microwaves, corresponding to a present-day temperature of about 2.725 Kelvin (-270.425°C).

Discovery of the CMB

The CMB was accidentally discovered in 1965 by Arno Penzias and Robert Wilson, two American radio astronomers at the Bell Telephone Laboratories in New Jersey.
While testing a microwave antenna, they detected a persistent background noise that could not be attributed to any known source — it was isotropic and constant in all directions. Simultaneously, theoretical physicists Robert Dicke and Jim Peebles at Princeton University had predicted such radiation as a remnant of the Big Bang.
Penzias and Wilson’s discovery confirmed this prediction, earning them the 1978 Nobel Prize in Physics and firmly establishing the Big Bang model of the universe over competing theories such as the Steady-State Theory.

Characteristics of the CMB

The Cosmic Microwave Background is remarkably uniform, yet it contains minute temperature fluctuations (anisotropies) that reveal critical information about the early universe.
Key characteristics include:

1. Temperature:
   • Average temperature: 2.725 K.
   • Variations: Only about ±0.0001 K (one part in 100,000).
2. Spectrum:
   • The CMB follows a perfect blackbody spectrum, peaking at a wavelength of around 1.9 mm in the microwave region.
   • This matches theoretical predictions of radiation from a hot, dense early universe.
3. Isotropy and Anisotropy:
   • While the CMB is nearly the same in all directions, tiny fluctuations correspond to regions of slightly different density in the early universe.
   • These density variations became the seeds for galaxy and large-scale structure formation.

Observations and Missions

Since its discovery, several space missions have mapped the CMB with increasing precision, transforming cosmology into a data-driven science.

1. COBE (Cosmic Background Explorer, 1989–1993):
   • First satellite to measure the CMB spectrum and detect small anisotropies.
   • Confirmed its blackbody nature and uniform temperature.
   • Earned the 2006 Nobel Prize in Physics for John Mather and George Smoot.
2. WMAP (Wilkinson Microwave Anisotropy Probe, 2001–2010):
   • Produced a detailed full-sky map of CMB temperature fluctuations.
   • Helped determine fundamental cosmological parameters such as the universe’s age, composition, and curvature.
3. Planck Mission (2009–2013):
   • Launched by the European Space Agency (ESA).
   • Measured the CMB with unprecedented accuracy, refining estimates of cosmological constants.
   • Determined the universe’s age to be about 13.82 billion years and its composition as:
     • 68% Dark Energy
     • 27% Dark Matter
     • 5% Ordinary (Baryonic) Matter

Information Derived from the CMB

The CMB serves as a cosmic blueprint, providing vital clues about the origin, composition, and geometry of the universe. Key insights include:

1. Age of the Universe: The time elapsed since the CMB’s release (13.8 billion years) marks the universe’s age.
2. Geometry and Curvature: The CMB data show that the universe is spatially flat, meaning parallel lines do not converge or diverge — a prediction of inflation theory.
3. Composition of the Universe: Analysis of temperature fluctuations reveals the proportions of dark energy, dark matter, and normal matter.
4. Density Fluctuations: Tiny variations in the CMB correspond to the primordial density fluctuations that eventually led to galaxies, clusters, and large-scale structures.
5. Baryon Acoustic Oscillations: Patterns in the CMB reflect sound waves (acoustic oscillations) in the early universe’s plasma, serving as a “cosmic ruler” to measure distances across the universe.
6. Inflationary Epoch: The CMB supports the theory that the universe underwent a brief period of exponential expansion (inflation) within a fraction of a second after the Big Bang.

Polarisation of the CMB

Besides temperature variations, the CMB is polarised — its light waves exhibit preferred orientations. This polarisation arises due to scattering of photons by electrons during recombination.
Two main types of polarisation patterns are studied:

• E-modes (even parity): Produced by density variations; already detected with high precision.
• B-modes (odd parity): Associated with gravitational waves from inflation; their detection would provide direct evidence for the inflationary model.

Theoretical Importance

The CMB is often described as the “cosmic Rosetta Stone” because it encodes fundamental information about the early universe. Its precise measurement allows cosmologists to test and refine theories of cosmic origin and evolution, including:

• Big Bang cosmology
• Cosmic inflation
• Structure formation
• Dark matter and dark energy models

It also provides one of the strongest refutations of the Steady-State Theory, which predicted no such background radiation.

Ongoing and Future Research

Modern research continues to focus on:

• Detecting primordial gravitational waves through CMB polarisation (experiments like BICEP, SPTpol, and LiteBIRD).
• Understanding the epoch of reionisation, when the first stars and galaxies formed and affected CMB photons.
• Studying CMB lensing, where light from the CMB is deflected by intervening matter, helping map dark matter distribution.