In 1964, while performing communications tests with the Holmdel horn antenna at Bell Labs, Penzias and Wilson detected a background noise of about 3 K that did not depend on direction. After ruling out instrumental and local sources they consulted Princeton physicists led by R. Dicke and concluded the signal was the cosmic microwave background (CMB) predicted by Big-Bang cosmology. The CMB originated when the universe was about 380 000 years old and atoms first formed, allowing light to travel freely. Its near-uniform temperature provided strong evidence that the universe began hot and dense, powerfully supporting the Big-Bang theory. The discovery transformed cosmology into a precision observational science.

The Holmdel antenna, built for 4.08 GHz satellite experiments, had a broad beam and nearly isotropic sensitivity. Penzias and Wilson used carefully calibrated reference loads to keep systematic error below 1 %, measuring a radiation temperature of 2.7 ± 0.3 K. The CMB exhibits a thermal black-body spectrum, later pinned down to 2.725 K by COBE, WMAP and Planck satellites. Observations of CMB anisotropies at the 10⁻⁵ level have become essential for determining dark-matter content and inflation parameters. The CMB dipole also serves as a yardstick for Earth’s velocity through space.

The Holmdel horn had a 2.3° × 2.3° beam pattern and fed a 1.35 cm waveguide to a HEMT receiver; zenith absorption corrections relied on atmospheric line models, holding T_sys ≈ 6 K. Their reported T_CMB = 3.5 ± 1.0 K matched the Planck spectrum I_ν = 2hν³/c²(e^{hν/kT}−1) within errors, consistent with post-recombination photon relic models. The absence of spectral distortions down to 10⁻⁵ constrained late energy injection (μ-distortions) and hence the photon-to-baryon ratio and Ω_b h². Subsequent spherical-harmonic C_ℓ measurements located the first acoustic peak at ℓ≈220, indicating Ω_k≈0 and underpinning ΛCDM cosmology.