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Grades 9–12 · Earth & Space

How can ancient light reveal a hotter early universe?

A microwave instrument looking in almost any direction detects a faint background glow. This cosmic microwave background, or CMB, has a nearly thermal spectrum: its brightness at different wavelengths closely matches the pattern expected from radiation in thermal equilibrium at one temperature. That shared pattern provides evidence of a time when matter and radiation interacted far more closely than they do across today's mostly transparent universe.

V013 visual explanationAncient radiation carries evidence of cooling; Frequent scattering; Fewer free electrons; Later expansion stretches the radiation’s wavelength; Schematic sequence; the CMB last scattered about 380,000 years after the start.Ancient radiation carries evidence of coolingFrequent scatteringFewer free electronsLater expansion stretches the radiation’s wavelengthSchematic sequence; the CMB last scattered about 380,000 years after the start.

In the diagramAncient radiation carries evidence of coolingFrequent scatteringFewer free electronsLater expansion stretches the radiation’s wavelengthSchematic sequence; the CMB last scattered about 380,000 years after the start.

A schematic sequence connects frequent electron scattering, the formation of neutral atoms and later wavelength stretching. The drawn waves are symbols, not a spatial map of the universe or a view of the first instant.

Early in the universe's hot expansion, matter included many free electrons. Photons repeatedly scattered from those electrons, changing direction rather than traveling freely over long distances. Expansion cooled the material. Eventually electrons could remain bound to nuclei in neutral atoms, greatly reducing the number of free electrons available to scatter the light.

Radiation could then travel much more freely. This transition, about 380,000 years after the hot expansion began, left the light whose later arrival we measure as the CMB. We are not seeing photons escaping from the first instant. We are receiving an ancient signal from a stage when the universe became much more transparent.

Continued expansion stretched the radiation's wavelengths. Its spectrum now corresponds to a temperature of about 2.7 kelvin, far cooler than conditions at the time of last scattering. The spectrum's form and the wavelength stretching fit together: a formerly hot radiation field has cooled as the universe expanded. This is different from ordinary starlight becoming faint simply because a star is far away.

Tiny temperature differences across the sky add another layer of evidence. They reflect early variations in matter and radiation and help constrain models of how later structures developed. Scientists compare their sizes and patterns with other measurements rather than treating a colored sky map as a complete history. The CMB gives observable evidence of an early hot, dense universe while leaving questions about the earliest moments and ultimate origin open.

About this explanation

Starting knowledge: Understand electromagnetic wavelengths, scattering by charged particles and the idea of cosmic expansion.

Connect this idea: Networks, Exchange & Time

Sources

  • NASA — Universe glossary: cosmic microwave background
  • NASA — COBE Science
  • NASA — Big Bang and the Evolution of the Universe
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