The first light: the cosmic microwave background
The universe cools enough to become transparent
Quick facts
- When
- About 380,000 years after the Big Bang
- What it is
- The oldest light we can observe, now stretched into microwaves
- Before it
- A fog of free electrons that scattered all light
- Mapped by
- NASA's WMAP (launched 2001)
What happened
For its first few hundred thousand years the universe was so hot and crowded that a vast number of free electrons scattered light in every direction, leaving it in a kind of glowing fog through which nothing could be seen. Around 380,000 years after the Big Bang, in a stage scientists call recombination, it finally cooled enough for those electrons to join with nuclei into neutral atoms. Light was suddenly free to travel in straight lines, and it streamed out across the whole sky at once. That ancient light is still arriving today, stretched by the expanding universe into faint microwaves called the cosmic microwave background, the oldest light we can observe.
Why it matters
The background glow is the earliest baby picture of the cosmos, a snapshot from when it was less than a thousandth of its current age. Reading the tiny temperature ripples frozen into it is how astronomers measured the universe's age, its overall shape, and what it is made of, so much of what we know about the whole of time comes from this one faint signal.
How we know
NASA describes recombination at about 380,000 years and the fog of electrons that came before it. NASA's WMAP spacecraft, launched on 30 June 2001 and operating until 2010, mapped the temperature differences across this background light over the entire sky, and those measurements are a large part of how the 13.8-billion-year age was fixed.
Sources
- NASA. The Universe: overview and history (NASA Science) (2024) · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- NASA. WMAP: mapping the cosmic microwave background (NASA) (2024) · Primary source (author-declared)science.nasa.gov · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
- European Space Agency (Planck). Planck and the cosmic microwave background (European Space Agency) · Reputable sourceesa.int · The domain "esa.int" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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