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The cosmic dark ages
1990· Cosmic event
Well supportedStrong evidence or math, but not fully tested.
The stretch from about 380,000 years to 100–200 million years after the start, when the universe held gas but no light sources.
See it on the Science map →How it works
After the microwave background was released, space was filled with neutral hydrogen and helium gas and nothing to light it. The leftover glow cooled from about 3,000 degrees, an orange-red, into infrared within a few million years, and to about 60 degrees above absolute zero by the end. To human eyes, space was black. Meanwhile gravity worked unseen: dark matter clumps grew and drew in gas, setting up the first stars.
Why it matters
It is the setup period for all cosmic structure and the largest unobserved stretch of history. Its signal could hold far more information than the microwave background.
How we know
Inferred from the microwave background and well-understood gas physics; not yet observed. The only signal is hydrogen's faint 21-centimeter radio line, now stretched to meters and buried under Earth's radio noise and the Milky Way's glow.
Still a mystery
Can the 21-centimeter signal ever be caught? Small radio experiments on the Moon's far side, shielded from Earth's noise, are planned; larger lunar telescopes may follow.
Connections (3)
▸Cosmic microwave background: the Big Bang's afterglow is the last light before The cosmic dark agesProven
The relationship
When atoms formed about 380,000 years in and the microwave background streamed free, the same event left gas neutral and with no new light sources. That glow was the last light for 100 million years or more. A few leftover free electrons kept the gas at the glow's temperature until about 10 million years; after that the gas cooled faster than the light, setting up the dark ages' radio signal.
How the link was found
Jim Peebles, and independently Yakov Zel'dovich, Vladimir Kurt and Rashid Sunyaev, calculated in 1968 how atoms formed and how few free electrons remained.
Why it's proven
The microwave background's measured ripple pattern and polarization match these calculations closely (Planck, 2018), confirming when and how the gas went neutral.
Open questions and mysteries
Forming atoms should have emitted faint extra spectral lines, now mixed into the microwave background. They are predicted but not yet detected; future instruments may find them.
▸The cosmic dark ages ends with The first starsWell supported
The relationship
Through the dark ages, dark matter clumps grew and merged. Once some reached about a million times the Sun's mass, around 100 to 200 million years, the gas inside was dense enough to form molecular hydrogen, which radiates heat away. Cooling let the gas collapse further until its densest cores ignited fusion. The first starlight marks 'cosmic dawn', the end of the dark ages.
How the link was found
Max Tegmark, Joseph Silk, Martin Rees and colleagues calculated in 1997 which clumps could cool and form stars. Tom Abel, Greg Bryan and Michael Norman simulated a first star's birth in 2002.
Is it proven?
Not directly observed. Supercomputer simulations from standard cosmology agree on this picture, and JWST galaxies at about 300 million years require that stars started earlier.
Open questions and mysteries
The start time is uncertain by about 100 million years. Gas streaming past dark matter, magnetic fields or unusual dark-matter properties could delay or speed it.
▸Dark matter gathers gas into clumps during The cosmic dark agesWell supported
The relationship
Dark matter had been clumping since before atoms formed, because light's pressure never held it back. When gas went neutral at 380,000 years, it was freed from light too and fell into dark matter's existing gravity wells. That head start was essential: the ordinary-matter ripples seen in the microwave background, about 1 part in 100,000, could not alone have grown into galaxies by now.
How the link was found
Jim Peebles showed in 1982 that cold, slow dark matter could reconcile the smooth microwave background with today's galaxies; George Blumenthal, Sandra Faber, Joel Primack and Martin Rees developed the picture in 1984.
Is it proven?
Simulations with cold dark matter reproduce galaxy timing and the cosmic web; without it, galaxies form far too late. Direct observations of the dark ages are still missing.
Open questions and mysteries
The 21-centimeter signal could show whether dark matter feels anything besides gravity. EDGES' 2018 report of an unexpectedly deep dip at cosmic dawn, about 180 million years, prompted such ideas; SARAS 3 (2022) found no sign of it.