The Primordial Era

The Primordial Era is the first age of the universe: the hot, dense phase from the Planck time, about 10^-43 seconds after the Big Bang, to roughly 100,000 years. There were no stars. The universe built its first protons, neutrons and light nuclei, and was dominated by radiation.

It is the first of the five eras in the Deep Time explorer, following the scheme of Fred Adams and Gregory Laughlin, who count time in cosmological decades: η = log₁₀(age in years). In their 1999 book The Five Ages of the Universe, the Primordial Era spans η = −50 to 5. Their 1997 review “A dying universe” called the same phase the Radiation Dominated Era and ended it at η = 4, when matter began to dominate.

What happened in the Primordial Era?

Almost everything that set up the later universe happened here, most of it in the first few minutes. The milestones below use the cosmological decades from Adams and Laughlin’s table of events, plus later measurements.

Event	When	η (log10 years)
Planck epoch: the limit of known physics	About 10-43 seconds	−50.5
Grand unification epoch (theoretical)	About 10-37 seconds	−44.5
Electroweak transition	About 10-10 seconds	−17.5
Quarks bind into protons and neutrons	About 10 microseconds	−12.5
Big Bang nucleosynthesis: helium, deuterium, lithium	Seconds to minutes	−6
Matter overtakes radiation	Tens of thousands of years	4
End of the era (book boundary)	About 100,000 years	5

The first fraction of a second

Before the Planck time, our physics cannot describe the universe: gravity and quantum mechanics would need a combined theory nobody has. Shortly after, according to the theory Alan Guth proposed in 1981, space went through cosmic inflation, an exponential growth spurt that smoothed the universe and stretched quantum ripples into the seeds of future galaxies. Planck measured the imprint of those ripples: a spectral index of 0.965 ± 0.004, close to but just below 1, as inflation predicts.

The first minutes

As the universe cooled, quarks bound into protons and neutrons. Then, in the first few minutes, protons and neutrons fused into helium nuclei, a little deuterium and a trace of lithium. The idea that the light elements were forged in a hot early universe goes back to the 1948 paper by Alpher, Bethe and Gamow. The heavier elements came much later, inside stars.

Tens of thousands of years of radiation

For its first tens of thousands of years the universe’s energy was mostly radiation. Adams and Laughlin place the switch to matter domination at about η = 4. The universe was still an opaque plasma: light could not travel far before scattering off free electrons.

How the Primordial Era ended

The era closes as the universe becomes ready for stars. About 380,000 years after the Big Bang (η ≈ 5.6), electrons and nuclei combined into neutral atoms and light could travel freely. That light is still arriving: the cosmic microwave background, discovered by Penzias and Wilson in 1965 and shown by COBE in 1990 to have an almost perfect blackbody spectrum. Planck mapped it in detail and used it to measure the age of the universe, 13.787 ± 0.020 billion years.

Adams and Laughlin note that stars cannot form before this recombination, at a redshift of about 1,000 (η ≈ 5.5). That is why their book draws the boundary between η = 5 and the start of the Stelliferous Era at η = 6. The boundaries are round numbers on a logarithmic scale, not sharp events: in reality the first stars took tens to hundreds of millions of years longer to ignite.

Why the Primordial Era looks so long

In ordinary years the Primordial Era is the shortest of the five eras, about 100,000 years. On the logarithmic clock it covers 55 cosmological decades, from −50 to 5. That is because so much physics changed so quickly: each tenfold step in time in the first second brought a new state of matter.

It is also the era we know best by direct evidence, apart from our own. The cosmic microwave background, the abundance of helium (about a quarter of ordinary mass) and the pattern of fluctuations measured by WMAP and Planck all come from it. Everything after it, from galaxies to the far future in our guide to the future of the universe, grew from conditions set here.

Key numbers

• Start: the Planck time, about 5.4 × 10^-44 seconds (η ≈ −50.8; Adams and Laughlin round to −50).
• End: about 10⁵ years (η = 5).
• Recombination: about 380,000 years, redshift about 1,090.
• Today’s temperature of its leftover light: 2.725 kelvin.

Next era: the Stelliferous Era, the age of stars.