Cosmology · Origins
The First Three Minutes: A Timeline of the Early Universe
S
Editorial Team
September 19, 2026 · 9 min read
The Big Bang is often described as an explosion. It was not. There was no point in space where it happened, no surrounding void into which the universe expanded, no moment at which a pre-existing space was suddenly filled with matter. The Big Bang was the beginning of space, time, and energy simultaneously — a hot, dense state from which all subsequent structure emerged.
What we call the Big Bang is better understood as the earliest moment from which our physical theories can make reliable predictions. We have no theory of quantum gravity, so we cannot describe the universe at times earlier than roughly 10⁻⁴³ seconds (the Planck time). But from that moment forward, we have a coherent, observationally tested framework — the standard cosmological model, ΛCDM — that traces the universe from a hot plasma to the galaxies and filaments we observe today.
The evidence for the Big Bang is not a single observation but a convergence from multiple independent lines:
- Hubble's law — galaxies are receding from us, and more distant ones faster. Running the expansion backward gives a finite age.
- Big Bang nucleosynthesis — the predicted primordial abundances of hydrogen, helium, deuterium, and lithium match observations precisely.
- The cosmic microwave background (CMB) — a uniform glow of microwave radiation, at 2.725 K, filling the entire sky.
- The large-scale structure — the distribution of galaxies on the largest scales matches simulations of structure growing from tiny initial fluctuations in an expanding universe.
The Timeline: From Planck Epoch to Recombination
0 to 10⁻⁴³ seconds — The Planck Epoch
Before 10⁻⁴³ seconds, density and temperature were so extreme that quantum gravitational effects dominated. Our physics breaks down here. The four fundamental forces were likely unified into a single force. We have no predictive theory for this era.
10⁻³⁶ to 10⁻³² seconds — Cosmic Inflation
The universe underwent a period of exponential expansion, doubling in size roughly every 10⁻³⁸ seconds for 60 or more e-folds. By the end of inflation, the universe had expanded by a factor of at least 10²⁶. This inflation explains three otherwise puzzling observations: why the CMB temperature is the same in all directions (the universe was tiny and in thermal contact before inflation); why space appears geometrically flat (inflation stretched away any curvature); and why the large-scale structure looks the way it does (quantum fluctuations during inflation became the seeds of all future structure).
Inflation is not a single model but a class of models, not all equally favoured. The 2014 BICEP2 announcement of gravitational wave imprints in the CMB polarisation — which would have directly probed inflationary energy scales — was later found to be contaminated by galactic dust. The search for primordial gravitational waves in the CMB continues with experiments including BICEP Array, the South Pole Telescope, and future satellite missions.
10⁻¹² seconds — The Electroweak Transition
The electromagnetic and weak nuclear forces, which behave identically at very high energies, separate. The Higgs field gains a non-zero expectation value, giving W and Z bosons their mass. This transition is well understood — the Large Hadron Collider probes these energies directly.
10⁻⁶ seconds — The Quark-Hadron Transition
The temperature drops below roughly 10¹² Kelvin. Free quarks and gluons cool enough to be confined into hadrons: protons and neutrons. The universe is now a hot, dense soup of protons, neutrons, electrons, photons, and neutrinos — a plasma. There is a slight excess of matter over antimatter (about one extra particle per billion particle-antiparticle pairs), which is why there is a matter-dominated universe today. The origin of this baryon asymmetry is not yet fully explained.
"The cosmic microwave background is a photograph of the universe at 380,000 years old — the moment the fog cleared and light was first able to travel freely. Every point of that sky carries information about conditions when the universe was a thousandth of its current size."
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1 second — Neutrino Decoupling
The universe cools to roughly 10¹⁰ Kelvin. Neutrinos, which interact only weakly, stop colliding with other particles frequently enough to remain in thermal equilibrium. They decouple from the rest of the universe — streaming freely from this point onward — forming the cosmic neutrino background (CνB), an analogue of the CMB that has never been directly detected but whose existence is inferred from Big Bang nucleosynthesis calculations. About 330 neutrinos and antineutrinos per cubic centimetre pervade the universe today, left over from this epoch.
1–3 minutes — Big Bang Nucleosynthesis (BBN)
This is the era Steven Weinberg called "the first three minutes." As temperature drops below 10⁹ Kelvin, protons and neutrons can form nuclei without immediately being photo-dissociated by high-energy photons. Nuclear fusion proceeds rapidly:
- Deuterium (hydrogen-2) forms first: p + n → d + γ
- Deuterium fuses to form helium-3 and helium-4
- A small amount of lithium-7 forms
By the time the temperature drops too low for fusion to continue (about 20 minutes after the Big Bang), the universe's nuclear composition is fixed: roughly 75% hydrogen by mass, 25% helium-4, with traces of deuterium, helium-3, and lithium-7. This is exactly what we observe in the oldest, least-processed stars and gas clouds — a powerful confirmation of the model.
There is one tension: the predicted primordial lithium-7 abundance is about three times higher than observed in old stars. The cosmological lithium problem is real and unresolved. It may reflect poorly understood stellar physics that depletes lithium in old stars, or it may hint at physics beyond the standard model.
380,000 years — Recombination and the Last Scattering Surface
For the first 380,000 years, the universe was a plasma — electrons were not bound to nuclei, and photons scattered off free electrons constantly, making the universe opaque. When the temperature dropped to roughly 3,000 K, electrons and nuclei combined to form neutral atoms. Photons could then travel freely — the universe became transparent.
The photons released at this moment form the CMB. We observe them today, redshifted by a factor of about 1,100 (the universe has expanded by that factor since recombination), arriving at a temperature of 2.725 K. The COBE satellite detected this in 1992, earning John Mather and George Smoot the 2006 Nobel Prize. WMAP (2003–2010) and the Planck satellite (2009–2018) mapped its tiny temperature variations — one part in 100,000 — with exquisite precision.
What the CMB Tells Us
The CMB's temperature anisotropies — its slight patchiness — encode an extraordinary amount of physics. The angular power spectrum of those fluctuations (how the variance varies with angular scale) constrains:
- The age of the universe: 13.787 ± 0.020 billion years (Planck 2018)
- The Hubble constant: 67.4 ± 0.5 km/s/Mpc — in tension with local measurements (~73 km/s/Mpc)
- The density of ordinary matter, dark matter, and dark energy
- The geometry of space (flat, to within 0.4%)
- The amplitude and spectral index of primordial fluctuations from inflation
The Hubble tension — the discrepancy between the CMB-inferred Hubble constant and direct measurements from Type Ia supernovae and Cepheid variable stars — is currently at roughly 5 sigma significance. It may reflect systematic errors in one or both methods, or it may signal physics beyond ΛCDM. It is one of the most actively debated questions in cosmology.
What We Don't Know
The standard model is spectacularly successful, but its major constituents — dark matter and dark energy — are unknown. Dark matter makes up roughly 27% of the universe's energy budget and was essential for structure formation, but no direct detection experiment has found a particle. Dark energy, composing about 68%, drives the accelerating expansion discovered in 1998 through Type Ia supernova observations (Nobel Prize 2011), but its nature — whether a cosmological constant, a dynamical field, or a modification of gravity — remains entirely open. The first 10⁻⁴³ seconds of the universe's history remain inaccessible to us. And we have no explanation for why there was slightly more matter than antimatter in the early universe.
What we do have is a framework accurate from one second after the Big Bang to the present — tested across 13.8 billion years of cosmic history — that predicts the CMB, the primordial element abundances, the large-scale structure, and the expansion history with a precision that no prior cosmological model came close to achieving.
The Cosmic Dark Ages and the First Stars
After recombination at 380,000 years, the universe entered a period cosmologists call the Cosmic Dark Ages. There were no stars, no galaxies, no sources of light. The universe was filled with neutral hydrogen and helium, dark matter halos slowly growing by gravitational accretion, and the cooling afterglow of the CMB redshifting from infrared toward microwave frequencies. This era lasted from roughly 380,000 years to approximately 100–400 million years after the Big Bang.
Population III stars: the first light
The first stars — called Population III stars — formed when enough neutral hydrogen had accumulated in dark matter halos to cool, condense, and ignite nuclear fusion. They were almost entirely made of hydrogen and helium, with no heavier elements (which astronomers call "metals"). This low metallicity changed everything about how they formed:
- Without metals, gas cannot cool as efficiently via atomic and molecular line emission. Molecular hydrogen (H₂) and HD (hydrogen deuteride) were the primary coolants — less effective than the carbon and oxygen fine-structure lines that dominate cooling in later star formation.
- As a result, the characteristic mass of a collapsing gas cloud was higher — Population III stars are predicted to have had masses of tens to hundreds of solar masses, possibly up to 1,000 solar masses. None have been directly observed; they lived fast and died in supernovae millions of years ago.
- Their enormous UV and ionising radiation output began reionising the neutral intergalactic medium — the Epoch of Reionisation.
Cosmic Reionisation: Lighting up the Universe
From roughly 100 million to 1 billion years after the Big Bang, energetic radiation from the first stars and early galaxies progressively ionised the neutral hydrogen in the intergalactic medium. This transition — from a mostly neutral universe to the mostly ionised one we inhabit today — is called the Epoch of Reionisation (EoR).
We know reionisation was essentially complete by redshift z ≈ 6 (about 900 million years after the Big Bang) because quasars at z > 6 show a complete absorption of their blue-shifted light by neutral hydrogen — the Gunn-Peterson trough. The CMB polarisation measured by Planck constrains the midpoint of reionisation to z ≈ 7.7 (about 700 million years), but the detailed history — how patchy reionisation was, which sources drove it, how long it took — remains poorly constrained.
JWST is the primary instrument for studying the EoR. It can directly observe galaxies at z > 6–12 and characterise their stellar populations, ionising photon production efficiencies, and escape fractions — the key physical quantities needed to determine whether early galaxies can account for reionisation. The finding of unexpectedly massive early galaxies complicates this picture: if these galaxies existed earlier and were more prolific than expected, reionisation may have begun earlier or proceeded differently than standard models predict.
21-cm cosmology: seeing the Dark Ages directly
The neutral hydrogen that filled the universe during the Dark Ages emits (and absorbs) radio waves at a wavelength of 21 centimetres — the hyperfine transition of the hydrogen atom. This signal, redshifted to metre wavelengths today, is in principle a direct probe of the Dark Ages and the EoR. Mapping its fluctuations across the sky and in frequency (which corresponds to redshift, hence cosmic time) would be the most information-rich single measurement in cosmology.
This is the goal of 21-cm cosmology. The EDGES experiment reported a tentative detection of the global 21-cm signal from the cosmic dawn in 2018 — but the signal was about twice as strong as any standard model predicted, and whether it represents a true detection or an instrumental systematic remains disputed. Next-generation radio arrays, including the Square Kilometre Array (SKA) and HERA (Hydrogen Epoch of Reionization Array), aim to map 21-cm fluctuations in three dimensions — producing a movie of the universe lighting up, star by star and galaxy by galaxy, across the first billion years.
The Accelerating Universe and What We Still Cannot Explain
In 1998, two independent supernova survey teams — the Supernova Cosmology Project and the High-Z Supernova Search Team — announced that distant Type Ia supernovae were fainter than expected for a matter-dominated universe. The universe is not merely expanding; the expansion is accelerating. Something is counteracting gravity on the largest scales. This "something" was named dark energy, and it earned the Nobel Prize in Physics in 2011 for Saul Perlmutter, Brian Schmidt, and Adam Riess.
The simplest explanation is Einstein's cosmological constant Λ — a constant energy density of empty space, equivalent to a vacuum energy. But the value of Λ required by observations is roughly 10¹²⁰ times smaller than what quantum field theory predicts for vacuum energy. This enormous discrepancy — sometimes called the worst prediction in physics — is not resolved. Alternative explanations include dynamical dark energy (a scalar field that changes with time), modifications to general relativity on cosmological scales, or anthropic selection effects. The DESI results from 2024, which hinted at possible variation in the dark energy equation of state over cosmic time, could — if confirmed — be the first observational crack in the cosmological constant.
The other unexplained constituent is dark matter. Whatever it is, it makes up about 27% of the universe's total energy content, interacts gravitationally, and does not emit or absorb electromagnetic radiation. The leading candidates — weakly interacting massive particles (WIMPs), axions, sterile neutrinos — have each been searched for in dedicated experiments for decades without a confirmed detection. The nature of dark matter is the deepest open question in cosmology and particle physics simultaneously.
