How Did the First Elements Form? Big Bang Nucleosynthesis, the CMB and the Lithium Problem

Every hydrogen atom in your body was made in the first few minutes of the universe. Most of the helium in the cosmos was too. The heavier elements came later, cooked in stars and stellar explosions. The process that made the first light nuclei is called Big Bang nucleosynthesis, or BBN, and it is one of the most successful predictions in cosmology. Our sticker Primordix is the baker in that kitchen. This post explains how the first elements formed, how we test it, and the puzzles it still leaves open.

What is Big Bang nucleosynthesis?

Put simply: once the young universe cooled enough, protons and neutrons clumped together into nuclei of the light elements, hydrogen, helium and lithium, in a process known as Big Bang nucleosynthesis (PDG^[1]). The Particle Data Group’s review gives the full modern treatment, with the nuclear reaction rates, the predicted abundances and the observations (PDG^[1]).

Who came up with the idea?

In 1948 Ralph Alpher, Hans Bethe and George Gamow published “The Origin of Chemical Elements,” proposing that the elements were cooked in a hot early universe (Phys. Rev.^[2]). Their hope that all elements formed this way did not work out. As Kenneth Nollett explains in Physics World, there is no stable nucleus with mass number 5, which blocks the easy path to heavier elements (Physics World^[3]). The modern calculation keeps their core idea but limits it to the lightest nuclei.

How did the first elements form, step by step?

1. The first second. The universe is too hot for nuclei to survive. Protons and neutrons constantly turn into each other through weak-force reactions.
2. Freeze-out. As things cool, those reactions can no longer keep up. Because a neutron is slightly heavier than a proton, by 1.293 MeV (PDG^[1]), neutrons end up outnumbered, roughly one for every five or six protons.
3. The wait. The first step to helium is deuterium (one proton plus one neutron). But early on, energetic photons, which outnumber protons and neutrons by more than a billion to one, smash any deuterium apart. Meanwhile free neutrons decay.
4. Baking. A few minutes in, the universe has cooled enough for deuterium to survive. By then the ratio is about one neutron per seven protons. Almost every remaining neutron is rapidly locked into helium-4.

That last step gives a famous back-of-envelope result. If there are 2 neutrons for every 14 protons, you can make one helium-4 nucleus (2 protons + 2 neutrons) and have 12 protons left over. The helium share of the mass is then 4/16 = 0.25. Observations give 0.245 ± 0.003 for the primordial helium mass fraction (Aver et al.^[4]), in good agreement with the standard prediction (PDG^[1]). By mass, primordial matter came out roughly three-quarters hydrogen and one-quarter helium-4, with traces of deuterium, helium-3 and lithium (PDG^[1]). By number of atoms, helium is a much smaller share, because each helium nucleus is four times heavier.

How do we know this happened?

BBN makes precise, testable predictions from essentially one input: how many protons and neutrons there are compared with photons. Physics World notes that this ratio can be taken from the cosmic microwave background, and BBN then predicts the relative amounts of each light nucleus (Physics World^[3]). That cross-check between two completely different eras of the universe is the heart of the evidence.

What is the cosmic microwave background?

For its first few hundred thousand years, the universe was a hot plasma that light could not cross. When it cooled to about 3000 K, electrons and nuclei combined into atoms and the universe became transparent. The light released then is still arriving, stretched by cosmic expansion into microwaves. Arno Penzias and Robert Wilson found it in 1965, sharing the 1978 Nobel Prize “for their discovery of cosmic microwave background radiation” (NobelPrize.org^[5]). John Mather and George Smoot shared the 2006 prize for measuring its blackbody form and anisotropy with NASA’s COBE satellite (NobelPrize.org^[6]). Today its temperature is 2.72548 K (Fixsen 2009^[7]), and the Planck satellite’s maps pin down the density of ordinary matter with high precision (Planck 2018^[8]). That is our sticker Relicta: I was 3000 K once. Now I’m 2.7 K.

Where did heavier elements come from?

Carbon, oxygen, iron and the rest were made mostly in stars, and in the explosions and collisions of stars. The classic 1957 paper by Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle, “Synthesis of the Elements in Stars,” laid out the processes (Rev. Mod. Phys.^[9]). Fowler shared the 1983 Nobel Prize “for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the universe” (NobelPrize.org^[10]). Our Stellar Core tee is the stellar furnace; for the nuclear binding that makes fusion pay, see What Holds the Nucleus Together?

How do we know stars are mostly hydrogen in the first place? That discovery belongs to Cecilia Payne, whose 1925 thesis showed it, though she was cautious about her own result. We tell that story in Who Really Discovered It?

What is the lithium problem?

BBN gets deuterium and helium-4 right. Lithium-7 is the exception. Brian Fields’ review in the Annual Review of Nuclear and Particle Science summarises it: lithium-7 observations “lie below the BBN+WMAP prediction by a factor of three to four,” the so-called cosmic lithium problem (Annual Reviews^[11]).

The candidate explanations, as Nollett lays them out, are that a process is missing from BBN theory, that old stars have destroyed some of their surface lithium over billions of years, or that the stellar spectra are being misread (Physics World^[3]). Which is right is an open question.

How long does a neutron live?

Because neutrons decay while the universe waits to bake, the neutron lifetime matters to the helium yield. And it is disputed. Natalie Wolchover reported in Quanta that “bottle” experiments, which trap neutrons and count survivors, give about 14 minutes 39 seconds, while “beam” experiments, which count the protons from decays, give about 14 minutes 48 seconds (Quanta^[12]). The Particle Data Group’s 2024 listing gives a mean life of 878.4 ± 0.5 s from bottle measurements and notes the disagreement (PDG^[13]). The roughly nine-second gap is unresolved. Primordix wears it as her catchphrase: Three minutes. Neutrons won’t wait.

Common misconceptions

• “The Big Bang made all the elements.” It made hydrogen, helium and a little lithium. Nearly everything else came from stars (Rev. Mod. Phys.^[9]).
• “The CMB is light from the Big Bang itself.” It is light from when the universe became transparent, a few hundred thousand years later.
• “BBN is untested.” It is tested against measured abundances and against the CMB, and it passes, except for lithium (Annual Reviews^[11]).

Key terms in plain English

• Nucleosynthesis: the making of atomic nuclei by nuclear reactions.
• Baryon: a particle made of three quarks; for this story, protons and neutrons.
• Freeze-out: the moment a reaction becomes too slow to keep up with the cooling universe, fixing a ratio.
• Deuterium: heavy hydrogen, one proton plus one neutron, the first step to helium.
• Primordial abundance: the amount of an element made in the Big Bang, before stars changed it.
• Cosmic microwave background (CMB): light released when the universe became transparent, now at 2.72548 K (Fixsen 2009^[7]).
• Baryon-to-photon ratio: the single number that fixes BBN’s predictions, measurable from the CMB (Physics World^[3]).

Why does helium matter so much?

Helium-4 is the most sensitive test of the early universe’s expansion rate. If there had been extra kinds of light particles in the first second, the universe would have expanded faster, the weak reactions would have frozen out earlier, more neutrons would have survived, and more helium would have formed. That is why measured helium abundances put limits on new physics, a point developed at length in the Particle Data Group review (PDG^[1]). The near-agreement between the observed 0.245 and the predicted value (Aver et al.^[4]) is therefore not just a success story but also a constraint on what else might have existed in the first seconds.

How do astronomers measure primordial abundances?

They look for gas that stars have barely touched. Deuterium is measured in distant gas clouds seen in silhouette against quasars; helium in ionised gas in small, chemically primitive galaxies; lithium in the atmospheres of very old, metal-poor stars in our own galaxy (PDG^[1]). That last method is the one under suspicion in the lithium problem, because stars can destroy lithium: Nollett notes that lithium is rapidly destroyed at temperatures above about 2.5 million kelvin, so any mixing that carries surface material deeper inside a star would deplete it (Physics World^[3]).

A short timeline of the first elements

• 1925: Cecilia Payne’s thesis implies that stars are mostly hydrogen and helium.
• 1948: Alpher, Bethe and Gamow propose that elements were cooked in the hot early universe (Phys. Rev.^[2]).
• 1957: Burbidge, Burbidge, Fowler and Hoyle describe how stars make heavier elements (Rev. Mod. Phys.^[9]).
• 1965: Penzias and Wilson discover the cosmic microwave background (NobelPrize.org^[5]).
• 1990s: COBE confirms the CMB’s blackbody spectrum (NobelPrize.org^[6]).
• 2018–2020: Planck’s final results pin down the density of ordinary matter (Planck 2018^[8]).
• Still open: the lithium problem and the neutron-lifetime discrepancy (Annual Reviews^[11]; Quanta^[12]).

Read as a whole, the timeline shows how unusual BBN is as a theory: its predictions were made with nuclear physics measured in laboratories on Earth, and then checked decades later against light that has been travelling for most of the age of the universe.

The bottom line

The lightest elements were cooked in the first few minutes, in amounts that physics can predict from laboratory nuclear data and one number measured from the cosmic microwave background. Hydrogen and helium come out right. Lithium does not, and the neutron’s lifetime is still disputed. Both are open, and both are good reasons to keep looking.

FAQ

When did the first elements form?

In the first few minutes after the Big Bang, once the universe cooled enough for deuterium to survive (PDG^[1]).

What were the first elements?

Hydrogen, helium, and traces of lithium (PDG^[1]).

Why is a quarter of ordinary matter helium?

Because about one neutron per seven protons survived to the baking stage, and nearly all of those neutrons ended up in helium-4.

Is the lithium problem solved?

No. It remains open (Annual Reviews^[11]; Physics World^[3]).

See the On Cosmic Time sticker sheet, Red Shift and the Relativity & Cosmos collection. Related: Why Does Time Only Go Forward?

References

1. Particle Data Group (2024), Review: Big-Bang Nucleosynthesis. https://pdg.lbl.gov/2024/reviews/rpp2024-rev-bbang-nucleosynthesis.pdf
2. Alpher, Bethe & Gamow (1948), “The Origin of Chemical Elements,” Phys. Rev. 73, 803. https://doi.org/10.1103/PhysRev.73.803
3. Kenneth Nollett, “Testing the elements of the Big Bang,” Physics World. https://physicsworld.com/a/testing-the-elements-of-the-big-bang/
4. Aver, Olive & Skillman (2015) The effects of He I λ10830 on helium abundance determinations, JCAP 07, 011 (Y_p = 0.2449 ± 0.0040). https://doi.org/10.1088/1475-7516/2015/07/011
5. NobelPrize.org, The Nobel Prize in Physics 1978 (Kapitsa; Penzias & Wilson). https://www.nobelprize.org/prizes/physics/1978/summary/
6. NobelPrize.org, The Nobel Prize in Physics 2006 (Mather & Smoot). https://www.nobelprize.org/prizes/physics/2006/summary/
7. D. J. Fixsen (2009), “The Temperature of the Cosmic Microwave Background,” ApJ 707, 916. https://arxiv.org/abs/0911.1955
8. Planck Collaboration (2020), “Planck 2018 results. VI. Cosmological parameters,” A&A 641, A6. https://arxiv.org/abs/1807.06209
9. Burbidge, Burbidge, Fowler & Hoyle (1957), “Synthesis of the Elements in Stars,” Rev. Mod. Phys. 29, 547. https://doi.org/10.1103/RevModPhys.29.547
10. NobelPrize.org, The Nobel Prize in Physics 1983 (Chandrasekhar, Fowler). https://www.nobelprize.org/prizes/physics/1983/summary/
11. B. D. Fields (2011), “The Primordial Lithium Problem,” Annu. Rev. Nucl. Part. Sci. 61, 47. https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-102010-130445
12. Natalie Wolchover, “Neutron Lifetime Puzzle Deepens, but No Dark Matter Seen,” Quanta Magazine (2018). https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/
13. Particle Data Group (2024) — neutron listing (mean life 878.4 ± 0.5 s from ultracold-neutron bottles; beam–bottle disagreement noted). https://pdg.lbl.gov/2024/listings/rpp2024-list-n.pdf

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.