Astronomy & Celestial Mechanics
the sky, computed
Concept

Baryon-to-photon ratio — where it appears

The number of baryons in the universe per photon in the microwave background, about six in ten billion. It is the single free parameter of big-bang nucleosynthesis and it is fixed independently by the shape of the acoustic peaks, which is why the agreement between the two is a test.

Named by 6 essays across one field — each of them below, with the objects they name alongside it.

Four abundances, one free parameter. The abundances big-bang nucleosynthesis predicts, against the one number it is free to choose: η₁₀, the ratio of baryons to photons in units of 10⁻¹⁰. Four curves spanning nine decades, from a helium mass fraction of about a quarter down to a lithium abundance of one atom in ten billion, and they are not four independent predictions — they all come out of the same reaction network run at the same density. The horizontal bands are what is measured in the sky, each at its published one sigma. The measurement that matters is deuterium, because its curve is the steep one: inverting the drawn curve at D/H = 2.527e-5 gives η₁₀ = 6.11, and the ends of the observed interval give 6.06 to 6.15. The vertical band is what the microwave background gives, 6.13 ± 0.04, from the height of the second acoustic peak relative to the first. Those two agree to 0.4 per cent, and they have nothing whatever in common: one is a nuclear-reaction network run in the first three minutes and read off a quasar absorption line, the other is a fluid oscillation at four hundred thousand years read off a sky map. Lithium is the exception and it is not a small one — the network predicts 4.70e-10 at the microwave background's density and the halo stars show 1.60e-10, a factor of 2.9 too much, which is unresolved.

Four abundances and one free parameter

In the first three minutes the universe ran a nuclear reaction network with exactly one adjustable number in it. That number predicts four abundances spanning nine orders of magnitude, three of them are observed and match, and the fourth is wrong by a factor of three and has been for twenty-five years.

cosmology · Nucleosynthesis The helium abundance as a count of neutrino species. The primordial helium mass fraction against the number of light neutrino species, integrated at a deuterium-bottleneck temperature of 0.0855 MeV and a neutron lifetime of 877.75 s. The curve rises at 0.01348 in Y_p per species near the standard model, and it is not quite a line: freeze-out temperature goes as the sixth root of g_*, so a species added at N_eff = 4.5 buys 25 per cent less helium than one added at 2 — a slope ratio of 0.749 against the 0.769 that scaling requires. The reading below is therefore taken off the drawn curve rather than off a slope. The horizontal band is the measurement — ⁴He (Aver 2015), Y_p = 0.2449 ± 0.004, taken from recombination lines in metal-poor dwarf galaxies and extrapolated to zero metallicity. Where the band crosses the line is the answer: N_eff = 2.84, with the ends of the observed interval giving 2.56 to 3.13. The standard model has three, and 3.046 rather than 3 because the neutrinos are not quite decoupled when the electron–positron pairs annihilate and take a sliver of the heat. The strength of this is not its precision, which is a third of a species and worse than the microwave background's; it is that the two constraints come from utterly different epochs, and that this one is a laboratory result about particle content obtained from an emission line in a galaxy.

A particle count taken from a dwarf galaxy

Nearly every neutron that survives the first three minutes ends inside a helium nucleus, so the primordial helium abundance is not chemistry — it is the reading of a race between the weak interaction and the expansion. The expansion rate carries the square root of the number of relativistic species, which is why an emission line in a metal-poor galaxy counts neutrinos.

cosmology · Nucleosynthesis The deuterium burning rate against the expansion, at three baryon densities. The rate at which a deuteron is destroyed, divided by the expansion rate, against temperature. Temperature falls to the right, so the picture reads left to right as time, and the horizontal line at one is where the burning stops mattering: above it a deuteron is destroyed many times over before the universe doubles in size, below it the reaction has effectively ceased. The rate drawn is D(p,γ)³He at the NACRE parameterisation, multiplied by the free-proton density — three quarters of the baryons by number once ⁴He has taken the rest — and the expansion rate is the same 1.66√g* T²/mPl the freeze-out calculation raced the weak interactions against. The three curves differ in one number and one only: the baryons per photon, 3, 6, 12 in units of 10⁻¹⁰. They are therefore vertical translations of each other, exactly in proportion to η, because the reaction is two-body and the expansion is not. That is the entire mechanism by which an abundance measures a density. A denser universe crosses the line later — 21.4 keV at η₁₀ = 3, 16.0 keV at η₁₀ = 6, 12.3 keV at η₁₀ = 12 — and every extra second below the crossing is deuterium that does not survive. What the figure does not do is predict the abundance itself: the residue depends on the whole reaction network and on the ⁷Be and ³He channels that feed back into it, and the curve the abundance is read off is a fit to that network rather than to this.

The residue that failed to burn

Deuterium's abundance is not a measure of what the first three minutes made. It is a measure of what escaped being used — a two-body destruction rate losing a race to a one-body expansion, which is why its curve against the baryon density is steep and helium's is flat.

cosmology · Nucleosynthesis A factor of 2.9, and the density that would remove it. The predicted ⁷Li abundance against the baryon density, with the halo-star measurement as a horizontal band and the microwave background's density as a vertical one. At η₁₀ = 6.13 the network gives 4.70e-10 and the stars show 1.60e-10, a factor of 2.94 — 0.47 in the logarithm, which is the number every proposed resolution has to produce. The obvious cure is drawn as the second marker: ⁷Li rises as η², so the density that would reproduce the observation is η₁₀ = 3.58, a third below the measured one. That is where the cure fails, and it fails on a different element. Deuterium falls as η^(−1.6), so at 3.58 the predicted D/H is 5.946e-5 against the 2.527e-5 measured in quasar absorbers — 2.35 times too much, and 114 times deuterium's own error bar. Helium, meanwhile, moves to 0.2445 and stays inside its measurement, because its curve is flat. The three light elements do not fail together, and that is what rules out a single wrong parameter: whatever is wrong is wrong about mass seven specifically.

A factor of three, and the flatness that prices every cure

The oldest stars in the Galaxy show a third of the lithium the first three minutes should have left. Three kinds of resolution have been offered, and the datum that rules on all three is not how much lithium is missing but how uniformly it is missing.

cosmology · Nucleosynthesis A bound on the baryon density from an abundance nobody can extrapolate. Primordial deuterium against the baryon density, with the upper bounds that the solar system's own deuterium and helium-3 place on it. The inequality is D_p ≤ D_obs + ³He_obs/g₃, and it holds for any star-formation history whatever: a deuteron entering a star becomes ³He, so the pair can only be moved from one member to the other and then destroyed, never increased. With pre-solar values of D/H = 2.0e-5 and ³He/H = 1.5e-5, the bounds are 3.50e-5 at g₃ = 1, 5.00e-5 at g₃ = 0.5, 8.00e-5 at g₃ = 0.25 — and because deuterium falls with density, each upper bound on the abundance is a lower bound on η: η₁₀ > 4.98, η₁₀ > 3.99, η₁₀ > 2.97. The weakest of them, at a survival fraction of 0.25, still excludes 52 per cent of the density range below the answer. That was the state of the measurement for most of the 1980s, and the shape of it is the thing worth carrying: an abundance too poorly understood to be extrapolated at all still constrained the quantity, because its direction of change under processing was known even though its magnitude was not. The microwave background's 6.13 sits above every bound drawn, which is not a coincidence and is not evidence — a bound that excluded the answer would have been an error, and a bound that admits it is only a bound.

An abundance with no direction to correct in

Every primordial abundance is measured today and extrapolated backwards, and the extrapolation works because processing moves each species one way. Helium-3 is made by small stars and destroyed by large ones, so the sign of its correction is not merely uncertain — it is unknown.

cosmology · Nucleosynthesis A lumpy universe makes more of both, and one of them was already too much. The deuterium–lithium plane, with the curve a homogeneous universe traces as its baryon density varies and the points a two-zone universe reaches at a fixed mean density of η₁₀ = 6.13. The dense zone occupies 15 per cent of the volume, and the contrast between the zones runs from 1 — which is the homogeneous case — to 100. Every mixture lies up and to the right of its own homogeneous point, and that is not a modelling choice: an abundance is measured per baryon, so what a telescope averages is η times the abundance — and for both deuterium and lithium that product is a convex function of the density, whose average therefore exceeds its value at the average. The excess is large. At a contrast of 100 the mixture gives D/H = 1.13e-4 against 2.51e-5 smooth, and ⁷Li/H = 1.77e-8 against 4.70e-10, a factor of 37.7. The extra deuterium is exactly what the proposal was for: it lets the mean baryon density be raised while the observed D/H is still matched, which in the 1980s was the one way to make the baryons account for all the matter that dynamics required. The extra lithium is what it costs. The measured abundance was already a factor of 2.9 below the homogeneous prediction, and every step toward the lumpy universe that fixes the density makes that discrepancy worse — which is why the answer to "the baryons are lumpy" turned out to be that the extra matter is not baryons.

The universe that was lumpy at one second

If the baryons were unevenly spread when the network fired, each region ran its own nucleosynthesis and what is observed is an average. For a decade that was the one way to make ordinary matter account for all the matter — and the reason it fails is a theorem about convex curves.

cosmology · Nucleosynthesis

Named alongside it

The objects these essays reach for when they reach for this one.

Big bang nucleosynthesisDeuterium abundanceBaryon densityFreeze outLithium problemPrimordial abundanceConvective envelopeDeuterium bottleneckGamow peakNeutron proton ratioPrimordial heliumQuasar absorption lines

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