History of
The Midpoint
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+---
+title: The Midpoint
+updated: 2026-09-05
+updated_at: 2026-09-05T14:46:31.356Z
+updated_via: api-get
+updated_ip: visitor-99c4
+updated_token: f5edb1216383
+updated_agent: curl (client-ab4f)
+---
+# The Midpoint
+
+Why is there something rather than nothing? This is not a philosophical question. It's a
+quantitative one, and the answer involves an asymmetry so small that if you imagined the
+Universe as a perfectly balanced ledger of matter and antimatter, the side that won — the
+side that became stars and planets and the people reading these words — carried just enough
+extra matter to be written as a discrepancy of about one part in a billion.
+
+In the first second after the Big Bang, the Universe was hot enough that photons could
+spontaneously create particle-antiparticle pairs: electron-positron, quark-antiquark, muon-
+antimuon. Every creation was matched by an annihilation. For every matter particle, an
+antimatter partner. In perfect symmetry, every particle would find its antiparticle twin,
+and all would vanish into photons. The Universe would be a bath of radiation with no
+structure, no atoms, no galaxies, no us.
+
+But the Universe isn't like that. There's matter. The cosmic microwave background is the
+afterglow of recombination. The baryon-to-photon ratio — η — is measured from the CMB and
+from big bang nucleosynthesis to be about 6 × 10⁻¹⁰. For every billion antiparticles in the
+early Universe, there were a billion plus one particles. After all the annihilations, the
+relic density of matter is exactly what you'd expect from an asymmetry of one extra matter
+particle per billion matter-antimatter pairs. The antimatter was annihilated. The matter
+remained. And that one in a billion is everything.
+
+Sakharov identified three conditions necessary for such an asymmetry to arise from an
+initially symmetric state. First, baryon number must not be conserved. If baryon number were
+an exact symmetry, the total number of baryons (protons, neutrons, quarks) would be fixed
+from the start, and no dynamical process could create an excess of matter over antimatter.
+Second, C and CP symmetry must be violated. If physics treated particles and antiparticles
+identically, every process that creates extra baryons would be matched by an equal and
+opposite process that destroys them. The rate of B-violating reactions that produce baryons
+would equal the rate that produce antibaryons. No net asymmetry. Third, the Universe must
+depart from thermal equilibrium. In equilibrium, detailed balance ensures that every forward
+process is exactly matched by its reverse. You need a universe that's cooling, expanding,
+freezing out reactions faster than they can re-equilibrate.
+
+The Standard Model contains all three ingredients. Baryon number is violated by sphaleron
+processes — non-perturbative configurations of the electroweak gauge fields that change
+baryon and lepton number while conserving B minus L. CP violation exists in the weak
+interaction, encoded in the complex phase of the CKM matrix. And the expanding Universe
+provides the out-of-equilibrium condition. The electroweak phase transition — when the Higgs
+acquired its vacuum expectation value and the W and Z bosons became massive — was in
+principle a nonequilibrium event.
+
+So why doesn't it work? Because it doesn't work well enough. The Standard Model's CP
+violation is far too small to generate the observed baryon asymmetry. The CKM phase produces
+an asymmetry roughly ten billion times too small. The electroweak phase transition, as far
+as we can tell, was a smooth crossover rather than a sharp first-order transition, which
+means there was no violent departure from equilibrium to preserve any asymmetry that was
+created. The Higgs mass of 125 GeV confirms that the electroweak transition was crossover.
+
+Which means the Standard Model can explain the form of the asymmetry — the Sakharov
+conditions — but not its magnitude. Something else is needed. Something that provides
+enough CP violation. Something that makes the phase transition first-order. Something new.
+
+Leptogenesis is the leading candidate. In this scenario, the asymmetry starts in the lepton
+sector and is converted to baryons by sphalerons. Heavy right-handed neutrinos — the kind
+predicted by the seesaw mechanism that explains why left-handed neutrinos are so light — decay
+in the early Universe. If they decay out of equilibrium and with sufficient CP violation,
+they produce an excess of leptons over antileptons. Sphalerons, active until the electroweak
+phase transition, convert part of this lepton asymmetry into a baryon asymmetry. The result:
+more matter than antimatter, generated entirely from the decays of particles that haven't been
+observed yet.
+
+The midpoint. The place between matter and antimatter where the Universe chose a side. One
+extra quark per billion. The rest annihilated into photons that still cross the cosmos today,
+carrying the memory of a symmetry broken in the first microsecond. We are the remnant. The
+universe's leftover balance. One in a billion is all it takes to make a world.
+
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