[Physics FAQ] - [Copyright]

Original by David Brahm.

Baryogenesis: Why Are There More Protons Than Antiprotons?

How do we really know that the universe is not matter-antimatter symmetric?

  1. The Moon: Neil Armstrong did not annihilate, therefore the moon is made of matter.
  2. The Sun: Solar cosmic rays are matter, not antimatter.
  3. The other Planets: We have sent probes to almost all.  Their survival demonstrates that the solar system is made of matter.
  4. The Milky Way: Cosmic rays sample material from the entire galaxy. In cosmic rays, protons outnumber antiprotons 104 to 1.
  5. The Universe at large: This is tougher.  If there were antimatter galaxies then we should see gamma emissions from annihilation.  Its absence is strong evidence that at least the nearby clusters of galaxies (e.g., Virgo) are matter-dominated.  At larger scales there is little proof.
    However, there is a problem, called the "annihilation catastrophe" which probably eliminates the possibility of a matter-antimatter symmetric universe.  Essentially, causality prevents the separation of large chucks of antimatter from matter fast enough to prevent their mutual annihilation in the early universe.  So the Universe is most likely matter dominated.

How did it get that way?

Annihilation has made the asymmetry much greater today than in the early universe.  At the high temperature of the first microsecond, there were large numbers of thermal quark-antiquark pairs.  Kolb and Turner estimate 30 million antiquarks for every 30 million and 1 quarks during this epoch. That's a tiny asymmetry.  Over time most of the antimatter has annihilated with matter, leaving the very small initial excess of matter to dominate the Universe.

Here are a few possibilities for why we are matter dominated today:

  1. The Universe just started that way. Not only is this a rather sterile hypothesis, but it doesn't work under the popular "inflation" theories, which dilute any initial abundance.
  2. Baryogenesis occurred around the Grand Unified (GUT) scale (very early). Long thought to be the only viable candidate, GUT's generically have baryon-violating reactions, such as proton decay (not yet observed).
  3. Baryogenesis occurred at the Electroweak Phase Transition (EWPT). This is the era when the Higgs first acquired a vacuum expectation value (vev), so other particles acquired masses.  Pure Standard Model physics.

In 1967 Sakharov enumerated 3 necessary conditions for baryogenesis:

  1. Baryon number violation.  If baryon number (B) is conserved in all reactions, then the present baryon asymmetry can only reflect asymmetric initial conditions, and we are back to the first case in the previous list.
  2. C and CP violation.  Even in the presence of B-violating reactions, without a preference for matter over antimatter the B-violation will take place at the same rate in both directions, leaving only a very tiny statistical excess, perhaps only enough matter to make one star in the observable universe.
  3. Thermodynamic Nonequilibrium.  Because CPT guarantees equal masses for baryons and antibaryons, chemical equilibrium would drive the necessary reactions to correct for any developing asymmetry.

It turns out the Standard Model satisfies all 3 conditions:

  1. Though the Standard Model conserves B classically (no terms in the Lagrangian violate B), quantum effects allow the universe to tunnel between vacua with different values of B.  This tunnelling is very suppressed at energies/temperatures below 10 TeV (the "sphaleron mass"), may occur at future supercollider energies (controversial), and certainly occurs at higher temperatures.
  2. C-violation is commonplace.  CP-violation (that's "charge conjugation" and "parity") has been experimentally observed in kaon decays, though strictly speaking the Standard Model probably has insufficient CP-violation to give the observed baryon asymmetry.
  3. Thermal nonequilibrium is achieved during first-order phase transitions in the cooling early universe, such as the EWPT (at T = 100 GeV or so).  As bubbles of the "true vacuum" (with a nonzero Higgs vev) percolate and grow, baryogenesis can occur at or near the bubble walls.

A major theoretical problem, in fact, is that there may be too much B-violation in the Standard Model, so that after the EWPT is complete (and condition 3 above is no longer satisfied) any previously generated baryon asymmetry would be washed out.


  1. Kolb and Turner, The Early Universe
  2. Sakharov, JETP, 5, 32 (1967)
  3. Dine, Huet, Singleton & Susskind, Phys.Lett.B257:351 (1991)
  4. Dine, Leigh, Huet, Linde & Linde, Phys.Rev.D46:550 (1992).