[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?
- The Moon: Neil Armstrong did not annihilate, therefore the moon is made of
matter.
- The Sun: Solar cosmic rays are matter, not antimatter.
- The other Planets: We have sent probes to almost all. Their survival
demonstrates that the solar system is made of matter.
- The Milky Way: Cosmic rays sample material from the entire galaxy. In
cosmic rays, protons outnumber antiprotons 104 to 1.
- 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:
- 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.
- 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).
- 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:
- 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.
- 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.
- 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:
- 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.
- 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.
- 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.
References
- Kolb and Turner, The Early Universe
- Sakharov, JETP, 5, 32 (1967)
- Dine, Huet, Singleton & Susskind, Phys.Lett.B257:351 (1991)
- Dine, Leigh, Huet, Linde & Linde, Phys.Rev.D46:550 (1992).