[Physics FAQ] -
Original by David Brahm.
Baryogenesis: Why Are There More Protons Than Antiprotons?
How do we really know that the universe is not matter-antimatter
- 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.
But 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.
- 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).