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B. The many roles of cosmic phase transitions

Phase transitions and baryogenesis One of the insights we hope to gain from the application of particle physics to the early Universe is an explanation of the observed baryon asymmetry of the Universe. A crucial ingredient in any baryogenesis scenario is a period during which the relevant processes are out of equilibrium. Cosmic phase transitions provide an excellent opportunity to create out-of-equilibrium effects, and phase transitions are central to a wide variety of baryogenesis scenarios, from the GUT scale all the way down to the electroweak scale. Some scenarios involve topological defects, while others involve other out-of-equilibrium effects. A more extensive discussion of baryogenesis (including the connection with phase transitions) can be found in section VI.

Phase transitions and inflation As noted in subsection VA, phase transitions were crucial in creating the idea of cosmic inflation. They provided the first specific mechanism for how the Universe could enter an inflationary state, and also led to the monopole problem, which stimulated a fresh thinking about early Universe cosmology. Phase transitions continue to play a central role in the development of the inflationary scenario, and bubbles produced in a higher dimensional phase transition are at the core of a fascinating new alternative to inflation [Bucher(2001)].

Cosmic Rays Cosmic rays are the most energetic particles observed, and they have energies almost a billion times greater than particles in the Tevatron. The origin of these particles remains a mystery. Defects formed in cosmic phase transitions carry energy densities set by the energy scale of the phase transition, which could be upwards of 1016 GeV. Topologically stable defects would persist, to some degree at least, until the present day, and could perhaps produce ultra high energy cosmic rays. Thus cosmic rays could be providing us with a window on symmetry breaking at ultra-high energies. (Cosmic rays in general are discussed at length in the report of group P4.5.)

Gravity Waves Perhaps the most ambitious frontier of physics is the pursuit of gravity wave detection. Because the energy scales for cosmic defects can be extremely high, cosmic defects can be a significant source of observable gravitational waves. In fact, for topologically stable defects, the emission of gravity waves is often the only decay channel, and significant amounts of gravity waves are produced. (See the reports of the P4.3 and P4.6 groups for further discussion.)

Cosmic Magnetic Fields Magnetic fields of 10-6 Gauss are common within galaxies, and extragalactic magnetic fields are also present, although at lower strength. The origin of these fields, and especially of the small "seeds" that could be amplified by astrophysical processes, remains a mystery. One very interesting possibility is that primordial magnetic fields were generated in a cosmic phase transition [Berera et al.(1999), Olinto(1998)].

Exotic Objects Phase transitions produce dramatic out-of-equilibrium effects, and cosmic phase transitions can generate a wide range of exotic objects. Such objects could be a cosmological disaster (such as domain walls or monopoles), or they could help explain significant phenomena (for example, "WIMP-zillas", a candidate for the Dark Matter, could have formed in a cosmic phase transition [Kolb et al.(1998)]).

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