Phase transitions play a very important role in the evolution of our
universe. They may leave behind imprints on the canvas of our universe
that survive to the present period and may be observable now. An
interesting remnant of a phase transition are the topological defects.
Topological defects such as the cosmic strings may play an important
role in generating cosmological density perturbations.
Cosmic strings are topological defects formed due to some symmetry
breaking phase transition. Topological defects are points, lines, or
surfaces where the order parameter gets locked in the symmetric phase
when the rest of the system is in the broken symmetry phase. So these
are defects in the background broken symmetry phase. Clearly the system
will not want these defects since they are now in a phase which has a
higher free energy than the rest of the system. But it is
possible that a mapping of the region surrounding the defect into the
vacuum manifold is topologically non-trivial. Any such topological
constraints will then force a topological stability to the defects.
Topological defects play an important role in cosmology. It was pointed
out that domain walls formed during GUT phase transitions would have
enormous mass density which would spoil the homogeneity of the
universe, thereby conflicting with cosmological observations. Similar
troubles arose with the GUT monopole too. Only the cosmic strings
seemed to have some beneficial role. For some time it was believed that
density inhomogeneities generated by cosmic strings may seed structure
formation in the early universe, but recent observational data of the
CMBR have more or less ruled out these models. But even though they may
not have seeded the structure formation they may be important in
explaining other puzzles in the universe. Like, they may be responsible
for the generation of ultrahigh energy cosmic rays.
A good starting point to read about them is the book :
Cosmic strings and other topological
defects, by A. Vilenkin and E.P.S. Shellard
(CambridgeUniversity Press 1994).
There are also some material online:
1) Topological
Defects in Cosmology (Gangui).
2) Topological
defects (Poletti)
3) A site with
links to papers and workshops on topological defects in cosmology.
4) Topological
defects in cosmology. (Review by Ajit M. Srivastava)
5) COSMIC
STRINGS AND OTHER DEFECTS (Has very nice pictures.)
Cosmic strings move with very high velocities through the background
medium. Due to the conical metric of the string a wake forms
behind the moving strings. In the radiation dominated epoch the
background was a relativistic fluid of elementary particles. Cosmic
strings moving through this relativistic fluid can generate shocks. A
shock is formed
when the string velocity exceeds the velocity of sound and the
properties of the shock can be determined following the standard
treatment of shocks. We have studied the effect of these cosmic
string wakes on the quark-hadron transition, and a second order
electroweak phase transition.
Publications.
1) Cosmic string induced
sheet like baryon inhomogeneities at quark-hadron transition.
Biswanath
Layek, Soma
Sanyal, Ajit
M. Srivastava.
Journal-ref: Phys.Rev. D63
(2001) 083512
2) Baryogenesis via
Density Fluctuations with a Second Order Electroweak Phase Transition.
Biswanath
Layek, Soma
Sanyal, Ajit
M. Srivastava.
Journal-ref: Int.J.Mod.Phys. A18
(2003) 4851-4850
3) Baryon inhomogeneity
generation via cosmic strings at QCD scale and its effects on
nucleosynthesis.
Biswanath Layek, Soma
Sanyal, Ajit
M. Srivastava.
Journal-ref: Phys.Rev. D67 (2003)
083508
Conference Proceedings
Baryon inhomogeneities due to cosmic string wakes at the quark-hadron transition.
B. Layek, Soma Sanyal and A. M. Srivastava.
Proc. Physics and Astrophysics of Quark-Gluon Plasma, Jaipur
November,2001.
Pramana 60 (2002) 997.