Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2009-06-26
JCAP 0911:001,2009
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
41 pages, 12 figures
Scientific paper
10.1088/1475-7516/2009/11/001
Gravitational wave production induces a strong constraint on the amplitude of a primordial magnetic field. It has been shown that the nucleosynthesis bound for a stochastic gravitational wave background implies that causally generated fields cannot have enough power on large scales to provide the seeds necessary for the observed magnetic fields in galaxies and clusters, even by the most optimistic dynamo amplification. Magnetic fields generated at inflation can have high enough amplitude only if their spectrum is very red. Here we show that helicity, which leads to an inverse cascade, can mitigate these limits. In particular, we find that helical fields generated at the QCD phase transition or at inflation with red spectrum are possible seeds for the dynamo. Helical fields generated at the electroweak phase transition are instead excluded as seeds at large scales. We also calculate the spectrum of gravitational waves generated by helical magnetic fields.
Caprini Chiara
Durrer Ruth
Fenu Elisa
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