Primordial magnetic fields from a non-singular bouncing cosmology

Abstract

Although inflation is a natural candidate to generate the lengths of coherence of magnetic fields needed to explain current observations, it needs to break conformal invariance of electromagnetism to obtain significant magnetic amplitudes. Of the simplest realizations are the kinetically-coupled theories $f^2(\phi)F_{\mu\nu}F^{\mu\nu}$ (or $IFF$ theories). However, these are known to suffer from electric fields backreaction or the strong coupling problem. In this work we shall confirm that such class of theories are problematic to support magnetogenesis during inflationary cosmology. On the contrary, we show that a bouncing cosmology with a contracting phase dominated by an equation of state with $p>-\rho/3$ can support magnetogenesis, evading the {backreaction/strong-coupling problem}. Finally, we study safe magnetogenesis in a particular bouncing model with an ekpyrotic-like contracting phase. In this case we found that $f^2(\phi)F^2$-instabilities might arise during the final kinetic-driven expanding phase for steep ekpyrotic potentials.Comment: 33 pages, 7 figures, v2: references added, typos corrected, v3 Final version published in NP

Primordial magnetic fields from gravitationally coupled electrodynamics in nonsingular bounce cosmology

“…To investigate the PMF, a specific formula of f is required. As is mentioned above, f is a function of the curvature tensors or scalar fields, namely, a function of the scale factor a(η) and its derivatives a (η) and a (η), etc.. For simplicity, the coefficient f can be taken a function of the scale factor only, and is usually assumed to have a power-law dependance f ∝ a n (η) in the bouncing scenario [36,[41][42][43]. In this model, the standard electrodynamics cannot be recovered when the universe is flat a → ∞.…”

“…Hence, to generate PMF, a mechanism which violates the conformal invariance should be introduced in the early universe [19][20][21][22][23]. Typically, the break of conformal invariance can be realized by introducing a coupling of the electromagnetic field (EMF) to another field such as gravity [22,[24][25][26][27][28][29] and (pseudo-)scalar field [19,20,[30][31][32][33][34][35][36][37][38][39][40][41][42][43] (and see Ref. [5,[44][45][46][47][48] for more complicated mechanisms).…”

“…The PMF generation in bouncing scenario has been studied by Refs. [36,[41][42][43]57], and most of them are in the mechanism that the EMF couples to a scalar field. In this paper, we investigate the PMF generation within a different mechanism that EMF couples to gravity, in bouncing scenario.…”

Lorentz-violating inflationary magnetogenesis

“…(Such problems are successfully evaded also in a magnetogenesis model proposed by Membiela [74]. In such a model, however, the background cosmology is given by a nonstandard bouncing cosmological model instead of standard inflation.…”

Bounce Cosmology in Generalized Modified Gravities

“…In particular, they can be eas- * Electronic address: geminas@phys.uoa.gr † Electronic address: msaridak@phys.uoa.gr ‡ Electronic address: pstavrin@math.uoa.gr § Electronic address: alktrian@phys.uoa.gr ily acquired in the Pre-Big-Bang [13,14] and the Ekpyrotic [15,16] models, in gravity actions with higher order corrections [17,18], in f (R) gravity [19,20], in f (T) gravity [21], in braneworld scenarios [22,23], in non-relativistic gravity [24][25][26], in Galileon theory [27,28], in massive gravity [29], in Lagrange modified gravity [30], in loop quantum cosmology [31][32][33], etc. Moreover, a nonsingular bounce model which supports magnetogenesis at the inflationary epoch is presented in [34].…”