Inflationary cosmology- A new approach using Non-linear electrodynamics

We explore a new kind of field of nonlinear electrodynamics(NLED) which acts as a source of gravity and can accelerate the universe during the inflationary era. We propose a new type of NLED lagrangian which is charecterized by two paremetrs $\alpha$ and $\beta$. We investigate the classical stability and causality aspects of this model by demanding that the speed ($C_s = \frac{dP}{d\rho}$) of the sound wave $C_s^2 > 0$ and $C_s \le 1$ and find that $0 < C_s^2 < 1$ corresponds to $0.25 \le \alpha \le 0.4$ and $0.6 \le \beta B^2 \le 1$. A study of the deceleration parameter($q = \frac{1}{2}(1 + 3 \omega)$, $\omega = P/\rho$ being the equation of state parameter) suggests that the value $q < 0$ (i.e. $\omega < -1/3$ and $\ddot{a}(t) > 0$ ( the accelerating universe)) requires $\beta B^2 \ge 0.13$. During inflation, the energy density $\rho_B$ is found to be maximum and is given by $\rho_B^{max} = 0.65/\beta$ corresponding to $\alpha = 0.3$. The magnetic field necessary to trigger the inflation, is found to be $B (= B_{max}) \simeq \sqrt{\frac{0.4 \rho_B^{max}}{0.65}} = 4 \times 10^{51}~{\rm Gauss}$, where $\rho_{B}^{max}( =10^{64}~{\rm GeV}^4$) is the energy density of the universe during inflation. The model also predict the e-fold number $N = 71$, which agrees with the experimental result.