Constraining anisotropic models of the early Universe with WMAP9 data

We constrain several models of the early Universe that predict a statistical anisotropy of the cosmic microwave background (CMB) sky. We make use of WMAP9 maps deconvolved with beam asymmetries. As compared to previous releases of WMAP data, they do not exhibit the anomalously large quadrupole of statistical anisotropy. This allows to strengthen the limits on the parameters of models established earlier in the literature. In particular, the amplitude of the special quadrupole is constrained as |g_*|<0.072 at 95% C.L. (-0.046<g_*<0.048 at 68% C.L.) independently of the preferred direction in the sky. The upper limit is obtained on the total number of e-folds in anisotropic inflation with the Maxwellian term nonminimally coupled to the inflaton, namely N_{tot} <N_{CMB} +82 at 95% C.L. (+14 at 68% C.L.) for N_{CMB}=60. We also constrain models of the (pseudo)conformal universe. The strongest constraint is obtained for spectator scenarios involving a long stage of subhorizon evolution after conformal rolling, which reads h^2 < 0.006 at 95% C.L., in terms of the relevant parameter. The analogous constraint is much weaker in dynamical models, e.g., Galilean genesis.Comment: 28 pages, 8 figures; references added, matches published versio

Constraints on violation of Lorentz invariance from atmospheric showers initiated by multi-TeV photons

We discuss the effect of hypothetical violation of Lorentz invariance at high energies on the formation of atmospheric showers by very-high-energy gamma rays. In the scenario where Lorentz invariance violation leads to a decrease of the photon velocity with energy the formation of the showers is suppressed compared to the Lorentz invariant case. Absence of such suppression in the high-energy part of spectrum of the Crab nebula measured independently by HEGRA and H.E.S.S. collaborations is used to set lower bounds on the energy scale of Lorentz invariance violation. These bounds are competitive with the strongest existing constraints obtained from timing of variable astrophysical sources and the absorption of TeV photons on the extragalactic background light. They will be further improved by the next generation of multi-TeV gamma-ray observatories.Comment: 21 pages, 4 figures. References adde

On calculation of cross sections in Lorentz violating theories

We develop a systematic approach to the calculation of scattering cross sections in theories with violation of the Lorentz invariance taking into account the whole information about the theory Lagrangian. As an illustration we derive the Feynman rules and formulas for sums over polarizations in spinor electrodynamics with Lorentz violating operators of dimensions four and six. These rules are applied to compute the probabilities of several astrophysically relevant processes. We calculate the rates of photon decay and vacuum Cherenkov radiation along with the cross sections of electron-positron pair production on background radiation and in the Coulomb field. The latter process is essential for detection of photon-induced air showers in the atmosphere.Comment: 23 pages, 1 figur

Prospective constraints on Lorentz violation from ultrahigh-energy photon detection

We point out that violation of Lorentz invariance affects the interaction of high-energy photons with the Earth's atmosphere and magnetic field. In certain parameter region this interaction becomes suppressed and the photons escape observation passing through the atmosphere without producing air showers. We argue that a detection of photon-induced air showers with energies above 10^19 eV, implying the absence of suppression as well as the absence of photon decay, will put tight double-sided limits on Lorentz violation in the sector of quantum electrodynamics. These constraints will be by several orders of magnitude stronger than the existing ones and will be robust against any assumptions about the astrophysical origin of the detected photons.Comment: 6 pages, revtex; significant additions compared to the previous versio

First constraints on the strength of the extragalactic magnetic field from γ\gamma-ray observations of GRB 221009A

The extragalactic magnetic field (EGMF) could be probed with $\gamma$-ray observations of distant sources. Primary very high energy (VHE) $\gamma$-rays from these sources absorb on extragalactic background light photons, and secondary electrons/positrons from the pair production acts create cascade $\gamma$-rays. These cascade $\gamma$-rays could be detected with space $\gamma$-ray telescopes such as Fermi-LAT. The $\gamma$-ray burst GRB 221009A was an exceptionally bright transient well suited for intergalactic $\gamma$-ray propagation studies. Using publicly-available Fermi-LAT data, we obtain upper limits on the spectrum of delayed emission from GRB 221009A during the time window of 30 days after the burst, and compare these with model spectra calculated for various EGMF strengths $B$, obtaining lower limits on $B$. We show that the values of $B < 10^{-18}$ G are excluded. For some optimistic models of the VHE spectrum of GRB 221009A, the values of $B < 10^{-17}$ G are excluded.Comment: 5 pages, 6 figure