Inflation as an amplifier: The case of Lorentz violation

Modified gravity theories are supposed to incorporate low-energy quantum-gravity effects and, at the same time, they could shed light into the dark matter and dark energy problems. Here we study a particular modification of general relativity where local Lorentz invariance is spontaneously broken and whose physical effects, despite a decade-long effort, were unknown. We show that, during inflation, this modification produces anisotropies that would generate measurable effects on the cosmic microwave background. Then, by using empirical constraints on the B-mode polarization spectrum, we can estimate that the “coefficient” components absolute value have to be smaller than 10−43. This is a remarkably strong limit; in fact, it is 29 orders of magnitude better than the best constraints on similar coefficients. Thus, we propose that inflation could stringently test other modified gravity theories.