标题： 伽利略引力的速度 显示英文标题

标题： The Speed of Galileon Gravity

摘要： 我们分析了在方程状态$\omega_\phi=-1$的耦合伽利略模型中引力波的速度以及无鬼态的闵可夫斯基极限。 我们发现，除非这些模型是三次伽利略项的小扰动，并且伽利略能量密度次于主导的宇宙常数，否则引力波的传播速度远快于光速。 在这种情况下，双脉冲星对引力波速度的限制可以得到满足，并且当伽利略拉格朗日量的三次项系数与物质耦合相关时，方程状态可以接近 -1。 这严重限制了伽利略模型允许的宇宙学行为，我们被迫得出结论：具有稳定闵可夫斯基极限的伽利略场无法单独解释观测到的宇宙膨胀加速。 此外，我们宇宙中的任何次主导的伽利略组分必须由三次项主导。 对于引力子传播速度超过光速的此类模型，引力子可能变得不稳定，并可能衰变为光子对。 它们也可能通过切连科夫辐射发射光子。 我们表明，这种快速引力子衰变为光子以及切连科夫辐射的实际速率是可以忽略不计的。 此外，由爆炸性天体物理事件发出的引力信号和光之间的延迟时间可以作为引力在宇宙最大尺度上发生修改的验证。 显示英文摘要

摘要： We analyse the speed of gravitational waves in coupled Galileon models with an equation of state $\omega_\phi=-1$ now and a ghost-free Minkowski limit. We find that the gravitational waves propagate much faster than the speed of light unless these models are small perturbations of cubic Galileons and the Galileon energy density is sub-dominant to a dominant cosmological constant. In this case, the binary pulsar bounds on the speed of gravitational waves can be satisfied and the equation of state can be close to -1 when the coupling to matter and the coefficient of the cubic term of the Galileon Lagrangian are related. This severely restricts the allowed cosmological behaviour of Galileon models and we are forced to conclude that Galileons with a stable Minkowski limit cannot account for the observed acceleration of the expansion of the universe on their own. Moreover any sub-dominant Galileon component of our universe must be dominated by the cubic term. For such models with gravitons propagating faster than the speed of light, the gravitons become potentially unstable and could decay into photon pairs. They could also emit photons by Cerenkov radiation. We show that the decay rate of such speedy gravitons into photons and the Cerenkov radiation are in fact negligible. Moreover the time delay between the gravitational signal and light emitted by explosive astrophysical events could serve as a confirmation that a modification of gravity acts on the largest scales of the Universe.