Cosmic Tsunamis in Modified Gravity: Disruption of Screening Mechanisms from Scalar Waves

Extending general relativity by adding extra degrees of freedom is a popular approach for explaining the accelerated expansion of the Universe and to build high energy completions of the theory of gravity. The presence of such new degrees of freedom is, however, tightly constrained from several observations and experiments that aim to test general relativity in a wide range of scales. The viability of a given modified theory of gravity, therefore, strongly depends on the existence of a screening mechanism that suppresses the extra degrees of freedom. We perform simulations, and find that waves propagating in the new degrees of freedom can significantly impact the efficiency of some screening mechanisms, thereby threatening the viability of these modified gravity theories. Specifically, we show that the waves produced in the symmetron model can increase the amplitude of the fifth force and the parametrized post Newtonian parameters by several orders of magnitude.

Figure 1

The PPN parameter $|\gamma -1|$, plotted against distance from the center of the Galaxy. The curves show $|\gamma -1|$ in the quasistatic case (blue dashed line), as well as after a scalar wave has entered the halo (black solid line). The vertical (green dotted) line indicates the position of the Solar System, and the horizontal (red dotted) line indicates the highest allowed value of $\gamma -1$ in the Solar System from the Cassini experiment. When the wave enters the Milky Way, it increases the value of $|\gamma -1|$ by several orders of magnitude.

Figure 2

Maximum increase in the PPN parameter $|\gamma -1|$ due to incoming scalar field waves at the position of the Sun in the Galaxy (8 kpc from the center) as a function of amplitude and frequency of the incoming waves. The color indicates by which factor $|\gamma -1|$ is increased when compared to the quasistatic case with no waves.