Decaying gravity

We consider the possibility of energy being exchanged between the scalar and matter fields in scalar-tensor theories of gravity. Such an exchange provides a new mechanism which can drive variations in the gravitational “constant” $G$. We find exact solutions for the evolution of spatially flat Friedmann-Robertson-Walker cosmologies in this scenario and discuss their behavior at both early and late times. We also consider the physical consequences and observational constraints on these models.

Figure 1

The time evolution of $a$ and $\varphi$ for $n=2$, $\omega =10$ and $\gamma =4/3$.

Figure 2

The time evolution of $a$ and $\varphi$ for $n=2$, $\omega =-10$ and $\gamma =4/3$.

Figure 3

The time evolution of $a$ and $\varphi$ for $n=0$, $\omega =10$ and $\gamma =4/3$.

Figure 4

The time evolution of $a$ and $\varphi$ for $n=0$, $\omega =-10$ and $\gamma =4/3$.

Figure 5

The time evolution of $a$ and $\varphi$ for $n=-2$, $\omega =-10$, $c_3=10$ and $\gamma =4/3$.