Modified gravity makes galaxies brighter

We investigate the effect of modified gravity with screening mechanisms, such as the chameleon or symmetron models, upon the structure of main sequence stars. We find that unscreened stars can be significantly more luminous and ephemeral than their screened doppelgangers. By embedding these stars into dwarf galaxies, which can be unscreened for values of the parameters not yet ruled out observationally, we show that the cumulative effect of their increased luminosity can enhance the total galactic luminosity. We estimate this enhancement and find that it can be considerable given model parameters that are still under experimental scrutiny. By looking for systematic offsets between screened dwarf galaxies in clusters and unscreened galaxies in voids, these effects could form the basis of an independent observational test that can potentially lower the current experimental bounds on the model independent parameters of these theories by an order of magnitude or more.

Figure 1

The field profile inside a spherical overdensity embedded in a homogeneous background. The field minimizes its effective potential at a distance $r_{\mathrm{s}}$ from the center. Far from the overdensity, the field asymptotically approaches the value which minimizes the effective potential in the background, ${\varphi }_0$. The region $r>r_{\mathrm{s}}$ is known as the unscreened region and here the fifth force is $\mathcal{O}(1)$. In the region interior to $r_{\mathrm{s}}$ the fifth force is negligible and this region is said to be screened.

Figure 2

The ratio of the stellar luminosity of a partially screened star compared with one that is completely screened as a function of stellar mass $M$. $L_{\mathrm{std}}$ is the luminosity of a completely screened (general relativity) star. From top to bottom: ${\chi }_0=({10}^{-4},{10}^{-5},5\times {10}^{-6},{10}^{-6})$.

Figure 3

The Hertsprung-Russell diagram for stars of one solar mass with initial metallicity $Z=0.02$. From bottom to top: the black line shows the tracks for stars in general relativity while the red, blue, and green tracks correspond to stars in modified gravity with ${\chi }_0={10}^{-7}$, ${10}^{-6}$, and $5\times {10}^{-6}$, respectively. The radius and age at the point where the central hydrogen mass fraction has fallen to 0.5, 0.1, and ${10}^{-5}$ are shown for each star except the ${\chi }_0={10}^{-7}$ case.

Figure 4

The chameleon effective potential (solid line) for small and large local densities. The dashed lines show the contribution from the potential and the dotted lines show the contribution from the coupling.