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This theoretical study examines whether the Vainshtein mechanism, which is thought to suppress fifth force effects near massive objects in modified gravity theories, remains effective when the source is not perfectly spherical. The researchers found that in the effective field theory of dark energy, multipole moments of the fifth force are not adequately screened even in regions where the monopole component is suppressed, displaying characteristic oscillatory behavior instead. Even in special parameter configurations that minimize graviton decay, the Vainshtein mechanism fails to efficiently screen the fifth force around non-spherical sources.
Why it matters
This finding challenges a key assumption in modified gravity theories that attempt to explain cosmic acceleration. If fifth forces cannot be adequately screened around realistic non-spherical astronomical objects, these theories may produce observable signatures that could either constrain or rule out certain dark energy models through precise gravitational measurements.
Understand the Science
arXiv:2607.09098v1 Announce Type: cross
Abstract: It has been argued that degenerate higher-order scalar-tensor theories and the effective field theory (EFT) of dark energy are endowed with the Vainshtein mechanism, resulting in a screened fifth force in the exterior of a gravitational source. However, spherical symmetry has been assumed in most of the discussions so far. In this paper, we study whether the Vainshtein mechanism operates in the EFT of dark energy beyond spherical symmetry, focusing in particular on the role of the “beyond Horndeski” EFT parameters. Assuming that deviations from spherical symmetry are small, we compute multipole moments of the gravitational potential for a given nonspherical source. For a generic choice of the “beyond Horndeski” EFT parameters, it is shown that the multipole moments of the fifth force are not screened in the region where the monopole component is screened. Rather, the gravitational potential shows a characteristic oscillatory behavior in its multipole components. In the special case where the EFT parameters are tuned so that graviton decay into dark energy is practically absent, the behavior of the multipole moments is qualitatively different from that in the generic case. However, also in this case, the Vainshtein mechanism is not efficient enough to screen the fifth force around the source.
Source: Unscreened multipole moments of the fifth force in the EFT of dark energy