We construct an intersecting brane configuration in six-dimensional space with one extra spacelike and one extra time-like dimensions. With a certain additional symmetry imposed on the extra space-time we have found that effective four-dimensional cosmological constant vanishes automatically, providing the static solution with gravity fully localized at the intersection region as there are no propagating massive modes of graviton. In this way, the same symmetry allows us to eliminate tachyonic states of graviton from the spectrum of the effective four-dimensional theory, thus avoiding phenomenological difficulties comming from the matter instability usually induced in theories with extra time-like dimensions.

As is well known, there are various mass limits for compact stars. For example, the maximum mass for non-rotating white dwarfs is given by the famous Chandrasekhar limit about 1.4 M ☉ (solar masses). Although the mass limit for neutron stars is not so clear to date, one of the widely accepted values is about 2.1 M ☉. Recently, challenges to these mass limits appeared. Motivated by the super-Chandrasekhar mass white dwarfs with masses up to 2.4 ∼ 2.8 M ☉, and compact objects (probably neutron stars) in the mass gap (from 2.5 M ☉ or 3 M ☉ to 5 M ☉) inferred from gravitational waves detected by LIGO/Virgo in the third observing run (O3), we reconsider the mass limits for compact stars in the present work. Without invoking strong magnetic field and/or exotic equation of state (EOS), we try to increase the mass limits for compact stars in modified gravity theory. In this work, we propose an inverse chameleon mechanism, and show that the fifth-force mediated by the scalar field can evade the severe tests on earth, in solar system and universe, but manifest itself in compact stars such as white dwarfs and neutron stars. The mass limits for compact stars in the inverse chameleon mechanism can be easily increased to 3 M ☉, 5 M ☉ or even larger. We argue that the inverse chameleon mechanism might be constrained by the observations of exoplanets orbiting compact stars (such as white dwarfs and neutron stars), and gravitational waves from the last stage of binary compact star coalescence.

No abstract

No abstract