Equivalence Principle Implications of Modified Gravity Models

By L. Hui, Alberto Nicolis, C. Stubbs. 2009

196 citations recorded by the source corpus.

Theories that attempt to explain the observed cosmic acceleration by modifying general relativity all introduce a new scalar degree of freedom that is active on large scales, but is screened on small scales to match experiments. We demonstrate that if such screening occurs via the chameleon mechanism, such as in f(R) theory, it is possible to have order unity violation of the equivalence principle, despite the absence of explicit violation in the microscopic action. Namely, extended objects such as galaxies or constituents thereof do not all fall at the same rate. The chameleon mechanism can screen the scalar charge for large objects but not for small ones (large/small is defined by the depth of the gravitational potential, and is controlled by the scalar coupling). This leads to order one fluctuations in the ratio of the inertial mass to gravitational mass. We provide derivations in both Einstein and Jordan frames. In Jordan frame, it is no longer true that all objects move on geodesics; only unscreened ones, such as test particles, do. In contrast, if the scalar screening occurs via strong coupling, such as in the DGP braneworld model, equivalence principle violation occurs at a much reduced level. We propose several observational tests of the chameleon mechanism: 1. small galaxies should accelerate faster than large galaxies, even in environments where dynamical friction is negligible; 2. voids defined by small galaxies would appear larger compared to standard expectations; 3. stars and diffuse gas in small galaxies should have different velocities, even if they are on the same orbits; 4. lensing and dynamical mass estimates should agree for large galaxies but disagree for small ones. We discuss possible pitfalls in some of these tests. The cleanest is the third one where mass estimate from HI rotational velocity could exceed that from stars by 30% or more. To avoid blanket screening of all objects, the most promising place to look is in voids.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/2e8d7003100411a050b42d1b1f5df0ad02c2b50a

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Citations in the corpus

f(R) Theories (2010, background): “In this section we discuss the compatibility of f (R) models with local gravity constraints (see [469, 470, 245, 233, 154, 448, 251] for early works, and [31, 306, 134] for experimental constraints on viable f (R) dark energy models, and [101, 210, 330, 332, 471, 628, 149, 625, 329, 45, 511, 277, 53”
Single-field consistency relations of large scale structure part II: resummation and redshift space (2013, methodology): “an acceleration proportional to the mass and independent of the possible Vainshtein screening of the object [26].”
Probing dark energy with atom interferometry (2014, methodology): “In this Appendix we compute the force exerted by ball A on a test object, ball B, of mass MB , radius RB and density ρB , this discussion was first presented in Reference[16].”
Cosmology and Fundamental Physics with the Euclid Satellite (2012, background): “We mention here only that the presence of chameleon mechanisms [234, 887, 885, 235, 621, 236, 380] can, for example, modify the coupling depending on the environment.”
Cosmic Voids in Sloan Digital Sky Survey Data Release 7 (2011, methodology): “Beyond tests of the ΛCDM model, the properties of voids galaxies may even constrain alternative theories gravity (Hui et al. 2009).”
Cosmological tests of modified gravity (2015, background): “Therefore, the field inside the object loses knowledge of any exterior gradient and the fifth force generated by the external field, and thus once the object is screened, it does not feel any fifth force [167, 305, 306].”
Effective field theory of dark energy: A review (2019, unknown): “baryons and photons [130, 131].”
Symmetron Cosmology (2011, background): “The stars are oblivious to the symmetron whereas the hydrogen gas experiences an additional force, resulting in an enhanced rotational velocity for the gas [18].”
Observational probes of cosmic acceleration (2012, background): “Hui et al. (2009) and Jain and VanderPlas (2011) propose testing for differential acceleration of screened and unscreened objects in low density environments (e.g., stars vs. gas, or dwarf galaxies vs. giant galaxies), in effect looking for macroscopic and order unity violations of the equivalence…”
hi_class: Horndeski in the Cosmic Linear Anisotropy Solving System (2016, background): “2Modified gravity models may violate the equivalence principle via non-linear interactions [14].”