Equivalence principle implications of modified gravity models
Abstract
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 Dvali-Gabadadze-Porrati braneworld model, equivalence principle violation occurs at a much reduced level. We propose severalmore »
- Authors:
-
- Institute for Strings, Cosmology, and Astroparticle Physics (ISCAP), Department of Physics, Columbia University, New York, New York 10027 (United States)
- Publication Date:
- OSTI Identifier:
- 21308619
- Resource Type:
- Journal Article
- Journal Name:
- Physical Review. D, Particles Fields
- Additional Journal Information:
- Journal Volume: 80; Journal Issue: 10; Other Information: DOI: 10.1103/PhysRevD.80.104002; (c) 2009 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0556-2821
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; BRANES; COSMOLOGICAL MODELS; COUPLING; DEGREES OF FREEDOM; EQUIVALENCE PRINCIPLE; FLUCTUATIONS; GALAXIES; GENERAL RELATIVITY THEORY; GEODESICS; GRAVITATION; MASS; M-THEORY; ORBITS; POTENTIALS; STARS; STRONG-COUPLING MODEL; TEST PARTICLES; VELOCITY
Citation Formats
Hui, Lam, Nicolis, Alberto, Stubbs, Christopher W, and Department of Physics, Harvard University, Cambridge, Massachusetts 02138. Equivalence principle implications of modified gravity models. United States: N. p., 2009.
Web. doi:10.1103/PHYSREVD.80.104002.
Hui, Lam, Nicolis, Alberto, Stubbs, Christopher W, & Department of Physics, Harvard University, Cambridge, Massachusetts 02138. Equivalence principle implications of modified gravity models. United States. https://doi.org/10.1103/PHYSREVD.80.104002
Hui, Lam, Nicolis, Alberto, Stubbs, Christopher W, and Department of Physics, Harvard University, Cambridge, Massachusetts 02138. 2009.
"Equivalence principle implications of modified gravity models". United States. https://doi.org/10.1103/PHYSREVD.80.104002.
@article{osti_21308619,
title = {Equivalence principle implications of modified gravity models},
author = {Hui, Lam and Nicolis, Alberto and Stubbs, Christopher W and Department of Physics, Harvard University, Cambridge, Massachusetts 02138},
abstractNote = {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 Dvali-Gabadadze-Porrati 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 the 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.},
doi = {10.1103/PHYSREVD.80.104002},
url = {https://www.osti.gov/biblio/21308619},
journal = {Physical Review. D, Particles Fields},
issn = {0556-2821},
number = 10,
volume = 80,
place = {United States},
year = {Sun Nov 15 00:00:00 EST 2009},
month = {Sun Nov 15 00:00:00 EST 2009}
}