Amol Upadhye

Existence of relativistic stars in f ( R ) gravity

We refute recent claims in the literature that stars with relativistically deep potentials cannot exist in f(R) gravity. Numerical examples of stable stars, including relativistic (GM⋆/r⋆ � 0.1), constant density stars, are studied. As a star is made larger, non-linear “chameleon” effects screen much of the star’s mass, stabilizing gravity at the stellar center. Furthermore, we show that the onset of this chameleon screening is unrelated to strong gravity. At large central pressures P > �/3, f(R) gravity, like general relativity, does have a maximum gravitational potential, but at a slightly smaller value: GM⋆/r⋆|max = 0.345 < 4/9 for constant density and one choice of parameters. This difference is associated with negative central curvature R under general relativity not being accessed in the f(R) model, but does not apply to any known astrophysical object.

Dark energy fifth forces in torsion pendulum experiments

The chameleon scalar field is a matter-coupled dark energy candidate whose nonlinear self-interaction partially screens its fifth force at laboratory scales. Nevertheless, small-scale experiments such as the torsion pendulum can provide powerful constraints on chameleon models. Here we develop a simple approximation for computing chameleon fifth forces in torsion pendulum experiments such as Eot-Wash. We show that our approximation agrees well with published constraints on the quartic chameleon, and we use it to extend these constraints to a much wider range of models. Finally, we forecast the constraints which will result from the next-generation Eot-Wash experiment, and show that this experiment will exclude a wide range of quantum-stable models.

Laboratory constraints on chameleon dark energy and power-law fields.

We report results from a search for chameleon particles created via photon-chameleon oscillations within a magnetic field. This experiment is sensitive to a wide class of unexplored chameleon power-law and dark energy models. These results exclude 5 orders of magnitude in the coupling of chameleons to photons covering a range of 4 orders of magnitude in chameleon effective mass and, for individual models, exclude between 4 and 12 orders of magnitude in chameleon couplings to matter.

Constraining chameleon field theories using the GammeV afterglow experiments

The GammeV experiment has constrained the couplings of chameleon scalar fields to matter and photons. Here, we present a detailed calculation of the chameleon afterglow rate underlying these constraints. The dependence of GammeV constraints on various assumptions in the calculation is studied. We discuss the GammeV-CHameleon Afterglow SEarch, a second-generation GammeV experiment, which will improve upon GammeV in several major ways. Using our calculation of the chameleon afterglow rate, we forecast model-independent constraints achievable by GammeV-CHameleon Afterglow SEarch. We then apply these constraints to a variety of chameleon models, including quartic chameleons and chameleon dark energy models. The new experiment will be able to probe a large region of parameter space that is beyond the reach of current tests, such as fifth force searches, constraints on the dimming of distant astrophysical objects, and bounds on the variation of the fine structure constant.

Symmetron dark energy in laboratory experiments.

The symmetron scalar field is a matter-coupled dark energy candidate which effectively decouples from matter in high-density regions through a symmetry restoration. We consider a previously unexplored regime, in which the vacuum mass μ~2.4×10(-3) eV of the symmetron is near the dark energy scale, and the matter coupling parameter M~1 TeV is just beyond standard model energies. Such a field will give rise to a fifth force at submillimeter distances which can be probed by short-range gravity experiments. We show that a torsion pendulum experiment such as Eöt-Wash can exclude symmetrons in this regime for all self-couplings λ is < or approximately equal to 7.5.

Testing sub-gravitational forces on atoms from a miniature in-vacuum source mass

Atomic interferometry measurements of the gravitational force on free-falling atoms provide improved constraints on certain scalar field theories trying to explain dark energy.

Quantum Stability of Chameleon Field Theories

Chameleon scalar fields are dark-energy candidates which suppress fifth forces in high density regions of the Universe by becoming massive. We consider chameleon models as effective field theories and estimate quantum corrections to their potentials. Requiring that quantum corrections be small, so as to allow reliable predictions of fifth forces, leads to an upper bound m<0.0073(ρ/10 g cm(-3))(1/3) eV for gravitational-strength coupling whereas fifth force experiments place a lower bound of m>0.0042 eV. An improvement of less than a factor of two in the range of fifth force experiments could test all classical chameleon field theories whose quantum corrections are well controlled and couple to matter with nearly gravitational strength regardless of the specific form of the chameleon potential.

Large-scale structure formation with massive neutrinos and dynamical dark energy

Over the next decade, cosmological measurements of the large-scale structure of the Universe will be sensitive to the combined effects of dynamical dark energy and massive neutrinos. The matter power spectrum is a key repository of this information. We extend higher-order perturbative methods for computing the power spectrum to investigate these effects over quasilinear scales. Through comparison with N-body simulations, we establish the regime of validity of a time-renormalization group perturbative treatment that includes dynamical dark energy and massive neutrinos. We also quantify the accuracy of standard, renormalized and Lagrangian resummation (LPT) perturbation theories without massive neutrinos. We find that an approximation that neglects neutrino clustering as a source for nonlinear matter clustering predicts the baryon acoustic oscillation (BAO) peak position to 0.25% accuracy for redshifts

The GammeV suite of experimental searches for axion-like particles

1\ensuremath{\le}z\ensuremath{\le}3

Designing dark energy afterglow experiments

, justifying the use of LPT for BAO reconstruction in upcoming surveys. We release a modified version of the public Copter code which includes the additional physics discussed in the paper.