Astrophysical Tests of Modified Gravity: Constraints from Distance Indicators in the Nearby Universe (1204.6044v2)

Abstract: We use distance measurements in the nearby universe to carry out new tests of gravity, surpassing other astrophysical tests by over two orders of magnitude for chameleon theories. The three nearby distance indicators -- cepheids, tip of the red giant branch (TRGB) stars, and water masers -- operate in gravitational fields of widely different strengths. This enables tests of scalar-tensor gravity theories because they are screened from enhanced forces to different extents. Inferred distances from cepheids and TRGB stars are altered (in opposite directions) over a range of chameleon gravity theory parameters well below the sensitivity of cosmological probes. Using published data we have compared cepheid and TRGB distances in a sample of unscreened dwarf galaxies within 10 Mpc. As a control sample we use a comparable set of screened galaxies. We find no evidence for the order unity force enhancements expected in these theories. Using a two-parameter description of the models (the coupling strength and background field value) we obtain constraints on chameleon and symmetron screening scenarios. In particular we show that f(R) models with background field values fR0 above 5e-7 are ruled out at the 95% confidence level. We also compare TRGB and maser distances to the galaxy NGC 4258 as a second test for larger field values. While there are several approximations and caveats in our study, our analysis demonstrates the power of gravity tests in the local universe. We discuss the prospects for additional, improved tests with future observations.

• The paper uses distance indicators like Cepheids, TRGB, and water masers in nearby galaxies to test scalar-tensor gravity theories and screening mechanisms, enabling constraints beyond cosmological probes.
• The study finds no evidence for predicted force enhancements and rules out f(R) models with background field values above 5 x 10^-7, significantly strengthening previous limits.
• This method distinguishes between screened and unscreened environments, providing a framework for future tests using other distance indicators and upcoming observational data.

Astrophysical Tests of Modified Gravity: Implications and Interpretations

The paper of modified gravity theories has gained significant interest as a means to address various astrophysical phenomena that remain unexplained by general relativity (GR). In an insightful exploration of this topic, Jain, Vikram, and Sakstein present a rigorous analysis of scalar-tensor gravity theories by using astrophysical tests from distance indicators in the nearby universe. They aim to impose constraints on such theories beyond what traditional cosmological probes might reveal.

Methodology and Constraints

The authors utilize three key distance indicators: Cepheids, tip of the red giant branch (TRGB) stars, and water masers, each operating within gravitational fields of varying strengths. This unique approach enables differentiated testing of the scalar field coupling and screening mechanisms that may not be apparent on cosmological scales. Specifically, Cepheids are used for their tight Period-Luminosity (P-L) relation, TRGB stars for their nearly universal peak luminosity, and water masers for their geometric distance measurements. By comparing distances derived from these mechanisms in both screened and unscreened dwarf galaxies, they exploit the sensitivity of each indicator to the chameleon and symmetron screening scenarios.

Their results show no significant evidence for the order unity force enhancements predicted by some modified gravity theories. Notably, they rule out $f(R)$ models with background field values $f_{R0}$ above $5 \times 10^{-7}$ at the 95% confidence level, which are substantially more stringent than previous constraints obtained via large-scale structure and cluster analyses.

Numerical Results and Theoretical Implications

One of the critical outcomes of this paper is the ability to distinguish between screened and unscreened environments, enabling robust testing of models where scalar fields might mediate fifth forces. For example, their analysis of Cepheid and TRGB distances yields a fractional difference ($\Delta d/d$), allowing constraints on the coupling parameter $\alpha_c$ and the chameleon parameter $\chi_c$. These findings, when extrapolated, show potential exclusions of the parameter spaces for these theories far beyond existing astrophysical limits.

The use of the $P-L$ relation for Cepheids and TRGB magnitudes in unscreened environments as a test of gravity posits broader implications for the understanding of the universe's accelerated expansion. The work underscores the importance of local astrophysical phenomena in constraining fundamental physics, potentially influencing the theoretical modeling of gravity at both small and cosmological scales.

Prospects and Future Directions

The paper concludes with a discussion on the prospects for future observations and improved tests with upcoming data, emphasizing the role of space telescopes and sophisticated simulations. The anticipated advancements in measurement precision, particularly in the infrared spectrum, are expected to provide sharper insights and broader coverage of parameter space that could validate or further constrain modified gravity theories.

Furthermore, the paper provides a framework for additional tests involving other astronomical distance indicators like Type Ia supernovae. Such future work promises to extend the utility of these methodologies, potentially elucidating the underlying principles of gravity and its modifications across different cosmic regimes.

The research conducted offers not only immediate experimental constraints on specific modified gravity models but also extends an invitation to the scientific community to refine theoretical models and explore innovative observational strategies. As the quest for understanding the true nature of gravity continues, this paper serves as both a challenge and guidepost for future astrophysical investigations.