A cosmic glitch in gravity (2311.03028v3)

Abstract: We investigate a model that modifies general relativity on cosmological scales, specifically by having a `glitch' in the gravitational constant between the cosmological (super-horizon) and Newtonian (sub-horizon) regimes, as motivated e.g. in the Ho\v{r}ava-Lifshitz proposal or in the Einstein-aether framework. This gives a single-parameter extension to the standard $\Lambda$CDM model, which is equivalent to adding a dark energy component, but where the energy density of this component can have either sign. Fitting to data from the Planck satellite, we find that negative contributions are, in fact, preferred. Additionally, we find that roughly one percent weaker superhorizon gravity can somewhat ease the Hubble and clustering tensions in a range of cosmological observations, although at the expense of spoiling fits to the baryonic acoustic oscillation scale in galaxy surveys. Therefore, the extra parametric freedom offered by our model deserves further exploration, and we discuss how future observations may elucidate this potential cosmic glitch in gravity, through a four-fold reduction in statistical uncertainties.

• The paper introduces a cosmological model that modifies gravity on different scales by proposing a discrepancy between super-horizon and sub-horizon gravitational constants.
• It uses Planck satellite data to reveal a statistically significant negative dark energy component (Ωg ≈ -0.0087 ± 0.0046) which may help resolve the Hubble and matter clustering tensions.
• The model, built on Hořava-Lifshitz and Einstein-aether frameworks, extends the ΛCDM paradigm and encourages further tests with future CMB and galaxy surveys.

A Cosmological Model with a Gravitational Glitch: Implications and Observations

The paper presented in "A Cosmic Glitch in Gravity" investigates a novel cosmological model that proposes modifications to general relativity on cosmological scales. This model introduces a discrepancy between the gravitational constant on super-horizon scales—cosmic scales beyond the observable universe—and sub-horizon scales—those within our cosmic event horizon. This discrepancy is termed a "glitch" in the gravitational constant and is observed within the frameworks of both the Hořava-Lifshitz proposal and the Einstein-aether framework. This one-parameter extension to the prevailing $\Lambda$CDM model suggests an additional component to dark energy which can be of either sign.

Experimental Findings and Their Implications

The model was empirically examined using data from the Planck satellite. Remarkably, results indicate a preference for a negative contribution from this additional dark energy component. A roughly one percent weaker super-horizon gravity was found to offer potential resolutions to observational tensions concerning the Hubble constant and matter clustering. However, this comes at the expense of compromising fits with the baryonic acoustic oscillation (BAO) scale derived from galaxy surveys.

Numerical Implications:

• The introduction of a negative $\Omega_g$ resolves the Hubble tension by allowing higher $H_0$ values which aligns more closely with local measurements without impacting the $\Lambda$CDM existing parameters.
• The analysis reveals that current data from Planck suggests $\Omega_g = -0.0087 \pm 0.0046$, a statistically significant deviation that demands further exploration as additional observational evidence accumulates.

Theoretical and Practical Significance

The core theoretical implication centers on alternative gravitational theories, suggesting that the $\Lambda$CDM model may need an extension to fully encapsulate cosmological phenomena. This "cosmic glitch" provides an intriguing alternative by modifying gravitational dynamics only on cosmological scales, leaving local gravity unaffected. This can be likened to a varying strength in gravity depending on cosmic scale, a hypothesis previously suggested by certain theoretical models proposing Lorentz-violating gravity theories.

On a practical level, the implications of these results are significant in terms of resolving key tensions in cosmological observations. This parameter extension might aid in bridging the so-called Hubble tension—a notable divergence in measurements of the Hubble constant between local and early Universe observations—and the clustering tension, which involves variations in measurements of the large-scale structure.

Future Directions

Future research directions could consider the exploration of this anomaly in greater observational detail, primarily by employing next-generation cosmic microwave background (CMB) measurements and galaxy surveys. Notably, the researchers forecast that future projects like the Euclid mission and other next-generation telescopic surveys could drastically reduce uncertainty surrounding $\Omega_g$, potentially by a factor of four.

This model, while presenting compelling advantages and resolving some outstanding cosmological tensions, does present compatibility challenges with BAO scale observations and may even imply foundational changes to our understanding of gravitational dynamics on cosmic scales.

In summation, this proposed "cosmic glitch in gravity" offers a provocative extension to the $\Lambda$CDM model, bridging discrepancies in multiple cosmic observations and providing a fresh perspective on the enigma of dark energy through its redefinition on cosmic scales. How this impacts our broader understanding of cosmic evolution remains a fertile ground for both theoretical inquiry and empirical scrutiny.