Galileon Gravity in Light of ISW, CMB, BAO and $H_0$ data (1707.02263v3)

Abstract: Cosmological models with Galileon gravity are an alternative to the standard $\Lambda {\rm CDM}$ paradigm with testable predictions at the level of its self-accelerating solutions for the expansion history, as well as large-scale structure formation. Here, we place constraints on the full parameter space of these models using data from the cosmic microwave background (CMB) (including lensing), baryonic acoustic oscillations (BAO) and the Integrated Sachs-Wolfe (ISW) effect. We pay special attention to the ISW effect for which we use the cross-spectra, $C_\ell^{\rm T g}$, of CMB temperature maps and foreground galaxies from the WISE survey. The sign of $C_\ell^{\rm T g}$ is set by the time evolution of the lensing potential in the redshift range of the galaxy sample: it is positive if the potential decays (like in $\Lambda {\rm CDM}$), negative if it deepens. We constrain three subsets of Galileon gravity separately known as the Cubic, Quartic and Quintic Galileons. The cubic Galileon model predicts a negative $C_\ell^{\rm T g}$ and exhibits a $7.8\sigma$ tension with the data, which effectively rules it out. For the quartic and quintic models the ISW data also rule out a significant region of the parameter space but permit regions where the goodness-of-fit is comparable to $\Lambda {\rm CDM}$. The data prefers a non zero sum of the neutrino masses ($\sum m_\nu\approx 0.5$eV) with $ \sim ! 5\sigma$ significance in these models. The best-fitting models have values of $H_0$ consistent with local determinations, thereby avoiding the tension that exists in $\Lambda {\rm CDM}$. We also identify and discuss a $\sim ! 2\sigma$ tension that Galileon gravity exhibits with recent BAO measurements. Our analysis shows overall that Galileon cosmologies cannot be ruled out by current data but future lensing, BAO and ISW data hold strong potential to do so.

Overview of Galileon Gravity: Implications from CMB, BAO, and ISW Data

The paper "Galileon Gravity in Light of ISW, CMB, BAO and $H_0$ data" offers an insightful analysis of the covariant Galileon model, a form of modified gravity designed as an alternative to the Cosmological Constant ($\Lambda$) model, more commonly known as $\Lambda$CDM. The paper explores the implications of the model using an extensive set of cosmological observations, namely the Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations (BAO), and the Integrated Sachs-Wolfe (ISW) effect, alongside $H_0$ measurements.

Summary of the Framework

The Galileon model modifies gravity through a scalar field whose dynamics can drive the accelerated expansion of the Universe, aiming to explain dark energy phenomena without invoking a cosmological constant. It introduces multiple parameters distinct from $\Lambda$CDM, namely $c_2$, $c_3$, $c_4$, and $c_5$, in addition to a new parameter $\xi$, which characterizes the contribution of the Galileon field within the cosmological observations. The model does not have a $\Lambda$CDM limit, meaning it provides unique observational predictions.

Numerical Results and Claims

1. ISW Effect and Negative Predictions: For the Cubic Galileon model (with $c_4 = c_5 = 0$), there exists a significant tension with ISW data which effectively rules out this version. The predicted negative ISW amplitude shows a strong $7.8\sigma$ tension with observations, confirming the necessity to explore more complex forms of the model to obtain viable fits.
2. Neutrino Mass Preference: Unlike $\Lambda$CDM, Galileon gravity showcases a preference for non-zero neutrino masses, with combined estimates from CMB and BAO pointing towards $\sum m_\nu\approx 0.5$ eV with $\sim 5\sigma$ significance. This presents an important distinguishing factor that future laboratory measurements of neutrino masses could help explore further.
3. Consistency with Local $H_0$ Measurements: Galileon models project values of $H_0$ that align more closely with local measurements obtained via direct distance ladder methods, thereby potentially relieving the tension present in $\Lambda$CDM.
4. Future Potential of BAO Data: A tension exists between the model predictions and newer BAO datasets, but not at decisive significance. Nonetheless, this highlights BAO as a promising avenue for future constraints on modified gravity models.

Theoretical and Practical Implications

The paper reinforces the utility of the Galileon model as a potent working hypothesis to explore various types of observational signatures imparted by theories of modified gravity. With its rich phenomenology and deviation from $\Lambda$CDM predictions, it provides a valuable testbed for theoretical insights and observational strategies:

• Predictive Power: The model’s redshift-dependent ISW signals suggest using galaxy samples in narrow redshift bands as a valuable probe to test these predictions further.
• Constraints on Large-Scale Structure Formation: The alteration in the growth dynamics and the CMB lensing signal amplitude further highlight the areas where modified gravity could manifest observationally, offering new potential tests as lensing data improve.

Future Directions in AI and Cosmological Research

The paper directs attention to several future trajectories in cosmological research, leveraging ongoing advancements in astrophysical data collection:

• Advanced BAO Analysis: Continual improvement in BAO datasets and reconstruction methods could decisively rule out specific modified gravity models or further constrain their parameter space.
• Non-Cosmological Constraints: Observations of gravitational waves and further laboratory experiments to determine absolute neutrino masses may elucidate additional signatures predicted by the Galileon model.

In conclusion, while current data do not conclusively rule out more general forms of Galileon gravity, they prescribe several pathways for tighter constraints and highlight the model's role in broadening the landscape of gravitational theory beyond $\Lambda$CDM.