Challenges to the DGP Model from Horizon-Scale Growth and Geometry

TL;DR

This paper tests the self-accelerating DGP braneworld against combined cosmological data (SNLS, WMAP5, HST) using a PPF-based effective dark energy framework embedded in CAMB. It shows persistent tension between distance measurements and horizon-scale growth, with bulk perturbations enhancing the ISW effect and the low-$\ell$ CMB power, which cannot be cured by curvature or initial power modifications. In the flat case, the maximum-likelihood DGP fit is about 5.3σ worse than ΛCDM; allowing curvature slightly improves distance fits but strengthens growth-related discrepancies, leaving an overall ~4.8σ difference dominated by the low-$\ell$ TT spectrum. The study demonstrates the diagnostic power of combining growth and distance observables to test gravity on the largest scales and finds that the DGP self-accelerating scenario remains a poor alternative to dark energy within current data constraints.

Abstract

We conduct a Markov Chain Monte Carlo study of the Dvali-Gabadadze-Porrati (DGP) self-accelerating braneworld scenario given the cosmic microwave background (CMB) anisotropy, supernovae and Hubble constant data by implementing an effective dark energy prescription for modified gravity into a standard Einstein-Boltzmann code. We find no way to alleviate the tension between distance measures and horizon scale growth in this model. Growth alterations due to perturbations propagating into the bulk appear as excess CMB anisotropy at the lowest multipoles. In a flat cosmology, the maximum likelihood DGP model is nominally a 5.3 sigma poorer fit than Lambda CDM. Curvature can reduce the tension between distance measures but only at the expense of exacerbating the problem with growth leading to a 4.8 sigma result that is dominated by the low multipole CMB temperature spectrum. While changing the initial conditions to reduce large scale power can flatten the temperature spectrum, this also suppresses the large angle polarization spectrum in violation of recent results from WMAP5. The failure of this model highlights the power of combining growth and distance measures in cosmology as a test of gravity on the largest scales.

Figures (7)

• Figure 1: Equation of state of the effective dark energy $w_e$ for the self-accelerating DGP model with $\Omega_m=0.26$ and $\Omega_K=0$.
• Figure 2: Predictions for the power spectra of the CMB temperature anisotropies $C_{\ell}^{\rm TT}$ of the best-fit DGP (solid), QCDM with the same expansion history as DGP (short-dashed), and $\Lambda$CDM (dashed, coincident with QCDM at low $\ell$) models obtained by fitting to SNLS + WMAP5 (both temperature and polarization) + HST, assuming a flat universe. Bands represent the 68% and 95% cosmic variance regions for the DGP model. Points represent WMAP5 measurements; note that noise dominates over cosmic variance for $\ell \gtrsim 500$.
• Figure 3: Predictions for the power spectra of the CMB temperature anisotropies $C_{\ell}^{\rm TT}$ of the best-fit DGP (solid), QCDM with the same expansion history as DGP (short-dashed), and $\Lambda$CDM (dashed) models obtained by fitting to SNLS + WMAP5 (both temperature and polarization) + HST, allowing curvature.
• Figure 4: Predictions for the power spectra of the CMB temperature anisotropies $C_{\ell}^{\rm TT}$ of the best-fit DGP model as found in § \ref{['subsec:omk']} without cutting off any large scale primordial perturbations (solid) and with a cut-off scale of $k_{\rm min}=8\times 10^{-4}$ Mpc$^{-1}$ (dotted) -- the best-fit scale obtained when fitting to the WMAP 5 year TT data alone, while all other parameters are fixed at their best-fit values with $k_{\rm min}=0$.
• Figure 5: Predictions for the power spectra of the CMB E-mode polarization $C_{\ell}^{\rm EE}$ of the best-fit DGP model as found in § \ref{['subsec:omk']} without cutting off any large scale primordial perturbations (solid) and with a cut-off scale of $k_{\rm min}=8\times 10^{-4}$ Mpc$^{-1}$ (dotted) -- the best-fit scale obtained when fitting to the WMAP 5 year TT data alone, while all other parameters are fixed their best-fit values. Note here, according to Rfn. Noetal08, the reionization feature at the lowest-$\ell$ modes is preferred by the data through $\Delta \chi^2=19.6$.