Observational Evidence for Cosmological-Scale Extra Dimensions

TL;DR

The paper explores infrared-modified gravity from cascaded, infinite-volume extra dimensions in which the 4D graviton is a narrow resonance with Compton scale $r_c$. The longitudinal mode acts as an extra scalar force that enhances late-time structure growth on intermediate scales, potentially explaining anomalies such as the Lyman-$\alpha$ power excess, large bulk flows, and increased ISW cross-correlations, while a Vainshtein screening preserves GR near dense regions. A key feature is the near-$\Lambda$CDM background for $\alpha\approx0$, combined with a scale- and time-dependent perturbation evolution encoded via the PPF framework; with certain choices (e.g., $g\to1$ at late times) a cancellation between late-time ISW and primordial Sachs-Wolfe effects can account for the observed lack of large-angle CMB correlations. The model makes predictions for weak lensing, ISW cross-correlations, bulk flows, Ly-$\alpha$ forest, and SZ cluster signals, and highlights the need for explicit brane-world realizations and non-linear studies to fully test the scenario.

Abstract

We present a case that current observations may already indicate new gravitational physics on cosmological scales. The excess of power seen in the Lyman-alpha forest and small-scale CMB experiments, the anomalously large bulk flows seen both in peculiar velocity surveys and in kinetic SZ, and the higher ISW cross-correlation all indicate that structure may be more evolved than expected from LCDM. We argue that these observations find a natural explanation in models with infinite-volume (or, at least, cosmological-size) extra dimensions, where the graviton is a resonance with a tiny width. The longitudinal mode of the graviton mediates an extra scalar force which speeds up structure formation at late times, thereby accounting for the above anomalies. The required graviton Compton wavelength is relatively small compared to the present Hubble radius, of order 300-600 Mpc. Moreover, with certain assumptions about the behavior of the longitudinal mode on super-Hubble scales, our modified gravity framework can also alleviate the tension with the low quadrupole and the peculiar vanishing of the CMB correlation function on large angular scales, seen both in COBE and WMAP. This relies on a novel mechanism that cancels a late-time ISW contribution against the primordial Sachs-Wolfe amplitude.

Figures (14)

• Figure 1: 95% lower limits on $r_c$, the scale of transition to higher dimensional gravity, from background cosmology (see (\ref{['rcmax_wmap']})) and lunar laser ranging (see (\ref{['rclunar']})), as a function of $\alpha$. Standard DGP corresponds to $\alpha=1/2$, whereas its higher-dimensional extensions all have $\alpha\approx 0$.
• Figure 2: Dependence of $g(k,a)$ (see (\ref{['ginterpol']})) in our model on $k/aH$ in the matter-dominated era for $r_c = 0.3,1,3$, and $10$ Hubble radius. The number of space-time dimensions, $D$, is assumed to be $6$ (i.e. $2$ extra dimensions). General Relativity is recovered when $r_c \gg H^{-1}$ as, $g\rightarrow 0$.
• Figure 3: CMB angular power spectra for best-fit $\Lambda$CDM (blue, solid curve), $r_c = 600$ Mpc (green, dashed curve) and $r_c = 300$ Mpc (red, short-dashed curve). The vertical bars show the cosmic variance spread centered on the respective theoretical predictions. In the middle and bottom panels, the previous curves are shown for comparison. The data points are from the WMAP-5 data release Komatsu.
• Figure 4: Square of the correlation function of CMB temperature anisotropy, as a function of the cosine of the separation angle in the sky. The curves show $\Lambda$CDM (solid), $r_c = 300$ Mpc (dotted) and $r_c = 600$ Mpc (short-dashed). The long-dashed curve is the Legendre transform of the WMAP5 maximum likelihood power spectrum Nolta:2008ih. The observed correlation is systematically below the $\Lambda$CDM prediction for $\theta \gtrsim 60^{\circ}$, or $\cos\theta < 0.5$.
• Figure 5: The cumulative probability for $S_{1/2}$, defined in (\ref{['eq:s12']}), measuring the CMB correlation on scales $> 60^{\circ}$. The curves are for $\Lambda$CDM (solid), $r_c = 300$ Mpc (dotted) and $r_c = 600$ Mpc (dashed). The vertical lines show the observed values of $S_{1/2}$ from different estimators (see huterer2 for details).