A local resolution of the Hubble tension: The impact of screened fifth forces on the cosmic distance ladder

TL;DR

This work investigates whether partially screened fifth forces can locally alter Cepheid calibrations and hence the cosmic distance ladder to alleviate the Hubble tension. By embedding phenomenological screening models (e.g., chameleon, K-mouflage, Vainshtein) and a baryon–dark matter interaction proxy into the Cepheid–SN distance ladder, the authors propagate changes in the effective gravitational constant $G$ to Cepheid periods and luminosities and then to $H_0$ via SN calibrations. They derive consistency bounds on $\Delta G/G_N$ from Cepheid–TRGB comparisons, which constrain the viable parameter space, and show that if Cepheid cores are unscreened with $\Delta G/G_N \approx 0.03$–$0.05$, the local $H_0$ can be lowered to within roughly $2$–$3\sigma$ of the Planck value. Though not a complete resolution, the results demonstrate that physics beyond GR operating at galactic scales could provide a local resolution to the Hubble tension, motivating further forward-modeling and cross-checks with additional distance indicators and screening theories.

Abstract

The discrepancy between the values of the Hubble constant $H_0$ derived from the local distance ladder and the cosmic microwave background provides a tantalising hint of new physics. We explore a potential resolution involving screened fifth forces in the local Universe, which alter the Cepheid calibration of supernova distances. In particular, if the Cepheids with direct distance measurements from parallax or water masers are screened but a significant fraction of those in other galaxies are not, neglecting the difference between their underlying period--luminosity relations biases the local $H_0$ measurement high. This difference derives from a reduction in the Cepheid pulsation period and possible increase in luminosity under a fifth force. We quantify the internal and environmental gravitational properties of the Riess et al. distance ladder galaxies to assess their degrees of screening under a range of phenomenological models, and propagate this information into the $H_0$ posterior as a function of fifth force strength. We consider well-studied screening models in scalar--tensor gravity theories such as chameleon, K-mouflage and Vainshtein, along with a recently-proposed mechanism based on baryon--dark matter interactions in which screening is governed by local dark matter density. We find that a fifth force strength $\sim5-30\%$ that of gravity can alleviate (though not resolve) the $H_0$ tension in some scenarios, around the sensitivity level at which tests based on other distance ladder data can constrain this strength. Although our analysis is exploratory and based on screening models not necessarily realised in full theories, our results demonstrate that new physics-based local resolutions of the $H_0$ tension are possible, supplementing those already known in the pre-recombination era.

Figures (7)

• Figure 1: Left: The distance ladder. Each vertical segment represents a rung, as indicated at the very top. Distances to objects in the upper part are calibrated by means of the lower indicator. A fiducial screening status of each rung is shown under the distance axis: a rung is labelled as unscreened when that is the case for at least one indicator. In this work we consider SNe to be screened, although this may not be the case in some models; we consider this further in Appendix \ref{['Appendix:SN']}. Right: A schematic representation of the Cepheid period--luminosity relation (PLR) when various parts of a Cepheid are unscreened. The grey solid line shows the Newtonian relation as traced out by the screened Cepheids in the MW and N4258. The red dashed line shows the relation for Cepheids with unscreened envelopes and the blue dotdashed line for both envelope and core unscreened. The distances between the lines indicate that unscreening the core has a larger effect than unscreening the envelope. Assuming an unscreened Cepheid lies on the Newtonian PLR causes its luminosity and hence distance to be underestimated, as shown by the vertical line at fixed measured period. This causes the inferred $H_0$ to be biased high.
• Figure 2: The Hertzsprung--Russell track for a 5$M_\odot$ star with $\Delta G/G_{\rm N}=0$ (black solid), $\Delta G/G_{\rm N}=0.05$ (red dashed), and $\Delta G/G_{\rm N}=0.1$ (blue dotted). The black dashed lines bound the instability strip.
• Figure 3: The screening proxies of luminosity, mass estimated through hydrogen gas, halo virial mass and external potential, acceleration and curvature over the Cepheid host galaxies in the R16 sample. The anchors, MW and N4258, are shown separately by the red and green horizontal lines. Galaxies lying below these lines have weaker internal gravitational fields than the MW and N4258, and may therefore be unscreened. The ordering of the galaxies is as in Table \ref{['tab:galprops1']}, starting with M101. The subpanels of the environmental screening plots (right column) correspond to the three different distances $R_\text{max}$ out to which we include contributions from masses: $0.5$ Mpc (lower), $5$ Mpc (middle) and $50$ Mpc (upper). The asymmetric errorbars indicate the minimal width enclosing $68\%$ of the Monte Carlo model realisations.
• Figure 4: Normalised frequency distributions of dark matter densities $\rho_\text{DM}$ at the positions of the R16 Cepheids within their hosts, estimated using the halo properties from Table \ref{['tab:galprops2']}. Each curve corresponds to a different galaxy. The local dark matter density, $10^7 M_\text{sun}$ kpc$^{-3}$, is shown by the vertical red line: Cepheids to the left of this line live in lower density regions than the Solar System and may therefore be unscreened in the baryon--dark matter interaction model.
• Figure 5: Upper limit on $\Delta G/G_\text{N}$ from the comparison of Cepheid and TRGB distance estimates to 51 galaxies from NED-D, as a function of the fraction of unscreened galaxies. We show separately the cases of Cepheids entirely unscreened (left) and only Cepheids' envelopes unscreened (right). Dashed lines indicate typical unscreened fractions as shown in Table \ref{['tab:H0']}.