A 2.4% Determination of the Local Value of the Hubble Constant

TL;DR

This work refines the local measurement of the Hubble constant by expanding and unifying Cepheid-based calibrations in SN Ia hosts and tying them to multiple geometric anchors (NGC 4258 masers, MW parallaxes, LMC and M31 DEBs) using HST WFC3 data. A global, simultaneous fit of Cepheid PL relations and SN Ia luminosities yields H0 = 73.24 ± 1.74 km s−1 Mpc−1 (2.4% total uncertainty), with near-IR Cepheid data providing the lowest systematics. The result strengthens the apparent tension with Planck ΛCDM predictions and discusses possible cosmological explanations (e.g., dark radiation) alongside robust avenues for further reduction in uncertainty via improved parallaxes and Gaia data. Overall, the study demonstrates that sub-percent precision in the local H0 is approaching, while highlighting the continued tension between local and CMB-based measurements and its potential implications for new physics or systematics.

Abstract

We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to reduce the uncertainty in the local value of the Hubble constant (H_0) from 3.3% to 2.4%. Improvements come from new, near-infrared observations of Cepheid variables in 11 new hosts of recent SNe~Ia, more than doubling the sample of SNe~Ia having a Cepheid-calibrated distance for a total of 19; these leverage the magnitude-z relation based on 300 SNe~Ia at z<0.15. All 19 hosts and the megamaser system NGC4258 were observed with WFC3, thus nullifying cross-instrument zeropoint errors. Other improvements include a 33% reduction in the systematic uncertainty in the maser distance to NGC4258, more Cepheids and a more robust distance to the LMC from late-type DEBs, HST observations of Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW) Cepheids. We consider four geometric distance calibrations of Cepheids: (i) megamasers in NGC4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with parallaxes, and (iv) 2 DEBs in M31. H_0 from each is 72.25+/-2.51, 72.04+/-2.67, 76.18+/-2.37, and 74.50+/-3.27 km/sec/Mpc, respectively. Our best estimate of 73.24+/-1.74 km/sec/Mpc combines the anchors NGC4258, MW, and LMC, and includes systematic errors for a final uncertainty of 2.4%. This value is 3.4 sigma higher than 66.93+/-0.62 km/sec/Mpc predicted by LambdaCDM with 3 neutrinos with mass 0.06 eV and the Planck data, but reduces to 2.1 sigma relative to the prediction of 69.3+/-0.7 km/sec/Mpc with the combination of WMAP+ACT+SPT+BAO, suggesting systematic uncertainties in CMB measurements may play a role in the tension. If we take the conflict between Planck and H_0 at face value, one plausible explanation could involve an additional source of dark radiation in the early Universe in the range of Delta N_eff=0.4-1. We anticipate significant improvements in H_0 from upcoming parallax measurements.

Figures (16)

• Figure 1: Uncertainties in the determination of H$_0$. Uncertainties are squared to show their individual contribution to the quadrature sum. These terms are given in Table \ref{['tb:h0unc']}.
• Figure 2: HST observations of the host galaxies of ideal SNe Ia. The data used to observe Cepheids in 19 SN Ia hosts and NGC$\,$4258 have been collected over 20 years with 4 cameras and over 600 orbits of HST time. 60-90 day campaigns in F555W and F814W or in F350LP were used to identify Cepheids from their light curves with occasional reobservations years later to identify Cepheids with $P>60$ d. Near-IR follow-up observations in F160W are used to reduce the effects of host-galaxy extinction, sensitivity to metallicity, and breaks in the $P$--$L$ relation. Data sources: (1) HST SN Ia Calibration Project, Sandage:2006; (2) HST Key Project, Freedman:2001; (3) Riess:2005; (4) macri06; and (5) Mager:2013.
• Figure 3: Images of Cepheid hosts. Each image is of the Cepheid host indicated. The magenta outline shows the field of view of WFC3/IR, $2\farcm 7$ on a side. Red dots indicate the positions of the Cepheids. Compass indicates North (long axis) and East (short axis).
• Figure 4: Composite visual ( F555W) or white-light ( F350LP) Cepheid light curves. Each HST Cepheid light curve with $10<P<80$ days is plotted after subtracting the mean magnitude and determining the phase of the observation. Two fields (F1 and F2) are shown for M101.
• Figure 5: Example WFC3 F160W Cepheid scene model for each host. A random Cepheid in the period range of $30<P<70$ d was selected. The four panels of each host show a $1^{\prime\prime}$ region of the scene around each known Cepheid, the region after the Cepheid is fit and subtracted, the model of all detected sources, and the model residuals.