A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3 (1103.2976v1)

Abstract: We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope to determine the Hubble constant (H0) from optical and infrared observations of over 600 Cepheid variables in the host galaxies of 8 recent Type Ia supernovae (SNe Ia), providing the calibration for a mag-z relation of 253 SNe Ia. Increased precision over past measurements comes from: (1) more than doubling the number of infrared observations of Cepheids in nearby SN hosts; (2) increasing the sample of ideal SN Ia calibrators from six to eight; (3) increasing by 20% the number of Cepheids with infrared observations in the megamaser host NGC 4258; (4) reducing the difference in the mean metallicity of the Cepheid comparison samples from \Delta log [O/H] = 0.08 to 0.05; and (5) calibrating all optical Cepheid colors with one camera, WFC3, to remove cross-instrument zero-point errors. Uncertainty in H0 from beyond the 1st rung of the distance ladder is reduced from 3.5% to 2.3%. The measurement of H0 via the geometric distance to NGC 4258 is 74.8 \pm 3.1 km s- 1 Mpc-1, a 4.1% measurement including systematics. Better precision independent of NGC 4258 comes from two alternative Cepheid absolute calibrations: (1) 13 Milky Way Cepheids with parallaxes and (2) 92 Cepheids in the Large Magellanic Cloud with multiple eclipsing binary distances, yielding 74.4 \pm 2.5 km s- 1 Mpc-1, a 3.4% uncertainty with systematics. Our best estimate uses all three calibrations but a larger uncertainty afforded from any two: H0 = 73.8 \pm 2.4 km s- 1 Mpc-1 including systematics, a 3.3% uncertainty. The improvement in H0, combined with WMAP7yr data, results in a constraint on the EOS parameter of dark energy of w = -1.08 \pm 0.10 and Neff = 4.2 \pm 0.7 for the number of relativistic species in the early universe. It also rules out the best-fitting gigaparsec-scale void models, posited as an alternative to dark energy. (abridged)

• The paper achieves a 3% precise measurement of H0 (73.8 ± 2.4 km/s/Mpc) by refining the Cepheid-based distance ladder using HST/WFC3 data.
• Methodological advances include doubling infrared Cepheid observations and increasing SN Ia calibrators while reducing metallicity discrepancies.
• These results tighten dark energy constraints and suggest a slight excess in effective relativistic species, influencing cosmological models.

Determination of the Hubble Constant Utilizing a Refurbished Distance Ladder

The paper "A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3" by Riess et al. presents a rigorous methodology for calculating the Hubble constant, $H_0$, with enhanced precision, leveraging the capabilities of the Hubble Space Telescope (HST) and its Wide Field Camera 3 (WFC3). The paper capitalizes on improvements in both instrumentation and methodology to achieve a 3.3% uncertainty, marking a significant refinement over previous endeavors.

Methodological Enhancements

The researchers utilized observations from over 600 Cepheid variables located in the host galaxies of eight recent Type Ia supernovae (SNe Ia), thereby refining the calibration of the magnitude-redshift relation derived from 240 SNe Ia. Notably, the Hubble constant measurement was honed by:

1. Doubling the number of infrared observations of Cepheid variables in SN host galaxies.
2. Increasing the count of SN Ia calibrators from six to eight, incorporating new supernovae SN 2007af and SN 2007sr.
3. Expanding the sample of observed Cepheids in NGC 4258 by 20%.
4. Minimizing metallicity discrepancies between Cepheid comparison samples, reducing $\Delta \text{log} [\text{O/H}]$ differential from 0.08 to 0.05.
5. Unifying optical Cepheid measurements through WFC3 to eliminate cross-instrument zeropoint errors.

Numerical Results and Astrophysical Implications

The paper reports a robust measurement of the Hubble constant—$H_0$—accommodating systematic uncertainties to achieve a precision of 73.8 $\pm$ 2.4 km/s/Mpc when utilizing all three Cepheid calibrations, which include the geometric distance to NGC 4258, trigonometric parallaxes from Milky Way Cepheids, and distances using Large Magellanic Cloud Cepheids.

Integration of this new $H_0$ value with the Wilkinson Microwave Anisotropy Probe (WMAP) 7-year data provides an improved constraint on the dark energy equation-of-state parameter, observing $w = -1.08 \pm 0.10$. Additionally, these findings yield significant implications concerning early Universe properties, notably indicating $N_{\text{eff}} = 4.2 \pm 0.7$, hinting at a potential slight excess in the effective number of relativistic species, surpassing the minimally expected value resulting from known neutrino flavors.

Future Directions and Theoretical Impact

The paper underscores the pivotal role of addressing systematic uncertainties in celestial distance measures and advocates for further enhancements towards improving the precision of $H_0$ to within 1%. This can be achieved through reinforcement of the Cepheid distance scale using future advancements like the James Webb Space Telescope (JWST), which promises superior reach in operational wavelengths and spatial resolution, potentially extending observations to larger extragalactic volumes.

Overall, the research offers significant contributions to the broader cosmological discourse, particularly in refining the local expansion rate of the universe and enhancing our understanding of dark energy dynamics. In addition to its empirical robustness, this paper lays groundwork for developmental algorithms and instruments in observational cosmology, ultimately serving to sharpen future theoretical frameworks as well.