Hubble Constant: Building a Better Distance Ladder

A. Riess,

To measure distances across the vastness of space, astronomers often build distance “ladders” using nearer and more common objects to determine the distance to objects that are more rare and distant. At 100 megaparsecs and beyond, the ladder can be used to measure the Hubble constant (H0) which quantifies the expansion rate of the universe. A precision approaching 1% for H0 would be invaluable to help address important cosmic mysteries, such as the history of cosmic expansion, the nature of dark energy, and the general curvature of space. High precision in H0 is also needed to mount a cosmological challenge to the Standard Model of basic physics through derived constraints on cosmic neutrinos, and to frame the grand inventory of the cosmic objects gathered by Hubble Space Telescope and James Webb Space Telescope, which will extend back in time to the era of reionization.

The accompanying figure illustrates improving precision and accuracy of the measurement of H0.

The top two science objectives listed in NASA’s 1977 Announcement of Opportunity (AO) for the Hubble Space Telescope were “Precise determination of distances to galaxies out to expansion velocities ~104 km s-l and calibration of distance criteria applicable at cosmologically significant distances,” and “Determination of the rate of the deceleration of the Hubble expansion of the universe, its uniformity in different directions, and possibly its constancy with time.” One could say that Hubble was chartered from the outset to measure cosmic distances, particularly by finding and measuring distances with Cepheid variable stars in remote galaxies. Making qualitative advances would demand the superb photometric sensitivity and spatial resolution available only in space.

In Hubble’s first decade, its Wide Field and Planetary Camera 2 (WFPC2) was used to resolve Cepheids in hosts out to 20 Mpc to calibrate longer-range distance indicators such as type Ia supernovae, the Tully-Fisher relation, and the luminosity function of planetary nebulae. From ground-based telescopes, Cepheids are also readily visible in the Large Magellanic Cloud (LMC), our dwarf neighbor 50 kiloparsecs away, where their importance was first discovered by Henrietta Leavitt 100 years ago. This “Cepheid rung” completed a new distance ladder, resolving the existing factor-of-two uncertainty in H0 by measuring it to 10% precision. This true landmark in cosmology was achieved by two groups of astronomers—the Key Project team led by Wendy Freedman (Freedman et al. 2001) and the Sandage consortium led by Alan Sandage (Sandage et al. 2006). The residual 10% uncertainty in H0 in the 1990s was due in part to systematic uncertainties along the ladder. For example, at optical wavelengths, the luminosity of Cepheids depends on metallicity. The LMC is metal-poor and the Galaxy—as well as distant spiral galaxies—are metal-rich. To account for this difference, the Cepheid luminosities had to be corrected for their metallicity, and the greater the difference, the larger and more uncertain the correction. A challenge for future reduction of error was that shorter-period Cepheids are more common, but in more distant galaxies only the longer-period, brighter Cepheids can be seen, thus producing another correction—and its uncertainty from the mismatch in period between the Cepheids in the LMC and more distant galaxies. Another limitation was posed by the small volume in which WFPC2 could find and measure Cepheids—a sphere of only ~20 Mpc radius—which limited the number of rare SN Ia events in that volume to about one per decade. To improve statistics, astronomers needed to take advantage of a few SNe Ia recorded years ago on photographic plates or with high extinction, which required understanding the properties of analog data (Pierce and Jacoby 1995), the form of extragalactic extinction laws, and the differences in the zero points of flux for ground-based photometry and WFPC2 photometry.

After the installation of new Hubble instruments—Advanced Camera for Surveys (ACS), Near Infrared Camera and Multi-Object Spectrometer (NICMOS), and Wide Field Camera 3 (WFC3)—the SHOES (“Supernovae, H0, for the Equation of State of dark energy”) team led by Riess and Lucas Macri constructed a new pair of Cepheid rungs between NGC 4258, the maser host 7 Mpc away, with a geometric distance good to 3% by Humphreys et al. (2008, 2013), and the hosts of recent SNe Ia observed with CCDs to 40 Mpc. These new rungs reduced systematic errors of the prior by acquiring Cepheids of similar metallicity and period in both sets and by observing both with the same camera to eliminate the use of uncertain flux zero points. In addition, the Cepheids were all observed in the near infrared (NIR) to mitigate the effect on Cepheid fluxes of variations in host dust and chemistry. The factor of 8 increase in volume reached by the new rung provided a sample of 8 recent, nearby SNe Ia with the same high-quality CCD photometry used to measure the expanding universe to a few Gpc, about 25 times farther than other secondary distance indicators. The SHOES team reached a 5% uncertainty using ACS and NICMOS in 2009 (Riess et al. 2009), and just over 3% with WFC3 in 2011 (Riess et al. 2011).

Yet, just as the 2000s ladder was being completed, a third, more powerful ladder, enabled by high-precision astrometry, had begun construction.