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Determination of the Hubble Constant Proc. Natl. Acad. Sci. USA Vol. 96, pp. 11063–11064, September 1999 From the Academy This paper is a summary of a session presented at the first Chinese-American Frontiers of Science symposium, held August 28–30, 1998, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, CA. Determination of the Hubble constant WENDY L. FREEDMAN* AND LONG LONG FENG† *Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101; and †Center for Astrophysics, University of Science and Technology of China, Anhui, Hefei, 230026, People’s Republic of China ABSTRACT Establishing accurate extragalactic dis- precise determination of galaxy distances remains a longstand- tances has provided an immense challenge to astronomers ing fundamental problem in astronomy. In principle, measur- since the 1920s. The situation has improved dramatically as ing the distance of a distant galaxy relies on the following better detectors have become available, and as several new, property of propagation of light in space: the apparent bright- promising techniques have been developed. For the first time ness of a light source varies inversely with the square of in the history of this difficult field, relative distances to distance. Accordingly, the distance to an object may be galaxies are being compared on a case-by-case basis, and their determined by knowing its intrinsic luminosity and then com- quantitative agreement is being established. New instrumen- paring that with its apparent brightness. tation, the development of new techniques for measuring For reviews on recent progress in measuring distances, see, distances, and recent measurements with the Hubble Space for example, the conference proceedings for the Space Tele- telescope all have resulted in new distances to galaxies with scope Science Institute meeting on the Extragalactic Distance .(precision at the ؎5–20% level. The current statistical uncer- Scale edited by Donahue and Livio (2 tainty in some methods for measuring H0 is now only a few The Hubble Space Telescope (HST) Key Project to percent; with systematic errors, the total uncertainty is ap- Measure H proaching ؎10%. Hence, the historical factor-of-two uncer- 0 tainty in the value of the H0 is now behind us. The three Key Projects for the HST were selected by peer review to enable science to be undertaken that might require Though there has been remarkable progress in measuring the large amounts of telescope time. The HST H Key Project was Ϯ 0 cosmological parameters, the accuracy of these quantities is designed to measure H0 to 10% (3–5). Rather than concen- determined by the available technology and measurement trate on a single method (which might be affected by unknown, techniques and is still not sufficiently high to discriminate systematic effects), the goal of the Key Project is to undertake among the various existing world models (1). Because of the a comparison and a calibration of several different methods so fundamental dependence on the cosmological parameters in that cross-checks on both the absolute zero point as well as all of the models, accurate determinations are critical to make relative distances, and therefore on H0, can be obtained. reliable predictions based on the current models. For instance, The underlying basis of the Key Project is the discovery of a reliable value of the Hubble constant is required to constrain a class of well-understood stars, known as Cepheid variables. the density of baryons from nucleosynthesis at an early epoch These stars obey a tight correlation between their periods of of the universe. The Hubble constant sets the time and length oscillation and their luminosities (see the reviews, e.g., refs. 6 scale at the epoch of equality of the energy density of matter and 7). With a measurement of the period and observed brightness, and a calibration of the intrinsic brightness, the and radiation. In the structure formation paradigm based on distance is obtained according to the inverse-square law of gravitational instability, the horizon scale at matter-radiation light. The period-luminosity (P-L) relation is calibrated with equality specifies the critical range of the density perturbation aid of a small sample of nearby Cepheids whose absolute spectrum turnover, and an accurate knowledge of the Hubble distances are measured in an independent way. Given an constant allows a quantitative comparison of the anisotropies absolute calibration, distances to galaxies obtained by using in the cosmic background radiation and theories of the large- Cepheids can, in turn, be used to calibrate other methods for scale structure of the universe. In addition, in the issue distance determination that can be used beyond the reach of addressed in this session, that of the age of the universe, there the Cepheids; these methods often are referred to as second- is a direct confrontation between the expansion age inferred ary methods. from the Hubble constant in the standard model and age The H0 Key Project has been designed with three primary dating of the oldest objects in the universe. The reason for goals: (i) to discover Cepheids in galaxies located out to testing the cosmological model by using the age of the universe distances of about 20 Mpc (where 1 Mpc ϭ 3.09 ϫ 1022 m), and is obvious: there should be no astronomical object in the thereby measure accurate distances to spiral galaxies that are universe older than the universe itself. Consequently, the suitable for the calibration of several independent secondary oldest objects known provide the minimum age of the universe. methods, (ii) to provide a check on potential systematic errors What is required to measure an accurate value of H0? both in the Cepheid distance scale and the secondary methods, According to the Hubble law, what is needed are measure- and (iii) to make direct Cepheid measurements of distances to ments of both redshifts of galaxies (via spectral lines), and three spiral galaxies in each of the Virgo and Fornax clusters distances to galaxies (at sufficiently large distances where (located at approximately 16–18 Mpc). peculiar motions relative to the smooth Hubble flow are slow). ͞ The Hubble constant then follows immediately from the slope Measurement of Cepheid Distances Calibration of of correlation between the redshift and distance. However, the Secondary Methods The extragalactic distance scale at present is still faced with the PNAS is available online at www.pnas.org. undesirable situation that there exists no single distance 11063 Downloaded by guest on September 26, 2021 11064 From the Academy: Freedman and Feng Proc. Natl. Acad. Sci. USA 96 (1999) indicator for which local, geometric parallax measurements with improvements to the calibrating Cepheid distance scale, can be made and for which distances can be measured suffi- and large new numbers of Cepheid distances now available ciently far that the smooth, cosmic Hubble expansion is being from HST, have led to an enormous increase in the accuracy probed. Locally, the gravitational interaction between galaxies and precision with which the expansion rate, or Hubble and their neighbors, in addition to larger-scale, bulk motions constant, H0, can be measured. introduce noise or ‘‘peculiar motions’’ into the measured In the near future, the satellite experiments MAP, to be velocities of galaxies. launched by the National Aeronautics and Space Administra- Determination of H0 to an accuracy of 10% requires that tion in 2000 (http:͞͞map.gsfc.nasa.gov), and Planck (http:͞͞ measurements be acquired at great enough distances and in a astro.estec.esa.nl͞SA-general͞Projects͞Planck), to be variety of directions so that the average contribution from launched by the European Space Agency in 2007, will be able motions induced by the gravitational interaction of galaxies to measure the anisotropies of cosmic background radiation (peculiar motions) is significantly less than 10% of the overall (CBR) with unprecedented accuracy. If the physics underlying expansion velocity. The current limit for detection of Cepheids the formation of CBR anisotropies is confirmed by the detec- with HST is a distance of about 30 Mpc (or about 0.01% of the visible universe. At these distances peculiar motions still can tion of the first acoustic peak in the angular power spectrum contribute 10–20% of the observed velocity. Hence, the main of CBR anisotropies (14), along with ongoing galaxy and thrust of the Key Project is the calibration of secondary supernova Ia searches, these complementary experiments will distance indicators that can be applied out to distances sig- enable independent measurements of cosmological parame- nificantly greater than can be measured with Cepheids alone. ters. Though the final accuracy will depend on how well various With the database of Cepheid distances assembled as part of systematic errors can be controlled or eliminated, these up- the H0 Key Project, a number of secondary indicators can be coming, promising experiments are likely to resolve defini- directly calibrated and tested. Several of these methods can be tively the essential issues in the cosmological paradigm. applied to velocity distances of 10,000 km͞sec or greater. These include, for example, type Ia supernovae, type II Work on the H0 Key Project has been done in collaboration with the supernovae, and the Tully-Fisher relation. Type Ia supernovae Key Project team on the Extragalactic Distance Scale. W.L.F. would can be observed at velocity distances of beyond 30,000 km͞sec, like to acknowledge the contributions of R. Kennicutt, J. R. Mould or Ϸ10% of the visible universe. For details on other recent (co-principal investigators), F. Bresolin, S. Faber, L. Ferrarese, H. results from the Key Project, see refs. 8 and 9. This preliminary Ford, B. Gibson, J. Graham, J. Gunn, M. Han, P. Harding, J. Hoessel, ϭ Ϯ Ϯ J. Huchra, S. Hughes, G. Illingworth, D. Kelson, L.
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