A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders The Hubble Constant and the Expanding Universe A newly refined value of H0, the expansion rate of the universe, may herald a first step toward a new era of “precision“ cosmology Wendy Freedman may hold the distinction of being the fort. The goal of our work is nothing when the American astronomer Edwin Ionly astronomer to have been trapped less than trying to understand the for- Hubble discovered that our universe is in a cage at the top of a large telescope on mation and evolution of the universe. indeed expanding. He showed that the the 14,000-foot summit of Mauna Kea. (I We do this through observations and farther a galaxy is from us, the faster it know another astronomer who once fell experiments that ultimately provide is speeding away. This velocity- out of such a cage, but that is another sto- numbers as answers—the values of distance relation came to be called ry.) Twenty years ago, before most ob- cosmological parameters. These num- Hubble’s law, and the value that de- servers spent their nights in warm com- bers can tell us something important scribes its current rate of expansion is puter rooms, astronomers commonly about the universe: how much matter H0. Hubble was the first to measure H0 observed in very small cages at the there is, whether the universe is curved (which wasn’t named as such at the prime focus of giant telescopes. Al- or flat, and even how it might all end. time)—deriving a value of 500 kilome- though the long winter nights were al- Understanding the significance of these ters per second per megaparsec. (A most unbearably cold, we had spectacu- numbers and this curious cosmological parsec is equal to 3.26 light-years.) For lar views of the dark night sky, and we quest for parameters requires a brief di- various reasons, Hubble’s result was listened to music through headphones as version into some history. far off the mark, but even a couple of we recorded images and spectra of our The modern science of cosmology is years ago, estimates for H0 varied by a celestial targets. At the end of one night, founded on general relativity—Albert factor of two, generally ranging be- a faulty telescope position caused an ele- Einstein’s theory of gravity—whose tween 50 and 100 (the values are usual- vator to jam, making it impossible for me equations describe the global behavior ly stated without the units of measure). to leave the cage. This was no small in- of matter and energy, and space and This lack of precision was problem- convenience—the closest restroom time. Some solutions to these equa- atic because H0 is a key parameter was 40 feet below, and I was unpleas- tions, notably those devised by the needed to estimate both the age and antly confined within two snowsuits. Russian mathematician Alexander size of the universe. A twofold range in It was another seven hours before a Friedmann in the 1920s, suggest that H0 yields an unacceptably wide span group of engineers arrived from sea the universe originated from a very for the age of the universe—anywhere level (delayed by a flat tire), climbed hot, very dense state in a “big bang“ from 10 to 20 billion years. Such uncer- up the side of the telescope’s dome explosion and that it has been expand- tainty also puts few constraints on cos- and finally pried the elevator free with ing in size ever since. The dynamics of mological models. a crowbar. Now why would an as- the expansion are expressed by the so- But all of this is changing. The value tronomer want to subject herself to called Friedmann equation, which de- of the H0, along with some other cos- such indignities? scribes the evolution of the universe in mological parameters, is becoming in- As an observational cosmologist, I terms of its density and geometry (see creasingly accessible to accurate mea- can say that the rewards more than “Friedmann’s Equation and Cosmological surement as new technologies allow us compensate for the occasional discom- Parameters,” page 42). Applying Fried- to see farther into the universe than mann’s equation requires that we ever before. The Hubble Space Tele- Wendy Freedman is an astronomer at the Carnegie know something about a few parame- scope (HST), which was launched in Observatories in Pasadena, California. Her re- ters it contains—such as H, the Hubble 1990, is among these technological search interests include the evolution of galaxies parameter, which defines the expan- breakthroughs. One of the primary rea- and their stellar populations, and the extragalactic sion rate; Ω , the mass density of the sons the HST was built was to deter- distance scale and cosmology. She was recently m universe; and Ω , the curvature of the mine a more accurate value for H . elected a Fellow of the American Academy of Sci- k 0 ences, and this year was awarded the Magellanic universe. These numbers are not inher- This “Key Project” of the HST program Premium Award of the American Philosophical ently defined by the equation. Instead, was an enormous effort, involving 30 Society. Address: Carnegie Observatories, 813 they remain for us to measure. astronomers (I was one of three co- Santa Barbara Street, Pasadena, CA 91101. Inter- Some of the first efforts to make leaders), spanning eight years of work net: [email protected] these measurements date back to 1929, and about 1,000 hours of HST time. It © 2004 Sigma Xi, The Scientific Research Society. Reproduction 36 American Scientist, Volume 91 with permission only. Contact [email protected]. Figure 1. Spiral galaxy NGC 4414 is speeding away from us as it is carried by the expansion of the universe. The rate at which the universe is ex- panding, described by the Hubble constant, is determined simply by measuring the velocities and distances of galaxies. In practice, however, making accurate measurements to distant galaxies can be extremely difficult. The launch of the Hubble Space Telescope in 1990 made such mea- surements much easier, allowing astronomers to determine the distances to galaxies with an unprecedented level of accuracy. After nearly a decade of measurements, astronomers have derived a value for the Hubble constant that is accurate enough to be used meaningfully in various cosmological and astrophysical calculations. NGC 4414 is nearly 19.1 megaparsecs away (roughly 62 million light-years), and it is receding from us with a speed of about 1,400 kilometers per second. (Image courtesy of NASA, Hubble Heritage Team, STSci/AURA and Wendy Freedman.) was the largest project tackled by HST ing universe and this effect must be ac- number of galaxies distributed across in its first decade, and it was finally counted for or minimized. the sky so that the peculiar motions completed in 2001. A galaxy’s velocity is calculated can be averaged out. from the observed shift of lines in its Measuring distances presents a A Variable and a Constant spectrum (the pattern of electromag- greater challenge. The universe is so In principle, the Hubble constant netic radiation it emits at different large that there is no direct way to should be a straightforward calcula- wavelengths). Galaxies that are mov- measure its full size. There is no cos- tion. It only requires the measurement ing away from us emit light that is mological equivalent to a land survey- of a galaxy’s distance and velocity. In shifted to longer (redder) wavelengths or’s rangefinder—no single method practice, however, devising a method because it is stretched, or “redshifted,” can provide a measure of the uni- to measure distances over cosmologi- by the recession. The greater the shift verse’s absolute size. Instead, as- cal scales is far from trivial. Even rela- in wavelength, the faster the galaxy’s tronomers rely on a series of tech- tively simple velocity measurements velocity. Since the velocity of recession niques, each of which is suitable for a are complicated by the fact that galax- is proportional to its distance (Hub- certain range of distances, and together ies tend to have other galaxies as ble’s law again), the farther the dis- these methods constitute the “cosmo- neighbors, and so they interact gravita- tance measurements can be made, the logical distance ladder.” tionally, perturbing one another’s mo- smaller the proportional impact of pe- For the nearest stars, distances can tions. These peculiar velocities are dis- culiar velocities on the overall expan- be measured by trigonometric parallax, tinct from the recession velocities (the sion velocity. Astronomers can further which uses the baseline of the Earth’s Hubble flow) of galaxies in the expand- reduce the uncertainty by observing a orbit for triangulating a star’s distance © 2004 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 2003 January–February 37 A Distant Candle in the Dark luminosity faint bright a Cepheid’s periodicity Cepheid period-luminosity relation time (days) 131030 100 period (days) Cepheids are massive stars (at least three times greater than the Sun) that have reached an unstable point in their evolution. During the Cepheid phase, the outer atmospheres of these stars pulsate—changing in size and color (left)—and therefore in their brightness, or luminosity, over a very regular period, ranging from 2 to more than 100 days.