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The Expansion Rate and Size of the Universe by Wendy L A UNIVERSAL VIEW The Expansion Rate and Size of the Universe by Wendy L. Freedman ANDROMEDA GALAXY is a prime example of why calculating the expansion rate of the universe is difficult. Andromeda is 2.5 million light-years away from Earth, but it still feels the gravitational pull of our own galaxy. Consequently, its relative motion has little to do with the expansion of the universe. By observing more distant galaxies, astronomers can detect the expansion, but they do not know its precise rate because it is difficult to measure distances to remote galaxies. JIM RIFFLE 92 Scientific American Presents Copyright 1998 Scientific American, Inc. ur Milky Way and all other galaxies are moving Astronomers began measuring the expansion rate of the away from one another as a result of the big universe some 70 years ago. In 1929 the eminent astronomer bang, the fiery birth of the universe. As we near Edwin P. Hubble of the Carnegie Institution’s observatories the end of the millennium, it is interesting to reflect made the remarkable observation that the velocity of a gal- that during the 20th century, cosmologists discovered this axy’s recession is proportional to its distance. His observations expansion, detected the microwave background radiation provided the first evidence that the entire universe is expanding. Ofrom the original explosion, deduced the origin of chemical elements in the universe The Hubble Constant and mapped the large- scale structure and motion ubble was the first to determine the expansion rate. The age, evolution of galaxies. Despite these Later this quantity became known as the Hubble advances, elementary constant: the recession velocity of the galaxy divided and fate of the questions remain. When by its distance. A very rough estimate of the Hubble con- universe depend on did the colossal expansion Hstant is 100 kilometers per second per megaparsec. (Astron- begin? Will the universe omers commonly represent distances in terms of mega- just how fast it is expand forever, or will parsecs, where one megaparsec is the distance light travels gravity eventually halt its in 3.26 million years.) Thus, a typical galaxy at a distance expanding. By expansion and cause it to of 50 megaparsecs moves away at about 5,000 kilometers collapse back on itself? (3,000 miles) per second. A galaxy at 500 megaparsecs there- measuring the size For decades, cosmol- fore moves at about 50,000 kilometers per second, or more ogists have attempted to than 100 million miles per hour! of the universe answer such questions by For seven decades, astronomers have hotly debated the pre- using a variety of measuring the universe’s cise value of the expansion rate. Hubble originally obtained size-scale and expansion- a value of 500 kilometers per second per megaparsec new techniques, rate. To accomplish this (km/s/Mpc). After Hubble’s death in 1953, his protégé task, astronomers must Allan R. Sandage, also at Carnegie, continued to map the ex- astronomers have determine both how fast pansion of the universe. As Sandage and others made more galaxies are moving and accurate and extensive observations, they revised Hubble’s recently improved how far away they are. original value downward into the range of 50 to 100 Techniques for measuring km/s/Mpc, thereby indicating a universe far older and larger estimates of the the velocities of galaxies than suggested by the earliest measurements. are well established, but During the past two decades, new estimates of the Hubble expansion rate estimating the distances to constant have continued to fall within this same range, but galaxies has proved far preferentially toward the two extremes. Notably, Sandage more difficult. During the and his longtime collaborator Gustav A. Tammann of the past decade, several independent groups of astronomers University of Basel have argued for a value of 50 km/s/Mpc, have developed better methods for measuring the distances whereas the late Gérard de Vaucouleurs of the University to galaxies, leading to completely new estimates of the expan- of Texas advocated a value of 100 km/s/Mpc. The contro- sion rate. Recently the superb resolution of the Hubble versy has created an unsatisfactory situation in which sci- Space Telescope has extended and strengthened the calibra- entists have been free to choose any value of the Hubble tion of the extragalactic distance scale, leading to new esti- constant between the two extremes. mates of the expansion rate. In principle, determining the Hubble constant is simple, At present, several lines of evidence point toward a high requiring only a measurement of velocity and distance. expansion rate, implying that the universe is relatively Measuring a galaxy’s velocity is straightforward: Astron- young, perhaps only 10 billion years old. The evidence also omers disperse light from a galaxy and record its spectrum. suggests that the expansion of the universe may continue A galaxy’s spectrum has discrete spectral lines, which occur indefinitely. Still, many astronomers and cosmologists do at characteristic wavelengths caused by emission or absorp- not yet consider the evidence definitive. We actively debate tion of elements in the gas and stars making up the galaxy. the merits of our techniques. For a galaxy receding from Earth, these spectral lines shift to An accurate measurement of the expansion rate is essential longer wavelengths by an amount proportional to the vel- not only for determining the age of the universe and its fate ocity—an effect known as redshift. but also for constraining theories of cosmology and models If the measurement of the Hubble constant is so simple in of galaxy formation. Furthermore, the expansion rate is principle, then why has it remained one of the outstanding important for estimating fundamental quantities, from the problems in cosmology for almost 70 years? In practice, density of the lightest elements (such as hydrogen and helium) measuring the Hubble constant is extraordinarily difficult, to the amount of nonluminous matter in galaxies, as well as primarily for two reasons. First, although we can measure clusters of galaxies. Because we need accurate distance mea- their velocities accurately, galaxies interact gravitationally surements to calculate the luminosity, mass and size of with their neighbors. In so doing, their velocities become per- astronomical objects, the issue of the cosmological distance turbed, inducing “peculiar” motions that are superimposed scale, or the expansion rate, affects the entire field of extra- onto the general expansion of the universe. Second, estab- galactic astronomy. lishing an accurate distance scale has turned out to be The Expansion Rate and Size of the Universe Magnificent Cosmos 93 Copyright 1998 Scientific American, Inc. Why Cepheid Variables Pulsate discovered that the period of a Cepheid correlates closely with its everal times more massive than the sun, a Cepheid variable is a relatively young star brightness. She found that the longer Swhose luminosity changes in a periodic way: a Cepheid brightens and then dims the period, the brighter the star. more slowly over a period of a few days to months. It pulsates because the force of grav- This relation arises from the fact ity acting on the atmosphere of the star is not quite balanced by the pressure of the hot that a Cepheid’s brightness is pro- gases from the interior of the star. portional to its surface area. Large, The imbalance occurs because of changes in the atmosphere of a Cepheid. An impor- bright Cepheids pulsate over a long tant ingredient in the at- period just as, for example, large mosphere is singly ion- bells resonate at a low frequency (or ized helium (that is, heli- longer period). um atoms that have lost a By observing a Cepheid’s varia- single electron). As radia- tions in luminosity over time, astron- tion flows out of the inte- omers can obtain its period and rior of a Cepheid, singly average apparent luminosity, thereby AND NASA ionized helium in the at- calculating its absolute luminosity ies or at mosphere absorbs and (that is, the apparent brightness the v bser scatters radiation, and it star would have if it were a standard s O ’ may become doubly ion- distance of 10 parsecs away). Furth- ized (that is, each helium ermore, they know that the apparent nstitution atom releases a second luminosity decreases as the distance negie I ar electron). Consequently, it travels increases—because the ap- C the atmosphere becomes parent luminosity falls off in pro- more opaque, making it portion to the square of the distance difficult for radiation to es- to an object. Therefore, we can com- Y L. FREEDMAN cape from the atmo- pute the distance to the Cepheid WEND sphere. This interaction from the ratio of its absolute bright- CEPHEID VARIABLE in the galaxy M100 is shown here at three between radiation and ness to its apparent brightness. different times in its light cycle (top). As seen from left to right, matter generates a pres- During the 1920s, Hubble used the star brightens. sure that forcefully pushes Cepheid variables to establish that out the atmosphere of the other galaxies existed far beyond the star. As a result, the Cepheid variable increases in size and in brightness. Milky Way. By measuring apparent Yet as the atmosphere expands, it also cools, and at lower temperatures the helium re- brightnesses and periods of faint, turns to its singly ionized state. Hence, the atmosphere allows radiation to pass through starlike images that he discovered on more freely, and the pressure on the atmosphere decreases. Eventually, the atmosphere photographs of objects such as the collapses back to its initial size, and the Cepheid returns to its original brightness. The cy- Andromeda Nebula (also known as cle then repeats.
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