转自科学美国人最新评论 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" cosmologyWendy Freedman
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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 expanding, 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 measurements 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.
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Figure 2. Measures of the velocities and distances of galaxies in this Hubble diagram, based solely on Cepheid variable stars in 22 galaxies, yield a Hubble constant (H0) value of 75 kilometers per second per megaparsec (slope of the solid line). Because Cepheids can only be accurately measured to a distance of 20 or 30 megaparsecs, several other methods were combined to determine a more accurate value for H0 (see Figure 4). (Error values of plus or minus about 10 percent are shown by the dashed lines.)
Barbara Aulicino
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Figure 3. Type Ia supernovae, such as supernova 1994D (bright spot, lower left) in the spiral galaxy NGC 4526, can be used as a distance marker by determining its relative brightness. Such supernovae have played a dual role in cosmology. Type Ia supernovae within 400 megaparsecs are used to estimate H0, but beyond this distance the Hubble-diagram plot of their velocities and distances is no longer linear. This break from linearity in the distant universe reveals that the expansion rate of the universe is actually accelerating--a phenomenon that few scientist expected.
Image courtesy of High-Z Supernova Search Team, HST and NASA.
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Figure 4. Secondary distance indicators are used to measure the Hubble constant far out in the universe (up to 400 megaparsecs or 1.3 billion light-years away). Each of these four methods provides an independent way of measuring the relative distance to a galaxy. In each instance, some measurable property is correlated with the intrinsic brightness or relative distance of the object. Thus, the light curves of intrinsically brighter type Ia supernovae have a shallow decay (upper left), the stars in intrinsically brighter elliptical galaxies have a wider range of velocities (upper right), intrinsically brighter spiral galaxies rotate more quickly (lower left), and distant elliptical galaxies have a smoother appearance than near ones (lower right). The relative distances determined with these methods are calibrated (directly or indirectly) to the Cepheid distance scale.
Tom Dunne
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Figure 5. Hubble diagram of five secondary-distance indicators reveals a combined average H0 value of 72, the final result of the Hubble Key Project. Integrating this H0 value into the Friedmann equation (see page 42) suggests that the universe has been expanding for about 13 billion years.
Barbara Aulicino
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Figure 6. Value of the Hubble constant converges toward 72 as the recession velocities of galaxies are measured to ever greater distances by the secondary distance indicators. Beyond a recession velocity of about 5,000 kilometers per second (about 70 megaparsecs away), the effects of a galaxy's peculiar motions (caused by gravitational interactions with other galaxies) is reduced to less than 5 percent of its overall velocity. These distant galaxies provide a more accurate measure of H0.
Barbara Aulicino
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Figure 7. Evolutionary course of the expanding universe depends on the total amount of matter it contains. The parameter Ωm is the ratio of the actual density of the matter in the universe relative to the critical value, which marks the dividing line between a universe that expands forever and one that ultimately collapses again. The critical value is defined as Ωm, where there is just enough matter in the universe to prevent its eternal expansion or eventual collapse. In a "closed" universe, with Ωm > 1, the gravitational pull of matter ultimately causes the universe to collapse in a "big crunch." In an "open" universe, with Ωm < 1, there is not enough matter to gravitationally halt the expansion, and so it continues forever. Astronomers have determined that we live in a universe with Ωm < 1, but that a dark (or vacuum) enter (ΩΛ > 0) appears to be causing the expansion to accelerate. Each of these models also implies a different age for the universe, based on its current expansion rate. Tom Dunne, adapted from a NASA illustration.
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