Supernovae, Dark Energy, and the Accelerating Universe Using very distant supernovae as standard candles, one pansion of the universe since the time the light was emitted. A collection of can trace the history of cosmic expansion and try to find such measurements, over a sufficient out what’s currently speeding it up. range of distances, would yield an en- tire historical record of the universe’s expansion. Saul Perlmutter Conceptually, this scheme is a re- markably straightforward means to a or millennia, cosmology has been a theorist’s domain, profound prize: an empirical account of the growth of our Fwhere elegant theory was only occasionally endangered universe. A spectroscopically distinguishable class of ob- by inconvenient facts. Early in the 20th century, Albert jects with determinable intrinsic brightness would do the Einstein gave us new conceptual tools to rigorously ad- trick. In Edwin Hubble’s discovery of the cosmic expansion dress the questions of the origins, evolution, and fate of the in the 1920s, he used entire galaxies as standard candles. universe. In recent years, technology has developed to the But galaxies, coming in many shapes and sizes, are diffi- point where these concepts from general relativity can be cult to match against a standard brightness. They can substantiated and elaborated by measurements. For ex- grow fainter with time, or brighter—by merging with other ample, measurement of the remnant glow from the hot, galaxies. In the 1970s, it was suggested that the brightest dense beginnings of the expanding universe—the cosmic member of a galaxy cluster might serve as a reliable stan- microwave background—is yielding increasingly detailed dard candle. But in the end, all proposed distant galactic data about the first half-million years and the overall candidates were too susceptible to evolutionary change. geometry of the cosmos (see the news story on page 21 of As early as 1938, Walter Baade, working closely with this issue). Fritz Zwicky, pointed out that supernovae were extremely The standard model of particle physics has also begun promising candidates for measuring the cosmic expansion. to play a prominent role in cosmology. The widely accepted Their peak brightness seemed to be quite uniform, and idea of exponential inflation in the immediate aftermath they were bright enough to be seen at extremely large dis- of the Big Bang was built on the predicted effect of certain tances.1 In fact, a supernova can, for a few weeks, be as putative particle fields and potentials on the cosmic ex- bright as an entire galaxy. Over the years, however, as pansion. Measuring the history of cosmic expansion is no more and more supernovae were measured, it became easy task, but in recent years, a specific variety of super- clear that they were a rather heterogeneous group with a novae, type Ia, has given us a first glimpse at that his- wide range of intrinsic peak brightnesses. tory—and surprised us with an unexpected plot twist. In the early 1980s, a new subclassification of super- novae emerged. Supernovae with no hydrogen features in Searching for a standard candle their spectra had previously all been classified simply as In principle, the expansion history of the cosmos can be de- type I. Now this class was subdivided into types Ia and Ib, termined quite easily, using as a “standard candle” any dis- depending on the presence or absence of a silicon absorp- tinguishable class of astronomical objects of known in- tion feature at 6150 Å in the supernova’s spectrum.2 With trinsic brightness that can be identified over a wide that minor improvement in typology, an amazing consis- distance range. As the light from such beacons travels to tency among the type Ia supernovae became evident. Their Earth through an expanding universe, the cosmic expan- spectra matched feature-by-feature, as did their “light sion stretches not only the distances between galaxy clus- curves”—the plots of waxing and waning brightness in the ters, but also the very wavelengths of the photons en route. weeks following a supernova explosion.3,4 By the time the light reaches us, the spectral wavelength The uniformity of the type Ia supernovae became even l has thus been redshifted by precisely the same incre- more striking when their spectra were studied in detail as mental factor z Dl/l by which the cosmos has been they brightened and then faded. First, the outermost parts stretched in the time interval since the light left its source. of the exploding star emit a spectrum that’s the same for That time interval is the speed of light times the object’s all typical type Ia supernovae, indicating the same ele- distance from Earth, which can be determined by com- mental densities, excitation states, velocities, and so forth. paring its apparent brightness to a nearby standard of the Then, as the exploding ball of gas expands, the outermost same class of astrophysical objects. layers thin out and become transparent, letting us see the The recorded redshift and brightness of each such ob- spectral signatures of conditions further inside. Eventu- ject thus provide a measurement of the total integrated ex- ally, if we watch the entire time series of spectra, we get to see indicators that probe almost the entire explosive Saul Perlmutter is a senior scientist at the Lawrence Berkeley event. It is impressive that the type Ia supernovae exhibit National Laboratory and leader of the Supernova Cosmology so much uniformity down to this level of detail. Such a “su- Project. pernova CAT-scan” can be difficult to interpret. But it’s © 2003 American Institute of Physics, S-0031-9228-0304-030-4 April 2003 Physics Today 53 Figure 1. Light curves of nearby, low-redshift type Ia super- –20 a novae measured by Mario Hamuy and coworkers.7 (a) Ab- solute magnitude, an inverse logarithmic measure of intrinsic –19 brightness, is plotted against time (in the star’s rest frame) be- fore and after peak brightness. The great majority (not all of them shown) fall neatly onto the yellow band. The figure –18 emphasizes the relatively rare outliers whose peak brightness or duration differs noticeably from the norm. The nesting of the light curves suggests that one can deduce the intrinsic –17 brightness of an outlier from its time scale. The brightest supernovae wax and wane more slowly than the faintest. (b) Simply by stretching the time scales of individual light –16 curves to fit the norm, and then scaling the brightness by an ABSOLUTE MAGNITUDE amount determined by the required time stretch, one gets all the type Ia light curves to match.5,8 –15 –20 b –19 clear that essentially the same physical processes are oc- –18 curring in all of these explosions. The detailed uniformity of the type Ia supernovae im- plies that they must have some common triggering mech- –17 anism (see the box on page 56). Equally important, this uniformity provides standard spectral and light-curve SCALED MAGNITUDE –16 templates that offer the possibility of singling out those su- pernovae that deviate slightly from the norm. The complex natural histories of galaxies had made them difficult to –15 standardize. With type Ia supernovae, however, we saw –20 0 20 40 60 the chance to avoid such problems. We could examine the DAYS rich stream of observational data from each individual ex- plosion and match spectral and light-curve fingerprints to recognize those that had the same peak brightness. tantalizing prospect that we could find such standard- Within a few years of their classification, type Ia su- candle supernovae more than ten times farther away and pernovae began to bear out that expectation. First, David thus sample the expansion of the universe several billion Branch and coworkers at the University of Oklahoma years ago. Measurements using such remote supernovae showed that the few type Ia outliers—those with peak might actually show the expected slowing of the expansion brightness significantly different from the norm—could rate by gravity. Because that deceleration rate would de- generally be identified and screened out.4 Either their r pend on the cosmic mean mass density m, we would, in ef- spectra or their “colors” (the ratios of intensity seen fect, be weighing the universe. through two broadband filters) deviated from the tem- If mass density is, as was generally supposed a decade plates. The anomalously fainter supernovae were typically ago, the primary energy constituent of the universe, then redder or found in highly inclined spiral galaxies (or both). the measurement of the changing expansion rate would Many of these were presumably dimmed by dust, which also determine the curvature of space and tell us about absorbs more blue light than red. whether the cosmos is finite or infinite. Furthermore, the Soon after Branch’s work, Mark Phillips at the Cerro fate of the universe might be said to hang in the balance: Tololo Interamerican Observatory in Chile showed that If, for example, we measured a cosmic deceleration big the type Ia brightness outliers also deviated from the tem- enough to imply a r exceeding the “critical density” r plate light curve—and in a very predictable way.5 The su- m c (roughly 10–29 gm/cm3), that would indicate that the uni- pernovae that faded faster than the norm were fainter at verse will someday stop expanding and collapse toward an their peak, and the slower ones were brighter (see figure 1). In fact, one could use the light curve’s time scale to pre- apocalyptic “Big Crunch.” dict peak brightness and thus slightly recalibrate each su- All this sounded enticing: fundamental measure- pernova.