Dark energy appears to account for over three-quarters of the stuff in the Universe, and it’s pushing all the rest – ordinary matter and dark matter – farther apart at an ever-increasing rate. But what is dark energy? Although theories abound, the short answer is that nobody knows.
We know it exists because of an experimental technique that uses specific types of exploding stars, or supernovae, as “standard candles.” A dozen years ago measurements of these supernova at increasing distances from Earth led to the unexpected discovery of dark energy; observations of supernovae continue to increase in power and precision in ongoing studies.
Independent evidence from measurements of the cosmic microwave background and other estimates of the matter density of the Universe provided early support for the radical idea of dark energy. Newer and quite different techniques, including weak lensing and baryon acoustic oscillations, are now poised to offer unique insights into what Nobel Prize-winner Frank Wilczek has called “the most fundamentally mysterious thing in basic science.”
Type Ia Supernovae: The Best Standard Candles
During the 1980s and 90s, the Supernova Cosmology Project (SCP), co-founded by Saul Perlmutter and Carl Pennypacker and based at Berkeley Lab, demonstrated that Type Ia supernovae were excellent standard candles for measuring the expansion history of the Universe. Although the idea had been circulating within the astronomical community for years, says Perlmutter, a Berkeley Lab astrophysicist and professor of physics at UC Berkeley, “In the early days, people thought measuring expansion with supernovae would be too hard.”
The SCP went on to show that distant supernovae, short-lived and unpredictable as they are, can nevertheless be collected “on demand,” allowing observers to schedule telescope time in advance and accumulate enough data to make confident estimates of expansion.
“In retrospect it seems obvious, but we realized that the whole process could be systematized,” Perlmutter explains. “By searching the same group of galaxies three weeks apart, we could find supernovae candidates that had appeared in the meantime. We could guarantee four to eight supernovae each time, and all of them would be on the way up” growing brighter instead of already fading.
Type Ia supernovae are among the brightest things in the Universe; what’s more, they are all almost the same brightness, with differences that can be standardized to less than 10 percent. Thus a supernova’s apparent brightness shows how far away it is and, because light takes time to travel, how far back in time it exploded.
The supernova’s redshift – the shifting of spectral lines (signals of specific elements in the exploding star) toward the red end of the spectrum – is a direct measure of how much the space through which the light has traveled has stretched.
The idea is simple on paper: by comparing brightness to redshift for numerous Type Ia supernovae, from nearby to very distant, an observer can tell how the rate of expansion of the Universe has changed over time.
Members of the Supernova Cosmology Project expected to find, as did their rivals in the High-Z Supernova Search Team, that the farther away (the farther back in time) a supernova was, the brighter (closer) it would appear relative to its redshift — an indication that expansion has been slowing. Instead both teams found the opposite.
“The chain of analysis was long, and the Universe can be devious, so at first we were reluctant to believe our result,” Perlmutter explains. “But the more we analyzed it, the more it wouldn’t go away.”
Perlmutter described the evidence for accelerating expansion at an American Astronomical Society meeting in January 1998. At first both teams thought the cause was a form of Einstein’s “cosmological constant,” assumed to be an unknown form of energy that uniformly, as its name suggests, counteracts the mutual gravitational attraction of the matter in the cosmos.
But within weeks a flurry of alternative explanations and theories were put forth, including ideas for a dynamical, not constant, form of energy, or for an odd cosmos in which our Universe bounces back and forth between expansion and contraction — or perhaps most radical of all, that Einstein’s General Theory of Relativity, the best explanation of gravitation we have, is flawed.
One way to sort out some of these competing theories is to collect a much larger sample of supernovae and measure them with greater precision. That way, scientists would be able to tell whether dark energy has indeed been constant and expansion has followed a smooth curve, or whether at different eras expansion has proceeded faster or slower than at present, and dark energy is dynamic.
To gather a lot more supernovae, especially more distant supernovae, it’s necessary for a telescope to escape the limitations of Earth’s atmosphere. In 1999, Berkeley Lab physicists and astronomers formed an international collaboration to design the SuperNova/Acceleration Probe (SNAP), a satellite dedicated to the study of dark energy. In 2003 the U.S. Department of Energy (DOE) and NASA formed the Joint Dark Energy Mission (JDEM) and solicited additional ideas. The DOE JDEM Project Office is located at Berkeley Lab.
Better measurements of Type Ia supernovae require reducing or eliminating uncertainties in measuring their brightness and spectra. Brighter Type Ia supernovae wax and wane more slowly than fainter ones, for example, but when these individual “light curves” are stretched to fit the norm, and brightness is scaled according to the stretch, most can be made to match. This “classic” method has been used to standardize intrinsic brightness to within 8 to 10 percent.
To reduce these error bars and other uncertainties, more high-quality spectra are needed, beginning with “nearby” supernova, those whose spectra have not been shifted so far into the red that parts are hard to recover or no longer visible. Since its founding in 2002, the Nearby Supernova Factory (SNfactory), a collaboration of Berkeley Lab, a consortium of French laboratories, and Yale University, has amassed an enormous database of some 2,500 spectra.
With this data, SNfactory researcher Stephen Bailey found that simply by measuring the ratio of brightness between two specific regions in the spectrum of a Type Ia supernova taken on a single night, that supernova’s distance can be determined to better than 6 percent uncertainty.
Berkeley Lab cosmologist Greg Aldering, a founder and leader of the SNfactory, says, “This is an example of exactly what we designed the Nearby Supernova Factory to do. It underlines the vital role of detailed spectrometry in discoveries of cosmic significance.”
But supernovae alone cannot provide the whole answer. Baryon acoustic oscillation is a new technique that provides a “cosmic ruler” to measure the expansion history of the Universe.
Courtesy: Paul Preus, Lawrence Berkeley National Laboratory
If dark energy is real and not a flaw in our understanding of gravity, then the best way to understand it is by studying the expansion history of the Universe, according to Martin White, an astrophysicist at Berkeley Lab and a professor of astronomy and physics at UC Berkeley. “One way is with ‘standard candles’ – that is, supernovae,” White says. “Another way is with a ‘standard ruler.’”
Baryon acoustic oscillation, or BAO, may provide the ideal standard ruler. The scale is calibrated by the cosmic microwave background (CMB), which recorded the state of the Universe roughly 400,000 years after the Big Bang. At this early epoch, the standard ruler for BAO is detectable as periodic, minute variations in the temperature of the CMB. More recently, the ruler’s scale is evident in the regular clustering of galaxies and intergalactic gas, and is also present in the clumping of invisible dark matter. These oscillations can be measured both across the sky and in the line of sight (back in time).
Both signals have the same origin. The early universe was a liquid-like plasma of protons and electrons in which light was trapped, with dark matter also part of the mix. “Baryons” (ordinary matter) moved in “acoustic oscillations” (sound waves) through the plasma. When the Universe cooled enough for the protons and electrons to combine into hydrogen atoms, the photons were freed and the Universe became transparent. The dark matter stayed invisible, but variations in density left their mark in the CMB and were the seeds of large-scale structure in today’s universe, such as clusters of galaxies.
The first clear detection of a BAO signal was announced in 2005 by Daniel Eisenstein of the University of Arizona and his colleagues, who analyzed data from about 50,000 luminous red galaxies in the Sloan Digital Sky Survey (SDSS), plus some from a separate survey by the Two Degree Field Galaxy Redshift Survey based in Australia.
In a paper published in July 2007, Nikhil Padmanabhan, David Schlegel, and Uroš Seljak, all by then at Berkeley Lab, presented their work with colleagues in SDSS and members of the Australian survey to extend the analysis to 600,000 luminous red galaxies at distances up to 5.6 billion light-years. This report benefited from exploring the largest volume of space ever used for galaxy clustering measurements – approaching halfway back in time to the origin of the CMB. Although the survey did not depend on the redshifts of individual galaxies (instead estimating distance on the basis of the specific colors of the galaxies), it was able to establish a specific scale for the markings on the standard ruler: 450 light-years.
“Unfortunately it’s an inconveniently sized ruler, to put it mildly,” says Schlegel, a staff scientist in the Physics Division. “We had to sample a huge volume of the Universe just to fit the ruler inside.”
Big as the map was, and important as it was in establishing the real possibility of using the BAO standard ruler to measure the expansion history of the Universe, precision measurements were still a long way off. “We showed there was an effective ruler,” says Schlegel. “Now we had to use it.”
That effort got underway in September 2009, when the Baryon Oscillation Spectroscopic Survey, BOSS, recorded “first light” in what will be a five-year search to record the individual spectra of two million galaxies and quasars, plus variations in the density of the intergalactic gas, across a quarter of the entire sky. The oscillations of these “baryons” in many different forms, revealed in the periodicity of the large-scale structures of the Universe, will establish the accuracy of the cosmic ruler to a precision of one percent.
BOSS is the flagship survey of the third phase of the Sloan Digital Sky Survey, known as SDSS-III, which is headed by Daniel Eisenstein. Among other Berkeley Lab contributors to the collaboration, Schlegel is the principal investigator for BOSS, White is its survey scientist, and Natalie Roe of Berkeley Lab’s Physics Division is its instrument scientist.
The correlated results from supernovae and BAO measurements will home in on the expansion history of the Universe and make it possible to differentiate among the competing theories of its nature. But what if dark energy is an illusion, and what we take as evidence for dark energy is really just a flaw in our understanding of gravity?
Another technique, which relies upon the idea that large amounts of mass — whether visible or invisible — can distort our view of the light waves from even more distant objects beyond in known ways, may be the answer.
One way to find out is to measure the growth of structure in the Universe by means of weak gravitational lensing.
“Einstein’s theory of General Relativity is how we understand gravity,” says Martin White, “and it’s never a good idea to bet against Einstein.” Nevertheless, astronomers know a good way to test whether there’s a breakdown in General Relativity.
Gravitational lensing arises directly from Einstein’s realization that what we call gravity is the fact that mass curves the space-time fabric of the Universe. If there is a chunk of matter between us and a distant object – say, our Sun between us and a distant star – then the intervening mass of the Sun acts as a lens, bending space so as to enlarge and outwardly displace the distant star’s apparent position. In fact it was the displacement of stars during a solar eclipse, measured by Sir Arthur Eddington in 1919, that provided the first experimental evidence for Einstein’s revolutionary theory.
Obvious visible displacement is characteristic of a phenomenon known as strong gravitational lensing. Weak lensing is less obvious, but still measurable, as a statistical estimate of the distortion of the apparent size and shape (shear) of background galaxies behind an intervening mass – which may be a single galaxy, a cluster of galaxies, or a concentration of invisible dark matter.
“Because weak lensing can trace the evolution of all the matter in the Universe, visible and dark, a lot of people have been excited about using it to measure dark energy since at least the 1990s,” says Uroš Seljak, a member of Berkeley Lab’s Physics Division and a professor of astronomy and physics at UC Berkeley. “The problem is that for a long time it was difficult to implement on telescopes. It’s a subtle effect, and all sorts of distortions – in the atmosphere, the cameras, or the telescopes themselves – can interfere with good measurements.”
Weak lensing directly detects matter, including dark matter, but it can also be used to study dark energy, says Seljak, “because dark energy will affect how matter grows in time.”
In fact, says Seljak, “dark energy slows the growth of structure.” This is because structure results from the mutual gravitational attraction of matter; dense regions progressively grow more dense. But dark energy is “a sea of smooth energy” that uniformly expands space and everywhere acts against increasing density.
Seljak describes two ways that weak lensing can be measured. One, called “shear-shear” correlations, measures the combined effects of all the matter between us and the distant galaxies being observed – not the effect of any particular structure alone. “The farther away the galaxies we’re looking at are, the earlier in the history of the Universe we can observe,” he says. Effects of weak lensing have been detected at great distances, and there are even hints of weak lensing in the cosmic microwave background itself.
A different technique, for which Seljak uses the shorthand “galaxy-shear,” looks at weak lensing around galaxies or clusters of galaxies. This method is much less sensitive to spurious distortions than the cumulative shear-shear method, because with this technique many of the spurious distortions are self-canceling.
The two techniques are complementary; both require enormous databases, powerful computers, and programs that can derive the mass signature from the shear signal and find the constraints thus imposed on possible cosmologies, so that the results can be compared to the predicted effects of different theories of dark energy. Among the Berkeley Lab cosmologists who are investigating applications of weak lensing are Alexie Leauthaud and Reiko Nakajima of the Physics Division.
“Weak lensing is a powerful method for measuring the growth of structure over time,” says White. “General Relativity makes a unique prediction about the growth of structures. At a particular expansion rate, did structures grow according to Einstein? If Relativity is wrong, we’ll see it directly.”
More to come
Supernovae, baryon acoustic oscillations, and weak lensing are not the only techniques proposed for studying dark energy, although at present they are the most mature. So far, Type Ia supernovae have been the most used for measuring the expansion history of the Universe.
In addition to the Joint Dark Energy Mission satellite, which will apply all three methods, Berkeley Lab is participating in a number of other dark energy missions such as the multi-agency, multi-institutional, multinational Dark Energy Survey, in which a special red-sensitive camera will be mounted on a four-meter telescope in Chile with the stated goal of answering the question, “is the dark energy a cosmological constant?”
As in the beginning, the cosmological constant remains the favorite form of dark energy among many – although far from all – astronomers, astrophysicists, and cosmologists. But answering the question – cosmological constant or something else? – will be only the beginning of investigation into the nature of the biggest mystery in 21st-century physics.
Courtesy: Paul Preus, Lawrence Berkeley National Laboratory