Supernovae, Dark Energy, and theAccelerating Universe
pansion of the universe since the time
the light was emitted. A collection ofsuch measurements, over a sufficient
range of distances, would yield an entire historical record of the universe’s
expansion.
Conceptually, this scheme is a remarkably straightforward means to a
profound prize: an empirical account of the growth of ouruniverse. A spectroscopically distinguishable class of objects with determinable intrinsic brightness would do thetrick. In Edwin Hubble’s discovery of the cosmic expansion
in the 1920s, he used entire galaxies as standard candles.
But galaxies, coming in many shapes and sizes, are difficult to match against a standard brightness. They cangrow fainter with time, or brighter—by merging with othergalaxies. In the 1970s, it was suggested that the brightest
member of a galaxy cluster might serve as a reliable standard candle. But in the end, all proposed distant galactic
candidates were too susceptible to evolutionary change.
As early as 1938, Walter Baade, working closely with
Fritz Zwicky, pointed out that supernovae were extremely
promising candidates for measuring the cosmic expansion.
Their peak brightness seemed to be quite uniform, andthey were bright enough to be seen at extremely large distances.1 In fact, a supernova can, for a few weeks, be as
bright as an entire galaxy. Over the years, however, as
more and more supernovae were measured, it became
clear that they were a rather heterogeneous group with awide range of intrinsic peak brightnesses.
In the early 1980s, a new subclassification of supernovae emerged. Supernovae with no hydrogen features intheir spectra had previously all been classified simply astype I. Now this class was subdivided into types Ia and Ib,
depending on the presence or absence of a silicon absorption feature at 6150 Å in the supernova’s spectrum.2 With
that minor improvement in typology, an amazing consistency among the type Ia supernovae became evident. Their
spectra matched feature-by-feature, as did their “light
curves”—the plots of waxing and waning brightness in the
weeks following a supernova explosion.3,4The uniformity of the type Ia supernovae became evenmore striking when their spectra were studied in detail as
they brightened and then faded. First, the outermost partsof the exploding star emit a spectrum that’s the same forall typical type Ia supernovae, indicating the same elemental densities, excitation states, velocities, and so forth.
Then, as the exploding ball of gas expands, the outermost
layers thin out and become transparent, letting us see thespectral signatures of conditions further inside. Eventually, if we watch the entire time series of spectra, we getto see indicators that probe almost the entire explosive
event. It is impressive that the type Ia supernovae exhibit
so much uniformity down to this level of detail. Such a “supernova CAT-scan” can be difficult to interpret. But it’sUsing very distant supernovae as standard candles, one
can trace the history of cosmic expansion and try to findout what’s currently speeding it up.
Saul Perlmutteror millennia, cosmology has been a theorist’s domain,F
where elegant theory was only occasionally endangeredby inconvenient facts. Early in the 20th century, Albert
Einstein gave us new conceptual tools to rigorously address the questions of the origins, evolution, and fate of theuniverse. In recent years, technology has developed to thepoint where these concepts from general relativity can besubstantiated and elaborated by measurements. For example, measurement of the remnant glow from the hot,dense beginnings of the expanding universe—the cosmic
microwave background—is yielding increasingly detailed
data about the first half-million years and the overall
geometry of the cosmos (see the news story on page 21 ofthis issue).
The standard model of particle physics has also begun
to play a prominent role in cosmology. The widely acceptedidea of exponential inflation in the immediate aftermathof the Big Bang was built on the predicted effect of certain
putative particle fields and potentials on the cosmic expansion. Measuring the history of cosmic expansion is noeasy task, but in recent years, a specific variety of supernovae, type Ia, has given us a first glimpse at that history—and surprised us with an unexpected plot twist.

Searching for a standard candle
In principle, the expansion history of the cosmos can be determined quite easily, using as a “standard candle” any distinguishable class of astronomical objects of known intrinsic brightness that can be identified over a wide
distance range. As the light from such beacons travels to
Earth through an expanding universe, the cosmic expansion stretches not only the distances between galaxy clusters, but also the very wavelengths of the photons en route.
By the time the light reaches us, the spectral wavelengthl has thus been redshifted by precisely the same incremental factor z  Dl/l by which the cosmos has been
stretched in the time interval since the light left its source.
That time interval is the speed of light times the object’s
distance from Earth, which can be determined by comparing its apparent brightness to a nearby standard of thesame class of astrophysical objects.
The recorded redshift and brightness of each such object thus provide a measurement of the total integrated exSaul Perlmutter is a senior scientist at the Lawrence Berkeley
National Laboratory and leader of the Supernova Cosmology
Project.

© 2003 American Institute of Physics, S-0031-9228-0304-030-4

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Physics Today

53

–20

ABSOLUTE MAGNITUDE

Figure 1. Light curves of nearby, low-redshift type Ia supernovae measured by Mario Hamuy and coworkers.7 (a) Absolute magnitude, an inverse logarithmic measure of intrinsic
brightness, is plotted against time (in the star’s rest frame) before and after peak brightness. The great majority (not all ofthem shown) fall neatly onto the yellow band. The figure
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
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
curves to fit the norm, and then scaling the brightness by anamount determined by the required time stretch, one gets all
the type Ia light curves to match.5,8

a

–19
–18
–17

–16
–15
–20

clear that essentially the same physical processes are occurring in all of these explosions.
The detailed uniformity of the type Ia supernovae implies that they must have some common triggering mechanism (see the box on page 56). Equally important, thisuniformity provides standard spectral and light-curve
templates that offer the possibility of singling out those supernovae that deviate slightly from the norm. The complex
natural histories of galaxies had made them difficult tostandardize. With type Ia supernovae, however, we saw
the chance to avoid such problems. We could examine therich stream of observational data from each individual explosion and match spectral and light-curve fingerprints torecognize those that had the same peak brightness.
Within a few years of their classification, type Ia supernovae began to bear out that expectation. First, David
Branch and coworkers at the University of Oklahoma
showed that the few type Ia outliers—those with peakbrightness significantly different from the norm—could
generally be identified and screened out.4 Either theirspectra or their “colors” (the ratios of intensity seen
through two broadband filters) deviated from the templates. The anomalously fainter supernovae were typicallyredder or found in highly inclined spiral galaxies (or both).
Many of these were presumably dimmed by dust, which
absorbs more blue light than red.
Soon after Branch’s work, Mark Phillips at the Cerro
Tololo Interamerican Observatory in Chile showed thatthe type Ia brightness outliers also deviated from the template light curve—and in a very predictable way.5 The supernovae that faded faster than the norm were fainter attheir peak, and the slower ones were brighter (see figure1). In fact, one could use the light curve’s time scale to predict peak brightness and thus slightly recalibrate each supernova. But the great majority of type Ia supernovae, as
Branch’s group showed, passed the screening tests andwere, in fact, excellent standard candles that needed nosuch recalibration.6

Cosmological distances
When the veteran Swiss researcher Gustav Tammann andhis student Bruno Leibengut
first reported the amazing
unduniformity of type Ia supernovae, there was immediate interest in trying to use them to determine the Hubble constant, H0, which measures the present expansion rate of
the cosmos. That could be done by finding and measuring
a few type Ia supernovae just beyond the nearest clustersof galaxies, that is, explosions that occurred some 100 million years ago. An even more challenging goal lay in the54

April 2003

Physics TodaySCALED MAGNITUDE

b
–19
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–17

–16
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0

20
DAYS

40

60

tantalizing prospect that we could find such standardcandle supernovae more than ten times farther away andthus sample the expansion of the universe several billion
years ago. Measurements using such remote supernovae
might actually show the expected slowing of the expansion
rate by gravity. Because that deceleration rate would depend on the cosmic mean mass density rm, we would, in effect, be weighing the universe.
If mass density is, as was generally supposed a decadeago, the primary energy constituent of the universe, thenthe measurement of the changing expansion rate would
also determine the curvature of space and tell us about
whether the cosmos is finite or infinite. Furthermore, thefate of the universe might be said to hang in the balance:If, for example, we measured a cosmic deceleration big
enough to imply a rm exceeding the “critical density” rc(roughly 10–29 gm/cm3), that would indicate that the universe will someday stop expanding and collapse toward an
apocalyptic “Big Crunch.”
All this sounded enticing: fundamental measurements made with a new distance standard bright enoughto be seen at cosmological distances. The problem was thattype Ia supernovae are a pain in the neck, to be avoided ifanything else would do. At the time, a brief catalog of reasons not to pursue cosmological measurement with type Ia
supernovae might have begun like this:왘 They are rare. A typical galaxy hosts only a couple oftype Ia explosions per millennium.
왘 They are random, giving no advance warning of where
to look. But the scarce observing time at the world’s largest
telescopes, the only tools powerful enough to measurethese most distant supernovae adequately, is allocated on
the basis of research proposals written more than six
months in advance. Even the few successful proposals are
granted only a few nights per semester. The possible ochttp://www.physicstoday.org

Brightness

currence of a chance supernova doesn’t makefor a compelling proposal.
왘 They are fleeting.
50–100After exploding, they
fieldsScheduled follow-up spectroscopy
must be discovered Almost 1000
at Keck and ESO telescopes
promptly and measured galaxies per
Scheduled follow-up
multiple times within a fieldimaging by
few weeks, or they will
Hubble, Cerro
already have passed
Tololo, WIYN, Isaac
the peak brightness
Newton and ESO telescopes
Hubblethat is essential for calibration. It’s too late tosubmit the observing
proposal after you’ve
discovered the supernova.
This was a classic
catch-22. You couldn’t
TIME
preschedule telescope
time to identify a suA dozen type 1a supernovae
pernova’s type or follow
discovered while still
it up if you couldn’t
brightening
guarantee one. But you
couldn’t prove a technique for guaranteeing
type Ia supernova discoveries without prescheduling telescope
time to identify them Figure 2. Observing strategy that guarantees batches of about a dozen fresh supernovae on demand. A first set of images of adjacent patches of sky containing tens of thousands of galaxies isspectroscopically.
The list of prob- made just after a new moon, and then these patches are reimaged just before the next new moon.
lems didn’t stop there. New bright spots pinpoint supernovae explosions since the first exposures. The timing ensures
The increasing red- that the supernovae are discovered before or near peak brightness. Scientists can preschedule, forshifting of supernova moonless nights just after the second field imaging, the spectral observations at the large telespectra with distance scopes in Hawaii and Chile needed to confirm supernova type. By searching through many galaxmeans that the bright- ies, we can guarantee a dozen or so new supernovae discovered on the second visit. That allows
ness of a very distant advance scheduling of time on the Hubble Space Telescope and other telescopes around thesupernova measured world, to monitor the light curves as they fade over several months.
through a given filter ishard to compare withthe brightness of amuch closer supernova measured through the same filter. supernova discovery. Contemporary computing and net(Astronomers call this the K-correction problem.) Dust in working advances just barely made possible the next-day
a supernova’s host galaxy can dim the explosion’s light. analysis that would let us catch supernovae as they first
And there were doubts that the spectra of faint distant su- brightened.
pernovae could be reliably identified as type Ia.
Finding our first supernova in 1992, we also found a
In fact, the results from the first search for very dis- solution to the K-correction problem by measuring the sutant type Ia supernovae were not encouraging. In the late pernova in a correspondingly redshifted filter. By playing
1980s, a Danish team led by Hans Nørgaard-Nielsen this trick with two redshifted filter bands, one could also
found only one type Ia supernova in two years of inten- expect to recognize dust absorption by its wavelength desive observing, and that one was already several weeks pendence. But we still hadn’t solved the catch-22 telescope
past its peak.
scheduling problem. We couldn’t preschedule follow-up observations of our first supernova, so couldn’t obtain its
A systematic solution
Daunting as these problems appeared, it seemed crazy to identifying spectrum.
In retrospect, the solution we found seems obvious—
let the logistics stand in the way, when the tools were atthoughmuch effort was needed to implement it and prove
hand for measuring such fundamental properties of theuniverse: its mass density, its large-scale curvature, and it practical. By specific timing of the requested telescopeits fate. After all, we didn’t have to build anything nearly schedules (see figure 2), we could guarantee that our wideas formidable as the gargantuan accelerators and detec- field imager would harvest a batch of about a dozen freshly
tors needed for particle physics. In a project that Carl Pen- exploded supernovae, all discovered on a pre-specified obnypacker and I began in Richard Muller’s group at the serving date during the dark phase of the moon. (A bright
University of California, Berkeley, just before the Danish moon is an impediment to the follow-up observation.) We
team’s 1988 supernova discovery, we started by building a first demonstrated this supernovae-on-demand methodolwide-field imager for the Anglo–Australian Observatory’s ogy in 1994. From then on, our proposals for time at major
4-meter telescope. The imager would let us study thou- ground-based telescopes could specify discovery dates andsands of distant galaxies in a night, upping the odds of a roughly how many supernovae would be found and folhttp://www.physicstoday.org

April 2003

Physics Today55

Supernova Cosmology
Project
High-Z Supernova
Search

24
0.001
22
0.01
0.1
1

Hamuy et al.

0

gy

er

20
0.2

18

0.4 0.6

um

en

cu

1

a
hv

y

erg

t
wi

m

16

u
cu

en

Em
Mass density pty

260.0001OBSERVED MAGNITUDE

RELATIVE BRIGHTNESS

Figure 3. Observed magnitude
versus redshift is plotted forwell-measuresd distant12,13 and(in the inset) nearby7 type Ia supernovae. For clarity, measurements at the same redshift are
combined. At redshifts beyond
z = 0.1 (distances greater thanabout 109 light-years), the cosmological predictions (indicated by the curves) begin todiverge, depending on the assumed cosmic densities ofmass and vacuum energy. The
red curves represent models
with zero vacuum energy andmass densities ranging from thecritical density rc down to zero(an empty cosmos). The best fit(blue line) assumes a massdensity of about rc /3 plus a
vacuum energy density twicethat large—implying an accelerating cosmic expansion.

rc

a

14
0.01
22

tv

ou

0.02

0.04

ith

w

0.1

Accelerating
universe
Decelerating
universe

21

20

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0.4
REDSHIFT z

0.6

1.0

0.8
0.7
0.6
0.5
LINEAR SCALE OF THE UNIVERSE RELATIVE TO TODAY

lowed up. This approach also made it possible to use the
Hubble Space Telescope for follow-up light-curve observations, because we could specify in advance the one-squaredegree patch of sky in which our wide-field imager wouldfind its catch of supernovae. Such specificity is a requirement for advance scheduling of the HST. By now, theBerkeley team, had grown to include some dozen collaborators around the world, and was called Supernova Costhe
mology Project (SCP).

By the end of the year, the error bars began to tighten,
as both groups now submitted papers with a few more supernovae, showing evidence for much less than the expected slowing of the cosmic expansion.9–11 This was beginning to be a problem for the simplest inflationarymodels with a universe dominated by its mass content.
Finally, at the beginning of 1998, the two groups presented the results shown in figure 3.12,13

A community effort

The faintness—or distance—of the high-redshift supernovae in figure 3 was a dramatic surprise. In the simplest

Meanwhile, the whole supernova community was making
progress with the understanding of relatively nearby supernovae. Mario Hamuy and coworkers at Cerro Tololo
took a major step forward by finding and studying many
nearby (low-redshift) type Ia supernovae.7 The resulting
beautiful data set of 38 supernova light curves (someshown in figure 1) made it possible to check and improveon the results of Branch and Phillips, showing that typeIa peak brightness could be standardized.6,7
The new supernovae-on-demand techniques that permitted systematic study of distant supernovae and the improved understanding of brightness variations among
nearby type Ia’s spurred the community to redouble its efforts. A second collaboration, called the High-Z Supernova
Search and led by Brian Schmidt of Australia’s Mount
Stromlo Observatory, was formed at the end of 1994. The
team includesd many veteran supernova experts. The tworival teams raced each other over the next few years—occasionally covering for each other with observations when
one of us had bad weather—as we all worked feverishly tofind and study the guaranteed on-demand batches ofsupernovae.
At the beginning of 1997, the SCP team presented theresults for our first seven high-redshift supernovae.8 These
first results demonstrated the cosmological analysis techniques from beginning to end. They were suggestive of an
expansion slowing down at about the rate expected for thesimplest inflationary Big Bang models, but with error bars
still too large to permit definite conclusions.
56

April 2003

Physics Today

What’s wrong with faint supernovae?

Exploding White Dwarfs
plausible, though unconfirmed, scenario would explainA
how all type Ia supernovae come to be so much alike,given the varied range of stars they start from. A lightweightstar like the Sun uses up its nuclear fuel in 5 or 10 billion
years. It then shrinks to an Earth-sized ember, a white dwarf,
with its mass (mostly carbon and oxygen) supported against
further collapse by electron degeneracy pressure. Then it
begins to quietly fade away.
But the story can have a more dramatic finale if the whitedwarf is in a close binary orbit with a large star that is still
actively burning its nuclear fuel. If conditions of proximity
and relative mass are right, there will be a steady stream ofmaterial from the active star slowly accreting onto the whitedwarf. Over millions of years, the dwarf’s mass builds up
until it reaches the critical mass (near the Chandrasekhar
limit, about 1.4 solar masses) that triggers a runaway thermonuclear explosion—a type Ia supernova.
This slow, relentless approach to a sudden cataclysmic
conclusion at a characteristic mass erases most of the original differences among the progenitor stars. Thus the light
curves (see figure 1) and spectra of all type Ia supernovae
are remarkably similar. The differences we do occasionallysee presumably reflect variations on the common theme—
including differences, from one progenitor star to the next,of accretion and rotation rates, or different carbon-to-oxygen ratios.
http://www.physicstoday.org

}

0.01
0.1
1

0.001

0.0001

}

1.0

0

0.0
–20

s

de

cel
e

ra
t es

REDSHIFT z

d

te
ra

alw
ays

n
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t
irs
nf
sio

a
ler
ece

le
cce
na
e
, th
tes

–10
0
BILLIONS OF YEARS FROM TODAY

cosmological models, the expansion history of the cosmos
is determined entirely by its mass density. The greater thedensity, the more the expansion is slowed by gravity. Thus,in the past, a high-mass-density universe would have been
expanding much faster than it does today. So one shouldn’t have to look far back in time to especially distant (faint)supernovae to find a given integrated expansion (redshift).
Conversely, in a low-mass-density universe one would
have to look farther back. But there is a limit to how low
the mean mass density could be. After all, we are here, andthe stars and galaxies are here. All that mass surely puts
a lower limit on how far—that is, to what level of faintness—we must look to find a given redshift. The highredshift supernovae in figure 3 are, however, fainter thanwould be expected even for an empty cosmos.
If these data are correct, the obvious implication isthat the simplest cosmological model must be too simple.
The next simplest model might be one that Einstein entertained for a time. Believing the universe to be static, hetentatively introduced into the equations of general relativity an expansionary term he called the “cosmologicalconstant” (L) that would compete against gravitational collapse. After Hubble’s discovery of the cosmic expansion,
Einstein famously rejected L as his “greatest blunder.” Inlater years, L came to be identified with the zero-point
vacuum energy of all quantum fields.
It turns out that invoking a cosmological constant allows us to fit the supernova data quite well. (Perhaps there
was more insight in Einstein’s blunder than in the best efforts of ordinary mortals.) In 1995, my SCP colleague Ariel
Goobar and I had found that, with a sample of type Ia supernovae spread over a sufficiently wide range of distances, it would be possible to separate out the competing
effects of the mean mass density and the vacuum-energy
density.14
The best fit to the 1998 supernova data (see figures 3
and 4) implies that, in the present epoch, the vacuum energy density rL is larger than the energy density attributable to mass (rmc2). Therefore, the cosmic expansion is now
accelerating. If the universe has no large-scale curvature,
http://www.physicstoday.org

0.5
1
1.5
2
3

. . . or

0.5

Ex

LINEAR SCALE OF UNIVERSE RELATIVE TO TODAY

1.5

Eternal expansion

Eventual collapse

RELATIVE BRIGHTNESS OF SUPERNOVAE+10

Figure 4. The history of cosmic
expansion, as measured by thehigh-redshift supernovae (the black
data points), assuming flat cosmic
geometry. The scale factor R of theuniverse is taken to be 1 at present, so it equals 1/(1 + z). The
curves in the blue shaded region
represent cosmological models inwhich the accelerating effect of
vacuum energy eventually overcomes the decelerating effect ofthe mass density. These curves assume vacuum energy densities
ranging from 0.95 rc (top curve)
down to 0.4 rc. In the yellow
shaded region, the curves represent models in which the cosmic
expansion is always decelerating
due to high mass density. They assume mass densities ranging (left toright) from 0.8 rc up to 1.4 rc. In
fact, for the last two curves, the expansion eventually halts and reverses into a cosmic collapse.

as the recent measurements of the cosmic microwave background strongly indicate, we can say quantitatively thatabout 70% of the total energy density is vacuum energy
and 30% is mass. In units of the critical density rc, one
usually writes this result as
WL  rL/rc  0.7 and Wm  rm/rc  0.3.

Why not a cosmological constant?
The story might stop right here with a happy ending—acomplete physics model of the cosmic expansion—were it
not for a chorus of complaints from the particle theorists.
The standard model of particle physics has no naturalplace for a vacuum energy density of the modest magnitude required by the astrophysical data. The simplest estimates would predict a vacuum energy 10120 times greater.
(In supersymmetric models, it’s “only” 1055 times greater.)So enormous a L would have engendered an acceleration
so rapid that stars and galaxies could never have formed.
Therefore it has long been assumed that there must be
some underlying symmetry that precisely cancels the vacuum energy. Now, however, the supernova data appear torequire that such a cancellation would have to leave a remainder of about one part in 10120. That degree of fine tuning is most unappealing.
The cosmological constant model requires yet anotherfine tuning. In the cosmic expansion, mass density becomes ever more dilute. Since the end of inflation, it has
fallen by very many orders of magnitude. But the vacuum
energy density rL, a property of empty space itself, stays
constant. It seems a remarkable and implausible coincidence that the mass density, just in the present epoch, is
within a factor of 2 of the vacuum energy density.
Given these two fine-tuning coincidences, it seems
likely that the standard model is missing some fundamental physics. Perhaps we need some new kind of accelerating energy—a “dark energy” that, unlike L, is not constant. Borrowing from the example of the putative“inflaton” field that is thought to have triggered inflation,
theorists are proposing dynamical scalar-field models andother even more exotic alternatives to a cosmological conApril 2003

Physics Today57

Pursuing the elusive dark energy
The dark energy evinced by the accelerating cosmic expansion grants us almost no clues to its identity. Its tiny
density and its feeble interactions presumably preclude
identification in the laboratory. By construction, of course,
it does affect the expansion rate of the universe, and different dark-energy models imply different expansion rates
in different epochs. So we must hunt for the fingerprintsof dark energy in the fine details of the history of cosmic
expansion.
The wide-ranging theories of dark energy are often
58

April 2003

Physics TodayNo Big Bang

2

Supernovae

1

CMB
r
s foreve
Cosmos expand
y
ll
a
tu
n
ve
Recollapses e

0

Clusters

Fl

at

–1

0

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2
MASS DENSITY Wm

B
to ig
o B
re an
ce g
nt

stant, with the goal of solving the coincidence problems.
(See the Reference Frame article by Michael Turner on
page 10 of this issue.)
The experimental physicist’s life, however, is dominated by more prosaic questions: “Where could my measurement be wrong, and how can I tell?” Crucial questionsof replicability were answered by the striking agreement
between our results and those of the competing team, butthere remain the all-important questions of systematic uncertainties. Most of the two groups’ efforts have been devoted to hunting down these systematics.15,16 Could thefaintness of the supernovae be due to intervening dust?
The color measurements that would show color-dependentdimming for most types of dust indicate that dust is not amajor factor.12,13 Might the type Ia supernovae have been
intrinsically fainter in the distant past? Spectral comparisons have, thus far, revealed no distinction between the
exploding atmospheres of nearby and more distant supernovae.9,12
Another test of systematics is to look for even moredistant supernovae, from the time when the universe wasso much more dense that rm dominated over the dark energy and was thus still slowing the cosmic expansion. Supernovae from that decelerating epoch should not get as
faint with increasing distance as they would if dust or intrinsic evolutionary changes caused the dimming. The firstfew supernovae studied at redshifts beyond z = 1 have already begun to constrain these systematic uncertainties.17(See PHYSICS TODAY, June 2001, page 17.)
By confirming the flat geometry of the cosmos, the recent measurements of the cosmic microwave backgroundhave also contributed to confidence in the accelerating-universe results. Without the extra degree of freedom provided
by possible spatial curvature, one would have to invoke improbably large systematic error to negate the supernova results. And if we include the low rm estimates based on inventory studies of galaxy clusters, the Wm–WL parameter
plane shows a reassuring overlap for the three independent kinds of cosmological observations (see figure 5).

3

VACUUM ENERGY DENSITY WL

Figure 5. In the cosmological parameter space of the normalized mass and vacuum energy densities Wm and WL,sthree independent sets of obervations—high-redshift
supernovae, galaxy cluster inventories, and the cosmic microwave
background, converge nicely near Wm = 0.3 and WL = 0.7.
The small yellow contour in this region indicates how well
we expect the proposed SNAP satellite experiment to further
narrow down the parameters. The inflationary expectation
constraint of a flat cosmos (Wm + WL = 1) is indicated by theblack diagonal. The red curve separates an eternallyexpanding cosmos from one that ends in a “Big Crunch.”3

characterized by their equation-of-state parameterw  p/r, the ratio of the dark energy’s pressure to its
energy density. The deceleration (or acceleration) of an
expanding universe, given by the general relativistic
equation᝽᝽ /R = –4/3pGr(1 + 3w),
R
depends on this ratio. Here R, the linear scale factor of the
expanding universe, can be thought of as the mean distance between galaxy clusters not bound to each other.
Thus the expansion accelerates whenever w is more negative than –1/3, after one includes all matter, radiation,and dark-energy components of the cosmic energy budget.
Each of the components has its own w: negligible fornonrelativistic matter, +1/3 for radiation and relativistic
matter, and –1 for L. That is, L exerts a peculiar negative
pressure! General relativity also tells us that each component’s energy density falls like R–3(1 + w) as the cosmos expands. Therefore, radiation’s contribution falls away first,
so that nonrelativistic matter and dark energy now predominate. Given that the dark-energy density is now about
twice the mass density, the only constraint on dark-energy
models is that w must, at present, be more negative than
–1/2 to make the cosmic expansion accelerate. However,most dark-energy alternatives to a cosmological constant
have a w that changes over time. If we can learn more
about the history of cosmic expansion, we can hope to discriminate among theories of dark energy by better determining w and its time dependence.
Unfortunately, the differences between the expansion
histories predicted by the current crop of dark-energy models are extremely small. Distinguishing among them will
require measurements an order of magnitude more accurate than those shown in figure 3, and extending twice as
http://www.physicstoday.org

far back in time.
There is no shortage of type Ia supernovae; one explodes somewhere in the sky every few seconds. In principle, then, the job is simply to study a hundred times as
many supernovae as we have so far. That’s a difficult but
not prohibitive task, if we install dedicated wider-field imagers and improved spectrographs on dedicated large telescopes. However, it’s not just a matter of improving thequantity of measurements. The quality must also take adramatic step forward, because the current measurement
accuracy is not limited simply by statistical errors. Even
with the number of supernovae we already have in hand,
our statistical uncertainties are already close to the systematic uncertainties.

A new challenge
The next generation of supernova projects has alreadybegun. Telescope scheduling committees have dramatically increased the time allotted them on the largest telescopes. With biweekly monitoring of patches of sky for several years on end at two 4-meter telescopes, it will bepossible to collect almost complete light curves for hundreds of 5-billion-year-old type Ia supernovae. Smaller telescopes will study the time-varying spectra of much closer
supernovae,. And
and imagers on the HST and the 8-m Subaru
Telescope in Hawaii are now revealing handfuls of 10-billion-year-old supernovae. A number of large new telescopes are devoting extensive observing programs to follow-up measurements of this plethora of supernovae. At
the most extreme distances, only the Hubble telescope canjust barely follow the fading supernovae, redshifted into
the infrared. With this array of effort,s,we may know, before too long, whether the time-averaged behavior of thedark energy is consistent with a cosmological constant.
The still harder goal of the third generation of supernova work, which also has already begun, is to look fortime variations in the dark energy. For this higher-precision work, the systematic uncertainties must be reduced
dramatically. The physical details of each individual supernova explosion must be pinned down with extensive
and exacting spectral and photometric monitoring. Intervening dust must be measured with wavelength coverage
extending into the near-infrared. Host galaxies must beclassified to control for environmental effects on the typeIa standard candle. And we will have to study enough supernovae in each redshift range to take account of possible gravitational lensing by foreground galaxies that canbrighten or dim a supernova.
These very exacting requirements have pushed us towork above the atmosphere and design a new orbiting optical and near-infrared telescope called SNAP (the SuperNova/Acceleration Probe). With a 2-meter mirror, ahalf-billion-pixel imager, and a high-throughput spectrograph, this space mission can accomplish the unprecedented suite of measurements required for measuring
thousands of supernovae with adequately constrained
systematic uncertainties.18
We live in an unusual time, perhaps the first golden
age of empirical cosmology. With advancing technology, we
have begun to make philosophically significant measurements. These measurements have already brought surprises. Not only is the universe accelerating, but it apparently consists primarily of mysterious substances. We’ve
already had to revise our simplest cosmological models.
Dark energy has now been added to the already perplexing question of dark matter. One is tempted to speculatethat these ingredients are add-ons, like the Ptolemaic
epicycles, to preserve an incomplete theory,.. With the next
decade’s new experiments, exploiting not only distantApril 2003

Physics Today

59

supernovae, but also the cosmic microwave background,gravitational lensing of galaxies, and other cosmologicalobservations, we have the prospect of taking the next step
toward that “Aha!” moment when a new theory makes
sense of the current puzzles.
In references 12 and 13, I have listed in full the members on
the High-Z Supernova Search and Supernova Cosmology
Project teams, because each of these scientists should be recognized for important contributions to the discoveries described here. It has been both an honor and a pleasure towork closely with my SCP colleagues, who dedicated themselves to this work for years on end, providing creativity andleadership.

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18. For more information on SNAP, see http://snap.lbl.gov.
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