Origins: Fourteen Billion Years of Cosmic Evolution (7 page)

In 1998, two
rival teams of astronomers announced new observations implying the existence of a non-zero cosmological constant—not (of course) the very number that Einstein had proposed in order to keep the universe static, but another, quite different value, one that implies that the universe will expand forever at an ever-increasing rate.

If theorists had proposed this for yet another model universe, the world would have little noted nor long remembered their effort. Here, however, reputable experts in observing the real universe, had mistrusted one another, checked on their rivals’ suspicious activities, and discovered that they agreed on the data and their interpretation. The observational results not only implied a cosmological constant different from zero but also assigned to that constant a value that makes space flat.

What’s that you say? The cosmological constant flattens space? Aren’t you suggesting, like the Red Queen in
Alice in Wonderland
, that we each believe six impossible things before breakfast? More mature reflection may, however, convince you that if apparently empty space does contain energy (!), that energy must contribute mass to the cosmos, just as Einstein’s famous equation,
E = mc
2
, implies. If you’ve got some
E
, you can conceive it as a corresponding amount of
m
, equal to
E
divided by
c
2
. Then the total density must equal the total of the density contributed by matter, plus the density contributed by energy.

The new total density is what we must compare with the critical density. If the two are equal, space must be flat. This would satisfy the inflationary model’s prediction of flat space, for it does not care whether the total density in space arises from the density of matter, or the matter equivalent provided by the energy in empty space, or a combination of the two.

The crucial evidence
suggesting a non-zero cosmological constant, and thus the existence of dark energy, came from astronomers’ observations of a particular type of exploding star or supernova, stars that die spectacular deaths in titanic explosions. These supernovae, called either Type Ia or SN Ia’s, differ from other types, which occur when the cores of massive stars collapse after exhausting all possibilities of producing more energy by nuclear fusion. In contrast, SN Ia’s owe their origin to white dwarf stars that belong to binary star systems. Two stars that happen to be born close to one another will spend their lives performing simultaneous orbits around their common center of mass. If one of the two stars has more mass than the other, it will pass more rapidly through its prime of life, and in most cases will then lose its outer layers of gas, revealing its core to the cosmos as a shrunken, degenerate “white dwarf,” an object no larger than Earth but containing as much mass as the Sun. Physicists call the matter in white dwarfs “degenerate” because it has such a high density—more than a hundred thousand times the density of iron or gold—that the effects of quantum mechanics act on matter in bulk form, preventing it from collapsing under its enormous self-gravitational forces.

A white dwarf in mutual orbit with an aging companion star attracts gaseous material that escapes from the star. This matter, still relatively rich in hydrogen, accumulates on the white dwarf, growing steadily denser and hotter. Finally, when the temperature rises to 10 million degrees, the entire star ignites in nuclear fusion. The resulting explosion—similar in concept to a hydrogen bomb but trillions of times more violent—blows the white dwarf completely apart and produces a Type Ia supernova.

SN Ia’s have proven particularly useful to astronomers by possessing two separate qualities. First, they produce the most luminous supernova explosions in the cosmos, visible across billions of light-years. Second, nature sets a limit to the maximum mass that any white dwarf can have, equal to about 1.4 times the Sun’s mass. Matter can accumulate on a white dwarf’s surface only until the white dwarf’s mass reaches this limiting value. At that point, nuclear fusion blasts the white dwarf apart—and the blast occurs in objects with the same mass and the same composition, strewn throughout the universe. As a result, all of these white dwarf supernovae attain nearly the same maximum energy output, and they all fade away at almost the same rate after they achieve their maximum brightness.

These dual attributes allow SN Ia’s to provide astronomers with highly luminous, easily recognizable “standard candles,” objects known to achieve the same maximum energy output wherever they appear. Of course, the distance to the supernovae affects their brightnesses as we observe them. Two SN Ia’s, seen in two faraway galaxies, will appear to reach the same maximum brightness only if they have the same distance from us. If one has twice the distance of the other, it will attain only one quarter of the other’s maximum apparent brightness, because the brightness with which any object appears to us diminishes in proportion to the square of its distance.

Once astronomers learned how to recognize Type Ia supernovae, based on their detailed study of the spectrum of light from each of these objects, they had a golden key with which to unlock the riddle of determining accurate distance. After measuring (through other means) the distances to the closest of the SN Ia’s, they could estimate much greater distances to other Type Ia supernovae, simply by comparing the brightnesses of the relatively near and distant objects.

Throughout the 1990s, two teams of supernova specialists, one centered at Harvard and the other at the University of California at Berkeley, refined this technique by finding how to compensate for the small but real differences among the SN Ia’s, which the supernovae reveal to us through the details in their spectra. In order to use their newly forged key to unlock the distances to faraway supernovae, the researchers needed a telescope capable of observing distant galaxies with exquisite precision, and they found one in the Hubble Space Telescope, refurbished in 1993 to correct its primary mirror that had been ground to the wrong shape. The supernova experts used ground-based telescopes to discover dozens of SN Ia’s in galaxies billions of light-years from the Milky Way. They then arranged for the Hubble Telescope, for which they could obtain only a modest fraction of the total observing time, to study these newfound supernovae in detail.

As the 1990s drew toward a close, the two teams of supernova observers competed keenly to derive a new and expanded “Hubble diagram,” the key graph in cosmology that plots galaxies’ distances versus the speeds at which the galaxies are moving away from us. Astrophysicists calculate these speeds through their knowledge of the Doppler effect (described in Chapter 13), which changes the colors of the galaxies’ light by small amounts that depend on the velocities at which the galaxies are receding from us.

Each galaxy’s distance and recession velocity specify a point on the Hubble diagram. For relatively nearby galaxies these points march upward in lockstep, since a galaxy twice as distant from us as another turns out to be receding twice as fast. The direct proportionality between galaxies’ distances and recession velocities finds algebraic expression in Hubble’s law, the simple equation that describes the basic behavior of the universe:
v = H
o
x
d
. Here
v
stands for recession velocity,
d
for distance, and
H
o
is a universal constant, called Hubble’s constant, that describes the entire universe at any particular time. Alien observers throughout the universe, studying the cosmos 14 billion years after the big bang, will find galaxies receding at speeds that follow Hubble’s law, and all of them will derive the same value for Hubble’s constant, though they will probably give it a different name. This assumption of cosmic democracy underlies all of modern cosmology. We cannot prove that the entire cosmos follows this democratic principle. Perhaps, far beyond the farthest horizon of our vision, the cosmos behaves quite differently from what we see. But cosmologists reject this approach, at least for the observable universe. In that case,
v = H
o
x
d
represents universal law.

With time, however, the value of Hubble’s constant can and does change. A new and improved Hubble diagram, one that extended to include galaxies many billions of light-years away, will reveal not only the value of today’s Hubble constant
H
o
(embodied in the slope of the line that runs through the points representing galaxies’ distances and recession velocities) but also the way in which the universe’s current rate of expansion differs from its value billions of years ago. The latter value would be revealed by the details of the upper reaches of the graph, whose points describe the most distant galaxies ever observed. Thus a Hubble diagram extending to distances of many billion light-years would reveal the history of the expansion of the cosmos, embodied in its changing rate of expansion.

In striving for this goal, the astrophysics community struck a mother lode of good fortune in having two competing teams of supernova observers. The supernova results, first announced in February 1998, had an impact so great that no single group could have survived the natural skepticism of cosmologists to the overthrow of their widely accepted models of the universe. Because the two observing teams directed their skepticism primarily at each other, they brilliantly searched for errors in the other team’s data or interpretation. When they pronounced themselves satisfied, despite their human prejudices, that their competitors were careful and competent, the cosmological world had little choice but to accept, albeit with some restraint, the news from the frontiers of space.

What was that news? Just that the most distant SN Ia’s turned out a bit fainter than expected. This implies that the supernovae are somewhat farther away than they ought to be, which in turn shows that something made the universe expand a bit more rapidly than it should. What provoked this additional expansion? The only culprit that fits the facts is the “dark energy” that lurks in empty space—the energy whose existence corresponds to a non-zero value for the cosmological constant. By measuring the amount by which distant supernovae turned out to be fainter than expected, the two teams of astronomers measured the shape and fate of the universe.

When the two
supernova teams achieved consensus, the cosmos turned out to be flat. To understand, we must engage in a bit of rough and tumble in Greek. A universe with a non-zero cosmological constant requires one additional number to describe the cosmos. To the Hubble constant, which we write as
H
0
to denote its value at the present time, and to the average density of matter, which alone determines the curvature of space if the cosmological constant is zero, we must now add the density equivalent provided by the dark energy, which, by Einstein’s formula
E = mc
2
, must possess the equivalent of mass (
m
) because it has energy (
E
). Cosmologists express the densities of matter and dark energy with the symbols
Ω
M
and
Ω
Λ
, where
Ω
(the Greek capital letter omega) stands for the ratio of the cosmic density to the critical density.
Ω
M
represents the ratio of the average density of all the matter in the universe to the critical density, while
Ω
Λ
stands for the ratio of the density equivalent provided by the dark energy to the critical density. Here
Λ
(Greek capital lambda) represents the cosmological constant. In a flat universe, which has zero curvature of space, the sum of
Ω
M
and
Ω
Λ
always equals 1, because the total density (of actual matter plus the matter equivalent provided by the dark energy) exactly equals the critical density.

The observations of distant Type Ia supernovae measure the difference between
Ω
M
and
Ω
Λ
. Matter tends to slow the expansion of the universe, as gravity pulls everything toward everything else. The greater the density of matter, the more this pull will slow things down. Dark energy, however, does something quite different. Unlike pieces of matter, whose mutual attraction slows the cosmic expansion, dark energy has a strange property: it tends to make space expand, and thus accelerate the expansion. As space expands, more dark energy comes into existence, so that the expanding universe represents the ultimate free lunch. The new dark energy tends to make the cosmos expand still faster, so the free lunch grows ever larger as time goes on. The value of
Ω
Λ
is a measure of the size of the cosmological constant and gives us the magnitude of dark energy’s expansionist ways. When astronomers measured the relationship between galaxies’ distances and their recession velocities, they found the result of the contest between gravity’s pulling things together and dark energy’s pushing them apart. Their measurements implied that
Ω
Λ
–
Ω
M
=
0.46, plus or minus about 0.03. Since astronomers had already determined that
Ω
M
equals approximately 0.25, this result sets
Ω
Λ
at about 0.71. Then the sum of
Ω
Λ
and
Ω
M
rises to 0.96, near the total predicted by the inflationary model. Recent new results have sharpened these values and brought this sum even closer to 1.

Despite the agreement between the two competing groups of supernova experts, some cosmologists remained cautious. It is not every day that a scientist abandons a long-held belief, such as the conviction that the cosmological constant ought to be zero, and replaces it with a strikingly different one, such as the conclusion that dark energy fills every cubic centimeter of empty space. Almost all the skeptics who had followed the ins and outs of cosmological possibilities finally pronounced themselves convinced after they had digested new observations from a satellite designed and operated to observe the cosmic background radiation with unprecedented accuracy. That satellite, the all-important WMAP described in Chapter 3, began to make useful observations in 2002, and by early 2003 had accumulated sufficient data for cosmologists to make a map of the entire sky, seen in the microwaves that carry most of the cosmic background radiation. Although earlier observations had revealed the basic results to be derived from this map, they had observed only small portions of the sky or shown much less detail. WMAP’s whole-sky map provided the capstone to the mapping effort, and has determined, once and for all, the most important features of the cosmic background radiation.