How did the Universe begin? Does time have a beginning and an
end? Does space have edges? Einstein's theory of relativity replied to these
ancient questions with three startling predictions: that the Universe is
expanding from a Big Bang; that black holes so distort space and time that time
stops at their edges; and that a dark energy could be pulling space apart,
sending galaxies forever beyond the edge of the visible Universe. Observations
confirm these remarkable predictions, the last finding only four years ago. Yet
Einstein's legacy is incomplete. His theory raises – but cannot answer – three
profound questions: What powered the Big Bang? What happens to space, time and
matter at the edge of a black hole? and, What is the mysterious dark energy
pulling the Universe apart? The Beyond Einstein program within NASA's office of
space science aims to answer these questions, employing a series of missions
linked by powerful new technologies and complementary approaches to shared
science goals. The program also serves as a potent force with which to enhance
science education and science literacy.
INFINITE BEGINNINGS Pushing the limits of theory and
imagination in true Einsteinian fashion, cosmologists are daring to speculate
that ours is not the only universe. The big bang that created everything we
know of space and time could be just one of an infinite number of beginnings,
yielding a never ending sequence of universes. The scenario, shown in this
artist’s concept, emerges from inflation theory, a descendent of Einstein’s
general theory of relativity. Relativity implies that space and time can
stretch to vast dimensions from a tiny starting point; inflation describes how
our universe ballooned in its first moments and suggests that the same thing
can happen anywhere, at any time. The result: an eternal expanse of space
erupting with bubbles of energy, or big bangs, each the seed of a universe. Not
all universes will be alike. While a cosmos like our own glows with galaxies
(at lower right) others may contain more dimensions or different forms of
matter. In some, even the laws of physics work differently (twisted universe at
upper left).
On January 29, 1931, the world’s premier physicist,
Albert Einstein, and its foremost astronomer, Edwin Hubble, settled into the
plush leather seats of a sleek Pierce-Arrow touring car for a visit to Mount
Wilson in southern California. They were chauffeured up the long, zigzagging
dirt road to the observatory complex on the summit, nearly a mile above Pasadena.
Home to the largest telescope of its day, Mount Wilson was the site of Hubble’s
astronomical triumphs. In 1924 he had used the telescope’s then colossal
100-inch mirror to confirm that our galaxy is just one of countless “island
universes” inhabiting the vastness of space. Five years later, after tracking
the movements of these spiraling disks, Hubble and his assistant, Milton
Humason, had revealed something even more astounding: The universe is swiftly
expanding, carrying the galaxies outward.
On the peak that bright day in January, the 51-year-old
Einstein delighted in the telescope’s instruments. Like a child at play, he
scrambled about the framework, to the consternation of his hosts. Nearby was
Einstein’s wife, Elsa. Told that the giant reflector was used to determine the
universe’s shape, she reportedly replied, “Well, my husband does that on the
back of an old envelope.”
That wasn’t just wifely pride. Years before Hubble detected
cosmic expansion, Einstein had fashioned a theory, general relativity, that
could explain it. In studies of the cosmos, it all goes back to Einstein.
Just about anywhere astronomers’ observations take them—from
the nearby sun to the black holes in distant galaxies—they enter Einstein’s
realm, where time is relative, mass and energy are interchangeable, and space
can stretch and warp. His footprints are deepest in cosmol-ogy, the study of
the universe’s history and fate. General relativity “describes how our universe
was born, how it expands, and what its future will be,” says Alan Dressler of
the Carnegie Observatories. Beginning, middle, and end—“all are connected to
this grand idea.”
At the turn of the 20th century, 30 years before
Einstein and Hubble’s rendezvous at Mount Wilson, physics was in turmoil.
X-rays, electrons, and radioactivity were just being discovered, and physicists
were realizing that their trusted laws of motion, dating back more than 200
years to Isaac Newton, could not explain how these strange new particles flit
through space. It took a rebel, a cocky kid who spurned rote learning and had
an unshakable faith in his own abilities, to blaze a trail through this
baffling new territory. This was not the iconic Einstein—the sockless, rumpled
character with baggy sweater and fright-wig coiffure—but a younger, more
romantic figure with alluring brown eyes and wavy hair. He was at the height of
his prowess.
Among his gifts was a powerful physical instinct, almost a
sixth sense for knowing how nature should work. Einstein thought in images,
such as one that began haunting him as a teenager: If a man could keep pace
with a beam of light, what would he see? Would he see the electromagnetic wave
frozen in place like some glacial swell? “It does not seem that something like
that can exist!” Einstein later recalled thinking.
He came to realize that since all the laws of physics remain
the same whether you’re at rest or in steady motion, the speed of light has to
be constant as well. No one can catch up with a light beam. But if the speed of
light is identical for all observers, something else has to give: absolute time
and space. Einstein concluded that the cosmos has no universal clock or common
reference frame. Space and time are “relative,” flowing differently for each of
us depending on our motion.
Einstein’s special theory of relativity, published a hundred
years ago, also revealed that energy and mass are two sides of the same coin,
forever linked in his famed equation E = mc2. (E stands for energy, m for mass,
and c for the speed of light.) “The idea is amusing and enticing,” wrote
Einstein, “but whether the Almighty is ... leading me up the garden path—that I
cannot know.” He was too modest. The idea that mass could be transformed into
pure energy later helped astronomers understand the enduring power of the sun.
It also gave birth to nuclear weapons.
But Einstein was not satisfied. Special relativity was just
that—special. It could not describe all types of motion, such as objects in the
grip of gravity, the large-scale force that shapes the universe. Ten years
later, in 1915, Einstein made up for the omission with his general theory of
relativity, which amended Newton’s laws by redefining gravity.
General relativity revealed that space and time are linked in
a flexible four-dimensional fabric that is bent and indented by matter. In this
picture, Earth orbits the sun because it is caught in the space-time hollow
carved by the sun’s mass, much as a rolling marble would circle around a
bowling ball sitting in a trampoline. The pull of gravity is just matter sliding
along the curvatures of space-time.
Einstein shot to the pinnacle of celebrity in 1919, when
British astronomers actually measured this warping. Monitoring a solar eclipse,
they saw streams of starlight bending around the darkened sun. “Lights All
Askew in the Heavens. Stars Not Where They Seemed or Were Calculated to be, but
Nobody Need Worry,” proclaimed the headline in the New York Times.
With this new insight into gravity, physicists at last were
able to make actual predictions about the universe’s behavior, turning
cosmology into a science. Einstein was the first to try. Yet as events showed,
even Einstein was a fallible genius. A misconception about the nature of the
universe led him to propose a mysterious new gravitational effect—a notion he
soon rejected. But he may have been right for the wrong reasons, and his
“mistake” may yet turn out to be one of his deepest insights.
For Newton, space was eternally at rest, merely an inert
stage on which objects moved. But with general relativity, the stage itself
became an active player. The amount of matter within the universe sculpts its
overall curvature. And space-time itself can be either expanding or
contracting.
FAST FORWARD: THE BIG RIP? The death of the universe
could rival its birth in explosive drama if a puzzling form of energy continues
to accelerate the expansion of space-time. Since the 1920s astronomers have
thought the expansion was slowing down, but recent observations of distant
stars reveal that the stretching of space is actually speeding up. If it picks
up even more, the universe could be headed for a “big rip.” An artist’s
conception of this scenario—one of many possible fates—shows how, some 20
billion years from now, unchecked expansion coule tear matter apart, from
galaxies all the way down to atoms. The driving force is a mysterious “dark
energy” that counteracts gravity’s pull and might ultimately defeat all the
forces that bind matter. Einstein was the first to introduce the notion of
repulsive gravity, but he later disavowed it. Dark energy, says cosmologist
Michael S. Turner, who coined the term, “has the destiny of the universe in its
hands.” Although we live in the best of times, under a sky full of stars, it
will grow even darker and emptier as space-time expands.
Theorists have also dusted off his discarded cosmological
constant to explain a startling new discovery, and now Einstein’s “biggest
blunder” is starting to look like one of his greatest successes. Astronomers
had assumed that gravity is gradually slowing the expansion of the universe.
But in the late 1990s two teams, measuring the distances to faraway exploding
stars, found just the opposite. Like buoy markers spreading apart on ocean
currents, these supernovae revealed that space-time is ballooning outward at an
accelerating pace.
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