Early in the universe, the cosmos was compact and the density of
dark matter so high that it slowed down the expansion that began
with the Big Bang. At such early times, by comparison, dark energy’s
push was almost inconsequential. But as the universe grew bigger,
and the density of dark matter diminished, dark energy’s outward
push grew stronger than dark matter’s inward pull. Dark energy
began to dominate about 5 billion years ago. Ever since, the
universe has expanded at an accelerating rate.
we might find out what dark matter is,” says theorist Carlos
Frenk of the University of Durham in England.
Energy of darkness
But even if researchers soon unmask dark matter, a gloomier mystery remains. In 1998, astronomers were astonished to
find that the expansion of the universe has been speeding up.
Cosmologists call whatever is behind this accelerated expansion dark energy.
At a recent seminar at the Space Telescope Science Institute in Baltimore, Mario Livio did something perfectly ordinary. He threw his car keys up in the air. As expected, the keys
rose, slowed down and then fell, landing back in his hand.
Now, said Livio, a theorist at the institute, imagine if the
car keys kept accelerating skyward instead of returning to his
hand. “That’s how shocking dark energy is,” he exclaimed.
FACING PAGE: V. SPRINGEL ET AL./MILLENNIUM SIMULATION (BOTH); THIS PAGE: GRAPHIC: NASA, ADAPTED BY J.
KORENBLAT; GALAXIES IMAGE: NASA, ESA, J. BLAKESLEE/JOHNS HOPKINS, M. POSTMAN/STSCI, P. ROSATI/ESO
In fact, Einstein’s theory of relativity does allow gravity to
exert a cosmic push as well as the more familiar pull. According
to relativity, gravity has two sources:
the pressure exerted by a substance
as well as its mass. Ordinary pressure
contributes to gravitational attraction,
but dark energy exerts negative pressure, which pushes space apart. If the
push is strong enough, the needle on
the gravity meter swings from attraction to repulsion.
Dark energy seems to resemble
the cosmological constant, a space-filling energy represented by a term
that Einstein inserted into his equations to keep the universe balanced
bet ween expansion and collapse. The
most likely source of this constant
would be the energy associated with
the vacuum of space.
On the subatomic scale, the vacuum seethes with pairs of particles
and antiparticles popping in and out
of existence. But calculations of the
expected vacuum energy predict an
amount of dark energy 10120 times
larger than observations allow, notes
Robert Caldwell of Dartmouth College.
So despite thousands of papers written