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public with their findings. The
results shook the scientific world.
Using Einstein’s field equations for
general relativity, Adam Riess had
found that the results appeared to
give the universe a negative mass.
In other words, it appeared that
a kind of antigravity force was
pushing matter apart. This source
of energy was named dark energy,
because it was a complete mystery.
In 2016, new observations were
used to calculate a more accurate,
and slightly faster, figure for the
acceleration of the universe’s
expansion. If dark energy continues
to push the universe apart (it may
not, no one really knows), it will
disperse the galaxies so that
eventually they would all be too far
away to be seen from Earth (which
itself will no longer exist). Eventually,
it may scatter the stars within the
Milky Way until the sky goes dark.
The sun and the planets in the solar
system would be pulled apart, and
finally the particles in atoms will
also be scattered, resulting in a form
of heat death dubbed the Big Rip.
Reviving Einstein’s mistake
Dark energy may indicate that the
universe is not as homogenous as
cosmologists think it is, and that
the apparent acceleration seen is
due to the fact that it is inside a
region with less matter in it than
elsewhere. It may also be showing
that Einstein’s theory of gravity
is incorrect on the largest scales. On
the other hand, dark energy might
also be explained by a mathematical
device Einstein created in 1917
called the cosmological constant.
Einstein used this as a value that
would counteract the pull of gravity
and make the universe a static,
unchanging place. However,
when Lemaître used Einstein’s
own equations to show that the
universe could only be dynamic—
expanding or contracting—
Einstein dropped the constant from
his theories, calling it a mistake.
The value of Einstein’s
cosmological constant is set to
match the energy contained in
a vacuum—in empty space. This
was assumed to be zero. However,
according to quantum theory,
even a vacuum contains “virtual”THE TRIUMPH OF TECHNOLOGY
This discovery has led us
to believe that there is some
unknown form of energy that
is ripping the universe apart.
Brian Schmidtparticles, which exist for a Planck
time (10-43 seconds, the smallest
possible amount of time) and then
disappear again. Dark energy may
match this idea—a form of energy
arising from these virtual particles,
which creates a negative pressure
that pulls space apart, and
represents a nonzero value for
the cosmological constant.
The expansion was not always
accelerating. There was a time
when gravity and other forces
pulled matter together and was
more powerful than dark energy.
However, once the universe
became big and empty enough,
the effects of dark energy appear
to have become dominant. It may
be that a different force takes over
in the future, or dark energy’s
effects may continue to grow. One
suggestion is that a Big Rip would
be so powerful that dark energy will
tear apart spacetime itself, creating
a singularity—the next Big Bang. ■If the average
density of the
universe is above
a certain critical
value, it should
be closed, and
end with a Big
Crunch. The
critical value is
estimated to be
the equivalent of
five protons per
cubic meter.If the density
is equal to the
critical density,
the universe’s
geometry will
be flat, and the
universe ought to
continue into the
future, neither
expanding nor
contracting.If the density
is below the
critical value, the
universe should
be open and
expand forever,
to end eventually
in a heat death.Observations
suggest that
the universe’s
expansion is
accelerating due
to mysterious
“dark energy.”
The measured
density is very
close to the
critical density,
but dark energy
is accelerating
expansion.Four possible futures