The discovery in 1998 that the Universe is actually speeding up its
expansion was a total shock to astronomers. It just seems so
counter-intuitive, so against common sense. But the evidence has
The evidence came from studying distant Type Ia supernovae. This type
of supernova results from having a white dwarf star in a binary system. Matter
transfers from the normal star to the white dwarf until the white
dwarf attains a critical mass (the Chandrasekhar limit) and undergoes
a thermonuclear explosion. Because all white dwarfs achieve the same
mass before exploding, they all achieve the same luminosity and can be
used by astronomers as "standard candles." Thus by observing their
apparent brightness, astronomers can determine their distance using
the 1/r2 law.
By knowing the distance to these supernovae, we know how long ago they
occurred. In addition, the light from the supernova has been
red-shifted by the expansion of the Universe. By measuring this
redshift from the spectrum of the supernova, astronomers can
determine how much the Universe has expanded since the explosion.
By studying many supernovae at different distances, astronomers can
piece together a history of the expansion of the Universe.
In the 1990's two teams of astronomers, the Supernova Cosmology
Project (Lawrence Berkeley National Laboratory) and the High-Z Supernova Search (international), were looking for distant
Type Ia supernovae in order to measure the expansion rate of the
Universe with time. They expected that the expansion would be
slowing, which would be indicated by the supernovae being brighter
than their redshifts would indicate. Instead, they found the
supernovae to be fainter than expected. Hence, the expansion of the
Universe was accelerating!
In addition, measurements of the cosmic microwave background indicate
that the Universe has a flat geometry on large scales. Because there
is not enough matter in the Universe -- either ordinary or dark matter
-- to produce this flatness, the difference must be attributed to a
"dark energy". This same dark energy causes the acceleration of the
expansion of the Universe. In addition, the effect of dark energy
seems to vary, with the expansion of the Universe slowing down and
speeding up over different times.
Astronomers know dark matter is there by its gravitational effect on
the matter that we see, and
there are ideas about the kinds of particles it must be made of.
By contrast, dark energy remains a complete mystery. The name "dark
energy" refers to the fact that some kind of "stuff" must fill the
vast reaches of mostly empty space in the Universe in order to be able
to make space accelerate in its expansion. In this sense, it is a
"field" just like an electric field or a magnetic field, both of
which are produced by electromagnetic energy. But this analogy can
only be taken so far, because we can readily observe electromagnetic energy via the particle that carries it, the photon.
Some astronomers identify dark energy with Einstein's Cosmological
Constant. Einstein introduced this constant into his general
relativity when he saw that his theory was predicting an expanding
universe, which was contrary to the evidence for a static universe
that he and other physicists had in the early 20th century. This
constant balanced the expansion and made the Universe static. With
Edwin Hubble's discovery of the expansion of the Universe, Einstein
dismissed his constant. It later became identified with what quantum
theory calls the energy of the vacuum.
In the context of dark energy, the cosmological constant is a
reservoir which stores energy. Its energy scales as the Universe
expands. Applied to the supernova data, it would distinguish effects
due to the matter in the Universe from those due to the dark energy.
Unfortunately, the amount of this stored energy required is far more
than observed and would result in very rapid acceleration -- so much so
that the stars and galaxies would not form.
Physicists have suggested a new type of matter, "quintessence," which
would fill the Universe like a fluid which has a negative
gravitational mass. However, new constraints imposed on cosmological
parameters by Hubble Space Telescope data rule out at least simple
models of quintessence.
Other possibilities being explored are topological defects, time-varying forms of dark energy, or a dark energy that does not scale
uniformly with the expansion of the Universe.