Astronomers in the early 20th Century got the shocks of
their lives when they discovered that galaxies appeared
to be rushing away from us. They did this by taking
spectra of the galaxies, and then measuring the shift in
their spectrum due to their motion.
You're probably already familiar with this phenomenon,
called the Doppler Shift —it's the same principle
that makes a car horn change in pitch from high to
low as it approaches and passes you. The sound waves
are compressed as the car approaches you (resulting in
a higher pitch) and are stretched as it recedes (which
lowers the pitch). With light, an object approaching you
has its light waves compressed, shortening the wavelength.
This is called a blue shift ("blue" in this sense
doesn't necessarily mean the object gets bluer;
astronomers the word as a kind of
shorthand, since in visible light the shorter
wavelengths are blue). If the object is moving away, the
wavelengths are stretched, resulting in a red shift of
A similar change in the wavelength can occur if distances are
changing due to the expansion of space itself.
While the light from a distant galaxy travels through the Universe,
space is expanding, stretching the light with it, and increasing
By carefully measuring the spectrum of an
astronomical object, astronomers can tell how much the space between the
object and observer has stretched while the light was traveling through it.
Astronomer Vesto Slipher was the first to use
this technique on galaxies, and found that all the
galaxies he measured (with very few exceptions)
were redshifted. Moreover, the ones that were smaller and
fainter — and therefore, presumably, farther away —
had higher redshifts. Edwin Hubble expanded on this work,
and determined that the Universe itself was expanding. Albert
Einstein and other scientists were indeed able to explain
this phenomenon as the result of space itself expanding,
carrying galaxies along with it. This meant that in the
past, the Universe was smaller. In fact, extending this
idea backwards, the Universe was smaller, hotter
and denser in the past.
This indicated that the Universe had a beginning,
which at the time was a radical idea. There
must have been some singular event in which space and
time themselves were created, and subsequently the
Universe expanded and cooled. This idea, called the
Big Bang model, has since been confirmed by many experiments.
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The Big Bang, Extended
Over the years astronomers have had to modify the Big Bang
model. Inflation, for example — the idea that for the
tiniest fraction of a second, the Universe underwent
a super-expansion — was added to explain some problems that
cropped up in some astronomical observations.
Credit: NASA/WMAP Science Team
clusters of galaxies, as well as individual galaxies,
made it clear that much of the matter in the Universe
was not radiating light, and was invisible to our telescopes.
You can feel the gravitational effects of dark matter, but
you cannot see any light radiating from it.
Teams of astronomers
over many years have tried to identify what makes
up dark matter — dust, gas, planets, burnt-out stars,
black holes — and have come up empty. It's now
thought that dark matter is a new kind of exotic matter
that is not found in our current inventory of known particles.
At least 80%
of all matter must be of this exotic dark variety.
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But then, in 1998, astronomers got the biggest shock of
them all. Until then, it was thought that the expansion
of the Universe found by Slipher and Hubble was slowing,
since the gravity of all the combined matter in the Universe
pulled on that matter. But observations of distant
supernovae (exploding stars) led to the inevitable conclusion that
the expansion wasn't slowing down, it was speeding up!
Two teams of astronomers independently came to this conclusion.
They were reluctant at first to believe it, but over time
the evidence became overwhelming. As two team leaders said in a joint
paper, "Both samples [of supernovae] show that [supernovae]
are, on average, fainter than would be expected, even for
an empty Universe, indicating that the [expansion of the]
Universe is accelerating."
Credit: Adam Reiss, Hubblesite.org
Since the supernovae were too
faint, they must be farther away than their simple redshift
distances would indicate. After eliminating other possibilities,
the only one left was that something was causing the cosmic
expansion to step on the gas.
Astronomers called it dark energy. According to calculations,
it accounted for nearly 3/4 of the Universe's matter and energy budget!
At first (and to
some extent even now) it was met with much skepticism, as any new
hypothesis should be. But
around that same time, observations were being made
that would lend strong support to these findings.
For the first 400,000 years or so after the Big Bang,
the Universe was a hot, dense plasma, and
any beam of light couldn't get very far without being scattered
by matter. But the Universe cooled as it expanded,
and eventually became transparent. Light was able to freely
travel for long distances. The "imprint" of the conditions
in the early Universe are embedded in the light emitted at that
time, and astronomers can use that information to determine what
things were like back then.
For example, the temperature everywhere in the Universe was very
close to being constant, but
there were very slight differences, just hints at variations.
The size of these "hot
and cold spots" indicate that indeed, the Universe is filled
with some sort of invisible energy.
And, like dark matter, no one knows what it is.
Three leading theories are that
- dark energy is some sort of energy pervading space
change with time (usually called the cosmological constant),
- it is an energy field that changes in space and time
(sometimes called quintessence),
- it's some property of gravity we don't as yet understand;
an extension of Einstein's theories, such
as quantum gravity.
All three theories agree that dark energy will accelerate the expansion of the Universe,
but predict differences in how the expansion rate should change with time. Finding out which theory
is correct using astronomical observations, however, is very difficult.
Enter SNAP. It will precisely observe thousands of distant supernovae,
measuring cosmic expasion over the last 10 billion years.
It will also look for the very subtle effects of gravitational lensing —
the bending of light as it passes near foreground objects like galaxies
and galaxy clusters — to determine how or if dark energy
changes with time.
These two observations, when combined, zero in on the
"equation of state" of dark energy — the mathematical
relationship between the pressure dark energy
exerts and the amount of energy there is in the Universe.
The first two types of dark energy theories predict very different
state, and a modification of Einstein's gravity could be found by contrasting results
of the supernova and the weak gravitational lensing measurements.
SNAP's ability to distinguish between these three theories makes it
our best tool yet
to understand just what makes up the majority of the
Universe in which we live.
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