How Is SNAP Going to Learn About Dark Energy?
Before (right) and after (left) image of
When most people think of the word "supernova",
they imagine a giant star exploding. But there are two
kinds of supernovae! One is the explosion of a giant
star when it runs out of fuel, but there is another kind
that involves a star more like the Sun (don't worry,
though: the Sun won't explode).
As a star like the Sun runs out of hydrogen fuel,
two things happen. One is that the outer layers of the star
expand and blow off over many thousands or millions of years.
Think of it as a "super-solar wind", eventually blowing
all the outer parts of a star into space.
At the same time, deep within the star, its core
compresses, getting denser and denser. The immense pressure
squeezes it into a ball just a few thousand miles across,
roughly the size of the Earth. In a normal star, the heat
generated in the core supports it against its own gravity.
In the compressed core, the support is due to a weird
quantum mechanical effect called "degeneracy". The electrons
strongly resist being forced together, and repel each other
quite strongly. This repulsion, distinct from the electric
repulsion between charged particles and much stronger,
is what keeps the core from collapsing any further. Since
the outer layers of the star are gone, blown away in the
stellar wind, the core is exposed to space.
It's very hot, glowing white, but it's small and dense.
We call it a white dwarf.
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Type 1a Supernovae
For the Sun, the story will end here. In a few billion
years, it will become a white dwarf after shedding its outer layers.
But if the original star was closely orbiting another star,
things could be different. This is called a binary system.
Eventually, the companion star will also run out of hydrogen,
and its outer layers will expand away. As these layers expand,
some of the material from the star can be drawn onto the
white dwarf, creating a thin layer on its surface. If
the flow is really weak, the immense heat of the dwarf
can prevent this from happening, but if the flow is very
heavy, the matter piles up quickly, gets compressed by the
huge gravity of the white dwarf, and undergoes
explosive thermonuclear fusion. Bang! The star experiences a
pretty big explosion on its surface, but it survives.
Eventually matter begins to accumulate again, and the
process repeats itself. This is called a recurrent nova.
Artist conception of binary with a whte dwarf, prior
courtesy Don Dixon
But if the matter gets drawn from the normal star at just
the right rate, it can build up on the white dwarf and
not explode. It gets distributed all over the star.
But it still piles up, more and more. until it does
explode. But this time, the explosion is different.
And if conditions are just right, this leads to
catastrophe. As the mass piles up, the star reaches a critical
point. When the white dwarf tips the scales at about
1.44 times the mass of the Sun, it undergoes a massive
fusion event. Like a nuclear bomb, the entire star explodes,
fusing in one massive paroxysm. It detonates, releasing so
much energy it can be seen clear across the Universe.
This is a supernova. Technically, it.s called a Type Ia
supernova (a massive star exploding is a Type II). The explosion
energy seen in visible light is roughly 1044 Joules, as much
energy as the Sun puts out over its entire lifetime,
all in a few seconds.
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Standard Candles in the Dark
One of the most interesting aspects of this type of supernova is that the white dwarf needs to have a very specific mass to explode — 1.44 solar masses. This means that the explosion energy of a Type Ia is always roughly the same, no matter where we see it. This makes them incredibly useful to astronomers. Why? Because if you know how bright an object should be, and measure how bright it appears, you can determine its distance. Just like a distant car has faint headlights, but up close they are blindingly bright, a Type Ia supernova gets fainter with distance in a well-determined way.
Standard candles are objects which are the same absolute brightness,
and their distance can be determined by measuring their apparent
In 1998, two teams of astronomers compared measurements of the apparent brightnesses of many Type Ia supernovae to the "standard candle"
brightness to directly determine the distances to the supernovae. They then compared these distances to a different set of measurements of the distances derived from the redshifts of the galaxies in which the supernovae occurred. The interpretation of galactic redshifts as distance indicator, however, depends critically on the model used to describe the expansion history of the Universe. The scientists were shocked to discover that the two sets of measurements did not agree!
They went through an exhaustive checklist to make sure their results weren't influenced by other factors, like dust surrounding the supernovae, or differences in chemistry of very distant stars.
But it all checked out, and they were left with an incredible conclusion: the supernovae were actually farther away than their redshift distances naively indicated.
In 1998, two teams of astronomers exploited this fact
to compare the apparent brightnesses of many Type
Ia supernovae that were very far away — billions of
light years distant. They compared this to what was expected
from the distances as measured by
and were shocked to discover that the supernovae were all
fainter than expected. They went through an exhaustive
checklist to make sure their results weren't influenced
by other factors, like dust in the supernova, or
differences in chemistry of very distant stars. But it all
checked out, and they were left with an incredible conclusion:
the supernovae were actually farther away than their
redshift distances naively indicated.
If the supernovae are farther away than expected,
then the expansion of the Universe must be accelerating!
Most astronomers assumed it would be slowing down because
the gravity of all the combined objects in the Universe
would be hitting the cosmic brakes. Instead, the
opposite was happening. This was perhaps the biggest scientific
shock of the late 20th Century.
Before (left) and after (right) images of a supernova exploding
in a distant galaxy.
Using the tools available (such as giant ground-based
telescopes and the orbiting Hubble Space Telescope)
astronomers can see Type Ia supernovae out to a distance
of about 7 billion light years — a redshift of 1.
And even then it's very hard, since you never know when
or where one will pop off. But to really see the cosmic
acceleration well, and to measure it accurately,
astronomers need to see farther out.
This is where SNAP comes in. With its superior viewing
position of deep space, excellent optics, and
wide-field detector, it can detect supernovae at redshifts
of 1.7, almost 10 billion light years distant. This is
far enough away that the signature of the acceleration will be
easy to spot. The wide field allows astronomers to watch
large numbers of distant galaxies for the tell-tale signs of an
exploding star, to distinguish between the two types
of supernovae, and to get excellent data on the redshifts of
the events. Altogether, this makes SNAP the perfect tool
to measure these titanic explosions at cosmic distances.
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One of Albert Einstein's greatest contributions to science was the idea that space-time is not just a static stage in which objects move, but a dynamic player. Einstein realized that the force of gravity is due to the interaction of matter and space-time. Space-time is bent, warped, distorted where there is matter. The more matter there is within a certain amount of space, the more the distortion. Objects travel along curved paths through this warped space-time.
In this Hubble image, a cluster of galaxies
distorted the images of more distant
galaxies. The distant
galaxies have been warped
into long, thin arcs due to
You can think of this as matter telling space-time how to bend, and space-time telling matter how to move.
Imagine a large square sheet of rubber being held at
its corners by four people. It will be flat, but
if you put a heavy object in the center, it will create
a dimple in the sheet. If you roll a marble across the
sheet, its path will curve around the heavier object.
This is very much like how matter distorts space, and
how objects behave under that influence — except space isn't
flat like a sheet, it has three dimensions. This makes
it difficult to picture, but the sheet makes a good analogy.
But since space itself is bent by matter, Einstein realized
that any light traveling through that region of bent space
will have its path changed too. It takes a pretty hefty
mass to make a measurable change in the path of a beam
of light, but nature has provided us with galaxies massive
enough to do the trick.
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Through a Lens, Weakly
Imagine a very distant galaxy, with another galaxy or
a cluster of galaxies between us and it. The light
from the more distant galaxy will get bent around the
intervening object, distorting how we see its shape.
The galaxy in the middle acts like a lens, and in
fact it's called a "gravitational lens". Many such
examples have been found in astronomical images. In some cases
the lensing is very strong, and you can see arcs and
multiple images of objects. In other cases the lensing is weaker,
and the distant object's shape is only subtly distorted.
In most cases the actual galaxy doing the lensing is
invisible, but we can infer its existence from the
distortions it causes.
Imagine this grid of colors represents distant galaxies, with no
matter between them and us...
... then, if there are galaxies between us, this picture
represents how that grid gets distorted by the gravity of those
galaxies. That's lensing.
These weak lenses play a big role in cosmology. By mapping out
the distortion of distant objects, we can build up a
map of matter in the Universe that is causing the lensing.
The amount and shape of the distortion tells astronomers
quite a bit about the properties of that matter, too,
including its distance.
Why is this important? Distant supernovae allow SNAP to measure the expansion history of the Universe, but cannot tell us whether accelerated expansion is due to dark energy (either a cosmological constant or quintessence) or a property of gravity we don’t yet understand. Weak lensing, however, is very sensitive to both the distribution of matter and the details of gravity. Distortions from this weak lensing effect provide a map of the matter in the Universe not
just across the sky, but also provide a distance as well —
and at these distances, looking across space is the
same as looking back in time. The farther away the
object is, the longer it took for its light to get here,
and thus the younger the Universe was when the light left it.
Weak lensing gives astronomers a 3D map of the matter in Universe.
This is critical! Because dark energy pulls the Universe apart,
the ability of matter to clump together to form galaxies
and clusters changes as dark energy changes. By looking at
how matter clumps now versus how it clumped, say, 5 billion
years ago, astronomers can determine if the amount of dark
energy in the Universe is constant or increases with time.
Observations of supernovae and weak lensing together can provide the
key to distinguishing between
the three models of dark energy.
While there are other observational methods that
can be used to determine these two factors, combining supernovae
and weak lensing is the best and most accurate way to do it.
This is just what SNAP is designed to do. It will take
the measure of both supernovae and weak lensing, and by
the careful combination of the two, the nature of dark
energy may be unlocked.
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