How Is SNAP Going to Learn About Dark Energy?

Supernovae

Before and after image of a supernova, courtesy David Malin and
the Anglo Australian Observatory

Before (right) and after (left) image of Supernova 1987A

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.

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 to exploding,
courtesy Don Dixon: //ari.home.mchsi.com/supernova_research.htm

Artist conception of binary with a whte dwarf, prior
to exploding, 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.

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.

image of two light bulbs depicting standard candles

Standard candles are objects which are the same absolute brightness, and their distance can be determined by measuring their apparent brightness.

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 the redshifts 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 and after image of a supernova in a distant galaxy

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.

Weak Lensing

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
has strongly distorted the images of more distant
galaxies. The distant galaxies have been warped
into long, thin arcs due to gravitational lensing.

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.

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.