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Supernovae and Dark Energy: Part II

January 28, 2009 on 1:56 pm | In Dark Energy, cosmology | 30 Comments

On Monday, we told you what the Universe is doing, and it’s expanding faster than we can explain. This mysterious expansion’s cause is unknown, and until we figure it out, the name we give to it is called dark energy. But we do know how to measure what the expansion rate is, and despite what the occasional misinterpretation says, it’s extremely simple and straightforward to do.

Let’s make a simple analogy: start with a 100 Watt light bulb.

Now, you can put that light bulb anywhere in the Universe, and you’ll know how far away it is. How? Because you know how intrinsically bright it is, and you can measure how bright it appears, you can calculate how far away it must be. It’s as simple as this:

Well, guess what? This works all over the Universe. You find something that you know its intrinsic brightness, you measure how bright it appears, and you can figure out how far away it is! Well, there’s one special type of object that is the same everywhere in the Universe: a type Ia supernova. They all work the same exact way. Let’s show you:

Start with a white dwarf star. Many stars (including our Sun) will end up like this. When all the nuclear fuel of a star is used up, the core simply collapses and the star sheds its outer layers. What we see at first is a planetary nebula:

but the hot gas of the nebula dissipates after several thousand years. The white dwarf star, at the center of many of these nebulae, remains behind. This is the fate of our Sun. However, unlike our solar system, which only has one star, many star systems have two or more stars in them. If one of those stars is a white dwarf and the other one isn’t, something very neat can happen:

The white dwarf star, if it’s close enough, because it’s so dense, can start stealing mass from the other star! It can do this for a long time, but not indefinitely. As the white dwarf gets more and more massive, the pressure in its center increases. At some point, when the white dwarf reaches a mass of about 40% more than our Sun, the pressure gets too great, and starts to destroy the atoms in the center of the star:

And the collapsing atoms release a tremendous amount of energy, resulting in a type Ia supernova explosion!

Because it’s the same exact physics every time, these have the same brightness every time, and we can use them just the way we use a 100-Watt light bulb! You measure their brightness, figure out their distance, and for the last part, you measure how quickly they’re moving away from us! From this data, you can figure out what type of Universe we live in: open, closed, flat, or accelerating. Guess which one we see?

Closed is the red line, open is the green line (which is far enough away from the data at about 2-3 Gpc that it doesn’t work), flat is the black line, and the accelerating ones are purple. Which one works best? The accelerating one, definitely. We can make more complicated models, but they all need something to explain this. And that’s how we know that there’s dark energy in the Universe. Any questions?


Supernovae and Dark Energy: Part I

January 26, 2009 on 12:24 pm | In Dark Energy, cosmology | 19 Comments

You’ve heard the magic words before: dark energy. What is it? It’s our best explanation for why the Universe is expanding the way it is. Let’s remind you of how it all works today, and then on Wednesday I’ll tell you how we measure it.

Imagine the Big Bang the same way you would imagine a grenade exploding. After a big explosion, everything moves outward, away from the center. But our Universe is different from a grenade in that grenades are little with very tiny masses, but the Universe is huge and incredibly massive, with an estimated mass of about 1023 Suns! So just like a grenade, everything begins by flying apart, but unlike a grenade, gravity is so important that it tries to pull the entire explosion back together. Can it? Let’s look at the three obvious options:

1. Gravity Wins! Even though the Universe starts off expanding incredibly rapidly, if there’s enough mass and energy, gravity will pull everything back together again, resulting in a Big Crunch. A neat idea, but we need an awful lot of matter and energy to make it happen. What if we don’t have enough?

2. Expansion Wins! If there isn’t enough mass and energy, the expanding Universe just goes on forever. Gravity tries to slow the expansion rate down and manages to do so a little bit, but the Universe keeps on expanding, with gravity unable to stop it. Galaxies move farther and farther apart, the average density of the Universe drops asymptotically to zero, and the temperature of everything begins to freeze. This is called either the Big Chill or the Heat Death of the Universe. So what’s the third possibility?

3. Goldilocks? Instead of the Universe getting too hot (Big Crunch) or too cold (Heat Death), the Universe, like Goldilocks, could get it “just right,” and neither recollapse nor expand into an abyss. We don’t have a name for this case, but I like to call it the Big Coast, where the expansion rate asymptotes to zero, but never reverses and recollapses.

So those are the three classic fates of the Universe. The Big Crunch is what we call a closed Universe (shaped like a sphere), the Heat Death gives us an open Universe (shaped like a horse saddle), and the Big Coast gives us a flat Universe (shaped like a flat sheet).

What do we see? None of these. Instead, we find that for the first few billion years, the Universe looks like it’s doing the Goldilocks case, asymptoting in its expansion, and looking like it’s going to coast forever, like a flat Universe.

And then the fun starts. The unexpected happens. Something starts noticeably pushing galaxies farther apart! The expansion rate between any two galaxies in the Universe increases, like there’s some mysterious, repulsive force between them. If there’s an extra force (remember that F=ma?), that means there’s an extra acceleration in the Universe, and so something we didn’t predict at all happens:

The Universe expands faster and faster, and eventually all the billions and billions of galaxies we know of will disappear from view, leaving only us and Andromeda. Why is this happening? Well, folks, that is the mystery of dark energy. To be continued…


A New Hint at the Expanding Universe?

September 26, 2008 on 2:05 am | In Dark Energy, cosmology | 43 Comments

Dark Energy. You’ve heard the name before. What it really is, though, is the name we give to the expanding Universe that we don’t understand.

Imagine that the big bang, the birth of the Universe as we know it, is like a giant explosion in space:

So things start off moving away from one another very rapidly. Now you can imagine three different cases. Perhaps the energy of the explosion is so great that the Universe will expand forever, that its gravity will never pull it back together. We call this an “open” Universe. Or perhaps there’s enough matter and energy in the Universe for gravity to pull everything back together into a single point at some distant time in the future. We call this a “closed” Universe. Or, maybe we live on that finely balanced line between those two extremes, and we’ll just asymptote to some expansion rate where it never recollapses, but the expansion eventually slows to zero. We call this a “flat” Universe.

Well, over the last decade, our measurements have finally gotten good enough that we can discriminate, and determine the type of Universe we live in. The verdict? It’s NONE of these! What we actually observe looks like the “flat” case for a little while, but the expansion rate stops decreasing all of a sudden, and does this:

Now, this is bizarre. But there’s news that the story gets even weirder. Apparently, when you look at hundreds of distant galaxy clusters, you find that they’re all moving, peculiarly, towards the same spot in the sky:

Now, to be fair, this was measured with the WMAP satellite, which wasn’t really designed to measure this effect, and we also don’t really understand what we’re seeing, and we don’t understand the velocity flows that we see in our own local group, much less in clusters billions of light years away. So there is a lot of room for error. Still, this is, at the very least, something that needs to be explained. Know where I’d look to for either confirmation or refutation? Europe’s Planck satellite. Launch date? February 2009.

We won’t have to wait long, just another couple of years… and I swear, that’s actually short for a science experiment! And then we should be able to measure this effect with some real precision and accuracy. And just maybe, we can figure out what the heck is up with this expanding mess that, to be honest, nobody really understands.


Could It All End With A Rip?

July 28, 2008 on 1:06 pm | In Dark Energy, cosmology | 13 Comments

Here we are, nearly 14 billion years after the big bang, and we’re still trying to figure out where we’re headed. We know that the Universe is not only expanding, but that the expansion rate isn’t dropping to zero as the matter density drops. This, first off, is weird. After all, what determines the expansion rate of the Universe? Energy density, or the amount of energy you have in a given amount of space. As space expands, you’d expect that the energy density would go down, and it did for billions of years. This was because there was more matter in the Universe than anything else. But over the past three billion years or so, the matter density has dropped so low that we’ve discovered a new form of energy in the Universe, one that doesn’t appear to dilute the way matter does. This is what we call dark energy:

Now, one of the great unsolved mysteries of dark energy (other than what the hell it actually is) is how this dark energy density changes as the Universe expands. It could stay constant, which means as the Universe expands, the expansion due to dark energy stays the same, and the Universe will continue to expand at a rate that approaches a constant, of about 60 km/s for every Megaparsec (3,086,000 light-years) in distance. The data all point towards this as what’s going on.

But we don’t know for sure. As the Universe expands, we don’t have enough data to constrain how dark energy changes over time very well. It could, very slowly, dilute. So maybe when the Universe is 10 times the size it is now, dark energy will be 10% weaker. This is very different from matter, which will be 1000 times less dense when the Universe is 10 times the size it is now, but we don’t know whether dark energy really stays constant, or whether it decreases a little bit.

But there’s another possibility that’s really interesting: what if dark energy actually gets stronger as the Universe continues to expand? The Canadian Broadcasting Company did a radio show on this topic (and thanks to reader Brian for pointing this out), and what it would mean for the Universe. If dark energy gets stronger and stronger, that means the Universe will start to expand faster and faster. Instead of 60 km/s per Megaparsec, things can start expanding at 600, or 6000, or even 6,000,000 km/s for every Megaparsec they are apart.

Isn’t that faster than the speed of light? Yup. Because space doesn’t care about speed limits, it just expands based on the amount of energy it contains. You make the expansion rate large enough, and objects that were bound to each other fly apart. It’s a lot like spinning the Earth faster and faster: you spin the Earth fast enough and it starts to fly apart, much like this CD from mythbusters.

Dark energy, despite being a real form of energy, acts like a repulsive force. You increase a repulsive force and leave the rest of your forces (like gravity, electromagnetism, and the nuclear forces) the same, and eventually you overcome them. Galaxies fly apart into individual stars; solar systems lose their planets, individual planets are broken up into atoms, and eventually atoms themselves are destroyed as electrons are ripped off of their nuclei, and protons and neutrons are ripped apart into quarks and gluons. What a horrible ending to the Universe, and yet if dark energy is of this special type, called phantom energy, this is the fate of the Universe, called the big rip.

And then what happens to all that energy? Well, we don’t know, but if this happens, we can get back to the kinds of high energies that haven’t existed since… well… that other ‘big’ thing we all know…

Isn’t the Universe full of neat possibilities?


How to Count Up the Amount of Dark Energy

July 21, 2008 on 4:36 pm | In Dark Energy, Q & A, cosmology | 4 Comments

A couple of months ago, reader Scott Stuart was thinking about Dark Energy. For a quick review, here’s what dark energy is:

The Universe expands, like an exploding grenade, starting at the big bang. But there’s a lot of stuff in the Universe, and gravity tries to pull it back together. So the expansion rate slows down. But a few billion years ago, something very bizarre and unexpected happened:

The expansion rate changed, and didn’t slow down. Objects that should have receded more and more slowly from us recede more and more quickly instead. What causes this? Well, this is what we call dark energy. And with all of this in mind, Scott asks the following question:

Why does dark energy get added in with matter when calculating the density of the universe? It seems that dark energy has very different properties from matter (normal or dark) and in one important sense has essentially the opposite effect of matter: its gravitational effect is repulsive. So why would its density add to the density of matter to make the universe flat?

Let’s answer Scott’s question by answering a simpler question: what determines how the Universe expands? The expansion rate is given by the Hubble parameter (mistakenly called the Hubble constant, as it changes over time), H. There is an equation (one of the Friedmann equations) that tells us how to determine the expansion rate:

But the only important part of this equation is this fact, that H is proportional to ŌĀ1/2, or that the Universe’s expansion rate is dependent on all of the energy density in the Universe.

So if there’s anything that has energy in it, whether it’s photons and neutrinos (which dominate for the first few thousand years), matter (which dominates for the first few billion years), or this new mysterious dark energy (which dominates today), it causes the Universe to expand.

The big difference in the different types of energy is how their densities change as the Universe expand. For normal matter, its density changes as 1/Volume; as the Universe expands, the density decreases. For radiation like photons, the density changes as (1/volume)4/3, since the wavelengths of the photons also get stretched as the Universe expands.

But for dark energy, the energy density is constant, which means it doesn’t change as the Universe expands. (For this reason, it’s sometimes called a cosmological constant.) But the important thing about dark energy is that it’s still energy, and all of the energy of all different types, when you add it up, determines the expansion rate of the Universe.

And this is why, in modern cosmology, we try to measure the history of the Universe’s expansion. If we know how the Universe expanded at many different times, we can figure out what makes it up, and how much radiation, matter, dark energy, and anything else there is (or ever was) in it! And that’s how we know that today, our Universe is about 26% matter, 74% dark energy, and about 0.01% radiation.

And for those of you who are not sufficiently entertained, here’s a sign amidst the revelry from Portland International Beerfest; there was not even the slightest bit of photo editing other than cropping the image to fit on this page:

For those of you who can’t read the sign, it says, “Alcoholic Beverages Prohibited”. Go figure.


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