Starts With A Bang! » Dark Energy Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en Supernovae and Dark Energy: Part II Wed, 28 Jan 2009 20:56:19 +0000 ethan 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 Mon, 26 Jan 2009 19:24:33 +0000 ethan 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? Fri, 26 Sep 2008 09:05:58 +0000 ethan 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? Mon, 28 Jul 2008 20:06:38 +0000 ethan 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 Mon, 21 Jul 2008 23:36:37 +0000 ethan 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 ρ1/2, 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.

Is Dark Energy the same as Acceleration? Thu, 10 Apr 2008 16:52:07 +0000 ethan In a comment on my last post, What is Dark Energy, Kendall asks the following, which is such a good one I think it deserves its own post:

I thought the expansion was accelerating? Aren’t you saying that it is on its way down to 85% of its current rate? Sounds like expansion is slowing, but still leaves us with an open universe…

People do say the expansion of the Universe is accelerating. But that doesn’t mean that the expansion rate is accelerating. It means that if you take a look at any one galaxy that isn’t gravitationally bound to us in the Local Group (that is, any big galaxy that isn’t named Andromeda), it’s going to recede from us at a faster and faster speed. Since the expansion rate doesn’t drop to zero, but rather (according to the best measurements today) will drop to 60 km/s/Mpc, then if a galaxy is 10 Mpc away from us today, it’s moving away from us at 600 km/s. Later on, when that galaxy is 20 Mpc from us, it moves away from us at 1200 km/s. When it’s 100 Mpc away from us, it moves away at 6000 km/s. And when that galaxy finally gets to be 5000 Mpc away from us, it will move away from us at 300,000 km/s, or at the speed of light!

Is that even possible? Yes, but this one is going to be difficult to explain. Because it’s not that “the galaxy is moving faster than the speed of light,” but the space between galaxies is expanding, and since that isn’t made of any matter or energy, it isn’t subject to special relativity: there is no speed limit on the expansion rate of spacetime. So while it appears that this breaks the laws of special relativity, it’s perfect consistent in the framework of general relativity.

In conclusion, what this means is that if you look at the galaxy and measure “what is its motion relative to us,” you will find that it appears to accelerate and eventually move away faster than the speed of light. But if you put it in the context of an expanding Universe with General Relativity and Dark Energy, you get a Universe with a constant expansion rate where galaxies remain roughly stationary relative to space, and the space between them expands at this constant rate forever. Are you confused? Well, if you’re not, you’re doing better than most astrophysicists!

Got a hankering for more Space stuff than my website can provide you with? Check out this week’s Carnival of Space over at Will Gater’s website, where they highlight my post on whether all stars eventually explode. (Quick answer: No! Some just go “poof!”)

What is Dark Energy? Wed, 09 Apr 2008 20:49:53 +0000 ethan You’ve all heard these words before. Dark Energy. But what is it, and why are we stuck with it? Let me start by telling you a story.

Imagine, for a minute, that you have a candle. You know everything about this candle, including how bright it is and how far away it is from you. Like so:

Now if I move this candle twice as far away, I know it’s going to be one-fourth as luminous. If I move it three times as far away, I know it’s going to appear one-ninth as luminous. And if I move it a thousand times farther away, I know what I see is going to be one-millionth as luminous as the original candle.

Now in space, of course, we don’t have candles. But we do have a special type of event that has, as far as we can tell, the same intrinsic brightness (to within a few percent) everywhere in the Universe. And this special event is known as a type Ia supernova. When our Sun, and for that matter most stars that we know of, burns up all of its fuel, it will eventually become a white dwarf star. Our Sun will be made out of mostly carbon and oxygen, but white dwarfs can also contain helium, neon, and silicon. Here’s an image of one:

Now in our Solar System, there’s only one star. But many star systems have two or more stars. If one of those stars is a white dwarf, it can start to steal mass from one of the other stars. When this happens, it starts to grow in mass. Now, there’s a critical limit to how much mass a white dwarf can hold up before the very atoms themselves collapse. And when the atoms do collapse, that causes an explosion so violent it’s known as a type Ia supernova. Take a look at this movie, simulating one, and notice at the end how the other star gets kicked out of the star system by the violence of the explosion:

Well, when we see these supernova in different galaxies, we can measure their brightness, and we know their intrinsic brightness, so we can figure out how far away they are. We can also measure their redshifts. We can use that information to figure out how the Universe is expanding. Now, you can probably imagine three different possibilities for what the Universe can do, starting with the big bang. You start of with a bunch of matter and energy expanding away from one another, but gravity is trying to pull it back in on itself. Here’s what can happen:

  1. There’s so much matter and energy, and hence so much gravitational force, that gravity wins, and can eventually turn the expansion around, causing the Universe to recollapse on itself. (I.e., a closed Universe.)
  2. There isn’t enough matter and energy to overcome the expansion, and the Universe keeps on expanding forever. (I.e., an open Universe.)
  3. There is just enough matter and energy to counteract the expansion but not enough to turn it around, and so the Universe asymptotes to some state where the expansion rate drops to zero, but never recollapses. (I.e., a flat Universe.)

So now we look at the supernova, and see what they tell us the Universe is doing. Guess what? It isn’t doing any of those three things! It looks like it was doing the flat Universe thing for a while, but then all of a sudden the expansion rate stopped dropping, and now will not only never drop to zero but will become a constant at about 85% of its present value! Why is that? Well, to be honest, we have no idea. But there has to be some new physics going on to make this possible, and we give it the name “dark energy,” since if the Universe were full of a new type of energy that had a repulsive pressure, it would cause the expansion rate to speed up again. But it’s weird, it’s definitely happening, and we don’t know what the right explanation for it is. And that’s dark energy!

Say Goodbye to Virgo… Thu, 13 Mar 2008 18:11:23 +0000 ethan Aaah, the Virgo cluster. A huge cluster of hundreds of galaxies, and our closest large neighbor in the Universe. People have known for a long time that although Virgo is still redshifting away from us, it isn’t quite as fast as we would expect from the Hubble expansion rate of the Universe. Does this mean that we’re gravitationally bound to it, and some day, we’ll move into this dee-luxe apartment in the sky?

Nope. Dark energy is here to push it away from us, and we’ll unfortunately see this bright neighbor recede farther and farther from us, until it disappears from our sight. So say your farewells to Virgo, because in 100 billion years, it’ll be gone forever.

And while you’re around, check out this week’s Carnival of Space over at Observations from Missy’s Window. Lots of good stuff, as always!

The Universe is Accelerating? Wed, 12 Mar 2008 16:44:02 +0000 ethan Sure, there’s dark energy, but what does that really mean? First off, there’s the bizarre phenomenon we see: very distant objects appear dimmer than we expect in a Universe filled with just matter and space. This supernova (at right) should appear much brighter for how distant it is, based on what we know about supernova. This means one of three things are going on:

  1. Supernova were intrinsically different when they were younger, and inherently fainter.
  2. Some type of dust is blocking the light from distant supernova, making them seem fainter.
  3. These supernova are actually farther away than we had thought, meaning that the Universe is expanding at a different rate than a Universe merely filled with matter.

Well, supernova by themselves aren’t enough to necessarily figure this out, but we can add in lots of other data: the Baryon Acoustic Oscillation (right) data for one , or the Cosmic Microwave Background data, which both support the case of #3, that the expansion rate is different than we thought.

Furthermore, when we try to make models of the dust, it doesn’t work properly. For one thing, dust blocks light differently at different wavelengths, but the supernova light dims the same across all wavelengths. (The science-y term is that “dust” isn’t “grey”. Somehow, that sounds fancy?) So the last hope for not having dark energy, or needing to change the expansion rate of the Universe, is that supernova could be evolving. Well, the plot below indicates that might be possible in theory, although the scientists who work on supernova can’t make them less bright in the past:

When you look at this plot, the stringent constraints come from the points on the left, where the errors are small. You can see that the only examples that work are the solid and dashed pink lines and the dashed blue line. (Even the green line is off by too much for the data points on the left half of the graph.) So while evolving supernova are a possibility, the combined evidence from all sources is overwhelmingly in support of a Universe with dark energy.

But what this means is that the Hubble constant (which is 71 km/s/Mpc today), which would normally asymptote to zero as all the matter in the Universe expands away, actually asymptotes to about 60 km/s/Mpc. Now this is important for things which are 10s, 100s, or 1000s of Megaparsecs away from us, but for local things, like say, the Sun from the Earth, this causes an “extra” expansion of about one micron per second. In other words, this has virtually no effect on the local Universe, including our entire galaxy, even including Andromeda. But for everything farther away than that, the Universe will just keep on expanding and expanding, without slowing down. This is an acceleration relative to a Universe with no dark energy, but the expansion rate isn’t speeding up; it just isn’t slowing down.

WMAP results: Cosmology from the CMB Thu, 06 Mar 2008 22:37:19 +0000 ethan The cosmic microwave background is the radiation left over from the big bang. It’s very uniform, 2.725 Kelvin everywhere. We’re moving with respect to it, so there’s a doppler shift, and we see that as a dipole moment in the Temperature. When we subtract that out, we see variations on the order of 30 microKelvins! WMAP is a satellite (Wilkinson Microwave Anisotropy Probe) that measured these anisotropies, and they just released its year 5 data. First off, with the uniform and dipole parts subtracted out, and with the foreground from the galaxy also taken out, here’s the map of the microwave sky (a baby picture of the Universe, when it was only 380,149 years old), compared with what we used to know from the previous satellite, COBE:

As you can see, the WMAP data is far better. COBE’s angular resolution was about 7 degrees; by comparison, WMAP’s resolution is less than half of one degree. We can learn a lot about the Universe from this baby picture. Let me tell you, though, why some parts are slightly hotter and why some parts are slightly colder.

The Universe has an average density, and always did. Some places are slightly more dense; they have more energy. Other places are less dense; they have less energy. Since mass=energy, imagine it like this: to escape from the planet Earth, I need a certain amount of energy. If I move with a velocity of 11 kilometers per second, that will do it. But what if the Earth were more dense? Well, I’d need to expend more energy to escape. If it were less dense, I’d need less energy to escape. Earth is the densest planet; on the Moon, for example, escape velocity is only 2.4 kilometers per second.

Now, imagine that instead of a person, you’re a beam of light at the surface of our planet. You’re going to escape because you’re moving at the speed of light, but you have to lose energy to get out of that gravitational field.

If you’re on a denser planet, I lose more energy, and so my light gets colder than normal. If I’m on a less dense planet, I lose less energy, and the light is warmer than light that left a normal planet. Well, the light from the Cosmic Microwave Background is doing the same exact thing, except it’s leaving different regions of space instead of planets. Those blue spots are where there are overdense regions and the light we see is colder, and the red spots are underdense regions, hence that light appears warmer. And that’s why we see those patterns in the sky that we do.

But that’s not the end of the story. There’s a lot of information that we learn from looking at those patterns caused by different density regions. The whole bunch of details that we’ve learned (caution that it may get technical to those who read further) are below:

  • First off, Lambda-CDM (that the Universe is made up of a cosmological constant type of Dark Energy, Dark Matter, Baryons, and leftover radiation and neutrinos) works very well. They try lots of different models, including allowing curvature and allowing dark energy to have a different equation of state. Lambda-CDM always works, no matter what they try.
  • What are the stats of the Universe today?
    1. The present expansion rate (i.e., the Hubble constant) is 68.7 km/s/Mpc, with an error of only 3%.
    2. The age of the Universe today is 13.95 billion years, with an error of 2.4%. Comparatively, the age of the Universe when the CMB was emitted was 380,000 years, with an error of 1.5%.
    3. The composition of the Universe is 72.4% dark energy (with an uncertainty of 1.5%), 23.3% cold dark matter (with an uncertainty of 1.5%), and 4.8% normal matter (with an uncertainty of 0.3%).
    4. The size (i.e., radius) of the Universe to the CMB surface today is 14.3 Gigaparsecs, or 44.2 billion light years. The uncertainty on that number is only 1.3%.
  • The density of matter is 1/Volume, the density of a cosmological constant is constant, and what they allow is dark energy to scale as (1/volume)p, where p can be any power. They find that, -0.11 < p < 0.14, which is in pretty tight agreement with p=0, or a cosmological constant.
  • Furthermore, if there is energy in spatial curvature of the Universe, it’s less than 1.75% of the total energy. You can find more results here.
  • Also, while you’re here, check out this week’s Carnival of Space over at Bad Astronomy; you can find my post on why Mars is so dry along with a bunch of other neat stuff!