Starts With A Bang! » big bang Ethan Siegel's blog/video blog about Cosmology, the Universe, and everything else Sat, 04 Apr 2009 20:12:38 +0000 en Timeline of Natural History - Part 1 Wed, 04 Jun 2008 16:49:29 +0000 ethan Sometimes people ask me what I do, and if I’m being completely honest, I’ll tell them I’m a cosmologist. When they ask for more details (because it isn’t hair, nails, and makeup), I tell them that I study the Universe, and try to figure out and understand the story of how we came to live in the world we live in here and now. Then they either smirk and ask, “Is that all,” or tell me that we already know this, and they saw it on PBS.

But I think you might enjoy hearing the most up-to-date version of the story that we have. So, dear readers, I present to you the most accurate timeline ever composed of what has happened in the Universe to bring us to the present day, and telling you the age of the Universe at that time. I’ll do this in three parts, and so today I bring you part 1, from inflation to the first stars:

  1. 10-35 seconds: The Universe expands exponentially fast, stretching space to make it flat and giving it the same properties everywhere. We call this inflation.

  2. 10-30 seconds: Inflation ends, and all the energy that was stored in space, causing the exponential expansion now becomes an incredibly dense bath of hot particles of matter, anti-matter, and radiation. The birth of all the energy in what we know as our Universe is what we call the Big Bang.

  3. Somewhere between 10-30 and 10-10 seconds: natural processes create slightly more matter than anti-matter in the Universe. Even though there is only one extra matter particle for every one billion pairs of matter-antimatter particles, that’s enough to explain all the matter in the Universe today. We call this process baryogenesis.

  4. 3 minutes: After all the antimatter annihilates away with the matter, there’s a little bit of matter left over in a sea of radiation. At three minutes, the Universe cools enough so that protons and neutrons can fuse together to form heavier nuclei without being blasted apart. This is where nearly all the hydrogen, deuterium, helium, and lithium in the Universe is created. (Most of it is hydrogen, about 75%, and most of the rest is helium, about 25%. The rest is less than 0.01%.) We call this part Big Bang Nucleosynthesis.

  5. 380,000 years: The first neutral atoms form. Up until this point, all of the radiation energy in the Universe has been too cold to blast the nuclei of atoms apart, but that energy has also been too hot to allow neutral atoms to form. It takes almost 400,000 years for the Universe to expand and cool enough for the leftover radiation from the Big Bang to chill out. Finally, at this point, electrons and nuclei can meet to form neutral atoms. When this happens, that leftover radiation simply flies off in all directions; this is what we see as the Cosmic Microwave Background.

  6. 50 Million Years: The first stars in the Universe begin to form. It takes about 50 million years for gravity to collapse matter into volumes that are dense enough and massive enough to ignite nuclear fusion. These first stars are huge, hundreds or even thousands of times as massive as our Sun, and are responsible for creating many of the heavy elements and metals that our Universe has today. These stars will all die as supernovae or even hypernovae, and will blast their remnants all over the Universe.

Come back tomorrow for Part 2, where we’ll walk through the formation of galaxies to the creation of Earth and our Solar System!

Astronomers make use of… molecules? Wed, 14 May 2008 17:42:44 +0000 ethan When I think of molecules, I think of Conan O’Brien doing his skit where he plays Moleculo…

the molecular man! I don’t think of astronomy, and I certainly don’t think of the leftover radiation from the big bang (known as the cosmic microwave background)! But somebody over at the European Southern Observatory put these two together and made an incredibly tasty science sandwich.

See, we can measure the cosmic microwave background today, because we have photons (particles of light) coming at us in all directions at all locations, with a temperature of 2.725 Kelvin. Theoretical cosmology tells us that when the Universe was younger, it was also smaller. Because the expansion of space stretches the photons in it, causing them to lose energy, it means that photons were hotter when the Universe was younger.

But we’ve never been able to measure that, of course. After all, how can you measure the temperature of something in a place where you aren’t? (Hint: read the title of this post.) Use molecules as thermometers! Using a carbon monoxide molecule (CO to you chemists) in a distant galaxy, they were able to measure the temperature of the microwave background when the Universe was only about 3 billion years old! The temperature they measured was 9.15 +/- 0.70 Kelvins; and this compares pretty well with the predicted temperature of 9.315 Kelvin. Not bad! Here’s an incomprehensible graph for you to look at while you take it all in:

What’s nice about this is that, even though it’s just what we expected, it rules out or constraints a lot of crazy alternatives (such as theories where the fundamental constants vary), because the temperature of the microwave background evolves according to standard theoretical predictions. Here’s a link to the actual scientific paper, if you’re into that sort of thing.

By the way, while I’ve got you thinking about astronomy, NASA just announced that their X-ray satellite, Chandra, found a supernova in our own galaxy that went off in the 1800’s, making it the most recent supernova ever to occur in our galaxy! Why’d it take so long to find? Because the whole damned galaxy was in the way: the explosion happened on the opposite side!

How “Quantum” is the Big Bang? Tue, 13 May 2008 18:36:53 +0000 ethan There is a very techincal paper this morning by Martin Bojowald that asks the question, How Quantum Is The Big Bang? Let me break it down for you.

If you took a look at empty space and zoomed in on it, looking at spaces so small that they made a proton look like a basketball, you’d find that space wasn’t so empty after all, but was filled with stuff like this:

What are these? They’re little pairs of matter particles and anti-matter particles. They spontaneously get created, live for a brief fraction of a second, and then run into each other and disappear. That’s what happens on very small scales, in the quantum world. (This is known as the Heisenberg Uncertainty Principle, and it actually happens!)

Well, the Universe today is huge. But it wasn’t always; back when the Big Bang was happening, all the matter and energy in the Universe was concentrated into a volume so small that these quantum effects were important!

So now, we can ask the question: how important were these quantum effects at the time of the Big Bang? (FYI: this is talking about what happens at a singularity, so this is even before inflation!) And what he basically found is that at these super-high densities, you start to run into something very interesting. Remember the Pauli Exclusion Principle? It says that no two fermions (e.g., protons, neutrons, or electrons) can occupy the same quantum state. You put all the matter in the Universe into a small enough volume, and you wind up “squeezing” everything together!

And what he found, as best as I understand it, is that the quantum state of whatever’s in the Universe determines what type of Big Bang you get! Is it the same in all directions? Well, that depends on what the quantum state of the Universe is. Will it start expanding, contracting, or oscillating? Again, depends on the quantum state. We don’t know what that state is, especially in the context of inflation (which might wipe out all of that information), but this is what they’re trying to figure out! No definitive answers yet, but at least the quantum gravity people have gotten to the point where they can start to ask this question!

How to Destroy the Entire Universe Wed, 23 Apr 2008 15:49:52 +0000 ethan

Since the dawn of time man has yearned to destroy the sun. - C. M. Burns

There’s no need to stop at the Sun, though. Since yesterday was Earth day, I thought it was only appropriate to spend today telling you how not only to destroy the Earth, but to effectively destroy the entire Universe. To tell you this story, we have to go all the way back to the beginning, to just before the big bang.

The big bang was when the Universe was hot, dense, full of energy, and expanding very quickly. The Universe was also spatially flat and the same temperature everywhere, and full of both matter and antimatter. It may have looked something like this:

(Image credit: Stephen Van Vuuren, created from a simulation of 80,000 star images.) The thing is, we need something to make the Universe this way; we need something to set up the big bang. What makes the Universe flat? What forces the Universe to be the same temperature everywhere? What creates the fluctuations that allow stars, galaxies, and clusters to form from gravitational collapse? What pushes all the weird stuff that might have existed before the big bang away?

The best theory is cosmic inflation, or a theory that says that the Universe went through a period where space expanded exponentially fast. That expansion pushes everything that existed before away, removing it from what we know as our Universe. It takes whatever shape space is and stretches it flat. It takes a small, uniform area and stretches it, giving every point in our Universe the same temperature. And it takes tiny, quantum-scale fluctuations and stretches them across the Universe, creating those fluctuations that allow the formation of stars, galaxies, and clusters. It even gave the correct predictions for the amplitude and spectrum of those fluctuations, more than a decade before we were able to measure them!

So to destroy the Universe, all we have to do is make one tiny point near us expand exponentially fast again, even just for a tiny fraction of a second (~ 10-30 seconds), and that will remove everything we know of entirely, creating a new Universe in its wake. Kind of like a Phoenix (left).

Some of you may object. You may say that it’s wrong to do this; that this would be playing God. Look, people, if you want to destroy the Universe, there are some things you’re just going to have to suck up.

So how do we do this? In some sense, it’s as simple as pushing a ball up a hill; you just need enough energy. All we have to do is make the particle that causes inflation, called an inflaton, with enough energy to make the Universe inflate again. For instance, if we made an inflaton at the low energies we’re used to in big accelerators (you know, like 1012 Electron-Volts), we couldn’t get it out of the bottom of the valley that it’s stuck in, like this:

In fact, supermassive black holes produce cosmic rays that are about 1020 Electron-Volts, so we know we need more energy than that. But if we can get up to about 1026 Electron-Volts, we’re sure to do it. “Do it” means push that inflaton up the hill; push it up high enough, and you get inflation! And that’s how you destroy the Universe!

All you need is a bigger particle accelerator or stronger magnetic fields. We can get up to 1012 eV with a ring with 4 Tesla magnets and about a 1 km radius. So we’d really need a ring with a 1014 km radius (and the same magnetic field) to do this, or an accelerator ring about the radius of our distance to the nearest star. So support your particle physics research and the development of stronger magnetic fields, otherwise we’ll be doomed to celebrate many more Earth days!

Interesting note: Some of you were upset by my post that said string theory is untestable in principle. All you have to do is build a powerful enough accelerator. Guess what, the energy it takes to destroy the Universe like this is less than the energy it takes to test string theory (which is 1028 Electron-Volts). Have a nice day!

Genesis Teaser Trailer is Up! Fri, 18 Apr 2008 09:05:12 +0000 ethan What is the future of this website? I’m going to be creating videos for the web about the Universe. I’ll be answering questions ranging from what the Universe is like today to how it got to be that way. I’m going to address every step that we know of, from the Big Bang up to the present day.

And I’m going to do it naturally, by telling the story as the Universe tells it directly to us. I call this project Genesis. Check out the teaser trailer below, and tell your friends, because this is coming in January.

April Foo — oh, you were serious? Tue, 01 Apr 2008 21:02:56 +0000 ethan There was a paper posted today that I thought was an April Fools’ Day joke, entitled Was There a Big Bang? Ha ha, thought I, of course there was; I just wrote all about it two weeks ago. But no, this is a legitimate paper, or at least is attempting to be. (There was even a brief write-up on Universe Today.)

First off, let me tell you who the authors are: two retired scientists who used to work on space missions back in the 1970’s. They were instrument guys, taking data and making sure the spacecraft ran. But as they got closer to retirement, they started proposing explanations for cosmology that didn’t fit with the data. In 2001, they wrote a mess of a book called Dark Matter Illuminated, where they argued that dark matter was really just normal baryons in diffuse clumps of just a few atoms, called cosmoids (for cosmic meteoroids) make up all of the dark matter.

Now that’s wrong, because baryons don’t make extended halos; because they interact with one another and with photons, they collapse to form denser objects. But they go further anyway. They say that these cosmoids (which are made of hydrogen and helium, by the way) both cause the apparent redshift of far away objects by reddening the light from them little by little (in conflict with experiment, by the way), and cause the apparent “microwave background” by surrounding our solar system and radiating at 2.7 Kelvin.

First off, they fail to mention the third pillar of big bang cosmology; the abundance of hydrogen and helium. They also don’t mention why there is a decrease in the “microwave background” energy at radio frequencies, which would be impossible in their model. You’d need something powerful to absorb radio waves, and if you had that, you wouldn’t be able to see quasars! Why not? Because quasars come from QSRS, or Quasi-Stellar Radio Source. If you have something damping radio signals in our solar system, how are you going to see things billions of light years away in the radio?

The answer to all of this is that their theory conflicts with experiment. What’s worse is that this isn’t even a new idea, Fred Hoyle had the idea that the microwave background wasn’t cosmic but was local back in the 1970s. This was shown to be wrong by Jim Peebles back in 1991. The theory has stayed the same and the data has gotten better; the net result is that the idea is still crazy. So it’s kind of like an April Fools’ Day gag, except it’s laughing at someone, not with them. Yeesh.

And I still think it’s a good thing for people to be thinking about, but you have to listen to all of the relevant evidence, not just the parts that agree with your idea!

The Cosmic Background Radiation Thu, 20 Mar 2008 09:05:41 +0000 ethan This was the third and final piece of the puzzle that led to the acceptance of the Big Bang and the rejection of all alternatives: the discovery of the background radiation left over from the Big Bang.

The “leading theory” before this was discovered was the Steady-State theory. Sure, they knew of Hubble Expansion. But they hemmed and hawed and said, “well, the Universe is expanding, but there must be something that happens that keeps creating new matter for free, and that’s why the Universe can expand in a steady-state theory.” So, they made one thing up to explain that (which violates the conservation of energy, by the way). Then there were the light elements. They said, “Oh, well, since we make most of the heavy elements in stars, we can make the light ones, too, probably.” This was a young field, so they didn’t know that no, they couldn’t make that much helium that quickly.

But the Big Bang predicted leftover radiation. After all, if it was hot and dense, and all the Universe did was expand, then that radiation doesn’t just disappear, it sticks around, but evolves with the Universe, and becomes cold and diffuse. But they didn’t see it. Know why? It’s relatively faint, and you need a big antenna to see it. But a couple of guys from Bell Labs had a big antenna, and they were pointing it at different points in the sky. And they found there was this noise that you couldn’t get rid of, at about 3 Kelvin in Temperature. What the heck was it?

Well, their first guess was there was something dirty in the antenna. So they went up there, and it turned out it was filled with pigeon droppings.

So they cleaned it all out, and then looked again. Well, there was still all this noise! Everywhere in the sky! So they called up the smart guys at Princeton, who told them, “Oh-my-God you found the leftover radiation from the big bang!” So they got a Nobel Prize, and the guys from Princeton didn’t!

By chance, I met Arno Penzias (one of the two guys who found the leftover radiation) in Italy back in 2004, who told me most of this story. When they discovered this leftover radiation more than 40 years ago, people very quickly realized that the Big Bang was the only theory that explained this, the light elements, and the Hubble Expansion. That remains true to the present day. But one of the saddest stories is that of Fred Hoyle, who just couldn’t believe that his beautiful steady-state theory was wrong, and until his death in 2003, kept claiming, “We live in a fog,” referring to the cosmic microwave background as that mysterious fog.

Lesson? Learn from the evidence around you. And now you know all three reasons why the Big Bang is the only theory we have that explains why the Universe is the way it is!

The Abundance of Light Elements Wed, 19 Mar 2008 09:05:28 +0000 ethan This is the second key prediction of the Big Bang: the Universe was, before any stars formed, made up of about 75% Hydrogen and 25% Helium, and much less than 1% of all other elements combined. How does the Big Bang predict this, and how to we observe it?

Well, remember we said the Universe was hot and dense in the past. At some point, it was so hot that neutral atoms couldn’t form, because high energy photons would just come in and kick electrons off of nuclei. Like this:

Well, you know what? At some point, even before that, the Universe was hotter and denser, and you couldn’t even form stable nuclei, because things would bounce around with so much energy that they would dissociate even the smallest bound nuclei, like deuterium and helium. Well, when the Universe is hot enough, there are equal numbers of protons and neutrons. Then it cools down, and some of the neutrons decay radioactively, because neutrons are unstable. (That’s right, and that’s crazy. You take a typical proton, and it lives for at least 1034 years! But you take a typical neutron, and it’s gone within 15 minutes.)

So you work through the math, and when the Universe finally cools down enough that you can form stable deuterium (one proton + one neutron = one deuteron), there’s only about one neutron for every seven protons (the exact number depends on how many photons there are). Then all the neutrons start to fuse together, and go through reactions like this:

Well, let’s do the math. I need two neutrons and two protons to make one helium nucleus, everything else becomes hydrogen. So I have two neutrons for every fourteen protons, or I’ll wind up with one helium atom for every twelve hydrogen atoms.

Wait. Didn’t I say 75% hydrogen and 25% helium? Yes, yes I did. But 1:12 is not the same as 25:75! Here’s the deal: astronomers don’t count by numbers: they count by mass. (I was livid when I first found out about this. No joke.) Well, one helium atom is four times as massive as a hydrogen atom, so the ratio of mass is 4:12, which is 25% to 75%! In all models of the big bang, this critical temperature, where neutral atoms form, is somewhere between three and four minutes into the Universe. Wait a little longer, and you’d have too little Helium. Do it a little faster, and you’d have too much. But the amount we have matches exactly what we predict.

People used to argue that it could’ve all started out as hydrogen, and you can make as much helium as you want in stars. That part is true. But you also make a lot of heavier elements, like Carbon, Oxygen, and Nitrogen, and sometimes Silicon, Neon, and sometimes even Iron, Cobalt, and Nickel. (And when you get a supernova, you can make anything all the way up to Uranium!) When we look at primordial gas clouds, we see less than 1% of everything besides hydrogen and helium, so that is a real tough challenge for theories that aren’t the Big Bang; I haven’t found one that works satisfactorily. Come back tomorrow for the last piece of the puzzle: the Cosmic Microwave Background!

The Hubble Expansion of the Universe Tue, 18 Mar 2008 09:05:39 +0000 ethan This is the first of the key predictions of the Big Bang theory, that everything in the Universe will expand according to Hubble’s Law, or that the speed that other galaxies recede from us is proportional to their distance from us. Let’s jump into the details of why the Big Bang predicts it, and how we know it to be true.

We know that a static Universe is crazy. Sorry Einstein, I know you liked it, but it’s nuts. Why? Because gravity is unstable. Mass attracts more mass. Imagine setting up a perfect, evenly spaced, infinite grid of points, all with the same mass:

Well, according to the laws of gravity, this is stable. But if I move just one point, even slightly, the whole thing goes chaotic and collapses. On a much bigger grid, this winds up looking like this, due to gravity alone:

Not static. So a static Universe is unstable, which means either the Universe has to be expanding or collapsing. Well, we observe Hubble’s Law, which means we see things that are farther away moving at faster velocities, so we know the Universe is expanding. How did it get to be that way? Or maybe a better question is, if things are moving away from one another now, where were they in the past? What about billions of years ago?

Take anything that you see expanding today, and you know it had to be denser and hotter in the past. Extrapolate it all the way back to the beginning, and what do you get? The Big Bang! And that’s where the idea comes from. Sure, other things are consistent with an expanding Universe, but are they also consistent with the light elements and the background radiation? Come back over the next two days and I’ll tell you. (Okay, the answer is no. But come back anyway.)

Did It All Start With A Bang? Mon, 17 Mar 2008 09:05:40 +0000 ethan Here’s a really fundamental question, and yet one that I think that most people don’t know the answer to:

How do we know that the Big Bang is the right theory of the origin of the Universe?

There are a bunch of alternative theories out there, after all, like Plasma Cosmology, the Steady-State Theory, and Godel’s Universe. But the Big Bang Theory explains three things that none of the other model’s I’ve seen do, and they are these three:

  1. The Hubble Expansion of the Universe. Things that are close to us move away from us with a certain velocity, things that are twice as far move away twice as fast, 10x as far moves away 10x as fast… etc. The Big Bang gives space its initial expansion rate, and predicts that gravity slows it down at the observed rate.

  2. The Abundance of Light Elements. Why is the Universe 75% Hydrogen and 25% Helium, and much less than 1% everything else? After all, stars typically produce many heavy elements. The Big Bang tells us that The Early Universe, hot and dense, will have a bunch of free neutrons, which can fuse with the protons into Helium nuclei, but very little else. What we observe is an incredible match of theory and observations.
  3. The Cosmic Background Radiation. If the Universe started off hot and dense, all that energy, all that light, all those photons that were flying around, should still be flying around today. It should all be about the same temperature by today, and that temperature should be around 3 Kelvin. This one really sets the Big Bang apart from other theories; Fred Hoyle said, upon its discovery, “We live in a fog.” But it isn’t a fog at all, it’s a leftover echo from the Big Bang, and it’s really hard to predict without one! You can read about the initial discovery here.

And that’s the story. And no other theory predicts all of those three things; it’s that simple. Come back later this week for follow-ups on each of those three!