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A Special Place in the Universe?

January 30, 2009 on 1:48 pm | In cosmology | 63 Comments

Sometimes people over at the Straight Dope come up with some really interesting questions. It’s one of the only forums that I read, and occasional reader Stephen pointed me towards this thread, which basically asks whether we, living on the Earth in the Milky Way Galaxy, occupy a special place in the Universe? The questions was as follows:

If the Universe is infinite in size but contains a finite amount of matter, then the average density must be zero, and we living in a region where it’s not would have to be a very special region of the Universe indeed. The assumption that we occupy a special place in the Universe has had a very bad history, and it seems to go against everything that we can observe now.

First off, let’s look at the history of thinking we occupy a special place in the Universe. There was the oldest school of thought, that the Earth was the center of the Universe.

A nice idea, perhaps. Except if you wanted to explain what caused the motions of planets, stars, and the Sun and Moon, you needed gravity. And the Sun is much more massive than Earth. Things worked much better if you put the Sun at the center of our Solar System:

Ahh, and things were better. So then we thought the Sun was the center of the Universe, for a few hundred years. But we learned about other stars, and how they had the same properties as our Sun. In fact, many were larger, hotter, and intrinsically brighter than our Sun. Instead of our Universe being the size of our Solar System (several hundred million kilometers), we quickly discovered it was at least thousands of times bigger than that. Moreover, the Sun wasn’t even the center; the Sun was about 25,000 light years away from the center of this great collection of stars:

And so people decided that our Milky Way, our galaxy, must surely be the center of the Universe. And just as sure as they were that the Earth was unique, and that the Sun was unique, most people thought that our galaxy was the only one. But all that changed in the 1920s, when Edwin Hubble found that all of these objects, known at the time as spiral nebulae, were actually other galaxies, with billions of stars of their own! We now know that there are actually billions of other galaxies out there, and our local group, with the Milky Way, Andromeda, and a few smaller spiral galaxies, is a pretty insignificant cluster, considering there are ones like Coma (below) with thousands of huge galaxies in it:

So historically, we see that in all the ways we’ve thought of, we find nothing special about us, our place in the Universe, or the area around us.

But let’s get to Stephen’s main question: is the region of space that we live in, that we call “The Universe”, anything special? Some facts about our Universe, to get you started:

  1. Our Universe had a beginning. We call this the Big Bang. It happened 13.7 billion years ago, and so today, 13.7 billion years later, we can say that’s how old the Universe is.
  2. Because the speed of light is finite, and the age of the Universe is finite, the size of our Universe is finite. Let’s be clear, it’s huge, but it isn’t infinite. The current distance to the edge of our Universe is about 46 billion light years.
  3. Even though the Universe is 13.7 billion years old, it is bigger than 13.7 billion light years in size. Why? Because it’s been expanding this whole time, so things that we see now (the light from 13.7 billion years ago) have kept moving away from us, so they’re more than 13.7 billion light years away now.

One big source of confusion is that the Universe is expanding. Yes, it is expanding. But it’s expanding the same way a balloon expands. If you lived on the surface of a deflated balloon and inflated the balloon, you would see everything else expanding away from you. So it goes with our Universe: what we call our Universe is just one section of this balloon.

Now, the big question: is our section of the balloon, what we call the Universe, special? As far as we can tell, no. But, to be completely honest, we can’t see any other sections of the Universe that are outside of the 13.7 billion years we’ve had to see, just the 46 billion light years in every direction centered on us. Someone in a distant galaxy would see a different section of the Universe, just like someone living on a different dot on the balloon would be able to see a different part of the balloon.

As far as we can tell, everything we observe shows that every other part of the Universe that we can see is, more or less, just like ours.

Want some scientific details? There are some tests we can do to see whether the things that happen slightly outside of our Universe are similar to our Universe. As far as we can tell, they’re not significantly different, as this recent paper shows:

(If you’re wondering, the green dotted line is the only thing here that matches up with the data that we have; everything else is ruled out.)

So, as far as we can tell, our section of the Universe is just about the same in density as all other sections of the Universe. It has the same matter density, and the same amount of dark energy everywhere, too, as far as we can see. The average energy density of the Universe is small (about one proton per cubic meter, give-or-take), but definitely not zero.

And that’s perhaps the most spectacular find: that not only are the Earth, the Sun, the Galaxy, and the local group nothing special, but our Universe itself appears to be nothing special! We don’t know what lies beyond our Universe (since, you know, it’s not in our Universe for us to see), but whatever it is, and however far it goes on, it isn’t much different from us!


How to Spot a Crackpot?

January 29, 2009 on 1:24 pm | In Random Stuff | 8 Comments

Alright, here’s a quick bonus post for you all. As pointed out by Bad Astronomy, there’s an older article from the Chronicle that goes through the Seven Warning Signs of Bogus Science.

While this is good general advice, I’d like to point out that many legitimate scientists frequently do these things too. In fact, when there’s a theoretical revolution, many of these things need to happen. What are the seven telltale signs, according to Robert Park?

  1. The discoverer pitches the claim directly to the media.
  2. The discoverer says that a powerful establishment is trying to suppress his or her work.
  3. The scientific effect involved is always at the very limit of detection.
  4. Evidence for a discovery is anecdotal.
  5. The discoverer says a belief is credible because it has endured for centuries.
  6. The discoverer has worked in isolation.
  7. The discoverer must propose new laws of nature to explain an observation.

Now, some of these are almost always surefire signs of crackpottery. Specifically, numbers 3, 4, and 5. If you can’t measure a measurable effect significantly, you didn’t detect it. Go back and improve your experiment/observations until you can. (See N-rays for a classic example of this, or 1989’s Cold Fusion for a more recent one.) Evidence cannot be anecdotal, it must adhere to scientific rigor, and widespread belief is never a substitute for scientific evidence.

But numbers 1, 2, 6, and 7 are very grey areas. For example, consider Fritz Zwicky. Although he pitched his ideas to scientists and scientific journals, and his findings were published, there was a huge effort to suppress his work. Was it scientifically justified? Probably not. He worked mostly in isolation, he had to propose a new type of “invisible” matter, but his ideas were right.

Consider the Big Bang Theory. Due to the popularity of Arthur Eddington, no one could even seriously talk about this theory until after his death (in 1944), although people knew about an expanding Universe (and thus the possible validity of the Big Bang) since the 1920s. Akthough it isn’t my field, apparently his dogma in Stellar Astrophysics held sway with the field until his death also.

My point is not that crackpots don’t often do these things, since they do. It’s that legitimate theoretical advances that challenge the established order of things often have to go the same route. How can you tell the completely crazy from the idea that’s “out there” but could be right?

You need to find an honest expert. I’m happy to fill that role where I can, but I know that I have my limits to what I know and how “expert” my judgment is. There are plenty of scientists who are legitimate, good scientists that are working on theories that are not mainstream, possibly not free from errors, and probably not representative of physical reality, according to what we know at present. That doesn’t mean they aren’t right, and that certainly doesn’t mean they are crackpots. Some of them are people I really admire for the courage they display in supporting their own novel ideas, like John Moffat, Jakob Bekenstein, Bob McElrath, and Richard Woodard, to name a few theoretical physicists. They may be right, and for that possibility, their work is valuable. Perhaps they’ll someday be guilty of the sins numbered 1, 2, 6, and 7? And if so, you heard it here first that right or wrong, these people aren’t crackpots, and their ideas should be treated with respectful scrutiny. Just be aware that there’s a Universe of possibilities out there, and this is part of the process of figuring it out.


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 | 15 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…


Weekend Diversion: Where on Earth…?

January 24, 2009 on 3:48 pm | In Random Stuff | 11 Comments

Ever wonder what’s on the exact opposite side of Earth from you? When I was a kid, I always figured if you dug straight through the center of the Earth, you’d come out on the opposite side somewhere in China. But China isn’t opposite New York, China’s opposite Argentina. If you made a map of where the two opposing hemispheres correspond, it would look like this:

These two opposing points on our Earth are called antipodes. Well, are you curious about where the place opposite you on Earth is? Well, there’s an Antipode Map that allows you to find the antipode of any point on Earth! Where would I be?

In the Ocean, close to the French Southern and Antarctic Lands. Most of you will find yourselves in the Ocean; some of you who are very fortunate will find that you have some awesome antipodes! Examples:

  • Sevilla, Spain <---------------------------> Auckland, New Zealand
  • Xi’ an, China <----------------------------> Santiago, Chile
  • Taipei, Taiwan <--------------------------> Asuncion, Paraguay
  • Bogota, Colombia <----------------------> Djakarta, Indonesia

These are give or take a few miles, of course, but this is a pretty neat tool! Hope it amuses you; anyone else find anything fun?


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