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Today, I'll finish up my discussion on some of the science presented at last week's meeting of the American Astronomical Society by looking at the biggest scale we can -- the entire Universe.
These questions are easier to answer for dark matter than for dark energy. Certainly dark matter is fairly convincing. When we look at galaxies and clusters of galaxies, we can see that the pull of gravity is stronger than we expect based on the "normal" matter there. And emerging theories of physics predict dark matter particles that have properties that mimic what we surmise dark matter should act like. But these particles haven't been proven to exist yet, so they remain a very compelling hypothesis, not proven particles.
Dark energy is a little more subtle to explain the evidence for it. Most important is to realize that there are at least two independent lines of evidence for dark energy. The first method involves using what is known as a "Standard Candle," or something that we think we know exactly how bright it is. For dark energy, we use a certain type of supernova that always appears to be about the same brightness. We look and see how faint the supernova appears, we know how bright it actually is, and we use geometry to get a distance. Then we measure how fast the supernova is moving away from us due to the expansion of the Universe. And what we find is that the expansion of the universe is speeding up, when gravity should be causing the expansion of the universe to slow down. Since some form of energy has to be speeding the Universe up, we call that "dark energy."
The second piece of evidence for dark energy comes from a satellite called WMAP. WMAP explored the echoes of the Big Bang visible on the sky, and determined that the total amount of energy in the Universe. We add up all the energy we know about (light, visible matter, and dark matter -- remember, Einstein told us that matter is energy!), and we only get about 30% of the total energy measured by WMAP. The remaining 70% is called "dark energy."
Let me admit here that the distinct possibility exists that both dark matter and dark energy are related, and maybe they just indicate that we are missing some fundamental understanding of physics in the Universe. Dark matter and dark energy make up about 95% of the Universe, and we have very little clue what it is! So maybe our theories about gravity are incomplete. However, Einstein's General Relativity, which explains how gravity works, has worked so well in every experiment designed to test it, that most astronomers are not about to throw it out yet.
So, let's assume that our theories of gravity are correct. What could dark energy be? First, maybe it is a mistake. If the brightness of the supernovae we are looking at changes over the age of the Universe, then the supernovae are not "standard candles," and the measurements we make for them would naturally give us the wrong answer. This is where it is important that we have a second indicator or dark energy, the WMAP satellite. Those data are much harder (but not impossible) to misinterpret. Since both the satellite and supernovae give us the same answer, it seems likely that this is not a mistake.
So, many smart theorist astronomers have developed some ideas as to what dark energy could be. These theories explain all of our observations so far, but they make differing predictions about dark energy that better observations can test.
But there is one alternative theorists don't like, and that is Einstein's Cosmological Constant. The Cosmological Constant is a value that Einstein put into his General Relativity. Early on, Einstein saw that General Relativity predicted the Universe had to either be growing or shrinking. At the time, we had no evidence that it was doing either. The math of general relativity allowed Einstein to put the constant in, so he did. Later, when Einstein learned from astronomers that the Universe was indeed expanding, he set the constant equal to zero. But the cosmological constant, if it is not equal to zero, allows the universe to expand faster and faster, just like dark energy.
Astronomers don't like the cosmological constant, because it has no explanation -- it's just a number allowed by math. Let's suppose I ask you, "How long will it take a car to drive to El Paso if the car is moving at 60 miles per hour?" The answer depends on how far away from El Paso the car is. Now suppose I tell you that the car is 60 miles away. Simple! The answer is 1 hour!
But, then you can ask, why is the car 60 miles from El Paso? Did it just magically appear there? Is there an auto factory 60 miles from El Paso? Does the owner live 60 miles from El Paso? Or is the owner a vacationer from St. Louis, who just happens to be driving west at the time? For the purposes of math, these are silly questions. But if you truly want to understand all that there is to know about the car, these are important questions!
It is the same with the Universe and the Cosmological Constant. The Cosmological Constant is like a starting point. The value of the starting point affects the answers we get from doing math problems involving general relativity. But the math doesn't care where the value came from, it just wants to know what the value is.
For the astronomer and the physicist, that is not good enough. We want to know why the cosmological constant has the value that it does. Is it a fluke of nature? Is there some underlying process that we don't understand that sets the value? Does the value change over time?
So, back to the American Astronomical Society meeting. A few groups presented research trying to make detailed observations of "dark energy" and to determine if one of the existing theories was better than another, or if dark energy continues to act just like a cosmological constant. And the answer is, dark energy acts like the cosmological constant. Now, we can still make better measurements -- if dark energy is mostly like a cosmological constant, but slightly different, we can't yet measure that. But it makes a world of difference in understanding dark energy if the expanding universe acts just mostly like the cosmological constant, or exactly like a cosmological constant. And, if the answer is the latter, we have a lot of tough philosophical questions ahead.
In the meantime, though, astronomers are happy to say that we need to do better measurements first. And this means that you can all keep wondering, "What is Dark Energy? Is Dark Energy Real?" And we'll just smile, shrug our shoulders, and say, "Let me get back to you on that."