Dark Energy

Universe Dark Energy-1 Expanding Universe
This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.  Credit: NASA/STSci/Ann Feild






One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. 

Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. 

As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.


Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. 

But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. 

Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.  READ MORE...

Expanding Into What?

The universe is everything, so it isn't expanding into anything. It's just expanding. All of the galaxies in the universe are moving away from each other, and every region of space is being stretched, but there's no center they're expanding from and no outer edge to expand into anything else.

But that doesn't mean that the universe is infinite. That brings us to the long answer. To understand how something could be finite but have no edge, think of the fabric of the universe as the surface of a balloon. As the balloon inflates, the surface stretches and every point on that surface moves away from every other point, but a tiny being on the surface of that balloon could walk forever and never run into the edge of its balloon universe. There's no edge, yet that balloon universe has a finite volume.

The Shape of the Universe
But the balloon is just one example. Scientists aren't actually sure whether the universe is finite or infinite, or even what shape the universe is. There are three options: spherical, flat, or hyperbolic (that is, it curves upward). Evidence from the earliest light in the universe suggests that the second option is on the money, and the universe is, in fact, flat.

Even if the universe is flat and not balloon-shaped, however, it's still easy to think about how it could be finite with no edge. Think about a flat piece of paper. You could take two opposing edges and make them touch, creating a cylinder. If a tiny 2-dimensional rocket ship traveled from one of those edges to the other, it would arrive back where it started. You could do the same thing in the perpendicular direction: Connect the two ends of the tube to each other (pretend this is magically stretchy paper, for the sake of argument) and create a donut shape, also known as a torus. Now your 2-dimensional rocket ship could travel anywhere it likes, and it would never encounter an edge — even though your paper torus has finite volume.

But wait, you might be saying. Paper is flat; a torus is curved. Isn't that cheating? No, and that's because scientists have a very specific definition for the word "flat." When they say flat, they mean "Euclidean," which means that parallel lines always run parallel and the sum of the angles of a triangle is always exactly 180 degrees. This doesn't happen on a sphere or a hyperbola, but it does on a cylinder, a torus, and any other shape you can make out of a flat piece of paper.

This suggests something kind of exciting: If we live in a flat universe, you could potentially travel in one direction for long enough (or build a telescope that can see far enough) to end up right back where you started. Even cooler things happen when you think about other weird shapes — shapes that twist back on themselves could make you arrive back at a mirror image of where you started, for example.

But no matter what shape the universe is, it's not expanding into anything. There's nothing outside of the universe because the universe has no edge.  READ MORE