http://hubblesite.org/hubble_discoveries/dark_energy/
Stranger and Stranger
Most of the universe seems to consist of nothing we can see. Dark energy and dark matter, detectable only because of their effect on the visible matter around them, make up most of the universe.
We do know this: Since space is everywhere, this dark energy force is everywhere, and its effects increase as space expands. In contrast, gravity's force is stronger when things are close together and weaker when they are far apart. Because gravity is weakening with the expansion of space, dark energy now makes up over 2/3 of all the energy in the universe.
It sounds rather strange that we have no firm idea about what makes up 74% of the universe. It's as though we had explored all the land on the planet Earth and never in all our travels encountered an ocean. But now that we've caught sight of the waves, we want to know what this huge, strange, powerful entity really is.
The strangeness of dark energy is thrilling.
It shows scientists that there is a gap in our knowledge that needs to be filled, beckoning the way toward an unexplored realm of physics. We have before us the evidence that the cosmos may be configured vastly differently than we imagine. Dark energy both signals that we still have a great deal to learn, and shows us that we stand poised for another great leap in our understanding of the universe.
Did Einstein Predict Dark Energy?
Albert Einstein, 1947. Einstein used his "cosmological constant" to help describe a static universe. When he learned the universe was expanding, he discarded it.
Einstein theorized that mass warps the shape of space, creating the force we call gravity.
Oddly enough, dark energy — for all the surprise around its discovery — is not an entirely new concept in physics. There is historical background for this idea, and it comes from the preeminent astronomer of the 20th century, Albert Einstein.
In 1917, Einstein was applying his new theory of general relativity to the structure of space and time. General relativity says that mass affects the shape of space and the flow of time. Gravity results because space is warped by mass. The greater the mass, the greater the warp.
But Einstein, like all scientists at that time, did not know that the universe was expanding. He found that his equations didn't quite work for a static universe, so he threw in a hypothetical repulsive force that would fix the problem by balancing things out, an extra part that he called the "cosmological constant."
Then, in the 1920s, astronomer Edwin Hubble, using a type of star called a Cepheid variable as a "standard candle" to measure distances to other galaxies, discovered that the universe was expanding. The idea of the expanding universe revolutionized astronomy. If the universe was expanding, it must at one time have been smaller. That concept led to the Big Bang theory, that the universe began as a tiny point that suddenly and swiftly expanded to create everything we know today.
Dark Energy, Dark Matter
In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow.
The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.
What Is Dark Energy?
More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 70% of the Universe is dark energy. Dark matter makes up about 25%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the Universe.
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.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
What Is Dark Matter?
By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~70% dark energy, ~25% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is.
First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations.
Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them.
Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter.
Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
Galaxies, Clusters, and Superclusters
Photo Andromeda Galaxy
Galaxies are the building blocks of the universe. Clusters of galaxies, and clusters of clusters of galaxies, called superclusters, make up the structures in the geography of the universe. In this section, we'll explore these structures and take a look at our address on these scales.
Galaxies are titanic swarms of tens of millions to trillions of stars. Between the stars, there can be vast interstellar clouds of gas and dust. Spiral galaxies have a thin, pancake-shaped disk, with a spherical bulge at the center. Within the disk, the brightest stars trace out the characteristic spiral pattern. Elliptical galaxies are shaped roughly like watermelons, some round and some elongated. Large galaxies are approximately 100 thousand light-years across (a light-year is the distance light travels in one year: about six trillion miles). In rather plain fashion, the smallest galaxies are called dwarfs while the largest are called giants.
Our Milky Way Galaxy is one of three large galaxies in the Local Group of Galaxies. The other large galaxies are the Andromeda Galaxy, and Messier 33 (the 33rd entry in Charles Messier's catalog of fuzzy things in the sky). Also in the Local Group are a couple dozen dwarf galaxies. Several of these dwarf galaxies are satellite galaxies, orbiting around the large galaxies. The Milky Way has two prominent satellite galaxies, called the Large and Small Magellanic Clouds. (The Magellanic Clouds are visible on Earth only from the southern hemisphere; their existence was recorded by the voyage of Magellan).
Larger clusters of galaxies can contain hundreds of galaxies. Galaxies within a cluster are generally considered to be bound together by their mutual gravitational pulls. They each orbit around their common center of mass. Because the density of galaxies is high within clusters, galaxy collisions occur. One such collision can be seen in the lower right of the image of the Virgo cluster below.
Galaxies at the center of the Virgo Cluster of Galaxies
The enormous gravity of the Virgo Cluster makes it the center of a larger structure, called the Local Supercluster. This collection of nearly 100 clusters, and thousands of galaxies, stretches across a hundred million light-years. The Local Group is just a tiny member of the Virgo Supercluster, located on the outskirts. Just as we have found that Earth is not the center of the solar system, and the Sun is just another star in the Milky Way, so too have we found that our galaxy holds no special place in the universe.
Galaxies observed in the Hubble Deep Field image
The Hubble Deep Field, a special observation made with the Hubble Space Telescope can see farther into the universe and uncover more galaxies than perhaps any other observation ever made. From the hundreds of galaxies we can see in a very small patch of sky, we can estimate that there are about 50 billion galaxies in the universe.
