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The Most Abundant Substance in the Universe: What Is Dark Energy?
The universe is expanding beyond the intuitive thought that gravity should make it contract. After the big bang, and inflation, scientists used to think that gravity should make the universe shrink in size until everything was back to where it began. This thought was proven false. In the 1930s when Edwin Hubble found that galaxies were red-shifted, meaning the light was being stretched into the redder part of the spectrum, that the galaxies were moving away from the Earth. He also found that the further away these galaxies were, the more the light was stretched. This brought him to the conclusion that the universe was expanding.
This puzzled scientists, and was counter-intuitive. Gravity should be pulling everything together yet something was pushing it apart. This brought on the idea that the galaxies were not just moving apart, but the actual space between galaxies is expanding.
Dark matter and dark energy are two things we measure in the universe that are making things happen, and we have no idea what the cause is.— Neil deGrasse Tyson
What Is Dark Energy?
Dark energy is a mostly hypothetical construct. It is so rarefied that that one meter cubed of it would weigh approximately 10-27 kilograms. That is .0000000000000000000000000001 kilograms in an area about the size of an average person. Yet this substance is thought to make up about 70% of the energy in the universe. This is due to the fact that even what we consider empty space would be full of the stuff. And as the universe continues to expand, this energy density remains the same everywhere. So over time, this percentage will continue to rise.
The energy inherent to empty space has been experimentally proven to exist. The famous Casimir experiment took two plates, and found that they would be pushed together in a vacuum if they were only a few nanometers apart. This is thought to be from the force of virtual particles coming in contact with the outside of the plates. Yet on the inside, there is not enough room for these particles to exert their force. So the plates will experience a inward pressure from empty space.
The Casimir Effect.
During his work on general relativity, Einstein did not like that his equations produced a dynamic universe. As a man of god, Einstein wanted his universe to be static, and unchanging. To this end he added a cosmological constant to his equations. This constant added in a force that represented the mass of empty space, however, this actually was found to be unstable. If the universe were to expand, this would release energy, and the universe would expand more. If the opposite were to happen, then it would contract.
This so called mistake, was to Einstein, his greatest blunder. But today scientists are using this to try and prove that it is in fact an inherent part of our universe. That would mean that Einstein was right, again, even when he thought he was wrong!
The term "dark energy", I think, is a bit of a misnomer. We do not know enough about this force to give it a name. But I guess we have to call it something, and dark energy will do for now.
Ever since the discovery that the universe is not only expanding, and speeding up this expansion, scientists have been searching for the cause. The entire universe is based on cause, and effect. We observe this effect everyday, and assume that it will continue this way.
If the universe continues to exponentially expand, then in the future there will be a time that the observable universe will just be our own galaxy. This is a terrible thought. If somehow the knowledge of our universe is lost in the future, then people will be even more puzzled by their place in it. Not knowing that there are billions of galaxies out there with billions of stars. The universe would become a small place with no beginning, and an uncertain end.
What do you think?