When Albert Einstein applied his equations of General Relativity to the observable universe, he found something that he didn't like. His equations were correct, but for some reason, his equations predicted that the universe was dynamic when he and everyone else though the universe was static. This was in 1917, before the Big Bang theory and before Edwin Hubble found that the universe was expanding. To account for what he felt was incorrect, he introduced a fudge factor to take away the dynamic universe solution. He called his fudge factor, the cosmological constant. He hoped and felt that in time, physics and astronomy would be able to allow the cosmological constant to go away. When Hubble found the expansion of the universe and the Big Bang theory were proposed, Einstein thought that his cosmological constant was his biggest blunder. But was it?
Now, with the introduction of dark energy to help explain the expansion of the universe, the cosmological constant was reintroduced. As explained last time, if the dark energy density is constant, the universe will be open and expand forever. With a constant dark energy density, this implies that the universe is homogeneous in both space and time. Remember that this is referred to the Perfect Cosmological Principle which was briefly mentioned here. In other words, the universe appears static and therefore, the cosmological constant may be a physical quantity describing the dark energy density of the universe. Unfortunately, we still don't know what the dark energy density is doing and it may be centuries or millennia before we know.
Showing posts with label expansion. Show all posts
Showing posts with label expansion. Show all posts
10 March 2015
09 March 2015
Dark Energy
What is dark energy?
First, what isn't dark energy? It isn't energy used by either Sauron or Lord Voldemort. They use dark magic, which isn't the same. You also shouldn't confuse it with negative energy, which is strange phenomena in itself. We will look at negative energy later.
Dark energy is defined as the energy that permeates space and drives the expansion of the universe. At the present time, the amount of dark energy in the universe is seeming to accelerate the expansion rate of the universe. It should be noted, however, that this does not mean that the universe is open. Just because the universe's rate of expansion is accelerating now, it does not mean that sometime in the future, the rate of expansion can slow, stop, or reverse.
Dark energy is thought to be one of two things: a constant energy density over time and space (static) or a scalar field density that has a value that can change with time or space (dynamic).
If the energy density is constant over time and space, that means as the universe expands, the amount of energy (not including mass) must increase. The only way this happens without violating the conservation of energy is that mass must be converted to energy in some way. This could be done in the normal way (matter-antimatter collisions) or in some way that we don't know. This constant energy density is referred to as the Cosmological Constant, and was first introduced by Albert Einstein. We will discuss this more later. If this is what dark energy is, then we live in an open universe.
The other is that dark energy density is a changing quantity which in the future could either slow down (but not stop - open universe), stop (flat universe), or reverse (closed universe) the expansion, depending on how the density changes over time.
At our current knowledge, dark energy is 68.3% of the total mass-energy density, dark matter is 26.8%, and ordinary matter is only 4.5%. So you can see, what we don't know about the universe is a heck of a lot more than what we do know.
First, what isn't dark energy? It isn't energy used by either Sauron or Lord Voldemort. They use dark magic, which isn't the same. You also shouldn't confuse it with negative energy, which is strange phenomena in itself. We will look at negative energy later.
Dark energy is defined as the energy that permeates space and drives the expansion of the universe. At the present time, the amount of dark energy in the universe is seeming to accelerate the expansion rate of the universe. It should be noted, however, that this does not mean that the universe is open. Just because the universe's rate of expansion is accelerating now, it does not mean that sometime in the future, the rate of expansion can slow, stop, or reverse.
Dark energy is thought to be one of two things: a constant energy density over time and space (static) or a scalar field density that has a value that can change with time or space (dynamic).
If the energy density is constant over time and space, that means as the universe expands, the amount of energy (not including mass) must increase. The only way this happens without violating the conservation of energy is that mass must be converted to energy in some way. This could be done in the normal way (matter-antimatter collisions) or in some way that we don't know. This constant energy density is referred to as the Cosmological Constant, and was first introduced by Albert Einstein. We will discuss this more later. If this is what dark energy is, then we live in an open universe.
The other is that dark energy density is a changing quantity which in the future could either slow down (but not stop - open universe), stop (flat universe), or reverse (closed universe) the expansion, depending on how the density changes over time.
At our current knowledge, dark energy is 68.3% of the total mass-energy density, dark matter is 26.8%, and ordinary matter is only 4.5%. So you can see, what we don't know about the universe is a heck of a lot more than what we do know.
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