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Asked by sawyerbean to Matthew, Mark, Mike, Paul, Sabina on 12 Mar 2014. This question was also asked by .
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Matthew Malek answered on 12 Mar 2014:
Uh-oh. I’m not sure that I can answer this one… because no one really knows what dark energy is!
I can tell you what we do know, though:
In 1998, two different groups made observations of a certain type of supernova to measure the expansion of the universe. The most likely result expected was that the expansion was slowing, due to the gravitational pull of all the matter in the universe. They were trying to measure the rate at which the expansion was slowing down, which could teach us about the ultimate fate of the universe. Would the universe expand forever, at an ever-slower pace? Would it eventually start contracting again, leading to a “Big Crunch”? These are the sort of questions that they were seeking to answer.
What they found was rather surprising — their observations showed that the expansion of the universe is NOT slowing down at all. Instead, it is speeding up! That was completely unexpected — what could be causing the expansion to accelerate?
A slow-down could be understood by the attractive force of gravity countering the expansion. But what force could be making the universe expand faster?? That’s where dark energy comes in.
Dark energy appears to be a repulsive force that is enhancing the expansion of the universe. What’s more, just as gravity comes from matter (i.e., mass), dark energy seems to come from empty space. So as the universe expands, it creates more empty space… which means there is more dark energy from that empty space… which makes the universe expand faster… creating even MORE empty space!
In this picture, it would appear that the expansion of the universe will go on forever, with everything getting further and further apart. At early times, when the universe was young, there was very little dark energy; now we believe that about 70% of the energy / matter of the universe is made of dark energy! And that number will only go up as the expansion continues to produce more dark energy.
Pretty amazing stuff, isn’t it?
We don’t know what the dark energy is yet, and this is an ongoing hot topic of research. One possibility is that it could be the “cosmological constant” that Einstein proposed nearly a hundred years ago.
When Einstein developed his theory of gravity (General Relativity) in the early 1900s, he added an extra term, which he called a “cosmological constant”. Einstein believed that the universe was static — neither expanding nor contracting — and so this term represented a repulsive force that would counterbalance the attaction of gravity.
In the 1930s, Edwin Hubble discovered the universe was NOT static, it was expanding. Einstein’s assumption had been wrong, so he ditched the cosmological constant and called it his greatest “blunder”. Now it may be that Einstein was right all along, and his “cosmological constant” is what we have discovered with dark energy. Time will tell!
Hope this makes this a little clearer for you!
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Paul Coxon answered on 13 Mar 2014:
No one really knows, but a lot of our current understanding of the universe relies on its existence.
The universe is expanding, getting bigger and bigger. Not only is it getting bigger, but it’s accelerating and expanding at a faster rate!
To try to understand this, theorists came up with an idea of dark energy, which creates a field that causes the acceleration.
Dark energy is still a very mysterious concept, especially to me. That’s why I chose to study atoms and crystals instead 😉
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Mark Jackson answered on 15 Mar 2014:
If I knew the answer, I would have already gone to Stockholm to collect my Nobel Prize!
Like many things in science, if we give it a funny name we don’t really know what it is. The reason we think Dark Energy is there is because we have measured the expansion of the Universe to be accelerating. This means that not only is the Universe getting bigger, it’s speeding up how fast it’s getting bigger! Einstein’s General Theory of Relativity gives a relationship between the expansion of the Universe and the stuff that’s inside, and the only way it could be accelerating is for it to contain a very unusual energy field. We have a mathematical description of this type of energy but we’ve never been able to make it in the laboratory. The reason you don’t observe it around you is because it is so faint that it doesn’t have an effect on your surroundings, only becoming important on very very large distance scales.
One theory – and I stress that it is only a theory – is that our Universe is in a ‘False Vacuum’. The Vacuum refers to the state of the Universe when there are no particles present. This is normally the lowest energy state, but sometimes the system can temporarily reside in a non-lowest energy state. If this happens, it’s only a matter of time before it discovers that it could find a lowest energy state and do that. If this were true, our Universe will eventually transition to the lower energy vacuum state – and destroying everything in itself, since all the laws of physics change completely! But don’t worry, this probably won’t happen, at least not for a while. You can make plans for next week.
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