A stochasticly updated blog about interesting topics in Physics & Astronomy
Dark Energy is a very hot topic in physics at the moment, and has been for the last decade or so. So far, not a lot is known about what dark energy is, or where it comes from. We have a few ideas, but nothing solid yet. Before I talk about these ideas, I’m going to talk about why we think it exists.
As previously mentioned, it’s very hard to talk about modern ideas in physics and not end up back with Einstein at some point, and this story is no exception. Actually, this story starts with Einstein. 2 blog posts ago, I wrote about Einstein’s “greatest blunder”, as he called it. To refresh your memory, here’s what his blunder was in a few key points
This all happened in 1929. Einstein’s field equations went untouched for the next 69 years. Then, in 1998, Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess discovered that the Universe’s expansion is accelerating. To make Einstein’s equations fit with how the Universe was behaving, it had to be altered again. And guess how? That’s right, scientists reintroduced the cosmological constant (albeit, with a COMPLETELY different value). The cosmological constant appears in Einstein’s field equations as a Λ.
So, after arriving at this amazing conclusion that the expansion of the Universe was accelerating, physicists were left the incredibly difficult problem of figuring out what was causing this acceleration? Previously, when people knew that the Universe was expanding but didn’t know about this acceleration, everyone assumed that the expansion would eventually stop as gravity would slow the expansion down and eventually start pulling everything together. Now, scientists had to figure out what was combating gravity – and what they came up with is dark energy.
Everything we can see in the Universe is composed of matter. For a long time, people assumed that this was everything in the Universe, and that there is a lot of empty space in the Universe – all you need to do is look up to see the amount of darkness compared with light to figure this out. But then, astrophysicists began looking at the structure of other galaxies. And what they found is that the galaxies they were observing should have a lot, lot more mass than we can see for them to maintain their structure. Which led to the idea of dark matter, which is exactly what it says on the tin – it’s matter that emits no light. It is completely invisible. Thankfully, we have another way of confirming if it exists. If you have a body with a lot of matter (like a blackhole, or a body of dark matter) then the body can bend light that passes close to it. This is known as gravitational lensing, and this lensing due to dark matter objects has been observed as these dark matter objects pass in between us and other galaxies.
Right now, scientists think that there’s approximately 4 times more dark matter in the Universe than there is normal matter. But wait a minute – this dark matter is only going to increase the strength of gravity, slowing the expansion of the Universe quite a lot. So if it’s actually accelerating, then this means that there is something powerful out there that’s overcome the combined gravity of matter and dark matter. And that thing is, we think, dark energy.
How it works is a bit of a mystery. What we currently think is that dark energy is everywhere in the Universe, but virtually undetectable by normal experiments. And since dark energy if everywhere, it must take up a fraction of the energy in the Universe – after all, matter is energy, as is dark matter, and there is a finite amount of energy in the Universe. Current measurements, taking into account the total amount of matter and dark matter in the Universe and using this along with the rate of acceleration of expansion of the Universe, show that the composure of the Universe is
That is quite a lot of something we can’t detect!
There are 2 popular ideas of what dark energy is. Firstly, it is a property of space itself. That is, empty space has an energy associated with it. This “energy-of-space” (as described in this article on the NASA site, and from which I’m paraphrasing quite a lot) forces space to expand slightly. And now that there is more space, there is more energy again, since the energy is associated with the space itself. So this causes the space to expand a bit more, a bit faster, and leads to our acceleration that we’ve been looking for. But, this idea is still not fully understood (or at all, really.)
To try and explain this a bit better (and with some calming music in the background) I’m going to hand you over to the Minute Physics youtube channel, to make sure we’re all on the same page.
Another idea for what dark energy might be is as follows – the vaccum of space is full of particles that pop into and out of existence. Sounds kind of made up- but it’s a phenomenon that has been observed before. So when scientists tried to figure out how much energy would be contained within the vaccum if it had these virtual particles, the answer was 10^120 (that’s 10 followed by 120 zeroes) bigger than what our model predicts.
So what’s going on? Honestly – we have no idea. None. This is an open research problem that physicists are crying out for people to try and solve.
I’m not going to lie, I found this article really hard to do. Dark Energy is just that strange of a phenomenon, and there is just too little about it known (actually known, not just guessed) to give you a real feel for what it is. But I hope you enjoyed it anyway! So send in your suggestions for Monday’s article. Also, if you haven’t, sign up to the mailing list on the right of the article. See you on Monday!