Dark Energy: Cool name, but we know nothing
Of all the weird things in our Universe that we don’t understand, dark energy is the weirdest. And we understand it the least. We don’t even really know if it is energy. And it’s just “dark” because we don’t know anything about it. Really, the Universe is expanding. And it’s expanding faster and faster. And we don’t know why. And we’ve called that dark energy. It’s a bit embarrassing really. Let’s dive in, to dark energy.
Dark Energy: The starting point
We discovered that the Universe was expanding almost 100 years ago. While it is a bit of a weird concept, but it is core to a lot of modern physics and pretty much taken for granted. If you point a telescope at a galaxy somewhere in the night sky you will see that it is moving away from you. If you then do the same thing for galaxies on the other side of the Universe, you find the same thing, it is also moving away from us. This all combined together means that all the stuff in the Universe is getting further away from each other. This observation is what lead to us discovering the Big Bang (see our post on that here).
However, the Universe has expanded at different speeds at different times. Initially, in the early Universe, the expansion was incredibly rapid. In the first billionth of a second, the Universe got more than one trillion times larger – there is almost no way to really conceptualize it (see our post here). Since then, it has slowed down significantly and galaxies are only moving away at a few thousand kilometers a second. That is still pretty fast, but much slower than the rapid faster-than-light expansion of the early Universe.
However, over the last few billion years, the Universe has started to expand faster again. It is hard to explain how we worked this out. Showing that the Universe is expanding is easy, we point a telescope at the sky and see that galaxies are moving away from us. However, to see the changes in the rate at which things have been expanding is much harder – you need to take much more precise measurements. Galaxies are a bit too irregular and change shape too much for us to measure really precise changes in speed – we need something cleaner. Instead, to measure changes in the rate of expansion, scientists measure supernovas. Supernovas are exploding stars – they are quite complicated, but certain types of supernovas explode in a very consistent way, meaning we can look at them and tell how fast they are moving.
When scientists first measured the supernova, some weird things happened. About 70 high-quality supernovae were measured. The closer supernova all made sense – they were bright and moving reasonably slowly. The older and further away supernova were moving much faster. This makes sense in terms of an expanding Universe. However, they were not quite as bright as was expected – implying that they were probably a bit further away than was expected. This meant that somewhere along the line, the expansion of the Universe had sped up.
Now you can think of a lot of other explanations for this evidence. To reliably measure the distances and speeds of stars more than 10 billion light-years away, moving at hundreds of thousands of kilometers a second, is really hard. We could be wrong about how bright the stars are, how fast they were moving, you can come up with a lot of holes in this evidence really easily. However, over about 20 years, hundreds of scientists all across the planet publishing hundreds of papers have eliminated all the obvious alternative explanations. We have also got a lot more other types of evidence, such as using the cosmic microwave background. The Universe is expanding. And its expanding at a faster and faster rate.
So things are expanding faster? What does this mean?
The fact that the rate of expansion of the Universe is increasing is really weird. Galaxies are big. It takes a lot of energy to make them move (i.e. the Big Bang was really big). They don’t just do pivots or start spinning weirdly like leaves floating in the breeze. Galaxies travel through space in very predictable ways. So for galaxies to be speeding up, there must be an enormous amount of energy propelling them to do so.
We call this unexplained acceleration of galaxies “dark energy”. And we know almost nothing else about it. We have some theories around why it might exist, Einstien had some interesting ideas around a “cosmological constant”, some free energy that permeates all space making things expand. However, we have no way of empirically testing any of these ideas and no real consensus around what is going on. It’s all just very smart and complex guesswork.
It is pretty easy to see why dark energy is so hard to study. What do you look for? At least with dark matter, we know vaguely what we should be looking for (see our post on dark matter here). It’s a thing that is clumping around the center of galaxies. With dark energy, we haven’t even come up with an experiment to test what it is or why it exists. Everything is just moving away from itself faster and faster? What can we point a telescope at? This is clearly a problem we have only just started to grapple with.
Dark Energy: A few more thoughts
We know that there is a lot of this “dark energy” in the Universe. From seeing how fast all the galaxies are moving away from us we can estimate how much of dark energy there must be. When we compare it to the amount of dark matter and normal matter there is in the Universe, we find there is a lot more dark energy. In fact, of the “stuff” in the Universe we have roughly 5% of ordinary matter, 25% dark matter and 70% dark energy.
This is just bizarre. The idea that there is this weird invisible matter at the center of all the galaxies that we can’t seem to detect or understand is weird enough. The idea that there is more of this stuff than there is of ordinary matter is wild. The idea that there is this third kind of thing in the Universe, dark energy that we don’t even know how to start trying to detect is just pure insanity. It almost feels like the Universe is laughing at us.
Some people often say that science “ruins the mystery” of the Universe. These people must have stopped reading the textbook on page two. There is so much strange, wonderful stuff that we do not understand still happening in our Universe. We hear The Universal Story, are amazed by it every day.
