Dark Energy: A Homage
Supernovae are the most energetic processes in the Universe that we have ever witnessed. They are one of the many ways in which stars can end their lives and they are definitely the most spectacular one. When a sun dies in a supernova it really goes out with a bang. A bang you can see from millions of light years away, galaxies away. For a very short time this one star shines with the light of millions of stars, as bright as a galaxy.
Supernovae come in various kinds and some of them are very well understood. In particular, for a so-called Type-I supernovae we understand exactly what kind of the light they emit. This special signal that they emit can be used to estimate, with great precision, how far away the star that went supernova is.
It has been known for about a century now that the Universe is not static, but expanding. This means that tomorrow it is going to be larger than it is today, and yesterday it was smaller than it is today. The way we notice this is by looking at the light from the distant objects and measuring something similar to the famous Doppler effect. Essentially what this means is that if an object is moving towards us it will appear a bit ‘bluer’ to us than it really is, and if it is receding away it will appear a bit more red. We call this being “blue-shifted” or “red-shifted”, respectivelly. Finally, by finding that all observable galaxies are receding away from ours, we’ve known for a long time that the Universe is expanding.
Now we come back to the magic of the supernovae. You can’t just pull out a yardstick and measure how far something in the Universe is. You have to be able to deduce how far away something is just by looking at its light. So you better understand its light. This whole procedure of measuring the distances is called constructing a ‘distance ladder’. Well supernovae are the last step on this distance ladder. They’re extremely bright, so they can be seen to enormous distances. They’re also beautifully described by the General Theory of Relativity a.k.a. Einstein’s theory, so we understand their light. Now add the above mentioned Doppler to the game, and you’ve got yourself a way to measure how fast the Universe is expanding depending on how far something is away from us.
Finally we come to the punchline. Speed of light is finite, which means that it takes some time for us to see distant objects. The farther the object you’re looking at, the farther away in the past you’re looking. This means that supernovae allow you to understand how the Universe was expanding through a big chunk of its life.
In 2011. a Nobel prize was awarded for a 1999. discovery, through the above described measurements based on supernovae, that the Universe is accelerating. Not expanding, that we knew already, but expanding with the ever increasing velocity.
This means that the Universe is expanding faster today than it was yesterday, and it is going to expand faster tomorrow than it does today.
For most people this was shocking, and I don’t mean just laymen. I mean it was shocking for most scientists, for most physics Nobel laureates, and I suspect also for the pope. We all know that gravity is that thing pulling us towards the centre of the earth and stopping us from flying off to space. It does so by ‘attracting’ us towards the earth. It also keeps Moon in orbit around the Earth, and both of them in orbit around the Sun, and so on. Gravity is supposed to glue things together, not push them away, or so we thought.
To this day we cannot tell for certain what dark energy is. We know it exists. It is what tells gravity to push things away instead of pull them together. It is what pushes our galaxy from all the others, and every galaxy from every other. By now existence of dark energy has been confirmed in other ways too. All these independent tests ascribe same properties to the dark energy, this makes the evidence of its existence much more convincing. Theoretical physicists have come up with a lot of models to explain what dark energy is and how it behaves. Most of them aren’t completely satisfying in a sense that they do not address all the issues which the existence and the properties of the dark energy raise. What you can be certain is that this is one of the biggest open questions in contemporary science, and one which will attract a lot of attention until it gets solved. What ever its solution may be, it will tell us a lot about the Universe and about its death.