In my last post I left off with the revelation that scientists have no idea what makes up 69% of the universe. This is what we call Dark Energy. As I explained before, the name is kind of misleading—dark energy isn’t directly related to dark matter in any way that we know of. It’s just called “Dark” because we have no idea what’s there. The pie chart of the universe might as well look like this:
If we have no idea what it is, how do we know it’s even there?
The short answer is that the universe is expanding, and this expansion is accelerating; everything is getting further and further apart at a faster and faster rate. Something has to be causing that expansion, and that cause is named Dark Energy.
First of all, how do we know the universe is expanding?
If the universe wasn’t expanding, when you look out from Earth at other galaxies, you would expect some of them to be moving toward us and some to be moving away from us. However, in 1929 when Edwin Hubble measured these velocities, he found that ALL the galaxies were moving away from us! Additionally, the further away the galaxies were, the faster they were moving away from us.
This was pretty surprising at the time, but then things got even weirder when in 1998, the Hubble Space Telescope (of course named for Edwin Hubble), looked really deep into space and found that this rate of expansion was slower in the early universe. To get a physical picture of what this means, NASA has a nice graphic below, where the shallower the curve, the faster the rate of expansion. The universe expanded rapidly right after the big bang, but this subsided and a slower expansion took place for several billion years. Then, this expansion started accelerating about 7.5 billion years ago. This discovery won astronomers Saul Perlmutter, Brian Schmidt, and Adam Riess the Nobel Prize in Physics for 2011.
What does this mean? Am I getting bigger, too? Is the universe going to rip apart?
If the whole universe is expanding at an accelerating rate, it seems logical to assume that the distance between all of the atoms in the universe is getting larger. However, this isn’t quite the case. The atoms in our bodies have other forces holding them together that are stronger than whatever is pushing the universe apart. So, our own galaxy isn’t expanding because everything in it is bound together- by gravity, electromagnetism, and by the strong force. What’s happening is that the distance between things- between galaxies- is expanding.
A good model to think about this is the raisin bread model of expansion. When you bake raisin bread, the bread rises and expands, meaning the distance between all the raisins is increasing- but the raisins themselves don’t get any bigger. In the same way, it’s space itself that’s expanding, not the things inhabiting it. This is only true for now, however; unless this expansion starts slowing down, eventually all atoms will start to get farther apart too.
As to what this means about the fate of the universe, no one has a clear picture of what will happen. One possibility is that everything will expand to the point where atoms are pulled apart from each other, resulting in a “Big Rip.” Another, possibility is something called the heat death of the universe. As galaxies continue to get further and further apart from each other, the universe could eventually reach a constant temperature- meaning that no more heat could move from one place to another. Stars would no longer be able to shine shine and the entire universe would essentially become a cold, dark void.
But you and I will be dead by then anyway.
What could it be?
This is one of the biggest questions in Physics currently. We know what it does- causes space to expand- but we don’t know how, which makes it difficult to tell what it is. Some hints can be found in Einstein’s equations of gravity, where a number called the cosmological constant appears.
This number describes the energy density of the vacuum of space. This is sort of a strange concept; it means that “empty space” just doesn’t exist—there’s no such thing as “nothing”. The value of the cosmological constant determines whether the universe is shrinking, static, or expanding. To match the observations of dark energy, it has to be positive, generating negative pressure which counteracts gravity on a cosmic scale.
However, no one has found an explanation for exactly what this constant means. The positive pressure terms are caused by the force of gravity, which we can explain (for the most part) with equations that match observations. But the negative pressure term has no known force associated with it, so we just call it Dark Energy. So, this leads to one of the ideas of what Dark Energy could be- the fifth fundamental force.
Another possibility tries to directly explain why the vacuum of space itself has energy. In quantum mechanics, virtual particles can pop up and disappear instantaneously in the fabric of space. However, calculations using this theory don’t match the observed strength of Dark Energy, so something’s still not right there.
The last explanation that I know of (and the one which I think is the least likely) is that Einstein’s equations are wrong, and gravity is even weirder than we already know it is. There are some theories that suggest you could modify the equations of gravity to still match the observations we know of, but also explain the expansion of the universe. None of them have quite succeeded, but it’s still an avenue of exploration.
What we need are more experiments to probe the properties of Dark Energy. The base level of what is known is much less than with Dark Matter, so this is a pretty big challenge, but the Physics community is up to the task I think.
So what are we going to do about it?
The newest tool in the search for more information about Dark Energy is the Dark Energy Survey. Much like the starship Enterprise, in August the DES embarked on a 5 year mission to explore the universe. Unlike the Enterprise, it will probably not be able to seek out new life, and it will complete its mission without ever leaving Earth, using the Victor M. Blanco Telescope in Chile. Its goal is to map the southern sky with unprecedented resolution and detail.
Fermilab has a somewhat more detailed article (still accessible to a general audience) about how the DES will search for more information about Dark Energy, but basically it’s going to be looking for the effects of this expansion to more carefully measure its strength and observe its effects on objects in the universe such as galaxy clusters and supernovae.
So, the mystery continues, but we’re poised to learn a lot more about the universe in the coming years. There’s a lot of excitement that we could find the truth is even more exotic than we could have predicted. And, as always, the search for answers will no doubt lead to more interesting questions.