The diagram above was first introduced by astrophysicists last year as a graphical way of representing the current state of the future of the Universe. You may have heard of exotic phrases like cosmic expansion, dark matter, dark energy and other grand terms. The Cosmic Triangle compresses all these different fates into a handy and colorful chart.
The Universe, as we know, is expanding. But, is it slowing down? Is it speeding up?What are those strange symbols on the edges of the chart and what do they have to do with it?
I will try to explain.
Omega-M: Matter Density
The Matter Density is a statement about how much gravitational matter is in the Universe. We have the tools today to estimate how much this is, although we also believe that Matter comes in two different types:
Visible Matter
Visible Matter is the Matter we are all famliar with. Matter is what we are made of. Matter is also light. It is also what we call Anti-Matter.
We can estimate how much of it there is in this Universe just by looking at the Stars. We know how heavy the Stars, like our own Sun, are and we know about how many Suns and other stuff is in a typical Galaxy and, finally, we know about how many Galaxies there are in the Cosmos. So, if we total all this up we get a number which is... not very much.
Dark Matter
It turns out that when Astrophysicists were modeling Galaxies they came up with something surprising. Galaxies tend to bunch up together and form Galactic Clusters. These Clusters are like big Gravitational systems, bound in the way that our Earth is bound to our Sun, but on a much grander scale.
But when they tried to model these Clusters they found that the Galaxies were moving in such a way that they seemed to be responding to more than the Matter we could see. The Galaxies in a Cluster are moving too fast! They should be flying off but they are not. Something is holding them together. But its not something that we can see. We call it Dark Matter but that doesnt mean that its like anything that we know. Its just a term for what should be there.
In any case, even if you calculate how much Dark Matter there is and add it to the stuff we can see, it is not enough to really affect the Universe in any significant way. That is, all other things being equal, all these objects in the Universe are not enough to slow the Universe down, to reign it in.
Omega-K: Curvature
The Curvature Parameter is a measure of the shape of our Universe. Our Universe has three dimensions but it also has a shape in a higher-dimensional space. The particular shape of the Universe strongly affects its ultimate fate. Essentially, there are three possibilities for the shape: Open, Closed and Flat. An Open Universe will keep expanding forever. A Closed Universe is about to shut its doors; the waiters are serving the last bits of coffee and someone has showed up with a mop. A Flat Universe sits right in between, expanding forever, but just barely.
All indications are that our Universe is or is nearly Flat (we measure this by looking at our Universe's past and how it has evolved). This means that it might be exactly Flat.
One of my old astrophysics teachers, a British guy who wore a tight collar and flailed his arms a lot, used to say: "If our Universe is nearly flat, then it must be Flat. Why would it be nearly-something if it were not that thing!"
It sounds convincing. But the same argument can be made about our Earth and how its orbit is nearly a circle. We have a history of searching for perfection and then discovering that its the deviations which are more fascinating.
Omega-Lambda: Vacuum Energy
Now this is interesting. In recent years, it has been discovered that our Universe may not only be expanding but also accelerating! But, since Gravity and Matter can only slow the Universe down, what is making it speed up? The simple answer is: We don't know. But instead of admitting that, it is easier to give it a name and then talk about it. The name for this mysterious energy that may be propelling the Universe outward is: Dark Energy
If something is making the entire Universe expand then it is probably not something that sits at a particular place; It is likely everywhere, intrinsic to the Universe itself. And so this Dark Energy can also be thought of as Energy of the Vacuum, that is, a repulsive force which is part of the very nature of Space itself.
Dark Matter and Dark Energy have similar names but they have nothing in common. If anything, they are opposites of each other. Dark Matter, like regular Matter, is slowing the Universe down. Dark Energy, this anti-pressure, is speeding the Universe up.
The Cosmic Triangle
Based on what we currently know, our Universe sits at the circle labeled Lambda-CDM (Cold Dark Matter). That is, we live in a Universe that is fairly flat, has very little matter, some sort of mysterious repulsive force and, by a clear margin, looks like it will keep expanding forever.
But this is all subject to change. One of the major assumptions is that information we have about the Universe now is even valid when talking about its past or future. It is like those spaceships with tailfins from 1950's era futuristic visions: What we know about the future can quickly become outdated.
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