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Asked by to Aimee, Chris, Dave, Greig, Laurence on 20 Jun 2014. This question was also asked by .
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Laurence Perreault Levasseur answered on 20 Jun 2014:
That is a very interesting question, and also a bit tricky to answer.
The Universe is mainly made of 2 things. The fabric of space-time (that is all space AND time considered together, at the same time) and energy.With Einstein’s equation in general relativity, we know how energy tells space-time to curve and deform, and in return how space-time tell energy to move around (that’s what we call gravity!!).
The energy of the universe can be decomposed into many things.
1) First there is the ordinary matter. Often scientists called this the ‘Baryonic matter’ (because it’s mostly made of baryons, a type of particles!). That’s all the stuff we know and love. The planet, the rocks, the plants, the humans, the sun, the gas in the galaxy, the atmosphere of the Earth, the clouds, the water, my pet fish, my arms and legs, houses, cars, basically, everything around us that we can see. But that’s only 4.9% of the energy content of the Universe!!!!
2) Radiation. That’s basically light. it’s a really small fraction of the whole energy of the universe, so small that often it’s neglected.
3) Dark matter. That is a kind of matter we think makes up 26.8% of all the energy in the Universe, except that, because it’s dark (more precisely, it doesn’t interact with light: it neither emits nor absorbs it), we cannot see it at all. How do we know it’s there? Because it has a mass, so we observe it indirectly through the gravitational effects it has on ordinary matter and radiation! It’s a bit like looking outside on a sunny day and seeing a big shadow on the ground. Even without looking up or knowing what it is, I know there has to be something to make that shadow (a cloud, or something).
4) Dark energy. So if you add up what we have so far, we got 31.7% of all the Universe energy. That’s not good. The rest is basically something we have no idea what it is. So we call it dark energy. Not because it’s dark, but because we seriously have no clue what it is. People spend their careers trying to figure out where it could come from or what it is, but there is no satisfactory answers yet. The weirdest thing is how it acts: it acts like a fluid with negative pressure, and on earth we’ve never seen something like that (except MAYBE in the Casimir experiment, but it’s another story).
What this ‘negative pressure’ does is that it causes the fabric of space-time to accelerate much faster than what it’s supposed to if it wasn’t there. Even more weird, dark energy seems to come from empty space. So when it pushes the Universe to expand faster, it makes more empty space. This means there is even more dark energy from that empty space, which makes the universe expand EVEN faster, which makes MORE empty space, and so EVEN MORE dark energy!!Some scientists actually thing that the solution to this ‘dark energy’ mystery is to modify Einstein’s equations of general relativity, but so far they haven’t managed to do it in a convincing way, really. Isn’t that fascinating? 😀
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Dave Jones answered on 20 Jun 2014:
In terms of the stuff we can see, almost all of it is in the form of Hydrogen, with a little bit of Helium. All the other elements, like Carbon and Oxygen (the really important things for us on Earth!) make up an extremely tiny fraction of this matter. But, most of the Universe is in stuff we can’t see, or not directly at least.
When we observe the movements of stars in our Galaxy and others, we can tell that there is something there that isn’t shining like the stars. This material interacts only through gravity, and doesn’t interact with light so we can’t see it. That means it doesn’t reflect, absorb or shine light, ever. Impossible to see with your eyes, but we can see how it makes the stars move because of its gravity. This stuff which we call Dark Matter (dark because we can’t see it with light) makes up quite a lot of the rest of the material in the Universe.
After that, we can tell by the way that galaxies move that our Universe is expanding, and that it is expanding faster and faster every day. To do this there must be something else in the Universe besides Dark Matter and all the stuff in stars and planets and big gas clouds. We call that Dark Energy because again we can’t see it with light, but it acts more than an extra energy causing the Universe to expand. Turns out that this is the most important part and makes up about 70% of the stuff in our Universe. The dark matter is another 25% and finally, the stars and all the things we can see make up only about 5%
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Aimee Hopper answered on 21 Jun 2014:
As far as we know, our universe is made up of matter and energy. Einstein showed that these are essentially the same thing (E=mc^2) . I say “matter” to distinguish it from “antimatter”, which we dont see a huge amount of… anywhere in the universe!
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