The Huge
Dark Energy
Dark energy is used to refer to the observational fact that the expansion of the cosmos is accelerating in recent epochs compared to the distant past. How do we know this? Type IA supernovas, that’s how. They act as so-called “standard candles”. In the field of lighting, a candle, formally candela, is a standard unit of luminosity. It radiates a specific amount of light. If it appears dimmer, it must therefore be farther away; brighter, then it’s closer. Same with type IA supernovae. Redshift is another candle. Only it measures speed, not distance. As a light-emitting object moves away, its light appears slightly redder. As it moves toward an observer, it appears bluer. On average all the other galaxies appear redder, so they are all moving away from us. What makes us so special? Nothing. From the point of view of other galaxies the same thing is observed. How can this be? Only if the farther out you look, the faster the other galaxies are receding, then they are receding both from us and from each other. This is known as Hubble’s Law after its discoverer, Edwin Hubble. It one one of the most important discoveries in all astronomy. That’s why he had a space telescope named after him. The important thing here is that the closer galaxies move away faster with distance than the farther away ones do. The only way they can do this – to the degree that they do – is that if about 70% of empty space is filled with something that cannot be seen that is pushing the galaxies apart. That’s dark energy.
Dark Matter
Stars revolve around the centers of galaxies like planets do around stars. The more matter near the center of the galaxy means stronger gravity, which means stars have to move faster to stay in their orbits and not get sucked in. If they move too fast though, they will fly right out of the galaxy. It turns out that wherever we look, stars are moving too fast but not flying out. That must mean there is more matter than we can account for than just by looking at bright objects. So it must be accounted for by not-bright objects – dark matter. Can’t see it, don’t know what it is. That’s dark matter.
The Tiny
Atoms
Atoms are one or more electrons zipping around a nucleus. If a nucleus were the size of an apple, the closest electron, on average, would be over 5 miles away. Everything in between – empty space. 99.9999999999999% empty space. That’s an atom.
Nuclei
Although we can’t easily see inside the protons or neutrons that make up a nucleus, we know by ramming particles into it that there are tiny little parts inside they bounce off of. They are called partons, a fancy name for parts. Theory says they are quarks, but you can never see one alone, not so far anyway. Whatever they are, they are tiny. So even the part of the atom that has almost all the mass is itself mostly empty space. To be fair, there are fields in this empty space, but also to be fair, fields are theoretical constructs. All we know for sure is that there is energy there in empty space.