1.2 The 4% Universe

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Our recent advances in understanding the universe have been tremendous.

Deep connections have been established between quarks and the cosmos. These big ideas and powerful instruments have pushed cosmology forward.
We can trace the history of the universe from before the “quark soup” all the way to the galaxies and stars we see today.
We know the universe is uncurved (critical density $\Omega_0=1.005\pm0.006$), consistent with inflation.
One of the biggest remaining mysteries is about the composition of the universe. Everything we see in the universe, all things comprised of atoms, only describe about 4% of the universe.
- 4% Atoms: $\Omega_B=0.046\pm0.0016$
- 26% Dark Matter: $\Omega_M=0.273\pm0.014$
- 70% Dark Energy: $\Omega_{DE}=0.73\pm0.015$

Today's cosmology rests upon three mysterious pillars: Dark Matter, Dark Energy, and Inflation.

Much of the evidence we have for our theories about the universe comes from analyzing the cosmic microwave background.
The Planck CMB map represents slight variations in temperature of this radiation.
When we analyze the data from these variations, we see a strong alignment with the cosmological theories involving inflation, Dark Matter, and Dark Energy.
With inflation, we have this wonderful idea, and we may even have some evidence for it—but we’re out on a limb because we don’t know what caused inflation. Inflation is the most important idea in cosmology since the Big Bang theory itself.
In regard to Dark Matter and Dark Energy, we have evidence for them, but we don’t know what they are. That is not to say that we won’t find out, though.
We can trace the history of the universe back to ~10⁻³⁶ seconds, but the story is not complete and there will be some surprises.

When we look at the universe, we only can see the things that light up.

Consider the Hubble Deep Field—an image of an incredibly small region of the sky (about one 24-millionth), which exposed thousands and thousands of galaxies. And yet, this is just the so-called 4%. In fact, it’s not even quite 4%—not all 4% lights up.
The limit of how far we can see is not from the telescope; it’s from time. We’re looking back in time and seeing the formation of galaxies, but we’re missing most of the universe—the 96% that we can’t optically observe.
So how do we know that Dark Matter and Dark Energy are out there?

Dark Matter and Dark Energy 'control' the universe.

Dark Matter holds things together; Dark Energy pushes things apart and determines the destiny of the universe.
Dark Matter is an old problem. It dates back to 1933 with astrophysicist Fritz Zwicky and the Coma galaxy cluster.
What Zwicky and others noticed is that sometimes galaxies are closely clustered together and moving fast—thousands of kilometers per second. So, naturally, the assumption would be that the gravity of the stars in the galaxies is what holds them all together.
However, there isn’t enough gravity in all the stars in the cluster to hold everything together. In fact, it’s off by a factor of almost 100. Zwicky posited that clusters of galaxies must be held together by the gravity of unseen “dark matter.”
American astronomer Vera Rubin brought the dark matter puzzle closer to home, where she discovered exactly the same problem in individual galaxies.
In a galaxy, the stars orbit the center of mass, and gravity keeps them in this orbit. If the gravity present was only that of the stars, then the stars further from the center should be moving slower due to the inverse square dependence of Newton’s law of gravity:
$$\frac{GMm}{r^2}=\frac{mv^2}{r}\Rightarrow v\propto\frac{1}{\sqrt r}$$
But Rubin measured a flat rotation curve for the Andromeda galaxy—stars further away from the center were moving just as fast as the central stars, and even faster in some instances. It couldn’t be that all the mass present was solely from the stars.
Galaxies themselves have to be immersed in a “halo” of Dark Matter, responsible for keeping clusters together and explaining the flat galactic rotation curves.