In the second part of my brief history of the universe, I will cover the evolution of the overall properties of the universe.

## The Constituents of the Universe

As I stated in the first part, the presence of energy (including mass) affects the shape of space and time, allowing that shape to change. Just as smoothing out the details lets us deduce the options for the global geometry of the universe, smoothing out all the details of where energy is lets us see how that energy affects the global shape of the universe. The average energy density of the universe won’t cause any local effects on shape, so the only thing it can do is cause space to expand or contract. How this occurs is governed by the Friedmann equations.

It turns out that different types of energy affect the evolution of the universe in different ways. Everything we know about can be broadly categorized into four types of energy:

- Nonrelativistic energy (matter): basically all its energy is from mass. Its density just tracks the expansion or contraction of the universe in the most naive way possible: Double the volume, halve the density. Double the linear size, reduce the energy density by 2x2x2=8
- Relativistic energy (radiation): basically all its energy is from momentum. Its density tracks the expansion or contraction but it is also redshifted or blueshifted. In this case, if you double the linear size, the energy density is reduced by a factor of 16.
- Curvature: Not really an energy density, but acts like one. In this case, if you double the linear size of the universe the effective energy density is reduced by a factor of 4.
- Cosmological constant: An allowed term in Einstein’s field equations. Can be interpreted as a constant energy density throughout the universe. More typically called “dark energy.”

Matter and radiation tend to slow expansion or even cause accelerating contraction, curvature will tend to lead to a constant rate of expansion/contraction, while dark energy will lead to exponential growth (positive energy density) or contraction (negative energy density). What happens depends on the amounts of these different species.

## Our Universe

Recent measurements from the Planck satellite tell us that currently, the energy content of our universe is around 68% dark energy, 32% matter, with tiny amounts of radiation and no measurable curvature. These percentages change since the densities of the types of energy change in different ways with respect to changes in the size of the universe (ignoring for now the fact that something can change from one type of energy to another). Plugging this into the equations of general relativity, we get the following overall history of the universe:

- The early universe was radiation dominated for around the first hundred thousand years
- Radiation was then replaced by matter as the dominant form of energy
- Dark energy became dominant fairly recently – a few billion years ago. The expansion of the universe has been seen to be accelerating, a telltale sign of dark energy.

## The Even Earlier Universe

These models only work so long as we understand the physics involved. At some point, we need to merge general relativity with quantum field theory to make sense of what’s going on. There is no working theory of quantum gravity yet. String theory is the main candidate but a working string theory describing our universe hasn’t yet been found. Once quantum gravity is needed we can basically only guess. So, the Big Bang isn’t necessarily the beginning of the universe. It’s just the start of where physics we understand applies.

There are some hints of things that occurred before the radiation dominated era. Recall that we have a finite horizon. If we look toward the horizon opposite directions, what we find is that the universe seems too uniform. The temperature of the relics of the early universe is nearly the same everywhere and looks characteristic of a system at thermal equilibrium, yet particles near the horizon in opposite directions are outside each others’ horizons. They cannot have ever been at thermal equilibrium at any time that we can observe.

One way to solve this is to say that those points really were at thermal equilibrium in the very early universe. The universe then must have expanded so fast (even faster than the speed of light – this is allowed since nothing is ever locally moving faster than the speed of light, things aren’t really moving but space is dragging everything along) that the horizon actually shrunk. This extremely fast expansion is known as inflation and was proposed by Alan Guth and Andrei Linde. One model for inflation proposes that at some point in the very early universe a field giving a constant energy density (like dark energy) dominated, leading to an exponential expansion by many orders of magnitude over a tiny fraction of a second. The expansion made the observable universe appear very flat and uniform, regardless of the initial conditions. This field, the inflaton, then mysteriously turned off, leading to the standard cosmological history briefly described above. So, we just add an inflationary step prior to the radiation dominated era ensuring the apparent flatness and uniformity of the universe.

## The Fate of the Universe

From the Friedmann equations, we can see that there are several possibilities for what will happen in the end. If there is too much matter and radiation, the observable universe will slow down its expansion and eventually contract back to a point (the Big Crunch). In some cases, instead of a crunch, a Big Bounce occurs, where the size oscillates indefinitely. If conditions are just right, the universe’s expansion will just level off to a constant value. In this case, everything just keeps getting further apart and the universe more or less cools off and everything dies (Big Freeze). If dark energy continues to dominate, the universe may start expanding exponentially, as if it reenters an inflationary stage. In this case, the horizon eventually ends up shrinking to nothing. At some point the universe will expand so fast that everything will be torn apart (the Big Rip).

In each of these cases, our understanding of physics will eventually break down, leaving room for wild speculation of things like the generation of new universes, the decay of a metastable vacuum to another state, colliding branes (higher dimensional objects) and more.