The History of The Universe by David H. Lyth (Part II)

The observable Universe is expanding, and we can use Cosmic Microwave Background (CMB) to observe that. We can see through this that it's almost homogenous and remains so as we go back in time. The word expansion here means any two pieces of CMB are moving apart, and the process is isotropic (the same in all directions). The distance between any two pieces of the CMB is proportional to a scale factor, which is the same for any two measured pieces. 

Each object moves through the CMB with a velocity called peculiar velocity. Every galaxy has its peculiar velocity, and the distance between them is proportional to the scale factor. The distance between two galaxies is almost exactly proportional to their distance. And because of this, they stay so far apart that the peculiar velocities don't matter to us. 

Some people use the gas from galaxy clusters (aka cosmic gas) to measure the Universe's expansion. Like CMB, it's also homogeneous. However, galaxy clusters didn't always exist, and before they did, the Universe was nearly all homogenous gas expanding isotropically. 

To understand further, we need to know how cosmology describes density. In physics, it is the amount of something in a small area divided by the region's volume. Destiny on its own means density of mass. But in cosmology, we're concerned with the energy density, number density (number of particles) and the density of conserved quantities. 

Let's look at the table below to see what happened during the history of the Universe.

As the Universe expands, the distance between the particles increases, decreasing their energy because they don't collide as much. After about 1 second, collisions are so unlikely that the first stage is already over. After this, 30 seconds later, the photon's energy becomes less than the electron, and the reverse process removes all of the positrons and electrons. This leaves only 1 electron per photon. After another 5min, the neutrons combine with protons to form nuclei. Almost all neutrons are now inside the nuclei, which make up about 25% of the ordinary matter. The other 75% comes from single protons (aka hydrogen nuclei). Then, nothing happens for about 500,000 million years. Then, finally, electrons combine with the nucleus to form a helium or hydrogen atom. After atom formation, the photons existing are seen as Cosmic Microwave Background (CMB). 

While all that is happening, the Universe is nearly homogenous. After 100 million more years, some regions begin to collapse under the force of gravity into themselves, forming the first galaxies and the Universe we know and observe.