Big Bang

The universe expanded from a very high density and high temperature state. The instant of the Big Bang is associated with a singularity, a point of infinite density. Georges Lemaitre, who is considered to have proposed what became known as the Big Bang theory, referred to this initial states as a "primeval atom" or "cosmic egg".  Measurement on the rate of the universe's expansion, indicate that the universe, is about 13.8 billion years old. The age of the universe, would be the time that has passed, since the Big Bang. Current cosmological models of the Big Bang are well supported by astronomical observation. 

In 1915, Albert Einstein published his general theory of relativity, which relates gravitation to the geometry of 4-dimensional spacetime. In 1917, the first cosmological models were produced, based on general relativity. Einstein himself, wanted to remain consistent with a the "Steady State" idea of the universe. Albert Einstein's 1917 model of a static universe, however, will prove to be inconsistent with general relativity. The problem was that gravity, would cause a static universe to contract. Einstein introduced something known as a cosmological constant into his equations, to account for this contradiction, and achieve a static or steady state universe. The hope by Einstein was that the cosmological constant would hold gravity back, to achieve a static universe. Einstein's model of a static universe, however, will become abandoned, upon Edwin Hubble's observation that the universe is expanding.

In 1922, the prediction will come from a Russian mathematician, that the universe is not static, and must be either expanding or contracting. Alexander Friedman, pioneered a theory of an expanding universe, and, in-so-doing formulated his "Friedman equations", to describe the expansion of space.

In 1927, Georges Lemaitre, will also propose a theory of an expanding universe. 

Lastly, Arthur Eddington, will prove that Einstein's model of a static universe, is not stable. 

Our understanding that the universe is expanding, largely came from work by two people: Vesto Slipher and Edwin Hubble.

Vesto Slipher, proved the empirical basis for an expanding universe. Slipher observed 'recessional velocities' of galaxies. These are the rates at which astronomical objects move away from us. Slipher was the first to observe galactic redshifts. A redshift, is when, electromagnetic radiation, is increased in wavelength, hence, moved more toward the red end of the visible light portion of the spectrum of electromagnetic radiation. In 1912, Slipher observed that most spiral galaxies, had a redshift. This was how these recessional velocities were measured, by these shifts in spectral lines.

These ideas will be combined with those of Edwin Hubble, who found the relationship between the redshift of a galaxy, with it's distance away from Earth. The implication was that: the farther away two galaxies were, the faster they were moving apart from one another. Thus, the redshift, of an observed galaxy, could be interpreted as it's relative velocity away from Earth. This relationship is to be known as Hubble's law, and it is our first real observational basis for an expanding universe. Hubble showed that the universe is expanding and that means that it was once smaller and denser. 

The abundance of light elements.

Some of the first atomic nuclei was formed, about, within the first 3 minutes of the Big Bang. However, it took thousands of years before the first electrically neutral atoms were formed after the instant of the Big Bang. The Big Bang produced:

• Hydrogen

• Helium

• and some traces of Lithium

Gravity, worked to bring these clouds of light elements together into stars, where the heavier elements will be made, via, stellar nucleosynthesis. 

The CMB or cosmic microwave background:

The CMB is the electromagnetic radiation that is left over from an earlier stage in the history of the universe. This is also, the oldest electromagnetic radiation in the universe. This confirmed that the universe was significantly hotter in the past. This radiation can be dated to the epoch of recombination. Recombination was the epoch, where the first electrons and protons combined to form the first electrically neutral hydrogen atoms. The recombination epoch took place about 378,000 years after the Big Bang. The CMB was discovered by Arno Penzias and Robert Wilson in 1964. 

The large scale structure of the universe:

The structure and large scale of the universe is organized into a hierarchical structure:

Stars < Galaxies < Galaxy groups < Galaxy clusters < Superclusters < Filaments (separated by voids) that makes up the "cosmic web". 

Star - A gravitationally bound, luminous sphere of plasma.

Galaxy - A gravitationally bound system of stars (and other stellar material).

Galaxy groups - An aggregation of about 50 or fewer galaxies.

Galaxy clusters - A gravitationally bound system of anywhere to hundreds to thousands of galaxies.

Superclusters - A large group of galaxy groups or galaxy clusters.

Filaments - These are the largest structures in the universe and also consists of gravitationally bound galaxies. They are massive, thread-like structures. They are estimated to be about 50 to 80 megaparsecs. 

Void - These are the vast spaces that exist between the filaments. Voids contain few, or no galaxies.

Hubble's law

This is the property where: the recessional velocity, or the velocity that an object is moving away from the Earth, increases, as it moves farther away from the Earth. The farther they are from us, the faster they are moving away from us. This began with Hubble's observation that faraway galaxies' distance to us, had a special relationship to their redshifts. The conclusion could be drawn that the universe was expanding, at a constant rate.