The Big Bang theory of cosmology hypothesizes that all matter in the Universe was created at the same time, at around 13.8 billion years ago, from the expansion of a very small ball with a very high temperature and density. It is the current leading cosmological model of the Universe and how it began. The Big Bang theory was first developed in the early twentieth century when several different deep space observations and theoretical considerations all pointed to the fact the universe is expanding. In 1912, an American astronomer named Vesto Slipher was able to measure the Doppler shift of several spiral galaxies, that were believed to be nebulae at the time, and his observations showed that almost all of the galaxies were moving away from ours (phys.org). In 1922, a Russian mathematician and cosmologist named Alexander Friedmann was able to develop what are know called the Friedmann Equations, which showed a constantly expanding universe, contrary to the static universe and Cosmological Constant that Einstein was advocating at the time. Furthermore, in 1929, Edwin Hubble, an American astronomer, was able to measure the redshift of several distant galaxies, which is occurs when an object’s light increases in wavelength, or is shifted to the red end of the spectrum. He also measured their relative distances by determining, from each of the galaxies, the apparent brightness of a type of star called Cepheids, which were stars that repeatedly dim and brighten that could be used to find how far the stars were located. He found that when redshift was plotted against relative distance, the redshift would increase linearly, which could only be explained by an expanding universe (skyserver.org). In doing so, Hubble also proved that Andromeda, which was believed to be a nebula inside the Milky Way at the time, actually was its own galaxy beyond the edges of the Milky Way because his measured relative distance was greater than the estimated diameter of the Milky Way (Vox). Finally, in 1927, a Belgian Roman Catholic priest and physicist, Georges Lemaitre, independently derived the Friedmann Equations and went on to suggest in 1931 that as you go back in time, the smaller and smaller the Universe must be, and at some point in time, the “entire mass of the universe would have been concentrated into a single point from which the very fabric of space and time originated”, which he called the “primeval atom” (phys.org). However, at around the same time, a new hypothesis began to emerge, known as Steady State Theory, in order to explain some problems in the Big Bang Theory. Steady State Theory states that, according to the University of Oregon, the universe is constantly expanding but maintain the same density because matter is being continuously created with the same rate that other matter becomes unobservable. It also maintains that the universe has always existed and never will not exist, and will remain the same forever. It began to gain a larger following in the mid 1900’s, due to a reformulation by Thomas Gold, Hermann Bondi, and Fred Hoyle, and its main purpose was to account for a few problems from the Big Bang Theory. The largest problem with the Big Bang Theory was that Hubble had calculated the universe to be nearly 1.8 billion years younger than our solar system, which is impossible. However, it was later disproved for several reasons: one was the discovery of quasars. Because quasars are billions of light years away, the way we see them today is effectively viewing what the universe looked like billions of years ago, and since the structure of quasars is differs greatly from the universe’s structure today, the Steady State Theory can’t be true because the universe has changed from what it was billions of years ago. The Cosmic Microwave Background Radiation (CMB) is, according to The Scientific American, “a faint glow of light that fills the universe, falling on Earth from every direction with nearly uniform intensity” (scientificamerican.com). It is now known to be residual heat from the Big Bang and is the oldest light we can see, more than 14 billion years old. It was first predicted in 1948 by Robert Herman and Ralph Alpher, but was first discovered on May 20th, 1964, by two American radio astronomers named Robert Wilson and Albert Penzias. At their location at Bell Telephone Laboratories in New Jersey, the laboratory’s Horn Antenna had picked up an unexplainable buzzing noise from every direction in the sky at all times. The two astronomers originally believed it was some type of interference, and tried to eliminate all possible sources of it, including cleaning pigeon droppings and catching the pigeons nesting in the area. When even this did not remove the buzzing, they came to the conclusion that this must be some type of cosmic microwave radiation. Now, because CMB was formed only a short time after the Big Bang, it is very important to astrophysicists who want to learn more about what the universe was like immediately after the Big Bang, before the formation of stars and galaxies. Now we know that it was still a “soup” of atomic nuclei and electrons, colliding with photons in the background radiation, which at that time, had a temperature of over 3000 K. After a considerable amount of expansion, the CMB cooled enough for the electrons and nuclei could combine to form atoms (skyserver.org). Very precise measurements of CMB has now found it to be 2.72548±0.00057 K, and is very close to being uniform in all directions. This high degree of uniformity in the observable universe strongly supports the Big Bang Theory. There are many pieces of evidence that support the Big Bang, but some of the most important are cosmological redshift, CMB, and the abundance of light elements. Cosmological redshift supporting the Big Bang Theory is based on the fact that as light travels through the expanding universe, its wavelength is also stretched out. Therefore, because we know that the speed of light is constant, the amount of observed redshift can be used to calculate the distance of the object. Hubble’s observations found that redshift was pretty much proportional to distance, which gives strong evidence for the expanding universe. CMB also strongly supports the Big Bang Theory because it was hypothesized that if there were a Big Bang, the expanding universe would have cooled and stretched the very high-energy radiation of the newly formed universe into the microwave part of the electromagnetic spectrum, and lower the temperature from about 3000 K to about 3 K. Finally, the abundance of light elements in the universe (i.e. Hydrogen and Helium) that were formed from free neutrons and protons in the early universe also supports the Big Bang Theory. Because the universe expanded very quickly and therefore cooled quickly, light elements were able to form, and heavier elements didn’t have the chance to until later, in stellar cores and supernovae. Cosmological inflation was a theory first proposed by Alan Guth in 1971 in order to explain the creation of the large scale structure of the universe. The theory hypothesizes, in the early universe, an exponential expansion of space that lasted from 10?36 seconds after the Big Bang singularity to between 10?33 and 10?32 seconds after the singularity. After this inflationary period, the universe has continued expand, but at a slower rate. This resolves two important issues with the early Big Bang Theory, the horizon problem and the flatness problem. Dark matter and dark energy are incorporated into the theory because the estimated 13.7 billion years since the universe’s creation is not enough for many of the huge structures of the universe today to form. This could only be possible if there were much more matter in the universe than our estimates predict, which leads to the speculation of the existence of “dark matter” which is an unknown substance that does not emit heat, light, radiation, or radio waves. It is estimated that dark matter must make up around 85% of the mass making up galaxies, which is nearly exactly the additional amount of matter needed to allow the huge structures of today’s universe to form (physicsoftheuniverse.com). Dark energy is a hypothesized unknown form of energy that permeates through all of space, while also tending to make the expansion of the universe faster, which helps explain many observations and calculations since the 1990’s that indicates that rate at which the universe expands is accelerating. According to the best current measurements, 68.3% of the total energy in the current observable universe is dark energy.