Cosmology is the study of how the universe began, how it continues to exist, and how it will end.

Science brings forth a cosmological theory from scientific evidence that the universe once began to expand and continues to expand until today. This theory was eventually called the Big Bang theory or Big Bang model. Astronomers George Lemaître and Edwin Hubble were some of its first proponents.

Key Stages of the Big Bang model :

a. The universe may have begun as an infinitely hot and dense initial singularity, a point with all of space, time, matter and energy. This means that there was no where, when or what. There is no space around the singularity – just nothingness.

b. All of it then began to rapidly expand in a process known as inflation. Space itself expanded faster than the speed of light. In this still hot and dense mass of the universe, pairs of matter and antimatter (quarks and antiquarks) were formed from energy, but these pairs cancelled each other back into energy (annihilation).

c. The universe cooled down as it expanded. An excess of matter (electrons, protons, neutrons and other particles) somehow came to be in a highly energetic “plasma soup.” Photons (light particles) were being scattered everywhere in this “soup”. Protons and neutrons came together to form different types of nuclei by nucleosynthesis or nuclear fusion.

d. Much later on, electrons started to bind to ionized protons and nuclei forming neutral atoms in a process called recombination. The bound particles no longer scattered photons so light and energy moved freely across space. The period was hence known as the “dark ages”.

e. Gravity caused these atoms to collapse onto one another to form stars and galaxies and eventually, other matter. This still happens until today. Space also continues to expand at an accelerating rate, thus increasing the distance between the matters inside it.

Using different instruments can enable us to detect the light of other galaxies.

This light is found to be redshifted (the light looks “stretched”). This suggests that other galaxies are moving farther away from ours. It was later determined that they are not moving away. Instead, space itself is expanding in all directions causing all the galaxies to be relatively farther apart. From this “redshift” we learn how fast the universe is expanding. Redshift is the first piece of evidence for the Big Bang model.

During nucleosynthesis, free protons and neutrons would routinely combine and separate from each other due to the high energies at the time.

Compositions of the nuclei :

D = 1 p+, 1 n T = 1 p+, 2 n 3He = 2 p+, 1 n
4He = 2 p+, 2 n 7Be = 4 p+, 3 n 7Li = 3 p+, 4 n

D and T are isotopes of hydrogen namely deuterium and tritium, respectively.

Due to the rapid cooling due to expansion, nucleosynthesis ground to a halt after the Big Bang occurred. This left us with mostly H isotopes (p, D and T), He isotopes and a very tiny bit of other elements like Li. The relative abundance of He and H did not change much until today.


Significant nuclear reactions during Big Bang nucleosynthesis

The relative abundance of light elements in the universe is the second piece of evidence to prove that the Big Bang occurred. Through measurements, we find that around 24% of the universe’s ordinary matter is currently comprised of helium, about 74% hydrogen, and 2% of other elements. These figures only make sense if nucleosynthesis in the Big Bang model actually occurred since no chemical process significantly changes these percentages.

Third part of evidence for the Big Bang model: cosmic microwave background, or the energy (thermal radiation) that was left as a result of recombination. Atoms became neutral due to the binding of nuclei and electrons. The remaining radiation then began to scatter. This is seen by scientists as a faint microwave glow not emitted by any object in space.

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