The Big Bang Theory: One of the most popular and widely accepted cosmological explanations of how the universe began.
According to the Big Bang theory, the universe was born from a singularity with infinite density and temperature.
It expanded from that point and cooled resulting in the formation of atoms, stars and galaxies.
Inflationary Universe: According to this theory, the universe went through a period of rapid inflation and expansion in the moments after the Big Bang.
This is why the universe appears flat and why the cosmic microwave background radiation is uniform.
Quantum fluctuation model: This theory suggests that the universe began from a quantum fluctuation in the vacuum.
Emergent universe model: These models suggest a low-activity, past-eternal era before the Big Bang.
Meaning that there is a possibility that the universe was stable before the Big Bang and then transitioned into the expanding universe we observe today.
Cyclic models: This model proposes that the universe is going through a process of continuous expansion and contraction. Each cycle ends with a big crunch and begins with a 'big bang'.