1. Steady State universe model

This is an old model that claims new matter is constantly being generated in the universe, but it lost its popularity now and left its place to a more popular Big Bang theory.

2. Oscillating universe

This is also not a popular model anymore. In this model, the universe begins with a Big Bang and ends with a Big Crunch. The Big Crunch causes another Big Bang and the cycle continues.

3. Stephen Hawkings’ Quantum Cosmology

This idea proposes that the universe is finite but boundless, just like the surface of a sphere. Since the spacetime is finite but boundless, there is no starting point. (Due to the warpage of spacetime).

4. Chaotic Inflation model

In this model, the universe entered a super inflation phase 10 to the -35 seconds after Big Bang. And this inflation causes another inflation, so more than one universe forms. Each universe creates other universes when they inflate, and they create their baby universes, etc. In this model, universes chaotically cause each other.

5. Quantum fluctuations in vacuum

It is known that there are quantum fluctuations in the vacuum. Particles constantly come into existence and then get annihilated.

According to this model, our universe is a quantum fluctuation that occurs in a parent universe. There are other quantum fluctuations as well, so there are other universes.

6. Unstability of vacuum energy

According to this idea, vacuum is a state where energy is unstable, and it has a tendency to create matter by spontaneous symmetry breaking, which is a known process in physics. Vacuum is a state that is ready to be broken into more stable matter and energy. In this model, since the vacuum is in a supersymmetrical structure, the breaking of this symmetry and the creation of more complex structures is almost inevitable. In this idea, the Big Bang doesn’t need to happen more than once. It may have happened only once.

7. String theory and the ‘Big Splat’

String theory, or what it turned into (M theory), is one of the most popular theories in modern physics and it is the major candidate to become the unified physics theory.

According to one explanation that originates from this theory, our universe is a 3 dimentional sub universe (4 dimentions with time) of a larger 11 dimentional universe. These pieces of the larger universe that has fewer dimensions are called ‘branes’ in this theory, and our universe is a ‘brane’.

According to this theory, Big Bang is just one brane colliding with another in the larger universe. According to calculations, this type of a brane collision can create the amount of energy that is observed in Big Bang. And these collisions can happen countless times in the larger universe. So this model also has the idea of other universes.

8. Quantum Loop Gravity Theory and the ‘Big Bounce’

According to this theory, just like the matter being modeled as discrete structures (atoms and molecules), the space and time should also be modeled by discrete units. So in this view, there are such things as ‘minimum possible distance’ and ‘shortest possible time interval’. According to this theory, when space and time are analyzed using discrete units, gravity becomes repulsive rather than attractive in very short distances. It is thought that this might explain Big Bang.

According to the cosmological theory that originates from quantum loop gravity, we shouldn’t talk about Big Bang, but we should talk about Big Bounce, since this process will repeat constantly in this model.

9. Uncaused universe

In quantum physics, it is known that there are quantum fluctuations in vacuum. So, uncaused things can happen in the quantum world. So maybe, we should change our way of thinking, what we call causality, which is something that limits our understanding and forces us to consider everything in terms or cause and effect. Maybe no cause is needed to explain the existence of the universe. This is also a possibility.

10. Timeless universe

This is also a view similar to the one above.

Famous physicist Barbour created a timeless physics. In his theory, time is not a parameter in his equations. Time is an illusion according to Barbour. This view, since it also changes the way we comprehend causality, also changes the nature of the problem. Similar to the uncaused universe explanation, this theory also eliminates the need for a cause to the universe.

Source: http://turkishatheist.net/?p=1