“Our whole universe was in a hot, dense state . Then nearly fourteen billion years ago expansion started, wait”
The Big Bang model is the most widespread and popular theory of the development of the universe. Its fundamental feature is the creation of the universe from a state of superhigh temperature and density that happened 13.8 billion years ago.
Evidence
Cosmologists made several presumptions about the universe relying on theory and observation. The first one is that the laws of physics are universal and remain unchanged with time or location in space. The second one is that the universe is homogenous, or about the same in every direction (but not necessarily the entire time). The last one is that we, as humans, do not observe universe from an exclusive location such as its very heart.
When these assumptions are implemented into Albert Einstein’s equations, they suggest that the universe has some universal properties – it expands, it emerged from a superhot dense state, its lightest elements were created first and it has an electromagnetic radiation (cosmic microwave background) that fills it.
If any of the primary assumptions about the universe are wrong, the Big Bang theory could not explain the characteristics and properties of the universe. Could it be possible that a Big Bang never actually happened?
Steady State Theory
One alternate theory of the development of the universe is the Steady State model. Emerging as one of the first opponents to the Big Bang theory, Steady State postulates continuous creation of matter all over the universe to explain its discernible expansion. This sort of universe would be infinite, without a beginning or an end. Be that as it may, the abundance of evidence and data collected since the mid-1960s successfully disproves this theory to this day.
Bouncing Cosmology
Better known as the Big Bounce theory agrees with the accelerated expansion of the universe proposed by the Big Bang theory. However, the Big Bounce states that the universe has been subjected to a state of expansion and compression, one after another recurrently, bouncing each time it contracts to a certain volume. Some suggest that the universe only bounced once. In this model, the cosmos had been shrinking before the bounce, since the innumerable past, and it will expand for the rest of the time.
Eternal Inflation
Another interesting alternative is the Eternal Inflation theory. Following the Big Bang, the universe expanded rapidly during a short period called inflation. The Eternal Inflation theory, on the other hand, presupposes that inflation never stopped, but continued to happen for an infinite timespan. Out there, somewhere, even today, new universes are being created in an immense complex called the multiverse. What is more, those numerous universes could have quite different physical laws from the ones we know.
The Cyclic Model
Also known as the Oscillating model of the universe implicated an endless streak of Big Bangs, later on accompanied by Big Crunches that reignited the cycle, perpetually. The Steinhardt–Turok cyclic model includes two parallel orbifold planes (also known as M-branes) that collide at intervals in a higher-dimensional space. In this theory, the discernable four-dimensional universe rests on one of these branes.
The Black Hole Theory
The Black Hole theory says that our universe came from a black hole of another universe. Therefore, we are living outside its event horizon. In an alternate version of the theory, the observable universe is the inside of a black hole subsiding as one of potentially many within a larger parent universe, or multiverse.
In this respect, we can claim that a new universe lies behind every black hole. Given that we cannot pass the event horizon of a black hole, there is no way to clearly prove or disprove this theory.
Merely a Simulation
Some insinuations found in string theory and quantum gravity persuasively suggest the universe is not at all as it seems. It is possible that the entire universe is just a flat hologram projected onto a spherical surface. Others suggest it could be an entirely digital simulation running in an enormous computer.
Even though the Big Bang theory holds the most generally accepted explanation for of the development of the universe, one must also realize that nothing in science is forever and what was once purely fiction is common knowledge nowadays. Who knows what the future holds?
[1] Tate, Karl. “Alternatives to the Big Bang Theory Explained (Infographic)” Space.com [online] Available at: www.space.com/24781-big-bang-theory-alternatives-infographic.html [Accessed on: 10 Jun. 2020]
[2] Vyas, Kashyap. “5 Alternatives to the Big Bang Theory.” Interesting Engineering [online] Available at: interestingengineering.com/5-alternatives-to-the-big-bang-theory [Accessed on: 10 Jun. 2020]