The universe can be defined as the entirety of space, time, matter, and energy that exists. It includes all celestial bodies, such as planets, stars, galaxies, and the vast expanse of empty space between them.
The origins of the universe are still not completely understood, and several theories attempt to explain its beginning. The most prominent theories that explain the origin of the universe are:
1. Big Bang Theory: This theory suggests that the universe originated from a singularity, a tiny, hot, and dense point, approximately 13.8 billion years ago. It proposes that the universe rapidly expanded and continues to expand to this day.
2. Inflationary Theory: Based on the Big Bang Theory, the inflationary theory proposes that the universe experienced an extremely rapid expansion just moments after the initial singularity. This rapid expansion helps explain the uniformity and flatness of space observed at large scales.
3. Multiverse Theory: This hypothesis posits that our universe is just one of many in a vast multiverse. Each universe within the multiverse may have its own set of physical laws, leading to the creation of different universes with different properties.
4. Steady State Theory: This theory offers an alternative to the Big Bang model, suggesting that the universe has always existed in a steady state, with a continual creation of matter to maintain a constant average density.
5. String Theory: A branch of theoretical physics, string theory proposes that the fundamental building blocks of the universe are not point-like particles but rather tiny, vibrating strings. It also suggests the existence of extra dimensions beyond the four we perceive.
6. Ekpyrotic Theory: According to this hypothesis, our universe originated from a collision between two branes (sheets of multidimensional space). The collision released energy, leading to the formation of our universe.
7. Cyclic or Oscillating Universe Theory: This concept suggests that the universe goes through cycles of expansion and contraction. After each contraction, the universe rebounds and undergoes a new phase of expansion, leading to an endless cycle of cosmic activity. According to this idea, the universe could have gone through multiple Big Bangs and subsequent collapses, with each cycle lasting for billions of years.
8. Quantum Fluctuation Theory: Quantum mechanics suggests that at the smallest scales, particles and energy can appear and disappear randomly. Some theories propose that the universe originated from a quantum fluctuation, where a small patch of space rapidly expanded to form our universe.
9. Cosmic Creation Through Black Holes: This idea suggests that black holes could serve as “seeds” for the birth of new universes. Inside a black hole, the extreme conditions could trigger the formation of an entirely new universe.
10. Simulated Universe Hypothesis: Drawing inspiration from virtual reality and computer simulations, this theory suggests that our universe could be an elaborate simulation created by an advanced civilization or higher-dimensional beings.
11. Eternal Inflation Theory: This hypothesis suggests that our universe is just one of many bubble-like “universes” embedded in a larger “multiverse.” It proposes that the universe undergoes eternal inflation, where new universes continuously emerge and exist alongside our own.
Summary
The Universe is the entirety of space, time, matter, and energy that exists. It is a vast expanse that contains galaxies, stars, planets, and all forms of matter and energy. The reason why the Universe exists is a question that has puzzled humans for centuries. According to some perspectives from physics, the Universe exists because it is formally inevitable that it does, given the laws of physics that we perceive in it[1]. The philosopher Bertrand Russell took the view that the Universe is just there, and that’s all, and that it is a brute fact that does not have an explanation[2]. Others believe that the Universe exists because it is good that it exists[3]. While there is still much that remains unknown about the Universe, we do know that it exists with such properties that an intelligent observer could have possibly evolved within it[5].
Citations:
[1] https://writings.stephenwolfram.com/2021/04/why-does-the-universe-exist-some-perspectives-from-our-physics-project/
[2] https://earthsky.org/space/why-does-universe-exist/
[3] https://philosophy.fas.harvard.edu/files/phildept/files/parfit_-_why_does_the_universe_exist.pdf
[4] https://www.reddit.com/r/space/comments/37jkl0/how_and_why_is_the_universe_here/?rdt=54401
[5] https://bigthink.com/starts-with-a-bang/we-exist/