The universe is the total space and time that exists, including all matter and energy, as well as the physical laws and constants that describe them. The universe is about 13.8 billion years old and has gone through many changes over that time. From a high-energy, hot universe to the dark ages it experienced, stars continued to grow larger as time progressed. The most important change was when the universe went from being a chaotic mess to becoming ordered and stable under the influence of fundamental forces, such as gravity and the weak nuclear force. The universe went from a cake batter of particles and radiation to the one we experience today. Many theories explain how our unique existence comes about, including the big bang theory, the multiverse theory, the cyclical universe theory, and the steady state theory.
According to the National Aeronautics and Space Administration, the big bang theory states that the universe was once very dense, hot, and extremely smooth. As it expands, and the universe continues to grow older, its structure becomes less dense; the fabric of space and time becomes less smooth; matter starts to clump together and form galaxies; stars, planets, and eventually life emerge (Dumančić, 2021). Today, the hot, dense universe of the past is referred to as the initial state, also known as the concave-in universe, and the large collection of galaxies and particles that exist today is called our observable universe, also known as the convex-out universe. The big bang theory states that our current universe will continue to expand until it eventually stops expanding and collapses back in on itself (Dumančić, 2021). This collapse would result at the end of everything we know and a new big bang.
Despite that, in the multiverse theory, multiple universes exist alongside our own. They may appear identical to ours but could be slightly different due to random chance or other factors that can affect a universe’s size and matter distribution (Cleaver, 2019). The multiple universes may have started at different times, but the one we observe is the same age as our universe. All these universes are part of a multiverse, a collection of many universes. They are constantly interacting with ours, and that could be the reason we experience the heat and motion of the universe (Cleaver, 2019). Colliding bodies in certain universes could cause some collisions to travel to our universe, allowing us to view objects that are normally not visible from our perspective. Each universe has its laws of physics, stars, planets, and life. There may be millions of other universes that are too far away for us to see but exist all the same.
Also, in the cyclical universe theory, it is believed that the universe will expand and then contract with no end in sight. It could start expanding or contracting, depending on how its universal force is distributed after a hypothetical big bang (Ahmed & Alamri, 2019). The cycle could be more complex than that; the universe could start expanding and then close in on itself. Then, it could expand again for some time before contracting again. Many scientists believe that the universe is expanding and that the universe has already expanded, with no end in sight. As time progresses, the forces of gravity are seemingly stronger and stronger along with time. In addition, it is thought that all this energy expands outward like a bubble as our universe expands. This process would continue forever with no end in sight (Ahmed & Alamri, 2019). So, from our perspective, we would not see anything unique about now that makes it different from the past or future.
Additionally, in the steady state theory, the universe has existed forever and continues to expand. New material is always being created as the result of quantum fluctuations (Cummings, 2019). Another possibility is that new matter is created in space-time at a rate that balances out the old matter as it dies out. A steady-state universe continues to expand because new particles are being created at a constant rate to replace particles that have decayed (Cummings, 2019). Also, it explains how the universe is expanding, and yet stars in the sky seem to remain the same size. New galaxies are constantly being created as old ones die out. As new galaxies are created, more matter is made to replace that which has been destroyed by old age or other forces. That is why the universe appears to be expanding.
Many contrasting theories, including the big bang theory, the multiverse theory, the steady state theory, and the cylindrical universe theory are discussed, and they all agree that the universe is either expanding or has already expanded. It could be any other of these four theories based on observations made about our universe which has evolved through time and into its current state. For now, we have learned to live in a universe that has gone through many changes and continues to change as time progresses.
Ahmed, N., & Alamri, S. Z. (2019). A cyclic universe with varying cosmological constant in f (R, T) gravity. Canadian Journal of physics, 97(10), 1075-1082. https://cdnsciencepub.com/doi/abs/10.1139/cjp-2018-0635
Cleaver, G. (2019). Multiverse Theories: Philosophical and Religious Perspectives. In Oxford Research Encyclopedia of Religion. https://oxfordre.com/view/10.1093/acrefore/9780199340378.001.0001/acrefore-9780199340378-e-157
Cummings, S. (2019). The steady state theory: Recalibrating the quiddity of ambient music. Music Beyond Airports: Appraising Ambient Music, 83-118. https://search.informit.org/doi/pdf/10.3316/informit.692928639966528
Dumančić, D. (2021). Intertextual elements in The Big Bang Theory (Doctoral dissertation, Josip Juraj Strossmayer University of Osijek. Faculty of Humanities and Social Sciences. Department of English Language and Literature). https://repozitorij.unios.hr/islandora/object/ffos:5906