The idea of a cyclic universe, also known as an oscillating universe or a recurring universe, proposes that the universe undergoes a series of cycles of expansion and contraction, with each cycle giving rise to a new "big bang" and subsequent expansion. While the concept of a cyclic universe is intriguing, there are several reasons why it is considered implausible based on our current understanding:
Second Law of Thermodynamics: The second law of thermodynamics states that the entropy, or disorder, of a closed system tends to increase over time. In each cycle of a cyclic universe, the universe would go through a contraction phase, leading to a highly compressed and disordered state. The subsequent expansion phase, which resembles a new big bang, would result in a low entropy, highly ordered state. The ability of the universe to reset its entropy to initiate a new cycle is challenging to reconcile with the second law of thermodynamics.
Expansion Acceleration: Observations from the late 1990s provided strong evidence that the universe's expansion is currently accelerating. This acceleration is attributed to a mysterious entity called dark energy. If dark energy exists and continues to drive the expansion, it would prevent the universe from collapsing and make it increasingly difficult for a cyclic model to occur.
Singularities and Cosmic Microwave Background: Singularities are points of infinite density and curvature that are predicted by general relativity within black holes and during the initial singularity of the big bang. In a cyclic universe, each cycle would begin with a singularity, which is difficult to explain and understand within our current understanding of physics. Furthermore, the observations of the cosmic microwave background radiation, which provides strong evidence for the hot, dense early universe, are consistent with a single inflationary event rather than multiple cycles.
Baryon Asymmetry and Dark Matter: The observed imbalance between matter and antimatter in the universe, known as the baryon asymmetry, poses a challenge for a cyclic model. Each cycle would create an equal amount of matter and antimatter, leading to a cancellation and the absence of matter in the subsequent cycles. Additionally, the existence of dark matter, which is necessary to explain various astrophysical observations, is not easily accommodated in a cyclic model.
While the cyclic model has been explored in theoretical physics, these and other challenges make it currently implausible based on our understanding of fundamental physics and observations of the universe. However, it's important to note that scientific knowledge is continually evolving, and future discoveries could potentially lead to new insights or modifications to our understanding of the universe's evolution.