Cyclic model

A cyclic model, also known as an oscillating model, is a cosmological theory about the history of our universe. Instead of the universe having a single beginning and a single end, these models suggest it follows infinite, self-sustaining cycles. In these scenarios, the universe undergoes a repetitive process of expansion and contraction. This allows for a universe that could exist indefinitely through an eternal series of oscillations. Scientists study these models to understand if the cosmos has a beginning or if it has always been repeating.

One of the earliest versions of this idea involved a process called the Big Crunch. In this mechanism, the universe begins with a Big Bang and expands for a long period. Eventually, the gravitational attraction of all the matter in space becomes strong enough to pull everything back together. This causes the universe to collapse inward. When the collapse reaches a certain point, it undergoes a "bounce" that triggers a new Big Bang. This cycle then repeats, creating a continuous loop of expansion and contraction.

There are several different types of cyclic models proposed by physicists. One is the Steinhardt–Turok model, which is based on brane cosmology. This theory suggests that our four-dimensional universe exists on one of two parallel planes called M-branes. These branes exist in a higher-dimensional space and periodically collide with one another. Another version is the Baum–Frampton model, which relies on a concept called phantom energy. There are also other theories, such as conformal cyclic cosmology, which relies on general relativity, and loop quantum cosmology, which involves a "quantum bridge" between different stages of the universe.

Humans have been exploring these possibilities for over a century. In 1922, Alexander Friedmann introduced the Oscillating Universe Theory. By 1930, Albert Einstein was also briefly considering a cyclic model as an alternative to an expanding universe. However, in 1934, Richard C. Tolman identified a major obstacle known as the cyclic problem. He used the second law of thermodynamics to show that entropy, or disorder, must always increase. This meant that each cycle would necessarily become longer and larger than the last. If you traced this backward, the cycles would become smaller and shorter, eventually leading back to a single Big Bang rather than an infinite loop.

Modern science has found new ways to address the issues Tolman discovered. In 2011, a massive survey of 200,000 galaxies spanning 7 billion years of cosmic time confirmed that dark energy is driving the universe apart at accelerating speeds. This discovery provided new hope for consistent cyclic cosmologies. For example, the Steinhardt–Turok model avoids the entropy problem because it results in a net expansion during each cycle. This prevents entropy from building up in a way that would break the cycle. The model suggests that after billions of years of expansion, the universe will eventually begin to contract again.

Some models use very specific, extreme numbers to explain how a cycle can restart. The Baum–Frampton model proposes that a special type of dark energy called phantom energy causes a "Big Rip." However, just a septillionth of a second (or $10^{-24}$ seconds) before this happens, a turnaround occurs. Only one tiny "causal patch" of the universe is kept. This patch is empty, containing no quarks, leptons, or force carriers. Because this tiny patch contains only dark energy, its entropy vanishes. This allows the universe to "come back empty," which helps avoid the problem of too many black holes or structures forming during a contraction phase.

These theories connect many different fields of physics, including string theory and general relativity. While the Steinhardt–Turok model uses the idea of colliding branes, it is still being studied by string theorists. Another perspective, proposed by Nikolai Gorkavyi, looks at how black holes might influence these cycles. This idea suggests that the merger of black holes converts about 5% of their mass into energy through gravitational waves. This process could change the balance between gravity and antigravity forces, helping to drive the expansion and compression of the universe in each cycle.