Cyclic model

The universe might move in cycles. It grows big and then gets small. This could happen over and over.

One idea says the universe starts with a big burst. Then it grows for a long time. Next, it pulls back in. It gets small again.

This pulling back is caused by a force. This force acts like a magnet. It pulls everything together.

After it gets small, it starts to grow again. This makes a new burst. It is like a giant wave. This could go on forever. The universe might never stop its cycles.

Some scientists think the universe moves in cycles. This is called a cyclic model. In this idea, the universe repeats itself forever.

One old idea was the oscillating universe theory. It was studied by Albert Einstein. This theory says the universe starts with a Big Bang. Then, it grows larger and larger. Later, gravity pulls everything back together. This is called a Big Crunch. Then, the universe bounces and starts again.

But there was a problem with this idea. A rule called entropy says things change in one way. This would make each cycle bigger than the last. This made the math hard to fix.

New ideas use dark energy to solve this. Dark energy is a force that pushes the universe apart. One model uses things called branes. These are flat parts in a big space. When two branes hit each other, a new universe starts. This might happen over and over. Another idea uses phantom energy. This model says the universe comes back empty to start again. Scientists are still studying these big questions.

A cyclic model is a way to imagine the universe. It suggests the universe follows infinite, self-sustaining cycles. Instead of one beginning and one end, it repeats. Some theories say the universe expands and then collapses. This happens through a series of oscillations. Each cycle starts with a Big Bang. Then, it ends with a Big Crunch. The universe expands for a long time in between. Eventually, gravity pulls everything back together for a bounce.

How these cycles work can be quite complex. In one model, two flat planes called M-branes collide. These planes exist in a higher-dimensional space. Our visible universe lives on one of these branes. When the branes hit, a new universe is born. This collision causes a reversal from contraction to expansion. It turns a Big Crunch into a new Big Bang. This process might happen repeatedly over a very long time.

Scientists have studied these ideas for many years. In 1922, Alexander Friedmann introduced the Oscillating Universe Theory. Albert Einstein also considered a cyclic model in 1930. Later, Richard C. Tolman studied these ideas in 1934. He found a problem called the cyclic problem. He used the second law of thermodynamics to explain it. This law says that entropy, or disorder, can only increase. This meant cycles would grow longer and larger each time.

Newer ideas use dark energy to solve these old problems. In 2011, a survey of 200,000 galaxies provided new clues. It showed that dark energy drives the universe apart. This happens at accelerating speeds. In 2001, Paul Steinhardt and Neil Turok proposed a brane model. They suggested the collisions explain certain energy levels. Another model was proposed in 2007 by Lauris Baum and Paul Frampton. This model uses a special type of phantom energy.

These models help us connect big ideas. They link the start of the universe to physics rules. For example, the Baum-Frampton model says the universe "comes back empty." It suggests a tiny patch of the universe stays behind. This patch contains no matter or black holes. This helps avoid problems with growing too much structure. Other ideas include loop quantum cosmology and conformal cyclic cosmology. These theories all try to explain our cosmic history.

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.