Cyclic model

A cyclic model is a way scientists think about how the universe might work over very long periods of time. In these models, the universe goes through repeating cycles, like breathing in and out, over and over again without end.

One famous idea called the oscillating universe was thought about by the famous scientist Albert Einstein in 1930. This theory suggested that the universe starts with a big explosion called the Big Bang. After a very long time, the force that holds everything together, called gravity, might pull everything back together again in an event called the Big Crunch. After that, the universe could start again with another Big Bang, and the cycle would continue forever.

These ideas help scientists think about big questions, like how old the universe is, how it began, and what might happen in the far future. Even though we now know more about the universe, cyclic models are still interesting because they show one possible way the universe could keep going through endless cycles of expansion and contraction. Studying these models helps us understand the forces and rules that shape the cosmos.

Overview

In the 1920s, scientists like Albert Einstein asked if the universe could go through repeating cycles instead of just expanding forever. They called this the Oscillating Universe Theory. But they found a problem: each cycle would need to be bigger and longer than the last.

In the early 2000s, new discoveries about dark energy brought new ideas for these cyclic models. One idea, called the brane cosmology model, suggests the universe could explode into existence many times. This helps explain why the universe is spreading out faster. Other theories, like conformal cyclic cosmology and loop quantum cosmology, also look at the idea of repeating cycles in the universe.

The Steinhardt–Turok model

Main article: Ekpyrotic universe

In this cyclic model, two parallel planes called branes move through a higher-dimensional space and crash into each other sometimes. These crashes mark the shift from the universe shrinking back together to expanding outward again, similar to a Big Crunch followed right away by a Big Bang. The matter and energy we see around us today were created during the most recent crash.

Over many billions of years, the universe grows and changes until it will eventually start to shrink once more. A special kind of energy, called dark energy, acts as a force between these branes. This model suggests the universe could continue these cycles forever into the past and future. However, there are still many mysteries about how these branes behave during their collisions.

The Baum–Frampton model

The Baum–Frampton model is a new idea about the universe from 2007. It says that a special kind of dark energy, called phantom energy, might change how the universe works. Usually, this dark energy could make the universe end in something called a Big Rip. But in this model, if certain things happen, the universe could reset itself instead.

After the reset, the universe would be empty, with only dark energy left. This helps avoid problems if the universe got smaller while matter was still in it. The model uses ideas from string theory but does not need it completely.

Other cyclic models

Some ideas about how the universe might go through repeating cycles include Conformal cyclic cosmology, a theory by Roger Penrose. This idea suggests that the universe expands until everything turns to light, and then it can start a new cycle.

Another idea is Loop quantum cosmology, which talks about a "quantum bridge" connecting the end of one cycle to the start of the next. There are also theories that explain the universe’s cycles using the behavior of black holes and gravity.