Cyclic model facts for kids

A cyclic model is a fascinating idea in cosmology, which is the study of the universe. These models suggest that our universe might go through endless cycles, like a repeating song. Imagine the universe starting with a Big Bang, expanding outwards, and then slowly pulling back together into a Big Crunch. After the Big Crunch, it could then "bounce" back into another Big Bang, starting a new cycle! This idea is sometimes called an oscillating model.

Exploring the Universe's Cycles

The idea of a universe that expands and contracts in cycles has been around for a while. In 1922, a scientist named Alexander Friedmann first introduced a theory like this. However, early versions of these ideas faced a big problem.

The Entropy Challenge

A scientist named Richard C. Tolman showed in 1934 that these early models had a flaw. It's related to something called entropy. Think of entropy as the universe's tendency to get more and more "messy" or spread out. The second law of thermodynamics says that entropy can only increase over time. This means that each new cycle would have to be bigger and last longer than the one before it. If you go back in time, the cycles would get smaller and shorter, eventually leading back to a single Big Bang, not an endless series of bounces. This puzzle stumped scientists for many years.

Dark Energy and New Ideas

But then, in the early 2000s, scientists made an exciting discovery: dark energy. This mysterious force seems to be pushing the universe apart, making its expansion speed up! This new understanding gave scientists hope for creating new, more consistent cyclic models. In fact, by 2011, surveys of hundreds of thousands of galaxies confirmed that dark energy is indeed speeding up our universe's expansion.

One of these newer ideas is called the brane cosmology model. It was suggested in 2001 by Paul Steinhardt and Neil Turok. This theory suggests the universe doesn't just explode into existence once, but many times over and over. It might even help explain why dark energy is much weaker than some scientists expected. Other cyclic ideas include models involving phantom energy, conformal cyclic cosmology, and loop quantum cosmology.

By the mid-2020s, these cyclic models gained more attention. This was because some observations from projects like DESI didn't perfectly match predictions from the standard model of the universe. This left the door open for the possibility of a Big Bounce and a universe that cycles.

The Steinhardt–Turok Brane Model

In this exciting model, imagine our universe isn't the only thing out there. Instead, picture two special, flat "surfaces" or "branes" existing in a higher-dimensional space. Our entire visible four-dimensional universe lives on one of these surfaces!

These two "branes" periodically collide with each other. Each collision is like a cosmic reset button! It causes the universe to stop shrinking (a "Big Crunch") and immediately start expanding again (a "Big Bang"). The stars, planets, and everything we see today were created during the most recent collision. After billions of years of expansion, the universe will eventually start to contract again, leading to another collision and another Big Bang.

In this model, dark energy acts like a force between these two "branes." It plays a very important role in explaining why the universe is so smooth and flat, and why it looks the same in all directions. This model suggests that these cycles can go on forever, both into the past and into the future.

Earlier cyclic models had a problem with entropy (the "messiness" of the universe) building up. But the Steinhardt–Turok model tries to solve this. It suggests that the universe expands a little bit more each cycle, preventing entropy from piling up too much. However, scientists are still working to fully understand how these "brane" collisions work and how they create the tiny ripples that grow into galaxies.

The Baum–Frampton Phantom Energy Model

Another interesting cyclic model was proposed in 2007 by Lauris Baum and Paul Frampton. This model uses a very unusual type of dark energy called phantom energy. Phantom energy is so strange that it could cause the universe to expand so fast that everything would eventually be torn apart.

However, in the Baum–Frampton model, something incredible happens just a tiny fraction of a second (less than one-septillionth of a second!) before this could occur. The universe suddenly changes direction and starts to shrink! But here's the clever part: as it shrinks, it somehow gets rid of almost all matter, including quarks, leptons, and even black holes.

This means the universe "comes back empty" for the next cycle. By getting rid of all the matter and black holes, the universe also gets rid of its "messiness" (entropy). This allows a new cycle to begin with a very clean slate, avoiding the problems that plagued earlier cyclic models. It's a bold idea that helps explain how cycles could truly repeat without getting progressively messier.

Other Cyclic Universe Ideas

Scientists have explored several other fascinating ideas about a cycling universe:

• Conformal cyclic cosmology: This theory, from Roger Penrose, suggests that the universe expands until everything in it, even matter, eventually turns into light. At this point, there's nothing left with any sense of size or time. This allows it to essentially become like a new Big Bang, starting the next cycle.
• Loop quantum cosmology: This idea uses quantum mechanics to describe the very early universe. It suggests there's a "quantum bridge" that connects a contracting universe to an expanding one, allowing for a bounce.
• Nikolai Gorkavyi's Oscillating Universe: This model proposes that the universe's expansion and contraction are driven by black holes. It suggests that expansion happens when black holes merge and release gravitational waves, reducing the total mass and creating an "antigravity" effect. Contraction then occurs as black holes grow by absorbing gravitational waves, increasing gravity. This model even suggests a giant black hole exists today, billions of light-years across!

See also

• Big Bounce
• Conformal cyclic cosmology