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The Big Crunch vs. The Big Freeze: From the Beginning to the End of the Universe

Astronomers once believed that the universe would collide in a Big Crunch. Most people now believe it will end in a Big Freeze.

How will the universe come to an end? For thousands of years, humanity has pondered this question. And science now has the knowledge and tools to make an attempt at an answer.

Until recently, astronomers believed the universe would expand and contract infinitely in an endless cycle of cosmic death and rebirth. The best evidence, however, points to a distant Armageddon filled with greater existential dread than the Book of Revelation. Long after Earth has been destroyed, the universe will drift apart for trillions of years until galaxy and star formation ceases. Stars will gradually fade away, leaving the night sky black. Black holes will consume all remaining matter until there is nothing left. The last traces of heat will eventually vanish.

Rather than being destroyed by fire and brimstone, the universe will most likely die of "heat death." Astronomers refer to it as the Big Freeze.

The Alpha and Omega

The universe did not always appear to be destined to end in this manner. A century ago, astronomers believed that our Milky Way Galaxy was the centre of the universe. Our universe appeared to be static — it had always been and would always be roughly the same. However, while developing his theories of relativity, Albert Einstein noticed signs of something strange. His equations suggested a universe that was expanding or contracting. As a result, Einstein introduced a fudge factor — a cosmological constant — that kept the universe in a more appealing steady state.

Around the same time, astronomers began to accept that some of the fuzzy spiral-shaped nebulae seen through telescopes were not collections of stars in our galaxy. They were entirely different galaxies. When Edwin Hubble meticulously measured their motions, he discovered that these galaxies were, in fact, moving away from our own. The universe is expanding, as humanity has discovered.

Rewinding that expansion revealed that the entire universe was born 13.8 billion years ago in a violent Big Bang. After establishing its foundations, cosmology moved on to the next big question: How will the universe end?

An expanding universe can die in two ways: it can eventually collapse back in on itself, or it can continue to expand indefinitely. Astronomers had to fast-forward the universe's evolution to find out which is correct.

The Big Crunch

Alexander Friedmann, a Russian physicist and mathematician, developed a famous set of equations known as the Friedmann equations in 1922. These calculations revealed that the fate of our universe is determined by its density, and that it could either expand or contract rather than remain in a steady state. With enough matter, gravity would eventually halt the expansion of the universe, causing it to collapse inward.

When astronomers added up all the matter in the known universe in the 1960s and 1970s, they calculated there was enough mass that the universe would eventually collapse to an infinitely dense state, or perhaps even a monstrous black hole.
Some speculated that once the universe was compressed into an infinitely small point — the Big Crunch — it would kickstart yet another expansion, or Big Bounce.

Physicist John Wheeler, who helped coin the term "black hole," became a leading proponent of the Big Crunch in the 1970s and 1980s. It seemed like a foregone conclusion to him. A black hole revolution was underway, and Wheeler saw each one as a "experimental model" of the universe's final state.

However, Wheeler admitted that his fondness for Big Crunch was partially motivated by aesthetics. It was simple to visualise.

The Big Freeze

Unfortunately, reality isn't always so easy to understand. Just because we find a cold, empty universe to be an unappealing future doesn't mean that's where things are heading.

In the late 1990s, two groups of scientists were surveying the distant universe, studying dying stars known as type Ia supernovae, which serve as standard candles for determining cosmic distances. They discovered that distant blasts appeared dimmer and thus farther away than expected. The expansion of the universe was accelerating rather than slowing. The teams had discovered dark energy on their own, shattering existing universe models.

The unexpected discovery of dark energy demonstrated that the universe was very unlikely to collapsing in a Big Crunch. Gravity will never be strong enough to overcome the inflating effect of dark energy, even if all the matter in the universe pulls inward. In other words, the expanding universe is doomed to a Big Freeze.

Astronomers now believe that ordinary matter accounts for only 5% of the universe's contents. Meanwhile, dark matter accounts for approximately 26% of the total, with dark energy accounting for the remaining 69 percent. As it turns out, dark energy appears to be the real-world force behind Einstein's cosmological constant, which plays a significant role in preventing a Big Crunch-style collapse.

Astronomers now believe that ordinary matter accounts for only 5% of the universe's contents. Meanwhile, dark matter accounts for approximately 26% of the total, with dark energy accounting for the remaining 69 percent. As it turns out, dark energy appears to be the real-world force behind Einstein's cosmological constant, which plays a significant role in preventing a Big Crunch-style collapse.

The last objects in the universe, supermassive black holes, will finish evaporating via Hawking radiation about a googol years from now — that's a 1 followed by 100 zeroes. Following this, the universe enters a period known as the Dark Era, in which matter becomes a distant memory.

The second law of thermodynamics states that entropy will continue to increase in a system (such as the universe) until it reaches a maximum. In practise, this means that the universe will eventually reach a state in which all energy — and thus heat — is uniformly distributed. The final temperature of the universe will be slightly higher than absolute zero.

As a result, rather than resembling Revelation, the death of our universe will most likely resemble the beginning of Genesis: everything will be empty and dark.

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