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A bleak, frigid, and monotonous end. A harsh and brutal outcome, or perhaps a conclusion that leads to a new beginning?
These are some of the most prominent theories regarding the fate of the universe in the distant future – or perhaps, a fate that may never arrive to pass.
The destiny of the universe remains one of the most profound questions in science. Even experts acknowledge that there are more questions than answers. But to understand the possible ends, it’s necessary to first understand how it began. The exploration of these concepts pushes the boundaries of our understanding of cosmology and the fundamental laws of physics.
The Beginning of Everything
NASA defines the universe as everything – space, matter (anything that has mass and takes up space), energy, and even time itself.
The origin of the universe is often attributed to what astronomers call the “Big Bang” theory.
This theory posits that approximately 13.8 billion years ago, the universe existed in an extremely hot and dense state, and has been expanding ever since.
The universe continues to expand, and over vast stretches of time, structures like galaxies, stars, and planets formed.
And this expansion is ongoing today.
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But fundamentally, we still know relatively little about it.
“Our work is like observing the behavior and flow of an entire river, without evaluating each water molecule that passes through in detail,” explains Professor Alexandre Zabot, a physicist at the State University of Santa Catarina, Brazil.
1. The Big Freeze
The Greenwich Royal Observatory in the UK explains that if the universe continues to expand, energy will become so dispersed that galaxies will gradually move further apart, and eventually, stars will cease to form, and existing stars will burn out.
Then, over trillions of years, the universe will become increasingly dark, eventually reaching a cold, dark, and nearly empty state.
This scenario is known as the Big Freeze, or Heat Death.
The Royal Institution notes that this concept suggests that eventually, all atoms in the universe will reach thermal equilibrium (the same temperature everywhere), and everything will come to a halt.
“Everything indicates that the universe will become empty, colder, and more distant,” says Associate Professor Raul Abramo, from the Institute of Physics at the University of São Paulo, Brazil.
“Galaxies will move further and further away, stars will age and die,” he adds. “It’s a final state where the universe would truly be a graveyard.”
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2. The Big Rip
If the above sounds unsettling, there’s another hypothesis, though it paints an even more violent picture.
The Greenwich Royal Observatory explains that this hypothesis is based on the idea that the universe is expanding at an accelerating rate due to dark energy. If this continues, the universe could expand so rapidly that gravity would no longer be able to hold anything together.
This scenario is known as the Big Rip, or the Great Rupture.
NASA explains that gravity is the force that attracts objects with mass toward each other.
This force is what keeps planets in orbit around the sun, holds the Earth together, and keeps humans on the ground instead of floating into space. Gravity holds systems together, such as stars, solar systems, galaxies, and clusters of galaxies.
Conversely, dark energy is more mysterious and appears to have the opposite effect.
“We still don’t know what it’s made of, but it’s known to cause a repulsive force, like a counterforce to gravity,” Zabot explains.
According to NASA, dark energy is causing the universe to expand at an accelerating rate. The space agency says that approximately 68.3% to 70% of the universe is composed of dark energy, and it was only discovered in the late 1990s.
Dark energy remained undiscovered for so long given that gravity is dominant at human, planetary, and even galactic scales. Dark energy is only noticeable at much larger scales – between galaxies.
“As the universe expands and becomes larger in volume, the repulsive force associated with dark energy becomes more significant,” Zabot explains.
“It’s possible that as the universe gets bigger, dark energy will play an increasingly vital role at smaller scales,” he adds.
Unstable dark energy could lead to a Big Rip, in which the universe expands so violently that stars, planets, and even atoms are “torn apart,” according to NASA.
“That’s why the English name for this theory has a double meaning. It’s called the Big Rip, which means a break, a tear, a disruption, or a great rupture,” Zabot says. “But ‘rip’ is also an abbreviation for ‘rest in peace.’”
3. The Big Crunch
There are several other, even more unsettling, theories that have been considered.
NASA states that if dark energy weakens and eventually reverses direction, gravity will pull the universe back together at a single point, leading to a “Big Crunch” where the universe collapses in on itself.
Another theory suggests that the universe could contract back into a single point, before a new Big Bang occurs and a new universe is born. This is known as the Big Bounce, suggesting that our universe and future universes may be caught in an endless cycle of contraction and expansion.
“But this model is a completely bizarre one, for which we have no data or evidence,” Abramo explains.
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So, is it possible to estimate when the universe will end?
“No, we have no evidence to suggest that there will be a crack or a Big Crunch in the future,” Abramo explains.
“Some estimates of this end speak of times of trillions of years, while others predict even longer periods,” Zabot adds.
If the universe is at least 13.8 billion years old, it still has 986,200 million years before reaching 1 trillion years of age.
A recent study by Radboud University in the Netherlands suggests the universe is decaying faster than previously thought. Researchers calculated that the last stellar remnants may take approximately 10^78 years (a 1 followed by 78 zeros) to extinguish.
Our species likely won’t be around to witness it.
Earth will likely be long gone by then, as the sun is expected to turn into a red giant in approximately 6 billion years.
“The truth is that we still know very little about cosmology,” Abramo explains. He says that the limited tools currently available make it tricky to measure the universe accurately.
And this opens up opportunities to explore more speculative theories, such as the multiverse – the idea that there are more universes than just our own.
