Looking at our universe today, it is very easy to be delighted with what you see. The stars in our night sky are only a small fraction, a few thousand out of hundreds of billions of what is present in our Milky Way. The Milky Way itself is just one of the trillions of galaxies present in the observable universe, which stretches in all directions for about 46 billion light years. And it all started about 13.8 billion years ago from a hot, dense, fast, expanding state known as the Big Bang.
It is from the Big Bang that we get the opportunity to describe our Universe as full of matter and radiation and to connect the well-known laws of physics explaining the modern form of the cosmos. But the universe continues to expand. New stars appear, space evolves. How will it end? Let's ask science.
What is the end of the universe
For a long time, scientists who have studied the structure and evolution of the universe have considered three possibilities based on the simple physics of general relativity and the context of the expansion of the universe. On the one hand, gravity is actively pulling everything together; it is an attractive force controlled by matter and energy in all their forms that are present in the universe. On the other hand, there is an initial expansion rate that pulls everything apart.
The Big Bang was a shot, after which the greatest race of all time began: between gravity and the expansion of the universe. Who will win in the end? The answer to this question will determine the fate of our world.
We thought the Universe had these options:
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- The Universe will collapse in the Great Compression. Expansion will begin quickly and large amounts of matter and radiation will be torn apart. If there is more than enough matter and energy, the universe will expand to a certain maximum size, the expansion will reverse the contraction and the universe will collapse again.
- The universe will expand forever and lead to the Great Freeze. Everything will start the same as above, but this time the amount of matter and energy will not be enough to resist expansion. The universe will expand forever as the expansion rate continues to fall, but never reaches zero.
- The expansion of the Universe tends asymptotically to zero. Imagine a borderline situation between the two examples above. One more proton - and we collapse; one less - we expand infinitely. In this critical case, the Universe is expanding forever, but at the lowest possible speed.
To find out which option is correct, we just had to measure how fast the universe is expanding and how the rate of expansion changed over time. The rest is a matter of physics.
This has been one of the greatest challenges in astrophysics today. Measure the speed at which the universe was expanding and find out how the fabric of space is changing today. Measure how the expansion rate has changed over time and find out how the fabric of space has changed in the past.
Combine these two pieces of information and how the expansion rate has changed and what it was will allow you to determine what the universe is made of and in what proportions.
As far as we know, based on these measurements, we determined that the universe consists of 0.01% radiation, 0.1% neutrinos, 4.9% ordinary matter, 27% dark matter, 68% dark energy. This quest, which for some began back in the 1920s, received an unexpected answer in the late 1990s.
So if dark energy dominates the expansion of the universe, what does this mean for our destiny? It all depends on how - or if - dark energy evolves over time. Here are five options.
Dark energy is a cosmological constant dominant in expansion. This is the default and takes into account our best data. While matter becomes less dense as the universe expands, dilutes as volume expands, dark energy represents the non-zero amount of energy inherent in the fabric of space itself. As the universe expands, the density of dark energy remains constant, which causes the expansion to always remain positive.
This results in an exponentially expanding universe and will eventually push anything that isn't part of our local group. Already 97% of the visible Universe becomes inaccessible in such conditions.
Dark energy is dynamic and becomes more powerful over time. Dark energy appears to be a new form of energy that is inherent in space itself, which implies that it has a constant energy density. But it can also change over time. One of the possible ways to change is that it gradually increases, which will lead to an acceleration of the expansion rate of the universe.
Remote objects will not only move away from us, but do it faster and faster. Worse, objects that are now gravitationally bound - like clusters of galaxies, individual galaxies, solar systems, and even atoms - will one day untie as dark energy hardens. In the last moments of the universe's existence, subatomic particles and the fabric of space-time itself will be torn apart. This fate - the Big Rip - is our second option.
Dark energy is dynamic and weakens over time. How else can dark energy change? Instead of strengthening, it may weaken. Of course, the expansion rate is consistent with a constant amount of energy belonging to space itself, but this energy density can also decrease.
If it weakens to zero, everything will come to one of the possibilities described above: The Great Freeze. The universe will expand, but without enough matter and other forms of energy to help it collapse again.
If the decay turns negative, it could lead to another possibility: the Big Shrink. The universe will be filled with energy inherent in space, which will suddenly change signs and cause space to contract. This option is also possible.
Dark energy will transform into another form of energy that rejuvenates the universe. If dark energy does not disintegrate, but remains constant or even intensifies, another possibility arises. This energy inherent in the fabric of space may not always remain in this form. Instead, it can turn into matter and radiation, similar to what it was when the cosmic inflation ended and the Big Bang began.
If dark energy remains constant up to this point, it will create a very, very cold and diffuse version of the incandescent Big Bang, in which only neutrinos and photons can create themselves. But if the intensity of dark energy increases, it could lead to a state similar to inflation, followed by a new, truly red-hot Big Bang. This is the easiest way to rejuvenate the Universe with the given parameters.
Dark energy is associated with the zero energy of the quantum vacuum and will decay, destroying our universe. This is the most destructive opportunity of all. What if dark energy is not the true amount of empty space in the lowest energy configurations, but is the result of symmetries in the early Universe, when they were in a configuration with a false minimum?
If so, there must be a way to create a quantum tunnel into a lower energy state by changing the laws of physics and eliminating all bound states (i.e. particles) of quantum fields today. If the quantum vacuum is unstable in this sense, then wherever this decay occurs, the result will be the destruction of everything in the universe through a bubble that propagates at the speed of light. If such a signal reaches us, we too will end.
While we don't know which of these possibilities will hold true for our Universe, the data is simply voting frantically in favor of the first option: dark energy is indeed a constant. Right now, our observations of how the universe has evolved - especially thanks to the cosmic microwave background radiation and the large-scale structure of the universe - impose severe limits on how much wiggle room for dark energy to change.
Ilya Khel