There are several questions that keep us awake at night, and they relate to the ultimate fate of the entire cosmos. Stars light up, they are replaced by new ones, they also burn out, and everything repeats until the Universe runs out of fuel. The galaxies will merge and eject matter, and the space between groups and clusters of galaxies will expand forever. Dark energy is causing this expansion to be not only inexorable, but also accelerating. But will this be the end? Could this "big gap" (when everything ends up at an infinitely distant distance from each other) lead to a new "big bang"? When the Universe is expanding fast enough to tear apart the atoms and separate the quarks from them … Will a quark-gluon soup be formed?
At stake is the fate of the universe, whatever one may say.
What's in store for the universe at the end?
If you look at a distant, random galaxy in the universe, chances are high that you will see that its glow is redder than that of the stars that glow in our galaxy. Back in the 1920s, scientists discovered that this pattern persisted as a whole: the further away the galaxy is from you, the redder its light. In the context of general relativity, it quickly became clear that this was due to the expansion of the fabric of space itself over time.
The next step was to quantify how fast the universe was expanding and how that rate changed over time. The reason this was important, from a theoretical point of view, is that the history of the expansion of the universe determined what was in it. If you want to know what your universe is made of on the largest scales, measuring how the universe has expanded over cosmic time will help you.
If your universe is filled with matter, you would expect the rate of expansion to decrease in proportion to how much matter is diluted. If it is filled with radiation, the expansion rate will drop even more, because the radiation itself is redshifted and loses additional energy. A universe with spatial curvature, cosmic strings or energy inherent in space itself, will still develop in a different way, depending on the ratios of all energy components.
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Based on the full set of measurements that we were able to make, including variable stars, galaxies of different types and properties, and type Ia supernovae, as well as the cosmic microwave background and galaxy clustering and correlation, we were able to pinpoint exactly what the universe is made of. In particular, it consists of:
- 68% from dark energy;
- 27% dark matter;
- 4.9% from ordinary matter;
- 0.09% neutrinos;
- 0.01% radiation.
Plus or minus an adjustment of a few tenths of a percent in each case.
Our Universe, which is dominated by dark energy, is especially interesting because this component did not exist in the Universe, let alone its predominance. And yet, we are here, 13.8 billion years after the Big Bang, living in a universe in which dark energy is driving the expansion of the universe.
There are so many questions surrounding dark energy. What is its nature? Where does it come from? Is it constant or changing over time? There are no definitive answers, but everything indicates that dark energy is a cosmological constant. In other words, it behaves like a new form of energy inherent in space itself. As the universe expands, it creates a new space that contains the same uniform amount of dark energy.
Anyway, this is our best view so far. From a theoretical point of view, there are several known ways to create the cosmological constant, and therefore this explanation - as long as the data agree with it - will remain the preferred one. But there is no reason why dark energy couldn't be more complex.
It can be something that erodes over time, becoming less and less dense, albeit a little. It could be something that changes sign in the distant future and leads to the re-creation of the Universe in a Big Squeeze. It could also be something that gets stronger over time, accelerating and expanding the universe over time. It is this variation that leads to the Big Rip scenario.
When we talk about any component of energy in the Universe, we are talking about its equation of state, which describes how it evolves over time in the Universe. Astrophysicists use the parameter w for this, where w = 0 corresponds to matter, w = 1/3 corresponds to radiation, w = -1 corresponds to the cosmological constant.
Dark energy appears to have w = -1, but this is not accurate. For example, new work from the Subaru Hyper Suprime-Cam collaboration has added new constraints to the dark energy equation of state. While dark energy matches w = -1 quite convincingly, there is also speculation that it could be even more negative. If it really is - if it turns out that w <-1 and not equal to -1 - then the Big Rip is inevitable.
If the Big Rip is imminent, not only the expanding Universe, but also distant objects will accelerate from us faster and faster over time (due to dark energy). But objects that are held together by some fundamental force will eventually be torn apart by the increasing force of dark energy.
Many billions of years in the future, our local group will see how the stars on the outskirts will be thrown into space, as they will be untied gravitationally from our future distant galaxy: Milkomed. As time goes on, more and more stars will be thrown outward until the structures we know as galaxies collapse and become a collection of billions of unrelated stars and stellar corpses.
Over time, the planets will be ejected from their solar systems as dark energy will intensify and then even the planets themselves will be torn apart. In the very last moments, objects held by atomic and molecular forces will be torn apart, electrons will be torn from their atoms, atomic nuclei will crumble, and even the quarks themselves will be separated. And then they will burst.
Are we waiting for a new Big Bang?
If the Big Rip is a correct model for the development of the Universe, everything in the Universe will be reduced to the most fundamental components, in some ways strongly corresponding to the first stages of the Big Bang.
However, this quark-gluon plasma will be different from what it was during the Big Bang. First, the Big Bang is hot and dense, and the Big Rip will be extremely cold and diffuse. Second, the Big Bang is characterized by the fact that all matter and energy in the Universe is compressed into a tiny volume of space, but in the Big Rip they will be dispersed over trillions of light years. In addition, the Big Bang represents a state of relatively low entropy, but in the Big Bang the entropy will be 10 (to the power of 35) times more than in the Big Bang.
But there is hope.
Perhaps the dark energy that will lead to the Big Rip can restart the universe. If the strength of dark energy increases, this dark energy is inherent in the fabric of space itself, which means it can be completely analogous to the early period in the history of our Universe, when space was expanding at a tremendous rate: cosmic inflation. Inflation eliminates all pre-existing matter and energy in the universe, leaving behind only the fabric of space. After a period of inflation, energy is somehow converted into particles, antiparticles and radiation, which leads to the Big Bang. This scenario has been considered before and is known as a rejuvenated universe.
If the Big Rip is the true scenario of the end of the Universe, it will simply tear all matter apart and the Universe will be very empty, but with a huge amount of energy inherent in space itself. If the energy is very large, it is possible that the very fabric of space will break - but this is a completely different scenario.
Ilya Khel