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Photo Source: the Pixabay universe has expanded for 13.7 billion years since its birth. From the beginning of an infinitesimal singularity, to today's incomparably huge scale. However, some scientists predict that this long expansion will be completely over within 100 million years, when the universe begins to contract. And it is the famous dark energy that causes this.

According to the conclusion of standard cosmology, our universe has existed for about 137 billion years. In this long years, the universe has gradually expanded from an infinitesimal singularity at the beginning to an incomparably large scale today. Today's universe continues to expand, or even accelerates, forcing physicists to introduce dark energy to explain this unusual acceleration. We know very little about the nature of dark energy. A possible state of dark energy may soon stop the universe from expanding and slowly begin to contract, according to a previous study published in the Proceedings of the National Academy of Sciences (PNAS). And this possible time node is not as far as we think-in 6500 million years, the expansion of the universe will no longer accelerate, but in 100 million years the expansion of the universe will stop completely and begin to contract slowly.

If you think this time is still too far away, you can compare: dinosaurs just became extinct 6500 million years ago on Earth. )

The annoying acceleration of Einstein's creation of the general theory of relativity in 1915 made cosmology a real science. The Einstein field equation contains the answer to the question of the origin and evolution of the universe. But there is a problem in the equation that puzzles physicists. It cannot give a static solution under the cosmological model, and the universe described is always expanding or contracting. Limited to the view of the universe at that time, Einstein believed that the universe should be in an immutable static state, so he modified the equation and added a term with a "cosmological constant" to obtain a static cosmological solution.

However, subsequent astronomical observations took everyone by surprise. In 1929, astronomer Edwin Hubble (Edwin Hubble) observed redshifts in extragalactic galaxies, which meant that these galaxies were moving away from us. The expansion of the universe predicted by Einstein's field equation can explain this phenomenon. In this way, the equation will not be a problem without a static solution, and the addition of cosmological constants is superfluous. Einstein later withdrew the modification of the field equation and admitted that the introduction of cosmological constants was the biggest mistake he had ever made.

However, the problem does not end there. Since our universe is expanding, as the universe becomes larger, the density of matter in the universe becomes smaller, and so does the corresponding energy density. At this time, according to the prediction of Einstein's field equation, the expansion of the universe should gradually slow down. However, astronomical observations once again surprised everyone. In 1998, astronomers calculated the rate of expansion of the universe based on Ia supernova observations and found that the universe was not decelerating, but accelerating.

Physicists are puzzled by the discovery that the rate of expansion of the universe increases rather than decreases. In order to give an explanation, physicists believe that there is still a class of unknown things in the universe whose energy density does not decrease as the universe grows, but remains constant. So when the universe expands to twice its original size, the total amount of this energy doubles. Bringing this unknown energy into the cosmological model is tantamount to picking up the cosmological constant abandoned by Einstein, which enables Einstein's field equation to give an accelerated expansion solution.

Of all the contents in the universe today, 68.3% are dark energy, 26.8% are dark matter, and only 4.9% are known ordinary matter. (photo Source: Szczureq / Wikipedia) this unknown energy is now known as "dark energy". Physicists estimate that dark energy should account for about 70% of the entire contents of the universe today to be consistent with observations of the accelerated expansion of the universe. However, there is no consensus in the academic circle on what this mysterious energy is and how to describe it in physics. The understanding of dark energy is related to our prediction of the future fate of the universe. If the energy density of dark energy is not a constant, but will change with time, then the universe is likely to stop expanding and begin to contract in the future. It will take at least 100 million years.

The fate of 100 million years later in 1998, Paul Steinhart (Paul Steinhardt) and three other physicists published a paper that put forward the hypothesis that dark energy is a kind of "quintessence". Such dark energy is no longer a fixed energy density as described by the cosmological constant, but a scalar field that can change over time. Whether the scalar field is attractive or repulsive depends on the ratio of kinetic energy to potential energy. The researchers believe that the scalar field became repulsive about 3.5 billion years after the birth of the universe, but at that time it was the energy density of matter that dominated the expansion of the universe, so the rate of expansion gradually decreased. It was not until about 98 billion years after the birth of the universe that the energy density of matter decreased as the universe became larger, and dark energy played a leading role, accelerating the expansion of the universe.

A schematic diagram of the evolution of the universe for 137 billion years since the Big Bang, during which the universe has been expanding, and the expansion has accelerated in the last few billion years. (photo source: NASA / WMAP Science Team) but if dark energy is really a scalar field that changes over time, changes in its composition may cause exclusion to gradually disappear. In a recent paper published in PNAS, Steinhart and two colleagues Anna Ijjas and Cosmin Andrei built a model of the universe containing "refined" dark energy. They adjusted the parameters of the model to match the known history of cosmic expansion. The researchers then used this model to simulate what would happen to such dark energy in the future. They found that the energy density of dark energy decreases over time and eventually behaves like ordinary matter. This transformation can be achieved in 6500 million years, when the expansion of the universe will no longer accelerate. 100 million years later, dark energy becomes attractive, causing the universe to contract.

Although according to this model, the time when the universe began to contract is not far away, the researchers say that the initial contraction rate of the universe is very slow, and it may take billions of years for the universe to reach half its current size. On the other hand, all our observations of cosmic expansion come from celestial bodies ranging from millions to billions of light-years, which can tell us about the past of the universe, not the present or future of the universe. So, the universe may have begun to contract, and it will take a long time for us on Earth to realize this.

However, there is no way to determine whether dark energy is indeed a "essence" or whether the expansion of the universe has begun to slow. The work of Steinhart and others is based on a special dark energy model. Is this understanding of dark energy correct? Maybe we have to wait for future observations to tell us the answer.

Links to papers:

Https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.80.1582

Https://www.pnas.org/doi/full/10.1073/pnas.2200539119

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by Bai Defan, revised by Wang Yu

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