To be honest, we still know very little about dark energy.

We all know that the universe is composed of ordinary matter, dark matter and dark energy. Among them, ordinary matter is the most familiar to us, which is matter composed of protons, neutrons, and electrons that participate in all four interactions. Dark matter is almost the same as ordinary matter. The only thing is that it does not participate in electromagnetic interactions. This means that our traditional electromagnetic wave observation astronomy cannot do anything about it. However, with the development of gravitational wave observatories, astronomical observation has entered the multi-messenger era. We can still use gravity Interaction to observe evidence of the existence of dark matter. Not only that, people can also roughly infer possible candidates for dark matter, the most popular of which is WIMP (Weakly Interacting Massive Particles).

But dark energy is very special. People have not yet been able to propose the possible composition of dark energy, and there is not even any direct evidence. In fact, the original and only purpose of proposing dark energy was to explain the accelerating expansion of the universe.

Since Hubble discovered the expansion of the universe and Gamow proposed the Big Bang theory, people generally believe that the expansion of the universe should be decelerating. Especially after Linde and others proposed the inflation theory and pointed out that the power of the big bang came from a special substance "inflaton (actually a scalar field)", people thought that when inflation ended, the inflaton was no longer in a vacuum. Instead, they appear in large numbers in the form of particles, and finally decay into ordinary matter such as quarks. Then the subsequent expansion should be "inertial effect". Since there is gravity between matter in the universe (whether ordinary matter or dark matter), the expansion of the universe will naturally slow down.

However, in the 1990s, two groups exploring the use of type Ia supernova distance measurement simultaneously and independently discovered that the photometric distance (that is, the observation distance) of supernovae is generally farther than the distance based on the red shift and the actual distance. The calculated distances of some Big Bang universe models are far away, which means that the Big Bang models at that time were inconsistent with observations and needed to be modified. It is worth noting that the gap between the observed distance and the theoretically predicted distance becomes larger as the distance increases, indicating that the expansion of the universe is actually accelerating.

The best estimate now is that after the end of inflation, the universe did decelerate and expand, but it only lasted for more than 7 billion years. About 6 billion years ago, the universe changed from decelerating expansion to accelerating expansion.

In order to explain this difficulty, people assume that there is a repulsive substance or energy in the universe (matter and energy are essentially the same), and due to the uniformity of the expansion of the universe, this substance (or energy) should It is evenly distributed and does not clump. This very different property from ordinary matter and dark matter suggests that dark energy (the name was coined by physicist Turner after dark energy) may be closely related to vacuum, or vacuum energy.

In a sense, dark energy and inflatons are really similar, like a weakened version of inflatons. What people can know now is that the universe has three dominance periods, namely the radiation dominance period, the matter dominance period and the dark energy dominance period.

This is related to their nature. Photons have positive pressure and will do positive work as space-time expands. Therefore, as space-time expands, its density is proportional to one-fourth of the fourth power of the radius of the universe R. The pressure of matter is 0 and it does no work during expansion. Its density is inversely proportional to the expansion volume of the universe, that is, proportional to one-third of the cube of R. Dark energy has strong negative pressure, and the expansion of space-time will do work on it. As space-time expands, the density of dark energy remains unchanged (this is only the conclusion of the mainstream cosmological constant model, and there are some changes, such as the essence model, Elf model, tachyon model, etc., but not mainstream).

So, as the universe expands, the density of light, that is, radiation, decreases fastest, followed by matter (including ordinary matter and dark matter), while dark energy remains unchanged. In this way, at the beginning of the universe, there were the most photons, and it was a period of radiation dominance. Then came the period of dominance of matter until about 6 billion years ago. As the universe expanded, the density of matter became smaller and smaller, and finally became smaller than the density of dark energy. The repulsive force of dark energy defeated the gravity of matter, and the universe began to accelerate. expanded.

You may be curious, as the universe expands, the density of dark energy remains unchanged and the volume increases, whether there is more and more dark energy. Yes, this also obeys the conservation of energy. The increase in dark energy is caused by work done on it by space-time, so the increased dark energy comes from the energy of space-time itself. In the future, our universe will be completely dominated by dark energy, and the Big Rip may be the ultimate fate of our universe.