Figure 1: The material composition of the universe. Dark energy accounts for about 68% of the total composition. （www.fas37.org）
However, this dark energy, which occupies most of the universe, has completely different and even unbelievable properties from other matter. Unlike the particle properties of dark matter, dark energy is often thought of as being in the form of an energy state. Its most striking feature is that it has the repulsive effect of "negative pressure", which can be regarded as "universal repulsion" or "anti-gravitational force". It is precisely because of this peculiar nature that dark energy has been controversial since it was proposed, wandering through physics and astronomy like a cosmic ghost.
Theoretically, the theory of dark energy can be traced all the way back to Albert Einstein. The cosmological constant, one of the most important candidates for dark energy, was proposed by Albert Einstein in 1917 in order to balance the gravitational field equation to obtain a "steady-state" universe that was generally accepted at the time. However, subsequent studies have found that the addition of this constant does not keep the universe "stable", but rather accelerates expansion or contraction under small perturbations. In particular, in 1929, Edwin Hubble observed the expansion of the universe for the first time, and confirmed the "unsteady" properties of the universe. When Einstein learned of this observation, he regretted it and called the cosmological constant his "biggest blunder".
Figure 2: Schematic diagram of the history of the expansion of the universe. Observations show that the late universe began to expand at a gradual acceleration. （*www.cantorsparadise.com/）
Dramatically, however, Einstein's "greatest mistake" is by far the simplest and most consistent explanation for the vast majority of cosmological observations!
Although dark energy accounts for most of the energy components of the universe, because it fills the entire universe, its density is extremely low (about ten minus 30 g cm), which is 100 billion times lower than the density of air at normal temperature and pressure! At such low densities, it is clearly unrealistic to detect and study the nature of dark energy in the laboratory. Therefore, the current detection of dark energy is mainly concentrated in the cosmological observation of large samples in the large celestial region, that is, it is hoped that the nature of dark energy will be tried to be detected at a very large scale and a large number of samples through large-scale statistics.
As early as the 80s and early 90s of the 20th century, through the observation of galaxy clusters and globular clusters, combined with the theory of inflation, people have begun to realize that the general material composition in the universe may only account for a small part of the total mass of the universe, and indirectly deduce that there should be a kind of "repulsive" energy that occupies a large part of the universe, that is, dark energy. In 1998 and 1999, two observation teams in the United States, namely Saul Perlmutter and Adam Reese, respectively used the observed Type IA supernovae (SN IA) data to accurately measure the distances of different redshifts in the universe, and found that the universe is currently in the stage of accelerated expansion!
Figure 3: Schematic diagram of a type IA supernova. White dwarfs erupt violently by absorbing companion material beyond the Chandrasekhar limit, forming SN IA. SNIA eruptions are relatively consistent and can be used as "standard candlelight" to measure distances. （*esa）
This is considered a direct evidence of the existence of dark energy. This is because, in addition to dark energy, according to the calculations of general relativity, other matter or energy in the universe will be dominated by gravity on a cosmological scale, that is, they will attract each other due to gravity. In this case, the universe can only expand (or contract) at a faster rate, and there is no way to accelerate expansion unless there is some kind of "repulsion"!
In further cosmological observations, it is found that the energy density of dark energy does not change significantly with time through a variety of current cosmological observation methods and probes, such as cosmic microwave background radiation (CMB), baryon acoustic wave oscillation (BAO), weak gravitational lensing, etc. In other words, as the universe expands, dark energy will not be "diluted" due to the increase in volume like ordinary matter, but will remain "concentrated" unchanged.
This peculiar phenomenon of "creating something out of nothing" does not mean that the law of conservation of energy has been violated. According to the general theory of relativity, the pressure of matter is also a "kind of energy" (i.e., part of the active tensor). This phenomenon can be allowed as long as the pressure p of the dark energy is negative and the absolute value is exactly equal to the energy density r (i.e., the equation of state w=p rc2 is equal to -1 and c is the speed of light). And this is precisely the nature of cosmological constants.
One of the most intuitive physical equivalents of cosmological constants is "vacuum energy" or "zero point energy". This is very easy to understand because the vacuum energy or zero point energy, like the cosmological constant, is uniformly distributed in space and the energy density does not change with time. The effect of vacuum energy can be observed in experiments such as the Casimir effect, which has been widely accepted by physicists. Logically, as the universe expands, the vacuum increases, and the energy density of the vacuum energy does not change, so the vacuum energy in the universe also increases correspondingly. This is consistent with the observed nature of dark energy.
Figure 4: Schematic diagram of the vacuum "quantum sea". （*nasa/cxc/m.weiss）
However, there are currently great theoretical challenges to cosmological constants. According to current observations, the energy density of the cosmological constant does not exceed ten minus 9 joules per cubic meter. And according to the estimation of quantum mechanics, the theoretical value of vacuum energy should be higher than 113 joules per cubic meter of ten, that is, the observed value is at least 122 orders of magnitude smaller than the theoretical value! Such a huge difference raises the question of whether the current theory of quantum mechanics is too far wrong, or whether dark energy is not actually a cosmological constant or vacuum energy.
Therefore, another major theory of dark energy has also been proposed, which is scalar field dark energy. The basic idea of the scalar field dark energy theory is to introduce a scalar field that can evolve over time with minimal coupling to the matter field, and to describe the potential of this scalar field, and to explain the accelerated expansion of the universe in this way. A similar idea has been proposed in the theory of the early inflation of the universe, but the relationship between the scalar field that drives the inflation and the scalar field that drives the accelerated expansion of the universe in the later period is not theoretically clear.
Unlike cosmological constants or vacuum energy, scalar field dark energy can evolve over time, can be spatially unevenly distributed, and can manifest as "attraction" or "repulsion" of different intensities depending on the relative magnitude of its kinetic and potential energy terms. Therefore, compared with the cosmological constant, there are more "degrees of freedom" in theory, which can avoid the problems of "fine tuning" and "coincidence" in the cosmological constant.
Among the scalar field dark energy models, quintessence, phantom and quintom are widely discussed scalar field dark energy models. These models are basically based on the equation of state w>-1,w
Figure 5: The Effect of Different Dark Energy on the Outcome of the Universe. If dark energy is a cosmological constant, the universe will continue to expand at an accelerated rate; In the case of phantom, the universe will cause a "big rip" in a "crazy" acceleration of expansion; If the density of dark energy starts to decline significantly at some point in the future, it could cause the universe to "big crunch". （nasa/gsfc）
Perhaps the most incredible or "crazy" of these is the ghostly dark energy. In this model, the equation of state w is always less than -1. This means that the energy density of dark energy will neither remain the same as the cosmological constant, nor gradually decrease like the essence of dark energy, but will continue to increase dramatically as the universe expands, thus further pushing the universe to expand! This "positive feedback" mechanism will eventually lead to the expansion rate of the universe approaching infinity in a finite time (because it is a spatial expansion, so it does not conflict with the theory of relativity), that is, the "big rip" of the universe. At that time, galaxies, stars, and even molecules, atoms, and subatomic particles in the universe will eventually be torn apart and cease to exist, and finally the universe will be in an incomprehensible state of "crazy" expansion due to this "ghost"!
Of course, in addition to scalar field dark energy, many dark energy models have been proposed. For example, K-essence drives the accelerated expansion of the universe through kinetic energy terms, tachyon, whose mass is an imaginary number and always exceeds the speed of light, Chaplygin gas, which can be used as both dark matter and dark energy, and holographic dark energy inspired by quantum gravity theory.
Through the above discussion, we can understand that dark energy is undoubtedly successful in explaining the accelerated expansion of the universe and the evolution of large-scale structure, but it has also been theoretically challenged as a new form of energy. There is still no convincing physical theory that can fully explain the origin and nature of dark energy.
On the other hand, it is not necessarily an additional matter or energy that causes the accelerated expansion of the universe, but may also be due to the fact that gravity is not nearly the same as what we currently measure on the solar system or subscale at a scale such as galaxies or the universe. It is precisely because of this difference that it is possible to produce results that are different from those of general relativity on a large scale, and even cause the accelerated expansion of the universe. Based on this possibility, a modification of the current general theory of relativity, i.e., a modification of the theory of gravity, is, in principle, a possible explanation for the accelerated expansion of the universe.
Figure 6: Schematic diagram of the main modifications to the classification of the various theories of gravity. （*shankaranarayanan & johnson, 2022, general relativity and gr**itation, 54, 44）
The current modification of the theory of gravity is mainly aimed at modifying the general theory of relativity, for example, by introducing an additional field coupled with the gauge tensor, which can be a scalar field, a vector field, and a tensor field. or assume that some basic laws and understandings can be broken, such as introducing additional dimensions, destroying Lorentz invariance, introducing "non-locality", etc.; It can also be assumed that gravitons have static mass, not statically massless particles with spin of 2, and so on.
Of course, the phenomena and effects produced by different modifications to the theory of gravity are different, and there are also different degrees of predictions from the dark energy theory. Therefore, future terrestrial and space telescopes are needed to accurately measure different phenomena and effects, accurately distinguish different theoretical models, and test general relativity with high precision. The next generation of cosmological survey observations in the future is currently in full swing, including the Euclid Telescope (EUCLID) and the Roman Telescope (RST) in space, the Survey Space Telescope (CSST) in China, and the Rubin Observatory (LSST) on the ground.
Does dark energy exist? What is its essence? It is believed that with the powerful observation capabilities and a variety of detection methods in the future, we will be able to unravel the mystery of dark energy, the ghost of the universe.
References. 1]eisenstein, d. j. et al., 2005, apj, 633, 560-574
2]gong, y. et al., 2019, apj, 883, 203
3]percival, w. j. et al., 2007, mnras, 381, 1053-1066
4]perlmutter s. et al., 1999, apj, 517, 565
5]phillips, m. m., 1993, apj, 413, l105-l108
6]planck collaboration, 2020a, a&a 641, a6
7]planck collaboration, 2020b, a&a 641, a8
8]riess, a. g. et al., 1998, apj, 116, 1009-1038
9]riess, a. g. et al., 2004, apj, 607, 665
Author: Emerald Cucumber loves to eat potato chips and is often electrocuted.
Editor-in-Chief: Chen Xuelei.
Censorship: He Jia.
Review: Tian Bin.
Approval: Lu Ye.
*: National Astronomical Observatories of the Chinese Academy of Sciences.
Edit: SDK