Theory – II. Dark Matter, Dark Energy, and Late-Time Universe
The late-time universe is serving as the largest and the longest-standing laboratory of low-energy/large-scale physics, especially for the gravity at the cosmological scales. The “experimental data” from this Lab are mainly about the cosmic expansion (evolution of background space-time) and the structure formation (growth of density perturbations). Observational results have clearly manifested the need of extra attractive gravity to help the structure formation and the need of anti-gravity to drive the accelerating expansion. The sources of these two opposite types of mysterious gravity are still unknown. These are the most important unknowns in cosmology in the 21st century. For the convenience of communication, the energy sources of them have respectively been dubbed DARK MATTER and DARK ENERGY, while another possible origin of anti-gravity is MODIFIED GRAVITY.
Observations show that the dark side of the universe controls 95% of the gravity at the present time. In other words, it contributes 95% of the energy contents of the present universe, speaking in the scenario of dark matter and dark energy. Dark matter and dark energy together control the cosmic expansion and structure formation at late times. Dark matter is important at almost all astrophysical scales, while dark energy is more influential at larger scales.
Loudly and clearly have observations told us the need of the extra gravity which forms and holds cosmic structures. However, we know little about its origin or the nature of dark matter. In such ignorant situation, the solid research in this field is to get more information about dark matter from observations such as galaxy surveys, galaxy cluster surveys, weak and strong lensings. The observational results are mainly about the distribution of matter at various scales, such as the distribution or correlations of matter at cosmological scales and the density profiles of dark matter in galaxies or galaxy clusters.
The large-scale distribution of light (left) and dark matter (right) from Millennium Simulation.
Although the identity of dark matter is unknown, the PARTICLE scenario of dark matter is popular, especially to the particle physics society. In this scenario, the possible value of the dark matter mass is diverse, spanning many orders of magnitude from those beyond the Standard Model to the inverse of the galactic length scale. If one further requires dark matter be thermal relics, i.e. they were once in thermal equilibrium with the Standard Model particles, a favored value of mass will be around TeV, a scale one can probe and close to the Standard Model scale. In this (sub)scenario, the need of dark matter can be read as a hint of a new fundamental particle beyond the Standard Model with a mass scale not beyond our reach. Accordingly, the thermal relic scenario, particularly WIMPs (weakly interacting massive particles), have attracted much attention. [Note that the meaning of the phrase "weakly interacting" is literal; it has nothing to do with the Weak Interaction of the Standard Model, although the latter is a candidate of the former.] For the convenience of dark matter detection, some particle physicists further hope dark matter particles do interact with the Standard Model particles, hopefully nucleons, with a cross section similar to that of the Weak Interaction. That is a fairy tale one may like to tell in the particle physics society, who has been searching for new particles and interactions over decades and has done a great job in building the Standard Model.
This is an interesting interplay between cosmology and particle physics. Cosmology brings a challenge: dark matter; particle physics provides solutions: some hypothetical particles beyond the Standard Model that had been established before the challenge came. To cosmology, WIMP is just one of many possibilities. (None of the possibilities is particularly convincing. Therefore getting more information from observations is more essential than conjecturing possibilities or sticking in one of them.) To particle physics, the need of dark matter gives a strong motivation and a helpful guideline for the detection of new particles and interactions among the vast possibilities of the beyond-Standard-Model physics. (Note that the discovery of a new particle following the dark matter guideline does not guarantee the discovered particle is the dominant dark matter.)
In short, particle physics needs dark matter, but cosmology may not need WIMP. No matter how good in building models the theorists are, after all, what cosmology needs is to let observations speak.
We know the need of anti-gravity from observations. However, we know little about it nature and origin; we do not know the identity of dark energy. Like dark matter, in such ignorant situation, what we truly need is to let observations speak. To cosmology, this is the solid research being carried on.
To fundamental physics, even without knowing the details, the need of anti-gravity has already given a large impact. It gives a hint of a new energy form or modified gravity; it also reinforces the cosmological constant problem. The cosmological constant problem is truly profound. It is a manifestation of the conflict between gravity and quantum nature, which is the most important problem in fundamental physics. The problem is about a huge discrepancy in the size of the cosmological constant, between the expectation from quantum vacuum energy in quantum field theory and that required by observations. We expect that in the reconciliation of gravity and quantum nature, this problem will be solved or automatically disappear. Moreover, the cosmological constant problem is not simply a problem of high energies, but also of low energies: Even the quantum fluctuations at the micron scale (a well-known scale close to our daily life) can contribute too large vacuum energy and ruin our universe. Thus, the solution to the cosmological constant problem cannot be purely of high energies; it should also manifest in well-known, low-energy physics. Accordingly, the knowledge and the tool of solving this problem may have already existed in the known low-energy physics, waiting for our call, rather than hiding in unknown and unreachable physics of extremely high energies. Hereby we get a useful CRITERION: A feasible reconciliation between gravity and quantum nature should give a low-energy solution to the cosmological constant problem.
In short, to cosmology one needs to make observations speak. To fundamental physics, the need of anti-gravity is shedding bright light on the most important problem: the conflict between gravity and quantum nature. Use it wisely.