LCDM Cosmology: Pros and Cons

Dear Colleagues,

The ΛCDM model, or Concordance Model, is a successful model, describing the Universe’s large-scale structure formation and evolution, the state of the early Universe, and the abundance of the different forms of matter and energy. It is in agreement with a plethora of observations, and its predictive power has been already checked against old and new discoveries (e.g., lensing of the CMB, B-mode polarization, and the kinetic SZ effect). However, these successes do not allow for us to state that the model is correct. It merely implies that deviations from ΛCDM are too small compared with the current observational uncertainties to be inferred from cosmological data alone. Moreover, such success cannot hide the tensions at large and small scales that the precision data are revealing, such as the following: (1) The perturbation amplitude σ₈ and Hubble parameter, H₀, from the 2013 and 2015 Planck parameters versus the cluster number counts and CFHTLenS weak lensing, and versus the SNIa data, respectively. (2) The large-angle anomalies of CMB (i.e., quadrupole-octupole alignment, power hemispherical asymmetry, cold spot). (3) The small scales problems, namely, the cusp-core problem, missing satellite problem, too-big-to-fail problem, the satellites planes problem, and so on. (4) A lacking of detection of DM particles. Decades of direct and indirect searching of DM particles, a fundamental part of the model, did not achieve any positive results. (5) The “cosmological constant problem”, the 100 orders of magnitude gap between the predicted quantum field theories vacuum energy and present cosmological upper bounds, which gives rise to an extreme fine-tuning problem at the Planck scale era, thus in the initial conditions of dark energy. At the same time, it opens up the “cosmic coincidence problem” connected to the DE and DM energy densities, which are approximately equal nowadays.

Issues 1–5 seeded the push for several new modified gravity theories, in order to understand our Universe without DM, or at least by connecting the accelerated expansion to some new features of gravity. However, the large amount of different astrophysical evidence makes it difficult for modified gravity to account for all of these observations. On the other hand, changing the Einstein–Hilbert action brings new degrees of freedom that can behave like DM. Concerning the small scale problems (point 3), cosmological solutions have been proposed, based either on modifying the power spectrum at small scales, on modifying the kinematic or dynamic gravitational behavior of the constituent DM particles, or an astrophysical solution related to the heating of DM due to the supernovae feedback or the interaction of baryon and DM through dynamical friction.

The timeliness of this Special Issue essentially proceeds from the great amount of high precision data, either currently obtained or scheduled for the upcoming years (CMB-S4, LSST, SKA, CTA, etc.), which is a fundamental tool to understand which of the approaches better describes our Universe.

Thus, in the Special Issue “ΛCDM Cosmology: Pros and Cons”, we would like to focus on the comprehension of the Universe the ΛCDM model gives us, and on a discussion of the solution to the problems of the model.

The main topics of this Special Issue are (but are not restricted to) as follows:

1. Pros and cons of ΛCDM of the ΛCDM model
2. DM and DE: theory, observations, experiments, and constraints
3. DM and DE through modified gravity theories
4. The cosmological constant problems
5. ΛCDM with baryonic effects
6. Small scale problems of the ΛCDM model

Prof. Dr. Antonino Del Popolo
Prof. Dr. Morgan Le Delliou
Guest Editors

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• ΛCDM model
• DM and DE
• modified gravity theories