Universe: Feature Papers–Cosmology and Gravitation

Dear Colleagues,

In the last few decades, the standard model of cosmology, based on general relativity, has been shown to be able to explain a large number of observations 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. However, such success cannot hide the tensions at both large and small scales that precision data are revealing. At the large scale, there are tensions of unknown origin between the value of the Hubble parameter, H0, and SNe Ia data, the 2013 Planck parameters, and σ8 obtained from cluster number counts and weak lensing. Also the Planck 2015 data are in tension with σ8 growth rate, and with CFHTLenS weak lensing data. Moreover, a quadrupole–octupole alignment, a power hemispherical asymmetry, and a cold spot are presented in the large angle fluctuations in the CMB. To these problems, we should add the absence of detection particles constituting dark matter, the so-called ”small scale problems” of the LCDM, the “cosmological constant fine tuning problem,” and the “cosmic coincidence problem.”

The quoted issues motivated the investigations of other explanations to the dark matter and dark energy problems. These alternative models generate the dark matter and dark energy effects through additional matter fields (e.g., quintessence), or modified gravity (MG) models. In some cases, the quoted theories tried to explain dark matter and the accelerated expansion as the manifestation of extra dimensions, or higher-order corrections effects, as in the Dvali-Gabadadze-Porrati (DGP) model and in scalar-tensor and f(R) gravity. Disentangling between the plethora of models is not an easy task. An interpretation of cosmic acceleration as resulting from the backreaction effects of matter inhomogeneities in inhomogeneous universe models has also been proposed.

However, some recent results, such as the discovery of gravitational waves, may be used to reject some modified gravity theories thanks to the fact that they predict gravitational waves to move at speeds different to that of light, in disagreement with observations. A discussion on comparison of the predictions of modified theories of gravitation and those of general relativity are of paramount importance.

By converse, the previous discussion also urges us to try to improve our understanding of gravitational interaction, both at the classical and quantum levels. It is well known that to date no theory has been able to have gravity and quantum mechanics speak the one to the other.  A better understanding of gravity also leads us to look for solutions of the gravitational field equations describing compact object that in contradiction with the “cosmic censorship” idea have no horizons, and can reproduce the behaviour of some known solutions, as for example gravitational waves.

 It aims to set itself at the cutting edge of the most recent advances in the following areas:

1) our theoretical, phenomenological and experimental understanding of the gravitational interaction and its even more numerous and intertwined ties with other fields at all relevant distance and energy scales.

2) the disagreement between local measurements of Hubble's constant, and the value obtained from the ΛCDM model in conjunction with the data extracted from the Planck satellite mission.

3) the status of modified gravity theories after the discovery, with a high degree of accuracy, that the speeds of propagation of gravitational and light waves are equal.

5) Interpretation of the dark components of the universe in the framework of inhomogeneous universes.

6) The cosmological probes of the nature of the dark component of the universe.

7) the search for alternative solutions of gravitational field equations representing horizonless compact objects able to mimic the gravitational wave signals attributed to black holes.

You are welcome to send short proposals for submissions of feature papers to our Editorial Office ([email protected]). They will be evaluated by Editors first, and the selected papers will be thoroughly and rigorously peer reviewed.

Prof. Dr. Antonino Del Popolo
Prof. Dr. Yi-Fu Cai
Prof. Dr. Jean-Michel Alimi
Guest Editors

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• Problems of the LCDM model
• Large-scale structure formation
• Modified gravity theories
• Gravity at classical and quantum level