Astrophysics, Cosmology with Gravitational Waves and Symmetry

Dear Colleagues,

On the one hand, we are in an era where we have observational constraints on a variety of parameters describing the early and late universe. These include parameters such as the scalar spectral index, the tensor-to-scalar ratio, the running of index which describes the early universe, as well as the late-time EoS and Om(z), the amplitude of the magnetic fields and the strength of cosmological GWs today. On the other hand, modern cosmology is yet to provide satisfactory answers to a variety of questions such as: Did the universe really start its expansion from a big bang singularity or did the universe undergo a bounce? What is the mechanism of inflation? What drives bounces? What is dark energy? What are the dynamics during the epoch of reheating? Does the universe pass through an instantaneous reheating or does the reheating phase have a non-zero e-fold number? What is dark matter? Why does the universe undergo an accelerated stage at a very large as well as at a very low curvature scale?

Primordial gravitational waves (GWs) carry the imprints of the dynamics of the universe during its earliest stages; this is connected with the isotropic and homogeneous symmetry of the universe. Beside the primordial GWs, the induced GWs generated from enhanced scalar perturbations over low scales as well as from a primordial magnetic field are of considerable interest at present. With a variety of GW detectors being proposed to operate over a wide range of frequencies, there is great expectation that observations of primordial GWs (or secondary GWs as well) can provide us with an unprecedented window to the physics operating during inflation (or bounce) and reheating. Regarding the reheating, this is one of the most important phases of the early universe. It essentially links our standard thermal universe with the pre-thermal state, such as the inflationary universe, through a complex non-linear process. Over the years, major cosmological observations have given us ample data to understand the theoretical as well as observational aspects of both the thermal and the very early non-thermal inflationary universe cosmology to an unprecedented level. However, our understanding of the intermediate reheating phase is still at a preliminary stage both in terms of theory and observations. As far as the reheating EoS parameter is concerned, it is generally assumed to be time-dependent during the decay of the inflation to the radiation field. The evolution of the EoS must reach the value 1/3 at the end of reheating in order to smoothly connect the reheating phase to the radiation-dominated epoch. However, the initial value of the EoS (i.e., at the start of reheating) is badly constrained, hence, the reheating dynamics remain poorly understood. The primordial gravitational waves (PGWs) generated during inflation may encode valuable information about the initial value of the reheating EoS as well as its dynamics as they evolve through the reheating phase. Modified theories of gravity can play an important role in helping us overcome various cosmological problems to a certain extent. The modifications of Einstein’s gravity can be achieved in one of the following manners: (1) the inclusion of higher curvature terms in addition to the Einstein–Hilbert term in the gravitational action, (2) by introducing extra spatial dimensions over the usual four dimensional spacetime, and (3) by introducing additional matter fields. Currently, the modified gravitational models have been constrained well over the CMB scales and cosmological observations, i.e., in respect to the Planck constraints. However, in order to understand the viability of these models, we should also investigate their imprints and compatibility with observations at smaller scales. We intend to investigate this particularly in the context of the evolution of PGWs in modified theories of gravitation. We expect that the constraints from the sensitivities of the various current and forthcoming GW observatories can help us narrow the window on viable models of gravitation.

This specific issue is broadly inclined to both theoretical and observational sides of gravitational waves. We hope that through this issue, people can think and understand more deeply about gravitational waves. The specific themes include (but are not confined to):

• Primordial gravitational waves generated from quantum fluctuations in the very early universe;
• Induced gravitational waves generated from either enhanced scalar perturbations (over low scales) in the context of primordial black holes or primordial magnetic field (over all scales) in the context of magnetogenesis;
• Identifying the reheating era more through primary and secondary GWs;
• Narrowing the window of modified gravity theories through the observation of PGWs;
• Modification of standard cosmology to be consistent with NANOGrav data;
• Other aspects that can be connected with gravitational waves.

We also welcome reviews on the progress we have reached so far and what more we can achieve within the upcoming 5–10 years.

Dr. Tanmoy Paul
Prof. Dr. Soumitra SenGupta
Dr. Debaprasad Maity
Guest Editors

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• primary and induced (or secondary) gravitational waves
• reheating era
• modification of standard cosmology
• modified gravity theories