Particle Production In The Horndeski Gravity Model And Dynamical Vacuum Model

In recent years,the detection of gravitational waves and related research have become the hottest topics in cosmology,and they also bring hope to the exploration of the early universe.Some physicists believe that stochastic gravitational waves originate from the inflaton decays and cosmic strings in the early universe,so the detection of stochastic gravitational waves is of great significance to the study of the early universe.Similar to stochastic gravitational waves,the creation of fundamental particles is also closely related to the beginning of the universe.Furthermore,studying the particle production could make up for the lack of the study of the early universe with gravitational waves,for example,particle production could study the thermodynamic properties of the universe which gravitational waves cannot provide.In addition,the conversion between particles could influence the evolution of the universe and the formation of the large-scale structure of the universe,and may even influence the ultimate fate of the universe.Therefore,the origin of particles and their mutual transformation are important topics in cosmology.In this thesis,we study three related issues of particle production in the universe in the Horndeski gravity model and dynamic vacuum model: the production of scalar particles;the production of relativistic particles in the early universe and the production of nonrelativistic particles in the late universe;are there any thermodynamic differences between the interactions with and without particle production?In the introduction,we give a profile about the history of particle production research from both microscopic and macroscopic aspects,and briefly review the relevant important work.In Chapter 2,we discuss the generation of scalar particles in the context of a spatially homogeneous and isotropic universe.In order to study analytically the evolution of particle production over time,we focus on a simple Horndeski gravity model.We first suppose that the universe is dominated by a scalar field and derive the energy conservation condition.Then from the thermodynamic point of view,the macroscopic non-conservation of the scalar field energy-momentum tensor is explained as an irreversible production or annihilation of the scalar particles.Based on the explanation,we obtain the scalar particle production rate and the corresponding entropy.Since the universe,in general,could be regarded as a closed system satisfying the laws of thermodynamics,one can impose some thermodynamic constraints on it.Therefore,according to the thermodynamic properties of the universe,we obtain additional constraints on the simple Horndeski gravity model.In the past,the constraint on the Horndeski gravity model mainly relied on the observation of the universe and each update of the data meant that different results would be obtained.However,using the thermodynamic effects of particle production to constrain the Horndeski gravity model can avoid such uncertainty.The research in this thesis is the first attempt to adopt the proposed method to constrain the Horndeski gravity model.In Chapter 3,we study particle production and the corresponding entropy increase in the context of cosmology with dynamical vacuum.We focus on a particular model called“Running Vacuum model”,and analyze general thermodynamical aspects of particle and entropy production of the model.We first study the entropy of particles in the whole comoving volume during the early universe and late universe of the model.Then,in order to compare with observations,we pay attention to the contribution of the interior and surface of the cosmological apparent horizon to the total entropy of the universe.On combining the inner volume(particle)entropy with the entropy on the horizon,we prove in detail that the evolution of the entropy of the universe satisfies the generalized second law of thermodynamics and the law of thermodynamic equilibrium,and we also prove that the essential reason for this is the existence of a positive cosmological constant in the vacuum energy density.Studies on the thermodynamic properties of the Running Vacuum model are also covered in some literature,but these works lack complete descriptions and systematic analyses of the early and late universe.Our research makes a comprehensive analysis of the thermodynamic properties of the Running Vacuum model in different epoches of the universe,and fills the deficiency of the previous research.In Chapter 4,we study particle production in a general dynamical vacuum model.The focus of our discussion is whether different forms of energy exchange between matters could lead to different entropy increases of the system.For the matter obtaining energy,there are two possible outcomes: one is that the total number of particles increases,while the other is that the energy of every particle increases.It is instructive to study whether the entropy increases caused by these two situations are identical for predicting the future evolution of the universe.We first consider an early universe composed of photon gas and vacuum.We find the entropy increases of the two cases are consistent.However,for an ideal-gas universe,the conclusion is that the entropy increases are generally different.We have not found any similar research in the past.The research of this thesis is the first attempt to solve such problem.Finally,in Chapter 5,we make a detailed summary of the full thesis and point out the areas where the research needs to be improved.There are many research directions and problems that need to be solved in the future,and we list some important directions and problems.