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Register a new userMatter-antimatter asymmetry and other cosmological puzzles via running vacuum cosmologies
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Added by
Douglas A. Singleton
Publication date
2018
fields
Physics
and research's language is
English
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Current astronomical observations are successfully explained by the present cosmological paradigm based on the concordance model ($Lambda_0$CDM + Inflation). However, such a scenario is composed of a heterogeneous mix of ingredients for describing the different stages of cosmological evolution. Particularly, it does not give an unified explanation connecting the early and late time accelerating inflationary regimes which are separated by many aeons. Other challenges to the concordance model include: a singularity at early times or the emergence of the Universe from the quantum gravity regime, the graceful exit from inflation to the standard radiation phase, as well as, the coincidence and cosmological constant problems. We show here that a simple running vacuum model or a time-dependent vacuum may provide insight to some of the above open questions (including a complete cosmic history), and also can explain the observed matter-antimatter asymmetry just after the initial deflationary period.
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We present a model for the Universe in which quantum anomalies are argued to play an important dual role: they are responsible for generating matter-antimatter asymmetry, but also provide time-dependent contributions to the vacuum energy density of running-vacuum type, which drive the Universe evolution. During the inflationary phase of a string-inspired Universe, and its subsequent exit, the existence of primordial gravitational waves induce gravitational anomalies, which couple to the Kalb-Ramond (KR)axion field of the massless gravitational string multiplet. Such anomalous CP violating interactions have two important effects: first, they lead to contributions to the vacuum energy density of the form appearing in the running vacuum model (RVM) framework, proportional to both the square and the fourth power of the effective Hubble parameter $H$, the $H^4$ terms being associated with anomaly-induced inflation within the RVM framework. Second, there is an undiluted KR axion at the end of inflation, which plays an important role in generating matter-antimatter asymmetry through baryogenesis via leptogenesis in models with heavy right handed neutrinos. As the Universe exits inflation and enters a radiation dominated era, the generation of chiral fermionic matter is responsible for the cancellation of gravitational anomalies, thus restoring diffeomorphism invariance for the matter/radiation (quantum) theory, as required for consistency. Chiral U(1) anomalies may remain uncompensated, though, during matter/radiation dominance, providing RVM-like $H^2$ and $H^4$ contributions to the Universe energy density. Finally, in the current era, gravitational anomalies resurface, leading to much weaker RVM-like $H_0^2$ contributions to the vacuum energy density.Our model favours axionic Dark Matter,the source of which can be the KR axion.
In this letter, we elaborate further on a Cosmological Running-Vacuum type model for the Universe, suggested previously by the authors within the context of a string-inspired effective theory in the presence of a Kalb-Ramond (KR) gravitational axion field which descends from the antisymmetric tensor of the massless gravitational string multiplet. In the presence of this field, which has anomalous CP violating interactions with the gravitons, primordial gravitational waves induce gravitational anomalies, which in turn are responsible for the appearance of $H^2$ and $H^4$ contributions to the vacuum energy density, these terms being characteristic of generic running-vacuum-model (RVM) type, where $H$ is the Hubble parameter. In this work we prove in detail the appearance of the $H^4$ terms due to gravitational-anomaly-induced condensates in the energy density of the primordial Universe, which can self-consistently induce inflation, and subsequent exit from it, according to the generic features of RVM. We also argue in favour of the robustness of our results, which were derived within an effective low-energy field theory approach, against Ultra Violet completion of the theory. During the radiation and matter-dominated eras, gravitational anomalies cancel, as required for the consistency of the quantum matter/radiation field theory. However, chiral and QCD-axion-type anomalies survive and have important consequences for both cosmic magnetogenesis and axionic dark matter in the Universe. Finally, the stringy RVM scenario presented here predicts quintessence-like dynamical dark energy for the current Universe, which is compatible with the existing fitting analyses of such model against observations
We investigate the dynamical features of a large family of running vacuum cosmologies for which $Lambda$ evolves as a polynomial in the Hubble parameter. Specifically, using the critical point analysis we study the existence and the stability of singular solutions which describe de-Sitter, radiation and matter dominated eras. We find several classes of $Lambda(H)$ cosmologies for which new analytical solutions are given in terms of Laurent expansions. Finally, we show that the Milne universe and the $R_{h}=ct$ model can be seen as perturbations around a specific $Lambda(H)$ model, but this model is unstable.
In this work a series of methods are developed for understanding the Friedmann equation when it is beyond the reach of the Chebyshev theorem. First it will be demonstrated that every solution of the Friedmann equation admits a representation as a roulette such that information on the latter may be used to obtain that for the former. Next the Friedmann equation is integrated for a quadratic equation of state and for the Randall--Sundrum II universe, leading to a harvest of a rich collection of new interesting phenomena. Finally an analytic method is used to isolate the asymptotic behavior of the solutions of the Friedmann equation, when the equation of state is of an extended form which renders the integration impossible, and to establish a universal exponential growth law.
We investigate the running vacuum model (RVM) in the framework of scalar field theory.This dynamical vacuum model provides an elegant global explanation of the cosmic history, namely the universe starts from a non-singular initial de Sitter vacuum stage, it passes smoothly from an early inflationary era to a radiation epoch (graceful exit) and finally it enters the dark matter and dark energy (DE) dominated epochs, where it can explain the large entropy problem and predicts a mild dynamical evolution of the DE. Within this phenomenologically appealing context, we formulate an effective {it classical} scalar field description of the RVM through a field $phi$, called the {it vacuumon}, which turns out to be very helpful for an understanding and practical implementation of the physical mechanisms of the running vacuum during both the early universe and the late time cosmic acceleration. In the early universe the potential for the vacuumon may be mapped to a potential that behaves similarly to that of the scalaron field of Starobinsky-type inflation at the {it classical} level, whilst in the late universe it provides an effective scalar field description of DE. The two representations, however, are not physically equivalent since the mechanisms of inflation are entirely different. Moreover, unlike the scalaron, vacuumon is treated as a classical background field, and not a fully fledged quantum field, hence cosmological perturbations will be different between the two pictures of inflation.
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