What kind of science is cosmology?

In recent years, by theory and observation cosmology has advanced substantially. Parameters of the concordance or ΛCDM cosmological model are given with unprecedented precision (“precision cosmology”). On the other hand, 95% of the matter content of the universe are of an unknown nature. This awkward situation motivates the present attempt to find cosmology's place among the (exact) natural sciences. Due to its epistemic and methodical particularities, e.g., as a mathematized historical science, cosmology occupies a very special place. After going through some of the highlights of cosmological modeling, the conclusion is reached that knowledge provided by cosmological modeling cannot be as explicative and secure as knowledge gained by laboratory physics.     

Quintom cosmology: Theoretical implications and observations

We review the paradigm of quintom cosmology. This scenario is motivated by the observational indications that the equation-of-state of dark energy across the cosmological constant boundary is mildly favored, although the data are still far from being conclusive. As a theoretical setup we introduce a no-go theorem existing in quintom cosmology, and based on it we discuss the conditions for the equation-of-state of dark energy realizing the quintom scenario. The simplest quintom model can be achieved by introducing two scalar fields with one being quintessence and the other phantom. Based on the double-field quintom model we perform a detailed analysis of dark energy perturbations and we discuss their effects on current observations. This type of scenario usually suffers from a manifest problem due to the existence of a ghost degree-of-freedom, and thus we review various alternative realizations of the quintom paradigm. The developments in particle physics and string theory provide potential clues indicating that a quintom scenario may be obtained from scalar systems with higher derivative terms, as well as from non-scalar systems. Additionally, we construct a quintom realization in the framework of braneworld cosmology, where the cosmic acceleration and the phantom divide crossing result from the combined effects of the field evolution on the brane and the competition between four- and five-dimensional gravity. Finally, we study the outsets and fates of a universe in quintom cosmology. In a scenario with null energy condition violation one may obtain a bouncing solution at early times and therefore avoid the Big Bang singularity. Furthermore, if this occurs periodically, we obtain a realization of an oscillating universe. Lastly, we comment on several open issues in quintom cosmology and their connection to future investigations.     

Physically self-consistent basis for modern cosmology

Cosmoparticle physics appeared as a natural result of internal development of cosmology seeking physical grounds for inflation, baryosynthesis, and nonbaryonic dark matter and of particle physics going outside the Standard Model of particle interactions. Its aim is to study the foundations of particle physics and cosmology and their fundamental relationship in the combination of respective indirect cosmological, astrophysical, and physical effects. The ideas on new particles and fields predicted by particle theory and on their cosmological impact are discussed, as well as the methods of cosmoparticle physics to probe these ideas, are considered with special analysis of physical mechanisms for inflation, baryosynthesis, and nonbaryonic dark matter. These mechanisms are shown to reflect the main principle of modern cosmology, putting, instead of formal parameters of cosmological models, physical processes governing the evolution of the big-bang universe. Their realization on the basis of particle theory induces additional model-dependent predictions, accessible to various methods of nonaccelerator particle physics. Probes for such predictions, with the use of astrophysical data, are the aim of cosmoarcheology studying astrophysical effects of new physics. The possibility of finding quantitatively definite relationships between cosmological and laboratory effects on the basis of cosmoparticle approach, as well as of obtaining a unique solution to the problem of physical candidates for inflation, mechanisms of baryogenesis, and multicomponent dark matter, is exemplified in terms of gauge model with broken family symmetry, underlying horizontal unification and possessing quantitatively definite physical grounds for inflation, baryosynthesis, and effectively multicomponent dark-matter scenarios.     

Primordial non-Gaussianities after Planck 2015: An introductory review

Deviations from Gaussian statistics of the cosmological density fluctuations, so-called primordial non-Gaussianities (NG), are one of the most informative fingerprints of the origin of structures in the universe. Indeed, they can probe physics at energy scales inaccessible to laboratory experiments, and are sensitive to the interactions of the field(s) that generated the primordial fluctuations, contrary to the Gaussian linear theory. As a result, they can discriminate between inflationary models that are otherwise almost indistinguishable. In this short review, we explain how to compute the non-Gaussian properties in any inflationary scenario. We review the theoretical predictions of several important classes of models. We then describe the ways NG can be probed observationally, and we highlight the recent constraints from the Planck mission, as well as their implications. We finally identify well motivated theoretical targets for future experiments and discuss observational prospects.     

宇宙学常数疑难

当代天文学的一系列观测事实都支持应该存在一个非零的正的宇宙学常数，但是，人们发现当前宇宙学常数值太小，而且宇宙学常数即真空能量密度与现在的物质密度巧合地具有相同的量级，然而现有物理学理论还无法给出合理的解释，因此宇宙学常数问题成为物理学和天文学上最重大的疑难之一。文章综述了近年来宇宙在加速膨胀这一重大的天文发现和宇宙学常数的观测结果以及当前理论物理学在宇宙学常数问题上的一些尝试。     

What can we learn from the tension between PLANCK and BICEP2 data?

Recently BICEP2 collaboration has announced the detection of the primordial gravitational waves at high confidence level.In light of the results of B-modes power spectrum from BICEP2 and using the basedΛCDM,a constraint on the tensor-to-scalar ratio r=0.20+0.07-0.05(68%C.L.)can be obtained,however,this result is in apparent tension with the limit on standard inflation models from the recent PLANCK measurement,r0.11(95%C.L.).Herein we review the recent progress on the cosmological studies after BICEP2 and discuss on different ways of reconciling the tension between PLANCK and BICEP2 data.We will discuss possible modifications on the standard cosmological model,such as including the running of scalar spectral index or other cosmological parameters correlated with inflationary cosmological parameters,or tilting the primordial power spectrum at large scales by introducing a cut off which can be predicted by bouncing cosmology.We will also comment on another possibility of generating extra B-modes of CMB polarization,namely by a non-zero polarization rotation angle during its transferring from the last scattering surface.     

TESLA: Inventor of the Electrical Age,by W. Bernard Carlson

The success of the standard picture of the Hot Big Bang scenario, emphasizing the assumptions that have to be made in the initial conditions, is reviewed. A possible model which could account for these is then described. The inflationary universe scenario is one of the most exciting theoretical ideas in cosmology, because it offers the opportunity of explaining much of the observed large scale structure. These observations and the constraints they imply for the scenario are discussed, with emphasis being placed on the recent remarkable results from the COsmic Background Explorer (COBE).     

Modular thermal inflation without slow-roll approximation

We study an inflationary scenario where thermal inflation is followed by fast-roll inflation. This is a rather generic possibility based on the effective potentials of spontaneous symmetry breaking in the context of particle physics models. We show that a large enough expansion could be achieved to solve cosmological problems. However, the power spectrum of primordial density perturbations from the quantum fluctuations in the inflaton field is not scale invariant and thus inconsistent with observations. Using the curvaton mechanism instead, we can obtain a nearly scale invariant spectrum, provided that the inflationary energy scale is sufficiently low to have long enough fast-roll inflation to dilute the perturbations produced by the inflaton fluctuations.     

The warm inflationary universe

In the past decade, the importance of dissipation and fluctuation to inflationary dynamics has been realized and has led to a new picture of inflation called warm inflation. Although these phenomena are common to condensed matter systems, for inflation models their importance has only recently started to be appreciated. The article describes the motivation for these phenomena during inflation and then examines their origins from first principles quantum field theory treatments of inflation models. Cosmology today is a data intensive field and this is driving theory to greater precision and predictability. This opens the possibility to consider tests for detecting observational signatures of dissipative processes, which will be discussed. In addition, it will be discussed how particle physics and cosmology are now working in tandem to push the boundaries of our knowledge about fundamental physics.     

Inflation,quantum fields,and CMB anisotropies

Inflationary cosmology has proved to be the most successful at predicting the properties of the anisotropies observed in the cosmic microwave background (CMB). In this essay we show that quantum field renormalization significantly influences the generation of primordial perturbations and hence the expected measurable imprint of cosmological inflation on the CMB. However, the new predictions remain in agreement with observation, and in fact favor the simplest forms of inflation. In the near future, observations of the influence of gravitational waves from the early universe on the CMB will test our new predictions.     

第一讲微波背景辐射的各向异性、偏振及宇宙电离的历史

文章对微波背景辐射的各向异性、偏振及宇宙电离的历史给出了评述性介绍．从大爆炸理论的预言，到观测的发现，到其各向异性及偏振的探测，微波背景辐射(CMB)向人们揭示了丰富的宇宙学信息．文章在对基本理论作了简单介绍后，着重讲述了最新的CMB的观测结果及其物理意义．特别对微波背景各向异性探测器(Wilkinson Microwave Anisotropy Probe，WMAP)的偏振观测及其对宇宙重新电离的限制给出了较详细的叙述．     

Quantum gravitational contributions to the cosmic microwave background anisotropy spectrum

We derive the primordial power spectrum of density fluctuations in the framework of quantum cosmology. For this purpose we perform a Born-Oppenheimer approximation to the Wheeler-DeWitt equation for an inflationary universe with a scalar field. In this way, we first recover the scale-invariant power spectrum that is found as an approximation in the simplest inflationary models. We then obtain quantum gravitational corrections to this spectrum and discuss whether they lead to measurable signatures in the cosmic microwave background anisotropy spectrum. The nonobservation so far of such corrections translates into an upper bound on the energy scale of inflation.     

Phantom inflation in little rip

We study the phantom inflation in little rip cosmology, in which the current acceleration is driven by the field with the parameter of state w<−1$w<-1$, but since w tends to −1 asymptotically, the rip singularity occurs only at infinite time. In this scenario, before the rip singularity is arrived, the universe is in an inflationary regime. We numerically calculate the spectrum of primordial perturbation generated during this period and find that the results may be consistent with observations. This implies that if the reheating happens again, the current acceleration might be just a start of phantom inflation responsible for the upcoming observational universe.     

Observational constraints on loop quantum cosmology

In the inflationary scenario of loop quantum cosmology in the presence of inverse-volume corrections, we give analytic formulas for the power spectra of scalar and tensor perturbations convenient to compare with observations. Since inverse-volume corrections can provide strong contributions to the running spectral indices, inclusion of terms higher than the second-order runnings in the power spectra is crucially important. Using the recent data of cosmic microwave background and other cosmological experiments, we place bounds on the quantum corrections.     

$$\Lambda $$CDM: Much More Than We Expected,but Now Less Than What We Want

The \(\Lambda \)CDM cosmological model is remarkable: with just six parameters it describes the evolution of the Universe from a very early time when all structures were quantum fluctuations on subatomic scales to the present, and it is consistent with a wealth of high-precision data, both laboratory measurements and astronomical observations. However, the foundation of \(\Lambda \)CDM involves physics beyond the standard model of particle physics: particle dark matter, dark energy and cosmic inflation. Until this ‘new physics’ is clarified, \(\Lambda \)CDM is at best incomplete and at worst a phenomenological construct that accommodates the data. I discuss the path forward, which involves both discovery and disruption, some grand challenges and finally the limits of scientific cosmology.     

Cosmological bootstrap

The extremely large value of the cosmological constant that is characteristic of particle physics and the inflation of the early universe are inherently interconnected. One can construct a superpotential that, after consideration for the leading effects due to supergravity, produces a flat potential of inflaton with a constant density of energy V = Λ⁴. The introduction of relatively small quantum loop corrections to the parameters of this superpotential naturally leads to a dynamical instability taking the form of an inflationary regime of relaxation of the cosmological constant. This pattern is phenomenologically consistent with observational data at Λ ∼ 10¹⁶ GeV.     

On quintessential cosmological models and exponential potentials

We first study dark energy models with a minimally-coupled scalar field and generalized exponential potentials, admitting exact solutions for the cosmological equations: actually, it turns out that for this class of potentials the Einstein field equations exhibit alternative Lagrangians, and are completely integrable and separable. We analyze their analytical solutions, especially discussing when they are compatible with a late time quintessential expansion of the universe. As a further issue, we discuss how quintessential scalar fields with exponential potentials can be connected to the inflationary phase, building up a quintessential inflationary scenario: actually, it turns out that the transition from inflation toward late-time exponential quintessential tail admits a kination period, which is an indispensable ingredient of this kind of theoretical models. All such considerations have been made by including also radiation into the model.     

Two-Fluid Dark Energy Models in Bianchi Type-III Universe with Variable Deceleration Parameter

Some new exact solutions of Einstein’s field equations have come forth within the scope of a spatially homogeneous and anisotropic Bianchi type-III space-time filled with barotropic fluid and dark energy by considering a variable deceleration parameter. We consider the case when the dark energy is minimally coupled to the perfect fluid as well as direct interaction with it. Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution of the universe, the equation of state (EoS) for dark energy ω ^(de), in both cases, tends to ?1 (cosmological constant, ω ^(de)=?1), by displaying various patterns as time increases, which is consistent with recent observations. The cosmic jerk parameter in our derived models are in good agreement with the recent data of astrophysical observations under appropriate condition. It is observed that the universe starts from an asymptotic Einstein static era and reaches to the ΛCDM model. So from recently developed Statefinder parameters, the behaviour of different stages of the universe has been studied. The physical and geometric properties of cosmological models are also discussed.