Possibly, the most general action in the background of isotropic and homogeneous space-time has been considered to study the quantum evolution of the early universe, apart from a cosmological constant. The hermiticity of the effective Hamiltonian operator in the presence of curvature squared terms suggests unitary time evolution of the quantum states, assuring conservation of probability. The oscillatory behaviour of the semi-classical wavefunction around a de-Sitter solution signals that the theory is classically allowed, and the universe enters an inflationary regime just after Planck's era. In view of a hierarchy of Hubble flow parameters, and using a redefined effective potential, the complicated classical field equations in the presence of several coupling parameters, reduce to standard general-relativistic equations with a single scalar field. As a result, inflation has been studied without considering any additional flow parameters. Inflationary parameters lie very much within the presently available Planck's data, and the model admits graceful exit from inflation.
We study the possibility to probe the spatial geometry of the Universe by supernova measurement of the cubic correction to the luminosity distance. We illustrate with an accelerating universe model with infinite-volume extra dimensions, for which the 1$σ$ level supernova results indicate that the Universe is closed.
The next generation of instruments designed to measure the polarization of the cosmic microwave background (CMB) will provide a historic opportunity to open the gravitational wave window to the primordial Universe. Through high sensitivity searches for primordial gravitational waves, and tighter limits on the energy released in processes like phase transitions, the CMB polarization data of the next decade has the potential to transform our understanding of the laws of physics underlying the formation of the Universe.
We investigate the reach of the LIGO/Virgo/KAGRA detectors in the search for signatures of first-order phase transitions in the early Universe. Utilising data from the first three observing runs, we derive constraints on the parameters of the underlying gravitational-wave background, focusing on transitions characterised by strong supercooling. As an application of our analysis, we determine bounds on the parameter space of two representative particle physics models. We also comment on the expected reach of third-generation detectors in probing supercooled phase transitions.
Classical and quantum gravitational instabilities, can, respectively, inflate and warm up a primordial Universe satisfying a superstring-motivated principle of "Asymptotic Past Triviality". A physically viable big bang is thus generated without invoking either large fine-tunings or a long period of post-big bang inflation. Properties of the pre-bangian Universe can be probed through its observable relics, which include: i) a (possibly observable) stochastic gravitational-wave background; ii) a (possible) new mechanism for seeding the galactic magnetic fields; iii) a (possible) new source of la…
Inflation models are numerous. It is extremely difficult, if possible at all, to identify the actual underlying inflation model of our primordial universe. Thus, for the purpose of proving/falsifying inflation and using inflation to probe new physics, model independent approaches are crucial. Massive fields play a uniquely important role in those missions. This short review is based on a talk by one of the authors (YW) in the 2nd LeCosPA Symposium.
We develop a nonperturbative quantum field formalism to describe scalar gauge-invariant metric flucturations in the early universe from a 5D apparent (Ricci flat) vacuum.
A unified field theory in ten dimensions, of all interactions, can describe high energy processes occuring in the early universe. In such a theory transitions that give properties of the universe can occur due to the presence of algebraic and geometric structures. A correspondence between theory and observations of the universe is made, to obtain a new interpretation and properties. This paper consists of a field theory and cosmological model of dark and normal energy and matter, cosmological constant, acceleration and inflation in the early universe.
These lectures discuss how the direct detection of gravitational waves can be used to probe the very early Universe. We review the main cosmological mechanisms which could have produced relic gravitational waves, and compare theoretical predictions with capabilities and time scales of current and upcoming experiments.
Global, symmetry-breaking phase transitions in the early universe can generate scaling seed networks which lead to metric perturbations. The acoustic waves in the photon-baryon plasma sourced by these metric perturbations, when Silk damped, generate spectral distortions of the cosmic microwave background (CMB). In this work, the chemical potential distortion ($μ$) due to scaling seed networks is computed and the accompanying Compton $y$-type distortion is estimated. The specific model of choice is the $O(N)$ nonlinear $σ$-model for $N\gg 1$, but the results remain the same order of magnitude f…
In the very early universe, a generalized Schwinger effect can create pairs from both electrical and gravitational fields. The expectation value of fermionic current induced by these newly created pairs has been recently computed in de Sitter spacetime. I will discuss different limiting cases of this result and some of its possible physical interpretations.
We carry out a simple analysis of (n+3)-dimensional gravity in the context of recent work on 'large' supplementary dimensions and deduce a formula for the expected compactification radius for the n additional dimensions in the universe, as a function of the Planck and the electro-weak scales. We argue that the correspondingly modified gravitational force gives rise to effects that might be within the detection range of dedicated neutron experiments. A scattering analysis of the corresponding modified gravitational forces suggests that slow neutron scattering off atomic nuclei with null spin ma…
Effects of the vacuum polarization leading to change of the effective gravitational constant and particle creation in the early Universe are discussed. Gauss-Bonnet type coupling to the curvature is considered. Renormalization methods are generalized for such coupling and the N-dimensional space-time. Calculations of creation of entropy and visible matter of the Universe by the gravity of dark matter are made.
We study early universe cosmologies derived from a scalar-tensor action containing cosmological constant terms and massless fields. The governing equations can be written as a dynamical system which contains no past or future asymptotic equilibrium states (i.e. no sources nor sinks). This leads to dynamics with very interesting mathematical behaviour such as the existence of heteroclinic cycles. The corresponding cosmologies have novel characteristics, including cyclical and bouncing behaviour possibly indicating chaos. We discuss the connection between these early universe cosmologies and tho…
Stochastic gravitational wave backgrounds, predicted in many models of the early universe and also generated by various astrophysical processes, are a powerful probe of the Universe. The spectral shape is key information to distinguish the origin of the background since different production mechanisms predict different shapes of the spectrum. In this paper, we investigate how precisely future gravitational wave detectors can determine the spectral shape using single and broken power-law templates. We consider the detector network of Advanced-LIGO, Advanced-Virgo and KAGRA and the space-based g…
Since the temperature fluctuations in cosmic microwave background (CMB) on large-angular scales probe length scales that were super-horizon sized at photon decoupling and hence insensitive to microphysical processes, the low-multipole CMB data are supposed to be a good probe to the physics of the primordial Universe. In this letter we will constrain the cosmological parameters in the base $Λ$CDM model with tensor perturbations by only using low-multipole CMB data, including Background Imaging of Cosmic Extragalactic Polarization (B2), Planck released in 2013 (P13) and Wilkinson Microwaves Anis…
Two new one-parameter tracking behavior dark energy representations $ω=ω_0/(1+z)$ and $ω=ω_0 e^{z/(1+z)}/(1+z)$ are used to probe the geometry of the Universe and the property of dark energy. The combined type Ia supernova (SN Ia), Sloan Digital Sky Survey (SDSS), and Wilkinson Microwave Anisotropy Probe (WMAP) data indicate that the Universe is almost spatially flat and that dark energy contributes about 72% of the matter content of the present universe. The observational data also tell us that $ω(0)\sim -1$. It is argued that the current observational data can hardly distinguish different da…
Many motivated extensions of the Standard Model predict the existence of cosmic strings. Gravitational waves originating from the dynamics of the resulting cosmic string network have the ability to probe many otherwise inaccessible properties of the early universe. In this study we show how the spectrum of gravitational waves from a cosmic string network can be used to test the equation of state of the early universe prior to Big Bang Nucleosynthesis (BBN). We also demonstrate that current and planned gravitational wave detectors such as LIGO, LISA, DECIGO/BBO, and ET/CE have the potential to …
We study the modified theory of gravity in Friedmann Robertson Walker universe composed of several perfect fluids. We consider the power law inflation and determine the equation of state parameters in terms of the parameters of modified gravity's Lagrangian in the early universe. We also discuss briefly the gravitational baryogenesis in this model.
I review the "Trans-Planckian Censorship Conjecture" (TCC) and its implications for cosmology, in particular for the inflationary universe scenario. Whereas the inflationary scenario is tightly constrained by the TCC, alternative early universe scenarios are not restricted.
After a brief summary of general relativity and cosmology, we present the basic concepts underlying inflation, the currently best motivated models for the early Universe. We describe the simplest inflation models, based on a single scalar field, and discuss how primordial cosmological perturbations are generated. We then review some recent developments concerning multi-field inflation models, in particular multi-field Dirac-Born-Infeld inflation.
We derive a curved space generalization of a scalar to fermion decay rate with a Yukawa coupling in expanding Friedmann-Robertson-Walker universes. This is done using the full theory of quantum fields in curved spacetime and the added-up transition probability method. It is found that in an expanding universe the usual Minkowskian decay rates are considerably modified for early times. For conformally coupled scalars the decay rate is modified by a positive additive term proportional to the inverse of mass and related to the expansion rate of the Universe. We compare and contrast our results wi…
In this thesis we study early universe in the frame work of M theory. In particular We assume that the early universe is homogeneous, anisotropic, and is dominated by the mutually BPS 22'55' intersecting branes of M theory. We find that, asymptotically, three spatial directions expand to infinity and the remaining spatial directions reach stabilised values. We give a physical description of the stabilisation mechanism.
As we follow our Universe to early times, we find that matter was crushed to high densities, somewhat similar to the behavior at a black hole singularity. String theory has made progress in explaining the internal structure of black holes, so we would like to extrapolate the ideas learnt from black holes to the early Universe. If we assume that we want the most probable state of the Universe at early times, then we should look at the kind of state that describes a generic black hole. This suggests a definite equation of state for the matter in the early Universe. Quantum effects can stretch ac…
Using a formalism recently introduced we study the decaying of the cosmological parameter during the early evolution of an universe, whose evolution is governed by a vacuum equation of state. We use a stochastic approach in a nonperturbative treatment of the inflaton field from a Noncompact Kaluza-Klein (NKK) theory, to study the evolution of energy density fluctuations in the early universe.
In the most distant reaches of the Universe, the 21-cm hyperfine transition in neutral hydrogen provides one of the only available tracers of large-scale structure. A number of instruments have been working and planned to measure the 21-cm line signals, and in particular, Experiment to Detect the Global EoR Signature (EDGES) recently has reported the first detection of an absorption signal, which corresponds to the 21-cm line global signal at the epoch of reionization (EoR). The future large radio telescope, Square Kilometre Array (SKA) will be able to deliver the high-precision measurement of…
We have explored the possibility that the universe at very early stage was dominated by (macroscopic) heterotic strings. We have found that the dimensionless parameter $Gμ$ for the heterotic strings varies from $10^{-2}$ to $10^{-4}$ as the universe evolve from the matter dominance to radiation dominance. This led to the interesting consequence of epoch dependent gauge coupling constant. The gauge coupling constant at early times was found to be much stronger than the present strong interaction.
Assuming that superstring theory is the fundamental theory which unifies all forces of Nature at the quantum level, I argue that there are key limitations on the applicability of effective field theory techniques in describing early universe cosmology.
Observational cosmology is in its "golden age" with a vast amount of recent data on the distribution of matter and light in the universe. This data can be used to probe theories of the very early universe. It is small amplitude cosmological fluctuations which encode the information about the very early universe and relate it to current data. Hence, a central topic in these lectures is the "theory of cosmological perturbations", the theory which describes the generation of inhomogeneities in the very early universe and their evolution until the current time. I will apply this theory to three cl…
This contribution gives a brief overview of the theoretical ideas underlying our current understanding of the early Universe. Confronting the predictions of the early Universe models with cosmological observations, in particular of the cosmic microwave background fluctuations, will improve our knowledge about the physics of the primordial Universe.
Cosmological inflation is not the only early universe scenario consistent with current observational data. I will discuss the criteria for a successful early universe cosmology, compare a couple of the proposed scenarios (inflation, bouncing cosmologies, and the {\it emergent} scenario), focusing on how future observational data will be able to distinguish between them. I will argue that we need to go beyond effective field theory in order to understand the early universe, and that principles of superstring theory will yield a nonsingular cosmology.
Near comoving wavenumber k, the gravitational-wave background (GWB) from inflation carries information about the physical conditions near two moments in cosmic history: the moment when k ``left the horizon'' during inflation, and the moment when it ``re-entered the horizon'' after inflation. We investigate the extent to which this information can be extracted if the GWB is measured by a combination of cosmic-microwave-background (CMB) polarization experiments on large scales and space-based laser-interferometer experiments on small scales. To disentangle this information, we derive a new gravi…
In these lectures I focus on early universe models which can explain the currently observed structure on large scales. I begin with a survey of inflationary cosmology, the current paradigm for understanding the origin of the universe as we observe it today. I will discuss some progress and problems in inflationary cosmology before moving on to a description of two alternative scenarios - the Matter Bounce and String Gas Cosmology. All early universe models connect to observations via the evolution of cosmological perturbations - a topic which will be discussed in detail in these lectures.
The braneworld model of Dvali-Gabadadze-Porrati (DGP) is a theory where gravity is modified at large distances by the arrested leakage of gravitons off our four-dimensional universe. Cosmology in this model has been shown to support both "conventional" and exotic explanations of the dark energy responsible for today's cosmic acceleration. We present new results for the gravitational field of a clustered matter source on the background of an accelerating universe in DGP braneworld gravity, and articulate how these results differ from those of general relativity. In particular, we show that orbi…
Solitons formation through classical dynamics of two scalar fields with the potential having a saddle point and one minimum in (2+1)-space-time is discussed. We show that under certain conditions in the early Universe both domain walls and strings can be formed even if scalar fields are inflaton ones.
Quantum electrodynamics predicts that photons undergo one-loop scattering. The combined effect of this on the behaviour of a photon gas for temperatures above 10^{10} K results in a softening of the equation of state. We calculate the effect this has on the effective equation of state in the early universe, taking into account all the species of the Standard Model. The change to the dynamics of the early universe is discussed.
Probing String-Theory-Inspired Topologies of the Early Universe through CMB Temperature and Polarization Anisotropies
National Technical University of Athens（雅典国立技术大学）；King’s College London（伦敦国王学院）；Instituto de Física Corpuscular (IFIC), CSIC-University of Valencia（瓦伦西亚大学-CSIC粒子物理研究所(IFIC)）；University of Valencia（瓦伦西亚大学）
We obtain new stronger bounds by orders of magnitude, on the ultimate temperature of the universe by exploiting the copious production of gravitinos at finite temperature.
In this talk I will describe the role of supersymmetric flat directions in the early Universe cosmology. Particularly I will discuss how supergravity effects lift the flat direction potential and leads to interesting cosmological consequences.
To probe the late evolution history of the Universe, we adopt two kinds of optimal basis systems. One of them is constructed by performing the principle component analysis (PCA) and the other is build by taking the multidimensional scaling (MDS) approach. Cosmological observables such as the luminosity distance can be decomposed into these basis systems. These basis are optimized for different kinds of cosmological models that based on different physical assumptions, even for a mixture model of them. Therefore, the so-called feature space that projected from the basis systems is cosmological m…
We examine in the context of supersymmetric models whether the usual cosmological upper bound on the axion decay constant can be relaxed by assuming a period of stronger QCD in the early universe. By evaluating the axion potential in the early universe and also taking into account the dilaton potential energy, it is argued that a stronger QCD is not useful for raising up the bound.
In the early Universe matter was crushed to high densities, in a manner similar to that encountered in gravitational collapse to black holes. String theory suggests that the large entropy of black holes can be understood in terms of fractional branes and antibranes. We assume a similar physics for the matter in the early Universe, taking a toroidal compactification and letting branes wrap around the cycles of the torus. We find an equation of state p_i=w_i rho, for which the dynamics can be solved analytically. For black holes, fractionation can lead to non-local quantum gravity effects across…
In this work, the Friedman equations for hadronic matter in the Robertson-Walker metric in the early Universe are obtained. We consider the hadronic phase, formed after the hadronization of the quark-gluon plasma, that means times from 10^{-6}s to 1s. The set of equations is derived and the behavior of the system is studied considering one approximate analytical solution.
Superstring theory, models with extra dimensions and other SUSY models generically predict that the coupling constants are in fact vacuum expectation values of fields like the dilaton, moduli etc. Assuming some of these fields are light during inflation, we get generation of small classical inhomogeneities in these fields from inflation. Consequently, coupling constants inherit small inhomogeneities at scales much larger than the causal horizon in the early universe. After the moduli get pinned down to their minima, the spatial variations of coupling constants in the late time universe will be…
In the context of current particle physics theories, it is quite likely that topological defects may be present in our universe. An observation of these fossils from the early universe would lead to invaluable insight into cosmology and particle physics, while their absence provides important constraints on particle-cosmology model building. I describe recent efforts to address cosmological issues in condensed matter systems such as He-3 and a possible solution to the magnetic monopole problem due to defect interactions. (Invited talk at the 1997 RESCEU Symposium, University of Tokyo.)
