Early Universe Cosmology Research Articles

Axion cosmology with early matter domination

The default assumption of early universe cosmology is that the postinflationary universe was radiation dominated until it was about 47000 years old. Direct evidence for the radiation dominated epoch extends back until nucleosynthesis, which began during the first second. However there are theoretical reasons to prefer a period of earlier matter domination, prior to nucleosynthesis, e.g. due to late decaying massive particles needed to explain baryogenesis. Axion cosmology is quantitatively affected by an early period of matter domination, with a different axion mass range preferred and greater inhomogeneity produced on small scales. In this work we show that such increased inhomogeneity can lead to the formation of axion miniclusters in axion parameter ranges that are different from those usually assumed. If the reheating temperature is below $58$ MeV, axion miniclusters can form even if the axion field is present during inflation and has been previously homogenized. The upper bound on the typical initial axion minicluster mass is raised from $10^{-10} M_{\odot}$ to $10^{-7} M_{\odot}$, where $M_{\odot}$ is a solar mass. These results may have consequences for indirect detection of axion miniclusters, and could conceivably probe the thermal history of the universe before nucleosynthesis.

On the difficulty of generating gravitational wave turbulence in the early universe

A recent article by Galtier and Nazarenko (2017 Phys. Rev. Lett. 119 221101) proposed that weakly nonlinear gravitational waves could result in a turbulent cascade, with energy flowing from high to low frequency modes or vice versa. This is an interesting proposition for early universe cosmology because it could suggest some ‘natural’ initial conditions for the gravitational background. In this paper we use the ADM formalism to show that, given some simple and, arguably, natural assumptions, such initial conditions lead to expansion (or collapse) of the spacetime on a timescale much faster than that of the turbulent cascade, meaning that the cascade is unlikely to have sufficient time to develop under general conditions. We suggest possible ways in which the expansion could be mitigated to give the cascade time to develop.

Semiclassical self consistent treatment of the emergence of seeds of cosmic structure. The second order construction

In this work we extend the results of [1] where, Semiclassical Selfconsistent Configurations (SSC) formalism was introduced. The scheme combines quantum field theory on a background space-time, semiclassical treatment of gravitation and spontaneous collapse theories. The approach is applied to the context of early universe cosmology using a formal description of the transition from an initial inflationary stage characterized by a spatially homogeneous and isotropic (H&I) universe, to another where inhomogeneities are present in association with quantum fluctuations of the field driving inflation. In that work two constructions are produced. One of them describes a universe that is completely spatially homogeneous and isotropic, and the other is characterized by a slight excitation of the particular inhomogeneous and anisotropic perturbation. Finally, a characterization of their gluing to each other is provided as representing the transition as a result from a spontaneous collapse of the state of the quantum field, following the hypothesis originally introduced in [2]. Specifically, in [1] this construction is carried out by using cosmological perturbation theory and working up to linear order in the perturbation. However, given the nonlinear nature of gravitation, we should in principle explore the application of the formalism in a nonlinear regime. To this end and as a first step, we study in this work the transition from a spatially homogeneous and isotropic (H&I) Semiclassical Self-Consistent Configuration (SSC-I) to one SSC-II that is not spatially (H&I), working this time up to second order in perturbation theory. We find that the self consistent construction now requires consideration of the so called tensor modes, as well as a nontrivial mixing of modes that made the analysis much more difficult and which could not a priori be warranted to work out in detail. The present work shows that this is indeed the case.

Towards the cold atom analog false vacuum

Analog condensed matter systems present an exciting opportunity to simulate early Universe models in table-top experiments. We consider a recent proposal for an analog condensed matter experiment to simulate the relativistic quantum decay of the false vacuum. In the proposed experiment, two ultra-cold condensates are coupled via a time-varying radio-frequency field. The relative phase of the two condensates in this system is approximately described by a relativistic scalar field with a potential possessing a series of false and true vacuum local minima. If the system is set up in a false vacuum, it would then decay to a true vacuum via quantum mechanical tunnelling. Should such an experiment be realized, it would be possible to answer a number of open questions regarding non-perturbative phenomena in quantum field theory and early Universe cosmology. In this paper, we illustrate a possible obstruction: the time-varying coupling that is invoked to create a false vacuum for the long-wavelength modes of the condensate leads to a destabilization of shorter wavelength modes within the system via parametric resonance. We focus on an idealized setup in which the two condensates have identical properties and identical background densities. Describing the system by the coupled Gross-Pitaevskii equations (GPE), we use the machinery of Floquet theory to perform a linear stability analysis, calculating the wavenumber associated with the first instability band for a variety of experimental parameters. However, we demonstrate that, by tuning the frequency of the time-varying coupling, it may be possible to push the first instability band outside the validity of the GPE, where dissipative effects are expected to damp any instabilities. This provides a viable range of experimental parameters to perform analog experiments of false vacuum decay.

Back-reaction of gravitational waves revisited

We study the back-reaction of gravitational waves in early universe cosmology, focusing both on super-Hubble and sub-Hubble modes. Sub-Hubble modes lead to an effective energy density which scales as radiation. Hence, the relative contribution of such gravitational waves to the total energy density is constrained by big bang nucleosynthesis. This leads to an upper bound on the tensor spectral slope nT which also depends on the tensor to scalar ratio r. Super-Hubble modes, on the other hand, lead to a negative contribution to the effective energy density, and to an equation of state of curvature. Demanding that the early universe is not dominated by the back-reaction leads to constraints on the gravitational wave spectral parameters which are derived.

Stability of infinite derivative Abelian Higgs models

Motivated by the stringy effects by modifying the local kinetic term of an Abelian Higgs field by the Gaussian kinetic term, we show that the Higgs field does not possess any instability; the Yukawa coupling between the scalar and the fermion, the gauge coupling, and the self interaction of the Higgs yields exponentially suppressed running at high energies, showing that such class of theory never suffers from vacuum instability. We briefly discuss its implications for the early Universe cosmology.

Cosmology and accelerator tests of strongly interacting dark matter

A natural possibility for dark matter is that it is composed of the stable pions of a QCD-like hidden sector. Existing literature largely assumes that pion self-interactions alone control the early universe cosmology. We point out that processes involving vector mesons typically dominate the physics of dark matter freeze-out and significantly widen the viable mass range for these models. The vector mesons also give rise to striking signals at accelerators. For example, in most of the cosmologically favored parameter space, the vector mesons are naturally long-lived and produce Standard Model particles in their decays. Electron and proton beam fixed-target experiments such as HPS, SeaQuest, and LDMX can exploit these signals to explore much of the viable parameter space. We also comment on dark matter decay inherent in a large class of previously considered models and explain how to ensure dark matter stability.

Calculating the Number of Universes in a Multiverse from First Principles with Linkage to Vacuum Energy and a Relationship with Nonlinear Early Universe B Fields. And the Early Universe B Field Connection to Non-Zero Graviton Mass

In this document, using a minisuperspace model, in the WdW equation we come up with an evaluation as to an equation which could fit a specialized set of boundary conditions for bounds to early universe vacuum energy. In addition, using the arguments of Golgov, as of a 1989 Moriond Astrophysics meeting of that date we ascertain the probability vacuum energy, as re-written as the cosmological constant, could lead to the production of a “baby” universe. The number, N, of that supposition is then analyzed in detail. This in turn has a relationship to a non-linear B field contribution in early universe cosmology whose consequences are explored and delineated. If there is a multiverse, the graviton mass is non-zero. Through arguments given in the document, the scale factor is also then, non-zero, which also portends to having the graviton not equal to zero if there is a corresponding initial non-linear B field, in the set up of the multiverse cosmology. If the Graviton has zero mass, this corresponds to a single repeating universe, with different morphology.

How to Determine a Jump in Energy Prior to a Causal Barrier, with an Attendant Current, for an Effective Initial Magnetic Field. In the Pre Planckian to Planckian Space-Time

We start where we use an inflaton value due to use of a scale factor . Also we use as the variation of the time component of the metric tensor in Pre-Planckian space-time. Our objective is to find an effective magnetic field, to obtain the minimum scale factor in line with Non Linear Electrodynamics as given by Camara, et al., 2004. Our suggestion is based upon a new procedure for an effective current based upon an inflaton time exp (i times (frequency) times (cosmological time)) factor as a new rescaled inflaton which is then placed right into a Noether Current scalar field expression as given by Peskins, 1995. This is before the Causal surface with which is, right next to a quantum bounce, determined by , with the next shift in the Hubble parameter as set up to be then . And is an initial degree of freedom value of about 110. Upon calculation of the current, and a resulting magnetic field, for the space time bubble, we then next obtain a shift in energy, leading to a transition from too. We argue then that the delineation of the term is a precursor to filling in information as to the Weyl Tensor for near singularity measurements of starting space-time. Furthermore, as evidenced in Equations ((26) and (27)) of this document, we focus upon a “first order” that checks into if a cosmological “constant” would be invariant in time, or would be along the trajectory of the time, varying Quinessence models. We close this document, with Maxwell equations as to Post Newtonian theory, for Gravity, with our candidates as to a magnetic field included in, with what we think this pertains to, as far as Gravo Electric and Gravo Magnetic fields, and then make suggestions as to a quantum version of this methodology for future gravitational wave physics research. This is Appendix G, this last topic, and deliberately set up future works paradigm which will be investigated in the coming year. It is based upon a Gravo Electric potential, and we make suggestions as to its upgrade in our future works, in early universe cosmology. In the reference by Poisson, and Will, they write and in this last section we come up with a value of U, based in part on the comparison with the alteration of velocity, due to a massive graviton, namely via the substitution, we write as , so as to come up with a post Newtonian approximation result for a magnetic field. We compare this magnetic field, as far as the Inflaton magnetic field, and use it to come up with observations with regards to the phenomenology of gravity in Pre Planckian to Planckian regime limits. We close, then with the observation given in Appendix H, of the inhomogeneity of Pre Planckian-to Planckian space time as a necessary condition for a Gravi-Magnetic field. We also reference an Appendix I, which does a summary of a 5th force calculation, and we then compare those results, with our temporary results of a Gravi Magneitc field, as we have tried to start up as a future works project.

Dark matter and neutrino mass from the smallest non-Abelian chiral dark sector

All pieces of concrete evidence for phenomena outside the standard model (SM) - neutrino masses and dark matter - are consistent with the existence of new degrees of freedom that interact very weakly, if at all, with those in the SM. We propose that these new degrees of freedom organize themselves into a simple dark sector, a chiral SU(3) x SU(2) gauge theory with the smallest nontrivial fermion content. Similar to the SM, the dark SU(2) is spontaneously broken while the dark SU(3) confines at low energies. At the renormalizable level, the dark sector contains massless fermions - dark leptons - and stable massive particles - dark protons. We find that dark protons with masses between 10-100 TeV satisfy all current cosmological and astrophysical observations concerning dark matter even if dark protons are a symmetric thermal relic. The dark leptons play the role of right-handed neutrinos and allow simple realizations of the seesaw mechanism or the possibility that neutrinos are Dirac fermions. In the latter case, neutrino masses are also parametrically different from charged-fermion masses and the lightest neutrino is predicted to be massless. Since the new "neutrino" and "dark matter" degrees of freedom interact with one another, these two new-physics phenomena are intertwined. Dark leptons play a nontrivial role in early universe cosmology while indirect searches for dark matter involve, decisively, dark matter annihilations into dark leptons. These, in turn, may lead to observable signatures at high-energy neutrino and gamma-ray observatories, especially once one accounts for the potential Sommerfeld enhancement of the annihilation cross-section, derived from the low-energy dark-sector effective theory, a possibility we explore quantitatively in some detail.

Sterile neutrino dark matter with supersymmetry

Sterile neutrino dark matter, a popular alternative to the WIMP paradigm, has generally been studied in non-supersymmetric setups. If the underlying theory is supersymmetric, we find that several interesting and novel dark matter features can arise. In particular, in scenarios of freeze-in production of sterile neutrino dark matter, its superpartner, the sterile sneutrino, can play a crucial role in early Universe cosmology as the dominant source of cold, warm, or hot dark matter, or of a subdominant relativistic population of sterile neutrinos that can contribute to the effective number of relativistic degrees of freedom Neff during Big Bang nucleosynthesis.

Dark catalysis

Recently it was shown that dark matter with mass of order the weak scale can be charged under a new long-range force, decoupled from the Standard Model, with only weak constraints from early Universe cosmology. Here we consider the implications of an additional charged particle C that is light enough to lead to significant dissipative dynamics on galactic times scales. We highlight several novel features of this model, which can be relevant even when the C particle constitutes only a small fraction of the number density (and energy density). We assume a small asymmetric abundance of the C particle whose charge is compensated by a heavy X particle so that the relic abundance of dark matter consists mostly of symmetric X and X̄, with a small asymmetric component made up of X and C. As the universe cools, it undergoes asymmetric recombination binding the free Cs into (XC) dark atoms efficiently. Even with a tiny asymmetric component, the presence of C particles catalyzes tight coupling between the heavy dark matter X and the dark photon plasma that can lead to a significant suppression of the matter power spectrum on small scales and lead to some of the strongest bounds on such dark matter theories. We find a viable parameter space where structure formation constraints are satisfied and significant dissipative dynamics can occur in galactic haloes but show a large region is excluded. Our model shows that subdominant components in the dark sector can dramatically affect structure formation.

Constraining cosmological ultralarge scale structure using numerical relativity

Cosmic inflation, a period of accelerated expansion in the early universe, can give rise to large amplitude ultra-large scale inhomogeneities on distance scales comparable to or larger than the observable universe. The cosmic microwave background (CMB) anisotropy on the largest angular scales is sensitive to such inhomogeneities and can be used to constrain the presence of ultra-large scale structure (ULSS). We numerically evolve nonlinear inhomogeneities present at the beginning of inflation in full General Relativity to assess the CMB quadrupole constraint on the amplitude of the initial fluctuations and the size of the observable universe relative to a length scale characterizing the ULSS. To obtain a statistically significant number of simulations, we adopt a toy model in which inhomogeneities are injected along a preferred direction. We compute the likelihood function for the CMB quadrupole including both ULSS and the standard quantum fluctuations produced during inflation. We compute the posterior given the observed CMB quadrupole, finding that when including gravitational nonlinearities, ULSS curvature perturbations of order unity are allowed by the data, even on length scales not too much larger than the size of the observable universe. Our results illustrate the utility and importance of numerical relativity for constraining early universe cosmology.

Constraining the break of spatial diffeomorphism invariance with Planck data

The current most accepted paradigm for the early universe cosmology, the inflationary scenario, shows a good agreement with the recent Cosmic Microwave Background (CMB) and polarization data. However, when the inflation consistency relation is relaxed, these observational data exclude a larger range of red tensor tilt values, prevailing the blue ones which are not predicted by the minimal inflationary models. Recently, it has been shown that the assumption of spatial diffeomorphism invariance breaking (SDB) in the context of an effective field theory of inflation leads to interesting observational consequences. Among them, the possibility of generating a blue tensor spectrum, which can recover the specific consistency relation of the String Gas Cosmology, for a certain choice of parameters. We use the most recent CMB data to constrain the SDB model and test its observational viability through a Bayesian analysis assuming as reference an extended ΛCDM+tensor perturbation model, which considers a power-law tensor spectrum parametrized in terms of the tensor-to-scalar ratio, r, and the tensor spectral index, nt. If the inflation consistency relation is imposed, r=−8 nt, we obtain a strong evidence in favor of the reference model whereas if such relation is relaxed, a weak evidence in favor of the model with diffeomorphism breaking is found. We also use the same CMB data set to make an observational comparison between the SDB model, standard inflation and String Gas Cosmology.

Kaluza-Klein towers in the early universe: Phase transitions, relic abundances, and applications to axion cosmology

We study the early-universe cosmology of a Kaluza-Klein (KK) tower of scalar fields in the presence of a mass-generating phase transition, focusing on the time-development of the total tower energy density (or relic abundance) as well as its distribution across the different KK modes. We find that both of these features are extremely sensitive to the details of the phase transition and can behave in a variety of ways significant for late-time cosmology. In particular, we find that the interplay between the temporal properties of the phase transition and the mixing it generates are responsible for both enhancements and suppressions in the late-time abundances, sometimes by many orders of magnitude. We map out the complete model parameter space and determine where traditional analytical approximations are valid and where they fail. In the latter cases we also provide new analytical approximations which successfully model our results. Finally, we apply this machinery to the example of an axion-like field in the bulk, mapping these phenomena over an enlarged axion parameter space that extends beyond those accessible to standard treatments. An important by-product of our analysis is the development of an alternate "UV-based" effective truncation of KK theories which has a number of interesting theoretical properties that distinguish it from the more traditional "IR-based" truncation typically used in the extra-dimension literature.

Early cosmology constrained

We investigate our knowledge of early universe cosmology by exploring how much additional energy density can be placed in different components beyond those in the ΛCDM model. To do this we use a method to separate early- and late-universe information enclosed in observational data, thus markedly reducing the model-dependency of the conclusions. We find that the 95% credibility regions for extra energy components of the early universe at recombination are: non-accelerating additional fluid density parameter ΩMR < 0.006 and extra radiation parameterised as extra effective neutrino species 2.3 < Neff < 3.2 when imposing flatness. Our constraints thus show that even when analyzing the data in this largely model-independent way, the possibility of hiding extra energy components beyond ΛCDM in the early universe is seriously constrained by current observations. We also find that the standard ruler, the sound horizon at radiation drag, can be well determined in a way that does not depend on late-time Universe assumptions, but depends strongly on early-time physics and in particular on additional components that behave like radiation. We find that the standard ruler length determined in this way is rs = 147.4 ± 0.7 Mpc if the radiation and neutrino components are standard, but the uncertainty increases by an order of magnitude when non-standard dark radiation components are allowed, to rs = 150 ± 5 Mpc.

On f(R) gravity in scalar–tensor theories

We study [Formula: see text] gravity models in the language of scalar–tensor (ST) theories. The correspondence between [Formula: see text] gravity and ST theories is revisited since [Formula: see text] gravity is a subclass of Brans–Dicke models, with a vanishing coupling constant ([Formula: see text]). In this treatment, four [Formula: see text] toy models are used to analyze the early-universe cosmology, when the scalar field [Formula: see text] dominates over standard matter. We have obtained solutions to the Klein–Gordon equation for those models. It is found that for the first model [Formula: see text], as time increases the scalar field decreases and decays asymptotically. For the second model [Formula: see text], it was found that the function [Formula: see text] crosses the [Formula: see text]-axis at different values for different values of [Formula: see text]. For the third model [Formula: see text], when the value of [Formula: see text] is small, the potential [Formula: see text] behaves like the standard inflationary potential. For the fourth model [Formula: see text], we show that there is a transition between [Formula: see text]. The behavior of the potentials with [Formula: see text] is totally different from those with [Formula: see text]. The slow-roll approximation is applied to each of the four [Formula: see text] models and we obtain the respective expressions for the spectral index [Formula: see text] and the tensor-to-scalar ratio [Formula: see text].

Consistent higher derivative gravitational theories with stable de Sitter and anti–de Sitter backgrounds

In this paper we provide the criteria for any generally covariant, parity preserving, and torsion-free theory of gravity to possess a stable de Sitter (dS) or anti--de Sitter (AdS) background. By stability we mean the absence of tachyonic or ghostlike states in the perturbative spectrum that can lead to classical instabilities and violation of quantum unitarity. While we find that the usual suspects, the $F(R)$ and $F(G)$ theories, can indeed possess consistent (A)dS backgrounds, $G$ being the Gauss-Bonnet term, another interesting class of theories, string-inspired infinite derivative gravitational theories, can also be consistent around such curved vacuum solutions. Our study should not only be relevant for quantum gravity and early universe cosmology involving ultraviolet physics, but also for modifications of gravity in the infrared sector vying to replace dark energy.

Gedanken Experiment for B Field Contributions to Initial Conditions for Relic Graviton Production Based upon an Initial Inflaton Value

This paper is to address using what a fluctuation of a metric tensor leads to, in Pre Planckian physics, namely . If so then, we pick the conditions for an equality, with a small , to come up with initial temperature, particle count and entropy affected by initial degrees of freedom in early Universe cosmology. This leads to an initial graviton production due to a minimum magnetic field, as established in our analysis. Which we relate to the inflaton as it initially would be configured and evaluated.

Nonsingular cosmology from an unstable Higgs field

The observed value of the Higgs mass indicates an instability of the Higgs scalar at large energy scales, and hence also at large field values. In the context of early universe cosmology, this is often considered to lead to problems. Here, we point out that we can use the instability of the Higgs field to generate an ekpyrotic phase of contraction. In the context of string theory, it is possible that at very high energy densities, extra states become massless leading to an [Formula: see text]-brane which causes the transition between a contracting phase in the past and the current expanding phase. Thus, the Higgs field may be useable to generate a nonsingular bouncing cosmology in which the anisotropy problem of usual bouncing scenarios is mitigated.