Hubble Scale Research Articles

No asymptotic acceleration without higher-dimensional de Sitter vacua

There has recently been considerable interest in the question whether and under which conditions accelerated cosmological expansion can arise in the asymptotic regions of field space of a d-dimensional EFT. We conjecture that such acceleration is impossible unless there exist metastable de Sitter vacua in more than d dimensions. That is, we conjecture that ‘Asymptotic Acceleration Implies de Sitter’ (AA⇒DS). Phrased negatively, we argue that the d-dimensional ‘No Asymptotic Acceleration’ conjecture (a.k.a. the ‘strong asymptotic dS conjecture’) follows from the de Sitter conjecture in more than d dimensions. The key idea is that the relevant field-space asymptotics almost always correspond to decompactification and that the only positive energy contribution which decays sufficiently slowly in this regime is the vacuum energy of a higher-dimensional metastable vacuum. This result is in agreement with recent Swampland bounds on the potential in the asymptotics in field space from e.g. the species bound, but is significantly more constraining. We further note that for our asymptotic decompactification limits based on higher-dimensional de Sitter, the Kaluza-Klein scale always falls below the Hubble scale asymptotically. In fact, this occurs whenever V′/V≤2d+k−2/kd−2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\left|{V}^{\\prime}\\right|/V\\le 2\\sqrt{\\left(d+k-2\\right)/k\\left(d-2\\right)} $$\\end{document} asymptotically, with k the number of decompactifying internal directions. This is steeper than what is needed for accelerated expansion.

Primordial black holes from a curvaton scenario with strongly non-Gaussian perturbations

We investigate the production of primordial black holes (PBHs) in a mixed inflaton-curvaton scenario with a quadratic curvaton potential, assuming the curvaton is in de Sitter equilibrium during inflation with 〈χ〉 = 0. In this setup, the curvature perturbation sourced by the curvaton is strongly non-Gaussian, containing no leading Gaussian term. We show that for m 2/H 2 ≳ 0.3, the curvaton contribution to the spectrum of primordial perturbations on CMB scales can be kept negligible but on small scales the curvaton can source PBHs. In particular, PBHs in the asteroid mass range 10-16 M ⊙ ≲ M ≲ 10-10 M ⊙ with an abundance reaching F PBH = 1 can be produced when the inflationary Hubble scale H ≳ 1012 GeV and the curvaton decay occurs in the window from slightly before the electroweak transition to around the QCD transition.

Parity-violating trispectrum from Chern-Simons gravity

A potential source for parity violation in the Universe is inflation. The simplest inflationary models have two fields: the inflaton and graviton, and the lowest-order parity-violating coupling between them is dynamical Chern-Simons (dCS) gravity with a decay constant f. Here, we show that dCS imprints a parity-violating signal in primordial scalar perturbations. Specifically, we find that, after dCS amplifies one graviton helicity due to a tachyonic instability, the graviton-mediated correlation between two pairs of scalars develops a parity-odd component. This correlation, the primordial scalar trispectrum, is then transferred to the corresponding curvature correlator and thus is imprinted in both large-scale structure (LSS) and the cosmic microwave background (CMB). We find that the parity-odd piece has roughly the same amplitude as its parity-even counterpart, scaled linearly by the degree of gravitational circular polarization Πcirc ∼ √ε[h 2/(M Pl f)] ≤ 1, with ε the slow-roll parameter, H the inflationary Hubble scale, and the upper bound saturated for purely circularly-polarized gravitons. We also find that, in the collapsed limit, the ratio of the two trispectra contains direct information about the graviton's spin. In models beyond standard inflationary dCS, e.g. those with multiple scalar fields or superluminal scalar sound speed, there can be a large enhancement factor F ≳ 106 to the trispectrum. We find that an LSS survey that contains N modes linear modes would place an nσ constraint on Πcirc r of ∼ 0.04 (n/3)(106/F)(106/N modes)1/2 from the parity-odd galaxy trispectrum, for tensor-to-scalar ratio r. We also forecast for several spectroscopic and 21-cm surveys. This constraint implies that, for high-scale single-field inflation parameters, LSS can probe very large dCS decay constants f ≲ 4 × 109 GeV(3/n)(F/106)(N modes/106)1/2. Our result is the first example of a massless particle yielding a parity-odd scalar trispectrum through spin-exchange.

Dynamical systems analysis of an interacting scalar field model in an anisotropic universe

In this paper, we investigate a non-canonical scalar field model in the background dynamics of anisotropic Locally Rotationally Symmetric (LRS) Bianchi type I universe where gravity is coupled minimally to scalar field which is taken as dark energy and pressureless dust as dark matter are the main matter content of the universe. We perform dynamical system analysis to characterize the cosmological evolution of the model with and without interaction in the dark sector separately. First, we convert the evolution equation into an autonomous system of ordinary differential equations by using a suitable choice of dimensionless variables, which are normalized over the Hubble scale. We choose scalar field coupling and potential in such a way that the autonomous system converted to a 2D system. Linear stability theory is employed to the extracted critical points to find the nature. From the analysis, we find some interesting cosmological scenarios, such as late-time scalar-field dominated solutions, which evolve in the quintessence era, cannot solve the coincidence problem. Accelerated scaling attractors are also obtained that correspond to the late phase evolution in agreement with present observational data, and these solutions also provide possible mechanisms to alleviate the coincidence problem. A complete cosmic evolution is obtained from early inflation to a late-time dark energy-dominated phase, connecting through a matter-dominated transient phase of the universe. Furthermore, we find that for different values of the interaction parameter α, the evolutionary trajectories of the Hubble parameter, and the distance modulus forecasted by the model are in quite well agreement with observational datasets.

Thermodynamic and cosmological parameters of early stages of the universe

The early Universe was characterized by the presence of heavy particles that decoupled at different temperatures leading to different phases of the Universe. This had some consequences on the time evolution of the thermodynamic and the cosmological parameters characterizing each phase of the early Universe. In this study, we derive the analytic expressions of the equations governing the time evolution of these parameters using the equations of state of the MIT bag model describing the quark–gluon plasma era. In addition, using the equations of state derived from considering the recent results of the lattice QCD simulations, we solve numerically the differential equation governing the time evolution of the energy density in the early Universe. The time evolution of the parameters under concern including the energy density, entropy density, temperature, pressure in addition to Hubble parameter and scale factor can then be estimated as will be presented in this work.

Opening up a Window on the Postinflationary QCD Axion.

The QCD axion cosmology depends crucially on whether the QCD axion is present during inflation or not. We point out that contrary to the standard criterion, the Peccei-Quinn (PQ) symmetry could remain unbroken during inflation, even when the axion decay constant, f_{a}, is (much) above the inflationary Hubble scale, H_{I}. This is achieved through the heavy lifting of the PQ scalar field due to its leading nonrenormalizable interaction with the inflaton, encoded in a high-dimensional operator which respects the approximate shift symmetry of the inflaton. The mechanism opens up a new window for the post-inflationary QCD axion and significantly enlarges the parameter space, in which the QCD axion dark matter with f_{a}>H_{I} could be compatible with high-scale inflation and free from constraints on axion isocurvature perturbations. There also exist nonderivative couplings, which still keep the inflaton shift symmetry breaking under control, to achieve the heavy lifting of the PQ field during inflation. Additionally, by introducing an early matter domination era, more parameter space of high f_{a} could yield the observed DM abundance.

Isocurvature constraints on scalar dark matter production from the inflaton

We investigate the production of a spectator scalar dark matter field that is directly coupled to the inflaton during inflation and reheating. We consider two specific inflationary potentials, namely the Starobinsky and T-model of inflation, which satisfy the constraints on the scalar tilt, $n_s$, and tensor-to-scalar ratio, $r$, measured by the Planck satellite. Excitation of light scalar dark matter during inflation may result in large isocurvature perturbations, which can be avoided by inducing a sizable effective dark matter mass during the inflationary phase. For purely gravitational production, the Planck isocurvature constraints require the dark matter mass to be larger than the Hubble scale at horizon exit, with $m_{\chi} \gtrsim 0.5H_*$. For small bare dark matter masses $m_{\chi} \ll H_*$, these constraints translate into a lower bound on the dark matter coupling to the inflaton. We argue that these constraints can be applied to a wide class of single-field slow-roll inflation models. We also derive isocurvature, dark matter abundance, and Lyman-$\alpha$ constraints on the direct coupling and bare dark matter mass.

The Warm Inflation Story

Warm inflation has normalized two ideas in cosmology, that in the early universe the initial primordial density perturbations generally could be of classical rather than quantum origin and that during inflation, particle production from interactions amongst quantum field, and its backreaction effects, can occur concurrent with inflationary expansion. When we first introduced these ideas, both were met with resistance, but today they are widely accepted as possibilities with many models and applications based on them, which is an indication of the widespread influence of warm inflation. Open quantum field theory, which has been utilized in studies of warm inflation, is by now a relevant subject in cosmology, in part due to this early work. In this review I first discuss the basic warm inflation dynamics. I then outline how to compute warm inflation dynamics from first-principles quantum field theory (QFT) and in particular how a dissipative term arises. Warm inflation models can have an inflaton mass bigger than the Hubble scale and the inflaton field excursion can remain sub-Planckian, thus overcoming the most prohibitive problems of inflation model building. I discuss the early period of my work in developing warm inflation that helped me arrive at these important features of its dynamics. Inflationary cosmology today is immersed in hypothetical models, which by now are acting as a diversion from reaching any endgame in this field. I discuss better ways to approach model selection and give necessary requirements for a well constrained and predictive inflation model. A few warm inflation models are pointed out that could be developed to this extent. I discuss how, at this stage, more progress would be made in this subject by taking a broader view on the possible early universe solutions that include not just inflation but the diverse range of options.

Cosmology as a weak gravitational field and the trans-Planckian problem

At momenta much higher than the Hubble scale, the cosmological expansion can be thought of as a weak gravitational field. We consider QFT in a particularly convenient set of coordinates that makes this manifest, so that, for those high momenta, the effects of the cosmological expansion can be dealt with using the standard tools of perturbation theory in Minkowski space. In this way, we re-derive standard results of QFT in a cosmological background, such as mode-stretching and gravitational particle production. We discuss the implications of our results for the trans-Planckian problem.

Meso-inflationary QCD axion

We study the possibility that the axion Peccei-Quinn symmetry is spontaneously broken after the beginning of inflation. This scenario interpolates between preinflationary and postinflationary axion cosmology with significant phenomenological differences from both. Since the axion is not present at the early stages of inflation large inflationary fluctuations are produced only at scales not constrained by cosmic microwave background (CMB), avoiding the strongest isocurvature constraints. The energy density in isocurvature perturbations at short scales however can be comparable with the adiabatic contribution from misalignment. These large overdensities can lead to the formation of axion miniclusters and also to constraints from Lyman-$\ensuremath{\alpha}$ forest and future CMB spectral distortion measurements. If Peccei-Quinn symmetry is broken during the first O(25) $e$-foldings of inflation no axions are produced from the string network but contributions from the annihilation of domain walls can further boost the abundance. This scenario is minimally realized if the Hubble scale during inflation drops below the Peccei-Quinn symmetry breaking scale but other realizations are possible.

Parity-odd and even trispectrum from axion inflation

The four-point correlation function of primordial scalar perturbations has parity-even and parity-odd contributions and the parity-odd signal in cosmological observations is opening a novel window to look for new physics in the inflationary epoch. We study the distinct parity-odd and even prediction from the axion inflation model, in which the inflaton couples to a vector field via a Chern-Simons interaction, and the vector field is considered to be either approximately massless (mA ≪ Hubble scale H) or very massive (mA ∼ H). The parity-odd signal arises due to one transverse mode of the vector field being predominantly produced during inflation. We adopt the in-in formalism to evaluate the correlation functions. Considering the vector field mode function to be dominated by its real part up to a constant phase, we simplify the formulas for numerical computations. The numerical studies show that the massive and massless vector fields give significant parity-even signals, while the parity-odd contribution is about one to two orders of magnitude smaller.

Scale-invariant enhancement of gravitational waves during inflation

The inflationary 1-loop tensor power spectrum from a spectator scalar field with a sharp peak is calculated. Recent studies on primordial black holes suggest that the inflationary curvature perturbation may be huge on small scales. An enhanced curvature perturbation may arise from a drastic enhancement of spectator scalar field fluctuations. In this paper, using the in-in formalism, we calculate 1-loop quantum corrections to primordial gravitational waves by such a spectator field with a sharp peak in momentum space. We find scale-invariant loop corrections in this full quantum setup, in contrast to the sharply peaked corrections in the previously calculated scalar-induced tensor modes. Especially on super Hubble scales, the primordial gravitational waves are also amplified, which can be understood as a Bogoliubov transformation of the vacuum due to the enhanced scalar field. This mechanism allows us to probe the scalar field properties on extremely short-distance scales with the current and future cosmic microwave background and gravitational wave experiments, opening a novel window for inflationary cosmology.

Role of the Hubble scale in the weak lensing versus CMB tension

We explore a reparametrization of the lensing amplitude tension between weak lensing (WL) and cosmic microwave background (CMB) data and its implications for a joint resolution with the Hubble tension. Specifically, we focus on the lensing amplitude over a scale of 12 Mpc in absolute distance units using a derived parameter ${S}_{12}$ and show its constraints from recent surveys in comparison with Planck 2018. In WL alone, we find that the absolute distance convention correlates ${S}_{12}$ with ${H}_{0}$. Accounting for this correlation in the 3D space ${S}_{12}\ifmmode\times\else\texttimes\fi{}{\ensuremath{\omega}}_{m}\ifmmode\times\else\texttimes\fi{}h$ reproduces the usual levels of $2\ensuremath{\sim}3\ensuremath{\sigma}$ tension inferred from ${S}_{8}\ifmmode\times\else\texttimes\fi{}{\mathrm{\ensuremath{\Omega}}}_{m}$. Additionally, we derive scaling relations in the ${S}_{8}\ifmmode\times\else\texttimes\fi{}h$ and ${S}_{12}\ifmmode\times\else\texttimes\fi{}h$ planes that are allowed by $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ and extrapolate target scalings needed to solve the ${H}_{0}$ and lensing-amplitude tensions jointly in a hypothetical beyond-$\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. As a test example, we quantify how the early dark energy scenario compares with these target scalings. Useful fitting formulas for ${S}_{8}$ and ${S}_{12}$ as a function of other cosmological parameters in $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ are provided, with 1% precision.

Hot Big Bang from inflation without the inflaton reheating

According to the standard lore, a prolonged inflation leaves a quantum field theory in a cold, low entropy state. Thus, some mechanism is needed to reheat this post-inflationary state, leaving a hot, thermal, radiation-dominated Universe. Typically, reheating is achieved coupling the inflaton field to the QFT degrees of freedom. We argue that the nonequilibrium dynamics of a non-conformal QFT in (post-)inflationary background space-time can produce hot quark-gluon plasma with the reheating temperature of order the inflationary Hubble scale, without the inflaton coupling.

Possible relation between the cosmological constant and standard model parameters

We propose possible properties of quantum gravity in de Sitter space, and find that they relate the value of the cosmological constant to parameters of the Standard Model. In de Sitter space we suggest (i) that the most sharply defined observables are obtained by scattering objects from the horizon and back to the horizon and (ii) that black holes of discrete charge are well defined states in the theory. For a black hole of minimal discrete electric charge, we therefore demand that a scattering process involving the black hole and a probe can take place within a Hubble time before evaporating away, so that the state of a discretely charged black hole is well defined. By imposing that the black hole's charge is in principle detectable, which involves appreciably altering the state of a scattered electron, we derive a relation between the Hubble scale, or cosmological constant, and the electron's mass and charge and order one coefficients that describe our ignorance of the full microscopic theory. This gives the prediction $\Lambda \sim 10^{-123 \pm 2} M_{Pl}^4$, which includes the observed value of dark energy. We suggest possible ways to test this proposal.

Wash-in leptogenesis after axion inflation

CP violation and the violation of baryon-minus-lepton number B−L do not necessarily have to occur simultaneously in order to accomplish successful leptogenesis. Instead, it suffices if new CP-violating interactions at high energies result in primordial charge asymmetries, which are then reprocessed into a nonvanishing B−L asymmetry by right-handed neutrinos (RHNs) at lower energies. In this paper, we study this novel mechanism known as wash-in leptogenesis, utilizing axion inflation as the source of high-scale CP violation. We specifically consider axion inflation coupled to the Standard Model hypercharge sector, which results in the dual production of hypermagnetic helicity and fermionic charge asymmetries. Although the survival of these charges is endangered by sphaleron processes, magnetic diffusion, and the chiral plasma instability, we find a large range of viable scenarios. We consistently account for RHN flavor effects and coherence among the Standard Model lepton flavors across a wide range of RHN masses. We find a lower bound of 105···9 GeV on the mass of the lightest RHN involved in wash-in leptogenesis, depending on the onset of turbulence in the chiral plasma and the Hubble scale of inflation. Our model is representative of a broader class of new leptogenesis scenarios and suggests interesting observational signatures with regard to intergalactic magnetic fields, primordial black holes, and gravitational waves.

Confinement in de Sitter space and the swampland

The relation between confinement scale Λc of a gauge theory and the Hubble scale H of the background quasi de Sitter space, required to satisfy the Festina Lente criteria (Λc ≳ H) from swampland principles, are investigated for a holographic theory using the Karch-Randall setup. A purely gravitational description admits an unstable de Sitter extremum, and the requirement of Festina Lente translates into requirements of a perturbative gravitational dual, but prohibits an arbitrarily small hierarchy for other parameters fixed. With an additional scalar sector, the theory admits metastable de Sitter minima, and the requirement of Festina Lente imposes constraints on the parameters. For some range of parameters where Festina Lente could be constraining, the de Sitter minimum is very close to decay, thus invalidating the applicability. Further consequences of the finite lifetime of the dS extrema and the relation with related swampland conjectures are investigated, and no contradictions are found. These results make case for the consistency and utility of the Karch-Randall setup, clarify the implications of Festina Lente for holographic theories, and connect Festina Lente to other swampland conjectures.

Dark energy density and Israel–Stewart (IS) bulk viscosity model

In this paper, we investigate the thermodynamics of a dark energy bulk viscosity model as a cosmic fluid. In this regard, the two theories of Eckart and Israel–Stewart (IS) are the bases of our work. Therefore, we first investigate the thermodynamics of cosmic fluids in the dark energy bulk viscosity model and the general relationships. Then, we express the thermodynamic relationships of Eckart’s theory. Due to the basic equations of Eckart’s theory and Friedmann’s equations, we consider two states, one is [Formula: see text] (standard) and the other is [Formula: see text] (non-standard). In the standard state, we define the pressure [Formula: see text], energy density [Formula: see text] and bulk viscosity coefficient [Formula: see text] of the cosmic fluid in terms of cosmic time and we obtain its relations. We also mention that in this standard state, because of [Formula: see text], the value of [Formula: see text] is zero, so [Formula: see text] is not defined in this state. But in the non-standard case [Formula: see text], the bulk viscosity coefficient [Formula: see text] is zero and only the scale factor, pressure and energy density of the cosmic fluid are defined. We also consider two states of constant and variable bulk viscosity coefficients and obtain three Hubble constant parameters and scale factor in terms of cosmic time, and energy density in terms of scale factor. In the state of variable bulk viscosity coefficient, we consider the viscosity coefficient as the power law from energy density [Formula: see text], which is [Formula: see text] and a constant. Following this, we discuss about the dissipative effects of cosmic fluids and examine the effects of energy density for dark energy in the IS theory. The results are comprehensively presented in Tables 1 and 2. Also, according to observational constraints, the results of the likelihood analysis for the IS viscous model are summarized in Table 3.

Cosmological bootstrap in slow motion

Speed matters. How the masses and spins of new particles active during inflation can be read off from the statistical properties of primordial density fluctuations is well understood. However, not when the propagation speeds of the new degrees of freedom and of the curvature perturbation differ, which is the generic situation in the effective field theory of inflationary fluctuations. Here we use bootstrap techniques to find exact analytical solutions for primordial 2-,3- and 4-point correlators in this context. We focus on the imprints of a heavy relativistic scalar coupled to the curvature perturbation that propagates with a reduced speed of sound cs, hence strongly breaking de Sitter boosts. We show that akin to the de Sitter invariant setup, primordial correlation functions can be deduced by acting with suitable weight-shifting operators on the four-point function of a conformally coupled field induced by the exchange of the massive scalar. However, this procedure requires the analytical continuation of this seed correlator beyond the physical domain implied by momentum conservation. We bootstrap this seed correlator in the extended domain from first principles, starting from the boundary equation that it satisfies due to locality. We further impose unitarity, reflected in cosmological cutting rules, and analyticity, by demanding regularity in the collinear limit of the four-point configuration, in order to find the unique solution. Equipped with this, we unveil that heavy particles that are lighter than H/cs leave smoking gun imprints in the bispectrum in the form of resonances in the squeezed limit, a phenomenon that we call the low speed collider. We characterise the overall shape of the signal as well as its unusual logarithmic mass dependence, both vividly distinct from previously identified signatures of heavy fields. Eventually, we demonstrate that these features can be understood in a simplified picture in which the heavy field is integrated out, albeit in a non-standard manner resulting in a single-field effective theory that is non-local in space. Nonetheless, the latter description misses the non-perturbative effects of spontaneous particle production, well visible in the ultra-squeezed limit in the form of the cosmological collider oscillations, and it breaks down for masses of order the Hubble scale, for which only our exact bootstrap results hold.

Stochastic formalism for U(1) gauge fields in axion inflation

We develop the stochastic formalism for U(1) gauge fields that has the Chern-Simons coupling to a rolling pseudo-scalar field during inflation.The Langevin equations for the physical electromagnetic fields are derived and the analytic solutions are studied. Using numerical simulation we demonstrate that the electromagnetic fields averaged over the Hubble scale continuously change their direction and their amplitudes fluctuate around the analytically obtained expectation values.Though the isotropy is spontaneously broken by picking up a particular local Hubble patch, each Hubble patch is understood independent and the isotropy is conserved globally byaveraging all the Hubble patches.