Inflationary Scale Research Articles

Gravitational-wave signatures of gravito-electromagnetic couplings

Gravitational waves (GWs) can undoubtedly serve as a messenger from the early Universe acting as well as a novel probe of the underlying gravity theory. In this work, motivated by one-loop vacuum-polarization effects on curved spacetime, we investigate a gravitational theory with non-minimal curvature-electromagnetic coupling terms of the form ξR/M Pl2 F μν F μν , where M Pl is the reduced Planck mass, R is the scalar curvature and F μν the Faraday tensor, being responsible for the generation of primordial electromagnetic fields. We study then the GW signatures of such coupling terms deriving in particular for the first time to the best of our knowledge the modified tensor modes equation of motion. Remarkably, we find a universal infrared (IR) frequency scaling f 5 of the electromagnetically induced GW (EMIGW) signal, which, depending on the energy scale of inflation, the duration of inflation and reheating as well as the dynamical behaviour of the coupling function ξ, can be well within the detection sensitivity bands of GW experiments such as SKA, LISA, ET and BBO, being thus potentially detectable in the future by GW observatories.

Boosting Nigeria's Bond Market: Evidence from Macroeconomic Perspective

Macroeconomics and finance drive bond markets in developing countries, allowing governments to raise money for businesses and infrastructure. However, many factors in developing countries like Nigeria hinder the growth of the bond market. This study investigates a novel contribution by focusing exclusively on the Nigerian bond market and considering a set of macroeconomic drivers that have not been studied collectively. The study applies the Autoregressive Distributive Lag (ARDL) model to examine the short-run dynamics between key macrofinancial drivers and the Nigerian bond market. The findings show that an increase in fiscal deficit does not support the development of the bond market in Nigeria. Similar results are found for GDP per capita, inflation, interest rates, and banking scale; all negatively affect bond market development. However, domestic debt and stock market development positively promote bond market development. The policy implications offered from these findings are to redirect their spending to projects that have the potential to stimulate economic activities that help the government generate more revenue. Policymakers should also cut unnecessary spending on recurrent expenditure, which is a significant part by implementing efficient fiscal discipline.

Magnetogenesis from axion-SU(2) inflation

We describe a novel proposal for inflationary magnetogenesis by identifying the non-Abelian sector of Spectator Chromo Natural Inflation (SCNI) with the SU(2)L sector of the Standard Model. This mechanism relies on the recently discovered attractor of SCNI in the strong backreaction regime, where the gauge fields do not decay on super-horizon scales and their backreaction leads to a stable new trajectory for the rolling axion field. The large super-horizon gauge fields are partly transformed after the electroweak phase transition into electromagnetic fields. The strength and correlation length of the resulting helical magnetic fields depend on the inflationary Hubble scale and the details of the SCNI sector. For suitable parameter choices we show that the strength of the resulting magnetic fields having correlation lengths around 1 Mpc are consistent with the required intergalactic magnetic fields for explaining the spectra of high energy γ rays from distant blazars.

Insights into gravitinos abundance, cosmic strings and stochastic gravitational wave background

In this paper, we investigate D-term inflation within the framework of supergravity, employing the minimal Kähler potential. Following previous studies that revealed that this model can overcome the η-problem found in F-term models, we explore reheating dynamics and gravitino production, emphasizing the interplay between reheating temperature, spectral index, and gravitino abundance. Our analysis indicates that gravitino production is sensitive to the equation of state during reheating, affecting the reheating temperature and subsequent dark matter relic density. Furthermore, we analyze gravitational waves generated by cosmic strings, providing critical constraints on early Universe dynamics and cosmic string properties, the energy scales of both inflation and string formation influence the stochastic gravitational wave background (SGWB) generated by these cosmic strings.

Minimal solution to the axion isocurvature problem from nonminimal coupling

The main limitation for preinflationary breaking of Peccei-Quinn (PQ) symmetry is the upper bound on the Hubble rate during inflation from axion isocurvature fluctuations. This leads to a tension between high scale inflation and QCD axions with grand unified theory scale decay constants, which reduces the potential for a detection of tensor modes at next generation cosmic microwave background (CMB) experiments. We propose a mechanism that explicitly breaks PQ symmetry via nonminimal coupling to gravity, that lifts the axion mass above the Hubble scale during inflation and has negligible impact on today’s axion potential. The initially heavy axion gets trapped at an intermediate minimum during inflation given by the phase of the nonminimal coupling, before it moves to its true CP-conserving minimum after inflation. During this stage, it undergoes coherent oscillations around an adiabatically decreasing minimum, which slightly dilutes the axion energy density, while still being able to explain the observed dark matter relic abundance. This scenario can be tested by the combination of next generation CMB surveys like CMB-S4 and LiteBIRD with haloscopes such as ABRACADABRA, DMRadio, or CASPEr-Electric. Published by the American Physical Society 2024

Choice of vacuum state and the relation between inflationary and Planck scales

Choice of vacuum state and the relation between inflationary and Planck scales

Gravitational waves from a curvature-induced phase transition of a Higgs-portal dark matter sector

The study of interactions between dark matter and the Higgs field opens an exciting connection between cosmology and particle physics, since such scenarios can impact the features of dark matter as well as interfering with the spontaneous breaking of the electroweak symmetry. Furthermore, such Higgs-portal models of dark matter should be suitably harmonised with the various epochs of the universe and the phenomenological constraints imposed by collider experiments. At the same time, the prospect of a stochastic gravitational wave background offers a promising new window into the primordial universe, which can complement the insights gained from accelerators. In this study, we examined whether gravitational waves can be generated from a curvature-induced phase transition of a non-minimally coupled dark scalar field with a portal coupling to the Higgs field. The main requirement is that the phase transition is of first order, which can be achieved through the introduction of a cubic term on the scalar potential and the sign change of the curvature scalar. This mechanism was investigated in the context of a dynamical spacetime during the transition from inflation to kination, while also considering the possibility for inducing electroweak symmetry breaking in this manner for a sufficiently low reheating temperature when the Higgs-portal coupling is extremely weak. We considered a large range of inflationary scales and both cases of positive and negative values for the non-minimal coupling, while taking into account the bound imposed by Big Bang Nucleosythesis. The resulting gravitational wave amplitudes are boosted by kination and thus constrain the parameter space of the couplings significantly. Even though the spectra lie at high frequencies for the standard high inflationary scales, there are combinations of parameter space where they could be probed with future experiments.

Supermassive gauginos in supergravity inflation with high-scale SUSY breaking

A model of supergravity inflation we recently proposed can produce slow roll inflation and a realistic spectrum of particles even without F-term supersymmetry breaking. Supersymmetry is broken only by a D-term induced by a recently discovered new type of Fayet-Iliopoulos (FI) term. Almost all supersymmetric partners of the standard model fields can get masses as high as the inflationary Hubble scale. The exception is gauginos, for which the vanishing of F-terms implies an exact cancellation that keeps their masses exactly zero. To cure this problem without spoiling the simplicity of our model we introduce a new term that further enlarges the space of supergravity effective actions. It is an F-term that, similarly to the new FI term, becomes singular in the supersymmetric limit. We show that this term can produce large gaugino masses without altering the spectrum of other states and without lowering the cutoff of the effecive theory.

Stringy constraints on primordial electromagnetic fields in axion inflation

We study primordial electromagnetic fields in effective actions of string theory. In contrast to a conventional scenario of producing primordial electromagnetic fields induced by the axion inflation, we deal with the Dirac-Born-Infeld action as a non-linear generation of Maxwell theory. It turns out that the intensity of generated electromagnetic fields is bounded from above by the string scale which can also be rewritten in terms of supersymmetry breaking scale in the context of type IIB Large Volume Scenario. The instability parameter ξ is constrained by the tadpole cancellation condition of D3-branes and a realization of hierarchy between the string scale and the Hubble scale of inflation. Hence, the magnetogenesis can be realized in the limited corner of the string landscape due to the O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(1) value of the coefficient of Chern-Simons coupling.

Hybrid inflation from supersymmetry breaking

We extend a recently proposed framework, dubbed inflation by supersymmetry breaking, to hybrid inflation by introducing a waterfall field that allows to decouple the supersymmetry breaking scale in the observable sector from the inflation scale, while keeping intact the inflation sector and its successful predictions: naturally small slow-roll parameters, small field initial conditions and absence of the pseudo-scalar companion of the inflaton, in terms of one free parameter which is the first order correction to the inflaton Kähler potential. During inflation, supersymmetry is spontaneously broken with the inflaton being the superpartner of the goldstino, together with a massive vector that gauges the R-symmetry. Inflation arises around the maximum of the scalar potential at the origin where R-symmetry is unbroken. Moreover, a nearby minimum with tuneable vacuum energy can be accommodated by introducing a second order correction to the Kähler potential. The inflaton sector can also play the role of the supersymmetry breaking ‘hidden’ sector when coupled to the (supersymmetric) Standard Model, predicting a superheavy superparticle spectrum near the inflation scale. Here we show that the introduction of a waterfall field provides a natural way to end inflation and allows for a scale separation between supersymmetry breaking and inflation. Moreover, the study of the global vacuum describing low energy Standard Model physics can be done in a perturbative way within a region of the parameter space of the model.

Gravitational waves from particle decays during reheating

Gravitational waves have become an irreplaceable tool for exploring the post-inflationary universe. Their cosmological and astrophysical origins have been attracting numerous attention. In this Letter, we point out a novel source of ultra-high frequency gravitational waves: the decay of particles produced during the reheating era. We highlight the decays of the inflaton and the Higgs bosons in the Standard Model and in the gauged B-L model as representative cases, showing how they yield a testable gravitational wave spectrum by future observations. We find that the peak frequency and the strain in the Higgs boson decay in the gauged B-L model, when the symmetry breaking scale is higher than the inflationary scale, can be greater than those in the inflaton decay scenario. Typically, the predicted spectrum lies beyond MHz in frequency and thus calls for non-interferometric experiments. Regardless of the shape of the spectrum, on the other hand, the total energy density of the produced gravitational waves can be reached by future Big Bang nucleosynthesis limits.

The rise and fall of the Standard-Model Higgs: electroweak vacuum stability during kination

In this paper we investigate the vacuum stability of the non-minimally coupled Standard-Model Higgs during a phase of kinetic domination following the end of inflation. The non-minimal coupling to curvature stabilises the Higgs fluctuations during inflation while driving them towards the instability scale during kination, when they can classically overcome the potential barrier separating the false electroweak vacuum from the true one at super-Planckian field values. Avoiding the instability of the Standard-Model vacuum sets an upper bound on the inflationary scale that depends both on the strength of the non-minimal interaction and on the top quark Yukawa coupling. Classical vacuum stability is guaranteed if the gravitationally-produced energy density is smaller than the height of barrier in the effective potential. Interestingly enough, thanks to the explosive particle production in the tachyonic phase, the Higgs itself can be also appointed to the role of reheaton field responsible for the onset of the hot Big Bang era, setting an additional lower bound on the inflationary scale Hinf\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\mathcal{H}}_{\ extrm{inf}} $$\\end{document} ≳ 105.5 GeV. Overall, these constraints favour lower masses for the top quark, in agreement with the current measurements of the top quark pole mass. We perform our analysis semi-analytically in terms of the one-loop and three-loop running of the Standard-Model Higgs self-coupling and make use of lattice-based parametric formulas for studying the (re)heating phase derived in arXiv:2307.03774. For a specific choice of mt = 171.3 GeV we perform also an extensive numerical scanning of the parameter space via classical lattice simulations, identifying stable/unstable regions and supporting the previous analytical arguments. For this fiducial value, the heating of the Universe is achieved at temperatures in the range 10−2–109 GeV.

Inflationary Vulnerability of The Modern World Economy

In recent years, the world economy has faced inflation that is unusual in its nature, specificity and consequences. The development of the world economy was affected by a number of specific but powerful shocks associated with the pandemic crisis and its consequences, but then the situation became more complicated due to the intensifi cation of military-political confl icts, which signifi cantly aff ected the previously established system of international economic cooperation. These phenomena have weakened or made uncertain the reliable tools of macroeconomic policy (monetary, fiscal) for targeting inflation and overcoming price volatility in the world markets of energy and food raw materials. Despite the fact that inflation growth has slowed down, the outlook for inflation expectations remains highly uncertain. The article discusses new determinants of infl ation growth in the context of the current crisis, in particular, speculation in energy markets, the dynamics of corporate profits, as well as the dispersion and differentiation of inflation in the eurozone countries. The possibilities of global economic growth are assessed taking into account current inflationary pressure, the main vulnerabilities of different groups of countries and the risks of a global recession are shown. To assess the scale and nature of inflation, this study draws parallels with the global crisis of 2008–2009. The prospects for the dynamics of slowing inflation as the growth rate of the global economy decreases are considered.

LiteBIRD and CMB-S4 sensitivities to reheating in plateau models of inflation

We study the sensitivity of LiteBIRD and CMB-S4 to the reheating temperature and the inflaton coupling in three types of plateau-potential models of inflation, namely mutated hilltop inflation, radion gauge inflation, and α-attractor T models. We first find relations between model parameters and CMB observables in all models. We then perform Monte Carlo Markov Chain based forecasts to quantify the information gain on the reheating temperature, the inflaton coupling, and the scale of inflation that can be achieved with LiteBIRD and CMB-S4, assuming a fiducial tensor-to-scalar ratio r̅ ∼ 0.02 and neglecting foreground contamination of the B-mode polarization spectrum. We compare the results of the forecasts to those obtained from a recently proposed simple analytic method. We find that both LiteBIRD and CMB-S4 can simultaneously constrain the scale of inflation and the reheating temperature in all three types of models. They can for the first time obtain both an upper and lower bound on the latter, comprising the first ever measurement of the big bang temperature. In the mutated hilltop inflation and radion gauge inflation models this can be translated into a measurement of the inflaton coupling in parts of the parameter space. Constraining this microphysical parameter will help to understand how these models of inflation may be embedded into a more fundamental theory of particle physics.

Holographic massive plasma state in Friedman universe: cosmological fine-tuning and coincidence problems

Massive particle and antiparticle pair production andoscillation on the horizon form a holographic and massive pair plasma state in the Friedman Universe. Via this state, the Einsteincosmology term (dark energy) interacts with matter and radiation and is time-varying Λ̃ in the Universe's evolution.It is determined by a close set of ordinary differential equations for dark energy, matter, and radiation energy densities. The solutions are unique, provided the initial conditions given by observations.In inflation and reheating, dark energy density decreases from the inflation scale, converting to matter and radiation energy densities.In standard cosmology, matter and radiation energy densities convert to dark energy density, reaching the present Universe. By comparing with ΛCDM, quintessence and dark energy interacting models, we show thatthese results can be the possible solutions for cosmological fine-tuning and coincidence problems.

Entangled states as a probe of early universe history: a Higgs case study

I explore whether distinguishing features of phase transitions and/or the inflationary energy scale can be imprinted on cosmological observables due to entanglement during inflation, given a spectator scalar field with a Higgs-like potential. As a consequence of this analysis, I also present results that illustrate the variety of features a Higgs-like spectator can imprint on the primordial power spectrum due to entanglement, as well as how easy it might be to distinguish such spectra from other similar scalar field results at the level of CMB residuals. I utilize the technical framework for dynamically generated entangled states developed in [1,2] to obtain my results.

Stringy spacetime uncertainty principle and a modified trans-Planckian censorship criterion

We study the implications of the stringy spacetime uncertainty relation (STUR) for inflationary cosmology. By demanding that no fluctuation modes that exit the Hubble radius are affected by the nonlocality resulting from the STUR, we find an upper bound on the number of e-foldings of inflation. The bound is a factor of 2 weaker than what results from the trans-Planckian censorship criterion (TCC). By demanding that the inflationary phase is simultaneously consistent with STUR and sufficiently long for inflation to provide a causal explanation of structure on the scale of the current Hubble radius, we find an upper bound on the energy scale of inflation. The bound is less restrictive than what follows from the TCC, but it remains in conflict with canonical single-field inflation models. Published by the American Physical Society 2024

Cosmological collider signatures of Higgs-R2 inflation

We study the cosmological collider signatures in the Higgs-R 2 inflation model. We consider two distinct types of signals: one originating from the inflaton coupling to Standard Model fermions and gauge bosons, and another arising from the isocurvature mode interaction with the inflaton. In the former case, we determine that the signal magnitude is likely too small for detection by upcoming probes, primarily due to suppression by both the Planck scale and slow-roll parameters. However, we provide a detailed computation of the signal which could be potentially applicable to various Higgs inflation variants. For the isocurvature mode signals, we observe that the associated couplings remain unsuppressed when the isocurvature mode is relatively light or comparable to the inflationary scale. In this case, we study the Higgs-R 2 inflation parameter space that corresponds to the quasi-single-field inflation regime and find that the signal strength could be as large as |f NL| > 1, making Higgs-R 2 inflation a viable candidate for observation by future 21-cm surveys.

Postinflationary production of particle dark matter: Hilltop and Coleman-Weinberg inflation

We investigate the production of nonthermal dark matter (DM), χ, during the postinflationary reheating era. For inflation, we consider two slow roll single field inflationary scenarios—generalized version of Hilltop (GH) inflation, and Coleman-Weinberg (CW) inflation. Using a set of benchmark values that comply with the current constraints from cosmic microwave background radiation (CMBR) data for each inflationary model, we explored the parameter space involving mass of dark matter particles, mχ, and coupling between inflaton and χ, yχ. For these benchmarks, we find that tensor-to-scalar ratio r can be as small as 2.69×10−6 for GH and 1.91×10−3 for CW inflation, both well inside 1−σ contour on scalar spectral index versus r plane from 2018+BICEP3+2018 dataset, and testable by future cosmic microwave background (CMB) observations, e.g., Simons Observatory. For the production of χ from the inflaton decay satisfying CMB and other cosmological bounds and successfully explaining total cold dark matter density of the present universe, we find that yχ should be within this range O(10−4)≳yχ≳O(10−20) for both inflationary scenarios. We also show that, even for the same inflationary scenario, the allowed parameter space on reheating temperature versus mχ plane alters with inflationary parameters including scalar spectral index, r, and energy scale of inflation. Published by the American Physical Society 2024

Cosmological collider signal from non-Bunch-Davies initial states

We investigate the cosmological collider (CC) signal arising from the tree-level exchange of a scalar spectator particle with a non-Bunch-Davies (BD) initial state. We decompose the inflaton correlators into seed integrals, which we compute analytically by solving the bootstrap equations. We show that the non-BD initial state eliminates the Hubble-scale Boltzmann suppression e−πm/H that usually affects the CC signal. Consequently, in this scenario, the CC can probe an energy scale much higher than the inflationary Hubble scale H. Published by the American Physical Society 2024