Stochastic Gravitational Background Research Articles

Probing flavor violation and baryogenesis via primordial gravitational waves

We show that observations of primordial gravitational waves of inflationary origin can shed light into the scale of flavor violation in a flavon model which also explains the mass hierarchy of fermions. The energy density stored in oscillations of the flavon field around the minimum of its potential redshifts as matter and is expected to dominate over radiation in the early universe. At the same time, the evolution of primordial gravitational waves acts as bookkeeping to understand the expansion history of the universe. Importantly, the gravitational wave spectrum is different if there is an early flavon dominated era compared to radiation domination expected from a standard cosmological model and this spectrum gets damped by the entropy released in flavon decays, determined by the mass of the flavon field mS and new scale of flavor violation ΛFV. We derive analytical expressions of the frequency above which the spectrum is damped, as-well-as the amount of damping, in terms of mS and ΛFV. We show that the damping of the gravitational wave spectrum would be detectable at BBO, DECIGO, U-DECIGO, μ−ARES, LISA, CE and ET detectors for ΛFV = 105−10 GeV and mS = OTeV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O}\\left(\ extrm{TeV}\\right) $$\\end{document}. Furthermore, the flavon decays can source the baryon asymmetry of the universe. We identify the mS – ΛFV parameter space where the observed baryon asymmetry η ~ 10−10 is produced and can be tested by gravitational wave detectors like LISA and ET. We also discuss our results in the context of the recently measured stochastic gravitational background signals by NANOGrav.

Inflation, superheavy metastable strings and gravitational waves in non-supersymmetric flipped SU(5)

Motivated by the NANOGrav 15 year data and other recent investigations of stochastic gravitational background radiation based on pulsar timing arrays, we show how superheavy strings survive inflation but the slightly heavier monopoles do not in a non-supersymmetric hybrid inflation model based on flipped SU(5). With the dimensionless string tension parameter Gμ ∼ 10-6, the gravitational wave spectrum emitted by the strings, which are metastable due to breaking caused by monopole-antimonopole quantum mechanical tunneling, is compatible with the latest NANOGrav measurement as well as the advanced LIGO-VIRGO third run data. The string network undergoes about 30 e-foldings of inflation which suppresses the spectrum in the LIGO-VIRGO frequency range. With the symmetry breaking chain SU(5)×U(1) X → SU(3) c ×SU(2) L ×U(1) Z ×U(1) X → SU(3) c ×SU(2) L ×U(1) Y , the estimated proton lifetime is of order 1036-1037 yrs.

Probing the Mass Relation between Supermassive Black Holes and Dark Matter Halos at High Redshifts by Gravitational Wave Experiments

Numerous observations have shown that almost all galaxies in our Universe host supermassive black holes (SMBHs), but there is still much debate about their formation and evolutionary processes. Recently, gravitational waves (GWs) have been expected to be a new and important informative observation, in particular, in the low-frequency region by making use of the Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays (PTAs). As an evolutionary process of the SMBHs, we revisit a dark matter (DM) halo–SMBH coevolution model based on the halo merger tree employing an ansatz for the mass relation between the DM halos and the SMBHs at z = 6. In this model, the mass of SMBHs grows through their mergers associated with the halo mergers, and hence, the evolutionary information must be stored in the GWs emitted at the mergers. We investigate the stochastic gravitational background from the coalescing SMBH binaries, which the PTAs can detect, and also the GW bursts emitted at the mergers, which can be detected by the mHz band observations such as LISA. We also discuss the possibility of probing the mass relation between the DM halos and the SMBHs at high redshift by future GW observations.

Primordial clocks within stochastic gravitational wave anisotropies

A first order phase transition in the early universe can give an observable stochastic gravitational background (SGWB), which will necessarily have primordial anisotropies across the sky. In multi-field inflationary scenarios, these anisotropies may have a significant isocurvature component very different from adiabatic fluctuations, providing an alternate discovery channel for high energy physics at inflationary scales. Here, we consider classically oscillating heavy fields during inflation that can imprint distinctive scale-invariance-breaking features in the power spectrum of primordial anisotropies. While such features are highly constrained in the cosmic microwave background, we show that their amplitude can be observably large in isocurvature SGWB, despite both probing a similar period of inflation. Measuring SGWB multipoles at the required level, ℓ ∼ \U0001d4aa(10-100), will be technologically challenging. However, we expect that early detection of a strong isotropic SGWB, and the guarantee of anisotropies, would spur development of next generation detectors with sufficient sensitivity, angular resolution, and foreground discrimination.

Dark Matter production from relativistic bubble walls

In this paper we present a novel mechanism for producing the observed Dark Matter (DM) relic abundance during the First Order Phase Transition (FOPT) in the early universe. We show that the bubble expansion with ultra-relativistic velocities can lead to the abundance of DM particles with masses much larger than the scale of the transition. We study this non-thermal production mechanism in the context of a generic phase transition and the electroweak phase transition. The application of the mechanism to the Higgs portal DM as well as the signal in the Stochastic Gravitational Background are discussed.

Gauge transformation of scalar induced tensor perturbation during matter domination

The scalar-induced secondary gravitational wave as the stochastic gravitational background is a useful tool to study the physics in the early universe. We study the scalar-induced tensor perturbations at second-order during matter domination in seven different gauges. We obtain the results in six other gauges from that in the Newtonian gauge using the gauge transformation law of the scalar-induced tensor perturbation. We find that the kernel functions ${I}_{\ensuremath{\chi}}$ in the synchronous and comoving orthogonal gauges are the same if the residual gauge modes in these two gauges are eliminated. By identifying the oscillating terms $\mathrm{sin}x$ and $\mathrm{cos}x$ in the scalar-induced tensor perturbations as the scalar-induced secondary gravitational waves, we find that its energy density is actually gauge independent. The energy density ${\ensuremath{\rho}}_{\mathrm{GW}}\ensuremath{\propto}{a}^{\ensuremath{-}4}$, or ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{GW}}\ensuremath{\propto}{a}^{\ensuremath{-}1}$ in the matter-dominated era, and the scalar-induced secondary gravitational waves behave as radiation.

Primordial black hole dark matter and the LIGO/Virgo observations

The LIGO/Virgo collaboration have by now detected the mergers of ten black hole binaries via the emission of gravitational radiation. The hypothesis that these black holes have formed during the cosmic QCD epoch and make up all of the cosmic dark matter, has been rejected by many authors reasoning that, among other constraints, primordial black hole (PBH) dark matter would lead to orders of magnitude larger merger rates than observed. We revisit the calculation of the present PBH merger rate. Solar mass PBHs form clusters at fairly high redshifts, which evaporate at lower redshifts. We consider in detail the evolution of binary properties in such clusters due to three-body interactions between the two PBH binary members and a third by-passing PBH, for the first time, by full numerical integration. A Monte-Carlo analysis shows that formerly predicted merger rates are reduced by orders of magnitude due to such interactions. The natural prediction of PBH dark matter formed during the QCD epoch yields a pronounced peak around 1M⊙ with a small mass fraction of PBHs on a shoulder around 30M⊙, dictated by the well-determined equation of state during the QCD epoch. We employ this fact to make a tentative prediction of the merger rate of ∼ 30M⊙ PBH binaries, and find it very close to that determined by LIGO/Virgo. Furthermore we show that current LIGO/Virgo limits on the existence of ∼ M⊙ binaries do not exclude QCD PBHs to make up all of the cosmic dark matter. Neither do constraints on QCD PBHs from the stochastic gravitational background, pre-recombination accretion, or dwarf galaxies pose a problem. Microlensing constraints on QCD PBHs should be re-investigated. We caution, however, in this numerically challenging problem some possibly relevant effects could not be treated.

Testing the Seesaw Mechanism and Leptogenesis with Gravitational Waves.

We present the possibility that the seesaw mechanism with thermal leptogenesis can be tested using the stochastic gravitational background. Achieving neutrino masses consistent with atmospheric and solar neutrino data, while avoiding nonperturbative couplings, requires right handed neutrinos lighter than the typical scale of grand unification. This scale separation suggests a symmetry protecting the right-handed neutrinos from getting a mass. Thermal leptogenesis would then require that such a symmetry be broken below the reheating temperature. We enumerate all such possible symmetries consistent with these minimal assumptions and their corresponding defects, finding that in many cases, gravitational waves from the network of cosmic strings should be detectable. Estimating the predicted gravitational wave background, we find that future space-borne missions could probe the entire range relevant for thermal leptogenesis.

Low temperature electroweak phase transition in the Standard Model with hidden scale invariance

We discuss a cosmological phase transition within the Standard Model which incorporates spontaneously broken scale invariance as a low-energy theory. In addition to the Standard Model fields, the minimal model involves a light dilaton, which acquires a large vacuum expectation value (VEV) through the mechanism of dimensional transmutation. Under the assumption of the cancellation of the vacuum energy, the dilaton develops a very small mass at 2-loop order. As a result, a flat direction is present in the classical dilaton-Higgs potential at zero temperature while the quantum potential admits two (almost) degenerate local minima with unbroken and broken electroweak symmetry. We found that the cosmological electroweak phase transition in this model can only be triggered by a QCD chiral symmetry breaking phase transition at low temperatures, T≲132 MeV. Furthermore, unlike the standard case, the universe settles into the chiral symmetry breaking vacuum via a first-order phase transition which gives rise to a stochastic gravitational background with a peak frequency ∼10−8 Hz as well as triggers the production of approximately solar mass primordial black holes. The observation of these signatures of cosmological phase transitions together with the detection of a light dilaton would provide a strong hint of the fundamental role of scale invariance in particle physics.

Phase transition and gravitational wave phenomenology of scalar conformal extensions of the Standard Model

Thermal corrections in classically conformal models typically induce a strong first-order electroweak phase transition, thereby resulting in a stochastic gravitational background that could be detectable at gravitational wave observatories. After reviewing the basics of classically conformal scenarios, in this paper we investigate the phase transition dynamics in a thermal environment and the related gravitational wave phenomenology within the framework of scalar conformal extensions of the Standard Model. We find that minimal extensions involving only one additional scalar field struggle to reproduce the correct phase transition dynamics once thermal corrections are accounted for. Next-to-minimal models, instead, yield the desired electroweak symmetry breaking and typically result in a very strong gravitational wave signal.

Space-time with a fluctuating metric tensor model

A presented physical time model is based on the assumption that time is a random Poisson process, the intensity of which depends on natural irreversible processes. The introduction of metric tensor space-time fluctuations allowing describing the impact of stochastic gravitational background has been demonstrated. The use of spectral lines broadening measurement for the registration of relic gravitational waves has been suggested.

Neutrino induced decoherence and variation in nuclear decay rates

Recent work has proposed that the interaction between ordinary matter and a stochastic gravitational background can lead to the decoherence of large aggregates of ordinary matter. In this work we point out that these arguments can be carried over to a stochastic neutrino background but with the Planck scale of the gravitational decoherence replaced by the weak scale. This implies that it might be possible to observe such neutrino induced decoherence on a small, microscopic system rather than a macroscopic system as is the case for gravitationally induced decoherence. In particular we suggest that neutrino decoherence could be linked with observed variations in the decay rates of certain nuclei. Finally we point out that this proposed neutrino induced decoherence can be considered the complement of the Mikheev-Smirnov-Wolfenstein (MSW) effect.

Stochastic gravitational background from inflationary phase transitions

We consider true vacuum bubbles generated in a first order phase transition occurring during the slow rolling era of a two field inflation: it is known that gravitational waves are produced by the collision of such bubbles. We find that the epoch of the phase transition strongly affects the characteristic peak frequency of the gravitational waves, causing an observationally interesting redshift in addition to the post-inflationary expansion. In particular it is found that a phase transition occurring typically 10$\div$20 $e-$foldings before the reheating at $kT\simeq 10^{15}$ GeV may be detected by the next Ligo gravity waves interferometers. Moreover, for recently proposed models capable of generating the observed large scale voids as remnants of the primordial bubbles (for which the characteristic wave lengths are several tens of Mpc), it is found that the level of anisotropy of the cosmic microwave background provides a deep insight upon the physical parameters of the effective Lagrangian.

Limits on the gravitational wave background from spacecraft Doppler experiments

Doppler tracking of an interplanetary spacecraft provides an unique opportunity to search for low frequency gravitational waves. In this paper, we describe in detail how Doppler experiments can be used to set an upper limit on a stochastic gravitational background, isotropically distributed. After reviewing the basic equations involved in this technique, we critically analyze three methods of data analysis, based on different assumptions about the physical properties of the signal.