arXiv subjects

Fabrizio Rompineve

Publications and source records attributed to Fabrizio Rompineve.

31 records · Page 2Linked to original sources

arXiv2020

Ultralight Scalar Decay and the Hubble Tension

We examine whether the Hubble tension, the mismatch between early and late measurements of $H_0$, can be alleviated by ultralight scalar fields in the early universe, and we assess its plausibility within UV physics. Since their energy density needs to rapidly redshift away, we explore decays to massless fields around the era of matter-radiation equality. We highlight a concrete implementation of ultralight pseudo-scalars, axions, that decay to an abelian dark sector. This scenario circumvents major problems of other popular realizations of early universe scalar models in that it uses a regular scalar potential that is quadratic around the minimum, instead of the extreme fine-tuning of many existing models. The idea is that the scalar is initially frozen in its potential until $H\sim m$, then efficient energy transfer from the scalar to the massless field can occur shortly after the beginning of oscillations due to resonance. We introduce an effective fluid model which captures the transition from the frozen scalar phase to the radiation dark sector phase. We perform a fit to a combined Planck 2018, BAO, SH$_0$ES and Pantheon supernovae dataset and find that the model gives $H_0=69.9_{-0.86}^{+0.84}$ km/s/Mpc with $Δχ^2 \approx -9$ compared to $Λ$CDM; while inclusions of other data sets may worsen the fit. Importantly, we find that large values of the coupling between fields is required for sufficiently rapid decay: For axion-gauge field models $ϕF\tilde{F}/Λ$ it requires $Λ\lesssim f/80$, where $2πf$ is the field range. We find related conclusions for scalar-scalar models $\simϕ\,χ^2$ and for models that utilize perturbative decays. We conclude that these sorts of ultralight scalar models that purport to alleviate the Hubble tension, while being reasonable effective field theories, require features that are difficult to embed within UV physics.

astro-ph.CO↗

arXiv2020

Detuning primordial black hole dark matter with early matter domination and axion monodromy

We present a scenario that ameliorates the tuning problems present in models of primordial black hole dark matter from inflation. Our setup employs the advantages of gravitational collapse in a long epoch of early matter domination with reheating temperature $\lesssim 10^{6}~\text{GeV}$. Furthermore, we make use of a string-inspired class of models where the inflaton is identified with a non-compact axion field. In this framework, the presence of multiple local minima in the inflaton potential can be traced back to an approximate discrete shift symmetry. This scenario allows the formation of primordial black holes in the observationally viable range of masses ($M_{\text{PBH}}\sim 10^{-13}M_{\odot}-10^{-16}M_{\odot}$) accounting for all dark matter, and in excellent agreement with the CMB. Crucially, we find a significant reduction in the required tuning of the parameters of the inflationary potential, in contrast to the standard case of primordial black hole formation during radiation domination. However, abundant formation of light PBHs during an early phase of matter domination can be more easily in conflict with evaporation bounds. We discuss how these can be avoided under mild assumptions on the collapsing energy density fraction.

astro-ph.CO↗

arXiv2020

Peccei-Quinn Phase Transition at LIGO

The LIGO observatories can potentially detect stochastic gravitational waves arising from phase transitions which happened in the early universe at temperatures around $T\sim 10^{8}$ GeV. This provides an extraordinary opportunity for discovering the phase transition associated with the breaking of the Peccei-Quinn symmetry, required in QCD axion models. Here we consider the simplest Peccei-Quinn models and study under which conditions a strong first-order phase transition can occur, analyzing its associated gravitational wave signal. To be detectable at LIGO, we show that some supercooling is needed, which can arise either in Coleman-Weinberg-type symmetry breaking or in strongly-coupled models. We also investigate phase transitions that interestingly proceed by first breaking the electroweak symmetry at large scales before tunneling to the Peccei-Quinn breaking vacuum. In this case, the associated gravitational wave signal is more likely to be probed at the proposed Einstein Telescope.

hep-ph↗

arXiv2020

Oscillons and Dark Matter

Oscillons are bound states sustained by self-interactions that appear in rather generic scalar models. They can be extremely long-lived and in the context of cosmology they have a built-in formation mechanism - parametric resonance instability. These features suggest that oscillons can affect the standard picture of scalar ultra-light dark matter (ULDM) models. We explore this idea along two directions. First, we investigate numerically oscillon lifetimes and their dependence on the shape of the potential. We find that scalar potentials that occur in well motivated axion-like models can lead to oscillons that live up to $10^8$ cycles or more. Second, we discuss the observational constraints on the ULDM models once the presence of oscillons is taken into account. For a wide range of axion masses, oscillons decay around or after matter-radiation equality and can thus act as early seeds for structure formation. We also discuss the possibility that oscillons survive up to today. In this case they can most easily play the role of dark matter.

hep-ph↗

arXiv2019

The Supercooled Universe

Strongly-coupled theories at the TeV can naturally drive a long period of supercooling in the early universe. Trapped into the deconfined phase, the universe could inflate and cool down till the temperature reaches the QCD strong scale. We show how at these low temperatures QCD effects are important and could trigger the exit from the long supercooling era. We also study the implications on relic abundances. In particular, the latent heat released at the end of supercooling could be the reason for the similarities between dark matter and baryon energy densities. The axion abundance could also be significantly affected, allowing for larger values of the axion decay constant. Finally, we discuss how a long supercooling epoch could lead to an enhanced gravitational wave signal.

hep-ph↗

arXiv2019

Primordial Black Holes from the QCD axion

We propose a mechanism to generate Primordial Black Holes (PBHs) which is independent of cosmological inflation and occurs slightly below the QCD phase transition. Our setup relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number $N_{DW}>1$ and Peccei-Quinn symmetry broken after inflation. In our framework, dark matter is mostly composed of axions in the meV mass range along with a small fraction, $Ω_{\text{PBH}} \gtrsim 10^{-6} Ω_{\text{CDM}} $ of heavy $M \sim 10^4-10^7 M_\odot$ PBHs. The latter could play a role in alleviating some of the shortcomings of the $Λ$CDM model on sub-galactic scales. The scenario has distinct signatures in ongoing axion searches as well as gravitational wave observatories.

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arXiv2017

The photo-philic QCD axion

We propose a framework in which the QCD axion has an exponentially large coupling to photons, relying on the "clockwork" mechanism. We discuss the impact of present and future axion experiments on the parameter space of the model. In addition to the axion, the model predicts a large number of pseudo-scalars which can be light and observable at the LHC. In the most favorable scenario, axion Dark Matter will give a signal in multiple axion detection experiments and the pseudo-scalars will be discovered at the LHC, allowing to determine most of the parameters of the model.

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arXiv2016

Axion Monodromy and the Weak Gravity Conjecture

Axions with broken discrete shift symmetry (axion monodromy) have recently played a central role both in the discussion of inflation and the `relaxion' approach to the hierarchy problem. We suggest a very minimalist way to constrain such models by the weak gravity conjecture for domain walls: While the electric side of the conjecture is always satisfied if the cosine-oscillations of the axion potential are sufficiently small, the magnetic side imposes a cutoff, $Λ^3 \sim m f M_{pl}$, independent of the height of these `wiggles'. We compare our approach with the recent related proposal by Ibanez, Montero, Uranga and Valenzuela. We also discuss the non-trivial question which version, if any, of the weak gravity conjecture for domain walls should hold. In particular, we show that string compactifications with branes of different dimensions wrapped on different cycles lead to a `geometric weak gravity conjecture' relating volumes of cycles, norms of corresponding forms and the volume of the compact space. Imposing this `geometric conjecture', e.g.~on the basis of the more widely accepted weak gravity conjecture for particles, provides at least some support for the (electric and magnetic) conjecture for domain walls.

hep-th↗

arXiv2016

Gravitational Waves from Axion Monodromy

Large field inflation is arguably the simplest and most natural variant of slow-roll inflation. Axion monodromy may be the most promising framework for realising this scenario. As one of its defining features, the long-range polynomial potential possesses short-range, instantonic modulations. These can give rise to a series of local minima in the post-inflationary region of the potential. We show that for certain parameter choices the inflaton populates more than one of these vacua inside a single Hubble patch. This corresponds to a dynamical phase decomposition, analogously to what happens in the course of thermal first-order phase transitions. In the subsequent process of bubble wall collisions, the lowest-lying axionic minimum eventually takes over all space. Our main result is that this violent process sources gravitational waves, very much like in the case of a first-order phase transition. We compute the energy density and peak frequency of the signal, which can lie anywhere in the mHz-GHz range, possibly within reach of next-generation interferometers. We also note that this "dynamical phase decomposition" phenomenon and its gravitational wave signal are more general and may apply to other inflationary or reheating scenarios with axions and modulated potentials.

hep-ph↗

arXiv2015

Tuning and Backreaction in F-term Axion Monodromy Inflation

We continue the development of axion monodromy inflation, focussing in particular on the backreaction of complex structure moduli. In our setting, the shift symmetry comes from a partial large complex structure limit of the underlying type IIB orientifold or F-theory fourfold. The coefficient of the inflaton term in the superpotential has to be tuned small to avoid conflict with Kahler moduli stabilisation. To allow such a tuning, this coefficient necessarily depends on further complex structure moduli. At large values of the inflaton field, these moduli are then in danger of backreacting too strongly. To avoid this, further tunings are necessary. In weakly coupled type IIB theory at the orientifold point, implementing these tunings appears to be difficult if not impossible. However, fourfolds or models with mobile D7-branes provide enough structural freedom. We calculate the resulting inflaton potential and study the feasibility of the overall tuning given the limited freedom of the flux landscape. Our preliminary investigations suggest that, even imposing all tuning conditions, the remaining choice of flux vacua can still be large enough for such models to provide a promising path to large-field inflation in string theory.

hep-th↗

arXiv2015

Winding out of the Swamp: Evading the Weak Gravity Conjecture with F-term Winding Inflation?

We present a new model of large field inflation along a winding trajectory in the field space of two axionic fields, where the 'axions' originate from the complex structure moduli sector of a Calabi-Yau 3-fold at large complex structure. The winding trajectory arises from fixing one combination of axions by bulk fluxes and allows for a transplanckian effective field range. The inflaton potential arises from small 'instantonic' corrections to the geometry and realises natural inflation. By working in a regime of large complex structure for two complex structure moduli the inflaton potential can be made subdominant without severe tuning. We also discuss the impact of the recent 'no-go theorems' for transplanckian axion periodicities on our work. Interestingly, our setup seems to realise a loophole pointed out in arXiv:1503.00795 and arXiv:1503.04783: our construction is a candidate for a string theory model of large field inflation which is consistent with the mild form of the weak gravity conjecture for axions.

hep-th↗

arXiv2014

Weak Scale Baryogenesis in a Supersymmetric Scenario with R-parity violation

We investigate the connection between R-parity violation (RPV) in supersymmetric models and Baryogenesis. First we discuss in detail the assumptions of a theorem by Nanopoulos and Weinberg on the CP asymmetry generated from the decay of massive particles. In light of this statement, we analyse some interesting models of Baryogenesis through RPV. We then explore, in the context of RPV SUSY, the possibility to generate the baryon asymmetry through the out-of-equilibrium decay of a metastable Weakly Interacting Massive Particle (WIMP), as proposed in \cite{Cui:2012jh}. This setting is also motivated by the observed coincidence between the abundances of dark and baryonic matter. In this framework, we propose two models of out-of-equilibrium decay of a would-be cold relic, and compute the associated CP asymmetry. With a TeV-scale parent, the observed baryon abundance can be reproduced in these models when the stop is in the multi-TeV region. Furthermore, annihilation of the metastable particle into SM states must be characterised by a very weak coupling, $g\sim 10^{-2}$ and by a heavy mediator $m_{med}\sim 10$ TeV. These models can also accommodate stop masses far from the TeV scale, at the expense of weakening the explanation of the $Ω_{B}-Ω_{DM}$ coincidence.

hep-ph↗

arXiv2014

Dark Radiation predictions from general Large Volume Scenarios

Recent observations constrain the amount of Dark Radiation ($ΔN_{\rm eff}$) and may even hint towards a non-zero value of $ΔN_{\rm eff}$. It is by now well-known that this puts stringent constraints on the sequestered Large Volume Scenario (LVS), i.e. on LVS realisations with the Standard Model at a singularity. We go beyond this setting by considering LVS models where SM fields are realised on 7-branes in the geometric regime. As we argue, this naturally goes together with high-scale supersymmetry. The abundance of Dark Radiation is determined by the competition between the decay of the lightest modulus to axions, to the SM Higgs and to gauge fields. The latter decay channel avoids the most stringent constraints of the sequestered setting. Nevertheless, a rather robust prediction for a substantial amount of Dark Radiation can be made. This applies both to cases where the SM 4-cycles are stabilised by D-terms and are small "by accident" as well as to fibred models with the small cycles stabilised by loops. Furthermore, we analyse a closely related setting where the SM lives at a singularity but couples to the volume modulus through flavour branes. We conclude that some of the most natural LVS settings with natural values of model parameters lead to Dark Radiation predictions just below the present observational limits. Barring a discovery, rather modest improvements of present Dark Radiation bounds can rule out many of these most simple and generic variants of the LVS.

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