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Ignatios Antoniadis

Publications and source records attributed to Ignatios Antoniadis.

At least 37 records · Page 2Linked to original sources
arXiv2025

How Accidental was Inflation?

Data on the cosmic microwave background (CMB) are discriminating between different models of inflation, disfavoring simple monomial potentials whilst being consistent with models whose predictions resemble those of the Starobinsky $R + R^2$ cosmological model. However, this model may suffer from theoretical problems, since it requires a large initial field value, threatening the validity of the effective field theory. This is quantified by the Swampland Distance Conjecture, which predicts the appearance of a tower of light states associated with an effective ultra-violet cutoff. This could be lower than the inflation scale for cases with an extended period of inflation, leading to an additional problem of initial conditions. No-scale supergravity models can reproduce the predictions of the Starobinsky model and accommodate the CMB data at the expense of fine-tuning of parameters at the level of $10^{-5}$. Here, we propose a solution to this problem based on an explicit realisation of the Starobinsky model in string theory, where this `deformation' parameter is calculable and takes a value of order of the one corresponding to the Starobinsky inflaton potential. Within this range, there are parameter values that accommodate more easily the combination of Planck, ACT and DESI BAO data, while also restricting the range of possible inflaton field values, thereby avoiding the swampland problem and predicting that the initial conditions for inflation compatible with the CMB data are generic.

hep-ph↗
arXiv2025

SUSY at the FPF

Experimental searches for supersymmetry (SUSY) are entering a new era. The failure to observe signals of sparticle production at the Large Hadron Collider (LHC) has eroded the central motivation for SUSY breaking at the weak scale. However, String Theory requires SUSY at the fundamental scale $M_s$ and hence SUSY could be broken at some high scale below $M_s$. Actually, if this were the case, the lack of experimental evidence for low-energy SUSY could have been anticipated, because most stringy models with high-scale SUSY breaking predict that sparticles would start popping up above about 10 TeV, well beyond the reach of current LHC experiments. We show that using next generation LHC experiments currently envisioned for the Forward Physics Facility (FPF) we could search for signals of neutrino-modulino oscillations to probe models with string scale in the grand unification region and SUSY breaking driven by sequestered gravity in gauge mediation. This is possible because of the unprecedented flux of neutrinos to be produced as secondary products in LHC collisions during the high-luminosity era and the capability of FPF experiments to detect and identify their flavors.

hep-ph↗
arXiv2024

Cosmological Constraints on Dark Neutrino Towers

We reexamine a dynamical dark matter model with Kaluza-Klein (KK) towers of gravitons and neutrinos fitting together in the dark dimension. We show that even though gravitational decays of neutrino KK towers have little impact in cosmology the weak decay channel could have significant cosmological effects. Taking conservative upper bounds on the dark matter decay rate into two photons before recombination and on the number of effective extra neutrino species $ΔN_{\rm eff}$ we derive constraints on the conversion rate from active to sterile species despite the dependence of the mixing angle on the KK mode mass. We also provide counterarguments to a recent claim suggesting that the bounds on $ΔN_{\rm eff}$ rule out micron-sized extra dimensions.

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arXiv2024

Through the Looking Glass into the Dark Dimension: Searching for Bulk Black Hole Dark Matter with Microlensing of X-ray Pulsars

Primordial black holes (PBHs) hidden in the incredible bulk of the dark dimension could escape constraints from non-observation of their Hawking radiation. Since these five-dimensional (5D) PBHs are bigger, colder, and longer-lived than usual 4D PBHs of the same mass $M$, they could make all cosmological dark matter if $10^{11} \lesssim M/{\rm g} \lesssim 10^{21}$, i.e., extending the 4D allowed region far down the asteroid-mass window. We show that these evasive PBHs could be search for by measuring their $X$-ray microlensing events from faraway pulsars. We also show that future $X$-ray microlensing experiments will be able to probe the interesting range ($10^{16.5} \lesssim M/{\rm g} \lesssim 10^{17.5}~{\rm g}$) where an all dark matter interpretation in terms of 4D Schwarzschild PBHs is excluded by the non-observation of their Hawking radiation.

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arXiv2024

Bulk Black Hole Dark Matter

The dark dimension provides a mechanism to resolve the cosmological hierarchy problem and assembles a colosseum for dark matter contenders. In a series of recent publications we investigated whether primordial black holes (PBHs) perceiving the dark dimension could constitute all of the dark matter in the universe. A key assumption of these investigations is that PBHs remain confined to the brane during the entire evaporation process. As a consequence, the abundance of PBHs evaporating at the present epoch is severely constrained by observations of both the extragalactic and Galactic $γ$-ray backgrounds. As a natural outgrowth of these investigations, herein we relax the assumption of brane localized PBHs and reexamine the evaporation process of PBHs which are allowed to escape into the dark dimension. We show that the escape of PBHs from the brane is almost instantaneous. Armed with this pivotal finding we reassess the allowed mass range of PBHs to assemble all cosmological dark matter, which is estimated to be $10^{11} \lesssim M_{\rm BH}/{\rm g} \lesssim 10^{21}$.

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arXiv2024

Thin-wall vacuum decay in the presence of a compact dimension

We study the problem of false vacuum decay in arbitrary dimensions, in the presence of gravity, and compute the transition probability within the thin-wall approximation, generalising the results of Coleman and de Luccia. In the particular case of one compact dimension, we present explicit formulae for the Euclidean Bounce configuration that drives the transition from a de Sitter to Minkowski or from a Minkowski to anti-de Sitter vacua.

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arXiv2024

From infinite to infinitesimal: Using the Universe as a dataset to probe Casimir corrections to the vacuum energy from fields inhabiting the dark dimension

Promptly after high-resolution experiments harbinger the field of precision cosmology low- and high-redshift observations abruptly gave rise to a tension in the measurement of the present-day expansion rate of the Universe ($H_0$) and the clustering of matter ($S_8$). The statistically significant discrepancies between the locally measured values of $H_0$ and $S_8$ and the ones inferred from observations of the cosmic microwave background assuming the canonical $Λ$ cold dark matter (CDM) cosmological model have become a new cornerstone of theoretical physics. $Λ_s$CDM is one of the many beyond Standard Model setups that have been proposed to simultaneously resolve the cosmological tensions. This setup relies on an empirical conjecture, which postulates that $Λ$ switched sign (from negative to positive) at a critical redshift $z_c \sim 2$. We reexamine a stringy model that can describe the transition in the vacuum energy hypothesized in $Λ_s$CDM. The model makes use of the Casimir forces driven by fields inhabiting the incredible bulk of the dark dimension scenario. Unlike the $Λ_s$CDM setup the model deviates from $Λ$CDM in the early universe due to the existence of relativistic neutrino-like species. Using the Boltzmann solver CLASS in combination with MontePython we confront predictions of the stringy model to experimental data (from the Planck mission, Pantheon+ supernova type Ia, BAO, and KiDS-1000). We show that the string-inspired model provides a satisfactory fit to the data and can resolve the cosmological tensions.

astro-ph.CO↗
arXiv2024

Flux vacua in type IIB compactifications on orbifolds: their finiteness and minimal string coupling

We perform a detailed study of (supersymmetric) moduli stabilisation in type IIB toroidal orientifolds with fluxes. We provide strong evidence towards exhaustion of the finite number of inequivalent vacua for a given total 3-form flux charge $N_{\rm flux}$. We also find that the minimal value of the string coupling $g_s$ is given in terms of $N_{\rm flux}$ and present strong evidence for the asymptotic relation $g_{s,\,\rm{min}}\sim c/N_{\rm flux}^α$, with $α=1,2$, valid for not too small $N_{\rm flux}$, where $c$ is an order 1 coefficient and $α$ depends on the orbifold. Imposing tadpole cancellation, $N_{\rm flux}$ is bounded from the number of orientifold O3-planes. Combined with the flux quantisation, this forbids orientifold vacua without anti-brane charge. On the other hand, the presence of negative D3-brane charge induced by magnetised D7-branes breaks supersymmetry and relaxes the bound, allowing significantly smaller values for $g_s$.

hep-th↗
arXiv2024

Anomalous $U(1)$ extension of the Standard Model

We present a set of example models in which the Standard Model (SM) symmetry group is extended by a new abelian symmetry. This additional symmetry appears anomalous in the effective low-energy theory; however, the anomalies cancel out when massive chiral fermions not present in the effective low-energy theory are taken into account. These chiral fermions under the new abelian gauge group, are chosen to be vector-like under the SM symmetries, and reside in the same representations as quarks and leptons. This allows us to quantitatively determine the magnitude of tree-level interactions between three vector bosons induced in low-energy effective field theory by the integration of chiral heavy fermions. We also examine the perturbativity constraints of the theory and the ultraviolet cut-off. We conclude by highlighting possible extensions of our work.

hep-ph↗
arXiv2024

Large extra dimensions from higher-dimensional inflation

We propose the possibility that compact extra dimensions can obtain large size by higher dimensional inflation, relating the weakness of the actual gravitational force to the size of the observable universe. Solution to the horizon problem implies that the fundamental scale of gravity is smaller than $10^{13}$ GeV which can be realised in a braneworld framework for any number of extra dimensions. However, requirement of (approximate) flat power spectrum of primordial density fluctuations consistent with present observations makes this simple proposal possible only for one extra dimension at around the micron scale. After the end of five-dimensional inflation, the radion modulus can be stabilised at a vacuum with positive energy of the order of the present dark energy scale. An attractive possibility is based on the contribution to the Casimir energy of right-handed neutrinos with a mass at a similar scale.

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arXiv2024

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.

hep-th↗
arXiv2024

Anti-de Sitter $\to$ de Sitter transition driven by Casimir forces and mitigating tensions in cosmological parameters

Over the last few years, low- and high-redshift observations set off tensions in the measurement of the present-day expansion rate $H_0$ and in the determination of the amplitude of the matter clustering in the late Universe (parameterized by $S_8$). It was recently noted that both these tensions can be resolved if the cosmological constant parametrizing the dark energy content switches its sign at a critical redshift $z_c \sim 2$. However, the anti-de Sitter (AdS) swampland conjecture suggests that the postulated switch in sign of the cosmological constant at zero temperature seems unlikely because the AdS vacua are an infinite distance appart from de Sitter (dS) vacua in moduli space. We provide an explanation for the required AdS $\to$ dS crossover transition in the vacuum energy using the Casimir forces of fields inhabiting the bulk. We then use entropy arguments to claim that any AdS $\to$ dS transition between metastable vacua must be accompanied by a reduction of the species scale where gravity becomes strong. We provide a few examples supporting this AdS $\to$ dS uplift conjecture.

hep-th↗
arXiv2024

More on Black Holes Perceiving the Dark Dimension

In the last two years the dark dimension scenario has emerged as focal point of many research interests. In particular, it functions as a stepping stone to address the cosmological hierarchy problem and provides a colosseum for dark matter contenders. We reexamine the possibility that primordial black holes (PBHs) perceiving the dark dimension could constitute all of the dark matter in the universe. We re-assess limits on the abundance of PBHs as dark matter candidates from $γ$-ray emission resulting from Hawking evaporation. We re-evaluate constraints from the diffuse $γ$-ray emission in the direction of the Galactic center which offer the best and most solid upper limits on the dark matter fraction composed of PBHs. The revised mass range which allows PBHs to assemble all cosmological dark matter is estimated to be $10^{15} \alt M_{\rm BH}/{\rm g} \alt 10^{21}$. We demonstrate that due to the constraints from $γ$-ray emission, quantum corrections due to the speculative memory burden effect do not modify this mass range. We also investigate the main characteristics of PBHs which are localized in the bulk. We show that PBHs localized in the bulk can make all cosmological dark matter if $10^{11} \alt M_{\rm BH}/{\rm g} \alt 10^{21}$. Finally, we comment on the black holes that could be produced if one advocates a space with two boundaries for the dark dimension.

hep-th↗
arXiv2024

Landscape, Swampland, and Extra Dimensions

By combining swampland conjectures with observational data, it was recently suggested that the cosmological hierarchy problem (i.e. the smallness of the dark energy in Planck units) could be understood as an asymptotic limit in field space, corresponding to a decompactification of one extra (dark) dimension of a size in the micron range. In these Proceedings we examine the fundamental setting of this framework and discuss general aspects of the effective low energy theory inherited from properties of the overarching string theory. We then explore some novel phenomenology encompassing the dark dimension by looking at potential dark matter candidates, decoding neutrino masses, and digging into new cosmological phenomena.

hep-th↗
arXiv2024

Cosmological perturbations from five-dimensional inflation

It was recently proposed that five-dimensional inflation can relate the causal size of the observable universe to the present weakness of gravitational interactions by blowing up an extra compact dimension from the microscopic fundamental length of gravity to a large size in the micron range, as required in the Dark Dimension proposal. Here, we compute the power spectrum of all primordial fluctuations emerging from a 5-dimensional inflaton in a slow-roll region of its potential, showing an interesting change of behaviour at large scales corresponding to angles larger than about 10 degrees in the sky.

hep-ph↗
arXiv2024

The Dark Dimension, the Swampland, and the Dark Matter Fraction Composed of Primordial Near-Extremal Black Holes

In a recent publication we studied the decay rate of primordial black holes perceiving the dark dimension, an innovative five-dimensional (5D) scenario that has a compact space with characteristic length-scale in the micron range. We demonstrated that the rate of Hawking radiation of 5D black holes slows down compared to 4D black holes of the same mass. Armed with our findings we showed that for a species scale of ${\cal O} (10^{9}~{\rm GeV})$, an all-dark-matter interpretation in terms of primordial black holes should be feasible for black hole masses in the range $10^{14} \lesssim M/{\rm g} \lesssim 10^{21}$. As a natural outgrowth of our recent study, herein we calculate the Hawking evaporation of near-extremal 5D black holes. Using generic entropy arguments we demonstrate that Hawking evaporation of higher-dimensional near-extremal black holes proceeds at a slower rate than the corresponding Schwarzschild black holes of the same mass. Assisted by this result we show that if there were 5D primordial near-extremal black holes in nature, then a PBH all-dark-matter interpretation would be possible in the mass range $10^{7}\sqrtβ \lesssim M/{\rm g} \lesssim 10^{21}$, where $β$ is a parameter that controls the difference between mass and charge of the associated near-extremal black hole.

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arXiv2024

Fuzzy Dark Matter and the Dark Dimension

We propose a new dark matter contender within the context of the so-called ``dark dimension'', an innovative 5-dimensional construct that has a compact space with characteristic length-scale in the micron range. The new dark matter candidate is the radion, a bulk scalar field whose quintessence-like potential drives an inflationary phase described by a 5-dimensional de Sitter (or approximate) solution of Einstein equations. We show that the radion could be ultralight and thereby serve as a fuzzy dark matter candidate. We advocate a simple cosmological production mechanism bringing into play unstable Kaluza-Klein graviton towers which are fueled by the decay of the inflaton.

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