arXiv subjects

Georges Obied

Publications and source records attributed to Georges Obied.

At least 19 recordsLinked to original sources

arXiv2025

Charged black holes in the 1/N expansion

We study some implications of $SU(N)\times U(1)$ theories coupled to gravity in the large-$N$ limit. We find that in theories with quarks transforming as $q\sim (\mathbf{N},1)$, black holes (BH) with charge larger than a critical value approach extremality $(M_{\rm BH}=\sqrt{2} M_{\rm Pl} Q_{\rm BH})$ as they evaporate. This occurs because BHs in this theory can only lose charge by emitting baryons, a process suppressed by a combinatoric factor $e^{-N\log N}$. Once extremality is reached the BH evolution halts for exponentially long times, $\tau_{\rm BH}\gtrsim e^{N\log N}$, without being protected by any symmetry. At times $t\gg\tau_{\rm BH}$ the BH forms a cloud of deconfined quarks around it, a \textit{quark corona}. The quarks are connected to the horizon by strings extending throughout distances that can be much larger than the inverse of the confinement scale, $\Lambda_N^{-1}$. Strikingly, due to energy conservation, these long strings cannot break even for light quarks. These effects can be further enhanced in clockwork-like theories based on Yang-Mills sectors of the type, $SU(N_1)\times SU(N_2)\times ...\times U(1)$. As an application to phenomenology, we give examples where BHs as light as one gram survive until today without evaporating, opening up regions of parameter space for primordial BH dark matter previously excluded by Hawking evaporation. This result relies only on large-$N$ combinatorics and is independent of the radiation mechanism from the BH, particle masses, and confinement scale. The theories we discuss pass non-trivial tests imposed by Swampland conjectures, including completeness of the spectrum as well as the weak gravity conjecture.

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arXiv2025

Evolving Dark Sector and the Dark Dimension Scenario

String theory naturally leads to the expectation that dark energy is not stable, and may be evolving as captured by the Swampland de Sitter conjectures. Moreover, motivated by the distance conjecture, a unification of dark sector has been proposed, where the smallness of dark energy leads to one extra dimension of micron size with dark matter being the Kaluza--Klein graviton excitations in this extra dimension. We consider the natural possibility that the radius of the dark dimension varies as the energy decreases, leading to the variation of the dark matter mass. This correlates the variation of the dark energy with the variation of the dark matter mass as they depend on the variations of a scalar field $\phi$ controlling the radius of the extra dimension. A simple realization of this idea for small range of $\phi$ is adequately captured by choosing a potential which is locally of the form $V=V_0\ {\rm exp}(-c\phi)$ and dark matter mass $m_{\rm DM}=m_0\ {\rm exp}(-c' \phi)$ as was proposed in Agrawal et al. (2019). We find excellent agreement with recent experimental data from DESI DR2 combined with SN measurements (from DES, Union3 or Pantheon+) and reproduces the same significance as CPL parametrization with the added benefit of providing a natural explanation for the apparent phantom behavior ($w<-1$) reported by DESI and DES based on a physical model. DESI and SN datasets independently favor non-zero values of $c'$ and $c$, respectively, both lying within the expected $\mathcal{O}(1)$ range suggested by the Swampland criteria. Moreover, our best fit value $c'\simeq 0.05 \pm 0.01$ is remarkably consistent with the experimental upper bound of $c'\lesssim 0.2$ demanded by the lack of detection of fifth force in the dark sector.

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arXiv2024

De Sitter space constraints on brane tensions and couplings

We argue for the existence of bounds on the tensions of $p$-branes in de Sitter space in terms of the Hubble rate and the strength of a class of Chern-Simons-like couplings. The world-volume couplings involve Abelian 1-form gauge fields in the bulk and possibly field strengths intrinsic to the brane. In many cases these couplings are the D-brane Chern-Simons terms present in string theory, while in other cases they are the interactions of axion domain walls with $U(1)$ fields. Our arguments use the same logic and assumptions as the recent Festina Lente proposal (thus utilizing the properties of Nariai de Sitter black holes) and generalize it to extended objects, thereby providing a bottom-up set of constraints independent of any particular UV completion. We compare these bounds to the properties of (wrapped) D-branes in Type II string theory in the weak coupling limit, under the assumption that these properties are not modified significantly in de Sitter constructions. We find that all constraints are satisfied by D-branes, providing further evidence for the Festina Lente conjecture. For the particular case of 2-branes with Chern-Simons interactions we obtain a bound, which however can be evaded if the theory contains a light axion. Similarly, we find the bounds do not apply to axion domain walls due to the presence of the axion.

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arXiv2024

Chern-Simons Induced Thermal Friction on Axion Domain Walls

We study the dynamics and interactions of the solitonic domain walls that occur in realistic axion electrodynamics models including the Chern-Simons interaction, $a\epsilon_{\mu\nu\lambda\sigma}F^{\mu\nu} F^{\lambda\sigma}$, between an axion $a(x)$ of mass $m_a$, and a massless U(1) gauge field, e.g. EM, interacting with strength $\alpha=e^2/4\pi$ with charged matter, e.g. electron-positron pairs. In particular, in the presence of a U(1) gauge-and-matter relativistic thermal plasma we study the friction experienced by the walls due to the Chern-Simons term. Utilizing the linear response method we include the collective effects of the plasma, as opposed to purely particle scattering across the wall (as is done in previous treatments) which is valid only in the thin wall regime that is rarely applicable in realistic cases. We show that the friction depends on the Lorentz-$\gamma$-factor-dependent inverse thickness of the wall in the plasma frame, $\ell^{-1} \sim \gamma m_a$, compared to the three different plasma scales, the temperature $T$, the Debye mass $m_D\sim\sqrt{\alpha} T$, and the damping rate $\Gamma \sim \alpha^2 T$, and elucidate the underlying physical intuition for this behavior. (For friction in the thin-wall-limit we correct previous expressions in the literature.) We further consider the effects of long-range coherent magnetic fields that are possibly present in the early universe and compare their effect with that of thermal magnetic fields. Finally, we briefly discuss the possible early universe consequences of our results for domain wall motion and network decay, stochastic gravitational wave production from domain wall networks, and possible primordial black hole production from domain wall collapse, though a more complete discussion of these topics is reserved for a companion paper.

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arXiv2024

Chern-Simons bubbles: Lopsided false vacuum decay in axion electrodynamics

We study axion electrodynamics, including the Chern-Simons interaction term, in the presence of parallel background electric and magnetic fields, as can for example occur in certain models of axion inflation and in the study of dyonic black holes. In this setup, we find a new back-reacted instanton solution which corresponds to the nucleation of an axion domain wall that screens the electromagnetic fields in a process analogous to Schwinger pair production, despite the absence of light charged particles. The full solution includes the effect of the Chern-Simons induced charges and currents on the axion domain wall arising from the Witten and Sikivie (anomalous Hall) effects, respectively. The Euclidean solution has a reduced $O(2)\times O(2)$ symmetry which describes the nucleation of a prolate bubble in its rest-frame. A unique feature of this solution is that the region of lower energy density is outside the bubble rather than inside. We also describe the time evolution of this initial configuration, showing how the bubble can become further elongated along the direction of the background electric and magnetic fields. We describe potential applications of this process in particle physics and cosmology.

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arXiv2023

Dark Dimension and Decaying Dark Matter Gravitons

We explore the cosmology of the Dark Dimension scenario taking into account perturbations in the linear regime. In the context of the Dark Dimension scenario, a natural candidate for dark matter in our universe is the excitations of a tower of massive spin-2 KK gravitons. These dark gravitons are produced in the early universe and decay to lighter KK gravitons during the course of cosmological evolution. The decay causes the average dark matter mass to decrease as the universe evolves. In addition, the kinetic energy liberated in each decay leads to a kick velocity for the dark matter particles leading to a suppression of structure formation. Using current CMB (Planck), BAO and cosmic shear (KiDS-1000) data, we put a bound on the dark matter kick velocity today $v_\mathrm{today} \leq 2.2 \times 10^{-4} c$ at 95\% CL. This leads to rather specific regions of parameter space for the dark dimension scenario. The combination of the experimental bounds from cosmology, astrophysics and table-top experiments lead to the range $l_5\sim 1- 10 \, μm$ for the size of the Dark Dimension. The Dark Dimension scenario is found to be remarkably consistent with current observations and provides signatures that are within reach of near-future experiments.

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arXiv2023

Astrophysical Constraints on Decaying Dark Gravitons

In the dark dimension scenario, which predicts an extra dimension of micron scale, dark gravitons (KK modes) are a natural dark matter candidate. In this paper, we study observable features of this model. In particular, their decay to standard matter fields can distort the CMB and impact other astrophysical signals. Using this we place bounds on the parameters of this model. In particular we find that the natural range of parameters in this scenario is consistent with these constraints and leads to the prediction that the mean mass of the dark matter today is close to a few hundred keV and the effective size of the extra dimension is around $1 - 30 \;\mu\mathrm{m}$.

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arXiv2022

Dark Dimension Gravitons as Dark Matter

We consider cosmological aspects of the Dark Dimension (a mesoscopic dimension of micron scale), which has recently been proposed as the unique corner of the quantum gravity landscape consistent with both the Swampland criteria and observations. In particular we show how this leads, by the universal coupling of the Standard Model sector to bulk gravitons, to massive spin 2 KK excitations of the graviton in the dark dimension (the "dark gravitons") as an unavoidable dark matter candidate. Assuming a lifetime for the current de Sitter phase of our universe of order Hubble, which follows from both the dS Swampland Conjecture and TCC, we show that generic features of the dark dimenson cosmology can naturally lead to the correct dark matter density and a resolution of the cosmological coincidence problem, where the matter/radiation equality temperature ($T\sim$ 1 eV) coincides with the temperature where the dark energy begins to dominate. Thus one does not need to appeal to Weinberg's anthropic argument to explain this coincidence. The dark gravitons are produced at $T\sim$ 4 GeV, and their composition changes as they mainly decay to lighter gravitons, without losing much total mass density. The mass of dark gravitons is $m_{\text{DM}}\sim 1-100$ keV today.

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arXiv2022

Swampland Bounds on Dark Sectors

We use Swampland principles to theoretically disfavor regions of the parameter space of dark matter and other darkly charged particles that may exist. The Festina Lente bound, the analogue of the Weak-Gravity conjecture in de Sitter, places constraints on the mass and charge of dark particles, which here we show cover regions in parameter space that are currently allowed by observations. As a consequence, a broad set of new ultra-light particles are in the Swampland, independently of their cosmic abundance, showing the complementarity of Quantum Gravity limits with laboratory and astrophysical studies. In parallel, a Swampland bound on the UV cutoff associated to the axion giving a Stückelberg photon its longitudinal mode translates to a new constraint on the kinetic mixings and masses of dark photons. This covers part of the parameter space targeted by upcoming dark-photon direct-detection experiments. Moreover, it puts astrophysically interesting models in the Swampland, including freeze-in dark matter through an ultra-light dark photon, as well as radio models invoked to explain the 21-cm EDGES anomaly.

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arXiv2021

A Tale of Two $U(1)$'s: Kinetic Mixing from Lattice WGC States

We point out that the states required by the Lattice Weak Gravity Conjecture, along with certain genericity conditions, imply the existence of non-vanishing kinetic mixing between massless Abelian gauge groups in the low-energy effective theory. We carry out a phenomenological estimate using a string-inspired probability distribution for the masses of superextremal states and compare the results to expectations from string theory and field theory, estimating the magnitude of kinetic mixing in each case. In the string case, we compute the kinetic mixing in an ensemble of 1858 MSSM-like heterotic orbifolds as well as in Type II supergravity on a Calabi-Yau manifold. From the field theory perspective, we consider compactifications of a 5D gauge theory. Finally, we discuss potential loopholes that can evade the bounds set by our estimates.

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arXiv2020

Topological Gravity as the Early Phase of Our Universe

Motivated by string dualities we propose topological gravity as the early phase of our universe. The topological nature of this phase naturally leads to the explanation of many of the puzzles of early universe cosmology. A concrete realization of this scenario using Witten's four dimensional topological gravity is considered. This model leads to the power spectrum of CMB fluctuations which is controlled by the conformal anomaly coefficients $a,c$. In particular the strength of the fluctuation is controlled by $1/a$ and its tilt by $c g^2$ where $g$ is the coupling constant of topological gravity. The positivity of $c$, a consequence of unitarity, leads automatically to an IR tilt for the power spectrum. In contrast with standard inflationary models, this scenario predicts $\mathcal{O}(1)$ non-Gaussianities for four- and higher-point correlators and the absence of tensor modes in the CMB fluctuations.

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arXiv2019

$H_0$ Tension, Swampland Conjectures and the Epoch of Fading Dark Matter

Motivated by the swampland dS conjecture, we consider a rolling scalar field as the source of dark energy. Furthermore, the swampland distance conjecture suggests that the rolling field will lead at late times to an exponentially light tower of states. Identifying this tower as residing in the dark sector suggests a natural coupling of the scalar field to the dark matter, leading to a continually reducing dark matter mass as the scalar field rolls in the recent cosmological epoch. The exponent in the distance conjecture, $\tilde{c}$, is expected to be an $\mathcal{O}(1)$ number. Interestingly, when we include the local measurement of $H_0$, our model prefers a non-zero value of the coupling $\tilde{c}$ with a significance of $2.8σ$ and a best-fit at $\tilde{c} \sim 0.3$. Modifying the recent evolution of the universe in this way improves the fit to data at the $2σ$ level compared to $Λ$CDM. This string-inspired model automatically reduces cosmological tensions in the $H_0$ measurement as well as $σ_8$.

astro-ph.CO↗

arXiv2018

Dark Energy and the Refined de Sitter Conjecture

We revisit the phenomenology of quintessence models in light of the recently refined version of the de Sitter Swampland conjecture, which includes the possibility of unstable de Sitter critical points. We show that models of quintessence can evade previously derived lower bounds on $(1+w)$, albeit with very finely-tuned initial conditions. In the absence of such tuning or other rolling quintessence fields, a field with mass close to Hubble is required, which has a generic prediction for $(1+w)$. Slow-roll single field inflation models remain in tension. Other phenomenological constraints arising from the coupling of the quintessence field with the Higgs or the QCD axion are significantly relaxed.

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arXiv2018

On the Cosmological Implications of the String Swampland

We study constraints imposed by two proposed string Swampland criteria on cosmology. These criteria involve an upper bound on the range traversed by scalar fields as well as a lower bound on $|\nabla_ϕ V|/V$ when $V >0$. We find that inflationary models are generically in tension with these two criteria. Applying these same criteria to dark energy in the present epoch, we find that specific quintessence models can satisfy these bounds and, at the same time, satisfy current observational constraints. Assuming the two Swampland criteria are valid, we argue that the universe will undergo a phase transition within a few Hubble times. These criteria sharpen the motivation for future measurements of the tensor-to-scalar ratio $r$ and the dark energy equation of state $w$, and for tests of the equivalence principle for dark matter.

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arXiv2018

De Sitter Space and the Swampland

It has been notoriously difficult to construct a meta-stable de Sitter (dS) vacuum in string theory in a controlled approximation. This suggests the possibility that meta-stable dS belongs to the swampland. In this paper, we propose a swampland criterion in the form of $|\nabla V|\geq\ c \cdot V$ for a scalar potential $V$ of any consistent theory of quantum gravity, for a positive constant $c$. In particular, this bound forbids dS vacua. The existence of this bound is motivated by the abundance of string theory constructions and no-go theorems which exhibit this behavior. We also extend some of the well-known no-go theorems for the existence of dS vacua in string theory to more general accelerating universes and reinterpret the results in terms of restrictions on allowed scalar potentials.

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arXiv2018

Inflationary vs. Reionization Features from Planck 2015 Data

Features during inflation and reionization leave corresponding features in the temperature and polarization power spectra that could potentially explain anomalies in the Planck 2015 data but require a joint analysis to disentangle. We study the interplay between these two effects using a model-independent parametrization of the inflationary power spectrum and the ionization history. Preference for a sharp suppression of large scale power is driven by a feature in the temperature power spectrum at multipoles $\ell \sim 20$, whereas preference for a component of high redshift ionization is driven by a sharp excess of polarization power at $\ell \sim 10$ when compared with the lowest multipoles. Marginalizing inflationary freedom does not weaken the preference for $z \gtrsim 10$ ionization, whereas marginalizing reionization freedom slightly enhances the preference for an inflationary feature but can also mask its direct signature in polarization. The inflation and reionization interpretation of these features makes predictions for the polarization spectrum which can be tested in future precision measurements especially at $10\lesssim \ell \lesssim 40$.

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arXiv2017

Inflationary Features and Shifts in Cosmological Parameters from Planck 2015 Data

We explore the relationship between features in the Planck 2015 temperature and polarization data, shifts in the cosmological parameters, and features from inflation. Residuals in the temperature data at low multipole $\ell$, which are responsible for the high $H_0\approx 70$ km s$^{-1}$Mpc$^{-1}$ and low $σ_8Ω_m^{1/2}$ values from $\ell<1000$ in power-law $Λ$CDM models, are better fit to inflationary features with a $1.9σ$ preference for running of the running of the tilt or a stronger $99\%$ CL local significance preference for a sharp drop in power around $k=0.004$ Mpc$^{-1}$ in generalized slow roll and a lower $H_0\approx 67$ km s$^{-1}$Mpc$^{-1}$. The same in-phase acoustic residuals at $\ell>1000$ that drive the global $H_0$ constraints and appear as a lensing anomaly also favor running parameters which allow even lower $H_0$, but not once lensing reconstruction is considered. Polarization spectra are intrinsically highly sensitive to these parameter shifts, and even more so in the Planck 2015 TE data due to an outlier at $\ell \approx 165$, which disfavors the best fit $H_0$ $Λ$CDM solution by more than $2σ$, and high $H_0$ value at almost $3σ$. Current polarization data also slightly enhance the significance of a sharp suppression of large-scale power but leave room for large improvements in the future with cosmic variance limited $E$-mode measurements.

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arXiv2016

Towards a comprehensive picture of powerful quasars, their host galaxies and quasar winds at z ~ 0.5

Luminous type-2 quasars in which the glow from the central black hole is obscured by dust are ideal targets for studying their host galaxies and the quasars' effect on galaxy evolution. Such feedback appears ubiquitous in luminous obscured quasars where high velocity ionized nebulae have been found. We present rest-frame yellow-band (~5000 Angstroms) observations using the Hubble Space Telescope for a sample of 20 luminous quasar host galaxies at 0.2 < z < 0.6 selected from the Sloan Digital Sky Survey. For the first time, we combine host galaxy observations with geometric measurements of quasar illumination using blue-band HST observations and [OIII] integral field unit observations probing the quasar winds. The HST images reveal bright merger signatures in about half the galaxies; a significantly higher fraction than in comparison inactive ellipticals. We show that the host galaxies are primarily bulge-dominated, with masses close to M*, but belong to < 30% of elliptical galaxies that are highly star-forming at z ~ 0.5. Ionized gas signatures are uncorrelated with faint stellar disks (if present), confirming that the ionized gas is not concentrated in a disk. Scattering cones and [OIII] ionized gas velocity field are aligned with the forward scattering cones being co-spatial with the blue-shifted side of the velocity field, suggesting the high velocity gas is indeed photo-ionized by the quasar. Based on the host galaxies' high star-formation rates and bright merger signatures, we suggest that this low-redshift outbreak of luminous quasar activity is triggered by recent minor mergers. Combining these novel observations, we present new quasar unification tests, which are in agreement with expectations of the orientation-based unification model for quasars.

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