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Constantinos Skordis

Publications and source records attributed to Constantinos Skordis.

At least 19 recordsLinked to original sources
arXiv2026

CMBolic: Symbolic emulators for the Cosmic Microwave Background. I. Lensing

We present the first installment of CMBolic: a suite of symbolic cosmic microwave background (CMB) emulators. In this instance, we emulate the CMB lensing potential power spectrum $C_\ell^{ϕϕ}$ for the widely used extended $Λ$CDM model which simultaneously includes massive neutrinos and evolving dark energy modelled using the Chevallier-Polarski-Linder (CPL) parameterization. We achieve comparable precision to existing neural network emulators, with the added benefit of simpler handling as our emulators are analytic functions of the model parameters and multipole $\ell$. On independent validation spectra evaluated in the range $2\leq \ell \leq 5500$, CMBolic achieves mean absolute fractional errors of $0.27\%$ in the $Λ$CDM subspace and $0.32\%$ across the full extended parameter space. This emulation error is well below even the most optimistic noise forecasts from CMB Stage 4 experiments. We apply CMBolic to cosmological parameter estimation with Bayesian inference using the lensing-only likelihoods from ACT DR6 and Planck. We show excellent agreement between the posteriors obtained by CMBolic and the Boltzmann code CLASS. This demonstrates the practical use of CMBolic on cosmological parameter estimation, reducing the runtime from 2 weeks to under 3 minutes.

astro-ph.CO↗
arXiv2025

The Cosmological Dipole in Tilted Anisotropic Universes

There is tentative evidence for a mismatch between the rest frames of matter and the cosmic microwave background, the "quasar dipole anomaly". We consider such a dipole in tilted anisotropic models, for a range of scenarios and sources: spatial curvature, cosmic heat flux, large scale electromagnetic fields and a Khronon field. Crucially, we determine the ancillary effects on other cosmological observables in each of these models and we show that, apart from the case of the Khronon field, it is unlikely that one can obtain a dipole with the amplitude that is being observed unless one considers additional exotica.

astro-ph.CO↗
arXiv2025

Scaling relations, dynamical heating and tidal disruption in spin $s$ ultralight dark matter models

We explore the impact of spin 0, spin 1 and spin 2 ultralight dark Matter (ULDM) on small scales by numerically solving the Schrödinger-Poisson system using the time-split method. We perform simulations of ULDM for each spin, starting with different numbers of identical initial solitons and analyse the properties of the resulting haloes after they merge. Our findings reveal that higher spin lead to broader, less dense haloes with more prominent Navarro-Frenk-White (NFW) tails, a characteristic that persists regardless of the number of solitons involved. Additionally, we study the process of dynamical heating for these haloes, and find that the heating time-scale for higher spin increases order an of magnitude compared to the spin 0 case. Then, we identify scaling relations that describe the density profile, core-NFW of spin~$s$ ULDM haloes as a function of the number of initial solitons $N_{\text{sol}}$. These relations allow us to construct equivalent haloes based on average density or total mass, for arbitrarily large $N_{\text{sol}}$, without having to simulate those systems. We simulate the orbit of an ULDM satellite in a constructed halo treated as an external potential, and find that for host haloes having the same average density, the disruption time of the satellite is as predicted for a uniform sphere regardless of the spin. However, satellites orbiting haloes having the same mass for each spin, result in faster disruption in the case of spin 0, whereas for haloes having the same core size result in faster disruption in the case of spin 2.

astro-ph.CO↗
arXiv2025

Sym-EFT: Accelerating Effective Field Theory of Large Scale Structure with Symbolic Regression

We present an emulator suite for the one- and two-loop cold dark matter power spectrum from the Effective Field Theory of Large Scale Structures (EFTofLSS). Specifically, we emulate separately the various contributions to the one- and two-loop parts of the power spectrum, leaving out the possible counterterms which can be added as multiplicative prefactors. By leaving the time-dependence of the counterterms unspecified at the emulation stage, our technique has the advantage of being extremely versatile in fitting any type of counterterm parametrisation to data, or to simulations, without having to change the emulator. We construct our emulators using the method of symbolic regression which results in functions that can be used directly in computer code, while achieving errors of better than $0.5\%$ within the $k$-range of validity of EFT and maintaining ultra-fast computational evaluation of less than $\sim5\times10^{-4}s$ on a single core.

astro-ph.CO↗
arXiv2025

Khronon-Tensor theory reproducing MOND and the cosmological model

We propose an alternative scalar-tensor theory based on the Khronon scalar field labeling a family of space-like three-dimensional hypersurfaces. This theory leads to modified Newtonian dynamics (MOND) at galactic scales for stationary systems, recovers GR plus a cosmological constant in the strong field regime, and is in agreement with the standard cosmological model and the observed cosmic microwave background anisotropies.

gr-qc↗
arXiv2025

Stealth black holes in Aether Scalar Tensor theory

The Aether Scalar Tensor (AeST) theory is an extension of general relativity(GR) successful at reproducing galactic rotational curves, gravitational lensing, linear large scale structure and cosmic microwave background power spectrum observations. We solve the most general static spherically symmetric vacuum equations in the strong-field regime of AeST and find two classes of stealth black hole solutions -- those with exact GR geometries -- containing non-trivial secondary hair. In particular, one of these can be continuously joined to the cosmological solution of AeST. We also derive a non-black hole solution with zero spatial component in the vector field. This result proves the existence of mathematically and observationally consistent candidates for black holes in AeST, and creates a basis for testing the theory in the strong-field regime.

gr-qc↗
arXiv2024

Aether scalar tensor theory: Hamiltonian Formalism

The Aether Scalar Tensor (AeST) theory is an extension of General Relativity (GR), proposed for addressing galactic and cosmological observations without dark matter. By casting the AeST theory into a $3+1$ form, we determine its full non-perturbative Hamiltonian formulation and analyse the resulting constraints. We find the presence of four first class and four second class constraints and show that the theory has six physical degrees of freedom at the fully nonlinear level. Our results set the basis for determining the propagation of perturbations on general backgrounds and we present the case of small perturbations around Minkowski spacetime as an example stemming from our analysis.

gr-qc↗
arXiv2024

Relativistic Khronon Theory in agreement with Modified Newtonian Dynamics and Large-Scale Cosmology

We propose an extension of General Relativity (GR) based on a space-time foliation by three-dimensional space-like hypersurfaces labeled by the Khronon scalar field $τ$. We show that this theory (i) leads to modified Newtonian dynamics (MOND) at galactic scales for stationary systems; (ii) recovers GR plus a cosmological constant in the strong field regime; (iii) is in agreement with the standard cosmological model and the observed cosmic microwave background anisotropies at linear cosmological scales, where the theory reduces to a subset of the generalized dark matter (GDM) model. We compute the second order action on a Minkowski background and show that it contains the usual tensor modes of GR and a scalar degree of freedom with dispersion relation $ω=0$. We find that the deconstrained Hamiltonian is bounded from below for wavenumbers larger than $\sim 10^{-31}\,\text{eV}$ and unbounded for smaller wavenumbers.

gr-qc↗
arXiv2023

Towards galaxy cluster models in Aether-Scalar-Tensor theory: isothermal spheres and curiosities

The Aether-Scalar-Tensor (AeST) theory is an extension of General Relativity (GR) which can support Modified Newtonian Dynamics (MOND) behaviour in its static weak-field limit, and cosmological evolution resembling $Λ$CDM. We consider static spherically symmetric weak-field solutions in this theory and show that the resulting equations can be reduced to a single equation for the gravitational potential. The reduced equation has apparent isolated singularities when the derivative of the potential passes through zero and we show how these are removed by evolving, instead, the canonical momentum of the corresponding Hamiltonian system that we find. We construct solutions in three cases: (i) vacuum outside a bounded spherical object, (ii) within an extended prescribed source, and (iii) isothermal gas in hydrostatic equilibrium, serving as a simplified model for galaxy clusters. We show that the oscillatory regime that follows the Newtonian and MOND regimes, obtained in previous works in the vacuum case, also persists for isothermal spheres, and we show that the gas density profiles in AeST may become more compressed than their Newtonian or MOND counterparts. We construct the Radial Acceleration Relation (RAR) in AeST for isothermal spheres and find that it can display a peak, an enhancement with respect to the MOND RAR, at an acceleration range determined by the value of the AeST weak-field mass parameter, the mass of the system and the boundary value of the gravitational potential. For lower accelerations, the AeST RAR drops below the MOND expectation, as if there is a negative mass density. Similar observational features of the galaxy cluster RAR have been reported. This illustrates the potential of AeST to address the shortcomings of MOND in galaxy clusters, but a full quantitative comparison with observations will require going beyond the isothermal case.

astro-ph.CO↗
arXiv2023

Scalar two-point functions at the late-time boundary of de Sitter

We calculate two-point functions of scalar fields of mass $m$ and their conjugate momenta at the late-time boundary of de Sitter with Bunch-Davies boundary conditions, in general $d+1$ spacetime dimensions. We perform the calculation using the wavefunction picture and using canonical quantization. With the latter one clearly sees how the late-time field and conjugate momentum operators are linear combinations of the normalized late-time operators $\alphaN$ and $\betaN$ that correspond to unitary irreducible representations of the de Sitter group with well-defined inner products. The two-point functions resulting from these two different methods are equal and we find that both the autocorrelations of $\alphaN$ and $\betaN$ and their cross correlations contribute to the late-time field and conjugate momentum two-point functions. This happens both for light scalars ($m<\frac{d}{2}H$), corresponding to complementary series representations, and heavy scalars ($m>\frac{d}{2}H$), corresponding to principal series representations of the de Sitter group, where $H$ is the Hubble scale of de Sitter. In the special case $m=0$, only the $\betaN$ autocorrelation contributes to the conjugate momentum two-point function in any dimensions and we gather hints that suggest $\alphaN$ to correspond to discrete series representations for this case at $d=3$.

hep-th↗
arXiv2023

Aether Scalar Tensor (AeST) theory: Quasistatic spherical solutions and their phenomenology

There have been many efforts in the last three decades to embed the empirical MOND program into a robust theoretical framework. While many such theories can explain the profile of galactic rotation curves, they usually cannot explain the evolution 15 the primordial fluctuations and the formation of large-scale-structures in the Universe. The Aether Scalar Tensor (AeST) theory seems to have overcome this difficulty, thereby providing the first compelling example of an extension of general relativity able to successfully challenge the particle dark matter hypothesis. Here we study the phenomenology of this theory in the quasistatic weak-field regime and specifically for the idealised case of spherical isolated sources. We find the existence of three distinct gravitational regimes, that is, Newtonian, MOND and a third regime characterised by the presence of oscillations in the gravitational potential which do not exist in the traditional MOND paradigm. We identify the transition scales between these three regimes and discuss their dependence on the boundary conditions and other parameters in the theory. Aided by analytical and numerical solutions, we explore the dependence of these solutions on the theory parameters. Our results could help in searching for interesting observable phenomena at low redshift pertaining to galaxy dynamics as well as lensing observations, however, this may warrant proper N-body simulations that go beyond the idealised case of spherical isolated sources.

astro-ph.CO↗
arXiv2022

Aether scalar tensor theory: Linear stability on Minkowski space

We have recently proposed a simple relativistic theory which reduces to modified Newtonian dynamics for the weak-field quasistatic situations applied to galaxies, and to cosmological behavior as in the $Λ$CDM model, yielding a realistic cosmology in line with observations. A key requirement of any such model is that Minkowski space is stable against linear perturbations. We expand the theory action to second order in perturbations on a Minkowski background and show that it leads to healthy dispersion relations involving propagating massive modes in the vector and the scalar sector. We use Hamiltonian methods to eliminate constraints present, demonstrate that the massive modes have Hamiltonian bounded from below and show that a nonpropagating mode with a linear time dependence may have unbounded Hamiltonian for wave numbers $k< μ$ and bounded otherwise. The scale $μ$ is estimated to be $\lesssim \mathrm{Mpc}^{-1}$ so that the low momenta instability may only play a role on cosmological scales.

gr-qc↗
arXiv2022

Cosmic backgrounds from the radio to the far-infrared: recent results and perspectives from cosmological and astrophysical surveys

Cosmological and astrophysical surveys in various wavebands, in particular from the radio to the far-infrared, offer a unique view of the universe's properties and the formation and evolution of its structures. After a preamble on the so-called tension problem, which occurs when different types of data are used to determine cosmological parameters, we discuss the role of fast radio bursts in cosmology, in particular for the missing baryon problem, and the perspectives from the analysis of the 21 cm redshifted line from neutral hydrogen. We then describe the Planck Legacy Archive, its wealth of scientific information and next developments, and the promising perspectives expected from higher resolution observations, in particular for the analysis of the thermal Sunyaev-Zel'dovich effect. Three cosmological results of the Planck mission are presented next: the implications of the map of Comptonization fluctuations, the dipole analysis from cross-correlating cosmic microwave background anisotropy and Comptonization fluctuation maps, and the constraints on the primordial tensor-to-scalar perturbation ratio. Finally, we discuss some future perspectives and alternative scenarios in cosmology, such as the study of the Lorentz invariance violation with the cosmic microwave background polarization, the introduction of new gravitational degrees of freedom to solve the dark matter problem, and the exploitation of the magnification bias with high-redshift sub-millimeter galaxies to constrain cosmological parameters.

astro-ph.CO↗
arXiv2021

A new relativistic theory for Modified Newtonian Dynamics

We propose a relativistic gravitational theory leading to modified Newtonian dynamics, a paradigm that explains the observed universal galactic acceleration scale and related phenomenology. We discuss phenomenological requirements leading to its construction and demonstrate its agreement with the observed cosmic microwave background and matter power spectra on linear cosmological scales. We show that its action expanded to second order is free of ghost instabilities and discuss its possible embedding in a more fundamental theory.

astro-ph.CO↗
arXiv2021

Dark Matter properties through cosmic history

We perform the first test of dark matter (DM) stress-energy evolution through cosmic history, using cosmic microwave background measurements supplemented with baryon acoustic oscillation data and the Hubble Space Telescope key project data. We constrain the DM equation of state (EoS) in 8 redshift bins, and its sound speed and (shear) viscosity in 9 redshift bins, finding no convincing evidence for non-$Λ$CDM values in any of the redshift bins. Despite this enlarged parameter space, the sound speed and viscosity are constrained relatively well at late times (due to the inclusion of CMB lensing), whereas the EoS is most strongly constrained around recombination. These results constrain for the first time the level of "coldness" required of DM across various cosmological epochs at both the background and perturbative levels. We show that simultaneously allowing time dependence for both the EoS and sound speed parameters shifts the posterior of the DM abundance before recombination to a higher value, while keeping the present day DM abundance similar to the $Λ$CDM value. This shifts the posterior for the present day Hubble constant compared to $Λ$CDM, suggesting that DM with time-dependent parameters is well-suited to explore possible solutions to persistent tensions within the $Λ$CDM model. We perform a detailed comparison with our previous study involving a vanishing sound speed and viscosity using the same datasets in order to explain the physical mechanism behind these shifts.

astro-ph.CO↗
arXiv2020

Unitarity at the Late time Boundary of de Sitter

The symmetry group of the de Sitter spacetime, accommodates fields of various masses and spin among its unitary irreducible representations. These unitary representations are labeled by the spin and the weight contribution to the scaling dimension and depending on the mass and spin of the field the weight may take either purely real or purely imaginary values. In this work, we construct the late time boundary operators for a massive scalar field propagating in de Sitter spacetime, in arbitrary dimensions. We show that contrary to the case of Anti de Sitter, purely imaginary weights also correspond to unitary operators, as well as the ones with real weight, and identify the corresponding unitary representations. We demonstrate that these operators correspond to the late time boundary operators and elucidate that all of them have positive definite norm.

hep-th↗
arXiv2019

A general class of gravitational theories as alternatives to dark matter where the speed of gravity always equals the speed of light

A number of theories of gravity have been proposed as proxies for dark matter in the regime of galaxies and cosmology. The recent observations of gravitational waves (GW170817) from the merger of two neutron stars, followed by an electromagnetic counterpart (GW170817a) have placed stringent constraints on the difference of the speed of gravity to the speed of light, severely restricting the phenomenological viability of such theories. We revisit the impact of these observations on the Tensor-Vector-Scalar (TeVeS) paradigm of relativistic Modified Newtonian Dynamics (MOND) and demonstrate the existence of a previously unknown class of this paradigm where the speed of gravity always equals the speed of light. We show that this holds without altering the usual (bimetric) MOND phenomenology in galaxies.

gr-qc↗