arXiv subjects

Cynthia Trendafilova

Publications and source records attributed to Cynthia Trendafilova.

13 recordsLinked to original sources

arXiv2026

Cosmic topology. Part Va. Information content of the observable Universe

If the Universe possesses a compact spatial topology with characteristic scale not much larger than the diameter of the observable Universe, then the associated symmetry transformations imprint specific statistical correlations on density perturbations. We derive the covariance between perturbation modes in the full topology volume and compute the induced covariance between modes in the observable subvolume. We then use this covariance to estimate the available information about topology contained in the full set of observable linear perturbations. For two sample Euclidean examples - the cubic three-torus ($E_1$) and a three-torus with a quarter turn ($E_3$) - the three-dimensional density field carries enough information to detect topology on scales up to 25 percent larger than those accessible from the two-dimensional cosmic microwave background anisotropies alone. Future probes of the cosmic density field using deep galaxy surveys or neutral-hydrogen 21-cm intensity mapping, combined with computationally intensive searches over the large parameter space of admissible topologies, thus offer a plausible avenue to extending the range of detectable cosmic topologies.

astro-ph.CO↗

arXiv2026

Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics

Neutrinos and other light relics leave characteristic imprints in the cosmic microwave background anisotropies, making their observation a sensitive probe of the particle content and thermal history of the early universe. The energy density in these relativistic species is parameterized by their effective number $N_\mathrm{eff}$. Measuring this parameter at the percent level, which is a long-standing science goal of CMB-S4 and other experiments, would test a wide range of well-motivated physics within and beyond the Standard Model of particle physics. In this paper, we present Fisher-matrix forecasts of the projected sensitivity to $N_\mathrm{eff}$ of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches $\sigma(N_\mathrm{eff}) < 0.03$ over its seven-year observing period, while the revised configuration achieves the same precision over a longer timescale. We complement these results with a cosmic-variance-limited survey over the same multipole range to quantify the room for improvement accessible with additional instrumental, observational, and theoretical efforts. Finally, we discuss the broad implications of precise $N_\mathrm{eff}$ measurements for the radiation sector, big bang nucleosynthesis, light thermal relics, and other early-universe physics. The forecasts presented in this work are performed with the publicly released DRAFT (Dark Radiation Anisotropy Flowdown Team) tool. It provides an end-to-end pipeline from simulated foreground maps and component separation to delensing and projected sensitivities for any cosmological parameter, and it can be directly applied to other cosmic microwave background survey designs.

astro-ph.CO↗

arXiv2026

DerivKit: stable numerical derivatives bridging Fisher forecasts and MCMC

DerivKit is a Python package for derivative-based statistical inference. It implements stable numerical differentiation and derivative assembly utilities for Fisher-matrix forecasting and higher-order likelihood approximations in scientific applications, supporting scalar- and vector-valued models including black-box or tabulated functions where automatic differentiation is impractical or unavailable. These derivatives are used to construct Fisher forecasts, Fisher bias estimates, and non-Gaussian likelihood expansions based on the Derivative Approximation for Likelihoods (DALI). By extending derivative-based inference beyond the Gaussian approximation, DerivKit forms a practical bridge between fast Fisher forecasts and more computationally intensive sampling-based methods such as Markov chain Monte Carlo (MCMC).

astro-ph.IM↗

arXiv2025

The End of Easy Phenomenology for CMB Experiments: A Case Study in the Dark Sector

The precision of Cosmic Microwave Background (CMB) experiments, specifically its lensing reconstruction, has reached the limit where non-linear corrections cannot be ignored. Neglecting these corrections results in biased constraints on cosmological parameters. In this work, we use lensing data from Planck and the South Pole Telescope third generation camera (SPT-3G) taken in 2018 to highlight the impact of these biases in two ways. First, we estimate the shifts due to ignoring non-linear corrections in $Λ$CDM. We find 0.2-0.6$σ$ shifts in the Dark Matter (DM) fraction, the Hubble constant, and the amplitude of matter fluctuations at $8 h^{-1}$ Mpc. Second, we estimate the loss in constraining power for not including data sensitive to non-linear corrections. As a case study, we consider two interacting DM models, for which such corrections are not readily available in existing CMB Boltzmann codes. The first one is DM interacting with baryons, while the second is DM interacting with Dark Radiation (DR). For the former case, when we add primary CMB data from SPT-3G 2018 observations, we find that constraints on model parameters improve by 10-20% compared to previous studies. However, we forecast a further 50% improvement on these constraints if one could include current or upcoming SPT-3G lensing data. For the case of DM interacting with DR, no meaningful constraints on the model parameters are found without including information from CMB lensing. We also highlight that the codes used to calculate non-linear corrections in $Λ$CDM, specifically HaloFit and HMCode, provide unstable results when improperly used for these extended models. These outcomes constitute a reason for caution if using CMB lensing data when constraining such models, as well as a motivation for estimating their non-linear corrections.

astro-ph.CO↗

arXiv2023

Improving Constraints on Inflation with CMB Delensing

The delensing of cosmic microwave background (CMB) maps will be increasingly valuable for extracting as much information as possible from future CMB surveys. Delensing provides many general benefits, including sharpening of the acoustic peaks, more accurate recovery of the damping tail, and reduction of lensing-induced $B$-mode power. In this paper we present several applications of delensing focused on testing theories of early-universe inflation with observations of the CMB. We find that delensing the CMB results in improved parameter constraints for reconstructing the spectrum of primordial curvature fluctuations, probing oscillatory features in the primordial curvature spectrum, measuring the spatial curvature of the universe, and constraining several different models of isocurvature perturbations. In some cases we find that delensing can recover almost all of the constraining power contained in unlensed spectra, and it will be a particularly valuable analysis technique to achieve further improvements in constraints for model parameters whose measurements are not expected to improve significantly when utilizing only lensed CMB maps from next-generation CMB surveys. We also quantify the prospects of testing the single-field inflation tensor consistency condition using delensed CMB data; we find it to be out of reach of current and proposed experimental technology and advocate for alternative detection methods.

astro-ph.CO↗

arXiv2023

The Impact of Cross-Covariances Between the CMB and Reconstructed Lensing Power

Weak gravitational lensing of the Cosmic Microwave Background (CMB) changes CMB statistics in a nontrivial way, allowing for reconstruction of the lensing potential and the use of these reconstructed maps in determining cosmological parameters that affect the formation of intervening large-scale structures. Although in principle there are correlations between the primary CMB and the reconstructed lensing potential due to the lensing procedure itself, in practice CMB analyses treat these as negligible when combining these band powers in likelihoods. In this paper we quantify explicitly the impact on parameter constraints due to these cross-covariances between the lensed CMB and reconstructed lensing power, and we compare to the effect of including all lensing-induced non-Gaussian covariances, which have previously shown to impact parameter constraints on the order of 10%. We perform our analysis for a range of experimental setups, scanning over instrumental noise levels of 0.5 to 10.0 $μ$K-arcmin in temperature assuming fully polarized detectors, and using a fixed beam size of 1.4 arcmin. When the correlations between the lensed CMB and lensing power are neglected, we find that forecasted constraints shift by at most 3% of the error bar for a 6-parameter $Λ$CDM model, and for the noise levels considered in this paper. For some of the $Λ$CDM extensions considered here, however, these correlations have a nontrivial impact, in some cases more than 10% of the error bar, even for current experimental noise levels.

astro-ph.CO↗

arXiv2023

Beyond Fisher Forecasting for Cosmology

The planning and design of future experiments rely heavily on forecasting to assess the potential scientific value provided by a hypothetical set of measurements. The Fisher information matrix, due to its convenient properties and low computational cost, provides an especially useful forecasting tool. However, the Fisher matrix only provides a reasonable approximation to the true likelihood when data are nearly Gaussian distributed and observables have nearly linear dependence on the parameters of interest. Also, Fisher forecasting techniques alone cannot be used to assess their own validity. Thorough sampling of the exact or mock likelihood can definitively determine whether a Fisher forecast is valid, though such sampling is often prohibitively expensive. We propose a simple test, based on the Derivative Approximation for LIkelihoods (DALI) technique, to determine whether the Fisher matrix provides a good approximation to the exact likelihood. We show that the Fisher matrix becomes a poor approximation to the true likelihood in regions where two-dimensional slices of level surfaces of the DALI approximation to the likelihood differ from two-dimensional slices of level surfaces of the Fisher approximation to the likelihood. We demonstrate that our method accurately predicts situations in which the Fisher approximation deviates from the true likelihood for various cosmological models and several data combinations, with only a modest increase in computational cost compared to standard Fisher forecasts.

astro-ph.CO↗

arXiv2022

Gravitational Wave Timing Array

We describe the design of a gravitational wave timing array, a novel scheme that can be used to search for low-frequency gravitational waves by monitoring continuous gravitational waves at higher frequencies. We show that observations of gravitational waves produced by Galactic binaries using a space-based detector like LISA provide sensitivity in the nanohertz to microhertz band. While the expected sensitivity of this proposal is not competitive with other methods, it fills a gap in frequency space around the microhertz regime, which is above the range probed by current pulsar timing arrays and below the expected direct frequency coverage of LISA. The low-frequency extension of sensitivity does not require any experimental design change to space-based gravitational wave detectors, and can be achieved with the data products that would already be collected by them.

gr-qc↗

arXiv2021

The Benefits of CMB Delensing

The effects of gravitational lensing of the cosmic microwave background (CMB) have been measured at high significance with existing data and will be measured even more precisely in future surveys. Reversing the effects of lensing on the observed CMB temperature and polarization maps provides a variety of benefits. Delensed CMB spectra have sharper acoustic peaks and more prominent damping tails, allowing for improved inferences of cosmological parameters that impact those features. Delensing reduces $B$-mode power, aiding the search for primordial gravitational waves and allowing for lower variance reconstruction of lensing and other sources of secondary CMB anisotropies. Lensing-induced power spectrum covariances are reduced by delensing, simplifying analyses and improving constraints on primordial non-Gaussianities. Biases that result from incorrectly modeling nonlinear and baryonic feedback effects on the lensing power spectrum are mitigated by delensing. All of these benefits are possible without any changes to experimental or survey design. We develop a self-consistent, iterative, all-orders treatment of CMB delensing on the curved sky and demonstrate the impact that delensing will have with future surveys.

astro-ph.CO↗

arXiv2021

Searching for Gravitational Waves with Strongly Lensed Repeating Fast Radio Bursts

Since their serendipitous discovery, Fast Radio Bursts (FRBs) have garnered a great deal of attention from both observers and theorists. A new class of radio telescopes with wide fields of view have enabled a rapid accumulation of FRB observations, confirming that FRBs originate from cosmological distances. The high occurrence rate of FRBs and the development of new instruments to observe them create opportunities for FRBs to be utilized for a host of astrophysical and cosmological studies. We focus on the rare, and as yet undetected, subset of FRBs that undergo repeated bursts and are strongly gravitationally lensed by intervening structure. An extremely precise timing of burst arrival times is possible for strongly lensed repeating FRBs, and we show how this timing precision enables the search for long wavelength gravitational waves, including those sourced by supermassive black hole binary systems. The timing of burst arrival for strongly lensed repeating FRBs is sensitive to gravitational wave sources near the FRB host galaxy, which may lie at cosmological distances and would therefore be extremely challenging to detect by other means. Timing of strongly lensed FRBs can also be combined with pulsar timing array data to search for correlated time delays characteristic of gravitational waves passing through the Earth.

astro-ph.CO↗

arXiv2019

Addressing Astrophysical and Cosmological Problems With Secretly Asymmetric Dark Matter

We present a simple model of dark matter that can address astrophysical and cosmological puzzles across a wide range of scales. The model is an application of the Secretly Asymmetric Dark Matter mechanism, where several flavors of dark matter are fully asymmetric despite an exact dark matter number symmetry $U(1)_χ$. The dark matter relic abundance arises from these asymmetries, generated in the early universe through right-handed neutrino decays. The $U(1)_χ$ is gauged by a massless dark photon, and asymmetries with opposite signs in the different DM flavors result in the formation of bound states. Dark acoustic oscillations in the early universe lead to a suppression in the matter power spectrum for addressing the $σ_8$ problem. The dark photon as a relativistic degree of freedom contributes to $ΔN_{eff}$, easing the tension between the CMB and late-time $H_{0}$ measurements. Finally, scattering between the bound states after structure formation leads to a flattening of the dark matter distribution at the cores of haloes.

hep-ph↗

arXiv2016

Secretly Asymmetric Dark Matter

We study a mechanism where the dark matter number density today arises from asymmetries generated in the dark sector in the early universe, even though total dark matter number remains zero throughout the history of the universe. The dark matter population today can be completely symmetric, with annihilation rates above those expected from thermal WIMPs. We give a simple example of this mechanism using a benchmark model of flavored dark matter. We discuss the experimental signatures of this setup, which arise mainly from the sector that annihilates the symmetric component of dark matter.

hep-ph↗

arXiv2015

Lepton-Flavored Asymmetric Dark Matter and Interference in Direct Detection

In flavored dark matter models, dark matter can scatter off of nuclei through Higgs and photon exchange, both of which can arise from renormalizable interactions and individually lead to strong constraints from direct detection. While these two interaction channels can destructively interfere in the scattering amplitude, for a thermal relic with equal abundances for the dark matter particle and its antiparticle, this produces no effect on the total event rate. Focusing on lepton-flavored dark matter, we show that it is quite natural for dark matter to have become asymmetric during high-scale leptogenesis, and that in this case the direct detection bounds can be significantly weakened due to interference. We quantify this by mapping out and comparing the regions of parameter space that are excluded by direct detection for the symmetric and asymmetric cases of lepton-flavored dark matter. In particular, we show that the entire parameter region except for a narrow Higgs resonance window is ruled out in the symmetric case for fermion dark matter when the coupling to the Higgs dominates over the coupling to leptons, while large portions of parameter space are still allowed for the asymmetric case. The same is also true for a dark matter mass above 8 GeV for scalar dark matter when the coupling to leptons dominates over the coupling to the Higgs.

hep-ph↗