Cosmological Field Research Articles

Early and late time cosmology: the f(R) gravity perspective

Discrepancies between observations at early and late cosmic epochs, and the vacuum energy problem associated with the interpretation of cosmological constant, are questioning the $\Lambda$CDM model. Motivated by these conceptual and observational facts, extensions of Einstein's gravity are recently intensively considered in view of curing unsolved issues suffered by General Relativity at ultraviolet and infrared scales. Here, we provide a short overview of some aspects of $f(R)$ gravity, focusing, in particular, on cosmological applications. Specifically, Noether symmetries are adopted as a criterion to select viable models and investigate the corresponding dynamics. We thus find solutions to the cosmological field equations, analyzing the behaviour of selected models from the matter-dominated to the present epoch. Moreover, constraints coming from energy conditions and the so-called swampland criteria are also considered. In particular, we qualitatively discuss the possibility of $f(R)$ gravity to account for fixing cosmic tensions.

Prompt cusp formation from the gravitational collapse of peaks in the initial cosmological density field

ABSTRACT I present an analytic model for the early post-collapse evolution of a spherical density peak on the coherence scale of the initial fluctuations in a universe filled with collisionless and pressure-free ‘dust’. On a time-scale which is short compared to the peak’s collapse time t0, its inner regions settle into an equilibrium cusp with a power-law density profile, ρ ∝ r−12/7. Within this cusp, the circular orbit period P at each radius is related to the enclosed mass M by P = t0(M/Mc)2/3 where Mc is a suitably defined characteristic mass for the initial peak. The relaxation mechanism which produces this cusp gives insight into those which are active in high-resolution simulations of first halo formation in cold or warm dark matter universes, and, indeed, a simple argument suggests that the same power-law index γ = −12/7 should describe the prompt cusps formed during the collapse of generic peaks, independent of any symmetry assumption. Further work is needed to investigate the additional factors required to explain the slightly flatter exponent, γ ≈ −1.5, found in high-resolution numerical simulations of peak collapse.

Adiabatic regularization and preferred vacuum state for the λϕ4 field theory in cosmological spacetimes

We extend the method of adiabatic regularization by introducing an arbitrary parameter $\mu$ for a scalar field with quartic self-coupling in a Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetime at one-loop order. The subtraction terms constructed from this extended version allow us to define a preferred vacuum state at a fixed time $\eta = \eta_0$ for this theory. We compute this vacuum state for two commonly used background fields in cosmology. We also give a possible prescription for an adequate value for $\mu$.

Relativistic angular redshift fluctuations embedded in large scale varying gravitational potentials

We compute the linear order, general relativistic corrections to angular redshift fluctuations (ARF), a new cosmological observable built upon density-weighted two-dimensional (2D) maps of galaxy redshifts. We start with an existing approach for galaxy/source counts developed in the Newtonian gauge, and generalize it to ARF, modifying for this purpose a standard Boltzmann code. Our calculations allow us identifying the velocity terms as the leading corrections on large scales, emphasizing the sensitivity of ARF to peculiar, cosmological velocity fields. Just like for standard 2D clustering, the impact of gravitational lensing on ARF is dominant on small angular scales and for wide redshift shells, while the signatures associated to gravitational potentials are extremely small and hardly detectable. The ARF also present interesting correlation properties to anisotropies of the Cosmic Microwave Background (CMB): they are highly correlated to CMB lensing potential fluctuations, while also exhibiting a significant (S/N∼ 4–5) anti-correlation with the Integrated Sachs-Wolfe effect (ISW). This negative ARF×ISW signal is quite complementary to the standard 2D clustering×ISW correlation, since the former appears mostly at higher redshift (z ∼ 2) than the latter (z ≲ 1), and the combination of the two observables significantly increases the χ 2 statistics testing the null (no ISW) hypothesis. We conclude that ARF constitute a novel, alternative, and potentially powerful tool to constrain the nature of Dark Energy component that gives rise to the ISW.

Complex scalar fields in scalar-tensor and scalar-torsion theories

In this paper, we investigate the cosmological dynamics in a spatially flat Friedmann–Lemaître–Robertson–Walker geometry in scalar-tensor and scalar-torsion theories where the nonminimally coupled scalar field is a complex field. We derive the cosmological field equations and we make use of dimensionless variables in order to determine the stationary points and determine their stability properties. The physical properties of the stationary points are discussed while we find that the two-different theories, scalar-tensor and scalar-torsion theories, share many common features in terms of the evolution of the physical variables in the background space.

Translation and rotation equivariant normalizing flow (TRENF) for optimal cosmological analysis

ABSTRACT Our Universe is homogeneous and isotropic, and its perturbations obey translation and rotation symmetry. In this work, we develop translation and rotation equivariant normalizing flow (TRENF), a generative normalizing flow (NF) model which explicitly incorporates these symmetries, defining the data likelihood via a sequence of Fourier space-based convolutions and pixel-wise non-linear transforms. TRENF gives direct access to the high dimensional data likelihood p(x|y) as a function of the labels y, such as cosmological parameters. In contrast to traditional analyses based on summary statistics, the NF approach has no loss of information since it preserves the full dimensionality of the data. On Gaussian random fields, the TRENF likelihood agrees well with the analytical expression and saturates the Fisher information content in the labels y. On non-linear cosmological overdensity fields from N-body simulations, TRENF leads to significant improvements in constraining power over the standard power spectrum summary statistic. TRENF is also a generative model of the data, and we show that TRENF samples agree well with the N-body simulations it trained on, and that the inverse mapping of the data agrees well with a Gaussian white noise both visually and on various summary statistics: when this is perfectly achieved the resulting p(x|y) likelihood analysis becomes optimal. Finally, we develop a generalization of this model that can handle effects that break the symmetry of the data, such as the survey mask, which enables likelihood analysis on data without periodic boundaries.

Rydberg Microwave-Frequency-Comb Spectrometer

The developing frequency-comb spectral technologies have potential applications and prospects in the wide fields of cosmology, meteorology, and microwave measurement. Here, we demonstrate a Rydberg microwave-frequency-comb spectrometer via multiple-microwave field dressing, providing precise microwave measurement. The Rydberg microwave-frequency-comb spectrum provides a real-time and absolute-frequency measurement with a range of 125 MHz and gives the relative phase of a single-frequency microwave signal. The reported experiment helps real-time sensing of an unknown microwave signal over a wide range using Rydberg atoms, which are vital for Rydberg-atom-based microwave metrology.

Variations on Violence in Greek and Akkadian Succession Myths

Abstract In this article I explore a selection of texts from Greece and Mesopotamia that either recount or comment on the succession myth. I argue that representations of violence in those texts differ considerably within the same culture and time period. I explain these variations as social deixis, positing that ancient authors and interpreters of the succession myth used different representations of violence to present themselves as innovative figures. I argue that both mythmakers and myth-interpreters increased and decreased the intensity and number of violent features to mark a position in the competitive field of cosmological knowledge. Through the comparison of the sources, I show that there was as much competition and innovation in Greece as in Mesopotamia within the field of cosmology. The similarity of social contexts and practices may explain the cross-cultural transfer of knowledge between specialists of these two regions.

A possible subclassification of fast radio bursts

Although fast radio bursts (FRBs) have been an active field in astronomy and cosmology, their origin is still unknown to date. One of the interesting topics is the classification of FRBs, which is closely related to the origin of FRBs. Different physical mechanisms are required by different classes of FRBs. In the literature, they usually could be classified into non-repeating and repeating FRBs. Well motivated by the observations, here we are interested in the possible subclassification of FRBs. By using the first CHIME/FRB catalog, we propose to subclassify non-repeating (type I) FRBs into type Ia and Ib FRBs. The distribution of type Ia FRBs is delayed with respect to the cosmic star formation history (SFH), and hence they are probably associated with old stellar populations, while the distribution of type Ib FRBs tracks SFH, and hence they are probably associated with young stellar populations. Accordingly, the physical criteria for this subclassification of type I FRBs have been clearly determined. We find that there are some tight empirical correlations for type Ia FRBs but not for type Ib FRBs, and vice versa. These make them different in physical properties. Similarly, we suggest that repeating (type II) FRBs could also be subclassified into type IIa and IIb FRBs. A universal subclassification scheme is given at the end. This subclassification of FRBs might help us to reveal quite different physical mechanisms behind them, and improve their applications in astronomy and cosmology.

Painlevé Analysis of the Cosmological Field Equations in Weyl Integrable Spacetime

We apply a singularity analysis to investigate the integrability properties of the gravitational field equations in Weyl Integrable Spacetime for a spatially flat Friedmann–Lemaître–Robertson–Walker background spacetime induced by an ideal gas. We find that the field equations possess the Painlevé property in the presence of the cosmological constant, and the analytic solution is given by a left Laurent expansion.

Numerical investigation of non-Gaussianities in the phase and modulus of density Fourier modes

ABSTRACT We numerically investigate the non-Gaussianities in the late-time cosmological density field in Fourier space. We explore various statistics, including the two- and three-point probability distribution function (PDF) of phase and modulus, and their two- and three-point correlation function. Significant non-Gaussianity is observed for certain configurations. Comparing the measurement from simulation with the theoretical expansion prediction, we find that for (600 Mpc h-1)3 volume, the $\mathcal {O}(V^{-1/2})$ order term alone is sufficiently accurate to describe all the measured non-Gaussianities in not only the PDFs, but also the correlations. We also numerically find that the phase-modulus cross-correlation contributes $\sim 50{{\ \rm per\ cent}}$ to the bispectrum, further verifying the accuracy of the $\mathcal {O}(V^{-1/2})$ order prediction. This work demonstrates that the non-Gaussianity of cosmic density field is simpler in Fourier space, and may facilitate the data analysis in the era of precision cosmology.

Towards an optimal estimation of cosmological parameters with the wavelet scattering transform

Optimal extraction of the non-Gaussian information encoded in the Large-Scale Structure (LSS) of the universe lies at the forefront of modern precision cosmology. We propose achieving this task through the use of the Wavelet Scattering Transform (WST), which subjects an input field to a layer of non-linear transformations that are sensitive to non-Gaussianity in spatial density distributions through a generated set of WST coefficients. In order to assess its applicability in the context of LSS surveys, we apply the WST on the 3D overdensity field obtained by the Quijote simulations, out of which we extract the Fisher information in 6 cosmological parameters. It is subsequently found to deliver a large improvement in the marginalized errors on all parameters, ranging between $1.2-4\times$ tighter than the corresponding ones obtained from the regular 3D cold dark matter + baryon power spectrum, as well as a $50 \%$ improvement over the neutrino mass constraint given by the marked power spectrum. Through this first application on 3D cosmological fields, we demonstrate the great promise held by this novel statistic and set the stage for its future application to actual galaxy observations.

Sky coverage assessment for the European ELT: a joint evaluation for MAORY/MICADO and HARMONI

The instruments developed for the upcoming Extremely Large Telescopes (ELTs) will need efficient adaptive optics (AO) systems to correct the effects of the atmospheric turbulence and allow imaging at the highest angular resolution. One of the most important requirements for ELT AO-assisted instruments will be to deliver diffraction-limited images in a significant part of the sky. For that, the instruments will be equipped with laser guide stars (LGSs) providing most of the information required by AO instruments. But even with LGSs, AO systems still require the use of natural guide stars (NGSs) to compensate for image motion (jitter) and some low order aberrations. These NGSs are eventually limiting the fraction of the sky that can be achieved by AO systems, the so-called sky coverage (SC). We first present the SC assessment methods used for high angular resolution monolithic optical and near-infrared integral field spectrograph (HARMONI) and multiconjugate adaptive optics relay/multi-AO imaging camera for deep observations (MAORY/MICADO), that are both instruments for the ELT of the European Southern Observatory (ESO). They are based on a semianalytical description of the main contributors in the AO error budget, allowing for a fast estimation of the residual jitter. As such, these methods are well suited for statistical estimation of the SC on multiple science fields and/or to efficiently explore the system parameter space. We then compute the SC of the two instruments in cosmological fields from the cosmic assembly near-IR deep extragalactic legacy survey catalog. The goal is to provide an insight on the possibilities given by two different types of tomographic AO systems, i.e., laser tomography AO with HARMONI and multiconjugate AO with MAORY, on the same telescope. In particular, we show that HARMONI and MAORY/MICADO are complementary, meaning that the overall SC of ESO’s ELT is much improved for applications common to both systems.

The large-scale 21-cm power spectrum from reionization

ABSTRACT Radio interferometers, such as the Low-Frequency Array and the future Square Kilometre Array, are attempting to measure the spherically averaged 21-cm power spectrum from the epoch of reionization. Understanding of the dominant physical processes which influence the power spectrum at each length-scale is therefore crucial for interpreting any future detection. We study a decomposition of the 21-cm power spectrum and quantify the evolution of its constituent terms for a set of numerical and semi-numerical simulations of a volume of (714 Mpc)3, focusing on large scales with k ≲ 0.3 Mpc−1. We find that after ∼10 per cent of the universe has been ionized, the 21-cm power spectrum follows the power spectrum of neutral hydrogen fluctuations, which itself beyond a certain scale follows the matter power spectrum. Hence the signal has a two-regime form where the large-scale signal is a biased version of the cosmological density field, and the small-scale power spectrum is determined by the astrophysics of reionization. We construct a bias parameter to investigate the relation between the large-scale 21-cm signal and the cosmological density field. We find that the transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionizing photons (λMFP), and is characterised by the empirical formula ktrans ≈ 2/λMFP. Furthermore, we show that the numerical implementation of the mean free path effect has a significant impact on the shape of this transition. Most notably, the transition is more gradual if the mean free path effect is implemented as an absorption process rather than as a barrier.

Exact general solutions for cosmological scalar field evolution in a background-dominated expansion

We derive exact general solutions (as opposed to attractor particular solutions) and corresponding first integrals for the evolution of a scalar field $\phi$ in a universe dominated by a background fluid with equation of state parameter $w_B$. In addition to the previously-examined linear [$V(\phi) = V_0 \phi$] and quadratic [$V(\phi) = V_0 \phi^2$] potentials, we show that exact solutions exist for the power law potential $V(\phi) = V_0 \phi^n$ with $n = 4(1+w_B)/(1-w_B) + 2$ and $n = 2(1+w_B)/(1-w_B)$. These correspond to the potentials $V(\phi) = V_0 \phi^6$ and $V(\phi) = V_0 \phi^2$ for matter domination and $V(\phi) = V_0 \phi^{10}$ and $V(\phi) = V_0 \phi^4$ for radiation domination. The $\phi^6$ and $\phi^{10}$ potentials can yield either oscillatory or non-oscillatory evolution, and we use the first integrals to determine how the initial conditions map onto each form of evolution. The exponential potential yields an exact solution for a stiff/kination ($w_B = 1$) background. We use this exact solution to derive an analytic expression for the evolution of the equation of state parameter, $w_\phi$, for this case.

Stellar populations of a sample of far-infrared AGN and non-AGN green valley galaxies

ABSTRACT We present a study on the stellar populations and stellar ages of a subsample of far-infrared (FIR) active galactic nucleus (AGN) and non-AGN green valley galaxies at ${0.6\, \lt \, z\, \lt \, 1.0}$ using the data from the Cosmological Evolution Survey field. We used long-slit spectroscopy and derived stellar populations and stellar ages using the stellar population synthesis code ‘starlight’ and analysed the available Lick/IDS indices, such as Dn4000 and HδA. We find that both FIR AGN and non-AGN green valley galaxies are dominated by intermediate stellar populations (67 per cent and 53 per cent, respectively). The median stellar ages for AGNs and non-AGNs are ${\log t\, =\, 8.5}$ and ${\, 8.4\, \rm yr}$, respectively. We found that majority of our sources (62 per cent of AGNs and 66 per cent of non-AGNs) could have experienced bursts and continuous star formation (SF). In addition, most of our FIR AGNs (38 per cent) compared to FIR non-AGNs (27 per cent) might have experienced a burst of SF more than 0.1 Gyr ago. We also found that our FIR AGN and non-AGN green valley galaxies have similar quenching time-scales of ∼ 70 Myr. Therefore, the results obtained here are in line with our previous results where we do not find that our sample of FIR AGNs in the green valley shows signs of negative AGN feedback, as has been suggested previously in optical studies.

Constraints on prerecombination early dark energy from SPT-3G public data

Early dark energy (EDE) is a proposed solution to the Hubble tension in which a new cosmological field accelerates cosmic expansion prior to recombination and reduces the physical size of the sound horizon. In previous work, a slight preference for a nonzero EDE contribution was found in the latest Atacama Cosmology Telescope data (ACT DR4), while the Planck satellite legacy data alone do not show evidence for it. In this work, we use the most recent public data from the South Pole Telescope (SPT-3G) to constrain the parameters of the EDE scenario. We find that at the current precision level of SPT-3G, an EDE contribution to the total energy density of the universe prior to recombination of $\ensuremath{\sim}10%$ cannot be ruled out, but that the data are also consistent with no EDE. The combination of ACT DR4 and SPT-3G with the Planck large-scale temperature anisotropy measurement shows a hint ($2.6\ensuremath{\sigma}$) for nonzero EDE; however, this preference disappears when the full Planck 2018 dataset is included.

Measuring the cosmological density field twice: A novel test of dark energy using the CMB quadrupole

The scattering of cosmic microwave background (CMB) radiation in galaxy clusters induces polarization signals according to the quadrupole anisotropy in the photon distribution at the cluster location. This `remote quadrupole' derived from the measurements of the induced polarization provides an opportunity for reconstructing primordial fluctuations on large scales. We discuss that comparing the local CMB quadrupoles predicted by these reconstructed primordial fluctuations and the direct measurements done by CMB satellites may enable us to test the dark energy beyond cosmic variance limits.

Numerical Relativity as a New Tool for Fundamental Cosmology

Advances in our understanding of the origin, evolution, and structure of the universe have long been driven by cosmological perturbation theory, model building, and effective field theory. In this review, numerical relativity is introduced as a powerful new complementary tool for fundamental cosmology. To illustrate its power, applications of numerical relativity are discussed to studying the robustness of slow contraction and inflation in homogenizing, isotropizing, and flattening the universe beginning from generic unsmooth initial conditions. In particular, it is described how recent numerical relativity studies of slow contraction have revealed a novel, non-linear smoothing mechanism based on ultralocality that challenges the conventional view on what is required to explain the large-scale homogeneity and isotropy of the observable universe.

Accurate Determination of Hubble Attenuation and Amplification in Expanding and Contracting Cold-Atom Universes.

In the expanding universe, relativistic scalar fields are thought to be attenuated by "Hubble friction," which results from the dilation of the underlying spacetime metric. By contrast, in a contracting universe this pseudofriction would lead to amplification. Here, we experimentally measure, with fivefold better accuracy, both Hubble attenuation and amplification in expanding and contracting toroidally shaped Bose-Einstein condensates, in which phonons are analogous to cosmological scalar fields. We find that the observed attenuation or amplification depends on the temporal phase of the phonon field, which is only possible for nonadiabatic dynamics. The measured strength of the Hubble friction disagrees with recent theory [Gomez Llorente etal., Phys. Rev. A 100, 043613 (2019)PLRAAN2469-992610.1103/PhysRevA.100.043613 and Eckel etal., SciPost Phys. 10, 64 (2021)SPCHCW2542-465310.21468/SciPostPhys.10.3.064].