Astrophysics

We extend the work of Contaldi et al. and derive analytical approximations for primordial power spectra arising from models of inflation which include primordial spatial curvature. These analytical templates are independent of any specific inflationary potential and therefore illustrate and provide insight into the generic effects and predictions of primordial curvature, manifesting as cut-offs and oscillations at low multipoles and agreeing with numerical calculations. We identify through our analytical approximation that the effects of curvature can be mathematically attributed to shifts in the wavevectors participating dynamically.

In the separate universe approach, an inhomogeneous universe is rephrased as a set of glued numerous homogeneous local patches. This is the essence of the gradient expansion and the δN formalism, which have been widely used in solving a long wavelength evolution of the universe. In this paper, we show that the separate universe approach can be generically used, as long as a theory under consideration is local and preserves the spatial diffeomorphism invariance. Focusing on these two conditions, we also clarify the condition for the existence of the so-called Weinberg's adiabatic mode. Remarkably, the separate universe approach and the δN formalism turn out to be applicable also to models with shear on large scales and also to modified theories of gravity, accepting violation of four-dimensional diffeomorphism invariance. The generalized δN formalism enables us to calculate all the large scale fluctuations, including gravitational waves. We also argue several implications on anisotropic inflation and ultra slow-roll inflation.

We propose a novel technique to analyse spacings of the observed CMB temperature angular power spectrum (APS) motivated by the behaviour of level spacings in the presence or absence of correlations between eigenvalues of a random matrix. We use data from WMAP 9 year ILC and 2018 Planck maps (Commander, NILC and SMICA) and estimate minimum, maximum, average spacings and ratios of maximum to minimum spacings between consecutive multipoles of APS measures ( C ??and ????1) 2? C ??). Anomalous multipole ranges given by ??max and ??min exist for multipoles ?�[2,31] of this work. Sans parity distinctions, average spacing of 8?��???1 for C ??'s, and maximum and average spacings of ??min ?�[8,12] of ????1) 2? C ??'s are anomalously low for all maps, with most anomalously low maximum spacing at 99.93% C.L. for ????1) 2? C ?? (Planck NILC) . With parity distinctions, even multipoles indicate anomalously low maximum and average spacings relative to odd multipoles, most consistently for ??max ?�[6,30] of C ??'s of all maps, the most outstanding being the even multipole average spacing of 2?��???7 for ????1) 2? C ?? (WMAP) , at 99.60% C.L. For spacing ratios, multipole ranges are similar to those for anomalous spacings, and odd multipole spacing ratios are mostly anomalous, with odd multipole spacing ratio of 2?��??? being consistently low for all maps. Overall, we infer an unusual deficit of correlations between APS measures and deviations from isotropy due to consistently low, parity-biased anomalous spacings, which can be argued to have arisen primordially. The physical origin of these findings may be probed in future work.

Intensity mapping of the H I 21 cm line and the CO 2.61 mm line from the epoch of reionization has emerged as powerful, complementary, probes of the high-redshift Universe. However, both maps and their cross-correlation are dominated by foregrounds. We propose a new analysis by which the signal is unbiased by foregrounds, i.e. it can be measured without foreground mitigation. We construct the antisymmetric part of two-point cross-correlation between intensity maps of the H I 21 cm line and the CO 2.61 mm line, arising because the statistical fluctuations of two fields have different evolution in time. We show that the sign of this new signal can distinguish model-independently whether inside-out reionization happens during some interval of time. More importantly, within the framework of the excursion set model of reionization, we demonstrate that the slope of the dipole of H I-CO cross-power spectrum at large scales is linear to the rate of change of global neutral fraction of hydrogen in a manner independent of reionization parameters, until the slope levels out near the end of reionization, but this trend might possibly depend on the framework of reionization modelling. The H I-CO dipole may be a smoking-gun probe for the speed of reionization, or "standard speedometer" for cosmic reionization. Observations of this new signal will unveil the global reionization history from the midpoint to near the completion of reionization.

Genetic Algorithm (GA) -- motivated by the natural evolutionary process -- is a robust method to estimate the optimal solutions of problems involving one or more objective functions. In this article, for the first time, we apply GA to reconstruct the cleaned CMB temperature anisotropy map over large angular scales of the sky using (internal) linear combination (ILC) of observations from the final-year WMAP and Planck satellite missions. To avoid getting trapped into a local minimum, we implement the GA with generous diversity in the populations. This is achieved by introducing a small but significant amount of mutation of genes during crossover and selecting pairs with diverse fitness coefficients. We find that the new GA-ILC method produces a cleaned CMB map which agrees very well with the CMB map obtained using the exact and analytical expression of weights in the ILC method. By performing extensive Monte Carlo simulations of the CMB reconstruction using the GA-ILC algorithm we find that residual foregrounds in the cleaned map are minimal and mostly tend to occupy localized regions along the central galactic plane. The CMB angular power spectrum shows no indication of any bias in the entire multipole range 2?��???2 studied in this work. The error in the CMB angular power spectrum is minimal as well and given entirely by the cosmic variance induced error. Our results agree well with those obtained by various other reconstruction methods by different research groups. This problem-independent robust GA-ILC method provides a flexible way towards the complex and challenging task of CMB component reconstruction in cosmology.

We study the approach to scaling in axion string networks in the radiation era, through measuring the root-mean-square velocity v as well as the scaled mean string separation x . We find good evidence for a fixed point in the phase-space analysis in the variables (x,v) , providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. The string length has also been commonly expressed in terms of a dimensionless string length density ζ , proportional to the number of Hubble lengths of string per Hubble volume. In simulations with initial conditions far from the fixed point ζ is still evolving after half a light-crossing time, which has been interpreted in the literature as a long-term logarithmic growth. We show that all our simulations, even those starting far from the fixed point, are accounted for by a VOS model with an asymptote of ζ ??=1.20±0.09 (calculated from the string length in the cosmic rest frame) and v ??=0.609±0.014 .

Exploring the possibility that fundamental constants of Nature might vary temporally or spatially constitutes one of the key science drivers for the European Southern Observatory's ESPRESSO spectrograph on the VLT and for the HIRES spectrograph on the ELT. High-resolution spectra of quasar absorption systems permit accurate measurements of fundamental constants out to high redshifts. The quality of new data demands completely objective and reproducible methods. We have developed a new fully automated Artificial Intelligence-based method capable of deriving optimal models of even the most complex absorption systems known. The AI structure is built around VPFIT, a well-developed and extensively-tested non-linear least-squares code. The new method forms a sophisticated parallelised system, eliminating human decision-making and hence bias. Here we describe the workings of such a system and apply it to synthetic spectra, in doing so establishing methods of importance for future analyses of VLT and ELT data. The results show that modelling line broadening for high-redshift absorption components should include both thermal and turbulent components. Failing to do so means it is easy to derive the wrong model and hence incorrect parameter estimates. We also argue that model non-uniqueness can be significant, such that it is not feasible to expect to derive an unambiguous estimate of the fine structure constant alpha from one or a small number of measurements. No matter how optimal the modelling method, it is a fundamental requirement to use a large sample of measurements to meaningfully constrain temporal or spatial alpha variation.

The geometry of the Universe may be probed using the Alcock-Paczynski (AP) effect, in which the observed redshift size of a spherical distribution of sources relative to its angular size varies according to the assumed cosmological model. Past applications of this effect have been limited, however, by a paucity of suitable sources and mitigating astrophysical factors, such as internal redshift-space distortions and poorly known source evolution. In this Letter, we introduce a new test based on the AP effect that avoids the use of spatially bound systems, relying instead on sub-samples of quasars at redshifts z < 1.5 in the Sloan Digital Sky Survey IV, with a possible extension to higher redshifts and improved precision when this catalog is expanded by upcoming surveys. We here use this method to probe the redshift-dependent expansion rate in three pertinent Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmologies: LCDM, which predicts a transition from deceleration to acceleration at z ~ 0.7; Einstein-de Sitter, in which the Universe is always decelerating; and the R_h=ct universe, which expands at a constant rate. LCDM is consistent with these data, but R_h=ct is favoured overall.

The N-body problem has become one of the hottest topics in the fields of computational dynamics and cosmology. The large dynamical range in some astrophysical problems led to the use of adaptive time steps to integrate particle trajectories, however, the search of optimal strategies is still challenging. We quantify the performance of the hierarchical time step integrator Hamiltonian Splitting (HamSp) for collisionless multistep simulations. We compare with the constant step Leap-Frog (LeapF) integrator and the adaptive one (AKDK). Additionally, we explore the impact of different time step assigning functions. There is a computational overhead in HamSp however there are two interesting advantages: choosing a convenient time-step function may compensate and even turn around the efficiency compared with AKDK. We test both reversibility and time symmetry. The symmetrized nature of the HamSp integration is able to provide time-reversible integration for medium time scales and overall deliver better energy conservation for long integration times, and the linear and angular momentum are preserved at machine precision. We address the impact of using different integrators in astrophysical systems. We found that in most situations both AKDK and HamSp are able to correctly simulate the problems. We conclude that HamSp is an attractive and competitive alternative to AKDK, with, in some cases, faster and with better energy and momentum conservation. The use of recently discussed Bridge splitting techniques with HamSp may allow to reach considerably high efficiency.

A promising method for measuring the cosmological parameter combination fsigma_8 is to compare observed peculiar velocities with peculiar velocities predicted from a galaxy density field using perturbation theory. We use N-body simulations and semi-analytic galaxy formation models to quantify the accuracy and precision of this method. Specifically, we examine a number of technical aspects, including the optimal smoothing length applied to the density field, the use of dark matter halos or galaxies as tracers of the density field, the effect of noise in the halo mass estimates or in the stellar-to-halo mass relation, and the effect of finite survey volumes. We find that for a Gaussian smoothing of 4 Mpc/h, the method has only small systematic biases at the level of 5%. Cosmic variance affects current measurements at the 5% level due to the volume of current redshift data sets.