arXiv subjects

Raul Jimenez

Publications and source records attributed to Raul Jimenez.

At least 73 records · Page 4Linked to original sources

arXiv2020

The distribution of dark galaxies and spin bias

In the light of the discovery of numerous (almost) dark galaxies from the ALFALAFA and LITTLE THINGS surveys, we revisit the predictions of Jimenez et al. 1997, based on the Toomre stability of rapidly-spinning gas disks. We have updated the predictions for $Λ$CDM with parameters given by Planck18, computing the expected number densities of dark objects, and their spin parameter and mass distributions. Comparing with the data is more challenging, but where the spins are more reliably determined, the spins are close to the threshold for disks to be stable according to the Toomre criterion, where the expected number density is highest, and reinforces the concept that there is a bias in the formation of luminous galaxies based on the spin of their parent halo.

astro-ph.GA↗

arXiv2020

Can Dark Matter be Geometry? A Case Study with Mimetic Dark Matter

We investigate the possibility of dark matter being a pure geometrical effect, rather than a particle or a compact object, by exploring a specific modified gravity model: mimetic dark matter. We present an alternative formulation of the theory, closer to the standard cosmological perturbation theory framework. We make manifest the presence of arbitrary parameters and extra functions, both at background level and at first order in perturbation theory. We present the full set of independent equations of motion for this model, and we discuss the amount of tuning needed to match predictions of the theory to actual data. By using the matter power spectrum and cosmic microwave background angular power spectra as benchmark observables, we explicitly show that since there is no natural mechanism to generate adiabatic initial conditions in this specific model, extra fine-tuning is required. We modify the publicly available Boltzmann code \texttt{CLASS} to make accurate predictions for the observables in mimetic dark matter. Our modified version of \texttt{CLASS} is available on GitHub. We have used mimetic dark matter as an illustration of how much one is allowed to change the initial conditions before contradicting observations when modifying the laws of gravity as described by General Relativity but we point out that modifying gravity without providing a natural mechanism to generate adiabatic initial conditions will always lead to highly fine-tuned models.

astro-ph.CO↗

arXiv2020

How Gaussian can the Sky be? Primordial Non-Gaussianity from Quantum Information

Using the quantum information picture to describe the early universe as a time dependent quantum density matrix, with time playing the role of a stochastic variable, we compute the non-gaussian features in the distribution of primordial fluctuations. We use a quasi de Sitter model to compute the corresponding quantum Fisher information function as the second derivative of the relative entanglement entropy for the density matrix at two different times. We define the curvature fluctuations in terms of the time quantum estimator. Using standard quantum estimation theory we compute the non-gaussian features in the statistical distribution of primordial fluctuations. Our approach is model independent and only relies on the existence of a quasi de Sitter phase. We show that there are primordial non-gaussianities, both in the form of squeezed and equilateral shapes. The squeezed limit gives a value of $f_{\rm NL} \sim n_s-1$. In the equilateral limit we find that $f_{\rm NL} \sim 0.03$. The equilateral non-gaussianity is due to the non-linearity of Einstein's equation. On the other hand, the squeezed one is due to the quantum nature of clock synchronization and thus real and cannot be gauged away as a global curvature. We identify a new effect: {\it clock bias} which is a pure quantum effect and introduces a bias in the spectral tilt and running of the power spectrum of order $\sim 10^{-4}$, which could be potentially measurable and yield precious information on the quantum nature of the early Universe.

astro-ph.CO↗

arXiv2020

Cosmology from Quantum Information

We describe inflation in terms of a time dependent quantum density matrix with time playing the role of a stochastic variable. Using a quasi-de Sitter model we compute the corresponding quantum Fisher information function as the second derivative of the relative entanglement entropy for the density matrix at two different times. Employing standard quantum estimation theory we evaluate the minimal variance of quantum scalar fluctuations that reproduces the power spectrum and the corresponding tilt in the slow roll limit. The Jeffreys prior associated with such Fisher information can be used to define the probabilities on the set of initial conditions defined by the slow roll parameter $ε$ and the initial Shannon information.

hep-th↗

arXiv2019

Measuring the Homogeneity of the Universe Using Polarization Drift

We propose a method to probe the homogeneity of a general universe, without assuming symmetry. We show that isotropy can be tested at remote locations on the past lightcone by comparing the line-of-sight and transverse expansion rates, using the time dependence of the polarization of Cosmic Microwave Background photons that have been inverse-Compton scattered by the hot gas in massive clusters of galaxies. This probes a combination of remote transverse and parallel components of the expansion rate of the metric, and we may use radial baryon acoustic oscillations or cosmic clocks to measure the parallel expansion rate. Thus we can test remote isotropy, which is a key requirement of a homogeneous universe. We provide explicit formulas that connect observables and properties of the metric.

astro-ph.CO↗

arXiv2019

The local and distant Universe: stellar ages and $H_0$

The ages of the oldest stellar objects in our galaxy provide an independent test of the current cosmological model as they give a lower limit to the age of the Universe. Recent accurate parallaxes by the Gaia space mission, accurate measurements of the metallicity of stars, via individual elemental abundances, and advances in the modelling of stellar evolution, provide new, higher-precision age estimates of the oldest stellar populations in the galaxy: globular clusters and very-low-metallicity stars. The constraints on the age of the Universe, $t_U$, so obtained are determined from the local Universe and at late time. It is well known that local and early-Universe determinations of another cosmological parameter closely related to the age of the Universe, the Hubble constant $H_0$, show a $\gtrsim 3 σ$ tension. In the standard cosmological model, $Λ$CDM, $t_U$ and $H_0$ are related by the matter density parameter $Ω_{m,0}$. We propose to combine local $t_U$ constraints with late-time $Ω_{m,0}$ estimates in a $Λ$CDM framework, to obtain a low-redshift $H_0$ determination that does not rely on early Universe physics. A proof-of-principle of this approach with current data gives $H_0=71\pm2.8$ ($H_0= 69.3 \pm 2.7$) km s$^{-1}$ Mpc$^{-1}$ from globular clusters (very-low-metallicity stars) with excellent prospects for improved constraints in the near future.

astro-ph.CO↗

arXiv2018

Setting the Stage for Cosmic Chronometers. I. Assessing the Impact of Young Stellar Populations on Hubble Parameter Measurements

The expansion history of the Universe can be constrained in a cosmology-independent way by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter $H(z)$ as a function of redshift. The most reliable cosmic chronometers known so far are extremely massive and passively evolving galaxies. Age-dating these galaxies is, however, a difficult task, and even a small contribution of an underlying young stellar population could, in principle, affect the age estimate and its cosmological interpretation. We present several spectral indicators to detect, quantify and constrain such contamination in old galaxies, and study how their combination can be used to maximize the purity of cosmic chronometers selection. In particular, we analyze the CaII H/K ratio, the presence (or absence) of H$α$ and [OII] emission lines, higher order Balmer absorption lines, and UV flux; each indicator is especially sensitive to a particular age range, allowing us to detect young components ranging between 10 Myr and 1 Gyr. The combination of these indicators minimizes the contamination to a level below 1% in the case of ideal data. More importantly, it offers a way to control the systematic error on $H(z)$ as a function of the contamination by young stellar populations. We show that for our previous measurements of the Hubble parameter, the possible bias induced by the presence of a younger component is well below the current errors. We envision that these indicators will be instrumental in paving the road for a robust and reliable dating of the old population and its cosmological interpretation.

astro-ph.CO↗

arXiv2018

Measuring the Energy Scale of Inflation with Large Scale Structures

The determination of the inflationary energy scale represents one of the first step towards the understanding of the early Universe physics. The (very mild) non-Gaussian signals that arise from any inflation model carry information about the energy scale of inflation and may leave an imprint in some cosmological observables, for instance on the clustering of high-redshift, rare and massive collapsed structures. In particular, the graviton exchange contribution due to interactions between scalar and tensor fluctuations leaves a specific signature in the four-point function of curvature perturbations, thus on clustering properties of collapsed structures. We compute the contribution of graviton exchange on two- and three-point function of halos, showing that at large scales $k\sim 10^{-3}\ \mathrm{Mpc}^{-1}$ its magnitude is comparable or larger to that of other primordial non-Gaussian signals discussed in the literature. This provides a potential route to probe the existence of tensor fluctuations which is alternative and highly complementary to B-mode polarisation measurements of the cosmic microwave background radiation.

astro-ph.CO↗

arXiv2018

Supergravity, $α$-attractors and primordial non-Gaussianity

We compute in detail how deviations from Einstein gravity at the inflation energy scale could appear as non-Gaussian features in the sky. To illustrate this we use multi-field $α-$attractor models in the framework of supergravity to realise inflation. We find no obvious obstacle for having choices of model's parameters that generate non-Gaussian features of the equilateral and local type at the $\cal O$(1) level in the $f_{\rm NL}$ non-Gaussianity parameter, thus being potentially detectable in future cosmological surveys. This non-Gaussianity has its origin in either the non-canonical kinetic term (which, in turn, is an immediate consequence of assuming an hyperbolic geometry of the moduli space), the interactions of the fields in the potential or the $α-$parameter, or a combination of these three. This opens up the exciting possibility of constraining the law of gravity at energy scales close to the Planck one.

astro-ph.CO↗

arXiv2018

Implications for the missing low-mass galaxies (satellites) problem from cosmic shear

The number of observed dwarf galaxies, with dark matter mass $\lesssim 10^{11}$ M$_{\odot}$ in the Milky Way or the Andromeda galaxy does not agree with predictions from the successful $Λ$CDM paradigm. To alleviate this problem a suppression of dark matter clustering power on very small scales has been conjectured. However, the abundance of dark matter halos outside our immediate neighbourhood (the Local Group) seem to agree with the $Λ$CDM--expected abundance. Here we connect these problems to observations of weak lensing cosmic shear, pointing out that cosmic shear can make significant statements about the missing satellites problem in a statistical way. As an example and pedagogical application we use recent constraints on small-scales power suppression from measurements of the CFHTLenS data. We find that, on average, in a region of $\sim $Gpc$^3$ there is no significant small-scale power suppression. This implies that suppression of small-scale power is not a viable solution to the `missing satellites problem' or, alternatively, that on average in this volume there is no `missing satellites problem' for dark matter masses $\gtrsim 5 \times 10^9$ M$_{\odot}$. Further analysis of current and future weak lensing surveys will probe much smaller scales, $k > 10h$ Mpc$^{-1}$ corresponding roughly to masses $M < 10^9 M_{\odot}$.

astro-ph.CO↗

arXiv2018

Peering beyond the horizon with standard sirens and redshift drift

An interesting test on the nature of the Universe is to measure the global spatial curvature of the metric in a model independent way, at a level of $|Ω_k|<10^{-4}$, or, if possible, at the cosmic variance level of the amplitude of the CMB fluctuations $|Ω_k|\approx10^{-5}$. A limit of $|Ω_k|<10^{-4}$ would yield stringent tests on several models of inflation. Further, improving the constraint by an order of magnitude would help in reducing "model confusion" in standard parameter estimation. Moreover, if the curvature is measured to be at the value of the amplitude of the CMB fluctuations, it would offer a powerful test on the inflationary paradigm and would indicate that our Universe must be significantly larger than the current horizon. On the contrary, in the context of standard inflation, measuring a value above CMB fluctuations will lead us to conclude that the Universe is not much larger than the current observed horizon, this can also be interpreted as the presence of large fluctuations outside the horizon. However, it has proven difficult, so far, to find observables that can achieve such level of accuracy, and, most of all, be model-independent. Here we propose a method that can in principle achieve that, this is done by making minimal assumptions and using distance probes that are cosmology-independent: gravitational waves, redshift drift and cosmic chronometers. We discuss what kind of observations are needed in principle to achieve the desired accuracy.

astro-ph.CO↗

arXiv2017

Election forensic analysis of the Turkish Constitutional Referendum 2017

With a majority of 'Yes' votes in the Constitutional Referendum of 2017, Turkey continues its transition from democracy to autocracy. By the will of the Turkish people, this referendum transferred practically all executive power to president Erdogan. However, the referendum was confronted with a substantial number of allegations of electoral misconducts and irregularities, ranging from state coercion of 'No' supporters to the controversial validity of unstamped ballots. In this note we report the results of an election forensic analysis of the 2017 referendum to clarify to what extent these voting irregularities were present and if they were able to influence the outcome of the referendum. We specifically apply novel statistical forensics tests to further identify the specific nature of electoral malpractices. In particular, we test whether the data contains fingerprints for ballot-stuffing (submission of multiple ballots per person during the vote) and voter rigging (coercion and intimidation of voters). Additionally, we perform tests to identify numerical anomalies in the election results. We find systematic and highly significant support for the presence of both, ballot-stuffing and voter rigging. In 6% of stations we find signs for ballot-stuffing with an error (probability of ballot-stuffing not happening) of 0.15% (3 sigma event). The influence of these vote distortions were large enough to tip the overall balance from 'No' to a majority of 'Yes' votes.

stat.AP↗

arXiv2017

Strong Bayesian Evidence for the Normal Neutrino Hierarchy

The configuration of the three neutrino masses can take two forms, known as the normal and inverted hierarchies. We compute the Bayesian evidence associated with these two hierarchies. Previous studies found a mild preference for the normal hierarchy, and this was driven by the asymmetric manner in which cosmological data has confined the available parameter space. Here we identify the presence of a second asymmetry, which is imposed by data from neutrino oscillations. By combining constraints on the squared-mass splittings with the limit on the sum of neutrino masses of $Σm_ν< 0.13$ eV, and using a minimally informative prior on the masses, we infer odds of 42:1 in favour of the normal hierarchy, which is classified as "strong" in the Jeffreys' scale. We explore how these odds may evolve in light of higher precision cosmological data, and discuss the implications of this finding with regards to the nature of neutrinos. Finally the individual masses are inferred to be $m_1 = 3.80^{+26.2}_{-3.73} \, \text{meV}, m_2 = 8.8^{+18}_{-1.2} \, \text{meV}, m_3 = 50.4^{+5.8}_{-1.2} \, \text{meV}$ ($95\%$ credible intervals).

astro-ph.CO↗

arXiv2017

The Limits of Cosmic Shear

In this paper we discuss the commonly-used limiting cases, or approximations, for two-point cosmic shear statistics. We discuss the most prominent assumptions in this statistic: the flat-sky (small angle limit), the Limber (Bessel-to-delta function limit) and the Hankel transform (large l-mode limit) approximations; that the vast majority of cosmic shear results to date have used simultaneously. We find that the combined effect of these approximations can suppress power by >1% on scales of l<40. A fully non-approximated cosmic shear study should use a spherical-sky, non-Limber-approximated power spectrum analysis; and a transform involving Wigner small-d matrices in place of the Hankel transform. These effects, unaccounted for, would constitute at least 11% of the total budget for systematic effects for a power spectrum analysis of a Euclid-like experiment; but they are unnecessary.

astro-ph.CO↗

arXiv2017

Hiding neutrino mass in modified gravity cosmologies

Cosmological observables show a dependence with the neutrino mass, which is partially degenerate with parameters of extended models of gravity. We study and explore this degeneracy in Horndeski generalized scalar-tensor theories of gravity. Using forecasted cosmic microwave background and galaxy power spectrum datasets, we find that a single parameter in the linear regime of the effective theory dominates the correlation with the total neutrino mass. For any given mass, a particular value of this parameter approximately cancels the power suppression due to the neutrino mass at a given redshift. The extent of the cancellation of this degeneracy depends on the cosmological large-scale structure data used at different redshifts. We constrain the parameters and functions of the effective gravity theory and determine the influence of gravity on the determination of the neutrino mass from present and future surveys.

astro-ph.CO↗

arXiv2016

Relation between halo spin and cosmic web filaments at z=3

We investigate the spin evolution of dark matter haloes and their dependence on the number of connected filaments from the cosmic web at high redshift (spin-filament relation hereafter). To this purpose, we have simulated $5000$ haloes in the mass range $5\times10^{9}h^{-1}M_{\odot}$ to $5\times10^{11}h^{-1}M_{\odot}$ at $z=3$ in cosmological N-body simulations. We confirm the relation found by Prieto et al. 2015 where haloes with fewer filaments have larger spin. We also found that this relation is more significant for higher halo masses, and for haloes with a passive (no major mergers) assembly history. Another finding is that haloes with larger spin or with fewer filaments have their filaments more perpendicularly aligned with the spin vector. Our results point to a picture in which the initial spin of haloes is well described by tidal torque theory and then gets subsequently modified in a predictable way because of the topology of the cosmic web, which in turn is given by the currently favoured LCDM model. Our spin-filament relation is a prediction from LCDM that could be tested with observations.

astro-ph.CO↗

arXiv2016

Early Cosmology Constrained

We investigate our knowledge of early universe cosmology by exploring how much additional energy density can be placed in different components beyond those in the $Λ$CDM model. To do this we use a method to separate early- and late-universe information enclosed in observational data, thus markedly reducing the model-dependency of the conclusions. We find that the 95\% credibility regions for extra energy components of the early universe at recombination are: non-accelerating additional fluid density parameter $Ω_{\rm MR} < 0.006$ and extra radiation parameterised as extra effective neutrino species $2.3 < N_{\rm eff} < 3.2$ when imposing flatness. Our constraints thus show that even when analyzing the data in this largely model-independent way, the possibility of hiding extra energy components beyond $Λ$CDM in the early universe is seriously constrained by current observations. We also find that the standard ruler, the sound horizon at radiation drag, can be well determined in a way that does not depend on late-time Universe assumptions, but depends strongly on early-time physics and in particular on additional components that behave like radiation. We find that the standard ruler length determined in this way is $r_{\rm s} = 147.4 \pm 0.7$ Mpc if the radiation and neutrino components are standard, but the uncertainty increases by an order of magnitude when non-standard dark radiation components are allowed, to $r_{\rm s} = 150 \pm 5$ Mpc.

astro-ph.CO↗

arXiv2016

The length of the low-redshift standard ruler

Assuming the existence of standard rulers, standard candles and standard clocks, requiring only the cosmological principle, a metric theory of gravity, a smooth expansion history, and using state-of-the-art observations, we determine the length of the "low-redshift standard ruler". The data we use are a compilation of recent Baryon acoustic oscillation data (relying on the standard ruler), Type 1A supernovæ (as standard candles), ages of early type galaxies (as standard clocks) and local determinations of the Hubble constant (as a local anchor of the cosmic distance scale). In a standard $Λ$CDM cosmology the "low-redshift standard ruler" coincides with the sound horizon at radiation drag, which can also be determined --in a model dependent way-- from CMB observations. However, in general, the two quantities need not coincide. We obtain constraints on the length of the low-redshift standard ruler: $r^h_{\rm s}=101.0 \pm 2.3 h^{-1}$ Mpc, when using only Type 1A supernovæ and Baryon acoustic oscillations, and $r_{\rm s}=150.0\pm 4.7 $ Mpc when using clocks to set the Hubble normalisation, while $r_{\rm s}=141.0\pm 5.5 $ Mpc when using the local Hubble constant determination (using both yields $r_{\rm s}=143.9\pm 3.1 $ Mpc). The low-redshift determination of the standard ruler has an error which is competitive with the model-dependent determination from cosmic microwave background measurements made with the {\em Planck} satellite, which assumes it is the sound horizon at the end of baryon drag.

astro-ph.CO↗