arXiv subjects

Raul Jimenez

Publications and source records attributed to Raul Jimenez.

At least 91 records · Page 5Linked to original sources

arXiv2016

Dark energy from the motions of neutrinos

We demonstrate that a scalar field is unable to reverse its direction of motion while continuously exchanging energy with another fluid. If the rate of transfer is modulated by the scalar's acceleration, the field can undergo a rapid process of freezing, despite being displaced from the local minimum of its potential. This enables dark energy to form from any potential, regardless of its shape. The field's equation of state mimicks that of a cosmological constant. We present a physically motivated realisation in the form of a derivative neutrino-majoron coupling. Coherent motions, which form only once the neutrinos become non-relativistic, could be responsible for instigating the freezing process. This would provide a natural resolution to the dark energy coincidence problem, while avoiding the dynamical instabilities associated with mass-varying neutrino models. Finally we discuss possible means by which this model could be experimentally verified.

astro-ph.CO↗

arXiv2016

Constraints on deviations from $Λ$CDM within Horndeski gravity

Recent anomalies found in cosmological datasets such as the low multipoles of the Cosmic Microwave Background or the low redshift amplitude and growth of clustering measured by e.g., abundance of galaxy clusters and redshift space distortions in galaxy surveys, have motivated explorations of models beyond standard $Λ$CDM. Of particular interest are models where general relativity (GR) is modified on large cosmological scales. Here we consider deviations from $Λ$CDM+GR within the context of Horndeski gravity, which is the most general theory of gravity with second derivatives in the equations of motion. We adopt a parametrization in which the four additional Horndeski functions of time $α_i(t)$ are proportional to the cosmological density of dark energy $Ω_{DE}(t)$. Constraints on this extended parameter space using a suite of state-of-the art cosmological observations are presented for the first time. Although the theory is able to accommodate the low multipoles of the Cosmic Microwave Background and the low amplitude of fluctuations from redshift space distortions, we find no significant tension with $Λ$CDM+GR when performing a global fit to recent cosmological data and thus there is no evidence against $Λ$CDM+GR from an analysis of the value of the Bayesian evidence ratio of the modified gravity models with respect to $Λ$CDM, despite introducing extra parameters. The posterior distribution of these extra parameters that we derive return strong constraints on any possible deviations from $Λ$CDM+GR in the context of Horndeski gravity. We illustrate how our results can be applied to a more general frameworks of modified gravity models.

astro-ph.CO↗

arXiv2016

A 6% measurement of the Hubble parameter at $z\sim0.45$: direct evidence of the epoch of cosmic re-acceleration

Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae and Baryon Acoustic Oscillations, providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 to provide new constraints on the Hubble parameter $H(z)$ using the em cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent $H(z)$ measurements in the redshift range $0.3<z<0.5$, with an accuracy of $\sim$11-16\% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6\% accuracy constraint of $H(z=0.4293)=91.8\pm5.3$ km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring $z_{t}=0.4\pm0.1$, and to significantly disfavor the null hypothesis of no transition between decelerated and accelerated expansion at 99.9\% confidence level. This analysis highlights the wide potential of the cosmic chronometers approach: it permits to derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model, it can constrain cosmologies beyond -and including- the $Λ$CDM model.

astro-ph.CO↗

arXiv2016

Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers

We use the latest compilation of observational H(z) measurements obtained with cosmic chronometers in the redshift range $0<z<2$ to place constraints on cosmological parameters. We consider the sample alone and in combination with other state-of-the art cosmological probes: CMB data from the latest Planck 2015 release, the most recent estimate of the Hubble constant $H_{0}$, a compilation of recent BAO data, and the latest SNe sample. Since cosmic chronometers are independent of the assumed cosmological model, we are able to provide constraints on the parameters that govern the expansion history of the Universe in a way that can be used to test cosmological models. We show that the H(z) measurements obtained with cosmic chronometer from the BOSS survey provide enough constraining power in combination with CMB data to constrain the time evolution of dark energy, yielding constraints competitive with those obtained using SNe and/or BAO. From late-Universe probes alone we find that $w_0=-0.9\pm0.18$ and $w_a=-0.5\pm1.7$, and when combining also CMB data we obtain $w_0=-0.98\pm0.11$and $w_a=-0.30\pm0.4$. These new constraints imply that nearly all quintessence models are disfavoured, only phantom models or a pure cosmological constant being allowed. For the curvature we find $Ω_k=0.003\pm0.003$, including CMB data. Cosmic chronometers data are important also to constrain neutrino properties by breaking or reducing degeneracies with other parameters. We find that $N_{eff}=3.17\pm0.15$, thus excluding the possibility of an extra (sterile) neutrino at more than $5σ$, and put competitive limits on the sum of neutrino masses, $Σm_ν< 0.27$ eV at 95% confidence level. Finally, we constrain the redshift evolution of dark energy, and find w(z) consistent with the $Λ$CDM model at the 40% level over the entire redshift range $0<z<2$. [abridged]

astro-ph.CO↗

arXiv2016

Discrepancies between CFHTLenS cosmic shear & Planck: new physics or systematic effects?

There is currently a discrepancy in the measured value of the amplitude of matter clustering, parameterised using $σ_8$, inferred from galaxy weak lensing, and cosmic microwave background data, which could be an indication of new physics, such as massive neutrinos or a modification to the gravity law, or baryon feedback. In this paper we make the assumption that the cosmological parameters are well determined by Planck, and use weak lensing data to investigate the implications for baryon feedback and massive neutrinos, as well as possible contributions from intrinsic alignments and biases in photometric redshifts. We apply a non-parametric approach to model the baryonic feedback on the dark matter clustering, which is flexible enough to reproduce the OWLS and Illustris simulation results. The statistic we use, 3D cosmic shear, is a method that extracts cosmological information from weak lensing data using a spherical-Bessel function power spectrum approach. We analyse the CFHTLenS weak lensing data and, assuming best fit cosmological parameters from the Planck CMB experiment, find that there is no evidence for baryonic feedback on the dark matter power spectrum, but there is evidence for a bias in the photometric redshifts in the CFHTLenS data, consistent with a completely independent analysis by Choi et al. (2015), based on spectroscopic redshifts; and that these conclusions are robust to assumptions about the intrinsic alignment systematic. We also find an upper limit on the sum of neutrino masses conditional on other $Λ$CDM parameters being fixed, of $< 0.28$ eV ($1σ$).

astro-ph.CO↗

arXiv2016

Neutrino footprint in Large Scale Structure

Recent constrains on the sum of neutrino masses inferred by analyzing cosmological data, show that detecting a non-zero neutrino mass is within reach of forthcoming cosmological surveys, implying a direct determination of the absolute neutrino mass scale. The measurement relies on constraining the shape of the matter power spectrum below the neutrino free streaming scale: massive neutrinos erase power at these scales. Detection of a lack of small-scale power, however, could also be due to a host of other effects. It is therefore of paramount importance to validate neutrinos as the source of power suppression at small scales. We show that, independent on hierarchy, neutrinos always show a footprint on large, linear scales; the exact location and properties can be related to the measured power suppression (an astrophysical measurement) and atmospheric neutrinos mass splitting (a neutrino oscillation experiment measurement). This feature can not be easily mimicked by systematic uncertainties or modifications in the cosmological model. The measurement of such a feature, up to 1% relative change in the power spectrum, is a smoking gun for confirming the determination of the absolute neutrino mass scale from cosmological observations. It also demonstrates the synergy of astrophysics and particle physics experiments.

astro-ph.CO↗

arXiv2016

Cosmic explosions, life in the Universe and the Cosmological Constant

Galactic Gamma-Ray Bursts (GRBs) are copious sources of gamma-rays that can pose a threat to complex life. Using recent determinations of their rate and the probability of GRBs causing massive extinction, we explore what type of universes are most likely to harbour advanced forms of life. For this purpose we use cosmological N-body simulations to determine at what time and for what value of the cosmological constant ($Λ$) the chances of life being unaffected by cosmic explosions are maximised. We find that $Λ-$dominated universes favour the survival of life against GRBs. Within a $Λ$CDM cosmology, the parameters that govern the likelihood of life survival to GRBs are dictated by the value of $Λ$ and the age of the Universe. We find that we seem to live in a favorable point in this parameter phase space which minimises the exposure to cosmic explosions, yet maximises the number of main sequence (hydrogen-burning) stars around which advanced life forms can exist.

astro-ph.CO↗

arXiv2015

Enhancing the Cosmic Shear Power Spectrum

Applying a transformation to a non-Gaussian field can enhance the information content of the resulting power spectrum, by reducing the correlations between Fourier modes. In the context of weak gravitational lensing, it has been shown that this gain in information content is significantly compromised by the presence of shape noise. We apply clipping to mock convergence fields, a technique which is known to be robust in the presence of noise and has been successfully applied to galaxy number density fields. When analysed in isolation the resulting convergence power spectrum returns degraded constraints on cosmological parameters. However substantial gains can be achieved by performing a combined analysis of the power spectra derived from both the original and transformed fields. Even in the presence of realistic levels of shape noise, we demonstrate that this approach is capable of reducing the area of likelihood contours within the $Ω_m - σ_8$ plane by more than a factor of three.

astro-ph.CO↗

arXiv2015

The relative and absolute ages of old globular clusters in the LCDM framework

Old Globular Clusters (GCs) in the Milky Way have ages of about 13 Gyr, placing their formation time in the reionization epoch. We propose a novel scenario for the formation of these systems based on the merger of two or more atomic cooling halos at high-redshift (z>6). First generation stars are formed as an intense burst in the center of a minihalo that grows above the threshold for hydrogen cooling (halo mass M_h~10^8 Msun) by undergoing a major merger within its cooling timescale (~150 Myr). Subsequent minor mergers and sustained gas infall bring new supply of pristine gas at the halo center, creating conditions that can trigger new episodes of star formation. The dark-matter halo around the GC is then stripped during assembly of the host galaxy halo. Minihalo merging is efficient only in a short redshift window, set by the LCDM parameters, allowing us to make a strong prediction on the age distribution for old GCs. From cosmological simulations we derive an average merging redshift =9 and narrow distribution Dz=2, implying average GC age =13.0+/-0.2 Gyr including ~0.2 Gyr of star formation delay. Qualitatively, our scenario reproduces other general old GC properties (characteristic masses and number of objects, metallicity versus galactocentric radius anticorrelation, radial distribution), but unlike age, these generally depend on details of baryonic physics. In addition to improved age measurements, direct validation of the model at z~10 may be within reach of ultradeep gravitationally lensed observations with the James Webb Space Telescope.

astro-ph.GA↗

arXiv2015

Signatures of Horndeski gravity on the Dark Matter Bispectrum

We present a detailed study of second-order matter perturbations for the general Horn- deski class of models. Being the most general scalar-tensor theory having second-order equations of motion, it includes many known gravity and dark energy theories and General Relativity with a cosmological constant as a specific case. This enables us to estimate the leading order dark matter bispectrum generated at late-times by gravitational instability. We parametrize the evolution of the first and second-order equations of motion as proposed by Bellini and Sawicki (2014), where the free functions of the theory are assumed to be proportional to the dark energy density. We show that it is unnatural to have large 10% ( 1%) deviations of the bispectrum introducing even larger ~ 30% (~ 5%) deviations in the linear growth rate. Considering that measurements of the linear growth rate have much higher signal-to-noise than bispectrum measurements, this indicates that for Horndeski models which reproduce the expansion history and the linear growth rate as predicted by GR the dark matter bispectrum kernel can be effectively modelled as the standard GR one. On the other hand, an observation of a large bispectrum deviation that can not be explained in terms of bias would imply either that the evolution of perturbations is strongly different than the evolution predicted by GR or that the theory of gravity is exotic (e.g., breaks the weak equivalence principle) and/or fine-tuned.

astro-ph.CO↗

arXiv2015

Calibrating the cosmic distance scale ladder: the role of the sound horizon scale and the local expansion rate as distance anchors

We exploit cosmological-model independent measurements of the expansion history of the Universe to provide a cosmic distance ladder. These are supernovae type Ia used as standard candles (at redshift between 0.01 and 1.3) and baryon acoustic oscillations (at redshifts between 0.1 and 0.8) as standard rulers. We calibrate (anchor) the ladder in two ways: first using the local $H_0$ value as an anchor at $z$ = 0 (effectively calibrating the standard candles) and secondly using the cosmic microwave background-inferred sound-horizon scale as an anchor (giving the standard ruler length) as an inverse distance ladder. Both methods are consistent, but the uncertainty in the expansion history $H(z)$ is smaller if the sound horizon scale is used. We present inferred values for the sound horizon at radiation drag $r_d$ which do not rely on assumptions about the early expansion history nor on cosmic microwave background measurements but on the cosmic distance ladder and baryon acoustic oscillations measurements. We also present derived values of $H_0$ from the inverse distance ladder and we show that they are in very good agreement with the extrapolated value in a $Λ$CDM model from Planck cosmic microwave background data.

astro-ph.CO↗

arXiv2015

The luminosity and stellar mass functions of GRB host galaxies: Insight into the metallicity bias

Long-Duration Gamma-Ray Bursts (GRBs) are powerful probes of the Universe star formation history, but correlation between the two depends on the highly debated presence/strength of a metallicity bias. To investigate this correlation, we use a phenomenological model that successfully describes star formation rates, luminosities and stellar masses of star forming galaxies, applying it to GRB production. We predict luminosities, stellar masses, and metallicities of host galaxies depending on the metallicity bias. Our best-fitting model includes a moderate metallicity bias, broadly consistent with the large majority of long-duration GRBs in metal-poor environments originating from collapsars (probability ~83%), but with a secondary contribution (~17%) from metal-independent production channels, such as binary evolution. Because of the mass-metallicity relation of galaxies, the maximum likelihood model predicts that the metal-independent channel becomes dominant at z<2, where hosts have higher metallicities and collapsars are suppressed. This possibly explains why some studies find no clear evidence of a metal-bias based on low-z samples. However, while metallicity predictions match observations well at high redshift, there is tension with low redshift observations, since a significant fraction of GRB hosts are predicted to have (near-)solar metallicity. This is in contrast to observations, unless obscured, metal-rich hosts are preferentially missed in current datasets, and suggests that lower efficiencies of the metal-independent GRB channel might be preferred following a comprehensive fit from complete samples. Overall, we are able to establish the presence of a metallicity bias for GRB production, but continued characterization of GRB host galaxies is needed to quantify its strength.

astro-ph.GA↗

arXiv2014

Standard rulers, candles, and clocks from the low-redshift Universe

We measure the length of the Baryon Acoustic Oscillation (BAO) feature, and the expansion rate of the recent Universe, from low-redshift data only, almost model-independently. We make only the following minimal assumptions: homogeneity and isotropy; a metric theory of gravity; a smooth expansion history, and the existence of standard candles (supernovæ) and a standard BAO ruler. The rest is determined by the data, which are compilations of recent BAO and Type IA supernova results. Making only these assumptions, we find for the first time that the standard ruler has length $103.9 \pm 2.3\, h^{-1}$ Mpc. The value is a measurement, in contrast to the model-dependent theoretical prediction determined with model parameters set by Planck data ($99.3 \pm 2.1 \, h^{-1}$ Mpc). The latter assumes $Λ$CDM, and that the ruler is the sound horizon at radiation drag. Adding passive galaxies as standard clocks or a local Hubble constant measurement allows the absolute BAO scale to be determined ($142.8\pm 3.7$ Mpc), and in the former case the additional information makes the BAO length determination more precise ($101.9\pm 1.9 \, h^{-1}\,$Mpc). The inverse curvature radius of the Universe is weakly constrained and consistent with zero, independently of the gravity model, provided it is metric. We find the effective number of relativistic species to be $N_{\rm eff} = 3.53\pm 0.32$, independent of late-time dark energy or gravity physics.

astro-ph.CO↗

arXiv2014

On the role of GRBs on life extinction in the Universe

As a copious source of gamma-rays, a nearby Galactic Gamma-Ray Burst (GRB) can be a threat to life. Using recent determinations of the rate of GRBs, their luminosity function and properties of their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place. Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 Gyr close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last 500 Myr causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, this probability drops below 50%. When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies and life can exist in only ~ 10% of galaxies. Remarkably, a cosmological constant is essential for such systems to exist. Furthermore, because of both the higher GRB rate and galaxies being smaller, life as it exists on Earth could not take place at $z > 0.5$. Early life forms must have been much more resilient to radiation.

astro-ph.HE↗

arXiv2014

The origin of spin in galaxies: clues from simulations of atomic cooling halos

In order to elucidate the origin of spin in both dark matter and baryons in galaxies, we have performed hydrodynamical simulations from cosmological initial conditions. We study atomic cooling haloes in the redshift range $100 > z > 9$ with masses of order $10^9{\rm M_{\odot}}$ at redshift $z=10$. We assume that the gas has primordial composition and that ${\rm H_2}$-cooling and prior star-formation in the haloes have been suppressed. We present a comprehensive analysis of the gas and dark matter properties of four halos with very low ($λ\approx 0.01$), low ($λ\approx 0.04$), high ($λ\approx 0.06$) and very high ($λ\approx 0.1$) spin parameter. Our main conclusion is that the spin orientation and magnitude is initially well described by tidal torque linear theory, but later on is determined by the merging and accretion history of each halo. We provide evidence that the topology of the merging region, i.e. the number of colliding filaments, gives an accurate prediction for the spin of dark matter and gas: halos at the center of knots will have low spin while those in the center of filaments will have high spin. The spin of a halo is given by $λ\approx 0.05 \times \left(\frac{7.6}{\rm number\,\,\, of \,\,\, filaments}\right)^{5.1}$.

astro-ph.CO↗

arXiv2014

Mild quasi-local non-Gaussianity as a signature of modified gravity during inflation

We show that modifications of Einstein gravity during inflation could leave potentially measurable imprints on cosmological observables in the form of non-Gaussian perturbations. This is due to the fact that these modifications appear in the form of an extra field that could have non-trivial interactions with the inflaton. We show it explicitly for the case $R+αR^2$, where nearly scale-invariant non-Gaussianity at the level of $f_{\rm NL} \approx -(1$ to $30)$ can be obtained, in a quasi-local configuration.

astro-ph.CO↗

arXiv2014

Disentangling the soft and hard components of the pp collisions using the sphero(i)city approach

A new method to extract information from the pp data is proposed. The approach is based on the use of the event structure variables: sphericity and spherocity, to split the data into enhanced soft and hard processes samples corresponding to events with large and low numbers of multi-parton interactions, respectively. The present study was developed in the framework of Pythia 8.180 for inelastic pp collisions at sqrt(s) =7 TeV. As an application of the method, a study of the identified particle transverse momentum spectra and their ratios; is presented for soft (isotropic) and hard (jetty-like) events. The flow-like effect on these observables due to multi-parton interactions and color reconnection is relevant for soft events suggesting that partons inside the jet do not feel color reconnection and its flow-like consequences.

hep-ph↗

arXiv2014

The bias of weighted dark matter halos from peak theory

We give an analytical form for the weighted correlation function of peaks in a Gaussian random field. In a cosmological context, this approach strictly describes the formation bias and is the main result here. Nevertheless, we show its validity and applicability to the evolved cosmological density field and halo field, using Gaussian random field realisations and dark matter N-body numerical simulations. Using this result from peak theory we compute the bias of peaks (and dark matter halos) and show that it reproduces results from the simulations at the ${\mathcal O}(10\%)$ level. Our analytical formula for the bias predicts a scale-dependent bias with two characteristics: a broad band shape which, however, is most affected by the choice of weighting scheme and evolution bias, and a more robust, narrow feature localised at the BAO scale, an effect that is confirmed in simulations. This scale-dependent bias smooths the BAO feature but, conveniently, does not move it. We provide a simple analytic formula to describe this effect. We envision that our analytic solution will be of use for galaxy surveys that exploit galaxy clustering.

astro-ph.CO↗