Publications (173)951.77 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: 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 lifethreatening (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 gigayears close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last 500×10^{6} years, 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, does this probability drop 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.Physical Review Letters 12/2014; 113(23):231102. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We exploit cosmologicalmodel 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 backgroundinferred soundhorizon 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 derived value from the stateofthe art cosmic microwave background data for a $\Lambda$CDM model.11/2014;  [Show abstract] [Hide abstract]
ABSTRACT: 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 starformation in the haloes have been suppressed. We present a comprehensive analysis of the gas and dark matter properties of four halos with very low ($\lambda \approx 0.01$), low ($\lambda \approx 0.04$), high ($\lambda \approx 0.06$) and very high ($\lambda \approx 0.1$) 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 $\lambda \approx 0.05 \times \left(\frac{7.6}{\rm number\,\,\, of \,\,\, filaments}\right)^{5.1}$.10/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Assuming the existence of standard clocks, standard candles and standard rulers, we determine the length of the Baryon Acoustic Oscillation (BAO) feature, and the expansion history of the recent Universe, from lowredshift data only, in a relatively modelindependent way. In this process we assume as little as possible, requiring only the cosmological principle, a metric theory of gravity, a smooth expansion history, and the existence of these standard objects; the rest is determined by the data. The data we use are a compilation of recent BAO data, Type \OneA supernov\ae\ and ages of earlytype galaxies. Making only these assumptions, we find for the first time that the standard ruler has a largely modelindependent length of $101.9 \pm 1.9 h^{1}$ Mpc. We also find that the standard candle has a brightness consistent with determinations from supernov\ae\ alone, and that the inverse curvature radius of the Universe is weakly constrained and consistent with zero, independently of the gravity model, provided it is metric. The most notable result is that of the standard ruler; we determine it without using the cosmic microwave background (CMB), and with an error which is marginally smaller than the modeldependent CMB determination from Planck data ($99.3 \pm 2.1 \, h^{1}$Mpc), which assumes it is the sound horizon at radiation drag in a $\Lambda$CDM universe.09/2014; 113(24).  [Show abstract] [Hide abstract]
ABSTRACT: As a copious source of gammarays, a nearby Galactic GammaRay 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 lifethreatening (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.09/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We show that modifications of Einstein gravity during inflation could leave potentially measurable imprints on cosmological observables in the form of nonGaussian perturbations. This is due to the fact that these modifications appear in the form of an extra field that could have nontrivial interactions with the inflaton. We show it explicitly for the case $R+\alpha R^2$, where nearly scaleinvariant nonGaussianity at the level of $f_{\rm NL} \approx (1$ to $30)$ can be obtained, in a quasilocal configuration.Physical review letters. 07/2014; 113(16).  [Show abstract] [Hide abstract]
ABSTRACT: 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 Nbody 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 scaledependent 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 scaledependent 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.Monthly Notices of the Royal Astronomical Society 04/2014; 443(1). · 5.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We use cosmologyindependent measurements of the expansion history in the redshift range 0.1 < z <1.2 and compare them with the Cosmic Microwave Backgroundderived expansion history predictions. The motivation is to investigate if the tension between the local (cosmology independent) Hubble constant H0 value and the Planckderived H0 is also present at other redshifts. We conclude that there is no tension between Planck and cosmology independentmeasurements of the Hubble parameter H(z) at 0.1 < z < 1.2 for the LCDM model (odds of tension are only 1:15, statistically not significant). Considering extensions of the LCDM model does not improve these odds (actually makes them worse), thus favouring the simpler model over its extensions. On the other hand the H(z) data are also not in tension with the local H0 measurements but the combination of all three datasets shows a highly significant tension (odds ~ 1:400). Thus the new data deepen the mystery of the mismatch between Planck and local H0 measurements, and cannot univocally determine wether it is an effect localised at a particular redshift. Having said this, we find that assuming the NGC4258 maser distance as the correct anchor for H0, brings the odds to comfortable values. Further, using only the expansion history measurements we constrain, within the LCDM model, H0 = 68.5 + 3.5 and Omega_m = 0.32 + 0.05 without relying on any CMB prior. We also address the question of how smooth the expansion history of the universe is given the cosmology independent data and conclude that there is no evidence for deviations from smoothness on the expansion history, neither variations with time in the value of the equation of state of dark energy.Physics of the Dark Universe. 03/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We have performed hydrodynamical simulations from cosmological initial conditions using the Adaptive Mesh Refinement (AMR) code RAMSES to study atomic cooling haloes (ACHs) at z = 10 with masses in the range 5 × 107 M⊙ ≲ M ≲ 2 × 109 M⊙. We assume the gas has primordial composition and H2cooling and prior star formation in the haloes have been suppressed. We present a comprehensive analysis of the gas and dark matter (DM) properties of 19 haloes at a spatial resolution of ˜10 (proper) pc, selected from simulations with a total volume of ˜2000 (comoving) Mpc3. This is the largest statistical hydrosimulation study of ACHs at z > 10 to date. We examine the morphology, angular momentum, thermodynamical state and turbulent properties of these haloes, in order to assess the prevalence of discs and massive overdensities that may lead to the formation of supermassive black holes (SMBHs). We find no correlation between either the magnitude or the direction of the angular momentum of the gas and its parent DM halo. Only three of the haloes form rotationally supported cores. Two of the most massive haloes, however, form massive, compact overdense blobs, which migrate to the outer region of the halo. These blobs have an accretion rate between ˜101 and 103 M⊙ yr1 (at a distance of 100 pc from their centre), and are possible sites of SMBH formation. Our results suggest that the degree of rotational support and the fate of the gas in a halo is determined by its largescale environment and merger history. In particular, the two haloes that form overdense blobs are located at knots of the cosmic web, cooled their gas early on (z > 17) and experienced many mergers. The gas in these haloes is thus lumpy and highly turbulent, with Mach numbers M≳ 5. In contrast, the haloes forming rotationally supported cores are relatively more isolated, located midway along filaments of the cosmic web, cooled their gas more recently and underwent fewer mergers. As a result, the gas in these haloes is less lumpy and less turbulent (Mach numbers M≲ 4), and could retain most of its angular momentum. The remaining 14 haloes have a diverse range of intermediate properties. If verified in a larger sample of haloes and with additional physics to account for metals and star formation, our results will have implications for observations of the highest redshift galaxies and quasars with James Web Space Telescope.Monthly Notices of the Royal Astronomical Society 12/2013; 436(3):23012325. · 5.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: While there are numerous indications that gammaray bursts (GRBs) arise from the deaths of massive stars, the GRB rate does not follow the global cosmic star formation rate and, within their hosts, GRBs are more concentrated in regions of very high star formation. We explain both puzzles here. Using the publicly available VESPA database of the Sloan Digital Sky Survey (SDSS) Data Release 7 spectra, we explore a multiparameter space in galaxy properties such as stellar mass, metallicity, and dust to find the subset of galaxies that reproduces the GRB rate recently obtained by Wanderman & Piran. We find that only galaxies with present stellar masses below <1010M ☉ and low metallicity reproduce the observed GRB rate. This is consistent with direct observations of GRB hosts and provides an independent confirmation of the nature of GRB hosts. Because of the significantly larger sample of SDSS galaxies, we compute their correlation function and show that they are antibiased with respect to dark matter: they are in filaments and voids. Using recent observations of massive stars in local dwarfs we show how the fact that GRB host galaxies are dwarfs can explain the observation that GRBs are more concentrated in regions of high star formation than are supernovae. Finally, we explain these results using new theoretical advances in the field of star formation.The Astrophysical Journal 08/2013; 773(2):126. · 6.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: There has been significant recent progress in observational cosmology. This, in turn, has provided an unprecedented picture of the early universe and its evolution. In this review I will present a (biased) view of how one can use these observational results to constraint fundamental physics and in particular physics beyond the standard model.07/2013;  [Show abstract] [Hide abstract]
ABSTRACT: It has been shown by Shchekinov & Vasiliev2006 (SV06) that HD molecules can be an important cooling agent in high redshift z >10 haloes if they undergo mergers under specific conditions so suitable shocks are created. Here we build upon Prieto et al. (2012) who studied in detail the mergergenerated shocks, and show that the conditions for HD cooling can be studied by combining these results with a suite of darkmatter only simulations. We have performed a number of dark matter only simulations from cosmological initial conditions inside boxes with sizes from 1 to 4 Mpc. We look for haloes with at least two progenitors of which at least one has mass M > M_cr (z), where M_cr (z) is the SV06 critical mass for HD overcooling. We find that the fraction of overcooled haloes with mass between M_cr (z) and 10^{0.2} M_cr (z), roughly below the atomic cooling limit, can be as high as ~ 0.6 at z ~ 10 depending on the merger mass ratio. This fraction decreases at higher redshift reaching a value ~0.2 at z ~ 15. For higher masses, i.e. above 10^{0.2} M_cr (z) up to 10^{0.6} M_cr (z), above the atomic cooling limit, this fraction rises to values ~ 0.8 until z ~ 12.5. As a consequence, a non negligible fraction of high redshift z > 10 minihaloes can drop their gas temperature to the Cosmic Microwave Background temperature limit allowing the formation of low mass stars in primordial environments.Monthly Notices of the Royal Astronomical Society 07/2013; · 5.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show how general initial conditions for small field inflation can be obtained in multifield models. This is provided by nonlinear angular friction terms in the inflaton that provide a phase of nonslowroll inflation before the slowroll inflation phase. This in turn provides a natural mechanism to star smallfield slowroll at nearly zero velocity for arbitrary initial conditions. We also show that there is a relation between the scale of SUSY breaking sqrt (f) and the amount of nongaussian fluctuations generated by the inflaton. In particular, we show that in the local nongaussian shape there exists the relation sqrt (f) = 10^{13} GeV sqrt (f_NL). With current observational limits from Planck, and adopting the minimum amount of nongaussian fluctuations allowed by singlefield inflation, this provides a very tight constraint for the SUSY breaking energy scale sqrt (f) = 37 x 10^{13} GeV at 95% confidence. Further limits, or detection, from next year's Planck polarisation data will further tighten this constraint by a factor of two. We highlight that the key to our approach is to identify the inflaton with the scalar component of the goldstino superfield. This superfield is universal and implements the dynamics of SUSY breaking as well as superconformal breaking.07/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We use the latest Planck constraints, and in particular constraints on the derived parameters (Hubble constant and age of the Universe) for the local universe and compare them with local measurements of the same quantities. We propose a way to quantify whether cosmological parameters constraints from two different experiments are in tension or not. Our statistic, T, is an evidence ratio and therefore can be interpreted with the widely used Jeffrey's scale. We find that in the framework of the LCDM model, the Planck inferred two dimensional, joint, posterior distribution for the Hubble constant and age of the Universe is in "strong" tension with the local measurements; the odds being ~ 1:50. We explore several possibilities for explaining this tension and examine the consequences both in terms of unknown errors and deviations from the LCDM model. In some oneparameter LCDM model extensions, tension is reduced whereas in other extensions, tension is instead increased. In particular, small total neutrino masses are favored and a total neutrino mass above 0.15 eV makes the tension "highly significant" (odds ~ 1:150). A consequence of accepting this interpretation of the tension is that the degenerate neutrino hierarchy is highly disfavoured by cosmological data and the direct hierarchy is slightly favored over the inverse.06/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We explore how local, cosmologyindependent measurements of the Hubble constant and the age of the Universe help to provide a powerful consistency check of the currently favored cosmological model (flat ΛCDM) and modelindependent constraints on cosmology. We use cosmic microwave background (CMB) data to define the modeldependent cosmological parameters, and add local measurements to assess consistency and determine whether extensions to the model are justified. At current precision, there is no significant tension between the locally measured Hubble constant and age of the Universe (with errors of 3% and 5%, respectively) and the corresponding parameters derived from the CMB. However, if errors on the local measurements could be decreased by a factor of two, one could decisively conclude if there is tension or not. We also compare the local and CMB data assuming simple extensions of the flat, ΛCDM model (including curvature, dark energy with a constant equation of state parameter not equal to −1, nonzero neutrino masses and a nonstandard number of neutrino families) and find no need for these extra parameters; in particular, we constrain the effective number of neutrino species to be less than 4 at 95% confidence. We show that local measurements provide constraints on the curvature and the equation of state of dark energy nearly orthogonal to those of the CMB. We argue that cosmologyindependent measurements of local quantities at the percent level would be very useful to explore cosmology in a modelindependent way.Physics of the Dark Universe. 06/2013; 2(2):65–71.  [Show abstract] [Hide abstract]
ABSTRACT: While there are numerous indications that GRBs arise from the death of massive stars, the GRB rate does not follow the global cosmic star formation rate and, within their hosts, GRBs are more concentrated in regions of very high star formation. We explain both puzzles here. Using the publicly available VESPA database of SDSS Data Release 7 spectra, we explore a multiparameter space in galaxy properties, like stellar mass, metallicity, dust etc. to find the subset of galaxies that reproduce the recently obtained GRB rate by Wanderman & Piran (2010). We find that only galaxies with present stellar masses below < 10^{10} Msun and low metallicity reproduce the observed GRB rate. This is consistent with direct observations of GRB hosts and provides an independent confirmation of the nature of GRB hosts. Because of the significantly larger sample of SDSS galaxies, we compute their correlation function and show that they are antibiased with respect to the dark matter: they are in filaments and voids. Using recent observations of massive stars in local dwarfs we show how the fact that GRB hosts galaxies are dwarfs can explain the observation that GRBs are more concentrated in regions of high star formation than SNe. Finally we explain these results using new theoretical advances in the field of star formation.03/2013; 
Article: Multivariate joint PDF for nonGaussianities: exact formulation and generic approximations
[Show abstract] [Hide abstract]
ABSTRACT: We provide an exact expression for the multivariate joint probability distribution function of nonGaussian fields primordially arising from local transformations of a Gaussian field. This kind of nonGaussianity is generated in many models of inflation. We apply our expression to the non Gaussianity estimation from Cosmic Microwave Background maps and the halo mass function where we obtain analytical expressions. We also provide analytic approximations and their range of validity. For the Cosmic Microwave Background we give a fast way to compute the PDF which is valid up to 7{\sigma} for fNL values (both true and sampled) not ruled out by current observations, which consists of expressing the PDF as a combination of bispectrum and trispectrum of the temperature maps. The resulting expression is valid for any kind of nonGaussianity and is not limited to the local type. The above results may serve as the basis for a fully Bayesian analysis of the nonGaussianity parameter.Journal of Cosmology and Astroparticle Physics 01/2013; 2013(06). · 6.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have performed cosmohydro simulations using the RAMSES code to study atomic cooling (ACHs) haloes at z=10 with masses 5E7Msun<~M<~2E9Msun. We assume primordial gas and H2cooling and prior starformation have been suppressed. We analysed 19 haloes (gas and DM) at a resolution of ~10 (proper) pc, selected from a total volume of ~2E3 (comoving) Mpc3. This is the largest statistical hydrosim. study of ACHs at z>10 to date. We examine the morphology, angular momentum (AM), thermodynamic, and turbulence of these haloes, in order to assess the prevalence of disks and supermassive black holes (SMBHs). We find no correlation between either the magnitude or the direction of the AM of the gas and its parent DM halo. Only 3 haloes form rotationally supported cores. Two of the most massive haloes form massive, compact overdense blobs. These blobs have an accretion rate ~0.5 Msun/yr (at a distance of 100 pc), and are possible sites of SMBH formation. Our results suggest that the degree of rotational support and the fate of the gas in a halo is determined by its largescale environment and merger history. In particular, the two haloes forming blobs are located at knots of the cosmic web, cooled early on, and experienced many mergers. The gas in these haloes is lumpy and highly turbulent, with Mach N. >~ 5. In contrast, the haloes forming rotationally supported cores are relatively more isolated, located midway along filaments, cooled more recently, and underwent fewer mergers. Thus, the gas in these haloes is less lumpy and less turbulent (Mach <~ 4), and could retain most of its AM. The remaining 14 haloes have intermediate properties. If verified in a larger sample of haloes and with additional physics, our results will have implications for observations of the highestredshift galaxies and quasars with JWST.01/2013; 
Dataset: 0804.3091v1
 [Show abstract] [Hide abstract]
ABSTRACT: Observationally confirming spatial homogeneity on sufficiently large cosmological scales is of importance to test one of the underpinning assumptions of cosmology, and is also imperative for correctly interpreting dark energy. A challenging aspect of this is that homogeneity must be probed inside our past lightcone, while observations take place on the lightcone. The star formation history (SFH) in the galaxy fossil record provides a novel way to do this. We calculate the SFH of stacked Luminous Red Galaxy (LRG) spectra obtained from the Sloan Digital Sky Survey. We divide the LRG sample into 12 equal area contiguous sky patches and 10 redshift slices (0.2 < z < 0.5), which correspond to 120 blocks of volume 0.04Gpc3. Using the SFH in a time period which samples the history of the Universe between lookback times 11.5 to 13.4 Gyrs as a proxy for homogeneity, we calculate the posterior distribution for the excess largescale variance due to inhomogeneity, and find that the most likely solution is no extra variance at all. At 95% credibility, there is no evidence of deviations larger than 5.8%.The Astrophysical Journal Letters 09/2012; 762(1). · 6.35 Impact Factor
Publication Stats
4k  Citations  
951.77  Total Impact Points  
Top Journals
Institutions

2014

Harvard University
Cambridge, Massachusetts, United States


2009–2014

University of Barcelona
 Instituto de Ciencias del Cosmos (ICCUB)
Barcino, Catalonia, Spain 
IEEC Institute of Space Studies of Catalonia
Barcino, Catalonia, Spain


2009–2013

CERN
Genève, Geneva, Switzerland


2009–2012

Catalan Institution for Research and Advanced Studies
Barcino, Catalonia, Spain


2010

Yale University
 Department of Physics
New Haven, CT, United States


1996–2009

The Royal Observatory, Edinburgh
Edinburgh, Scotland, United Kingdom


2008

National Institute of Standards and Technology
Maryland, United States 
Columbia University
New York City, New York, United States


2006–2008

Princeton University
 Department of Astrophysical Sciences
Princeton, New Jersey, United States


2003–2008

University of Pennsylvania
 Department of Physics and Astronomy
Philadelphia, PA, United States


2000–2008

Rutgers, The State University of New Jersey
 Department Physics and Astronomy
New Brunswick, NJ, United States


1998–2002

The University of Edinburgh
 Institute for Astronomy (IfA)
Edinburgh, SCT, United Kingdom
