arXiv subjects

Stefano Gariazzo

Publications and source records attributed to Stefano Gariazzo.

51 records · Page 3Linked to original sources

arXiv2018

EDGES result versus CMB and low-redshift constraints on ionization histories

We examine the results from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), which has recently claimed the detection of a strong absorption in the 21 cm hyperfine transition line of neutral hydrogen, at redshifts demarcating the early stages of star formation. More concretely, we study the compatibility of the shape of the EDGES absorption profile, centered at a redshift of $z \sim 17.2$, with measurements of the reionization optical depth, the Gunn-Peterson optical depth, and Lyman-$α$ emission from star-forming galaxies, for a variety of possible reionization models within the standard $Λ$CDM framework (that is, a Universe with a cosmological constant and cold dark matter). When, conservatively, we only try to accommodate the location of the absorption dip, we identify a region in the parameter space of the astrophysical parameters that successfully explains all of the aforementioned observations. However, one of the most abnormal features of the EDGES measurement is the absorption amplitude, which is roughly a factor of two larger than the maximum allowed value in the $Λ$CDM framework. We point out that the simple considered astrophysical models that produce the largest absorption amplitudes are unable to explain the depth of the dip and of reproducing the observed shape of the absorption profile.

astro-ph.CO↗

arXiv2018

Was there an early reionization component in our universe?

A deep understanding of the Epoch of Reionization is still missing in our knowledge of the universe. While future probes will allow us to test the precise evolution of the free electron fraction from redshifts between $z\simeq 6$ and $z\simeq 20$, at present one could ask what kind of reionization processes are allowed by present Cosmic Microwave Background temperature and polarization measurements. An early contribution to reionization could imply a departure from the standard picture where star formation determines the reionization onset. BBy considering a broad class of possible reionization parameterizations, we find that current data do not require an early reionization component in our universe and that only one marginal class of models, based on a particular realization of reionization, may point to that. In addition, the frequentist Akaike Information Criterion (AIC) provides strong evidence against alternative reionization histories, favoring the most simple reionization scenario, which describes reionization by means of only one (constant) reionization optical depth $τ$.

astro-ph.CO↗

arXiv2017

Neutrino clustering in the Milky Way

The Cosmic Neutrino Background is a prediction of the standard cosmological model, but it has been never observed directly. In the experiments with the aim of detecting relic CNB neutrinos, currently under development, the expected event rate depends on the local density of relic neutrinos. Since massive neutrinos can be attracted by the gravitational potential of our galaxy and cluster around it, a local overdensity of cosmic neutrinos should exist. Considering the minimal masses guaranteed by neutrino oscillations, we review the computation of the local density of relic neutrinos and we present realistic prospects for a PTOLEMY-like experiment.

astro-ph.CO↗

arXiv2017

Cold dark matter plus not-so-clumpy dark relics

Various particle physics models suggest that, besides the (nearly) cold dark matter that accounts for current observations, additional but sub-dominant dark relics might exist. These could be warm, hot, or even contribute as dark radiation. We present here a comprehensive study of two-component dark matter scenarios, where the first component is assumed to be cold, and the second is a non-cold thermal relic. Considering the cases where the non-cold dark matter species could be either a fermion or a boson, we derive consistent upper limits on the non-cold dark relic energy density for a very large range of velocity dispersions, covering the entire range from dark radiation to cold dark matter. To this end, we employ the latest Planck Cosmic Microwave Background data, the recent BOSS DR11 and other Baryon Acoustic Oscillation measurements, and also constraints on the number of Milky Way satellites, the latter of which provides a measure of the suppression of the matter power spectrum at the smallest scales due to the free-streaming of the non-cold dark matter component. We present the results on the fraction $f_{\rm ncdm}$ of non-cold dark matter with respect to the total dark matter for different ranges of the non-cold dark matter masses. We find that the 2$σ$ limits for non-cold dark matter particles with masses in the range 1--10 keV are $f_{\rm ncdm}\leq0.29$ (0.23) for fermions (bosons), and for masses in the 10--100 keV range they are $f_{\rm ncdm}\leq0.43$ (0.45), respectively.

astro-ph.CO↗

arXiv2017

CMB anomalies and the effects of local features of the inflaton potential

Recent analysis of the WMAP and Planck data have shown the presence of a dip and a bump in the spectrum of primordial perturbations at the scales $k=0.002$ Mpc${}^{-1}$ and $k=0.0035$ Mpc${}^{-1}$ respectively. We analyze for the first time the effects a local feature in the inflaton potential to explain the observed deviations from scale invariance in the primordial spectrum. We perform a best fit analysis of the cosmic microwave background (CMB) radiation temperature and polarization data. The effects of the features can improve the agreement with observational data respect to the featureless model. The best fit local feature affects the primordial curvature spectrum mainly in the region of the bump, leaving the spectrum unaffected on other scales.

astro-ph.CO↗

arXiv2017

Light sterile neutrinos and pseudoscalar interactions in cosmology

The Short BaseLine (SBL) neutrino oscillations anomalies hint at the presence of a sterile neutrino with a mass of around 1 eV. However, such neutrino is highly incompatible with the cosmological data, in particular from the Cosmic Microwave Background (CMB), if no new physics is assumed. An interesting possibility for reconciling the 1 eV sterile neutrino presence in cosmology is related to the existence of a new pseudoscalar interaction. If the sterile neutrinos experience such a pseudoscalar interaction, the cosmological analyses of the full CMB data prefer a sterile neutrino mass that is fully compatible with the SBL determinations. The additional interaction allows to obtain also an improved compatibility of the cosmological predictions with the local measurements of the Hubble parameter.

astro-ph.CO↗

arXiv2016

Pseudoscalar - sterile neutrino interactions: reconciling the cosmos with neutrino oscillations

The Short BaseLine (SBL) neutrino oscillation anomalies hint at the presence of a sterile neutrino with a mass of around 1 eV. However, such a neutrino is incompatible with cosmological data, in particular observations of the Cosmic Microwave Background (CMB) anisotropies. However, this conclusion can change by invoking new physics. One possibility is to introduce a secret interaction in the sterile neutrino sector mediated by a light pseudoscalar. In this pseudoscalar model, CMB data prefer a sterile neutrino mass that is fully compatible with the mass ranges suggested by SBL anomalies. In addition, this model predicts a value of the Hubble parameter which is completely consistent with local measurements.

astro-ph.CO↗

arXiv2016

Dark Radiation and Inflationary Freedom after Planck 2015

The simplest inflationary models predict a primordial power spectrum (PPS) of the curvature fluctuations that can be described by a power-law function that is nearly scale-invariant. It has been shown, however, that the low-multipole spectrum of the CMB anisotropies may hint the presence of some features in the shape of the scalar PPS, which could deviate from its canonical power-law form. We study the possible degeneracies of this non-standard PPS with the neutrino anisotropies, the neutrino masses, the effective number of relativistic species and a sterile neutrino or a thermal axion mass. The limits on these additional parameters are less constraining in a model with a non-standard PPS when only including the temperature auto-correlation spectrum measurements in the data analyses. The inclusion of the polarization spectra noticeably helps in reducing the degeneracies, leading to results that typically show no deviation from the $Λ$CDM model with a standard power-law PPS.

astro-ph.CO↗

arXiv2016

Constraints on the Coupling between Dark Energy and Dark Matter from CMB data

We investigate a phenomenological non-gravitational coupling between dark energy and dark matter, where the interaction in the dark sector is parameterized as an energy transfer either from dark matter to dark energy or the opposite. The models are constrained by a whole host of updated cosmological data: cosmic microwave background temperature anisotropies and polarization, high-redshift supernovae, baryon acoustic oscillations, redshift space distortions and gravitational lensing. Both models are found to be compatible with all cosmological observables, but in the case where dark matter decays into dark energy, the tension with the independent determinations of $H_0$ and $σ_8$, already present for standard cosmology, increases: this model in fact predicts lower $H_0$ and higher $σ_8$, mostly as a consequence of the higher amount of dark matter at early times, leading to a stronger clustering during the evolution. Instead, when dark matter is fed by dark energy, the reconstructed values of $H_0$ and $σ_8$ nicely agree with their local determinations, with a full reconciliation between high- and low-redshift observations. A non-zero coupling between dark energy and dark matter, with an energy flow from the former to the latter, appears therefore to be in better agreement with cosmological data.

astro-ph.CO↗

arXiv2016

New Developments in Cosmology

In this Thesis I discuss several recent results obtained using the CMB spectra measured by Planck and several other cosmological probes. Extensions of the $Λ$CDM model are studied, including the presence of an additional sterile neutrino (motivated by the short-baseline oscillation anomalies) and of a thermal axion. The degeneracies of the cosmological effects of these particles with the power spectrum of primordial perturbations are tested. We also show that the power spectrum of initial scalar perturbations can be degenerate with the presence of primordial non-Gaussianities, thus affecting the constraints on the non-Gaussianity parameter $f_{NL}$. Finally, an effective interaction between dark energy and dark matter is studied.

astro-ph.CO↗

arXiv2016

Light Sterile Neutrinos In Cosmology

We briefly discuss the main effects of the presence of a Light Sterile Neutrino (LS$ν$) in Cosmology and how its properties can be constrained by Cosmic Microwave Background (CMB) and other cosmological measurements.

astro-ph.CO↗

arXiv2015

Dark Radiation and Inflationary Freedom

A relaxed primordial power spectrum (PPS) of scalar perturbations arising from inflation can impact the dark radiation constraints obtained from Cosmic Microwave Background and other cosmological measurements. If inflation produces a non-standard PPS for the initial fluctuations, a fully thermalized light sterile neutrino can be favoured by CMB observations, instead of being strongly disfavoured. In the case of a thermal axion, the constraints on the axion mass are relaxed when the PPS is different from the standard power law.

astro-ph.CO↗

arXiv2015

Primordial Power Spectrum features and $f_{NL}$ constraints

The simplest models of inflation predict small non-gaussianities and a featureless power spectrum. However, there exist a large number of well-motivated theoretical scenarios in which large non-gaussianties could be generated. In general, in these scenarios the primordial power spectrum will deviate from its standard power law shape. We study, in a model-independent manner, the constraints from future large scale structure surveys on the local non-gaussianity parameter $f_{\rm NL}$ when the standard power law assumption for the primordial power spectrum is relaxed. If the analyses are restricted to the large scale-dependent bias induced in the linear matter power spectrum by non-gaussianites, the errors on the $f_{\rm NL}$ parameter could be increased by $60\%$ when exploiting data from the future DESI survey, if dealing with only one possible dark matter tracer. In the same context, a nontrivial bias $|δf_{\rm NL}| \sim 2.5$ could be induced if future data are fitted to the wrong primordial power spectrum. Combining all the possible DESI objects slightly ameliorates the problem, as the forecasted errors on $f_{\rm NL}$ would be degraded by $40\%$ when relaxing the assumptions concerning the primordial power spectrum shape. Also the shift on the non-gaussianity parameter is reduced in this case, $|δf_{\rm NL}| \sim 1.6$. The addition of Cosmic Microwave Background priors ensure robust future $f_{\rm NL}$ bounds, as the forecasted errors obtained including these measurements are almost independent on the primordial power spectrum features, and $|δf_{\rm NL}| \sim 0.2$, close to the standard single-field slow-roll paradigm prediction.

astro-ph.CO↗

arXiv2015

Robustness of cosmological axion mass limits

We present cosmological bounds on the thermal axion mass in an extended cosmological scenario in which the primordial power spectrum of scalar perturbations differs from the usual power-law shape predicted by the simplest inflationary models. The power spectrum is instead modeled by means of a "piecewise cubic Hermite interpolating polynomial" (PCHIP). When using Cosmic Microwave Background measurements combined with other cosmological data sets, the thermal axion mass constraints are degraded only slightly. The addition of the measurements of $σ_8$ and $Ω_m$ from the 2013 Planck cluster catalogue on galaxy number counts relaxes the bounds on the thermal axion mass, mildly favouring a $\sim 1$~eV axion mass, regardless of the model adopted for the primordial power spectrum.However, in general, such a preference disappears if the sum of the three active neutrino masses is also considered as a free parameter in our numerical analyses, due to the strong correlation between the masses of these two hot thermal relics.

astro-ph.CO↗

arXiv2014

Light sterile neutrinos after BICEP-2

The recent discovery of B-modes in the polarization pattern of the Cosmic Microwave Background by the BICEP2 experiment has important implications for neutrino physics. We revisit cosmological bounds on light sterile neutrinos and show that they are compatible with all current cosmological data provided that the mass is relatively low. Using CMB data, including BICEP-2, we find an upper bound of $m_s < 0.85$ eV ($2σ$ Confidence Level). This bound is strengthened to 0.48 eV when HST measurements of $H_0$ are included. However, the inclusion of SZ cluster data from the Planck mission and weak gravitational measurements from the CFHTLenS project favours a non-zero sterile neutrino mass of $0.44^{+0.11}_{-0.16}$ eV. Short baseline neutrino oscillations, on the other hand, indicate a new mass state around 1.2 eV. This mass is highly incompatible with cosmological data if the sterile neutrino is fully thermalised ($Δχ^2>10$). However, if the sterile neutrino only partly thermalises it can be compatible with all current data, both cosmological and terrestrial.

astro-ph.CO↗