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Register a new userEffects of neutrino mass and asymmetry on cosmological structure formation
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Light but massive cosmological neutrinos do not cluster significantly on small scales, due to their high thermal velocities. With finite masses, cosmological neutrinos become part of the total matter field and contribute to its smoothing. Structure formation in the presence of massive neutrinos is therefore impeded compared to that in the standard $Lambda$CDM cosmology with massless neutrinos. Neutrinos masses also distort the anisotropy power spectrum of cosmic microwave background (CMB). Furthermore, a finite chemical potential $mu$ for cosmological neutrinos, still allowed by current data, would have a non-negligible impact on CMB and structure formation. We consistently evaluate effects of neutrino masses and chemical potentials on the matter power spectrum by use of a neutrino-involved N-body simulation, with cosmological parameters obtained from a Markov-Chian Moonte-Carlo (MCMC) refitting of CMB data. Our results show that while a finite averaged neutrino mass $m_ u$ tends to suppress the matter power spectrum in a range of wave numbers, the neutrino degeneracy parameters ${xi_i equiv mu_i /T}$ ($i=$1, 2, 3) enhance the latter, leading to a large parameter degeneracy between $m_ u$ and $xi_i$. We provide an empirical formula for the effects on the matter power spectrum in a selected range of wave numbers induced by $m_ u$ and $eta equiv sqrt{sum_i xi^2_i}$. Observing a strong correlation between $m_ u$ and $eta$, we propose a single redshift-independent parameter $m_ u - frac{4}{3}eta^2$ to characterize the neutrino effects on the matter power spectrum.
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We present the results of cosmological simulations of large-scale structure formation with massive neutrinos. The phase-space distribution of the cosmic relic neutrinos is followed, for the first time, by directly integrating the six-dimensional Vlasov-Poisson equations. Our novel approach allows us to represent free streaming and clustering of neutrinos, and their gravitational interaction with cold dark matter accurately. We thus obtain solutions for the collisionless dynamics independent of conventional N-body methods. We perform a suite of hybrid N-body/Vlasov simulations with varying the neutrino mass, and systematically examine the dynamical effects of massive neutrinos on large-scale structure formation. Our simulations show characteristic large-scale clustering of the neutrinos and their coherent streaming motions relative to dark matter. The effective local neutrino temperature around massive galaxy clusters varies by several percent with respect to the cosmic mean; the neutrinos in clusters can be hotter or colder depending on the neutrino mass. We study a number of statistics of the large-scale structure and of dark matter halos in comparison with those obtained by N-body simulations and/or by perturbation theory. Our simulations mark an important milestone in numerical cosmology, and pave a new way to study cosmic structure formation with massive neutrinos.
We present here up-to-date neutrino mass limits exploiting the most recent cosmological data sets. By making use of the Cosmic Microwave Background temperature fluctuation and polarization measurements, Supernovae Ia luminosity distances, Baryon Acoustic Oscillation observations and determinations of the growth rate parameter, we are able to set the most constraining bound to date, $sum m_ u<0.09$ eV at $95%$~CL. This very tight limit is obtained without the assumption of any prior on the value of the Hubble constant and highly compromises the viability of the inverted mass ordering as the underlying neutrino mass pattern in nature. The results obtained here further strengthen the case for very large multitracer spectroscopic surveys as unique laboratories for cosmological relics, such as neutrinos: that would be the case of the Dark Energy Spectroscopic Instrument (DESI) survey and of the Euclid mission.
We investigate the effects of the change of cosmological parameters and star formation (SF) models on the cosmic SF history using cosmological smoothed particle hydrodynamics (SPH) simulations based on the cold dark matter (CDM) model. We vary the cosmological parameters within 1-sigma error from the WMAP best-fit parameters, and find that such changes in cosmological parameters mostly affect the amplitude of the cosmic SF history. At high redshift (hereafter high-z), the star formation rate (SFR) is sensitive to the cosmological parameters that control the small-scale power of the primordial power spectrum, while the cosmic matter content becomes important at lower redshifts. We also test two new SF models: 1) the `Pressure model based on the work by Schaye & Dalla Vecchia (2008), and 2) the `Blitz model that takes the effect of molecular hydrogen formation into account, based on the work by Blitz & Rosolowsky (2006). Compared to the previous conventional SF model, the Pressure model reduces the SFR in low-density regions and shows better agreement with the observations of the Kennicutt-Schmidt law. This model also suppresses the early star formation and shifts the peak of the cosmic SF history toward lower redshift, more consistently with the recent observational estimates of cosmic SFR density. The simulations with the new SF model also predict lower global stellar mass densities at high-z, larger populations of low-mass galaxies and a higher gas fraction in high-z galaxies. Our results suggest that there is room left in the model uncertainties to reconcile the discrepancy that was found between the theory and observations of cosmic SF history and stellar mass density. Nevertheless, our simulations still predict higher stellar mass densities than most of the observational estimates.
When combining cosmological and oscillations results to constrain the neutrino sector, the question of the propagation of systematic uncertainties is often raised. We address this issue in the context of the derivation of an upper bound on the sum of the neutrino masses ($Sigma m_ u$) with recent cosmological data. This work is performed within the ${{mathrm{Lambda{CDM}}}}$ model extended to $Sigma m_ u$, for which we advocate the use of three mass-degenerate neutrinos. We focus on the study of systematic uncertainties linked to the foregrounds modelling in CMB data analysis, and on the impact of the present knowledge of the reionisation optical depth. This is done through the use of different likelihoods built from Planck data. Limits on $Sigma m_ u$ are derived with various combinations of data, including the latest Baryon Acoustic Oscillations (BAO) and Type Ia Supernovae (SN) results. We also discuss the impact of the preference for current CMB data for amplitudes of the gravitational lensing distortions higher than expected within the ${{mathrm{Lambda{CDM}}}}$ model, and add the Planck CMB lensing. We then derive a robust upper limit: $Sigma m_ u< 0.17hbox{ eV at }95% hbox{CL}$, including 0.01 eV of foreground systematics. We also discuss the neutrino mass repartition and show that todays data do not allow one to disentangle normal from inverted hierarchy. The impact on the other cosmological parameters is also reported, for different assumptions on the neutrino mass repartition, and different high and low multipole CMB likelihoods.
Mass asymmetry effects on multifragmentation.
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