Neutrinos in Cosmology Alessandro Melchiorri Universita’ di Roma, “La Sapienza” INFN, Roma-1 NOW-2004, 16th September, 2004

Neutrino properties are among the most difficult to be probed experimentally, due to the weakness of their interactions. Fortunately, cosmology is quite sensitive to the neutrino sector and can shed light on several issues like the value of the neutrino absolute mass. However, these information are model dependent. Currently, the most popular cosmological model is the flat adiabatic  -CDM scenario, in which the present density of the universe is shared between baryons, Cold Dark Matter (CDM) and a cosmological constant. This model makes rather simplistic assumptions concerning the neutrino sector, consisting only of three ultra-relativistic neutrinos.

Brief History of ‘Neutrino Dark Matter’ 1970: Massive neutrinos, Zeldovich Pancakes * 1980 : Problems with structure formation * 1990 : CDM (80%) + HDM (20% ) * 2000 : Baryons (4%) + CDM (26%) +Lambda (70%): Massive neutrinos ? See e.g. Bond, Efstathiou, & Silk 1980, Melott 1982; Bond & Szalay 1983; White, Frenk & Davis 1983; Shandarin, Dorshkevich, & Zel’dovich 1983

Massive Neutrinos affect large scale structure !   We know the neutrino abundance in the universe:   Less clustering in universe with massive neutrinos. Cold Dark Matter (no neutrino mass) Hot + Cold Dark Matter (non-zero neutrino mass) S. Dodelson, 04

In practice, neutrinos affect the Power Spectrum of the 2-point density correlation function. P(k)=A k n T 2 (k) Neutrino Free Streaming  P(k)/P(k) = -8    m (Hu et al. 1998)  P(k)/P(k) = -8    m (Hu et al. 1998)

  Galaxy distribution   Lyman alpha forest   CMB Anisotropies Probes of the Power Spectrum

2dF Galaxy Survey   APM selected   Magnitude b J <   Median z  0.1   230 K measured

The 2dFGRS Power Spectrum

SDSS Survey   150K galaxies measured. Sloan Digital Sky Survey will eventually get redshifts of close to a million galaxies.   Photometry and spectroscopy done in same survey: helps with systematics Tegmark et al. 2003

Mathis, Lemson, Springel, Kauffmann, White & Dekel 2001 z = 0

z = 0.8

z = 2.4

Lyman alpha forest Photons with energy > (n=1 to n=2 transition energy) get absorbed along the line of sight as they lose energy due to cosmic redshift. Every absorption line corresponds to cloud of neutral hydrogen.

SDSS: Lyman Alpha Forest 3000 QSOs with absorption lines from z 2 to 4.2 Hui et al., 2003 Seljak et al., 2003   Each spectrum is a 1D probe of ~400 Mpc/h through the IGM (with full wavelength coverage)   Fluctuations in absorption trace the underlying mass distribution

Galaxies and Lya Power Spectrum S. Dodelson, 04

Degeneracies S. Dodelson, 04

CMB anisotropies CMB Anisotropies are weakly affected by massive neutrinos. However they measure very well the matter density and other parameters and, when combined with LSS data can break several degeneracies.

cosmic variance 1 deg

Riess et al., astro-ph/ Type Ia supernovae (SNe Ia) discovered with the Hubble Space Telescope during the course of the GOODS ACS Treasury program. 6 of the 7 highest-redshift SNe Ia known !

Neutrino mass from Cosmology DataAuthors  m i 2dFGRSElgaroy et al. 02 < 1.8 eV WMAP+2dF+…Spergel et al. 03 < 0.7 eV WMAP+2dFHannestad 03 < 1.0 eV SDSS+WMAPTegmark et al. 04 < 1.7 eV WMAP+2dF+ SDSS Crotty et al. 04 < 1.0 eV WMAP+SDSS LyaSeljak et al. 04< 0.43 eV Clusters +WMAPAllen et al eV All upper limits 95% CL, but different assumed priors !

Our Analysis Fogli, Lisi, Marrone,Melchiorri, Palazzo,Serra,Silk hep-ph/ We Analyzed the latest CMB, Galaxy Clusters, Ly-alpha (SDSS), SNI-IA data in order to constrain the sum of neutrino masses in cosmology. We restricted the analysis to three-flavour neutrino mixing. We assume the  -CDM model with primordial adiabatic and scalar invariant inflationary perturbations.

Fogli, Lisi, Marrone,Melchiorri, Palazzo,Serra,Silk hep-ph/

Conclusions   Cosmological constraints on neutrino mass are rapidly improving. If one includes bias information and Ly-alpha then  <0.5 eV. Sensitivity to  =0.1 eV is possible in the very near future.   Wide variety of techniques/experiments needed to eliminate systematics. The constraints are model dependent. Independent and more direct confirmations are needed.