273 research outputs found
Reducing sample variance: halo biasing, non-linearity and stochasticity
Comparing clustering of differently biased tracers of the dark matter
distribution offers the opportunity to reduce the cosmic variance error in the
measurement of certain cosmological parameters. We develop a formalism that
includes bias non-linearities and stochasticity. Our formalism is general
enough that can be used to optimise survey design and tracers selection and
optimally split (or combine) tracers to minimise the error on the
cosmologically interesting quantities. Our approach generalises the one
presented by McDonald & Seljak (2009) of circumventing sample variance in the
measurement of . We analyse how the bias, the noise,
the non-linearity and stochasticity affect the measurements of and explore
in which signal-to-noise regime it is significantly advantageous to split a
galaxy sample in two differently-biased tracers. We use N-body simulations to
find realistic values for the parameters describing the bias properties of dark
matter haloes of different masses and their number density.
We find that, even if dark matter haloes could be used as tracers and
selected in an idealised way, for realistic haloes, the sample variance limit
can be reduced only by up to a factor .
This would still correspond to the gain from a three times larger survey volume
if the two tracers were not to be split. Before any practical application one
should bear in mind that these findings apply to dark matter haloes as tracers,
while realistic surveys would select galaxies: the galaxy-host halo relation is
likely to introduce extra stochasticity, which may reduce the gain further.Comment: 21 pages, 13 figures. Published version in MNRA
BAO+BBN revisited -- Growing the Hubble tension with a 0.7km/s/Mpc constraint
The combination of Baryonic Acoustic Oscillation (BAO) data together with
light element abundance measurements from Big Bang Nucleosynthesis (BBN) has
been shown to constrain the cosmological expansion history to an unprecedented
degree. Using the newest LUNA data and DR16 data from SDSS, the BAO+BBN probe
puts tight constraints on the Hubble parameter (), resulting in a tension with the local distance
ladder determination from SH0ES in a CDM model. In the updated BAO
data the high- and low-redshift subsets are mutually in excellent agreement,
and there is no longer a mild internal tension to artificially enhance the
constraints. Adding the recently-developed ShapeFit analysis yields ( tension). For combinations with
additional data sets, there is a strong synergy with the sound horizon
information of the cosmic microwave background, which leads to one of the
tightest constraints to date, , in
tension with SH0ES. The region preferred by this combination is
perfectly in agreement with that preferred by ShapeFit. The addition of
supernova data also yields a tension with SH0ES for Pantheon, and a
tension for PantheonPLUS. Finally, we show that there is a degree
of model-dependence of the BAO+BBN constraints with respect to early-time
solutions of the Hubble tension, and the loss of constraining power in extended
models depends on whether the model can be additionally constrained from BBN
observations.Comment: 27 pages, 9 figures, 1 table. Comments are welcome
The Bispectrum of f(R) Cosmologies
In this paper we analyze a suite of cosmological simulations of modified
gravitational action f(R) models, where cosmic acceleration is induced by a
scalar field that acts as a fifth force on all forms of matter. In particular,
we focus on the bispectrum of the dark matter density field on mildly
non-linear scales. For models with the same initial power spectrum, the dark
matter bispectrum shows significant differences for cases where the final dark
matter power spectrum also differs. Given the different dependence on bias of
the galaxy power spectrum and bispectrum, bispectrum measurements can close the
loophole of galaxy bias hiding differences in the power spectrum.
Alternatively, changes in the initial power spectrum can also hide differences.
By constructing LCDM models with very similar final non-linear power spectra,
we show that the differences in the bispectrum are reduced (<4%) and are
comparable with differences in the imperfectly matched power spectra. These
results indicate that the bispectrum depends mainly on the power spectrum and
less sensitively on the gravitational signatures of the f(R) model. This weak
dependence of the matter bispectrum on gravity makes it useful for breaking
degeneracies associated with galaxy bias, even for models beyond general
relativity.Comment: 14 pages, 5 figures, Published in JCA
Beyond two-point statistics: using the minimum spanning tree as a tool for cosmology
Cosmological studies of large-scale structure have relied on two-point statistics, not fully exploiting the rich structure of the cosmic web. In this paper we show how to capture some of this cosmic web information by using the minimum spanning tree (MST), for the first time using it to estimate cosmological parameters in simulations. Discrete tracers of dark matter such as galaxies, N-body particles or haloes are used as nodes to construct a unique graph, the MST, that traces skeletal structure. We study the dependence of the MST on cosmological parameters using haloes from a suite of COmoving Lagrangian Acceleration (COLA) simulations with a box size of 250 hâ1Mpcâ , varying the amplitude of scalar fluctuations (As), matter density (Ωm), and neutrino mass (âmÎœ). The power spectrum P and bispectrum B are measured for wavenumbers between 0.125 and 0.5 hMpcâ1â , while a corresponding lower cut of âŒ12.6 hâ1Mpc is applied to the MST. The constraints from the individual methods are fairly similar but when combined we see improved 1Ï constraints of âŒ17 per cent (â âŒ12 per centâ ) on Ωm and âŒ12 per cent (â âŒ10 per centâ ) on As with respect to P (P + B) thus showing the MST is providing additional information. The MST can be applied to current and future spectroscopic surveys (BOSS, DESI, Euclid, PSF, WFIRST, and 4MOST) in 3D and photometric surveys (DES and LSST) in tomographic shells to constrain parameters and/or test systematics
An improved fitting formula for the dark matter bispectrum
In this paper we present an improved fitting formula for the dark matter
bispectrum motivated by the previous phenomenological approach of Scoccimarro &
Couchman (2001). We use a set of LCDM simulations to calibrate the fitting
parameters in the k-range of 0.03 h/Mpc<k<0.4 h/Mpc and in the redshift range
of 0<z<1.5. This new proposed fit describes well the BAO-features although it
was not designed to. The deviation between the simulations output and our
analytic prediction is typically less than 5% and in the worst case is never
above 10%. We envision that this new analytic fitting formula will be very
useful in providing reliable predictions for the non-linear dark matter
bispectrum for LCDM models.Comment: 16 pages, 5 figures. Published in JCA
The completed SDSS-IV extended baryon oscillation spectroscopic survey: Geometry and growth from the anisotropic void-galaxy correlation function in the luminous red galaxy sample
We present an analysis of the anisotropic redshift-space void-galaxy correlation in configuration space using the Sloan Digital Sky Survey extended Baryon Oscillation Spectroscopic Survey (eBOSS) Data Release 16 luminous red galaxy (LRG) sample. This sample consists of LRGs between redshifts 0.6 and 1.0, combined with the high redshift z > 0.6 tail of the Baryon Oscillation Spectroscopic Survey Data Release 12 CMASS sample. We use a reconstruction method to undo redshift-space distortion (RSD) effects from the galaxy field before applying a watershed void-finding algorithm to remove bias from the void selection. We then perform a joint fit to the multipole moments of the correlation function for the growth rate fÏ 8 and the geometrical distance ratio DM/DH, finding f Ï8 (zeff ) = 0.356 ± 0.079 and DM /DH (zeff ) = 0.868 ± 0.017 at the effective redshift zeff = 0.69 of the sample. The posterior parameter degeneracies are orthogonal to those from galaxy clustering analyses applied to the same data, and the constraint achieved on DM/DH is significantly tighter. In combination with the consensus galaxy BAO and full-shape analyses of the same sample, we obtain fÏ 8 = 0.447 ± 0.039, DM/rd = 17.48 ± 0.23, and DH/rd = 20.10 ± 0.34. These values are in good agreement with the ÎCDM model predictions and represent reductions in the uncertainties of 13 per cent, 23 per cent, and 28 per cent, respectively, compared to the combined results from galaxy clustering, or an overall reduction of 55 per cent in the allowed volume of parameter space
Study of dielectron production in C+C collisions at 1 AGeV
The emission of e+e- pairs from C+C collisions at an incident energy of 1 GeV
per nucleon has been investigated. The measured production probabilities,
spanning from the pi0-Dalitz to the rho/omega! invariant-mass region, display a
strong excess above the cocktail of standard hadronic sources. The
bombarding-energy dependence of this excess is found to scale like pion
production, rather than like eta production. The data are in good agreement
with results obtained in the former DLS experiment.Comment: submitted to Physics Letters
Meson and di-electron production with HADES
The HADES experiment, installed at GSI, Darmstadt, measures di-electron
production in A+A, p/pi+N and p/pi+A collisions. Here, the pi0 and eta Dalitz
decays have been reconstructed in the exclusive p+p reaction at 2.2 GeV to form
a reference cocktail for long-lived di-electron sources. In the C+C reaction at
1 and 2 GeV/u, these long-lived sources have been subtracted from the measured
inclusive e+e- yield to exhibit the signal from the early phase of the
collision. The results suggest that resonances play an important role in dense
nuclear matter.Comment: Invited plenary talk at the 10th International Workshop On Meson
Production, Properties And Interaction (MESON 2008) 6-10 Jun 2008, Cracow,
Polan
The completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Measurement of the BAO and growth rate of structure of the emission line galaxy sample from the anisotropic power spectrum between redshift 0.6 and 1.1
We analyse the large-scale clustering in Fourier space of emission line galaxies (ELG) from the Data Release 16 of the Sloan Digital Sky Survey IV extended Baryon Oscillation Spectroscopic Survey. The ELG sample contains 173â736 galaxies covering 1170âdeg^{2} in the redshift range 0.6 < z < 1.1. We perform a BAO measurement from the post-reconstruction power spectrum monopole, and study redshift space distortions (RSD) in the first three even multipoles. Photometric variations yield fluctuations of both the angular and radial survey selection functions. Those are directly inferred from data, imposing integral constraints which we model consistently. The full data set has only a weak preference for a BAO feature (1.4Ï). At the effective redshift z_{eff} = 0.845 we measure D_{V}(z_{eff})/r_{drag}=18.33\tfrac{+0.57}{â0.62â },with DV the volume-averaged distance and r_{drag} the comoving sound horizon at the drag epoch. In combination with the RSD measurement, at z_{eff} = 0.85 we find fÏ_{8}(z_{eff})=0.289\tfrac{+0.085}{â0.096â }, with f the growth rate of structure and Ï_{8} the normalization of the linear power spectrum, D_{H}(z_{eff})/r_{drag} = 20.0\tfrac{2.4}{-2.2} and D_{M}(z_[eff})/r_{drag} = 19.17 ± 0.99 with D_{H} and D_{M} the Hubble and comoving angular distances, respectively. These results are in agreement with those obtained in configuration space, thus allowing a consensus measurement of fÏ_{8}(z_{eff}) = 0.315 ± 0.095, D_{H}(z_{eff})/r_{drag} = 19.6\tfrac{+2.2}{â2.1} and D_{M}(z_{eff})/r_{drag} = 19.5 ± 1.0. This measurement is consistent with a flat ÎCDM model with Planck parameters
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