Effect Of Peculiar Velocities Research Articles

Gravitational wave luminosity distance-weighted anisotropies

Measurements of the luminosity distance of propagating gravitational waves can provide invaluable information on the geometry and content of our Universe. Due to the clustering of cosmic structures, in realistic situations we need to average the luminosity distance of events coming from patches inside a volume. In this work we evaluate, in a gauge-invariant and fully-relativistic treatment, the impact of cosmological perturbations on such averaging process. We find that clustering, lensing and peculiar velocity effects impact estimates for future detectors such as Einstein Telescope, Cosmic Explorer, the Big Bang Observer and DECIGO. The signal-to-noise ratio of the angular power spectrum of the average luminosity distance over all the redshift bins is 17 in the case of binary black holes detected by Einstein Telescope and Cosmic Explorer. We also provide fitting formulas for the corrections to the average luminosity distance due to general relativistic effects.

Peculiar velocity effects on the Hubble constant from time-delay cosmography

Peculiar velocity effects on the Hubble constant from time-delay cosmography

Relativistic distortions in galaxy density–ellipticity correlations: gravitational redshift and peculiar velocity effects

ABSTRACT We study relativistic effects, arising from the light propagation in an inhomogeneous universe. We particularly investigate the effects imprinted in a cross-correlation function between galaxy positions and intrinsic galaxy shapes (GI correlation). Considering the Doppler and gravitational redshift effects as major relativistic effects, we present an analytical model of the GI correlation function, from which we find that the relativistic effects induce non-vanishing odd multipole anisotropies. Focusing particularly on the dipole anisotropy, we show that the Doppler effect dominates at large scales, while the gravitational redshift effect originated from the halo potential dominates at the scales below 10–$30\, {\rm Mpc}\, h^{-1}$, with the amplitude of the dipole GI correlation being positive over all the scales. Also, we newly derive the covariance matrix for the modelled GI dipole. Taking into account the full covariance, we estimate the signal-to-noise ratio and show that the GI dipole induced by the relativistic effects is detectable in future large-volume galaxy surveys. We discuss how the measurement of dipole GI correlation could be helpful to detect relativistic effects in combination with the conventional galaxy–galaxy cross-correlation.

Cospatial 21 cm and metal-line absorbers in the epoch of reionization – I. Incidence and observability

ABSTRACTf Overdense, metal-rich regions, shielded from stellar radiation might remain neutral throughout reionization and produce metal as well as 21 cm absorption lines. Simultaneous absorption from metals and 21 cm can complement each other as probes of underlying gas properties. We use Aurora, a suite of high-resolution radiation-hydrodynamical simulations of galaxy formation, to investigate the occurrence of such ‘aligned’ absorbers. We calculate absorption spectra for 21 cm, O I, C II, Si II, and Fe II. We find velocity windows with absorption from at least one metal and 21 cm, and classify the aligned absorbers into two categories: ‘aligned and cospatial absorbers’ and ‘aligned but not cospatial absorbers’. While ‘aligned and cospatial absorbers’ originate from overdense structures and can be used to trace gas properties, ‘aligned but not cospatial absorbers’ are due to peculiar velocity effects. The incidence of absorbers is redshift dependent, as it is dictated by the interplay between reionization and metal enrichment, and shows a peak at z ≈ 8 for the aligned and cospatial absorbers. While aligned but not cospatial absorbers disappear towards the end of reionization because of the lack of an ambient 21 cm forest, aligned and cospatial absorbers are associated with overdense pockets of neutral gas that can be found at lower redshift. We produce mock observations for a set of sightlines for the next generation of telescopes like the ELT and SKA1-LOW, finding that given a sufficiently bright background quasar, these telescopes will be able to detect both types of absorbers throughout reionization.

Effects of peculiar velocities on the morphological properties of large-scale structure

It is known that the large-scale structure (LSS) mapped by a galaxy redshift survey is subject to distortions by galaxies' peculiar velocities. Besides the signatures generated in common N-point statistics, such as the anisotropy in the galaxy 2-point correlation function, the peculiar velocities also induce distinct features in LSS's morphological properties, which are fully described by four Minkowski functionals (MFs), i.e., the volume, surface area, integrated mean curvature and Euler characteristic (or genus). In this work, by using large suite of N-body simulations, we present and analyze these important features in the MFs of LSS on both (quasi-)linear and non-linear scales, with a focus on the latter. We also find the MFs can give competitive constraints on cosmological parameters compared to the power spectrum, probablly due to the non-linear information contained. For galaxy number density similar to the DESI BGS galaxies, the constraint on $\sigma_8$ from the MFs with one smoothing scale can be better by $\sim 50\%$ than from the power spectrum. These findings are important for the cosmological applications of MFs of LSS, and probablly open up a new avenue for studying the peculiar velocity field itself.

Effect of peculiar velocities of inhomogeneities on the shape of gravitational potential in spatially curved universe

We investigate the effect of peculiar velocities of inhomogeneities and the spatial curvature of the universe on the shape of the gravitating potential. To this end, we consider scalar perturbations of the FLRW metric. The gravitational potential satisfies a Helmholtz-type equation which follows from the system of linearized Einstein equations. We obtain analytical solutions of this equation in the cases of open and closed universes, filled with cold dark matter in presence of the cosmological constant. We demonstrate that, first, peculiar velocities significantly affect the screening length of the gravitational interaction and, second, the form of the gravitational potential depends on the sign of the spatial curvature.

Anti-symmetric clustering signals in the observed power spectrum

In this paper, we study how to directly measure the effect of peculiar velocities in the observed angular power spectra. We do this by constructing a new anti-symmetric estimator of Large Scale Structure using different dark matter tracers. We show that the Doppler term is the major component of our estimator and we show that we can measure it with a signal-to-noise ratio up to ∼ 50 using a futuristic SKAO HI galaxy survey. We demonstrate the utility of this estimator by using it to provide constraints on the Euler equation.

Redshift space three-point correlation function of IGM at z < 0.48

ABSTRACT Ly α forest decomposed into Voigt profile components allows us to study clustering properties of the intergalactic-medium and its dependence on various physical quantities. Here, we report the first detections of probability excess of low-z (i.e z < 0.48) Ly α absorber triplets over redshift-space scale of r∥ ≤ 8 pMpc (Mpc in physical units) with maximum amplitude of $8.76^{+1.96}_{-1.65}$ at a longitudinal separation of 1–2 pMpc. We measure non-zero three-point correlation ($\zeta = 4.76^{+1.98}_{-1.67}$) only at this scale with reduced three-point correlation Q = $0.95^{+0.39}_{-0.38}$. The measured ζ shows an increasing trend with increasing minimum H i column density (NH i) threshold while Q does not show any NH i dependence. About 88 per cent of the triplets contributing to ζ (at z ≤ 0.2) have nearby galaxies (whose distribution is known to be complete for ∼0.1L* at z < 0.1 and for ∼L* at z ∼ 0.25 within 20 arcsec to the quasar sightlines) within velocity separation of 500 km s−1 and median impact parameter of 405 pkpc. The measured impact parameters are consistent with majority of the identified triplets not originating from individual galaxies but tracing the underlying galaxy distribution. Frequency of occurrence of Broad-Ly α absorbers (b > 40 km s−1) in triplets (∼85 per cent) is factor ∼3 higher than that found among the full sample (∼32 per cent). Using four different cosmological simulations, we quantify the effect of peculiar velocities and feedback and show that most of the observed trends are broadly reproduced. However, ζ at small scales (r∥ < 1 pMpc) and its b-dependence found in simulations are inconsistent with observations. This could either be related to the failure of these simulations to reproduce the observed b and NH i distributions for NH i > 1014 cm−2 self-consistently or to the wide spread of signal-to-noise ratio in the observed data.

Peculiar velocities in the local Universe: comparison of different models and the implications for H0 and dark matter

ABSTRACT When measuring the value of the Hubble parameter, H0, it is necessary to know the recession velocity free of the effects of peculiar velocities. In this work, we study different models of peculiar velocity in the local Universe. In particular, we compare models based on density reconstruction from galaxy redshift surveys and kernel smoothing of peculiar velocity data. The velocity field from the density reconstruction is obtained using the 2M++ galaxy redshift compilation, which is compared to two adaptive kernel-smoothed velocity fields: the first obtained from the 6dF Fundamental Plane sample and the other using a Tully–Fisher catalogue obtained by combining SFI++ and 2MTF. We highlight that smoothed velocity fields should be rescaled to obtain unbiased velocity estimates. Comparing the predictions of these models to the observations from a few test sets of peculiar velocity data, obtained from the Second Amendment Supernovae catalogue and the Tully–Fisher catalogues, we find that 2M++ reconstruction provides a better model of the peculiar velocity in the local Universe than the kernel-smoothed peculiar velocity models. We study the impact of peculiar velocities on the measurement of H0 from gravitational waves and megamasers. In doing so, we introduce a probabilistic framework to marginalize over the peculiar velocity corrections along the line of sight. For the megamasers, we find H0 = 70.1 ± 2.9 km s−1 Mpc−1 using the 2M++ velocity field. We also study the peculiar velocity of the galaxy NGC 1052-DF2, concluding that a short ∼13 Mpc distance is not a likely explanation of the anomalously low dark matter fraction of that galaxy.

Specific Effect of Peculiar Velocities on Dark-energy Constraints from Type Ia Supernovae

Abstract Peculiar velocities of Type Ia supernova (SN Ia) host galaxies affect the dark-energy parameter constraints in a small but very specific way: the parameters are biased in a single direction in parameter space that is a priori knowable for a given SN Ia data set. We demonstrate the latter fact with a combination of inference from a cosmological N-body simulation with overwhelming statistics applied to the Pantheon SN Ia data set, then confirm it by simple quantitative arguments. We quantify small modifications to the current analyses that would ensure that the effect of cosmological parameters is essentially guaranteed to be negligible.

The effects of peculiar velocities in SN Ia environments on the local H0 measurement

ABSTRACT The discrepancy between estimates of the Hubble constant (H0) measured from local (z ≲ 0.1) scales and from scales of the sound horizon is a crucial problem in modern cosmology. Peculiar velocities (vpec) of standard candle distance indicators can systematically affect local H0 measurements. We here use 2MRS galaxies to measure the local galaxy density field, finding a notable z < 0.05 underdensity in the SGC-6dFGS region of 27 ± 2 per cent. However, no strong evidence for a ‘Local Void’ pertaining to the full 2MRS sky coverage is found. Galaxy densities are used to measure a density parameter, Δϕ+−, which we introduce as a proxy for vpec that quantifies density gradients along a supernova (SN) line of sight. Δϕ+− is found to correlate with local H0 estimates from 88 Pantheon Type Ia supernovae (SNe Ia; 0.02 < z < 0.05). Density structures on scales of ∼50 Mpc are found to correlate strongest with H0 estimates in both the observational data and in mock data from the MDPL2-Galacticus simulation. Using trends of H0 with Δϕ+−, we can correct for the effects of density structure on local H0 estimates, even in the presence of biased vpec. However, the difference in the inferred H0 estimate with and without the peculiar velocity correction is limited to < 0.1 per cent. We conclude that accounting for environmentally induced peculiar velocities of SN Ia host galaxies does not resolve the tension between local and CMB-derived H0 estimates.

Effect of peculiar velocities on the gravitational potential in cosmological models with perfect fluids

We consider a universe filled with perfect fluid with the constant equation of state parameter ω. In the theory of scalar perturbations, we study the effect of peculiar velocities on the gravitational potential. For radiation with ω=1/3, we obtain the expression for the gravitational potential in the integral form. Numerical calculation clearly demonstrates the modulation of the gravitational potential by acoustic oscillations due to the presence of peculiar velocities. We also show that peculiar velocities affect the gravitational potential in the case of the frustrated network of cosmic strings with ω=−1/3.

The impact of peculiar velocities on the estimation of the Hubble constant from gravitational wave standard sirens

ABSTRACT In this work, we investigate the systematic uncertainties that arise from the calculation of the peculiar velocity when estimating the Hubble constant (H0) from gravitational wave standard sirens. We study the GW170817 event and the estimation of the peculiar velocity of its host galaxy, NGC 4993, when using Gaussian smoothing over nearby galaxies. NGC 4993 being a relatively nearby galaxy, at ∼40 Mpc away, is subject to a significant effect of peculiar velocities. We demonstrate a direct dependence of the estimated peculiar velocity value on the choice of smoothing scale. We show that when not accounting for this systematic, a bias of ${\sim }200~{\rm km\, s^{-1}}$ in the peculiar velocity incurs a bias of ${\sim }4~{\rm km\, s^{-1}\, Mpc^{-1}}$ on the Hubble constant. We formulate a Bayesian model that accounts for the dependence of the peculiar velocity on the smoothing scale and by marginalizing over this parameter we remove the need for a choice of smoothing scale. The proposed model yields $H_0 = 68.6 ^{+14.0} _{-8.5}~{\rm km\, s^{-1}\, Mpc^{-1}}$. We demonstrate that under this model a more robust unbiased estimate of the Hubble constant from nearby GW sources is obtained.

Measuring cosmic velocities with 21 cm intensity mapping and galaxy redshift survey cross-correlation dipoles

We investigate the feasibility of measuring the effects of peculiar velocities in large-scale structure using the dipole of the redshift-space cross-correlation function. We combine number counts of galaxies with brightness-temperature fluctuations from 21 cm intensity mapping, demonstrating that the dipole may be measured at modest significance ($\ensuremath{\lesssim}2\ensuremath{\sigma}$) by combining the upcoming radio survey Canadian Hydrogen Intensity Mapping Experiment with the future redshift surveys of Dark Energy Spectroscopic Instrument (DESI) and Euclid. More significant measurements ($\ensuremath{\lesssim}10\ensuremath{\sigma}$) will be possible by combining intensity maps from the Square Kilometre Array (SKA) with these of DESI or Euclid, and an even higher significance measurement ($\ensuremath{\lesssim}100\ensuremath{\sigma}$) may be made by combining observables completely internally to the SKA. We account for effects such as contamination by wide-angle terms, interferometer noise and beams in the intensity maps, nonlinear enhancements to the power spectrum, stacking multiple populations, sensitivity to the magnification slope, and the possibility that number counts and intensity maps probe the same tracers. We also derive a new expression for the covariance matrix of multitracer redshift-space correlation function estimators with arbitrary orientation weights, which may be useful for upcoming surveys aiming at measuring redshift-space clustering with multiple tracers.

The effects of velocities and lensing on moments of the Hubble diagram

We consider the dispersion on the supernova distance–redshift relation due to peculiar velocities and gravitational lensing, and the sensitivity of these effects to the amplitude of the matter power spectrum. We use the Method-of-the-Moments (MeMo) lensing likelihood developed by Quartin et al., which accounts for the characteristic non-Gaussian distribution caused by lensing magnification with measurements of the first four central moments of the distribution of magnitudes. We build on the MeMo likelihood by including the effects of peculiar velocities directly into the model for the moments. In order to measure the moments from sparse numbers of supernovae, we take a new approach using Kernel density estimation to estimate the underlying probability density function of the magnitude residuals. We also describe a bootstrap re-sampling approach to estimate the data covariance matrix. We then apply the method to the joint light-curve analysis (JLA) supernova catalogue. When we impose only that the intrinsic dispersion in magnitudes is independent of redshift, we find |$\sigma _8=0.44^{+0.63}_{-0.44}$| at the one standard deviation level, although we note that in tests on simulations, this model tends to overestimate the magnitude of the intrinsic dispersion, and underestimate σ8. We note that the degeneracy between intrinsic dispersion and the effects of σ8 is more pronounced when lensing and velocity effects are considered simultaneously, due to a cancellation of redshift dependence when both effects are included. Keeping the model of the intrinsic dispersion fixed as a Gaussian distribution of width 0.14 mag, we find |$\sigma _8 = 1.07^{+0.50}_{-0.76}$|⁠.

Radiative transfer in cylindrical threads with incident radiation

We solved the radiative transfer and statistical equilibrium equations in a two-dimensional cross-section of a cylindrical structure oriented horizontally and lying above the solar surface. The cylinder is filled with a mixture of hydrogen and helium and is illuminated at a given altitude from the solar disc. We constructed simple models made from a single thread or from an ensemble of several threads along the line of sight. This first use of two-dimensional, multi-thread fine structure modelling combining hydrogen and helium radiative transfer allowed us to compute synthetic emergent spectra from cylindrical structures and to study the effect of line-of-sight integration of an ensemble of threads under a range of physical conditions. We analysed the effects of variations in temperature distribution and in gas pressure. We considered the effect of multi-thread structures within a given field of view and the effect of peculiar velocities between the structures in a multi-thread model. We compared these new models to the single thread model and tested them with varying parameters. These new computations show, for the first time, the effect of integrating the radiation emitted in H and He lines by several cylindrical threads that are static or moving along the line of sight. They can be used to interpret high-spatial and spectral resolutions of cylindrical structures found in the solar atmosphere, such as cool coronal loops or prominence threads.

The BOSS–WiggleZ overlap region – II. Dependence of cosmic growth on galaxy type

The anisotropic galaxy 2-point correlation function (2PCF) allows measurement of the growth of large-scale structures from the effect of peculiar velocities on the clustering pattern. We present new measurements of the auto- and cross- correlation function multipoles of 69,180 WiggleZ and 46,380 BOSS-CMASS galaxies sharing an overlapping volume of ~0.2 (Gpc/h)^3. Analysing the redshift-space distortions (RSD) of galaxy 2-point statistics for these two galaxy tracers, we test for systematic errors in the modelling depending on galaxy type and investigate potential improvements in cosmological constraints. We build a large number of mock galaxy catalogues to examine the limits of different RSD models in terms of fitting scales and galaxy type, and to study the covariance of the measurements when performing joint fits. For the galaxy data, fitting the monopole and quadrupole of the WiggleZ 2PCF on scales 24<s<80 Mpc/h produces a measurement of the normalised growth rate $f\sigma_8$(z=0.54)=0.409$\pm$0.059, whereas for the CMASS galaxies we found a consistent constraint of $f\sigma_8$(z=0.54)=0.466$\pm$0.074. When combining the measurements, accounting for the correlation between the two surveys, we obtain $f\sigma_8$(z=0.54)=0.413$\pm$0.054, in agreement with the LCDM-GR model of structure growth and with other survey measurements.

No evidence for bulk velocity from type Ia supernovae

We revisit the effect of peculiar velocities on low-redshift type Ia supernovae. Velocities introduce an additional guaranteed source of correlations between supernova magnitudes that should be considered in all analyses of nearby supernova samples but has largely been neglected in the past. Applying a likelihood analysis to the latest compilation of nearby supernovae, we find no evidence for the presence of these correlations, although, given the significant noise, the data is also consistent with the correlations predicted for the standard ΛCDM model. We then consider the dipolar component of the velocity correlations—the frequently studied ``bulk velocity''—and explicitly demonstrate that including the velocity correlations in the data covariance matrix is crucial for drawing correct and unambiguous conclusions about the bulk flow. In particular, current supernova data is consistent with no excess bulk flow on top of what is expected in ΛCDM and effectively captured by the covariance. We further clarify the nature of the apparent bulk flow that is inferred when the velocity covariance is ignored. We show that a significant fraction of this quantity is expected to be noise bias due to uncertainties in supernova magnitudes and not any physical peculiar motion.

21 cm signal from cosmic dawn: imprints of spin temperature fluctuations and peculiar velocities

The 21 cm brightness temperature $\delta T_{\rm b}$ fluctuations from reionization promise to provide information on the physical processes during that epoch. We present a formalism for generating the $\delta T_{\rm b}$ distribution using dark matter simulations and an one-dimensional radiative transfer code. Our analysis is able to account for the spin temperature $T_{\rm S}$ fluctuations arising from inhomogeneous X-ray heating and Ly$\alpha$ coupling during cosmic dawn. The $\delta T_{\rm b}$ power spectrum amplitude at large scales ($k \sim 0.1$ Mpc$^{-1}$) is maximum when $\sim 10\%$ of the gas (by volume) is heated above the CMB temperature. The power spectrum shows a "bump"-like feature during cosmic dawn and its location measures the typical sizes of heated regions. We find that the effect of peculiar velocities on the power spectrum is negligible at large scales for most part of the reionization history. During early stages (when the volume averaged ionization fraction $\lesssim 0.2$) this is because the signal is dominated by fluctuations in $T_{\rm S}$. For reionization models that are solely driven by stars within high mass ($\gtrsim 10^9\, \rm M_{\odot}$) haloes, the peculiar velocity effects are prominent only at smaller scales ($k \gtrsim 0.4$ Mpc$^{-1}$) where patchiness in the neutral hydrogen density dominates the signal. The conclusions are unaffected by changes in the amplitude or steepness in the X-ray spectra of the sources.

Non-Gaussianity and CMB aberration and Doppler

The peculiar motion of an observer with respect to the CMB rest frame induces a deflection in the arrival direction of the observed photons (also known as CMB aberration) and a Doppler shift in the measured photon frequencies. As a consequence, aberration and Doppler effects induce non trivial correlations between the harmonic coefficients of the observed CMB temperature maps. In this paper we investigate whether these correlations generate a bias on non-Gaussianity estimators fNL. We perform this analysis simulating a large number of temperature maps with Planck-like resolution (lmax = 2000) as different realizations of the same cosmological fiducial model (WMAP7yr). We then add to these maps aberration and Doppler effects employing a modified version of the HEALPix code. We finally evaluate a generalization of the Komatsu, Spergel and Wandelt non-Gaussianity estimator for all the simulated maps, both when peculiar velocity effects have been considered and when these phenomena have been neglected. Using the value v/c = 1.23 × 10−3 for our peculiar velocity, we found that the aberration/Doppler induced non-Gaussian signal is at most of about half of the cosmic variance σ for fNL both in a full-sky and in a cut-sky experimental configuration, for local, equilateral and orthogonal estimators. We conclude therefore that when estimating fNL it is safe to ignore aberration and Doppler effects if the primordial map is already Gaussian. More work is necessary however to assess whether a map which contains non-Gaussianity can be significantly distorted by a peculiar velocity.