Joint analysis of 6dFGS and SDSS peculiar velocities for the growth rate of cosmic structure and tests of gravity. (arXiv:2007.04993v1 [astro-ph.CO])

<a href="http://arxiv.org/find/astro-ph/1/au:+Said_K/0/1/0/all/0/1">Khaled Said</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Colless_M/0/1/0/all/0/1">Matthew Colless</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Magoulas_C/0/1/0/all/0/1">Christina Magoulas</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lucey_J/0/1/0/all/0/1">John R. Lucey</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hudson_M/0/1/0/all/0/1">Michael J. Hudson</a>

Measurement of peculiar velocities by combining redshifts and distance

indicators is a powerful way to measure the growth rate of cosmic structure and

test theories of gravity at low redshift. Here we constrain the growth rate of

structure by comparing observed Fundamental Plane peculiar velocities for 15894

galaxies from the 6dF Galaxy Survey (6dFGS) and Sloan Digital Sky Survey (SDSS)

with predicted velocities and densities from the 2M$++$ redshift survey. We

measure the velocity scale parameter $beta equiv {Omega_m^gamma}/b =

0.372^{+0.034}_{-0.050}$ and $0.314^{+0.031}_{-0.047}$ for 6dFGS and SDSS

respectively, where $Omega_m$ is the mass density parameter, $gamma$ is the

growth index, and $b$ is the bias parameter normalized to the characteristic

luminosity of galaxies, $L^*$. Combining 6dFGS and SDSS we obtain $beta=

0.341pm0.024$, implying that the amplitude of the product of the growth rate

and the mass fluctuation amplitude is $fsigma_8 = 0.338pm0.027$ at an

effective redshift $z=0.035$. Adopting $Omega_m = 0.315pm0.007$ as favoured

by Planck and using $gamma=6/11$ for General Relativity and $gamma=11/16$ for

DGP gravity, we get $S_8(z=0) = sigma_8 sqrt{Omega_m/0.3} =0.637 pm 0.054$

and $0.741pm0.062$ for GR and DGP respectively. This measurement agrees with

other low-redshift probes of large scale structure but deviates by more than

$3sigma$ from the latest Planck CMB measurement. Our results favour values of

the growth index $gamma > 6/11$ or a Hubble constant $H_0 >

70$,km,s$^{-1}$,Mpc$^{-1}$ or a fluctuation amplitude $sigma_8 < 0.8$ or

some combination of these. Imminent redshift surveys such as Taipan, DESI,

WALLABY, and SKA1-MID will help to resolve this tension by measuring the growth

rate of cosmic structure to 1% in the redshift range $0 < z < 1$.

Measurement of peculiar velocities by combining redshifts and distance

indicators is a powerful way to measure the growth rate of cosmic structure and

test theories of gravity at low redshift. Here we constrain the growth rate of

structure by comparing observed Fundamental Plane peculiar velocities for 15894

galaxies from the 6dF Galaxy Survey (6dFGS) and Sloan Digital Sky Survey (SDSS)

with predicted velocities and densities from the 2M$++$ redshift survey. We

measure the velocity scale parameter $beta equiv {Omega_m^gamma}/b =

0.372^{+0.034}_{-0.050}$ and $0.314^{+0.031}_{-0.047}$ for 6dFGS and SDSS

respectively, where $Omega_m$ is the mass density parameter, $gamma$ is the

growth index, and $b$ is the bias parameter normalized to the characteristic

luminosity of galaxies, $L^*$. Combining 6dFGS and SDSS we obtain $beta=

0.341pm0.024$, implying that the amplitude of the product of the growth rate

and the mass fluctuation amplitude is $fsigma_8 = 0.338pm0.027$ at an

effective redshift $z=0.035$. Adopting $Omega_m = 0.315pm0.007$ as favoured

by Planck and using $gamma=6/11$ for General Relativity and $gamma=11/16$ for

DGP gravity, we get $S_8(z=0) = sigma_8 sqrt{Omega_m/0.3} =0.637 pm 0.054$

and $0.741pm0.062$ for GR and DGP respectively. This measurement agrees with

other low-redshift probes of large scale structure but deviates by more than

$3sigma$ from the latest Planck CMB measurement. Our results favour values of

the growth index $gamma > 6/11$ or a Hubble constant $H_0 >

70$,km,s$^{-1}$,Mpc$^{-1}$ or a fluctuation amplitude $sigma_8 < 0.8$ or

some combination of these. Imminent redshift surveys such as Taipan, DESI,

WALLABY, and SKA1-MID will help to resolve this tension by measuring the growth

rate of cosmic structure to 1% in the redshift range $0 < z < 1$.

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