Prior-free cosmological parameter estimation of Cosmicflows-4

Abstract

As tracers of the underlying mass distributions, the peculiar velocities of galaxies are valuable probes of the Universe, allowing us to measure the Hubble constant or to map the large-scale structure and its dynamics. The catalogs of peculiar velocities, however, are noisy, scarce, and prone to various interpretation biases. We measured the radial and bulk flow directly from the largest available sample of peculiar velocities and did not impose a cosmological prior on the velocity field. Furthermore, a minimum assumption on the shape of the radial flow at large distances enabled us to estimate the local Hubble constant. To address these issues, we analyzed the Cosmicflows-4 catalog (CF4), the most extensive catalog of galaxy peculiar velocities, reaching a redshift $z=0.1$. Specifically, we constructed a forward-modeling approach assuming only a flat Universe, which reconstructs the radial and bulk flows of the velocity field directly from measurements of peculiar velocities. Our method was tested on a series of 64 simulated catalogs that mimicked the complex selection function of CF4 in space and in magnitude. Based on our mock data, we propose a simulation-based correction method that we applied to the CF4 data. Our method recovers the radial flow and the direction and magnitude of the bulk flow throughout the covered volume without bias. By estimating the (cosmic) variance of the density field at large distances from the $Λ$CDM model, we were able to extract a value of $75.9\pm1{\rm (stat) km/s/Mpc}$ from the radial inflow. With regard to the bulk flow, a 3$σ$ tension is found with $Λ$CDM around $140\ {\rm Mpc/h}$ and $240\ {\rm Mpc/h}$. In summary, our work confirms the existing tension on the Hubble constant measured locally and a significant tension in the local bulk flow with $Λ$CDM predictions.

Figures (11)

• Figure 1: Visualization of the CF4 data, split by redshift distance $d_z$. Some of the local clusters within the given range are marked with a black cross with their names. Top: Number of grouped galaxies within the HEALPix pixel. Bottom: Median of radial peculiar velocity for each direction. The pixels containing fewer than 10 data points are shaded in gray.
• Figure 2: 3D spatial distribution of the CF4 and mock catalog of each subtype shown as solid and dashed lines, respectively. The shade in a lighter color is the scatter among the mocks.
• Figure 3: Comparison between the average radial peculiar velocity from the simulation and the fit results. In the top panel, the black line represents the median of the true value across 64 mocks, with the shaded gray regions indicating 68%-95%-99.7% percentile intervals, equivalent to 1, 2, and 3$\sigma$. The purple error bars represent the median of the fit results, with 68% error bars. The middle and bottom rows show the difference $\Delta$ and ratio $R$ between the estimated and simulation values, respectively. For each of these parameters, the median across the 64 mocks is shown by the central light purple line, with error bars indicating the percentile interval covering the central 68% of the distribution.
• Figure 4: Same as \ref{['fig:mock-monopole']} but for the three Cartesian components of the bulk flow ($V_{X/Y/Z}$, first three columns) and its magnitude (right-most panel).
• Figure 5: Application of the corrections to the radial flow fit of the mocks. The median of the radial flow in the simulation, expected to be minimal, among the mocks is shown as a black line, with gray shades indicating the 68%-95%-99.7% intervals demonstrating the cosmic variance. Corrections based on differences and ratios are shown as dashed red and blue lines, respectively, with error bars indicating the 68% interval across the 64 corrected radial flows.