The quasi-linear nearby Universe
Cosmology is the science of the largest possible scales - temporal and spatial. Yet, the last two decades have seen the rise of a new approach, coined as near field cosmology, which advocates that studies of the ‘near field’, namely our local ‘patch’ of the Universe, can teach us about the Universe at large. Our paper presents a new approach to cosmography - the non-linear mapping of the total matter distribution of the local Universe, out to distances of roughly half a billion lightyears by means of constrained simulations. The paper brings the previously separated approaches: linear cosmography and massively non-linear simulations, and amalgamates them into one - the quasi-linear reconstruction of the near field. The Constrained Local UniversE Simulations (CLUES) project operates within this context. It aims at mapping - in space and time - the near field from observational data and within the framework of the standard model of cosmology. The data consists of the Cosmicflows database of galaxy distances and velocities and the essence of the standard model, namely a flat universe made of dark energy (Λ) and cold dark matter (the “ΛCDM” model). How local is ‘local’ is context dependent – for simulations such as those in the CLUES it ranges from scales as small as our Milky Way galaxy out to distances of roughly a billion lightyears from us.
The paper in Nature Astronomy is here: go.nature.com/2ujqsKh
The CLUES has traditionally operated along two lines of research, both using the Cosmicflows data and the ΛCDM model. One is cosmography, namely the mapping of the universe at the present epoch. This is done under the assumption of small (hence linear) deviations from an expending homogeneous and isotropic universe. The CLUES cosmography has robustly uncovered the main players that shape the local dynamics (e.g. the Laniakea supercluster and the Dipole Repellers). The other, and more ambitious, line of research consists of running cosmological simulations whose initial conditions are constrained by the Cosmicflows data to reproduce the main features of the near field. Such simulations have focused so far on the formation and present day structure of individual objects - our Local Group and the Virgo cluster of galaxies.
The structure of our local Universe is manifested in the paper by means of the underlying mass density and the three-dimensional velocity fields. Here we go one step further and present the cosmic web that characterizes the large-scale structure of the universe - matter, and thereby also galaxies, are distributed in a pattern made of extended (low density) voids, surrounded by planar sheets. The intersection of which form linear elongated filaments which end up in compact knots. The image shows the entangled web of filaments (in grey) and knots (nuances of red) that together with the voids and sheets, constitute the cosmic web.
The local cosmic web presented here is constructed from the quasi-linear density field, by the analysis of the eigenvalues of the tidal tensor. Yellow spheres in this image show the position of galaxy clusters from the Abell Catalog of rich clusters of galaxies. The central signpost provides scale and orientations with its 50Mpc/h-long arrows pointing to the SGX (red), SGY (green), SGZ (blue) directions of the Supergalactic Coordinate System.
Further three dimensional visualization is presented in this video.
The new algorithm opens a new window into the intricacies of our cosmological neighborhood and provides a link between the mapping of the universe at large and its small scale extremely non-linear dynamics.
Written by: Yehuda Hoffman (Hebrew University, Israel), Noam Libeskind (Leibniz-Institut fur Astrophysik (AIP), Potsdam, Germany) and Daniel Pomarède (CEA Université Paris-Saclay, France)