The cosmological principle asserts that on sufficiently large scales — vastly exceeding the dimensions of galaxies and their clusters — the universe is expected to be homogeneous. However, data obtained from the DESI experiment concerning the observable universe do not confirm this hypothesis, although they do not conclusively exclude it.
DESI data
A recent publication in the journal Nature by scientists presents findings that may potentially challenge one of the fundamental principles of cosmology — the cosmological principle. This assertion is based on their analysis of data obtained from the Dark Energy Spectroscopic Instrument (DESI).
DESI (Dark Energy Spectroscopic Instrument) is a spectroscopic complex installed on the 4-meter Mayall Telescope at Kitt Peak National Observatory. It collects images of objects in the night sky, but the field in which it is used is quite specific. Its main task is to collect light from 5000 galaxies simultaneously, obtain spectra and determine redshifts. The purpose of its creation is to understand how dark and visible matter are distributed globally in the Universe. Or more precisely, to create a three-dimensional map of the Universe and to investigate the nature of dark energy and the large-scale distribution of matter. To this end, it is trying to determine the spatial coordinates of as many galaxies as possible.
The results of the DESI project are notably impressive. Its most recent data release includes information on 47 million galaxies located within a radius of 11 billion light-years from our position. Consequently, it is imperative that we meticulously examine the pattern of their distribution.
Homogeneous or cellular structure
The cosmological principle states that on sufficiently large scales, the universe is homogeneous and isotropic. Homogeneity means that there are no special places or centers in it—its average properties are the same everywhere. Isotropic means that to an observer, the universe looks the same in all directions. That is why the statistical pattern of the distribution of galaxies in space should be the same no matter where the observer is or where he or she is looking.
It is a well-established fact among scientists that, on a small scale, the assertion does not hold true. Matter is congregated into discrete island galaxies. These galaxies are further organized into clusters, which in turn form extensive filaments that encircle largely vacant voids.
However, scientists are convinced that this structure persists only when considering regions measured in millions of light-years. Once the scale of the region expands to billions of light-years, all voids and cellular structures become negligible. Consequently, the universe, as it is expected to be, will exhibit homogeneity — or, as scientists describe, isotropy.
And now, finally, the DESI data have been released, allowing us to examine a significantly larger portion of the universe. It is evident that the filamentary cellular structure endures even at these scales. The universe continues to exhibit inhomogeneity and anisotropy. Using a new statistical method, the researchers found signs of anisotropy on scales of about one gigaparsec (over 3 billion light-years), which contradicts the classical assumption of isotropicity of the Universe at such distances. However, scientists are not hurried to conclude that the cosmological principle is not being fulfilled.
Ultimately, our perception encompasses only a limited fraction of the entire phenomenon. It is premature to assert that it is solely anisotropic. The fundamental issue lies in the fact that, owing to the constraints imposed by the speed of light, we are inherently incapable of determining the events occurring in the regions that remain beyond our observational reach.
Provided by: phys.org
