Local Galaxy Distribution Does Not Violate the Cosmological Principle
The cosmological principle asserts the statistical homogeneity and isotropy of the Universe on large scales. Earlier analysis of galaxy samples from DESI DR1 revealed coherent anisotropic features in the local distribution at distances up to a gigaparsec, which would contradict this principle. A re-analysis of the same data was performed using the correct comoving distance scale and comparison with the FLAMINGO hydrodynamic cosmological simulation within the standard ΛCDM model. It is shown that with the correct distance scale, the anomaly disappears. The observed structures are consistent with ΛCDM predictions, confirming the validity of the cosmological principle.
Context
The cosmological principle—a cornerstone of modern cosmology—asserts that the Universe is statistically homogeneous and isotropic on sufficiently large scales. Its foundations were laid in the early 20th century by Edwin Hubble and Georges Lemaître. Yet, periodically, studies challenge this, citing giant structures like the Sloan Great Wall. Recently, a group analyzing data from the first release of the spectroscopic DESI survey claimed to detect an anisotropic structure spanning billions of parsecs, which would directly threaten the standard cosmological model. These reports necessitated a thorough re-examination.
Methods
For verification, public DESI DR1 Bright Galaxy Sample data were used—a selection of bright galaxies with known redshifts, from which a continuous cylindrical volume with a radius of about 290 megaparsecs and a thickness of 40 megaparsecs was carved out. In parallel, geometrically identical mock catalogs were built from the largest cosmological hydrodynamical simulation, FLAMINGO, which includes dark matter, dark energy and baryonic physics. Special attention was paid to redshift-space distortions: in real observations, line-of-sight velocities of galaxies smear their positions along the line of sight, and this effect was carefully reproduced. Comparison was made using windowed 2D power spectra measured within a circular aperture without deconvolution, ensuring a direct comparison with no extra assumptions. Importantly, galaxy distances in DESI were computed directly from spectroscopic redshifts using the D3A cosmology, consistent with FLAMINGO.
Results
A visual comparison reveals a striking resemblance between the observed web of galaxies and the FLAMINGO mocks. Quantitatively, this is confirmed by the power spectra: the DESI DR1 curve lies within one standard deviation of the simulation median across most wavenumbers, accounting for redshift-space distortions. The discrepancy increases only on the largest scales due to cosmic variance, but remains statistically insignificant. Meanwhile, when published coordinates for the controversial S2 set are used, the power spectrum shows a giant excess of large-scale power—a deviation beyond 3σ—seemingly contradicting the standard model. A detailed analysis revealed that this excess is an artifact: the authors of S2 mistakenly took luminosity distances in megaparsecs as comoving distances in h⁻¹ Mpc. This stretched scales by a factor of 1.5–1.8 along the line of sight, introducing a spurious anisotropy. When correct comoving distances are restored, the anomaly disappears entirely, and the same structures, including the famous Sloan Great Wall, fall exactly where predicted.
Implications
The results have profound implications for fundamental physics. They demonstrate that the modern standard model ΛCDM with dark matter and dark energy continues to adequately describe large-scale structure up to gigaparsec scales. There's no need for exotic inhomogeneous cosmologies proposed to explain imaginary anomalies. At the same time, this work serves as a caution: even with large modern surveys, it's easy to make a systematic error by confusing distance types, and only direct comparison with simulations that include all selection effects can prevent false discoveries.
Future development
In the future, the theme will advance using even deeper surveys, such as the full DESI data and the upcoming spectroscopic sky-mapping project 4MOST, as well as accounting for subtle effects like gravitational lensing and the evolution of dark energy. Next-generation simulations with higher resolution and improved baryonic physics will allow quantitative characterization of nonlinear contributions to the power spectrum. Moreover, an important goal is developing robust statistical methods that do not rely on a priori distance assumptions, to automatically detect possible violations of isotropy.
Impact
The paper directly impacts cosmology, observational astrophysics, and structure formation theory. Its conclusions bolster confidence in the standard model and influence the interpretation of data from Hubble and other instruments.
Next steps
Immediate next steps include re-analyzing all DESI samples with correct distances and extending the comparison to full FLAMINGO mocks. Also planned is modeling the effect of a possible local void on measurements of dark energy.
Key open problems
This work highlights the delicacy of testing the cosmological principle—one of physics' unsolved questions tied to the deep nature of dark energy and the initial conditions of the Big Bang. Even a small systematic shift in distances can mimic an anomaly, masking real deviations from the standard model, if any exist.
🎯 The Sloan Great Wall, long considered the largest connected structure in the Universe, got 'inflated' to almost 600 h⁻¹ Mpc when using erroneous distances, turning into a gigaparsec monster—a neat example of how size matters, but only with the right ruler!
🎬 The idea of an inhomogeneous Universe with giant structures echoes the imagery of Ivan Yefremov's 'Andromeda Nebula', where the Great Ring or spiral arms of galaxies appear as colossal cosmic formations, challenging notions of uniformity.
Key numbers
- DESI cylinder radius: 290 h⁻¹ Mpc
- slice thickness: 40 h⁻¹ Mpc
- central redshift: 0.13
- galaxies in S2: 36,290
- distance overestimation in S2: by 1.5–1.8 times