Abstract
The cosmological principle (CP)—the notion that the Universe is spatially isotropic and homogeneous on large scales—underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive Λ-Cold-Dark-Matter (ΛCDM) model. Yet, tensions have emerged within the ΛCDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, H 0. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the H 0 tension may extend beyond ΛCDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.
| Original language | English |
|---|---|
| Article number | 094001 |
| Pages (from-to) | 1-103 |
| Number of pages | 103 |
| Journal | Classical and Quantum Gravity |
| Volume | 40 |
| Issue number | 9 |
| DOIs | |
| Publication status | Published - 4 May 2023 |
Bibliographical note
Funding Information:We thank the participants of the Asia Pacific Center for Theoretical Physics (APCTP) workshop ‘A Discussion on the Cosmological Principle’, 25-28 October 2021, for extensive discussion on the topic of FLRW. In particular, we thank Stephen Appleby for hosting the workshop and laying the foundations for this review. We also thank Yashar Akrami, Fernando Atrio-Barandela, Thomas Buchert, David Camarena, Guillem Domènech, Pablo Fosalba, Enqrique Gaztañaga, Asta Heinesen, István Horváth, Alexander Kashlinsky, Michael J Longo, Roy Maartens, Roya Mohayaee, Subir Sarkar, Dominik Schwarz, Nathan Secrest, Glenn Starkman, Christos Tsagas and Evgeny Zavarygin for correspondence, discussion and comments. • The work of CJAPM was funded by FEDER through COMPETE 2020 (POCI), and by Portuguese funds through FCT through Projects POCI-01-0145-FEDER-028987 and PTDC/FIS-AST/28987/2017; the author also acknowledges FCT and POCH/FSE (EC) support through Investigador FCT Contract 2021.01214.CEECIND/CP1658/CT0001. • EÓC was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT) (NRF-2020R1A2C1102899). • MMShJ would like to acknowledge SarAmadan Grant No. ISEF/M/400121. • LY was supported by the National Research Foundation of Korea funded through the CQUeST of Sogang University (NRF-2020R1A6A1A03047877). • PP wishes to acknowledge the support of ERC advanced grant ARThUs (Grant No. 740021). • DH acknowledges support from the F.R.S.-FNRS (Belgium) under Grant Nos. IISN 4.4503.19 and PDR T.0116.21. • WZ acknowledges support from the National Key R&D Program of China Grant Nos. 2021YFC2203100, NSFC Nos. 11903030 and 11903033, the Fundamental Research Funds for the Central Universities under Grant Nos. WK2030000036 and WK3440000004. • SJW is supported by the National Natural Science Foundation of China Grant No. 12105344 and the Science Research Grants from the China Manned Space Project with No. CMS-CSST-2021-B01. • NCMM receives funding from the European Union’s Horizon 2020 research and innovation programme, via a Marie Skłodowska-Curie grant (no. 101065772).
Publisher Copyright:
© 2023 IOP Publishing Ltd
Keywords
- Cosmological Principle
- FLRW
- anomalies