[Paper Review] Dark energy, gravitation and the Copernican principle

Jean–Philippe Uzan|ArXiv.org|Dec 30, 2009

Cosmology and Gravitation Theories10 references4 citations

TL;DR

This paper investigates whether the observed cosmic acceleration is due to dark energy or a violation of the Copernican principle, proposing that we might reside in a large underdense region rather than a cosmological constant. It argues that cosmological observations can test fundamental assumptions like general relativity and the Copernican principle, with implications for the nature of dark energy and the validity of the ΛCDM model.

ABSTRACT

This text aims at discussing the relations between the cosmic acceleration and the theory of gravitation and more generally with the hypotheses underlying the construction of our cosmological model, such as the validity of general relativity on astrophysical scales and the Copernican principle. We hope to illustrate that cosmological data have now the potential of testing these hypotheses, which go beyond the measurements of its parameters.

Motivation & Objective

• To assess whether the observed cosmic acceleration is due to dark energy or a violation of the Copernican principle.
• To evaluate the role of the Copernican principle in constructing cosmological models and its potential breakdown on Hubble scales.
• To explore whether large-scale inhomogeneities or new physics could explain cosmic acceleration without invoking a cosmological constant.
• To investigate whether cosmological observations can test the validity of general relativity and symmetry assumptions on cosmological scales.
• To determine whether the ΛCDM model remains viable or requires extension based on observational constraints on fundamental hypotheses.

Proposed method

• Analyzes the ΛCDM model as a reference cosmological framework assuming general relativity, a cosmological constant, and the Copernican principle.
• Examines the implications of abandoning the Copernican principle on observable scales, proposing that we may live in a large underdense region.
• Considers alternative explanations involving new physics or backreaction effects from inhomogeneities in the large-scale structure.
• Evaluates the role of averaging procedures in cosmology, particularly the need for effective stress-energy tensors on large scales.
• Assesses the viability of multiverse or super-Hubble inhomogeneity scenarios to restore the Copernican principle on larger scales.
• Proposes observational tests to distinguish between dark energy and modified gravity or inhomogeneous models.

Experimental results

Research questions

• RQ1 Can the observed cosmic acceleration be explained without invoking dark energy, by instead violating the Copernican principle on Hubble scales?
• RQ2 To what extent can cosmological observations test the validity of general relativity on cosmological scales?
• RQ3 Is the cosmological constant a fundamental constant, or is its small value a consequence of selection effects in a multiverse?
• RQ4 Can the Copernican principle be restored on super-Hubble scales without invoking a multiverse?
• RQ5 What observational signatures would distinguish a cosmological constant from inhomogeneous models or modified gravity?

Key findings

• The observed cosmic acceleration may not require dark energy if we live in a large underdense region, challenging the Copernican principle on Hubble scales.
• The cosmological constant problem—its value being 60 orders of magnitude smaller than theoretical expectations—remains unresolved and points to a deeper physical issue.
• The Copernican principle can be preserved on super-Hubble scales if there exists a statistical distribution of over- and under-dense regions of all sizes.
• Observational tests of gravity and symmetry hypotheses are essential to determine whether the ΛCDM model is a correct description of the universe.
• If all null tests of general relativity and the Copernican principle fail, the ΛCDM model may remain untestable and unrefined despite its empirical success.
• The possibility of backreaction from large-scale structures as a source of acceleration remains viable but requires deeper theoretical and observational investigation.