Results and open questions on two coasting cosmological models

The ΛCDM Model accurately reproduces most cosmological observations, including primordial nucleosynthesis, the cosmic microwave background radiation, and baryonic acoustic oscillations. However, standard baryonic matter constitutes only 5% of the total content of the ΛCDM universe, while the dominant components – cold dark matter (≈25%) and dark energy (≈70%) – are yet unobserved.

A peculiarity of the Standard Cosmological Model is that the evolution of the universe goes through different phases of acceleration and deceleration, depending on which component is dominant at a certain epoch. Several authors have noticed that a universe that neither accelerates nor decelerates (“coasting”) fares rather well in explaining many observational data.

Here, I will discuss two such coasting models. The first is the Dirac-Milne universe, where antimatter has a negative gravitational mass and is present in equal amounts as ordinary matter, leading to a gravitationally empty universe at large scales. Simulations have shown that the Dirac-Milne cosmology can reproduce with good accuracy both structure formation [1] and the MOND phenomenology [2]. However, its current theoretical basis is purely Newtonian, and including GR effects is a challenge.

The second model is a heuristic proposal to interpret the cosmological constant ∧ as an eigenvalue of the gravitational field equations [3]. This simple interpretation entails that the value of ∧ is no longer arbitrary, but determined by the boundary conditions of the system under consideration, leading to a time-dependent ∧ (compatible with current observations) and a coasting expansion. All this is achieved without appealing to any unobserved “new physics” – be it anti-gravity or modifications of GR. Again, this was done in a semi-Newtonian context, and extensions to GR are challenging.

[1] G. Manfredi, J.-L. Rouet, B. Miller, G. Chardin, Phys. Rev. D 98, 023514 (2018); Phys. Rev. D 102, 103518 (2020).

[2] G. Chardin, Y. Dubois, G. Manfredi, B. Miller, C. Stahl, A&A 652, A91 (2021).

[3] G. Manfredi, General Relativ. Gravit. 53, 31 (2021).


Lundi, 10 janvier, 2022 - 11:00 to 12:00

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CNRS – Institut de Physique et Chimie des Matériaux de Strasbourg

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  • Cosmologie

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