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).