K-mouflage Cosmology: the Background Evolution

Understand

We study the cosmology of K-mouflage theories at the background level.

• We show that the effects of the scalar field are suppressed at high matter density in the early Universe and only play a role in the late time Universe where the deviations of the Hubble rate from its $\Lambda$-CDM counterpart can be of the order five percent for redshifts $1 \lesssim z \lesssim 5$.
• Similarly, we find that the equation of state can cross the phantom divide in the recent past and even diverge when the effective scalar energy density goes negative and subdominant compared to matter, preserving the positivity of the squared Hubble rate.
• These features are present in models for which Big Bang Nucleosynthesis is not affected.