Model-independent reconstruction shows that early-universe modifications resolving the Hubble tension exist at the background level, requiring a smooth ~15% pre-recombination expansion rate enhancement.
Primordial gravitational waves and the H0-tension problem
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abstract
We analyse the H0-tension problem in the context of models of the early universe that predict a blue tilted spectrum of primordial gravitational waves (GW's). By considering the GW's contribution, Neff^GW, to the effective number of relativistic degrees of freedom, Neff, and assuming standard particle physics, we discuss the effects of Neff^GW on the background expansion, especially the constraints on the Hubble parameter H0. We analyse three scenarios which take into account the contribution of Neff^GW and perform a statistical study using recent data of cosmic microwave background, baryon acoustic oscillation, the latest measurement of the local expansion rate, along with the LIGO constraints on the tensor to scalar ratio and the tensor index. For the models explored, we show that an additional contribution from the primordial GW's background to Neff does not solve but alleviate the current H0-tension problem.
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Shear viscosity adds damping to primordial gravitational waves, producing an extra red tilt for constant viscosity-to-Hubble ratio and a k-dependent blue tilt from freeze-out in the electron-photon-baryon plasma with fractional difference of order 10^{-3}.
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Geometric Constraints on the Pre-Recombination Expansion History from the Hubble Tension
Model-independent reconstruction shows that early-universe modifications resolving the Hubble tension exist at the background level, requiring a smooth ~15% pre-recombination expansion rate enhancement.
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Freeze-out and spectral running of primordial gravitational waves in viscous cosmology
Shear viscosity adds damping to primordial gravitational waves, producing an extra red tilt for constant viscosity-to-Hubble ratio and a k-dependent blue tilt from freeze-out in the electron-photon-baryon plasma with fractional difference of order 10^{-3}.