Holographic inflation
  We apply the holographic principle at the early universe, obtaining an
inflation realization of holographic origin. Such a consideration has equal
footing with its well-studied late-time application, and moreover the decrease
of the horizons at early times naturally increases holographic energy density
at inflationary scales. Taking as Infrared cutoff the particle or future event
horizons, and adding a simple correction due to the Ultraviolet cutoff, whose
role is non-negligible at the high energy scales of inflation, we result in a
holographic inflation scenario that is very efficient in incorporating
inflationary requirements and predictions. We first extract analytically the
solution of the Hubble function in an implicit form, which gives a scale factor
evolution of the desired e-foldings. Furthermore, we analytically calculate the
Hubble slow-roll parameters and then the inflation-related observables, such as
the scalar spectral index and its running, the tensor-to-scalar ratio, and the
tensor spectral index. Confronting the predictions with Planck 2018
observations we show that the agreement is perfect and in particular deep
inside the 1$\sigma$ region.
