  We show how infinite derivative modifications of gravity impact on the
stochastic background of Gravitational Waves from early Universe. The generic
property of the ghost-free theory fixed on Minkowski space-time is the
emergence of an infinite number of complex mass states when other classical
backgrounds are considered. These additional states are shown to enhance the
power spectrum of scalar perturbations generated during inflation. Current and
future space-based and terrestrial interferometers offer indirect testing
methods for the infinite derivative gravity action, enabling the exploration of
new parameter spaces. In particular, we identify unconventional blue-tilted
Gravitational Wave spectra, presenting a novel approach for testing infinite
derivative quantum gravity in the future.
