  In this work, we show that the zero field excitation spectra in the quantum
spin ice candidate pyrochlore compound \ybti\ is a continuum characterized by a
very broad and almost flat dynamical response which extends up to $1-1.5$ meV,
coexisting or not with a quasi-elastic response depending on the wave-vector.
The spectra do not evolve between 50 mK and 2 K, indicating that the spin
dynamics is only little affected by the temperature in both the short-range
correlated and ordered regimes. Although classical spin dynamics simulations
qualitatively capture some of the experimental observations, we show that they
fail to reproduce this broad continuum. In particular, the simulations predict
an energy scale twice smaller than the experimental observations. This analysis
is based on a careful determination of the exchange couplings, able to
reproduce both the zero field diffuse scattering and the spin wave spectrum
rising in the field polarized state. According to this analysis, \ybti\ lies at
the border between a ferro and an antiferromagnetic phase. These results
suggest that the unconventional ground state of \ybti\ is governed by strong
quantum fluctuations arising from the competition between those phases. The
observed spectra may correspond to a continuum of deconfined spinons as
expected in quantum spin liquids.
