Echo of the quantum bounce

We identify a signature of quantum gravitational effects that survives from the early Universe to the current era: Fluctuations of quantum fields as seen by comoving observers are significantly influenced by the history of the early Universe. In particular, we show how the existence (or not) of a quantum bounce leaves a trace in the background quantum noise that is not damped and would be non-negligible even nowadays. Furthermore, we estimate an upper bound for the typical energy and length scales where quantum effects are relevant. We discuss how this signature might be observed and therefore used to build falsifiability tests of quantum gravity theories.

Figure 1

Scale factor as a function of the proper time for $l=1$, ${\pi }_{\phi }=1000$. The dashed blue curve represents the classical scale factor $a_{\mathrm{c}}(t)L$ and the solid red curve corresponds to the LQC effective scale factor $a_{\mathrm{q}}(t)L$. All quantities are expressed in Planck units, i.e. ${\ell }_{\mathrm{P}}=1$.

Figure 2

(a) Logarithmic plot of the relative difference of the averaged probabilities $E$ (with a regularized denominator) for the sudden switching ${\chi }_1(t)$ as a function of the parameter $l$, for $\mathrm{\Omega }\ll {{\ell }_{\mathrm{P}}}^2/l^3$ and ${\pi }_{\phi }=1000$. The detector is switched on at $T_0=0.01$ (some early time after the bounce). (b) Logarithmic plot of the difference estimator $E$ as a function of the parameter $l$ for the different switching functions, for a switching time scale $\delta =10$ in $1+1\mathrm{D}$. The behavior is identical in all four cases. All full-dimensional quantities are expressed in Planck units.