Sound Speed Resonance in the Gravitational Wave Background as a probe for non-standard early universe cosmologies

Abstract

Gravitational waves constitute a powerful probe of the underlying theory of gravity. In extensions of general relativity, additional degrees of freedom, such as scalar fields in the gravitational sector, can modify their propagation through changes in the effective friction term and propagation speed. These modifications may potentially induce resonant phenomena leading to distinctive signatures in the gravitational wave spectrum. One important aspect to be investigated is whether the resonances can be strong enough to enhance the underlying background of primordial tensor modes to levels detectable by upcoming gravitational wave detectors, such as LISA or the Einstein telescope. The characteristic peaks in the SBGW spectrum depend on the parameters of the resonant model as well as on the parameters of the primordial tensor spectrum, such as $r$ and $n_{t}$. Thus these resonance effects open a powerful pathway to explore physics of the very early Universe by amplifying otherwise feeble signals to experimentally detectable levels. Here we analyze how the signals of the primordial Universe can resonate in these scenarios, bringing the early universe physics into the realm of experimental access.

Figures (8)

• Figure 1: SGWB spectrum for the astrophysical foreground. We shown the unresolved Galactic WD in blue, the unresolved Extragalactic WD in red and the sum of the BBH and BNS in magenta. In Black, we show the LISA's sensitivity curve.
• Figure 2: Comparison of the resonant GW spectrum with $r = 0.035$ with the astrophysical foreground generated by the unresolved galactic white dwarf (blue); extragalactic white dwarf (red), the sum of the unresolved contribution to binary neutron star, binary black holes and black hole - neutron star binaries (magenta). In the left panel, the teal curve shows the resonance effect on a blue-tilted background with $n_t = 0.22$. In the right panel, the orange curve represents the resonance on a background with $n_t = 0.135$. The gray curve shows the resonance effect for a standard slow-roll inflationary scenario (where the spectral index follow the inflationary slow-roll consistency relation $n_t \sim 0$). The red shaded region represents the parameter space excluded by the BBN constraint.
• Figure 3: Plot comparing all the three models. In this plot, the background has $r = 0.035$. We show the case with $n_t \sim 0$ in gray, the blue tilted case with $n_t=0.135$ in orange and $n_t = 0.22$ in teal. The legend is the same as in Figure \ref{['fig:r=0.035-full']}.
• Figure 4: Comparison of GW spectra for the resonance effect with a background parametrized by $r=1 \cdot 10^{-3}$. The colors are the same as in Fig. \ref{['fig:r=0.035-full']}. In this case, the curve represents the same resonance effect happening in a blue-tilted background with $n_t = 0.336$ (teal) in the left panel and with $n_t = 0.240$ (orange) in the right panel. The red shaded region represents the excluded region by the BBN constraint and the astrophysical foreground is the same as before.
• Figure 5: Comparison of different values of tensor spectral index in the range $n_t \in [0,1]$. In this plot, we consider $r = 10^{-10}$. Specifically, we show the curves where $n_t = 0.73$ (in orange), which would be able to satisfy the BBN bound and the case with $n_t=0.81$ (in red), which the background signal would be inside LISA's sensitivity but is ruled out by the BBN constraint. LISA's sensitivity curve is in black. The light red shaded region represents the region excluded BBN constraints.