In another question dealing with the earliest explored phenomenon in the Universe the observing limit of the CMB was mentioned. Prior to CMB the Universe was opaque to photons, I believe. I'm wondering if conditions before "first light" would have allowed sufficient matter clumping to produce currently detectable, or potentially future detectable, gravitational signatures. The irregularities in the CMB do indicate clumping of some sort, do they not? I'm wondering if LIGO and it's successors will have a look back time farther than photon-based astronomy.
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Yes. Gravitational wave observatories like the proposed eLISA laser interferometer may be able to detect gravitational waves that originate from the early moments of the big bang itself. If some part of the big bang energy goes into gravitational waves then those waves will be redshifted by expansion and waves produced in the first $<10^{-10}$ s should be present in eLISA's frequency sensitivity window (Amaro-Seaone et al. 2012; Bartolo et al. 2016; Ricciardone et al. 2017). This might probe exotic physics like inflation, extra dimensions, phase transitions and cosmic strings.
