Speaker
Description
Fast radio bursts (FRBs) offer a promising new probe of primordial non-Gaussianity (PNG) through the scale-dependent bias signature imprinted on their dispersion measures (DMs). However, this signal is degenerate with the poorly-known IGM electron bias $b_e$, degrading constraints on $f_{\rm NL}$ by factors of 1.7–3.3 once this bias is properly marginalized. In this talk, I present a new method for breaking this degeneracy using only FRB data. By cross-correlating the DM field with Shapiro timing delays measured across multi-spacecraft interferometric baselines, we construct a bias-sensitive but power-spectrum-independent observable, $C_\ell^{D\Delta t}$, since the timing signal probes the gravitational potential directly rather than the biased electron density. I derive this cross-spectrum analytically in the Limber approximation and show that Limber corrections are negligible (below $10^{-6}$) at all relevant multipoles. A joint Fisher analysis over $\{f_{\rm NL},\, b_0^e,\, z_{\rm fb}\}$ shows that including the cross-spectrum reduces the uncertainty on the electron bias by a factor of 2.1–5.1, and recovers $\sigma(f_{\rm NL})$ to within a factor of 1.0–1.9 of the fixed-bias benchmark; compared to a 3.3-fold degradation when the cross-spectrum is omitted. I will discuss the resulting single-bin forecasts ($\sigma(f_{\rm NL}) \approx 790$–$1360$ for $N_{\rm FRB} = 10^4$), the minimum interferometric baseline needed for effective self-calibration ($\sim 125$–$200$ AU), and the roadmap; tomography, larger FRB samples, and multi-tracer combinations; toward competitive PNG constraints from this new observable.