Can a multi-tracer approach improve the constraints on the turnover scale at low redshift?

The turnover scale of the power spectrum is related to the size of the particle horizon at the matter-radiation equality, which can be used as a standard ruler to constrain cosmological parameters. In this work, we apply a model-independent method to mock datasets to forecast constraints on the turnover scale below a redshift of 0.5, investigating for the first time with a multi-tracer approach. We find that combining the galaxy density with peculiar velocity does not improve the turnover scale constraints for current or currently planned surveys because either the cosmological volume or the effective number density of peculiar velocities is too low. However, we demonstrate that when combining the galaxy power spectrum from 4HS with the HI power spectrum from SKA1-B2, the constraints on the turnover scale improve by $\sim30\%$ compared to using only a single tracer. We demonstrate for the first time that combining DESI, 4HS, and an SKA Phase 1 Band 2 survey could achieve a $\sim5\%$ level constraint on the turnover scale and a $\sim90\%$ probability of detecting the turnover below a redshift of 0.5. Lastly, we also demonstrate that combining the DESI BGS redshift sample with the LRG, ELG, and QSO samples could break the degeneracy between $r_H$ and $\Omega_m$ and improve their constraints by $\sim25\%$ and $\sim45\%$, respectively, compared to only using the high redshift samples. The constraints on the particle horizon at the matter-radiation equality $r_H$ and the matter density $\Omega_m$ could then further improve by $\sim20\%$ and $\sim30\%$, respectively, when combining the full set of DESI redshift tracers with 4HS and SKA1-B2.