Abstract
Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cosmological distances. The dispersion measures of FRBs, combined with the redshifts of their host galaxies, has very recently yielded a direct measurement of the baryon content of the universe, and has the potential to directly constrain the location of the “missing baryons”. The first results are consistent with the expectations of ΛCDM for the cosmic density of baryons, and have provided the first constraints on the properties of the very diffuse intergalactic medium (IGM) and circumgalactic medium (CGM) around galaxies. FRBs are the only known extragalactic sources that are compact enough to exhibit diffractive scintillation in addition to showing exponential tails which are typical of scattering in turbulent media. This will allow us to probe the turbulent properties of the circumburst medium, the host galaxy ISM/halo, and intervening halos along the path, as well as the IGM. Measurement of the Hubble constant and the dark energy parameter w can be made with FRBs, but require very large samples of localised FRBs (>103) to be effective on their own—they are best combined with other independent surveys to improve the constraints. Ionisation events, such as for He ii, leave a signature in the dispersion measure—redshift relation, and if FRBs exist prior to these times, they can be used to probe the reionisation era, although more than 103 localised FRBs are required.
Highlights
Cosmological simulations for large-structure formation indicate that the majority of the missing baryons reside in the diffuse Warm-Hot Intergalactic Medium (WHIM), where the intergalactic medium (IGM) has been heated by gravitational shocks and galactic feedback mechanisms to temperatures of 105 –107 K [89] in a cosmic web between clusters of galaxies [86,90,91]
Their “gold sample” of Fast radio bursts (FRBs) is over-plotted in the right panel of Figure 3 and is observed to be consistent with the predictions of ΛCDM cosmology which is shown as a black line
The existence of bright, short-duration FRBs makes it possible to probe the turbulence in the plasma through which they propagate over cosmological distances [74], constraining the turbulence properties in the IGM scale, the largest scale of turbulence in the universe [116]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. A detection of tentative periodicity of 157 days with a duty cycle of 56% was reported for FRB 121102, in which FRB activity is strongly modulated [57] The periodicity of these sources suggests a mechanism for periodic modulation either of the burst emission itself, or through external amplification or absorption [56] and leads to constraints on plausible binary systems in which the progenitor resides [57,58,59]. With just a few bursts, we are already able to detect and characterise the very diffuse intergalactic and circumgalactic media, which are nearly impossible to study using other techniques This is due to several propagation effects that the wavefront of the FRB undergoes as it traverses these media, experiencing effects such as “dispersion” (a frequency dependent propagation delay in the pulse), as well as pulse scattering, scintillation and Faraday rotation. We describe these effects in the following sub-sections and refer the reader to Petroff et al [25] for other propagation effects
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