Astronomers Yield a New Measurement Of The Hubble Constant Using Fast Radio Bursts (Astronomy)

Fast radio bursts (FRBs) are very short and bright transients visible over extragalactic distances. The mechanism causing the burst is still unknown, but at least some of them are associated with magnetars. A subset of bursts have been found to repeat, either with a fixed period or in cyclical phases of irregular activity. It is therefore possible that the observed FRB events actually fall into a mix of different populations or progenitor mechanisms. The observed radio bursts are bright enough to be visible over extragalactic distances, which opens up exciting possibilities to use FRBs for studying cosmological scales.

The radio pulse from the burst undergoes dispersion while travelling through the ionized intergalactic medium, which is caused by free electrons along the line of sight, most of which are associated with the large-scale structure (LSS). The total dispersion measure therefore increases with the line of sight and provides a distance estimate to the source.

Now, Hagstotz, Reischke and Lilow presented a new measurement of the Hubble constant 𝐻0 based on the dispersion measure – redshift relation of fast radio bursts (FRBs). The method is similar to the determination of 𝐻0 from the luminosity – redshift relation of calibrated SN Ia. The total dispersion measure (DM) is dominated by the cosmological signal for redshifts 𝑧 > 0.3. With the current small sample of nine FRBs with known host galaxies, they constrained the Hubble constant to 𝐻0 = 62.3±9.1 km s¯1 Mpc¯1.

Figure 1. PDFs for the Hubble constant 𝐻0 from the nine individual FRBs with known redshift (thin colored lines) and the joint constraint 𝐻0 = 62.3 ± 9.1 km s¯1 Mpc¯1 of the sample (solid black). They also included the constraint from the six well-identified gold sample FRBs alone (dotted black), which results in 𝐻0 = 62.5 ± 10.1 km s¯1 Mpc¯1. The 𝐻0 values with 2𝜎 error bars from Planck CMB measurements ( blue) and from Cepheid-calibrated supernovae by the SH0ES collaboration are shown as shaded bands for reference. Hagstotz et al.

They also limit the analysis to the six events with the most reliable host identification (the gold sample), with an almost identical result of 𝐻0 = 62.5 ± 10.1 km s¯1 Mpc¯1, since most of the excluded FRBs are located at low redshifts.

“The current main limitations lie in the very small number of available events with sufficient localisation and in the uncertainty about the DM contribution from the host galaxy. Both of these can be solved by a larger sample of localised FRBs. In fact, dedicated searches with excellent angular resolution are expected to detect hundreds of bursts and their host galaxies over the next years.”

— wrote authors of the study

They demonstrated with a forecast that, with the data available in the near future, it is possible to set precision constraints on 𝐻0 fully independent from the CMB or other cosmological measurements, while simultaneously determining the stochastic host halo contribution. Since the cosmological and the stochastic contributions to the DM scale differently with redshift, a sample of a few hundred FRBs can reliably distinguish the two effects. This demonstrates the potential of FRBs for precision measurements of cosmological parameters.

Reference: Steffen Hagstotz, Robert Reischke, Robert Lilow, “A new measurement of the Hubble constant using Fast Radio Bursts”, Arxiv pp. 1-6, 2021.

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