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Register a new userA census of baryons in the Universe from localized fast radio bursts
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More than three quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to observe, with only a small fraction directly observed in galaxies and galaxy clusters. Censuses of the nearby Universe have used absorption line spectroscopy to observe these invisible baryons, but these measurements rely on large and uncertain corrections and are insensitive to the majority of the volume, and likely mass. Specifically, quasar spectroscopy is sensitive either to only the very trace amounts of Hydrogen that exists in the atomic state, or highly ionized and enriched gas in denser regions near galaxies. Sunyaev-Zeldovich analyses provide evidence of some of the gas in filamentary structures and studies of X-ray emission are most sensitive to gas near galaxy clusters. Here we report the direct measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts (FRBs), thus utilizing an effect that measures the electron column density along each sight line and accounts for every ionised baryon. We augment the sample of published arcsecond-localized FRBs with a further four new localizations to host galaxies which have measured redshifts of 0.291, 0.118, 0.378 and 0.522, completing a sample sufficiently large to account for dispersion variations along the line of sight and in the host galaxy environment to derive a cosmic baryon density of $Omega_{b} = 0.051_{-0.025}^{+0.021} , h_{70}^{-1}$ (95% confidence). This independent measurement is consistent with Cosmic Microwave Background and Big Bang Nucleosynthesis values.
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Revealing the cosmic reionisation history is at the frontier of extragalactic astronomy. The power spectrum of the cosmic microwave background (CMB) polarisation can be used to constrain the reionisation history. Here we propose a CMB-independent method using fast radio bursts (FRBs) to directly measure the ionisation fraction of the intergalactic medium (IGM) as a function of redshift. FRBs are new astronomical transients with millisecond timescales. Their dispersion measure (DM$_{rm IGM}$) is an indicator of the amount of ionised material in the IGM. Since the differential of DM$_{rm IGM}$ against redshift is proportional to the ionisation fraction, our method allows us to directly measure the reionisation history without any assumption on its functional shape. As a proof of concept, we constructed mock non-repeating FRB sources to be detected with the Square Kilometre Array, assuming three different reionisation histories with the same optical depth of Thomson scattering. We considered three cases of redshift measurements: (A) spectroscopic redshift for all mock data, (B) spectroscopic redshift for 10% of mock data, and (C) redshift estimated from an empirical relation of FRBs between their time-integrated luminosity and rest-frame intrinsic duration. In all cases, the reionisation histories are consistently reconstructed from the mock FRB data using our method. Our results demonstrate the capability of future FRBs in constraining the reionisation history.
The $Lambda$CDM model successfully explains the majority of cosmological observations. However, the $ Lambda$CDM model is challenged by Hubble tension, a remarkable difference of Hubble constant $H_0$ between measurements from local probe and the prediction from Planck cosmic microwave background observations under $ Lambda$CDM model. So one urgently needs new distance indicators to test the Hubble tension. Fast radio bursts (FRBs) are millisecond-duration pulses occurring at cosmological distances, which are attractive cosmological probes. However, there is a thorny problem that the dispersion measures (DMs) contributed by host galaxy and the inhomogeneities of intergalactic medium cannot be exactly determined from observations. Previous works assuming fixed values for them bring uncontrolled systematic error in analysis. A reasonable approach is to handle them as probability distributions extracted from cosmological simulations. Here we report a measurement of ${H_0} = 64.67^{+5.62}_{-4.66} {rm km s^{-1} Mpc^{-1}}$ using fourteen localized FRBs, with an uncertainty of 8.7% at 68.3 per cent confidence. Thanks to the high event rate of FRBs and localization capability of radio telescopes (i.e., Australian Square Kilometre Array Pathfinder and Very Large Array), future observations of a reasonably sized sample ($sim$100 localized FRBs) will provide a new way of measuring $ H_0$ with a high precision ($sim$2.6%) to test the Hubble tension.
Nature of dark energy remains unknown. Especially, to constrain the time variability of the dark-energy, a new, standardisable candle that can reach more distant Universe has been awaited. Here we propose a new distance measure using fast radio bursts (FRBs), which are a new emerging population of $sim$ ms time scale radio bursts that can reach high-$z$ in quantity. We show an empirical positive correlation between the time-integrated luminosity (L$_{ u}$) and rest-frame intrinsic duration ($w_{rm int,rest}$) of FRBs. The L$_{ u}-w_{rm int,rest}$ correlation is with a weak strength but statistically very significant, i.e., Pearson coefficient is $sim$ 0.5 with p-value of $sim$0.038, despite the smallness of the current sample. This correlation can be used to measure intrinsic luminosity of FRBs from the observed $w_{rm int,rest}$. By comparing the luminosity with observed flux, we measure luminosity distances to FRBs, and thereby construct the Hubble diagram. This FRB cosmology with the L$_{ u}-w_{rm int,rest}$ relation has several advantages over SNe Ia, Gamma-Ray Burst (GRB), and well-known FRB dispersion measure (DM)-$z$ cosmology; (i) access to higher redshift Universe beyond the SNe Ia, (ii) high event rate that is $sim$ 3 order of magnitude more frequent than GRBs, and (iii) it is free from the uncertainty from intergalactic electron density models, i.e., we can remove the largest uncertainty in the well-debated DM-$z$ cosmology of FRB. Our simulation suggests that the L$_{ u}-w_{rm int,rest}$ relation provides us with useful constraints on the time variability of the dark energy when the next generation radio telescopes start to find FRBs in quantity.
We analyze the sources of free electrons that produce the large dispersion measures, DM $approx 300-1600$ (in units cm$^{-3}$ pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce DM $sim 25-60$ cm$^{-3}$ pc from ionized gas in their disk, disk-halo interface, and circumgalactic medium. Toward an FRB source at redshift $z$, a homogeneous IGM containing a fraction $f_{rm IGM}$ of cosmological baryons will produce DM $= (935~{rm cm}^{-3}~{rm pc}) f_{rm IGM} , h_{70}^{-1} I(z)$, where $I(z) = (2/3 Omega_m)[ { Omega_m(1+z)^3 + Omega_{Lambda} }^{1/2} - 1 ]$. A structured IGM of photoionized Ly-alpha absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, $f_b(N,z)$, of H I (Lya-forest) absorbers in column density and redshift with ionization corrections and scaling relations from cosmological simulations. An analytic formula for DM($z$) applied to observed FRB dispersions suggests that $z_{rm FRB} approx 0.2-1.5$ for an IGM containing a significant baryon fraction, $f_{rm IGM} = 0.6pm0.1$. Future surveys of the statistical distribution, DM($z)$, of FRBs identified with specific galaxies and redshifts, can be used to calibrate the IGM baryon fraction and distribution of Ly-alpha absorbers. Fluctuations in DM at the level $pm10$ cm$^{-3}$ pc will arise from filaments and voids in the cosmic web.
At present, we have almost as many theories to explain Fast Radio Bursts as we have Fast Radio Bursts observed. This landscape will be changing rapidly with CHIME/FRB, recently commissioned in Canada, and HIRAX, under construction in South Africa. This is an opportune time to review existing theories and their observational consequences, allowing us to efficiently curtail viable astrophysical models as more data becomes available. In this article we provide a currently up to date catalogue of the numerous and varied theories proposed for Fast Radio Bursts so far. We also launch an online evolving repository for the use and benefit of the community to dynamically update our theoretical knowledge and discuss constraints and uses of Fast Radio Bursts.
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