QUANTUM gravitational effects are usually ignored in calculations of the formation and evolution of black holes. The justification for this is that the radius of curvature of space-time outside the event horizon is very large compared to the Planck length (Għ/c3)1/2 ≈ 10−33 cm, the length scale on which quantum fluctuations of the metric are expected to be of order unity. This means that the energy density of particles created by the gravitational field is small compared to the space-time curvature. Even though quantum effects may be small locally, they may still, however, add up to produce a significant effect over the lifetime of the Universe ≈ 1017 s which is very long compared to the Planck time ≈ 10−43 s. The purpose of this letter is to show that this indeed may be the case: it seems that any black hole will create and emit particles such as neutrinos or photons at just the rate that one would expect if the black hole was a body with a temperature of (κ/2π) (ħ/2k) ≈ 10−6 (M/M)K where κ is the surface gravity of the black hole1. As a black hole emits this thermal radiation one would expect it to lose mass. This in turn would increase the surface gravity and so increase the rate of emission. The black hole would therefore have a finite life of the order of 1071 (M/M)−3 s. For a black hole of solar mass this is much longer than the age of the Universe. There might, however, be much smaller black holes which were formed by fluctuations in the early Universe2. Any such black hole of mass less than 1015 g would have evaporated by now. Near the end of its life the rate of emission would be very high and about 1030 erg would be released in the last 0.1 s. This is a fairly small explosion by astronomical standards but it is equivalent to about 1 million 1 Mton hydrogen bombs. It is often said that nothing can escape from a black hole. But in 1974, Stephen Hawking realized that, owing to quantum effects, black holes should emit particles with a thermal distribution of energies — as if the black hole had a temperature inversely proportional to its mass. In addition to putting black-hole thermodynamics on a firmer footing, this discovery led Hawking to postulate 'black hole explosions', as primordial black holes end their lives in an accelerating release of energy.

Black Hole Explosions

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

/pdf/chime-frb-discovery-of-eight-new-repeating-fast-radio-burst-a5pqsxwk15.pdf

CHIME/FRB Discovery of Eight New Repeating Fast Radio Burst Sources

Fast radio bursts (FRBs) are brief, bright, extragalactic radio flashes1,2. Their physical origin remains unknown, but dozens of possible models have been postulated3. Some FRB sources exhibit repeat bursts4–7. Although over a hundred FRB sources have been discovered8, only four have been localized and associated with a host galaxy9–12, and just one of these four is known to emit repeating FRBs9. The properties of the host galaxies, and the local environments of FRBs, could provide important clues about their physical origins. The first known repeating FRB, however, was localized to a low-metallicity, irregular dwarf galaxy, and the apparently non-repeating sources were localized to higher-metallicity, massive elliptical or star-forming galaxies, suggesting that perhaps the repeating and apparently non-repeating sources could have distinct physical origins. Here we report the precise localization of a second repeating FRB source6, FRB 180916.J0158+65, to a star-forming region in a nearby (redshift 0.0337 ± 0.0002) massive spiral galaxy, whose properties and proximity distinguish it from all known hosts. The lack of both a comparably luminous persistent radio counterpart and a high Faraday rotation measure6 further distinguish the local environment of FRB 180916.J0158+65 from that of the single previously localized repeating FRB source, FRB 121102. This suggests that repeating FRBs may have a wide range of luminosities, and originate from diverse host galaxies and local environments. Only one repeating fast radio burst has been localized, to an irregular dwarf galaxy; now another is found to come from a star-forming region of a nearby spiral galaxy.

A repeating fast radio burst source localized to a nearby spiral galaxy

More than three-quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to detect, with only a small fraction directly observed in galaxies and galaxy clusters1,2. Censuses of the nearby Universe have used absorption line spectroscopy3,4 to observe the ‘invisible’ baryons, but these measurements rely on large and uncertain corrections and are insensitive to most of the Universe’s volume and probably most of its mass. In particular, quasar spectroscopy is sensitive either to the very small amounts of hydrogen that exist in the atomic state, or to highly ionized and enriched gas4–6 in denser regions near galaxies7. Other techniques to observe these invisible baryons also have limitations; Sunyaev–Zel’dovich analyses8,9 can provide evidence from gas within filamentary structures, and studies of X-ray emission are most sensitive to gas near galaxy clusters9,10. Here we report a measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts; this technique determines the electron column density along each line of sight and accounts for every ionized baryon11–13. We augment the sample of reported arcsecond-localized14–18 fast radio bursts with four new localizations in host galaxies that have measured redshifts of 0.291, 0.118, 0.378 and 0.522. This completes a sample sufficiently large to account for dispersion variations along the lines of sight and in the host-galaxy environments11, and we derive a cosmic baryon density of $${\varOmega }_{{\rm{b}}}={0.051}_{-0.025}^{+0.021}{h}_{70}^{-1}$$ (95 per cent confidence; h70 = H0/(70 km s−1 Mpc−1) and H0 is Hubble’s constant). This independent measurement is consistent with values derived from the cosmic microwave background and from Big Bang nucleosynthesis19,20. The baryon density determined along the lines of sight to localized fast radio bursts is consistent with that determined from the cosmic microwave background and required by Big Bang nucleosynthesis.

A census of baryons in the Universe from localized fast radio bursts.

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.

A Living Theory Catalogue for Fast Radio Bursts

The dispersion measure---redshift relation of fast radio bursts (FRBs), $\mathrm{DM}(z)$, has been proposed as a potential new probe of the cosmos, complementary to existing techniques. In practice, however, the effectiveness of this approach depends on a number of factors, including (but not limited to) the intrinsic scatter in the data caused by intervening matter inhomogeneities. Here, we simulate a number of catalogues of mock FRB observations, and use Markov Chain Monte Carlo techniques to forecast constraints, and assess which parameters will likely be best constrained. In all cases, we find that any potential improvement in cosmological constraints are limited by the current uncertainty on the diffuse gas fraction, ${f}_{\mathrm{d}}(z)$. Instead, we find that the precision of current cosmological constraints allows one to constrain ${f}_{\mathrm{d}}(z)$ and possibly its redshift evolution. Combining Cosmic Microwave Background $+$ Baryon Acoustic Oscillations $+$ Supernovae $+$ ${H}_{0}$ constraints with just 100 FRBs (with redshifts), we find a typical constraint on the mean diffuse gas fraction of a few percent. A detection of this nature would alleviate the ``missing baryon problem,'' and therefore highlights the value of localization and spectroscopic follow-up of future FRB detections.

Probing diffuse gas with fast radio bursts

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) aims to improve constraints on the dark energy equation of state through measurements of large-scale structure at high redshift (0.8 < z < 2.5), while serving as a state-of-the-art fast radio burst detector. Bright galactic foregrounds contaminate the 400–800 MHz HIRAX frequency band, so meeting the science goals will require precise instrument characterization. In this paper we describe characterization of the HIRAX antenna, focusing on measurements of the antenna beam and antenna noise temperature. Beam measurements of the current HIRAX antenna design were performed in an anechoic chamber and compared to simulations. We report measurement techniques and results, which find a broad and symmetric antenna beam for ν<650MHz, and elevated cross-polarization levels and beam asymmetries for ν <700MHz. Noise temperature measurements of the HIRAX feeds were performed in a custom apparatus built at Yale. In this system, identical loads, one cryogenic and the other at room temperature, are used to take a differential (Y-factor) measurement from which the noise of the system is inferred. Several measurement sets have been conducted using the system, involving CHIME feeds as well as four of the HIRAX active feeds. These measurements give the first noise temperature measurements of the HIRAX feed, revealing a ∼60K noise temperature (relative to 30K target) with 40K peak-to-peak frequency-dependent features, and provide the first demonstration of feed repeatability. Both findings inform current and future feed designs.

/pdf/antenna-characterization-for-the-hirax-experiment-1cfk6dbf.pdf

Antenna characterization for the HIRAX experiment

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A Living Theory Catalogue for Fast Radio Bursts

A Living Theory Catalogue for Fast Radio Bursts

Probing diffuse gas with fast radio bursts

Antenna characterization for the HIRAX experiment