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Primordial Black Holes – Observational Characteristics of the Final Evaporation

The formation of Primordial Black Holes (PBHs) is predicted in many theories of the early universe. According to Ukwatta et al. PBHs could have masses that range from the Planck mass, m_p = 2.18 x 10^-8 kg to 10⁵ solar masses or higher depending on the size of the universe at its formation. A Black Hole (BH) has a Hawking temperature which is proportional to its mass. Therefore a black hole that is sufficiently small will radiate quasi-thermally particles at a rate that is ever increasing as emission reduces its mass and raises its temperature. According to Ukwatta et al., the final moments of this evaporation should be explosive, and the description of it is dependent on the particle physics model. This paper presents the results of the study by Ukwatta et al. investigating the final few seconds of this evaporation phase of a black hole, using the Standard Model and incorporating the most recent results obtained from the Large Hadron Collider (LHC), and provides a new parameterisation for the instantaneous emission spectrum. Ukwatta et al. calculate for the first time primordial black hole burst light curves in the GeV/TeV energy range that are energy dependent. They also explored the primordial black hole burst search methods and potential observational signals of primordial black holes. As a result of this study they have found a unique signature in the PBH burst light curves that may be detectable by GeV/TeV gamma ray (γ-ray) observatories such as the High Altitude Water Cerenkov (HAWC) observatory. Ukwatta et al. also discuss the implications of theories beyond the Standard Model on the PBH burst observational characteristics, including potential sensitivity of the instantaneous photon detection rate to a squark threshold in the 5-10 TeV range.

The production of PBHs is predicted by many current theories of the early universe (Carr et al., 2010). PBHs with masses in the order of, or smaller than, the size of the cosmological horizon at the time of formation could have been formed by cosmological density fluctuations, as well as other mechanisms, such as those associated with phase transitions in the early universe. PBHs could form at times from the Planck time to 1 second or later after the Big Bang, depending on the mechanism of formation. Therefore the initial mass of the PBH could be as small as the Planck mass or as massive as 10⁵ solar masses, or even higher.

It was shown in 1974 by Hawking that convolving quantum field theory, thermodynamics and general relativity that a black hole has a temperature that is inversely proportional to its mass and emits particle and photon radiation with thermal spectra (Hawking, 1974). The mass of the black hole decreases as this radiation is emitted leading to increases in the temperature and flux of the black hole. A primordial black hole that had an initial mass of ~ 5.0 x 10¹¹ kg when it formed in the early universe should, according to Ukwatta et al., be expiring at the present (MacGibbon, Carr & Page, 2008) with a burst of high energy particles, including gamma-rays in the MeV to TeV energy range. Therefore PBHs are candidates for gamma-ray burst (GRB) progenitors (Halzen, Zas, MacGibbon & Weekes, 1991).

Valuable insight into many areas of physics including the early universe, high energy particle physics, and the convolution of gravitation with thermodynamics, would be provided by confirmed detection of a PBH evaporation event. Conversely, important limits would be placed on models of the early universe by the non-detection of PBH evaporation events in sky searches. The constraining of the cosmological density fluctuation spectrum in the early universe on scales that are smaller than those that are constrained by the cosmic microwave background is one of the most important reasons to search for PBHs. Ukwatta et al. say there is particular interest in whether PBHs form from the quantum fluctuations that are associated with many different types of inflationary scenarios (Carr, Kohri, Sendouda & Yokoyama, 2010). Inflationary models can therefore be formed by the detection of upper limits on the number density of PBHs.

Ukwatta et al. suggest PBHs may be detectable by virtue of several effects. E.g., PBHs that have masses that are of planetary scale may be detectable by their gravitational effects in microlensing observations (Griest et al., 2011); or distinct, observable radiation may be produced by the accretion of matter onto PBHs in relatively dense environments (Trofimenko, 1990). It has been suggested, however, that such environments should be rare and therefore difficult to use as probes of the cosmological or local distributions of PBHs.

The properties of the final burst of a black hole depend on the physics that govern the production and decay of high-energy particles. The temperature of a black hole increases as the black hole evaporates and loses mass over its lifetime. The higher the number of fundamental degrees of freedom, the faster and more powerful the final burst from the black hole will be. The details of the spectra that have been predicted differ according to the high-energy particle physics model. In the Standard Evaporation Model (SEM) incorporating the Standard Model of particle physics, a black hole should directly Hawking-radiate the fundamental Standard Model particles which have de Broglie wavelengths that are of the order of the size of the black hole (MacGibbon & Webber, 1990). Once the radiation energy approaches the Quantum Chromodynamics (QCD) confinement scale (~ 200-300 MeV), quarks and gluons will be directly emitted (MacGibbon & Webber, 1990). The quarks and gluons should fragment and hadronise (analogous to jets observed in high-energy collisions in terrestrial accelerators) into particles stable on astrophysical timescales (MacGibbon, Carr & Page, 2008) as they stream away from the black hole. In the SEM therefore, the black hole that is evaporating is an astronomical burst of photons, neutrons, electrons, positrons, protons and anti-protons (and for sources that are close enough, neutrons and anti-neutrons (Smith et al., 2013; Keivani et al., 2015).

Sources & Further reading

1. Ukwatta, T. N., D. R. Stump, J. T. Linnemann, J. H. MacGibbon, S. S. Marinelli, T. Yapici and K. Tollefson (2016). "Primordial Black Holes: Observational characteristics of the final evaporation." Astroparticle Physics 80: 90-114.