Tag Archives: #primordialblackholes

What Is The Probability And Consequences Of Collision Of Primordial Black Holes With Earth? (Cosmology)

Sohrab Rahvar investigated the scenario of collision of primordial black holes (PBHs) with earth. He showed that this collision has different consequences as heating the interior of the earth through dynamical friction and accretion processes. The findings of this study recently appeared in Arxiv.

Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. They are plausible candidates of dark matter.

Now, by assuming that PBHs fill the dark content of the Milky Way Galaxy in the Galactic halo and dark disk, Sohrab Rahvar calculated the probability of collision of the PBHs with Earth. He showed that the black holes with the mass M < 1015 gr have the chance of more than one impact per billion years with Earth and the rate of black holes that could be trapped in the interior of the earth is almost zero. But, how can we say so confidently that, there are no primordial black holes trapped inside the earth?

“Well, since the velocity of black holes in the halo and the dark disk is high, the dissipation process inside the earth is not effective to decelerate the black holes and black holes do not sink inside the earth where in this case black holes could heat the interior of the earth and finally swallow the whole mass of earth. The number of this event for M ≃ 1015 gr is about 10¯11 Gyr¯1. This calculation assures that probability of primordial black holes being trapped inside the earth is almost zero.”

Another important point to be noted is that, those black holes that cross the earth, heat the interior of the earth through various mechanisms like through dynamical friction which can generate heat in the range of 105–108 J and through accretion process by about 1015 J.

Moreover, he calculated the energy released by a black hole collision with earth and compared it with the impact of asteroids on the earth. He have shown that the accretion process by the black holes is the dominant process for energy release. The amount of energy from this collision is comparable with a kilometer size asteroid where it happens four orders of magnitude more frequently than a black hole collision.

Finally, he concluded that the likelihood of the dangerous impact of primordial black holes with the earth is very low.


Reference: Sohrab Rahvar, “Possibility of Primordial black holes Collision with Earth and the Consequences”, Arxiv, pp. 1-5, 2021.
arXiv:2107.11139


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How Production Of Primordial Black Holes Take Place In NonMinimal Derivative Coupling Inflation? (Cosmology / Quantum)

Heydari and Karami investigated the generation of Primordial Black Holes (PBHs) with the aid of gravitationally increased friction mechanism originated from the NonMinimal field Derivative Coupling (NMDC) to gravity framework, with the quartic potential. By assigning a coupling parameter as a two-parted function of inflation field and fine-tuning of 4 parameter cases of the model they showed that they could acquire an epoch of ultra slow-roll inflation on scales smaller than CMB scale making the inflaton slow down, sufficient to generate PBHs. Their study recently appeared in Arxiv.

You may have heard the concept of primordial black holes (PBHs) generation from the over-dense regions of the early universe. For the generation of primordial black holes (PBHs) during Radiation Dominated (RD) era, production of a large enough amplitude of primordial curvature perturbations (R) during inflationary epoch is necessary. Overdense regions can be formed when superhorizon scales associated with the large amplitude of R become subhorizon during RD era, and gravitationally collapse of these overdensities generate PBHs.

Meaning, PBHs generation requires an enhancement in the power spectrum of R to order 10¯2 at scales smaller than CMB scales. Several techniques have been employed for multiplying the amplitude of the power spectrum of R at small scale by seven orders of magnitude in comparison with CMB scales. One of the proper ways to achieve a rise in the scalar power spectrum is a brief period of Ultra Slow-Roll (USR) inflation due to declining the speed of inflaton field via gravitationally enhanced friction. The framework of NonMinimal Derivative Coupling to gravity (NMDC) beside the fine-tuning of the parameters of the model can give rise to increase friction gravitationally.

The nonminimal derivative coupling model is a subclass of a generic scalar-tensor theory with second-order equations of motion namely Horndeski theory, which prevents the model from negative energy and pertinent instabilities. A characteristic of the nonminimal field derivative coupling to gravity is that the gravitationally increased friction mechanism can be applied for generic steep potentials such as quartic potential.

Now, Heydari and Karami investigated the generation of Primordial Black Holes (PBHs) with the aid of gravitationally increased friction mechanism originated from the NonMinimal field Derivative Coupling (NMDC) to gravity framework, with the quartic potential.

© Heydari and Karami

By assigning the coupling parameter as a two-parted function of inflaton field, and fine-tuning of the four parameter cases (A, B, C, and D) of the model (see Table I), they showed that, we could acquire an epoch of ultra slow-roll inflation on scales smaller than CMB scale making the inflaton slow down due to high friction. This enables them to achieve enough enhancement in the amplitude of curvature perturbations power spectra to generate PBHs with masses of order 10 M for Case A (stellar mass), 10¯6 M for Case B (earth mass), 10¯13 M for Case C, and 10¯15 M for Case D (asteroid mass).

Their results indicated that PBHs of case A is suitable to describe GWs and LIGO events, Case B can be useful to expound microlensing events in OGLE data, and PBHs of cases C and D can be interesting candidates for composing around 98.32% and 99.11% of dark matter (DM) content of the universe (see Table II and Fig. 1)

FIG. 1. The PBHs abundance (fPBH) in terms of PBHs mass (M) for Case A (purple line), Case B (green line), Case C (red line), and Case D (blue line). The red spots indicate the upper limit on the PBH abundance owing to the upper limit on the LIGO event merger rate. The brown shadowy zone signifies the permitted zone of PBH abundance from the ultrashort-timescale microlensing events in the OGLE data. The other shadowy areas demonstrate the current observational restrictions on the fractional abundance of PBHs comprising extragalactic gamma rays from PBH evaporation (EGγ), galactic center 511 keV γ-ray line (INTEGRAL), white dwarf explosion (WD), microlensing events with Subaru HSC (Subaru HSC), with the Kepler satellite (Kepler), with EROS/MACHO (EROS/MACHO), and accretion constraints from CMB © Heydari and Karami

Additionally, they inquired generation of the induced GWs subsequent to PBHs formation for all cases of their model. Their calculation of current density parameter spectra (ΩGW0) indicated that, all cases have climaxes at contrasting frequencies with nearly identical heights of order 10¯8. The climaxes of ΩGW0 for cases A and B have placed at frequencies 10¯10Hz and 10¯7 Hz, respectively, and both cases can be traced via the SKA detector. Moreover, the spectra of ΩGW0 for Cases A and B have climaxes localized at mHz and cHz bands which are tracked down by LISA, TaiJi, and TianQin (see Fig. 2). Hence, validity of their model can be assessed in view of GWs via the extricated data of these detectors.

Fig 2. The present induced GWs energy density parameter (ΩGW0) with respect to frequency. The solid purple, green, red, and blue lines associate with Cases A, B, C, and D of Table I, respectively. The power-law form of ΩGW0 is illustrated by black dashed line for Case D. © Heydari and Karami

Finally, they demonstrated that in the vicinity of climaxes, the spectra of density parameter behave as a power-law function with respect to frequency (ΩGW0 (f) ∼ (f/fc)^n). Also, in the infrared regime f<<fc, the power index satifies the relation n = 3 – 2/ ln(fc/f).


Reference: Soma Heydari, Kayoomars Karami, “Primordial black holes in nonminimal derivative coupling inflation driven by quartic potential”, Arxiv, pp. 1-28, 2021.
arXiv:2107.10550


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How Primordial Black Holes Forms From Inflation With Solo or Multiple Bumps? (Cosmology / Quantum Physics)

Ruifeng Zheng and colleagues investigated the formation of primordial black holes (PBHs) from inflation model with bumpy potential, which has multiple bumps. They found that, the potential can give rise to power spectrum with single or multiple peaks in small scales, which can in turn predict the production of primordial black holes. Their study recently appeared in Arxiv.

There are several inflation models which discuss the possibility of the production of primordial black holes in inflation process of the early universe. But, the production of PBHs requires the violation of slow-roll condition. For this reason, slow-roll violating models become an interesting alternative, including ultra-slow-roll (USR) inflation, inflation with inflection points or bumps and others.

Previous studies have already discussed the case of potential with one bump. Now, Ruifeng and colleagues discussed the extended case of multiple bumps, which can generate PBHs at different mass ranges.

Fig 1: The evolution of φ with the parameter N, which enters the USR-like stage three times near N = 20, N = 35 and N = 50. © Zheng et al.

Specifically, they considered the power-law potential as the basic potential, and add one or several bumps of Gaussian type which makes the inflaton roll from the slow-roll stage to USR-like stage.

“These multiple bumps are quite different from the solo-bumpy ones, we have to take care of not only the shape of each bump like height, width, etc., but also the relative distance of the bumps. This is important because, when the inflaton field passes through one bump, it will lose kinetic energy, and if the bumps are far from each other, it may not have enough energy to pass through the next ones. For this reason, we set the bumps close to each other.”

With the potential, they constructed the power spectrum with single or multiple peaks in small scales, while keeping the large scale power spectrum consistent with CMB data. Later, they numerically calculated the abundances of PBHs (fraction to dark matter) at the mass range given by the solo-bumpy potential, as well as three mass ranges given by the multi-bumpy potential. Finally, they found that, PBHs can be formed at different mass ranges, including asteroid mass range (10¯16 − 10¯14M), planet mass range (10¯6 − 10¯3M) and solar mass range (around 1M), some of which can reach significant abundance.

FIG. 2: They plot fP BH for potential (21) given in paper with p = 2 using different threshold densities δc, where the yellow line corresponds to δc = 0.41, the purple line corresponds to δc = 0.46, and the blue line corresponds to δc = 0.486. Their results are consistent with the constraints from current observations. © Zheng et al.

They also found that, the larger threshold energy density (δc) is, the smaller the abundance will be, and this is easy understanding: the larger the threshold energy is, the more difficult it is to form black holes. For the small value of threshold energy density, the abundance of PBHs can reach around 10% of dark matter. The mass range of the PBHs formed is around 10¯15 M, namely the asteroid mass.

Moreover, they also considered the possibility of formation of primordial black holes (PBHs) in the early universe, through ellipsoidal collapse instead of spherical collapse. The difference between these two collapse models is that the threshold density for forming PBH is different. Because compared with the spherical collapse, the PBHs formed by the ellipsoidal collapse will increase the ellipticity of the formed PBHs, which will lead to the correction of the threshold density. Thus, abudance of ellipsoidal PBHs is lower than that of spherical PBHs, due to difference in their threshold densities.

FIG. 3: The figures above show the constraints on primordial black holes acting as dark matter, in which the colored region is excluded by various observations. The blue line correspond to fPBH, and the red line correspond to fe-PBH. The plot is for potential and δc = 0.465. From left to right, the masses of PBHs are 3.6975 × 10¯27 M, 5.8601 × 10¯16 M and 3.6975 × 10¯3 M respectively. Constraints are obtained from the publicly available Python code PBHbounds. © Zheng et al.

Finally, it has been suggested that, considering the age of the universe, PBHs with initial mass less than 1015g (∼ 10¯18 M) has been completely evaporated today. But, the PBHs of mass 3.6975 × 10¯27 (as shown in figure 3 above) may actually be vanishing, and cannot explain the dark matter today. But, although they can’t explain today’s dark matter they may still have a significant impact on the early universe, such as the process of Big Bang Nucleosynthesis, reheating, baryogenesis and so on.

Ruifeng and colleagues suggested that, we can be able to detect the traces left by such PBHs with future observation techniques, to find more evidence of their existence. Meanwhile, for other mass ranges, the PBHs are hardly evaporated till now and thus can act as dark matter.

“We will explore further details on the influences of the PBHs in our model in the future work.”

— concluded authors of the study

Reference: Ruifeng Zheng, Jiaming Shi, Taotao Qiu, “On Primordial Black Holes generated from inflation with solo/multi-bumpy potential”, Arxiv, pp. 1-14, 2021. https://arxiv.org/abs/2106.04303


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How To Confirm The Existence of Primordial Black Holes? (Cosmology / Physics)

Primordial black holes (PBH’s) are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. They are non-baryonic and as such are plausible dark matter (DM) candidates. PBH’s emit Hawking radiation and evaporation process can give rise to observable signals. Now, Antonio Palazzo and colleagues for the first time proposed a possibility that, PBHs with masses in the range of [5 × 1014 − 5 × 1015]g, emit neutrinos during evaporation and these neutrinos can interact through the coherent elastic neutrino nucleus scattering process, producing an observable signals in the dark matter (DM) direct detection experiments (like XENONnT, DARWIN etc.) Their study recently appeared in Arxiv.

Coherent elastic neutrino-nucleus scattering (“CEvNS”) is a process involving the neutral-current scattering of a neutrino with an entire nucleus. It is only recently that, CEvNS process has been successfully observed by COHERENT, where a few kilograms of detector was exposed to an intense neutrino flux of artificial origin. The very same process involving neutrinos of natural origin, such as, from the Sun, diffuse supernovae and Earth’s atmosphere, constitute an irreducible background in DM direct searches. This background gives rise to the so-called “neutrino floor”, which applies only to direct detection experiments. These experiments search for the scattering of a dark matter particle like WIMP’s, off of a nucleus.

Fig 1: Impact of PBHs on the Neutrino floor. The black contour delimiting the yellow region represents the ordinary neutrino floor, while the upper border of the colored bands correspond to the modifications induced by neutrinos from PBHs with masses and DM fractions in the legend. These benchmark values lie on the 90% C.L. exclusion curve obtainable from a liquid xenon experiment with 200 t yr exposure © Antonio Palazzo et al.

Antonio Palazzo and colleagues, showed that, PBHs with masses in the range, I mentioned above, emit neutrinos with peak energy 10 MeV and 100 MeV, which may emerge as a signal on such a familiar background. As a result, it is possible to set prospective bounds on the PBHs fraction of dark matter (DM) in this mass range, by improving the existing neutrino limits obtained with Super-Kamiokande.

“We have shown that with the high exposures envisaged for the next-generation facilities, it will be possible to set bounds on the fraction of dark matter (DM) composed of PBHs, improving the existing neutrino limits obtained with Super-Kamiokande.”

— wrote authors of the study

Finally, they showed how the neutrino floor gets modified by the presence of a hypothetical signal from PBHs. The neutrinos emitted by PBHs would lie on top of an irreducible background. Therefore, the existence of even a minute fraction of PBHs in the DM content would modify the neutrino floor, making it higher.

“In the context of PBHs searches, the direct DM experiments would rather operate as indirect DM observatories. From this perspective, our study lends further support to the emerging role of such underground facilities as multi-purpose low-energy neutrino telescopes complementary to their high-energy “ordinary” counterparts, IceCube and KM3NeT.”

— concluded authors of the study

Reference: Roberta Calabrese, Damiano F.G. Fiorillo, Gennaro Miele, Stefano Morisi, Antonio Palazzo, “Primordial Black Hole Dark Matter evaporating on the Neutrino Floor”, Arxiv, pp. 1-8, 2021. https://arxiv.org/abs/2106.02492


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How Primordial Black Hole Forms? (Quantum / Cosmology)

Michael Baker and colleagues discussed the new mechanism of formation of primordial black holes (PBH’s) during a first-order phase transition in the early Universe. Their study recently appeared in Arxiv.

Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. There are several possible formation mechanisms of primordial black holes (PBH’s): the most widely studied is collapse of density perturbations generated during inflation, while the collapse of topological defects, the dynamics of scalar condensates, or collisions of bubble walls during a first-order phase transition are viable alternatives.

Now, Michael Baker and colleagues, proposed a new mechanism of PBH production during a first-order cosmological phase transition.

“While previous papers on this topic have only considered the energy density stored in the bubble wall, we focused on a population of particles that interact with the bubble wall and showed that during a first-order phase transition, the energy density of the reflected particles can reach sufficient densities to trigger collapse into PBHs.”

— wrote M. Baker and his collaborators

They considered a particle species that interact/collides with the bubble wall. The mass of these particles may increase significantly during phase transitions due to either confinement or a Higgs mechanism. High-momentum particles can pass through the bubble wall into the true vacuum and gain a large mass, while low-momentum χ particles are reflected due to energy conservation (as shown in fig 1 below). The build-up of reflected particles (in front of the walls) creates a density perturbation which may lead to PBH formation.

(article continues below image)

A cartoon picture of the late stage of a first-order cosmological phase transition: regions of true vacuum (blue) are expanding with speed vw and coalescing, leaving an approximately spherical bubble of false vacuum (light red). High-momentum χ particles can pass through the bubble wall into the true vacuum and gain a large mass, while low-momentum χ particles are reflected due to energy conservation. The build-up of χ particles creates a density perturbation which may lead to PBH formation. The local coordinate system is also shown, along with the bubble wall thickness, lw. © M. Baker et al.

They track this process quantitatively by solving a Boltzmann equation, and demonstrated that the mass and density of the PBHs depend on the temperature at which the phase transition occurs and the probability that a black hole will form in a given volume.


Reference: Michael J. Baker, Moritz Breitbach, Joachim Kopp, Lukas Mittnacht, “Primordial Black Holes from First-Order Cosmological Phase Transitions”, Arxiv, pp. 1-7, 2021. https://arxiv.org/abs/2105.07481


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Does Fall Of Inflaton Give Rise To Primordial Black Holes (PBH’s)? (Quantum Physics / Cosmology)

Summary:

  • According to physicists, e-folding is simply the amount of time it takes for space to expand by approximately 2.71828 times its original size. The number 2.71828 is eulers number.
  • In order to produce primordial black holes (PBH’s) from inflationary fluctuations, large deceleration of inflation is required.
  • Now, Inamoto and colleagues showed that a large enhancement of perturbations results when the inflation crosses a downward step in its potential in less than an e-fold.
  • In simple terms, during this step, inflation loses extra kinetic energy due to hubble friction and there are large enhancement of perturbations which produces primordial black holes and ultra-compact minihalos.

Primordial black holes (PBHs) are one of the most intriguing topics in modern cosmology, owing to their potential to explain dark matter (DM) and the BHs detected by the LIGO-Virgo collaboration. Also, PBHs might be related to other observational results, such as the existence of supermassive black holes, the OGLE results, the recent NANOGrav results, and the anomalous excess of 511 keV photons. PBHs can be produced when very large density perturbations enter the horizon in the early universe. In particular, the PBH scenarios for DM or LIGO-Virgo events can be associated with the large power spectrum of primordial curvature perturbations, PR ∼ 10¯2, on small scales.

Now, Inomata and colleagues, focused on single-field inflation models that can realize the large power spectrum on small scales for the PBH scenarios. Under the slow-roll approximation, the power spectrum is given by

where the subscript “∗” denotes evaluation at the horizon exit of the perturbation and

where N ≡ ∫ Hdt is the number of e-folds of inflationary expansion. From this relation, at first glance, the large power spectrum on small scales needed for the PBH scenarios seems to require a substantial decrease in ϵ, and hence the kinetic energy of the inflaton, from the horizon exit of CMB scales. This decrease is realized by a large negative value of η ≡ d ln ϵ/dN which violates the slow-roll assumption. This can be achieved with a very flat potential in a period of so called “ultra slow roll (USR)” when Hubble friction dominates over the potential slope. On the other hand since the slow-roll approximation must be violated, this invalidates the naive expectation of a decreased and leaves the possibility of alternative mechanisms.

“In our paper, we show that a decrease in the kinetic energy of the inflaton relative to that at CMB scales is not necessary for the large enhancement of perturbations required for the PBH scenarios. Equivalently, the inflation potential need not have a region that is flatter than it is at CMB scales. If the inflaton instead gains kinetic energy by rolling down a sufficiently sharp feature that it crosses in less than an e-fold, non-adiabatic particle production occurs.”

— told Inomata, first author of the study.

They showed that a large enhancement of perturbations results when the inflaton crosses a downward step in its potential in less than an e-fold, which counter-intuitively allows a sizable amount of PBHs to form in a model wherein the inflaton always possesses a velocity higher than its value at the horizon exit of CMB scales. The enhancement can be interpreted as particle production due to the non-adiabatic transition whose curvature fluctuations are then adiabatically enhanced to large values as the inflaton loses the extra kinetic energy from the step due to Hubble friction.

Finally, they mentioned that, depending on the height and the location of the downward step, their enhancement mechanism can generate seeds not only for PBHs with a variety of masses, but also for ultra-compact minihalos. Additionally, the enhancement can be probed (constrained or discovered) by a range of complementary observables, such as the gravitational waves induced by the scalar perturbations, and CMB spectral distortions.

“Future observations of PBHs and these varied observable probes will enable us to probe this characteristic feature in the inflaton potential.”

— told Inomata, first author of the study

Featured image: The inflaton potential of Eq. (14) given in paper that realizes the large enhancement of perturbations, with the steplike transition at φ1 ≤ φ ≤ φ2 highlighted and an inset for the full range. The parameters are ns = 0.97, ϵ1 = 7.43 × 10¯10, ϵ2 = 0.01, ϵ3 = 10¯9, and ∆Nstep = 0.5. φend denotes the end of inflation (red vertical dotted line) and corresponds to 50 e-folds from φCMB. © Inomata et al.


Reference: Keisuke Inomata, Evan McDonough, Wayne Hu, “Primordial Black Holes Arise When The Inflaton Falls”, Arxiv, pp. 1-6, 2021. https://arxiv.org/abs/2104.03972


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Does Something Prevents Primordial Black Holes To Evaporate Completely? PART 2: The Truth (Quantum)

Previously on “Does something prevents primordial black holes to evaporate completely: PART 1”, we saw that in 2003, Chen and Adler argued that primordial black hole doesn’t evaporate completely,. Instead, it exists in the form of Planck-mass remnant with a cross-section on the order of 10¯70 m² which makes direct detection nearly impossible. Such black hole remnants have been identified as possible cold dark matter candidates.

But now, in a recently published paper in Astronomical Journal, Samuel Kovacik argued that it’s not completely true, instead, the final stage of the evaporation has a recoil effect which would give the microscopic black hole velocity on the order of 10¯1 c which is in disagreement with the cold dark matter cosmological model.

Samuel Kovacik et al.

Yeah friends, the temperature as a function of mass, grows very rapidly in the vicinity of m0. This means that, when the black hole has the mass mTmax for which it reaches the maximal temperature, the radiation is so energetic that the mass difference ∆m = mTmax – m0 is radiated in a relatively small amount of quanta, Nq ≈ ∆m / Tmax ≤ 10². Each quantum carries momentum on the order of p ≈ ∆m/√Nq and due to the conservation law the black hole receives the opposite momentum. As the radiation is random so are the momentum impulses the black hole receives. As a result, it performs a random walk in the momentum space, and after radiating Nq quanta will carry momentum of magnitude pr ≈ ∆m/√Nq. As a result, its final recoiled velocity will be on the order of:

This is the recoil effect due to thermal Hawking radiation of Planck-size black holes.

For the considered cases of matter density, they have ∆m / m0 = 0.33/0.26/0.22 and Nαq = 88/8.4/3.8; therefore recoil velocity = 0.03/0.09/0.11. In all three cases, this is a considerable fraction of speed of light (recall that they used units in which c = 1).

Samuel Kovacik et al.

The recoil effect due to the Hawking radiation modified by the quantum structure of space discussed by us makes the Planck-size black holes improbable dark matter candidates as during the last moments of radiation they would obtain velocities large enough to be incompatible as cold dark matter. Velocities of the Planck-size black holes would also exceed escape velocities from most astronomical objects.

— told Samuel Kovacik, lead author of the study

However, their discussion has not been very detailed as they do not have a detailed description of the quantum gravity and the behaviour of the Hawking radiation on this scale. But, at least under current assumptions they think that the recoil effect due to thermal radiation of microscopic black holes should be taken into consideration.

This research was supported by VEGA 1/0703/20 and the MUNI Award for Science and Humanities funded by the Grant Agency of Masaryk University.


Reference: Samuel Kováčik et al., “Hawking-Radiation Recoil of Microscopic Black Holes”, Astronomical Journal, pp. 1-5, 2021. https://arxiv.org/abs/2102.06517


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Does Something Prevents Primordial Black Holes To Evaporate Completely?: PART 1 (Quantum / Cosmology)

Summary:

Hawking radiation would make microscopic black holes evaporate rapidly which excludes them from many astrophysical considerations. However, Chen and Adler in their paper argued that the quantum nature of space would alter this behaviour: the temperature of a Planck-size black hole vanishes and what is left behind is a Planck-mass remnant with a cross-section on the order of 10¯70 m² which makes direct detection nearly impossible. Such black hole remnants have been identified as possible dark matter candidates.


In 2003, Chen and Adler argued that, when the gravity effect is included, the generalized uncertainty principle (GUP) may prevent black holes from total evaporation in a similar way that the standard uncertainty principle prevents the hydrogen atom from total collapse.

In the standard view of black hole thermodynamics, based on the entropy expression of Bekenstein and the temperature expression of Hawking, a small black hole should emit black body radiation, thereby becoming lighter and hotter, leading to an explosive end when the mass approaches zero. However Hawking’s calculation assumes a classical background metric and ignores the radiation reaction, assumptions which must break down as the black hole becomes very small and light. Thus it does not provide an answer as to whether a small black hole should evaporate entirely, or leave something else behind, which we refer to as a black hole remnant (BHR).

Numerous calculations of black hole radiation properties have been made from different points of view, and some hint at the existence of remnants, but in the absence of a well-defined quantum gravity theory none appears to give a definitive answer.

A cogent argument against the existence of BHRs can be made: since there is no evident symmetry or quantum number preventing it, a black hole should radiate entirely away to photons and other ordinary stable particles and vacuum, just like any unstable quantum system.

Chen and Alder, in their paper, argued that, when the gravity effect is included, the generalized uncertainty principle (GUP) may prevent black holes from total evaporation in a similar way that the standard uncertainty principle prevents the hydrogen atom from total collapse.

Specifically, they derived the GUP to obtain a modified Hawking temperature, which indicated that there should exist non-radiating Planck-size remnants (BHR) with a cross-section on the order of 10¯70 m² which makes direct detection nearly impossible.

The temperature of such Planck-size black hole vanishes and what is left behind is a Planck-mass remnant with a cross-section on the order. In the ordinary space, small black holes evaporate rapidly. In quantum space, they can be eternal and are very difficult to detect due to their miniscule cross-section. If they contributed significantly to the overall dark matter density, proving it would be difficult as direct detection seems to be impossible.

— told Chen, Lead author of the study.

BHRs are an attractive candidate for cold dark matter since they are a form of weakly massive interacting particles. They also investigated an alternative cosmology in which primordial BHRs are the primary source of dark matter. Their study indicated that their scenario is not inconsistent with basic cosmological facts, but more scrutiny is required before it can become a viable option.

To be continued in next part..


Reference: Pisin Chen, Ronald J. Adler, “Black hole remnants and dark matter”, Nuclear Physics B – Proceedings Supplements, Volume 124, 2003, Pages 103-106, ISSN 0920-5632, https://doi.org/10.1016/S0920-5632(03)02088-7.
https://www.sciencedirect.com/science/article/pii/S0920563203020887


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A Technique to Sift Out The Universe’s First Gravitational Waves (Astronomy)

Identifying primordial ripples would be key to understanding the conditions of the early universe.

In the moments immediately following the Big Bang, the very first gravitational waves rang out. The product of quantum fluctuations in the new soup of primordial matter, these earliest ripples through the fabric of space-time were quickly amplified by inflationary processes that drove the universe to explosively expand.

Primordial gravitational waves, produced nearly 13.8 billion years ago in the moments following the Big Bang, still echo through the universe today. Credits: MIT News

Primordial gravitational waves, produced nearly 13.8 billion years ago, still echo through the universe today. But they are drowned out by the crackle of gravitational waves produced by more recent events, such as colliding black holes and neutron stars.

Now a team led by an MIT graduate student has developed a method to tease out the very faint signals of primordial ripples from gravitational-wave data. Their results are published this week in Physical Review Letters.

Gravitational waves are being detected on an almost daily basis by LIGO and other gravitational-wave detectors, but primordial gravitational signals are several orders of magnitude fainter than what these detectors can register. It’s expected that the next generation of detectors will be sensitive enough to pick up these earliest ripples.

In the next decade, as more sensitive instruments come online, the new method could be applied to dig up hidden signals of the universe’s first gravitational waves. The pattern and properties of these primordial waves could then reveal clues about the early universe, such as the conditions that drove inflation.

“If the strength of the primordial signal is within the range of what next-generation detectors can detect, which it might be, then it would be a matter of more or less just turning the crank on the data, using this method we’ve developed,” says Sylvia Biscoveanu, a graduate student in MIT’s Kavli Institute for Astrophysics and Space Research. “These primordial gravitational waves can then tell us about processes in the early universe that are otherwise impossible to probe.”

Biscoveanu’s co-authors are Colm Talbot of Caltech, and Eric Thrane and Rory Smith of Monash University.

A concert hum

The hunt for primordial gravitational waves has concentrated mainly on the cosmic microwave background, or CMB, which is thought to be radiation that is leftover from the Big Bang. Today this radiation permeates the universe as energy that is most visible in the microwave band of the electromagnetic spectrum. Scientists believe that when primordial gravitational waves rippled out, they left an imprint on the CMB, in the form of B-modes, a type of subtle polarization pattern.

A team led by an MIT graduate student has developed a method to tease out the very faint signals of primordial ripples from gravitational-wave data produced by more recent events, such as colliding black holes and neutron stars. Credits: Image: Carl Knox, OzGrav/ Swinburne

Physicists have looked for signs of B-modes, most famously with the BICEP Array, a series of experiments including BICEP2, which in 2014 scientists believed had detected B-modes. The signal turned out to be due to galactic dust, however.

As scientists continue to look for primordial gravitational waves in the CMB, others are hunting the ripples directly in gravitational-wave data. The general idea has been to try and subtract away the “astrophysical foreground” — any gravitational-wave signal that arises from an astrophysical source, such as colliding black holes, neutron stars, and exploding supernovae. Only after subtracting this astrophysical foreground can physicists get an estimate of the quieter, nonastrophysical signals that may contain primordial waves.

The problem with these methods, Biscoveanu says, is that the astrophysical foreground contains weaker signals, for instance from farther-off mergers, that are too faint to discern and difficult to estimate in the final subtraction.

“The analogy I like to make is, if you’re at a rock concert, the primordial background is like the hum of the lights on stage, and the astrophysical foreground is like all the conversations of all the people around you,” Biscoveanu explains. “You can subtract out the individual conversations up to a certain distance, but then the ones that are really far away or really faint are still happening, but you can’t distinguish them. When you go to measure how loud the stagelights are humming, you’ll get this contamination from these extra conversations that you can’t get rid of because you can’t actually tease them out.”

A primordial injection

For their new approach, the researchers relied on a model to describe the more obvious “conversations” of the astrophysical foreground. The model predicts the pattern of gravitational wave signals that would be produced by the merging of astrophysical objects of different masses and spins. The team used this model to create simulated data of gravitational wave patterns, of both strong and weak astrophysical sources such as merging black holes.

The team then tried to characterize every astrophysical signal lurking in these simulated data, for instance to identify the masses and spins of binary black holes. As is, these parameters are easier to identify for louder signals, and only weakly constrained for the softest signals. While previous methods only use a “best guess” for the parameters of each signal in order to subtract it out of the data, the new method accounts for the uncertainty in each pattern characterization, and is thus able to discern the presence of the weakest signals, even if they are not well-characterized. Biscoveanu says this ability to quantify uncertainty helps the researchers to avoid any bias in their measurement of the primordial background.

Once they identified such distinct, nonrandom patterns in gravitational-wave data, they were left with more random primordial gravitational-wave signals and instrumental noise specific to each detector.

Primordial gravitational waves are believed to permeate the universe as a diffuse, persistent hum, which the researchers hypothesized should look the same, and thus be correlated, in any two detectors.

In contrast, the rest of the random noise received in a detector should be specific to that detector, and uncorrelated with other detectors. For instance, noise generated from nearby traffic should be different depending on the location of a given detector. By comparing the data in two detectors after accounting for the model-dependent astrophysical sources, the parameters of the primordial background could be teased out.

The researchers tested the new method by first simulating 400 seconds of gravitational-wave data, which they scattered with wave patterns representing astrophysical sources such as merging black holes. They also injected a signal throughout the data, similar to the persistent hum of a primordial gravitational wave.

They then split this data into four-second segments and applied their method to each segment, to see if they could accurately identify any black hole mergers as well as the pattern of the wave that they injected. After analyzing each segment of data over many simulation runs, and under varying initial conditions, they were successful in extracting the buried, primordial background.

“We were able to fit both the foreground and the background at the same time, so the background signal we get isn’t contaminated by the residual foreground,” Biscoveanu says.

She hopes that once more sensitive, next-generation detectors come online, the new method can be used to cross-correlate and analyze data from two different detectors, to sift out the primordial signal. Then, scientists may have a useful thread they can trace back to the conditions of the early universe.

Paper: “Measuring the Primordial Gravitational-Wave Background in the Presence of Astrophysical Foregrounds

Provided by MIT