Tunnelling mechanism in non-commutative space with generalized uncertainty principle and Bohr-like black hole

The paper deals with non-thermal radiation spectrum by tunnelling mechanism with correction due to the generalized uncertainty principle (GUP) in the background of non-commutative geometry. Considering the reformulation of the tunnelling mechanism by Banerjee and Majhi, the Hawking radiation spectrum is evaluated through the density matrix for the outgoing modes. The GUP corrected effective temperature and the corresponding GUP corrected effective metric in non-commutative geometry are determined using Hawking's periodicity arguments. Thus, we obtain further corrections to the non-strictly thermal black hole (BH) radiation spectrum which give new final distributions. Then, we show that the GUP and the non-commutative geometry modify the Bohr-like BH recently discussed in a series of papers in the literature. In particular, we find the intriguing result that the famous law of Bekenstein on the area quantization is affected neither by non-commutative geometry nor by the GUP. This is a clear indication of the universality of Bekenstein's result. In addition, we find that both the Bekentsein-Hawking entropy and the total BH entropy to third order approximation are still functions of the BH quantum level.


Introduction
Hawking radiation [1] in the tunnelling mechanism [2 -11] is an elegant way to approach the particle creation caused by the vacuum fluctuations near the BH horizon. The virtual particle pair can be created either just inside the horizon or just outside the horizon. For both the possibilities the negative energy particle is absorbed by the BH, resulting a loss of the mass of the BH and the positive energy particle moves toward infinity, causing the subsequent emission of Hawking radiation. Considering contributions beyond the semiclassical approximation in the tunnelling process, Parikh and Wilczek [2,3] formulated the non-thermal spectrum of the radiation from BH which leads to interesting approaches [12,13] to resolve the information loss paradox of BH evaporation [14]. Also subsequently, by a novel formulation of the tunnelling formalism, Banerjee and Majhi [7], directly derived the black body spectrum for both bosons and fermions from a BH with standard Hawking temperature. The analysis in [7] was improved by one of us, C. Corda [15], who found as final result a non-strictly black body spectrum in agreement with the emission probability in [2,3]. This non-thermal spectrum is deeply interrelated to the underlying quantum gravity theory. As a result, the particle emission can be interpreted as a quantum transition of frequency ω between two discrete states [13,16,17]. Thus, the particle itself generates a tunnel through the BH horizon [3,15,16,17] having finite size. This solves a problem of the thermal approximation, namely in that case the tunnelling points have zero separation and hence there is no clear trajectory as there is no barrier [3,15,16,17]. Other aspects of tunnelling mechanism have been discussed in [41,42].
In this paper we analyse the corrections to the non-thermal spectrum of Parikh and Wilczek due to the non-commutative geometry and the GUP. It is shown that such corrections modify the Bohr-like BH recently discussed in a series of papers in the literature [13,[18][19][20][21][22]. An important result will be that the famous law of Bekenstein on the area quantization [23] is affected neither by non-commutative geometry nor by the GUP. We can consider this as a clear indication of the universality of Bekenstein's result. We also find that both the Bekentsein-Hawking entropy and the total BH entropy to third order approximation are still functions of the BH quantum level.
The physical reasons that motivate this work are two. On one hand, the background of non-commutative geometry is important because it can cure, in principle, some of the usual problems encountered in the description of the terminal phase of BH evaporation [24]. On the other hand, the GUP suggests a fundamental and discrete granular structure of space with important implications in quantum gravity [25,26,27]. For the sake of completeness, we take the chance to signal some recent important approach where GUP importance in BH physics is emphasized [44 -49].

Basic equations for non-commutative space
In non-commutative space the usual definition of mass density in the form of Dirac delta function does not hold due to position-position uncertainty relation. A particle mass, instead of being exactly localized at the point is diffused in a region of linear size √ δ. The intrinsic uncertainty can be expressed as the co-ordinate commutator: [x µ , x ν ] = i θ µν , where the antisymmetric matrix θ µν characterizes the fundamental cell discretization of space-time, analogous to the discretization of the phase-space by the Planck constant . It should be noted that the effect of non-commutativity is not visible at presently accessible energies, i.e. √ δ < 10 −16 cm. In particular, one should take into account the non-commutativity effect at distance r ≃ √ δ . As a result, there will be density of energy and momentum. Thus, in non-commutative geometry the mass density is described by a Gaussian distribution of minimal width √ δ as (we work in Planck units i.e. G = c = K B = = 1 4πG = 1 in the following) [24,28,29] Here the non-commutative parameter δ is a small (∼ unity in Planck units) positive number. Hence the mass of the analogous Schwarzschild BH is given by [29] m δ (r) =ˆr where the (lower) incomplete gamma function is defined as [29] γ(a, x) =ˆx 0 t a−1 e −t dt.
Thus, in the limit δ → 0, i.e. x → ∞, we have the usual gamma function and commutative geometry is recovered. Hence, the usual Schwarzschild metric, i.e. [15] takes the non-commutative form as [29] Note that the line element (5) can also be obtained as the solution of the Einstein field equations with equation (2) being the matter content. Thus, the event horizon can be obtained by solving g rr (r h ) = 0 as which clearly shows that r h cannot be solved in closed form.
For the sake of completeness, we stress that non-commutative corrections to different thermodynamical quantities have been discussed in [43].
3 Tunnelling approach to radiation spectrum: GUP corrections In strictly thermal approach, the probability of emission of Hawking quanta is where ω is the energy-frequency of the emitted radiation and T H = 1 8πM is the usual Hawking temperature. This probability of emission has been modified by Parikh and Wilezek in the tunnelling framework, considering contributions beyond the semi-classical approximation as [2,3] with α ∼ 1. For the sake of completeness, we stress that in [40] non thermal corrections to the Hawking effect were discussed through a different approach.
In that case, the structure of Γ is retained as exp −β (corr) ω , where β (corr) is given by [40] The non leading terms are the corrections to the temperature due to quantum effects and β H ≡ T −1 H , see [40] for details. Now, comparing the above two probabilities in eqs. (7) and (8) one can introduce the notion of the effective BH temperature T E as [13,15,16,17] Analogously, one can define the effective mass and effective horizon radius as [13,15,16,17] Further, these effective quantities can be interpreted as average values of the corresponding quantities before ( i.e. initial) and after ( i.e. final) the particle emission [13,15,16,17]. Accordingly, the effective temperature T E is the inverse of the average value of the inverses of the initial and final Hawking temperatures [13,15,16,17]. As a result, the effective Schwarzschild line element is [15] One can describe this line element as the BH dynamical geometry during the emission of the particle. Further, proceeding along the line of approach of one of us (C. Corda) [15], the final non-strictly thermal distributions considering the BH dynamical geometry take the form [15] where the suffices b and f represent boson and fermion particles respectively. This tunnelling approach has been recently finalized in [18].
On the other hand, by taking into account the non-commutative geometry discussed in Section 1 of this paper, starting from the non-commutative form of the Schwarzschild metric (5), eqs. from (7) to (13) must be replaced by their non-commutative counterparts as where T Hδ = 1 8πm δ is the Hawking temperature in the non-commutative geometry, Now, let us consider the modified Hawking temperature due to GUP corrections [25,26]. It can be expressed as [25,26] T As a consequence, we can introduce a GUP modified BH mass and a GUP modified horizon radius as [25,26] respectively. Thus, eq. (20) reads Using the Hawking's periodicity argument [15,25,30,31,40] one obtains the modified GUP Schwarzschild like line element [25] ds and [25] as the GUP modified surface gravity. The modified Schwarzschild solution (23) is obtained using the GUP in the background of non-commutative geometry. Thus, it is clear that the GUP Schwarzschild solution is not a solution of the Einstein field equation. It will be interesting to find modification of Einstein gravity for which the GUP Schwarzschild solution is a solution. This could be the subject of future works. Now, combining this GUP correction with the notion of effective temperature, one can introduce the GUP corrected effective temperature as [25] T the GUP corrected effective Boltzmann factor as [25] β and the GUP corrected effective mass and effective horizon radius as [25] M Note that the equations from (20) to (24) represent GUP corrections using Hamilton-Jacobi method beyond the semi-classical approximation. By taking into account the non-commutative geometry, the equations from (20) to (27) become Further, by Hawking's periodicity argument [15,25,30,31] one easily obtains the modified GUP non-commutative effective Schwarzschild like line element as Now, if one follows step by step the analysis in [15,25], then at the end one obtains the correct physical states for boson and fermions as out > (37) and the correct distributions as which, by using the first of eqs. (29) become The above expressions of the distributions clearly show that they are not thermal in nature as both the BH dynamical geometry during the emission of the particle, the GUP corrections to the semi classical Hawking temperature and the noncommutative geometry are taken into account.

Corrections to the Bohr-like black hole
The general conviction that BHs should be highly excited states representing the fundamental bricks of quantum gravity [32] has been shown to be correct in the recent works [13,[18][19][20][21][22]. In such papers, one of us (C. Corda) has indeed shown that the Schwarzschild BH is the gravitational analogous of the historical semi-classical Bohr's hydrogen atom [33,34]. The Bohr-like approach to BH quantum physics started with the pioneering works [16,17]. It works through the natural correspondence between Hawking radiation and BH quasi-normal modes (QNMs) [13,[18][19][20][21][22]. Considering an isolated BH (in the same way that Bohr considered an isolated hydrogen atom), the emissions of Hawking quanta and the absorptions of external particles "trigger" the BH QNMs [13,[18][19][20][21][22]. In this analogy, BH QNMs represent the "electron" jumping from a quantum level to another one. Hence, their absolute values are the energy "shells" of the "gravitational hydrogen atom" [13,[18][19][20][21][22]. Remarkably, the time evolution of the system permits to solve the BH information puzzle [13,20]. The results in [13,[18][19][20][21][22] are also consistent with previous results in the literature, included the historic result of Bekenstein on the area quantization [23]. The Bohr-like framework for BH quantum physics also finalizes the famous tunnelling approach of Parikh and Wilczek. One indeed finds the correct value of the pre-factor of the Parikh and Wilczek probability of emission, i.e. α in eq. (8), as [18] In this equation n max represents the maximum value of the principal quantum number n which can be found in Eq. (17) in [18], while m is the BH excited level. Thus, α depends on the BH quantum level, see [18] for details. This result permits to write down the probability of emission between two generic BH quantum levels m and n in the intriguing form [18] Γ m→n = α m exp − In a quantum mechanical framework, Hawking radiation can be physically interpreted in terms of quantum jumps among unperturbed levels [13,[16][17][18][19][20][21][22].
Now, let us see how the corrections due to the GUP and to non-commutative geometry change the model. As we use the modified GUP in non-commutative effective Schwarzschild line element (36) instead of the effective Schwarzschild line element (12), eq. (42) must be replaced by The solution of (43) in terms of |ω n | reads Clearly, a BH does not emit more energy than its total mass [13,[16][17][18][19][20][21][22]. In this case, we must take into account the corrections to the mass arising from the GUP and non-commutative geometry. Thus, the physical solution is the one obeying |ω n | < m δ that is E n is the value of total energy emitted by the BH when it is excited at a level n [13,[16][17][18][19][20][21][22]. Let us consider an emission from the ground state (i.e. a BH which is not excited) to a state with large n = n 1 . Then, by using Eq. (46), the GUP corrected mass of the analogous BH changes from m If now one considers a transition from n = n 1 to a different state with n = n 2 , having n 2 > n 1 , the GUP corrected mass of the analogous BH changes again from m ∆E n1→n2 in previous equation is given by and represents the jump between the two levels due to the emission of a particle having frequency ∆E n1→n2 . Such a discrete amount of energy corresponds to a quantum jump. The issue that for large n one finds independence on the other quantum numbers is perfectly consistent with the Correspondence Principle stated by Bohr [35]. This principle indeed claims that "transition frequencies at large quantum numbers should equal classical oscillation frequencies". We stress again the analogy with Bohr's hydrogen atom. In fact, in that model [33,34] energy is gained and loss by electrons through quantum jumps from one allowed energy shell to another. Hence, radiation can be absorbed or emitted and the energy difference of the levels respects the Planck relation E = hf (in standard units), where h is the Planck constant and f is the transition frequency. In the present GUP corrected analogous Bohr-like BH, QNMs (the "gravitational electrons") only gain and lose energy through jumps from one allowed energy shell to another with absorptions or emissions of Hawking quanta, but now the energy difference of the levels is governed by Eq. (49). Remarkably, one interpretes Eq. (46) in terms of a particle, the "electron", which is quantized on a circle of length This finalizes the cited similarity with Bohr's hydrogen atom. Eq. (50) represents indeed a perfect analogy with the electron travelling around the hydrogen nucleus with circular orbits in Bohr's approach [33,34] and is also similar to planets travelling around the Sun in our solar system. We stress that Bohr's hydrogen atom represents an approximated model with respect to the valence shell atom model of full quantum mechanics. In the same way, the present GUP corrected analogous Bohr-like BH should be a better approximated model with respect to previous results in [13,[18][19][20][21]. But it is still far from the final, currently unknown, BH model of a full unitary quantum gravity theory. Now, let us set n 1 = n − 1, n 2 = n in eq. (49). We get the emitted energy for a jump between two neighboring levels as Bekenstein [23] has shown that the Schwarzschild BH area quantum should be △A = 8π (the Planck length l p = 1.616 × 10 −33 cm is equal to one in Planck units). In Schwarzschild BHs the horizon area A is connected to the mass by the relation A = 16πM 2 . Hence, a variation △M of the mass enables the variation △A = 32πM △M (52) of the area. Setting △M = −∆E n−1→n (the case of an emission) if one uses Eqs. (47) and (51) one gets One can think that Eq. (53) gives the area quantum of an excited GUP corrected analogous BH for a jump from the level n − 1 to the level n in function of the "overtone" number n and of the initial GUP corrected analogous BH mass. But we see that one has a problem using eq. (53). An absorption from the level n to the level n − 1 is indeed possible, through the absorbed energy (54) Hence, if one sets △M = −∆E n→n−1 = ∆E n−1→n one gets a quantum of area Thus, one finds that the absolute value of the area quantum for an absorption between two levels is not the same as the absolute value of the area quantum for an emission between the same levels. This is because m (GUP ) δ n = m (GUP ) δ n−1 . Instead, we intuitively expect the area spectrum to be the equal for absorption and emission. [13,20]. One solves this problem considering the GUP corrected analogous effective mass which corresponds to the transitions between the two levels n and n − 1. This latter is indeed the same for emission and absorption: Thus, now one gets |△A n | = |△A n−1 |. Eqs. (54), (56) and some algebra give Thus, we find the very intriguing result that the famous law of Bekenstein on the area quantization [23] is affected neither by non-commutative geometry nor by the GUP. This is a clear indication of the universality of Bekenstein's result on the area quantization [23] .
If one puts A n−1 ≡ 16π m after the emission respectively. Hence, the Bekenstein-Hawking entropy can be written as a function of the QNMs principal quantum number, i.e. of the BH quantum excited state. Eqs. (62) and (63) permits to generalize the results in [13,[18][19][20][21][22] to the current case of the GUP corrected mass of the analogous BH. Now, we recall that the Bekenstein-Hawking entropy cannot be considered the definitive answer for a correct quantum theory of gravity [38]. It is indeed very important going beyond the Bekenstein-Hawking entropy and finding its sub-leading corrections [38]. By using the quantum tunnelling approach one remarkably arrives to the sub-leading corrections at third order approximation [39] S total = S BH − ln S BH + 3 2A In this approach the total BH entropy depends on four different parts: the standard Bekenstein-Hawking entropy, a logarithmic term, an inverse area term and an inverse squared area term [39]. Thus, one can find the formulas of the total BH entropy taking into account the sub-leading corrections at third order approximation and considering the GUP corrected mass of the analogous BH as (S total ) n−1 = 4π m after the emission, respectively. Therefore, the total BH entropy at third order approximation can be written as a function of the BH excited state n. Again, here we improve the results in [13,[18][19][20][21][22] to the current case of the GUP corrected mass of the analogous BH.

Summary and Concluding Remarks
The present work considers GUP correction of non-thermal radiation spectrum in the background of non-commutative geometry using the framework of tunnelling mechanism. At first, we formulated the line element for Schwarzschild BH in the context of non-commutative geometry. Then we introduced the notion of effective temperature, effective mass and effective horizon radius considering contributions beyond semi classical approximation. Also, following the idea of one of us, C. Corda, we have determined the non-strictly thermal distributions for bosons and fermions. Subsequently, we introduce the GUP correction to the BH dynamical geometry by finding the final distributions for the GUP corrected mass of the analogous BH. After that, it has been shown that the GUP and the non-commutative geometry modify the Bohr-like BH model, recently discussed in [13,[18][19][20][21][22]. In particular, we found the intriguing result that the famous law of Bekenstein on the area quantization [23] is affected neither by non-commutative geometry nor by the GUP. This is a clear indication of the universality of Bekenstein's result. Finally, it has been shown that both the Bekentsein-Hawking entropy and the total BH entropy to third order approximation are still functions of the BH quantum level, generalizing the results in [13,[18][19][20][21][22].