31943448
Extracting Energy from a Black Hole through Its Disk.
When some magnetic field lines connect a Kerr black hole with a disk rotating around it, energy and angular momentum are transferred between them. If the black hole rotates faster than the disk, ca&solm0;GMH>0.36 for a thin Keplerian disk, then energy and angular momentum are extracted from the black hole and transferred to the disk (MH is the mass and aMH is the angular momentum of the black hole). This way, the energy originating in the black hole may be radiated away by the disk. The total amount of energy that can be extracted from the black hole spun down from ca&solm0;GMH=0.998 to ca&solm0;GMH=0.36 by a thin Keplerian disk is approximately 0.15MHc2. This is larger than approximately 0.09MHc2, which can be extracted by the Blandford-Znajek mechanism.
118518653
Constraining fast radio burst progenitors with gravitational lensing
Fast Radio Bursts (FRBs) are new transient radio sources discovered recently. Because of the angular resolution restriction in radio surveys, no optical counter part has been identified yet so it is hard to determine the progenitor of FRBs. In this paper we propose to use radio lensing survey to constrain FRB progenitors. We show that, different types of progenitors lead to different probabilities for a FRB to be gravitationally lensed by dark matter halos in foreground galaxies, since different type progenitors result in different redshift distributions of FRBs. For example, the redshift distribution of FRBs arising from double stars shifts toward lower redshift than of the FRBs arising from single stars, because double stars and single stars have different evolution timescales. With detailed calculations, we predict that the FRB sample size for producing one lensing event varies significantly for different FRB progenitor models. We argue that this fact can be used to distinguish different FRB models and also discuss the practical possibility of using lensing observation in radio surveys to constrain FRB progenitors.
10326293
New light on time machines: Against the chronology protection conjecture.
I construct a spacetime with closed timelike curves and without closed causal geodesics. There is a noncompactly generated Cauchy horizon separating the region with closed timelike curves from that without closed causal curves. This spacetime might be stable against the vacuum fluctuation of matter fields. Whether such a spacetime is stable against the vacuum fluctuation of a gravitational field is an open question.
11909154
The X‐ray transient 080109 in NGC 2770: an X‐ray flash associated with a normal core‐collapse supernova
Although it is generally thought that long-duration gamma-ray bursts (LGRBs) are associated with core-collapse supernovae (SNe), so far only four pairs of GRBs and SNe with firmly established connection have been found. All the four GRB-SNe belong to special class of Type Ic – called the broad-lined SNe indicative of a large explosion energy, suggesting that only a small fraction of SNe Ibc have GRBs associated with them. This scheme has been refreshed by the discovery of a bright X-ray transient in NGC 2770 on 2008 January 9, which was followed by a rather normal Type Ib SN 2008D. In this paper, I argue that the transient 080109 is an X-ray flash (XRF, the soft version of a GRB) because of the following evidence. (1) The transient cannot be interpreted as a SN shock breakout event. (2) The GRB X-ray flare interpretation is not supported by the high-energy observation. I then show that XRF 080109 satisfies the well-known relation between the isotropic-equivalent energy and the peak spectral energy for LGRBs, which highly strengthens the XRF interpretation. Finally, I point out that the peak spectral energy of XRF 080109 and the maximum bolometric luminosity of SN 2008D agree with the E γ, peak–LSN,max relationship of Li, strengthening the validity of the relationship. I speculate that events like XRF 080109 may occur at a rate comparable to SNe Ibc, and a soft X-ray telescope devoted to surveying for nearby X-ray flares will be very fruitful in discovering them.
39373169
Can the universe create itself
The question of first-cause has troubled philosophers and cosmologists alike. Now that it is apparent that our universe began in a Big Bang explosion, the question of what happened before the Big Bang arises. Inflation seems like a very promising answer, but as Borde and Vilenkin have shown, the inflationary state preceding the Big Bang must have had a beginning also. Ultimately, the difficult question seems to be how to make something out of nothing. This paper explores the idea that this is the wrong question --- that that is not how the Universe got here. Instead, we explore the idea of whether there is anything in the laws of physics that would prevent the Universe from creating itself. Because spacetimes can be curved and multiply connected, general relativity allows for the possibility of closed timelike curves (CTCs). Thus, tracing backwards in time through the original inflationary state we may eventually encounter a region of CTCs giving no first-cause. This region of CTCs, may well be over by now (being bounded toward the future by a Cauchy horizon). We illustrate that such models --- with CTCs --- are not necessarily inconsistent by demonstrating self-consistent vacuums for Misner space and a multiply connected de Sitter space in which the renormalized energy-momentum tensor does not diverge as one approaches the Cauchy horizon and solves Einstein's equations. We show such a Universe can be classically stable and self-consistent if and only if the potentials are retarded, giving a natural explanation of the arrow of time. Some specific scenarios (out of many possible ones) for this type of model are described. For example: an inflationary universe gives rise to baby universes, one of which turns out to be itself. Interestingly, the laws of physics may allow the Universe to be its own mother.
11692409
Electromagnetic energy for a charged Kerr black hole in a uniform magnetic field
With the Komar mass formula we calculate the electromagnetic energy for a charged Kerr black hole in a uniform magnetic field. We find that the total electromagnetic energy takes the minimum when the Kerr black hole possesses a nonzero net charge $Q=2\ensuremath{\xi}{B}_{0}{J}_{H}$ where ${B}_{0}$ is the strength of the magnetic field, ${J}_{H}$ is the angular momentum of the black hole, and $\ensuremath{\xi}$ is a dimensionless parameter determined by the spin of the black hole. However, the Wald state with ${Q=2B}_{0}{J}_{H}$ does not minimize the electromagnetic energy.
14522537
Effects of the quantity sigma TS on the spin structure functions of nucleons in the resonance region.
In this paper, we investigate the effects of the quantity {sigma}{sub {ital TS}} on the spin structure functions of nucleons in the resonance region. The Schwinger sum rule for the spin structure function {ital g}{sub 2}({ital x},{ital Q}{sup 2}) at the real photon limit is derived for the nucleon treated as a composite system, and it provides a crucial constraint on the longitudinal transition operator which has not been treated consistently in the literature. The longitudinal amplitude {ital S}{sub 1/2} is evaluated in the quark model with the transition operator that generates the Schwinger sum rule. The numerical results of the quantity {sigma}{sub {ital TS}} are presented for both spin structure functions {ital g}{sub 1}({ital x},{ital Q}{sup 2}) and {ital g}{sub 2}({ital x},{ital Q}{sup 2}) in the resonance region. Our results show that this quantity plays an important role in the low {ital Q}{sup 2} region, which can be tested in future experiments at CEBAF. {copyright} {ital 1996 The American Physical Society.}
38207022
Complete screening of a magnetic flux string by vacuum polarization in 3+1 dimensions.
The feedback effects of the vacuum current induced by a magnetic flux string in 3 + 1 dimensions are studied. It is found that the induced magnetic field cancels the applied one completely; i.e., a magnetic flux string does not exist in 3 + 1 dimensions.
20647092
Anomalous but consistent gravity in two dimensions.
Two-dimensional gravity coupled to chiral fermions possesses a Lorentz anomaly, but it can be made consistent by allowing the gauge degree of freedom to be physical at the quantum level. The theory has massless excitations or a ghost particle depending on the sign of a counterterm. The possibility of adding the Wess-Zumino term in order to cancel the anomaly is also discussed.
42674415
PQCD analysis of inclusive semileptonic decays of B mesons.
We develop the perturbative QCD formalism for inclusive semileptonic {ital B} meson decays, which includes Sudakov suppression from the resummation of large radiative corrections near the high end of charged lepton energy. Transverse degrees of freedom of partons are introduced to facilitate the factorization of {ital B} meson decays. Ambiguities appearing in the quark-level analysis are then avoided. A universal distribution function, arising from the nonperturbative Fermi motion of the {ital b} quark, is constructed according to the heavy quark effective field theory based operator product expansion, through which the mean and the width of the distribution function are related to hadronic matrix elements of local operators. Charged lepton spectra of the {ital B}{r_arrow}{ital X}{sub {ital ul}}{nu} decay are presented. We find 50{percent} suppression near the end point of the spectrum. The overall suppression on the total decay rate is 8{percent} for the free quark model, and is less than 7{percent} for the use of smooth distribution functions. With our predictions, it is then possible to extract the Cabibbo-Kobayashi-Maskawa matrix element {parallel}{ital V}{sub {ital ub}}{parallel} from experimental data. We also discuss possible implications of our analysis when confronted with the rather small observed semileptonic branching ratio in {ital B} mesonmore » decays. {copyright} {ital 1996 The American Physical Society.}« less
38711007
Hybrid baryons via QCD sum rules.
Hybrid baryons, consisting of the three light quarks and a gluon, are investigated using the QCD sum rule method. We find that the mixed condensate 〈0\ensuremath{\Vert}\ensuremath{\psi}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\sigma}G\ensuremath{\psi}\ensuremath{\Vert}0〉=${\mathit{m}}_{0}^{2}$〈0\ensuremath{\Vert}\ensuremath{\psi}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\psi}\ensuremath{\Vert}0〉 plays a dominant role in hybrid baryons in contrast with the quark condensate in the nucleon case. For the standard value of ${\mathit{m}}_{0}^{2}$=0.8 ${\mathrm{GeV}}^{2}$, the mass of the lowest hybrid state is found to be approximately 1.5 GeV, about the observed mass of the Roper resonance.