37936938
Extracting Energy from Accretion into a Kerr Black Hole.
The highest efficiency of converting rest mass into energy by accreting matter into a Kerr black hole is approximately 31% (Thorne 1974). We propose a new process in which periods of accretion from a thin disk, and the associated spin-up of the black hole, alternate with the periods of no accretion and magnetic transfer of energy from the black hole to the disk. These cycles can repeat indefinitely, at least in principle, with the black hole mass increasing by approximately 66% per cycle and up to approximately 43% of accreted rest mass radiated away by the disk.
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.
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.
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.
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.
2293560
Kaon photoproduction on the neutron using deuterium.
The three processes \ensuremath{\gamma}+d\ensuremath{\rightarrow}${\mathrm{\ensuremath{\Lambda}}}^{0}$(${\mathrm{\ensuremath{\Sigma}}}^{0}$)+${\mathit{K}}^{0}$+p and \ensuremath{\gamma}+d\ensuremath{\rightarrow}${\mathrm{\ensuremath{\Sigma}}}^{\mathrm{\ensuremath{-}}}$+${\mathit{K}}^{+}$+p are investigated in the framework of the impulse approximation. The exclusive reactions d(\ensuremath{\gamma},Kp)Y where the proton is detected in coincidence with the ${\mathit{K}}^{0}$ or ${\mathit{K}}^{+}$ should provide unique signatures for each of the processes. In most kinematic situations the final-state Yp interaction is insignificant and the elementary process cross section for \ensuremath{\gamma}+n\ensuremath{\rightarrow}Y+K can be reliably extracted. Cross-section estimates are given for several models of the basic production operator.
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.
18117055
Threshold pion photoproduction of nucleons in the chiral quark model.
In this paper, we show that the low energy theorem (LET) of threshold pion photoproduction can be fully recovered in the quark model. An essential result of this investigation is that the quark-pion operators are obtained from the effective chiral Lagrangian, and the low energy theorem does not require constraints on the internal structures of the nucleon. Changing from pseudovector to pseudoscalar quark-pion couplings generates an additional term at order [mu]=[ital m][sub [pi]]/[ital M] only in the isospin amplitude [ital A][sup ([minus])]. The role of the transitions between the nucleon and the resonance [ital P][sub 33](1232) or [ital P]-wave baryons is also discussed; we find that the leading contributions to the isospin amplitudes at [ital O]([mu][sup 2]) are from the transition between the [ital P]-wave baryons and the nucleon, and from the charge radius of the nucleon. The leading contribution from the [ital P]-wave baryons only affects neutral pion production, and improves the agreement with data significantly. The transition between the resonance [ital P][sub 33](1232) and the nucleon only gives an order [mu][sup 3] correction to [ital A][sup ([minus])].
32917384
Analytic calculation of the vacuum wave function for (2+1)-dimensional SU(2) lattice gauge theory.
The long wavelength vacuum wave function of (2+1)-dimensional SU(2) lattice gauge theory is calculated by the method of truncated eigenvalue equation. Third order results are consistent with Monte Carlo measurement and display a good scaling behavior extending to the deep weak coupling region.
38608728
Comment on "Role of g2 in relating the Schwinger and Gerasimov-Drell-Hearn sum rules"