arXiv subjects

Sungwook E. Hong

Publications and source records attributed to Sungwook E. Hong.

62 records · Page 4Linked to original sources

arXiv2014

Field Target Allocation and Routing Algorithms for Starbugs

Starbugs are miniaturised robotic devices that position optical fibres over a telescope's focal plane in parallel operation for high multiplex spectroscopic surveys. The key advantage of the Starbug positioning system is its potential to configure fields of hundreds of targets in a few minutes, consistent with typical detector readout times. Starbugs have been selected as the positioning technology for the TAIPAN (Transforming Astronomical Imaging surveys through Polychromatic Analysis of Nebulae) instrument, a prototype for MANIFEST (Many Instrument Fiber System) on the GMT (Giant Magellan Telescope). TAIPAN consists of a 150-fibre Starbug positioner accessing the 6 degree field-of-view of the AAO's UK Schmidt Telescope at Siding Spring Observatory. For TAIPAN, it is important to optimise the target allocation and routing algorithms to provide the fastest configurations times. We present details of the algorithms and results of the simulated performance.

astro-ph.IM↗

arXiv2014

2D Genus Topology of 21-cm Differential Brightness Temperature During Cosmic Reionization

A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through the calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogen-ionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of the high-redshift Universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is crucial for this analysis. To this end, the Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.

astro-ph.CO↗

arXiv2014

Shock Waves and Cosmic Ray Acceleration in the Outskirts of Galaxy Clusters

The outskirts of galaxy clusters are continuously disturbed by mergers and gas infall along filaments, which in turn induce turbulent flow motions and shock waves. We examine the properties of shocks that form within $r_{200}$ in sample galaxy clusters from structure formation simulations. While most of these shocks are weak and inefficient accelerators of cosmic rays (CRs), there are a number of strong, energetic shocks which can produce large amounts of CR protons via diffusive shock acceleration. We show that the energetic shocks reside mostly in the outskirts and a substantial fraction of them are induced by infall of the warm-hot intergalactic medium from filaments. As a result, the radial profile of the CR pressure in the intracluster medium is expected to be broad, dropping off more slowly than that of the gas pressure, and might be even temporarily inverted, peaking in the outskirts. The volume-integrated momentum spectrum of CR protons inside $r_{200}$ has the power-law slope of $4.25 - 4.5$, indicating that the average Mach number of the shocks of main CR production is in the range of $\left< M_s \right>_{\rm CR} \approx 3 - 4$. We suggest that some radio relics with relatively flat radio spectrum could be explained by primary electrons accelerated by energetic infall shocks with $M_s >~ 3$ induced in the cluster outskirts.

astro-ph.CO↗

arXiv2012

The Possibility of Inflation in Asymptotically Safe Gravity

We examine the inflationary modes in the cubic curvature theories in the context of asymptotically safe gravity. On the phase space of the Hubble parameter, there exists a critical point which corresponds to the slow-roll inflation in Einstein frame. Most of the e-foldings are attained around the critical point for each inflationary trajectories. If the coupling constants $g_i$ have the parametric relations generated as the power of the relative energy scale of inflation $H_0$ to the ultraviolet cutoff $Λ$, a successful inflation with more than 60 e-foldings occurs near the critical point.

gr-qc↗

arXiv2012

Anthropic Likelihood for the Cosmological Constant and the Primordial Density Perturbation Amplitude

Weinberg et al. calculated the anthropic likelihood of the cosmological constant using a model assuming that the number of observers is proportional to the total mass of gravitationally collapsed objects, with mass greater than a certain threshold, at t \rightarrow \infty. We argue that Weinberg's model is biased toward small Λ, and to try to avoid this bias we modify his model in a way that the number of observers is proportional to the number of collapsed objects, with mass and time equal to certain preferred mass and time scales. Compared to Weinberg's model, this model gives a lower anthropic likelihood of Λ_0 (T_+(Λ_0) ~ 5%). On the other hand, the anthropic likelihood of the primordial density perturbation amplitude from this model is high, while the likelihood from Weinberg's model is low. Furthermore, observers will be affected by the history of the collapsed object, and we introduce a method to calculate the anthropic likelihoods of Λand Q from the mass history using the extended Press-Schechter formalism. The anthropic likelihoods for $Λ$ and Q from this method are similar to those from our single mass constraint model, but, unlike models using the single mass constraint which always have degeneracies between Λand Q, the results from models using the mass history are robust even if we allow both Λand Q to vary. In the case of Weinberg's flat prior distribution of Λ(pocket based multiverse measure), our mass history model gives T_+(Λ_0) ~ 10%, while the scale factor cutoff measure and the causal patch measure give T_+(Λ_0) \geq 30%.

astro-ph.CO↗

arXiv2010

The causal structure of dynamical charged black holes

We study the causal structure of dynamical charged black holes, with a sufficient number of massless fields, using numerical simulations. Neglecting Hawking radiation, the inner horizon is a null Cauchy horizon and a curvature singularity due to mass inflation. When we include Hawking radiation, the inner horizon becomes space-like and is separated from the Cauchy horizon, which is parallel to the out-going null direction. Since a charged black hole must eventually transit to a neutral black hole, we studied the neutralization of the black hole and observed that the inner horizon evolves into a space-like singularity, generating a Cauchy horizon which is parallel to the in-going null direction. Since the mass function is finite around the inner horizon, the inner horizon is regular and penetrable in a general relativistic sense. However, since the curvature functions become trans-Planckian, we cannot saymore about the region beyond the inner horizon, and it is natural to say that there is a 'physical' space-like singularity. However, if we assume an exponentially large number of massless scalar fields, our results can be extended beyond the inner horizon. In this case, strong cosmic censorship and black hole complementarity can be violated.

gr-qc↗

arXiv2009

Critical Reviews of Causal Patch Measure over the Multiverse

In this talk, the causal patch measure based on black hole complementarity is critically reviewed. By noticing the similarities between the causal structure of an inflationary dS space and that of a black hole, we have considered the complementarity principle between the inside and the outside of the causal horizon as an attractive way to count the inflationary multiverse. Even though the causal patch measure relieves the Boltzmann brain problem and stresses physical reality based on observations, it could be challenged by the construction of counterexamples, both on regular black holes and charged black holes, to black hole complementarity.

gr-qc↗

arXiv2008

Black hole complementarity with local horizons and Horowitz-Maldacena's proposal

To implement the consistent black hole complementarity principle, we need two assumptions: first, there exists a singularity near the center, and second, global horizons are the same as local horizons. However, these assumptions are not true in general. In this paper, the authors study a charged black hole in which the second assumption may not hold. From the previous simulations, we have argued that the event horizon is quite close to the outer horizon, and it seems not harmful to black hole complementarity; however, the Cauchy horizon can be different from the inner horizon, and a violation of complementarity will be possible. To maintain complementarity, we need to assume a selection principle between the singularity and the Hawking radiation generating surface; we suggest that Horowitz-Maldacena's proposal can be useful for this purpose. Finally, we discussed some conditions under which the selection principle may not work.

gr-qc↗