arXiv subjects

Stephen D. H. Hsu

Publications and source records attributed to Stephen D. H. Hsu.

At least 55 records · Page 3Linked to original sources

arXiv2010

Nonlinear Psychometric Thresholds for Physics and Mathematics

We analyze 5 years of student records at the University of Oregon to estimate the probability of success (as defined by superior undergraduate record; sufficient for admission to graduate school) in Physics and Mathematics as a function of SAT-M score. We find evidence of a nonlinear threshold: below SAT-M score of roughly 600, the probability of success is very low. Interestingly, no similar threshold exists in other majors, such as Sociology, History, English or Biology, whether on SAT combined, SAT-R or SAT-M. Our findings have significant implications for the demographic makeup of graduate populations in mathematically intensive subjects, given the current distribution of SAT-M scores.

physics.ed-ph↗

arXiv2010

Dark Energy, with Signatures

We propose a class of simple dark energy models which predict a late-time dark radiation component and a distinctive time-dependent equation of state $w(z)$ for redshift $z < 3$. The dark energy field can be coupled strongly enough to Standard Model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

astro-ph.CO↗

arXiv2010

Data Mining the University: College GPA Predictions from SAT Scores

We analyze a data set comprised of academic records of undergraduates at the University of Oregon from 2000-2004. We find correlations of roughly 0.35 to 0.5 between SAT scores and upper division, in-major GPA (henceforth, GPA). Interestingly, low SAT scores do not preclude high performance in most majors. That is, the distribution of SAT scores after conditioning on high GPA (e.g., 3.5 or even 4.0) typically extends below 1000 (the average among test takers). We hypothesize that "overachievers" overcome cognitive deficits through hard work, and discuss to what extent they can be identified from high school records. Only a few majors seem to exhibit a "cognitive threshold" -- such that high GPA (mastery of the subject matter) is very unlikely below a certain SAT threshold (i.e., no matter how dedicated or hard working the student). Our results suggest that almost any student admitted to university can achieve academic success, if they work hard enough. In addition to our primary result, we find that the best predictor of GPA is a roughly equally weighted sum of SAT and high school GPA, measured in standard deviation units. Using a sub-population of honors college students, we can estimate how students at elite universities would fare at a typical state university, allowing us to comment on issues such as grade inflation. Finally, we observe that 1) SAT scores fluctuate little on retest (very high reliability), 2) SAT and GRE scores (where available) correlate at roughly 0.75 (consistent with the notion that both tests measure a stable general cognitive ability) and 3) the SAT distribution of students that obtained a degree does not differ substantially from that of the entering class.

physics.ed-ph↗

arXiv2010

The black hole information paradox and macroscopic superpositions

We investigate the experimental capabilities required to test whether black holes destroy information. We show that an experiment capable of illuminating the information puzzle must necessarily be able to detect or manipulate macroscopic superpositions (i.e., Everett branches). Hence, it could also address the fundamental question of decoherence versus wavefunction collapse.

gr-qc↗

arXiv2010

Grand unification through gravitational effects

We systematically study the unification of gauge couplings in the presence of (one or more) effective dimension-5 operators cHGG/4MPl, induced into the grand unified theory by gravitational interactions at the Planck scale MPl. These operators alter the usual condition for gauge coupling unification, which can, depending on the Higgs content H and vacuum expectation value, result in unification at scales MX significantly different than naively expected. We find non-supersymmetric models of SU(5) and SO(10) unification, with natural Wilson coefficients c, that easily satisfy the constraints from proton decay. Furthermore, gauge coupling unification at scales as high as the Planck scale seems feasible, possibly hinting at simultaneous unification of gauge and gravitational interactions. In the Appendix we work out the group theoretical aspects of this scenario for SU(5) and SO(10) unified groups in detail; this material is also relevant in the analysis of non-universal gaugino masses obtained from supergravity.

hep-ph↗

arXiv2009

On the sign problem in dense QCD

We investigate the Euclidean path integral formulation of QCD at finite baryon density and temperature. We show that the partition function Z can be written as a difference between two sums Z+ and Z-, each of which defines a partition function with positive weights. We call the sign problem severe if the ratio Z-/Z+ is nonzero in the infinite volume limit. This occurs only if, and generically always if, the associated free energy densities F+ and F- are equal in this limit. We present strong evidence here that the sign problem is severe at almost all points in the phase diagram, with the exception of special cases like exactly zero chemical potential (ordinary QCD), which requires a particular order of limits. Part of our reasoning is based on the analyticity of free energy densities within their open phase regions. Finally, we describe a Monte Carlo technique to simulate finite-density QCD in regions where Z-/Z+ is small.

hep-th↗

arXiv2009

Life on moduli space?

While the number of metastable landscape vacua in string theory is vast, the number of supermoduli vacua which lead to distinct low energy physics is even larger, perhaps infinitely so. From the anthropic perspective it is therefore important to understand whether complex life is possible on moduli space -- i.e., in low energy effective theories with 1. exact supersymmetry and 2. some massless multiplets (moduli). Unless life is essentially impossible on moduli space as a consequence of these characteristics, anthropic reasoning in string theory suggests that the overwhelming majority of sentient beings would observe 1-2. We investigate whether 1 and 2 are by themselves automatically inimical to life and conclude, tentatively, that they are not. In particular, we describe moduli scenarios in which complex life seems possible.

hep-th↗

arXiv2009

Monsters, black holes and the statistical mechanics of gravity

We review the construction of monsters in classical general relativity. Monsters have finite ADM mass and surface area, but potentially unbounded entropy. From the curved space perspective they are objects with large proper volume that can be glued on to an asymptotically flat space. At no point is the curvature or energy density required to be large in Planck units, and quantum gravitational effects are, in the conventional effective field theory framework, small everywhere. Since they can have more entropy than a black hole of equal mass, monsters are problematic for certain interpretations of black hole entropy and the AdS/CFT duality. In the second part of the paper we review recent developments in the foundations of statistical mechanics which make use of properties of high-dimensional (Hilbert) spaces. These results primarily depend on kinematics -- essentially, the geometry of Hilbert space -- and are relatively insensitive to dynamics. We discuss how this approach might be adopted as a basis for the statistical mechanics of gravity. Interestingly, monsters and other highly entropic configurations play an important role.

gr-qc↗

arXiv2009

Black holes, information and decoherence

gr-qc↗

arXiv2009

What is the entropy of the universe?

Standard calculations suggest that the entropy of our universe is dominated by black holes, whose entropy is of order their area in Planck units, although they comprise only a tiny fraction of its total energy. Statistical entropy is the logarithm of the number of microstates consistent with the observed macroscopic properties of a system, hence a measure of uncertainty about its precise state. Therefore, assuming unitarity in black hole evaporation, the standard results suggest that the largest uncertainty in the future quantum state of the universe is due to the Hawking radiation from evaporating black holes. However, the entropy of the matter precursors to astrophysical black holes is enormously less than that given by area entropy. If unitarity relates the future radiation states to the black hole precursor states, then the standard results are highly misleading, at least for an observer that can differentiate the individual states of the Hawking radiation.

hep-th↗

arXiv2009

Dark radiation as a signature of dark energy

We propose a simple dark energy model with the following properties: the model predicts a late-time dark radiation component that is not ruled out by current observational data, but which produces a distinctive time-dependent equation of state w(z) for z < 3. The dark energy field can be coupled strongly enough to Standard Model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

astro-ph.CO↗

arXiv2008

Unitarity and the Hilbert space of quantum gravity

Under the premises that physics is unitary and black hole evaporation is complete (no remnants, no topology change), there must exist a one-to-one correspondence between states on future null and timelike infinity and on any earlier spacelike Cauchy surface (e.g., slices preceding the formation of the hole). We show that these requirements exclude a large set of semiclassical spacetime configurations from the Hilbert space of quantum gravity. In particular, the highest entropy configurations, which account for almost all of the volume of semiclassical phase space, would not have quantum counterparts, i.e. would not correspond to allowed states in a quantum theory of gravity.

hep-th↗

arXiv2008

Quantum gravity at a TeV and the renormalization of Newton's constant

We examine whether renormalization effects can cause Newton's constant to change dramatically with energy, perhaps even reducing the scale of quantum gravity to the TeV region without the introduction of extra dimensions. We examine a model that realizes this possibility and describe experimental signatures from production of small black holes.

hep-th↗

arXiv2008

Grand unification and enhanced quantum gravitational effects

In grand unified theories with large numbers of fields, renormalization effects significantly modify the scale at which quantum gravity becomes strong. This in turn can modify the boundary conditions for coupling constant unification, if higher dimensional operators induced by gravity are taken into consideration. We show that the generic size of these effects from gravity can be larger than the two-loop corrections typically considered in renormalization group analyses of unification. In some cases, gravitational effects of modest size can render unification impossible.

hep-ph↗

arXiv2008

Quantum Gravitational Effects and Grand Unification

In grand unified theories with large numbers of fields, renormalization effects significantly modify the scale at which quantum gravity becomes strong. This in turn can modify the boundary conditions for coupling constant unification, if higher dimensional operators induced by gravity are taken into consideration. We show that the generic size of, and the uncertainty in, these effects from gravity can be larger than the two-loop corrections typically considered in renormalization group analyses of unification. In some cases, gravitational effects of modest size can render unification impossible.

hep-ph↗

arXiv2008

Colorful quantum black holes at the LHC

We examine the LHC phenomenology of quantum black holes in models of TeV gravity. By quantum black holes we mean black holes of the smallest masses and entropies, far from the semiclassical regime. These black holes are formed and decay over short distances, and typically carry SU(3) color charges inherited from their parton progenitors. Based on a few minimal assumptions, such as gauge invariance, we identify interesting signatures for quantum black hole decay such as 2 jets, jet + hard photon, jet + missing energy and jet + charged lepton, which should be readily visible above background. The detailed phenomenology depends heavily on whether one requires a Lorentz invariant, low-energy effective field theory description of black hole processes.

hep-ph↗

arXiv2008

Precision cosmological measurements: independent evidence for dark energy

Using recent precision measurements of cosmological paramters, we re-examine whether these observations alone, independent of type Ia supernova surveys, are sufficient to imply the existence of dark energy. We find that best measurements of the age of the universe $t_0$, the Hubble parameter $H_0$ and the matter fraction $Ω_m$ strongly favor an equation of state defined by ($w < -1/3$). This result is consistent with the existence of a repulsive, acceleration-causing component of energy if the universe is nearly flat.

astro-ph↗

arXiv2007

TeV gravity in four dimensions?

We describe a model in which the fundamental scale M_\star of the theory which unifies gravity and quantum mechanics is in the TeV range, but without requiring additional spacetime dimensions. The weakness of gravity at low energies is due to a large vacuum expectation of a dilaton like field. The model requires a small dimensionless parameter (the self-coupling of the dilaton) but no fine-tuning. We discuss in detail the dynamical assumptions about nonperturbative quantum gravity required within the model. We observe that M_\star could be quite small, less than a TeV, and that the model could lead to copious strong coupling effects at the LHC. However, semiclassical black holes will not be produced.

hep-ph↗