Y. Jack Ng

Holographic foam, dark energy and infinite statistics

Abstract Quantum fluctuations of spacetime give rise to quantum foam, and black hole physics dictates that the foam is of holographic type. Applied to cosmology, the holographic model requires the existence of dark energy which, we argue, is composed of an enormous number of inert “particles” of extremely long wavelength. These “particles” necessarily obey infinite statistics in which all representations of the particle permutation group can occur. For every boson or fermion in the present observable universe there could be ∼ 10 31 such “particles”. We also discuss the compatibility between the holographic principle and infinite statistics.

Implications of spacetime quantization for the Bahcall–Waxman neutrino bound

There is growing interest in quantum-spacetime models in which small departures from Lorentz symmetry are governed by the Planck scale. In particular, several studies have considered the possibility that these small violations of Lorentz symmetry may affect various astrophysical observations, such as the evaluation of the GZK limit for cosmic rays, the interaction of TeV photons with the far infrared background and the arrival time of photons with different energies from cosmological sources. We show that the same Planck-scale departures from Lorentz symmetry that led to a modification of the GZK limit also have significant implications for the evaluation of the Bahcall–Waxman bound on the flux of high-energy neutrinos produced by photo–meson interactions.

From Computation to Black Holes and Space-Time Foam

We show that quantum mechanics and general relativity limit the speed nu of a simple computer (such as a black hole) and its memory space I to I(nu2) less, similar(t(-2))P, where t(P) is the Planck time. We also show that the lifetime of a simple clock and its precision are similarly limited. These bounds and the holographic bound originate from the same physics that governs the quantum fluctuations of space-time. We further show that these physical bounds are realized for black holes, yielding the correct Hawking black hole lifetime, and that space-time undergoes much larger quantum fluctuations than conventional wisdom claims-almost within range of detection with modern gravitational-wave interferometers.

Unimodular theory of gravity and the cosmological constant

The unimodular theory of gravity with a constrained determinant gμν is equivalent to general relativity with an arbitrary cosmological constant Λ. Within this framework Λ appears as an integration constant unrelated to any parameters in the Lagrangian. In a quantum theory the state vector of the universe is thus expected to be a superposition of states with different values of Λ. Following Hawking’s argument one concludes that the fully renormalized Λ=0 completely dominates other contributions to the integral over Λ in the vacuum functional. In this scenario of the unimodular theory of gravity the cosmological constant problem is solved. Furthermore, this formulation naturally provides an external (cosmic) time for time ordering of measurements so that the quantum version of the unimodular theory can have a normal ‘‘Schrodinger’’ form of time development, giving a simpler interpretation to the equation of the universe.

Cold dark matter with MOND scaling

We provide a holographic dual description of Milgroms scaling associated with galactic rotation curves. Our argument is partly based on the recent entropic reinterpretation of Newtons laws of motion. We propose a duality between cold dark matter and modified Newtonian dynamics (MOND). We introduce the concept of MONDian dark matter, and discuss some of its phenomenological implications. At cluster as well as cosmological scales, the MONDian dark matter would behave as cold dark matter, but at the galactic scale, the MONDian dark matter would act as MOND.

Observability of the bulk Casimir effect: Can the dynamical Casimir effect be relevant to sonoluminescence?

The experimental observation of intense light emission by acoustically driven, periodically collapsing bubbles of air in water (sonoluminescence) has yet to receive an adequate explanation. One of the most intriguing ideas is that the conversion of acoustic energy into photons occurs quantum mechanically, through a dynamical version of the Casimir effect. We have argued elsewhere that in the adiabatic approximation, which should be reliable here, Casimir or zero-point energies cannot possibly be large enough to be relevant. (About 10 MeV of energy is released per collapse.) However, there are sufficient subtleties involved that others have come to opposite conclusions. In particular, it has been suggested that bulk energy, that is, simply the naive sum of (1) /(2) {h_bar}{omega}, which is proportional to the volume, could be relevant. We show that this cannot be the case, based on general principles as well as specific calculations. In the process we further illuminate some of the divergence difficulties that plague Casimir calculations, with an example relevant to the bag model of hadrons. {copyright} {ital 1998} {ital The American Physical Society}

The Cosmological constant problem

The cosmological constant is a macroscopic parameter which controls the large-scale structure of the Universe. All observations to date have shown that it is very small. However, our modern microscopic theory of particle physics and gravity suggests that the cosmological constant should be very large. This discrepancy between theoretical expectation and empirical observation constitutes the cosmological constant problem. After a review of the problem, some approaches to its solution are briefly discussed, and then a possible solution is proposed. In this approach, the cosmological constant appears as a constant of integration, unrelated to any parameters in the Lagrangian. The solution makes crucial use of quantum mechanics.

A small but nonzero cosmological constant

Recent astrophysical observations seem to indicate that the cosmological constant is small but nonzero and positive. The old cosmological constant problem asks why it is so small; we must now ask, in addition, why it is nonzero (and is in the range found by recent observations), and why it is positive. In this essay, we try to kill these three metaphorical birds with one stone. That stone is the unimodular theory of gravity, which is the ordinary theory of gravity, except for the way the cosmological constant arises in the theory. We argue that the cosmological constant becomes dynamical, and eventually, in terms of the cosmic scale factor R(t), it takes the form Λ(t)=Λ(t0)(R(t0)/R(t))2, but not before the epoch corresponding to the redshift parameter z~1.

From spacetime foam to holographic foam cosmology

Abstract Due to quantum fluctuations, spacetime is foamy on small scales. For maximum spatial resolution of the geometry of spacetime, the holographic model of spacetime foam stipulates that the uncertainty or fluctuation of distance l is given, on the average, by ( l l P 2 ) 1 / 3 where l P is the Planck length. Applied to cosmology, it predicts that the cosmic energy is of critical density and the cosmic entropy is the maximum allowed by the holographic principle. In addition, it requires the existence of unconventional (dark) energy/matter and accelerating cosmic expansion in the present era. We will argue that a holographic foam cosmology of this type has the potential to become a full fledged competitor (with distinct testable consequences) for scalar driven inflation.

Energy-momentum uncertainties as possible origin of threshold anomalies in UHECR and TeV-γ events

Abstract A threshold anomaly refers to a theoretically expected energy threshold that is not observed experimentally. Here we offer an explanation of the threshold anomalies encountered in the ultra-high energy cosmic ray events and the TeV- γ events, by arguing that energy–momentum uncertainties due to quantum gravity, too small to be detected in low-energy regime, can affect particle kinematics so as to raise or even eliminate the energy thresholds.