3,990 research outputs found
Can effects of quantum gravity be observed in the cosmic microwave background?
We investigate the question whether small quantum-gravitational effects can
be observed in the anisotropy spectrum of the cosmic microwave background
radiation. An observation of such an effect is needed in order to discriminate
between different approaches to quantum gravity. Using canonical quantum
gravity with the Wheeler-DeWitt equation, we find a suppression of power at
large scales. Current observations only lead to an upper bound on the energy
scale of inflation, but the framework is general enough to study other
situations in which such effects might indeed be seen.Comment: 5 pages, 1 figure, essay awarded first prize in the Gravity Research
Foundation essay competition 201
Properties of cryogenically worked metals
A program was conducted to determine whether the mechanical properties of cryogenically worked 17-7PH stainless steel are suitable for service from ambient to cryogenic temperatures. It was determined that the stress corrosion resistance of the cryo-worked material is quite adequate for structural service. The tensile properties and fracture toughness at room temperature were comparable to titanium alloy 6Al-4V. However, at cryogenic temperatures, the properties were not sufficient to recommend consideration for structural service
Comments on Critical Electric and Magnetic Fields from Holography
We discuss some aspects of critical electric and magnetic fields in a field
theory with holographic dual description. We extend the analysis of
arxiv:1109.2920, which finds a critical electric field at which the Schwinger
pair production barrier drops to zero, to the case of magnetic fields. We first
find that, unlike ordinary weakly coupled theories, the magnetic field is not
subject to any perturbative instability originating from the presence of a
tachyonic ground state in the W-boson spectrum. This follows from the large
value of the 't Hooft coupling \lambda, which prevents the Zeeman interaction
term to overcome the particle mass at high B. Consequently, we study the next
possible B-field instability, i.e. monopole pair production, which is the
S-dual version of the Schwinger effect. Also in this case a critical magnetic
field is expected when the tunneling barrier drops to zero. These
Schwinger-type criticalities are the holographic duals, in the bulk, to the
fields E or B reaching the tension of F1 or D1 strings respectively. We then
discuss how this effect is modified when electric and magnetic fields are
present simultaneously and dyonic states in the spectrum can be pair produced
by a generic E - B background. Finally, we analyze finite temperature effects
on Schwinger criticalities, i.e. in the AdS-Schwarzshild black hole background.Comment: 33 pages, 4 figures; v2: refs added; v3: typos corrected, to appear
on JHE
What can quantum cosmology say about the inflationary universe?
We propose a method to extract predictions from quantum cosmology for
inflation that can be confronted with observations. Employing the tunneling
boundary condition in quantum geometrodynamics, we derive a probability
distribution for the inflaton field. A sharp peak in this distribution can be
interpreted as setting the initial conditions for the subsequent phase of
inflation. In this way, the peak sets the energy scale at which the
inflationary phase has started. This energy scale must be consistent with the
energy scale found from the inflationary potential and with the scale found
from a potential observation of primordial gravitational waves. Demanding a
consistent history of the universe from its quantum origin to its present
state, which includes decoherence, we derive a condition that allows one to
constrain the parameter space of the underlying model of inflation. We
demonstrate our method by applying it to two models: Higgs inflation and
natural inflation.Comment: 13 pages, 2 figures. Contribution to the Proceedings of the DICE14
meeting, Castiglioncello, September 201
Quantum cosmological consistency condition for inflation
We investigate the quantum cosmological tunneling scenario for inflationary
models. Within a path-integral approach, we derive the corresponding tunneling
probability distribution. A sharp peak in this distribution can be interpreted
as the initial condition for inflation and therefore as a quantum cosmological
prediction for its energy scale. This energy scale is also a genuine prediction
of any inflationary model by itself, as the primordial gravitons generated
during inflation leave their imprint in the B-polarization of the cosmic
microwave background. In this way, one can derive a consistency condition for
inflationary models that guarantees compatibility with a tunneling origin and
can lead to a testable quantum cosmological prediction. The general method is
demonstrated explicitly for the model of natural inflation.Comment: 1+16 pages, 3 figures. v2: typos corrected, minor improvement of the
discussio
Measurement Theory and General Relativity
The theory of measurement is employed to elucidate the physical basis of
general relativity. For measurements involving phenomena with intrinsic length
or time scales, such scales must in general be negligible compared to the
(translational and rotational) scales characteristic of the motion of the
observer. Thus general relativity is a consistent theory of coincidences so
long as these involve classical point particles and electromagnetic rays
(geometric optics). Wave optics is discussed and the limitations of the
standard theory in this regime are pointed out. A nonlocal theory of
accelerated observers is briefly described that is consistent with observation
and excludes the possibility of existence of a fundamental scalar field in
nature.Comment: LaTeX springer style lamu.cls, 2 figures, 16 pages, published in:
Black Holes: Theory and Observation: Proceedings of the 179th W.E. Heraeus
Seminar, held August 1997 in Bad Honnef, Germany. F.W. Hehl et al.(eds).
(Springer, Berlin Heidelberg 1998
Crogenic alloy screening Interim report
Evaluation of mechanical properties and fracture strength of aluminum alloys and stainless stee
Determination of low-temperature fatigue properties of structural metal alloys Final report, Jul. 1964 - Aug. 1965
Fatigue testing and determination of low temperature properties of structural metal alloys - aluminum alloy, stainless steel, and nickel alloy
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