86,039 research outputs found

### The Cosmological Constant Problem and Re-interpretation of Time

We abandon the interpretation that time is a global parameter in quantum
mechanics, replace it by a quantum dynamical variable playing the role of time.
This operational re-interpretation of time provides a solution to the
cosmological constant problem. The expectation value of the zero-point energy
under the new time variable vanishes. The fluctuation of the vacuum energy as
the leading contribution to the gravitational effect gives a correct order to
the observed "dark energy". The "dark energy" as a mirage is always seen
comparable with the matter energy density by an observer using the internal
clock time. Conceptual consequences of the re-interpretation of time are also
discussed.Comment: 9 pages, no figure; v3: improved discussion on remote simultaneity;
v4: improved discussion on coincidence problem, reproduced Einstein theory of
gravity from quantum reference frame, typos corrected, updated to the final
version published in Nuclear Physics

### Dark Energy from Quantum Uncertainty of Distant Clock

The observed cosmic acceleration was attributed to an exotic dark energy in
the framework of classical general relativity. The dark energy behaves very
similar with vacuum energy in quantum mechanics. However, once the quantum
effects are seriously taken into account, it predicts a completely wrong result
and leads to a severe fine-tuning. To solve the problem, the exact meaning of
time in quantum mechanics is reexamined. We abandon the standard interpretation
of time in quantum mechanics that time is just a global parameter, replace it
by a quantum dynamical variable playing the role of physical clock. We find
that synchronization of two spatially separated clocks can not be precisely
realized at quantum level. There is an intrinsic quantum uncertainty of distant
clock time, which implies an apparent vacuum energy fluctuation and gives an
observed dark energy density $\rho_{de}=\frac{6}{\pi}L_{P}^{-2}L_{H}^{-2}$ at
tree level approximation, where $L_{P}$ and $L_{H}$ are the Planck and Hubble
scale cutoffs. The fraction of the dark energy is given by
$\Omega_{de}=\frac{2}{\pi}$, which does not evolve with the internal clock
time. The "dark energy" as a quantum cosmic variance is always seen comparable
with the matter energy density by an observer using the internal clock time.
The corrected distance-redshift relation of cosmic observations due to the
distant clock effect are also discussed, which again gives a redshift
independent fraction $\Omega_{de}=\frac{2}{\pi}$. The theory is consistent with
current cosmic observations.Comment: 7 pages, no figure; v2:added discussion on distance-redshift
relation; v3:improved discussion on distance-redshift relation, an
independent calculation to the redshift variance over redshift squared is
given, dark energy fraction agrees with 2/pi; v4:typos corrected, updated to
the final version published in Journal of High Energy Physics, Volume 2015,
Issue

### Critical Relaxation and Critical Exponents

Dynamic relaxation of the XY model and fully frustrated XY model quenched
from an initial ordered state to the critical temperature or below is
investigated with Monte Carlo methods. Universal power law scaling behaviour is
observed. The dynamic critical exponent $z$ and the static exponent $\eta$ are
extracted from the time-dependent Binder cumulant and magnetization. The
results are competitive to those measured with traditional methods

### Radiation force on relativistic jets in active galactic nuclei

Radiative deceleration of relativistic jets in active galactic nuclei as the
result of inverse Compton scattering of soft photons from accretion discs is
discussed. The Klein-Nishina (KN) cross section is used in the calculation of
the radiation force due to inverse Compton scattering. Our result shows that
deceleration due to scattering in the KN regime is important only for jets
starting with a bulk Lorentz factor larger than 1000. When the bulk Lorentz
factor satisfies this condition, particles scattering in the Thomson regime
contribute positively to the radiation force (acceleration), but those
particles scattering in the KN regime are dominant and the overall effect is
deceleration. In the KN limit, the drag due to Compton scattering, though less
severe than in the Thomson limit, strongly constrains the bulk Lorentz factor.
Most of the power from the deceleration goes into radiation and hence the
ability of the jet to transport significant power (in particle kinetic energy)
out of the subparsec region is severely limited. The deceleration efficiency
decreases significantly if the jet contains protons and the proton to electron
number density ratio satisfies the condition $n_p/n_{e0}>2\gamma_{\rm
min}/\mu_p$ where $\gamma_{\rm min}$ is the minimum Lorentz factor of
relativistic electrons (or positrons) in the jet frame and $\mu_p$ is the
proton to electron mass ratio.Comment: 10 pages including 8 figures; accepted for publication in MNRA

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