221,724 research outputs found
Yang-Mills condensate dark energy coupled with matter and radiation
The coincidence problem is studied for the dark energy model of effective
Yang-Mills condensate in a flat expanding universe during the matter-dominated
stage. The YMC energy is taken to represent the dark energy, which
is coupled either with the matter, or with both the matter and the radiation
components. The effective YM Lagrangian is completely determined by quantum
field theory up to 1-loop order. It is found that under very generic initial
conditions and for a variety of forms of coupling, the existence of the scaling
solution during the early stages and the subsequent exit from the scaling
regime are inevitable. The transition to the accelerating stage always occurs
around a redshift . Moreover, when the Yang-Mills
condensate transfers energy into matter or into both matter and radiation, the
equation of state of the Yang-Mills condensate can cross over -1 around
, and takes on a current value . This is consistent with
the recent preliminary observations on supernovae Ia. Therefore, the
coincidence problem can be naturally solved in the effective YMC dark energy
models.Comment: 24 pages, 18 figure
Analytic Spectra of CMB Anisotropies and Polarization Generated by Relic Gravitational Waves with Modification due to Neutrino Free-Streaming
We present an analytical calculation of the spectra of CMB anisotropies and
polarizations generated by relic gravitational waves (RGWs). As a substantial
extension to the previous studies, three new ingredients are included in this
work. Firstly, the analytic and are given; especially the
latter can be useful to extract signal of RGWs from the observed data in the
zero multipole method. Secondly, a fitting formula of the decaying factor on
small scales is given, coming from the visibility function around the photon
decoupling. Thirdly, the impacts by the neutrino free-streaming (NFS) is
examined, a process that occurred in the early universe and leaves observable
imprints on CMB via RGWs.
It is found that the analytic and have profiles
agreeing with the numeric ones, except that in a range
and the trough of around have some deviations.
With the new damping factor, the analytic and match with
the numeric ones with the maximum errors only up to the first three
peaks for , improving the previous studies substantially. The
correspondence of the positions of peaks of and those of RGWs are
also demonstrated explicitly. We also find that NFS reduces the amplitudes of
by for and shifts slightly
their peaks to smaller angles. Detailed analyses show that the zero multipoles
, where crosses 0, are shifted to larger values by NFS. This
shifting effect is as important as those causedby different inflation models
and different baryon fractions.Comment: 17 pages, 7 figures. accepted by PR
Inflationary universe in loop quantum cosmology
Loop quantum cosmology provides a nice solution of avoiding the big bang
singularity through a big bounce mechanism in the high energy region. In loop
quantum cosmology an inflationary universe is emergent after the big bounce, no
matter what matter component is filled in the universe. A super-inflation phase
without phantom matter will appear in a certain way in the initial stage after
the bounce; then the universe will undergo a normal inflation stage. We discuss
the condition of inflation in detail in this framework. Also, for slow-roll
inflation, we expect the imprint from the effects of the loop quantum cosmology
should be left in the primordial perturbation power spectrum. However, we show
that this imprint is too weak to be observed.Comment: 21 pages, 4 figures; accepted for publication in JCA
Accelerating Universe from Extra Spatial Dimension
We present a simple higher dimensional FRW type of model where the
acceleration is apparently caused by the presence of the extra dimensions.
Assuming an ansatz in the form of the deceleration parameter we get a class of
solutions some of which shows the desirable feature of dimensional reduction as
well as reasonably good physical properties of matter. Interestingly we do not
have to invoke an extraneous scalar field or a cosmological constant to account
for this acceleration. One argues that the terms containing the higher
dimensional metric coefficients produces an extra negative pressure that
apparently drives the inflation of the 4D space with an accelerating phase. It
is further found that in line with the physical requirements our model admits
of a decelerating phase in the early era along with an accelerating phase at
present.Further the models asymptotically mimic a steady state type of universe
although it starts from a big type of singularity. Correspondence to Wesson's
induced matter theory is also briefly discussed and in line with it it is
argued that the terms containing the higher dimensional metric coefficients
apparently creates a negative pressure which drives the inflation of the
3-space with an accelerating phase.Comment: 0
Longitudinal LASSO: Jointly Learning Features and Temporal Contingency for Outcome Prediction
Longitudinal analysis is important in many disciplines, such as the study of
behavioral transitions in social science. Only very recently, feature selection
has drawn adequate attention in the context of longitudinal modeling. Standard
techniques, such as generalized estimating equations, have been modified to
select features by imposing sparsity-inducing regularizers. However, they do
not explicitly model how a dependent variable relies on features measured at
proximal time points. Recent graphical Granger modeling can select features in
lagged time points but ignores the temporal correlations within an individual's
repeated measurements. We propose an approach to automatically and
simultaneously determine both the relevant features and the relevant temporal
points that impact the current outcome of the dependent variable. Meanwhile,
the proposed model takes into account the non-{\em i.i.d} nature of the data by
estimating the within-individual correlations. This approach decomposes model
parameters into a summation of two components and imposes separate block-wise
LASSO penalties to each component when building a linear model in terms of the
past measurements of features. One component is used to select features
whereas the other is used to select temporal contingent points. An accelerated
gradient descent algorithm is developed to efficiently solve the related
optimization problem with detailed convergence analysis and asymptotic
analysis. Computational results on both synthetic and real world problems
demonstrate the superior performance of the proposed approach over existing
techniques.Comment: Proceedings of the 21th ACM SIGKDD International Conference on
Knowledge Discovery and Data Mining. ACM, 201
Quantum broadcast communication
Broadcast encryption allows the sender to securely distribute his/her secret
to a dynamically changing group of users over a broadcast channel. In this
paper, we just consider a simple broadcast communication task in quantum
scenario, which the central party broadcasts his secret to multi-receiver via
quantum channel. We present three quantum broadcast communication schemes. The
first scheme utilizes entanglement swapping and Greenberger-Horne-Zeilinger
state to realize a task that the central party broadcasts his secret to a group
of receivers who share a group key with him. In the second scheme, based on
dense coding, the central party broadcasts the secret to multi-receiver who
share each of their authentication key with him. The third scheme is a quantum
broadcast communication scheme with quantum encryption, which the central party
can broadcast the secret to any subset of the legal receivers
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