44 research outputs found
Oscillating universe in the DGP braneworld
With a method in which the Friedmann equation is written in a form such that
evolution of the scale factor can be treated as that of a particle in a
"potential", we classify all possible cosmic evolutions in the DGP braneworld
scenario with the dark radiation term retained. By assuming that the energy
component is pressureless matter, radiation or vacuum energy, respectively, we
find that in the matter or vacuum energy dominated case, the scale factor has a
minimum value . In the matter dominated case, the big bang singularity can
be avoided in some special circumstances, and there may exist an oscillating
universe or a bouncing one. If the cosmic scale factor is in the oscillating
region initially, the universe may undergo an oscillation. After a number of
oscillations, it may evolve to the bounce point through quantum tunneling and
then expand. However, if the universe contracts initially from an infinite
scale, it can turn around and then expand forever. In the vacuum energy
dominated case, there exists a stable Einstein static state to avoid the big
bang singularity. However, in certain circumstances in the matter or vacuum
energy dominated case, a new kind of singularity may occur at as a result
of the discontinuity of the scale factor. In the radiation dominated case, the
universe may originate from the big bang singularity, but a bouncing universe
which avoids this singularity is also possible.Comment: 25 pages, 24 figures. To appear in PR
Running cosmological constant with observational tests
We investigate the running cosmological constant model with dark energy
linearly proportional to the Hubble parameter, , in which the CDM limit is recovered by taking .
We derive the linear perturbation equations of gravity under the
Friedmann-Lema\"itre-Robertson-Walker cosmology, and show the power spectra of
the CMB temperature and matter density distribution. By using the Markov chain
Monte Carlo method, we fit the model to the current observational data and find
that and for coupled to matter and radiation-matter, respectively,
along with constraints on other cosmological parameters.Comment: 12 pages, 5 figures, version accepted by PL
Towards Open Temporal Graph Neural Networks
Graph neural networks (GNNs) for temporal graphs have recently attracted
increasing attentions, where a common assumption is that the class set for
nodes is closed. However, in real-world scenarios, it often faces the open set
problem with the dynamically increased class set as the time passes by. This
will bring two big challenges to the existing dynamic GNN methods: (i) How to
dynamically propagate appropriate information in an open temporal graph, where
new class nodes are often linked to old class nodes. This case will lead to a
sharp contradiction. This is because typical GNNs are prone to make the
embeddings of connected nodes become similar, while we expect the embeddings of
these two interactive nodes to be distinguishable since they belong to
different classes. (ii) How to avoid catastrophic knowledge forgetting over old
classes when learning new classes occurred in temporal graphs. In this paper,
we propose a general and principled learning approach for open temporal graphs,
called OTGNet, with the goal of addressing the above two challenges. We assume
the knowledge of a node can be disentangled into class-relevant and
class-agnostic one, and thus explore a new message passing mechanism by
extending the information bottleneck principle to only propagate class-agnostic
knowledge between nodes of different classes, avoiding aggregating conflictive
information. Moreover, we devise a strategy to select both important and
diverse triad sub-graph structures for effective class-incremental learning.
Extensive experiments on three real-world datasets of different domains
demonstrate the superiority of our method, compared to the baselines.Comment: ICLR 2023 Ora
Robust Outlier Detection Method Based on Local Entropy and Global Density
By now, most outlier-detection algorithms struggle to accurately detect both
point anomalies and cluster anomalies simultaneously. Furthermore, a few
K-nearest-neighbor-based anomaly-detection methods exhibit excellent
performance on many datasets, but their sensitivity to the value of K is a
critical issue that needs to be addressed. To address these challenges, we
propose a novel robust anomaly detection method, called Entropy Density Ratio
Outlier Detection (EDROD). This method incorporates the probability density of
each sample as the global feature, and the local entropy around each sample as
the local feature, to obtain a comprehensive indicator of abnormality for each
sample, which is called Entropy Density Ratio (EDR) for short in this paper. By
comparing several competing anomaly detection methods on both synthetic and
real-world datasets, it is found that the EDROD method can detect both point
anomalies and cluster anomalies simultaneously with accurate performance. In
addition, it is also found that the EDROD method exhibits strong robustness to
the number of selected neighboring samples, the dimension of samples in the
dataset, and the size of the dataset. Therefore, the proposed EDROD method can
be applied to a variety of real-world datasets to detect anomalies with
accurate and robust performances
Parameter-Efficient Conformers via Sharing Sparsely-Gated Experts for End-to-End Speech Recognition
While transformers and their variant conformers show promising performance in
speech recognition, the parameterized property leads to much memory cost during
training and inference. Some works use cross-layer weight-sharing to reduce the
parameters of the model. However, the inevitable loss of capacity harms the
model performance. To address this issue, this paper proposes a
parameter-efficient conformer via sharing sparsely-gated experts. Specifically,
we use sparsely-gated mixture-of-experts (MoE) to extend the capacity of a
conformer block without increasing computation. Then, the parameters of the
grouped conformer blocks are shared so that the number of parameters is
reduced. Next, to ensure the shared blocks with the flexibility of adapting
representations at different levels, we design the MoE routers and
normalization individually. Moreover, we use knowledge distillation to further
improve the performance. Experimental results show that the proposed model
achieves competitive performance with 1/3 of the parameters of the encoder,
compared with the full-parameter model.Comment: accepted in INTERSPEECH 202
The stability of Einstein static universe in the DGP braneworld
The stability of an Einstein static universe in the DGP braneworld scenario
is studied in this paper. Two separate branches denoted by of
the DGP model are analyzed. Assuming the existence of a perfect fluid with a
constant equation of state, , in the universe, we find that, for the branch
with , there is no a stable Einstein static solution, while, for
the case with , the Einstein static universe exists and it is
stable when . Thus, the universe can stay at this stable state
past-eternally and may undergo a series of infinite, non-singular oscillations.
Therefore, the big bang singularity problem in the standard cosmological model
can be resolved.Comment: 10 pages, 2 figures, to appear in PL