319 research outputs found
Falsifying Tree Level String Motivated Bouncing Cosmologies
The string effective action at tree level contains, in its bosonic sector,
the Einstein-Hilbert term, the dilaton, and the axion, besides scalar and gauge
fields coming from the Ramond-Ramond sector. The reduction to four dimensions
brings to scene moduli fields. We generalize this effective action by
introducing two arbitrary parameters, and , connected with the
dilaton and axion couplings. In this way, more general frameworks can be
analyzed. Regular solutions with a bounce can be obtained for a range of
(negative) values of the parameter which, however, exclude the pure
string configuration (). We study the evolution of scalar
perturbations in such cosmological scenarios. The predicted primordial power
spectrum decreases with the wavenumber with spectral index , in
contradiction with the results of the . Hence, all such effective string
motivated cosmological bouncing models seem to be ruled out, at least at the
tree level approximation.Comment: Latex file, 19 pages, 3 figures in eps forma
The radical left's turn towards civil society in Greece: One strategy, two paths
The Communist Party of Greece (KKE) and the Coalition of the Radical Left (SYRIZA) made remarkable ‘turns towards civil society’ over the last decade. It is argued that this was primarily a response aimed at strengthening their social legitimacy, which had reached its lowest point in the early 1990s. Differences in the way the two parties attempted to stabilise and engage their membership and re-establish links to trade unions and new social movements can be attributed to their distinct ideological and organisational legacies. Despite those differences, their respective linkage strategies were both successful until the game-changing 2012 Greek national elections, which brought about the remarkable rise of SYRIZA and the electoral demise of the KKE
The Hilbert space of Chern-Simons theory on the cylinder. A Loop Quantum Gravity approach
As a laboratory for loop quantum gravity, we consider the canonical
quantization of the three-dimensional Chern-Simons theory on a noncompact space
with the topology of a cylinder. Working within the loop quantization
formalism, we define at the quantum level the constraints appearing in the
canonical approach and completely solve them, thus constructing a gauge and
diffeomorphism invariant physical Hilbert space for the theory. This space
turns out to be infinite dimensional, but separable.Comment: Minor changes and some references added. Latex, 16 pages, 1 figur
Demixed principal component analysis of neural population data
Neurons in higher cortical areas, such as the prefrontal cortex, are often tuned to a variety of sensory and motor variables, and are therefore said to display mixed selectivity. This complexity of single neuron responses can obscure what information these areas represent and how it is represented. Here we demonstrate the advantages of a new dimensionality reduction technique, demixed principal component analysis (dPCA), that decomposes population activity into a few components. In addition to systematically capturing the majority of the variance of the data, dPCA also exposes the dependence of the neural representation on task parameters such as stimuli, decisions, or rewards. To illustrate our method we reanalyze population data from four datasets comprising different species, different cortical areas and different experimental tasks. In each case, dPCA provides a concise way of visualizing the data that summarizes the task-dependent features of the population response in a single figure
On the symmetries of BF models and their relation with gravity
The perturbative finiteness of various topological models (e.g. BF models)
has its origin in an extra symmetry of the gauge-fixed action, the so-called
vector supersymmetry. Since an invariance of this type also exists for gravity
and since gravity is closely related to certain BF models, vector supersymmetry
should also be useful for tackling various aspects of quantum gravity. With
this motivation and goal in mind, we first extend vector supersymmetry of BF
models to generic manifolds by incorporating it into the BRST symmetry within
the Batalin-Vilkovisky framework. Thereafter, we address the relationship
between gravity and BF models, in particular for three-dimensional space-time.Comment: 29 page
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