1,398 research outputs found
An Infinite Number of Closed FLRW Universes for Any Value of the Spatial Curvature
The Friedman-Lemaitre-Robertson-Walker (FLRW) cosmological models are based
on the assumptions of large-scale homogeneity and isotropy of the distribution
of matter and energy. They are usually taken to have spatial sections that are
simply connected; they have finite volume in the positive curvature case, and
infinite volume in the null and negative curvature ones. I want to call the
attention to the existence of an infinite number of models, which are based on
these same metrics, but have compact, finite volume, multiply connected spatial
sections. Some observational implications are briefly mentioned.Comment: 4 pages. Contribution to the 5th International Workshop on Astronomy
and Relativistic Astrophysics (Joao Pessoa, PB, Brazil, October 10-12, 2011)
and to the 1o. Simposio Jayme Tiomno (Brasilia, DF, Brazil, October 27-28,
2011). In version 2: a few minor corrections; two new references added. In
this version: title correction in Ref. 3; dedication paragraph at the en
Lessons From the Latest Data on U.S. Productivity
La croissance de la productivité est examinée par les macro-économistes car elle joue des rôles clés dans la compréhension de l’épargne dans le secteur privé, les sources des chocs macroéconomiques, l’évolution de la compétitivité internationale et la solvabilité des régimes de retraite publics. Toutefois, les estimations des taux de croissance de la productivité anticipées et conjoncturelles souffrent de deux problèmes potentiels : (i) les estimations des tendances récentes sont imprécises, et (ii) les données récemment publiées subissent souvent d’importantes révisions.
Cette étude met en évidence la (non-) fiabilité de plusieurs mesures de croissance de la productivité agrégée aux États-Unis en examinant la mesure dans laquelle elles sont révisées au fil du temps. Nous examinons également dans quelle mesure ces révisions contribuent aux erreurs dans les prévisions de croissance de la productivité des États-Unis.
Nous constatons que les révisions de données provoquent généralement des changements appréciables des estimations des taux de croissance de la productivité publiés à travers une gamme de différentes mesures de la productivité. D'importantes révisions surviennent souvent des années après la première publication des données, ce qui contribue significativement à l'incertitude générale à laquelle nos décideurs politiques doivent faire face. Cela souligne le besoin de moyens pour réduire l'incertitude à laquelle sont confrontés les décideurs politiques et les politiques robustes à l'incertitude sur les conditions économiques actuelles. La croissance de la productivité est examinée par les macro-économistes car elle joue des rôles clés dans la compréhension de l’épargne dans le secteur privé, les sources des chocs macroéconomiques, l’évolution de la compétitivité internationale et la solvabilité des régimes de retraite publics. Toutefois, les estimations des taux de croissance de la productivité anticipées et conjoncturelles souffrent de deux problèmes potentiels : (i) les estimations des tendances récentes sont imprécises, et (ii) les données récemment publiées subissent souvent d’importantes révisions.
Cette étude met en évidence la (non-) fiabilité de plusieurs mesures de croissance de la productivité agrégée aux États-Unis en examinant la mesure dans laquelle elles sont révisées au fil du temps. Nous examinons également dans quelle mesure ces révisions contribuent aux erreurs dans les prévisions de croissance de la productivité des États-Unis.
Nous constatons que les révisions de données provoquent généralement des changements appréciables des estimations des taux de croissance de la productivité publiés à travers une gamme de différentes mesures de la productivité. D'importantes révisions surviennent souvent des années après la première publication des données, ce qui contribue significativement à l'incertitude générale à laquelle nos décideurs politiques doivent faire face. Cela souligne le besoin de moyens pour réduire l'incertitude à laquelle sont confrontés les décideurs politiques et les politiques robustes à l'incertitude sur les conditions économiques actuelles.Productivité, analyses en temps réel, révisions de données, projections Greenbook projections , Productivité, analyses en temps réel, révisions de données, projections Greenbook projections
Extending invariant complex structures
We study the problem of extending a complex structure to a given Lie algebra
g, which is firstly defined on an ideal h of g. We consider the next
situations: h is either complex or it is totally real. The next question is to
equip g with an additional structure, such as a (non)-definite metric or a
symplectic structure and to ask either h is non-degenerate, isotropic, etc.
with respect to this structure, by imposing a compatibility assumption. We show
that this implies certain constraints on the algebraic structure of g.
Constructive examples illustrating this situation are shown, in particular
computations in dimension six are given.Comment: 22 pages, plus an Addendu
Normal frames and the validity of the equivalence principle. I. Cases in a neighborhood and at a point
A treatment in a neighborhood and at a point of the equivalence principle on
the basis of derivations of the tensor algebra over a manifold is given.
Necessary and sufficient conditions are given for the existence of local bases,
called normal frames, in which the components of derivations vanish in a
neighborhood or at a point. These frames (bases), if any, are explicitly
described and the problem of their holonomicity is considered. In particular,
the obtained results concern symmetric as well as nonsymmetric linear
connections.Comment: LaTeX2e, 9 pages, to be published in Journal of Physics A:
Mathematical and Genera
Can GDP measurement be further improved? Data revision and reconciliation
Recent years have seen many attempts to combine expenditure-side estimates of
U.S. real output (GDE) growth with income-side estimates (GDI) to improve
estimates of real GDP growth. We show how to incorporate information from
multiple releases of noisy data to provide more precise estimates while
avoiding some of the identifying assumptions required in earlier work. This
relies on a new insight: using multiple data releases allows us to distinguish
news and noise measurement errors in situations where a single vintage does
not.
Our new measure, GDP++, fits the data better than GDP+, the GDP growth
measure of Aruoba et al. (2016) published by the Federal Reserve Bank of
Philadephia. Historical decompositions show that GDE releases are more
informative than GDI, while the use of multiple data releases is particularly
important in the quarters leading up to the Great Recession
Can GDP Measurement Be Further Improved? Data Revision and Reconciliation
Recent years have seen many attempts to combine expenditure-side estimates of U.S. real output (GDE) growth with income-side estimates (GDI) to improve estimates of real GDP growth. We show how to incorporate information from multiple releases of noisy data to provide more precise estimates while avoiding some of the identifying assumptions required in earlier work. This relies on a new insight: using multiple data releases allows us to distinguish news and noise measurement errors in situations where a single vintage does not. We find that (a) the data prefer averaging across multiple releases instead of discarding early releases in favor of later ones, and (b) that initial estimates of GDI are quite informative. Our new measure, GDP(++), undergoes smaller revisions and tracks expenditure measures of GDP growth more closely than either the simple average of the expenditure and income measures published by the BEA or the GDP growth measure of Aruoba et al. published by the Federal Reserve Bank of Philadelphia
Broadband Meter-Wavelength Observations of Ionospheric Scintillation
Intensity scintillations of cosmic radio sources are used to study
astrophysical plasmas like the ionosphere, the solar wind, and the interstellar
medium. Normally these observations are relatively narrow band. With Low
Frequency Array (LOFAR) technology at the Kilpisj\"arvi Atmospheric Imaging
Receiver Array (KAIRA) station in northern Finland we have observed
scintillations over a 3 octave bandwidth. ``Parabolic arcs'', which were
discovered in interstellar scintillations of pulsars, can provide precise
estimates of the distance and velocity of the scattering plasma. Here we report
the first observations of such arcs in the ionosphere and the first broad-band
observations of arcs anywhere, raising hopes that study of the phenomenon may
similarly improve the analysis of ionospheric scintillations. These
observations were made of the strong natural radio source Cygnus-A and covered
the entire 30-250\,MHz band of KAIRA. Well-defined parabolic arcs were seen
early in the observations, before transit, and disappeared after transit
although scintillations continued to be obvious during the entire observation.
We show that this can be attributed to the structure of Cygnus-A. Initial
results from modeling these scintillation arcs are consistent with simultaneous
ionospheric soundings taken with other instruments, and indicate that
scattering is most likely to be associated more with the topside ionosphere
than the F-region peak altitude. Further modeling and possible extension to
interferometric observations, using international LOFAR stations, are
discussed.Comment: 11 pages, 17 figure
Frames of reference in spaces with affine connections and metrics
A generalized definition of a frame of reference in spaces with affine
connections and metrics is proposed based on the set of the following
differential-geometric objects:
(a) a non-null (non-isotropic) vector field,
(b) the orthogonal to the vector field sub space,
(c) an affine connection and the related to it covariant differential
operator determining a transport along the given non-null vector filed.
On the grounds of this definition other definitions related to the notions of
accelerated, inertial, proper accelerated and proper inertial frames of
reference are introduced and applied to some mathematical models for the
space-time. The auto-parallel equation is obtained as an Euler-Lagrange's
equation. Einstein's theory of gravitation appears as a theory for
determination of a special frame of reference (with the gravitational force as
inertial force) by means of the metrics and the characteristics of a material
distribution.
PACS numbers: 0490, 0450, 1210G, 0240VComment: 17 pages, LaTeX 2
- …