1,282 research outputs found
Linear Connections on Fuzzy Manifolds
Linear connections are introduced on a series of noncommutative geometries
which have commutative limits. Quasicommutative corrections are calculated.Comment: 10 pages PlainTex; LPTHE Orsay 95/42; ESI Vienna 23
Fuzzy Surfaces of Genus Zero
A fuzzy version of the ordinary round 2-sphere has been constructed with an
invariant curvature. We here consider linear connections on arbitrary fuzzy
surfaces of genus zero. We shall find as before that they are more or less
rigidly dependent on the differential calculus used but that a large number of
the latter can be constructed which are not covariant under the action of the
rotation group. For technical reasons we have been forced to limit our
considerations to fuzzy surfaces which are small perturbations of the fuzzy
sphere.Comment: 11 pages, Late
High Resolution Rotation Curves of Low Surface Brightness Galaxies
High resolution Halpha rotation curves are presented for five low surface
brightness galaxies. These Halpha rotation curves have shapes different from
those previously derived from HI observations, probably because of the higher
spatial resolution of the Halpha observations. The Halpha rotation curves rise
more steeply in the inner parts than the HI rotation curves and reach a flat
part beyond about two disk scale lengths. With radii expressed in optical disk
scale lengths, the rotation curves of the low surface brightness galaxies
presented here and those of HSB galaxies have almost identical shapes. Mass
modeling shows that the contribution of the stellar component to the rotation
curves may be scaled to explain most of the inner parts of the rotation curves,
albeit with high stellar mass-to-light ratios. On the other hand, well fitting
mass models can also be obtained with lower contributions of the stellar disk.
These observations suggest that the luminous mass density and the total mass
density are coupled in the inner parts of these galaxies.Comment: Accepted for publication in ApJ Letter
Linear connections on matrix geometries
A general definition of a linear connection in noncommutative geometry has
been recently proposed. Two examples are given of linear connections in
noncommutative geometries which are based on matrix algebras. They both possess
a unique metric connection.Comment: 14p, LPTHE-ORSAY 94/9
Topology at the Planck Length
A basic arbitrariness in the determination of the topology of a manifold at
the Planck length is discussed. An explicit example is given of a `smooth'
change in topology from the 2-sphere to the 2-torus through a sequence of
noncommuting geometries. Applications are considered to the theory of D-branes
within the context of the proposed (atrix) theory.Comment: Orsay Preprint 97/34, 17 pages, Late
Linear Connections in Non-Commutative Geometry
A construction is proposed for linear connections on non-commutative
algebras. The construction relies on a generalisation of the Leibnitz rules of
commutative geometry and uses the bimodule structure of . A special
role is played by the extension to the framework of non-commutative geometry of
the permutation of two copies of . The construction of the linear
connection as well as the definition of torsion and curvature is first proposed
in the setting of the derivations based differential calculus of Dubois-
Violette and then a generalisation to the framework proposed by Connes as well
as other non-commutative differential calculi is suggested. The covariant
derivative obtained admits an extension to the tensor product of several copies
of . These constructions are illustrated with the example of the
algebra of matrices.Comment: 15 pages, LMPM ../94 (uses phyzzx
Detection of the Red Giant Branch Stars in M82 Using the Hubble Space Telescope
We present color-magnitude diagrams and luminosity functions of stars in two
halo regions of the irregular galaxy in M82, based on F555W and F814W
photometry taken with the Hubble Space Telescope and Wide Field Planetary
Camera 2. The I-band luminosity function shows a sudden jump at I~23.95 mag,
which is identified as the tip of the red giant branch (TRGB). Adopting the Lee
et al. (1993) calibration of the TRGB based on the RR Lyrae distances to
Galactic globular clusters, we obtain the distance modulus of (m-M)_0=27.95 +-
0.14 (random) +- 0.16 (systematic) mag. This corresponds to a linear distance
of 3.9 +- 0.3 (random) +- 0.3 (systematicf) Mpc, which agrees well with the
distance of M81 deteremined from the HST observations of the Cepheid variable
stars. In addition, we observe a significant number of stars apparently
brighter than the TRGB. However, with the current data, we cannot rule out
whether these stars are blends of fainter stars, or are indeed intermediate-age
asymptotic giant branch stars.Comment: 8 figure
On Curvature in Noncommutative Geometry
A general definition of a bimodule connection in noncommutative geometry has
been recently proposed. For a given algebra this definition is compared with
the ordinary definition of a connection on a left module over the associated
enveloping algebra. The corresponding curvatures are also compared.Comment: 16 pages, PlainTe
Quantum Spacetimes in the Year 1
We review certain emergent notions on the nature of spacetime from
noncommutative geometry and their radical implications. These ideas of
spacetime are suggested from developments in fuzzy physics, string theory, and
deformation quantisation. The review focuses on the ideas coming from fuzzy
physics. We find models of quantum spacetime like fuzzy on which states
cannot be localised, but which fluctuate into other manifolds like .
New uncertainty principles concerning such lack of localisability on quantum
spacetimes are formulated.Such investigations show the possibility of
formulating and answering questions like the probabilty of finding a point of a
quantum manifold in a state localised on another one. Additional striking
possibilities indicated by these developments is the (generic) failure of
theorem and the conventional spin-statistics connection. They even suggest that
Planck's `` constant '' may not be a constant, but an operator which does not
commute with all observables. All these novel possibilities arise within the
rules of conventional quantum physics,and with no serious input from gravity
physics.Comment: 11 pages, LaTeX; talks given at Utica and Kolkata .Minor corrections
made and references adde
Noncommutative Geometry as a Regulator
We give a perturbative quantization of space-time in the case where the
commutators of the underlying algebra
generators are not central . We argue that this kind of quantum space-times can
be used as regulators for quantum field theories . In particular we show in the
case of the theory that by choosing appropriately the commutators
we can remove all the infinities by reproducing all the
counter terms . In other words the renormalized action on plus the
counter terms can be rewritten as only a renormalized action on the quantum
space-time . We conjecture therefore that renormalization of quantum
field theory is equivalent to the quantization of the underlying space-time
.Comment: Latex, 30 pages, no figures,typos corrected,references added .
Substantial amount of rewriting of the last section . Final interesting
remarks added at the end of the pape
- …