1,812 research outputs found
Dual Projection and Selfduality in Three Dimensions
We discuss the notion of duality and selfduality in the context of the dual
projection operation that creates an internal space of potentials. Contrary to
the prevailing algebraic or group theoretical methods, this technique is
applicable to both even and odd dimensions. The role of parity in the kernel of
the Gauss law to determine the dimensional dependence is clarified. We derive
the appropriate invariant actions, discuss the symmetry groups and their proper
generators. In particular, the novel concept of duality symmetry and
selfduality in Maxwell theory in (2+1) dimensions is analysed in details. The
corresponding action is a 3D version of the familiar duality symmetric
electromagnetic theory in 4D. Finally, the duality symmetric actions in the
different dimensions constructed here manifest both the SO(2) and
symmetries, contrary to conventional results.Comment: 20 pages, late
Damage spreading in two dimensional geometrically frustrated lattices: the triangular and kagome anistropic Heisenberg model
The technique of damage spreading is used to study the phase diagram of the
easy axis anisotropic Heisenberg antiferromagnet on two geometrically
frustrated lattices. The triangular and kagome systems are built up from
triangular units that either share edges or corners respectively. The
triangular lattice undergoes two sequential Kosterlitz-Thouless transitions
while the kagome lattice undergoes a glassy transition. In both cases, the
phase boundaries obtained using damage spreading are in good agreement with
those obtained from equilibrium Monte Carlo simulations.Comment: 7 pages, 4 figure
Let's talk about varying G
It is possible that fundamental constants may not be constant at all. There
is a generally accepted view that one can only talk about variations of
dimensionless quantities, such as the fine structure constant . However, constraints on the strength of
gravity tend to focus on G itself, which is problematic. We stress that G needs
to be multiplied by the square of a mass, and hence, for example, one should be
constraining , where is
the proton mass. Failure to focus on such dimensionless quantities makes it
difficult to interpret the physical dependence of constraints on the variation
of G in many published studies. A thought experiment involving talking to
observers in another universe about the values of physical constants may be
useful for distinguishing what is genuinely measurable from what is merely part
of our particular system of units.Comment: 6 pages, Gravity Research Foundation essa
Testing The Friedmann Equation: The Expansion of the Universe During Big-Bang Nucleosynthesis
In conventional general relativity, the expansion rate H of a
Robertson-Walker universe is related to the energy density by the Friedmann
equation. Aside from the present day, the only epoch at which we can constrain
the expansion history in a model-independent way is during Big-Bang
Nucleosynthesis (BBN). We consider a simple two-parameter characterization of
the behavior of H during BBN and derive constraints on this parameter space,
finding that the allowed region of parameter space is essentially
one-dimensional. We also study the effects of a large neutrino asymmetry within
this framework. Our results provide a simple way to compare an alternative
cosmology to the observational requirement of matching the primordial
abundances of the light elements.Comment: 18 pages, Final version to be published in Phys. Rev.
On the cosmological variation of the fine structure constant
A phenomenological model is proposed to explain the recent observed
cosmological variation of the fine structure constant as an effect of the
quantum vacuum, assuming a flat universe with cosmological constant
in the cases () equal to (0.3,0.7) and (1,0). Because
of the fourth Heisenberg relation, the lifetime of the virtual pairs of the
zero-point radiation must depend on the gravitational potential, so that the
quantum vacuum changes its density and acquires a relative permittivity
different from one. Since the matter was more concentrated in the past, the
gravitational potential of all the universe was stronger and the optical
density of the vacuum higher, the electron charge being then more renormalized
and smaller than now. The model is based on a first order Newtonian
approximation that is valid for the range of the observations, but not for very
high redshift, its prediction being that is
proportional to ,
being the scale factor. This agrees with the observations.Comment: 11 pages, one figure. In this new version, the effect of the
cosmological constant is considere
RXTE Studies of Long-Term X-ray Spectral Variations in 4U 1820-30
We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1820-30 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996-7. 4U 1820-30 is an ``atoll'' source X-ray burster
(XRB) located in the globular cluster NGC 6624. It is known to have an 11
minute binary period and a ~176 day modulation in its 2--12 keV flux.
Observations were made with the PCA and HEXTE instruments on RXTE at roughly
one-month intervals to sample this long-term period and study flux-related
spectral changes. There are clear correlations between our fitted spectral
parameters and both the broad-band (2--50 keV) flux and the position in the
color-color diagram, as described by the parameter S_a introduced by Mendez et
al. (1999). In addition, we find a strong correlation between the position in
the color-color diagram and the frequencies of the kilohertz quasi-periodic
oscillations (kHz QPOs) reported by Zhang et al. (1998). This lends further
support to the notion that evidence for the last stable orbit in the accretion
disk of 4U 1820-30 has been observed. For a model consisting of Comptonization
of cool photons by hot electrons plus an additional blackbody component, we
report an abrupt change in the spectral parameters at the same accretion rate
at which the kHz QPOs disappear. For a model consisting of a multicolor disk
blackbody plus a cut-off power law, we find that the inner disk radius reaches
a minimum at the same accretion rate at which the kHz QPO frequency saturates,
as expected if the disk reaches the last stable orbit. Both models face
theoretical and observational problems when interpreted physically for this
system.Comment: 39 pages, 11 figures, accepted to the Astrophysical Journa
Cosmology of a brane radiating gravitons into the extra dimension
We study in a self-consistent way the impact of the emission of bulk
gravitons on the (homogeneous) cosmology of a three-brane embedded in a
five-dimensional spacetime. In the low energy regime, we recover the well known
result that the bulk affects the Friedmann equation only via a radiation-like
term \C/a^4, called dark or Weyl radiation. By contrast, in the high energy
regime, we find that the Weyl parameter \C is no longer constant but instead
grows very rapidly as \C\propto a^4. As a consequence, the value of \C
today is not a free parameter as usually considered but is a fixed number,
which, generically, depends only on the number of relativistic degrees of
freedom at the high/low energy transition. Our estimated amount of Weyl
radiation satisfies the present nucleosynthesis bounds.Comment: 12 page
The Effect of Bound Dineutrons upon BBN
We have examined the effects of a bound dineutron, n2, upon big bang
nucleosynthesis (BBN) as a function of its binding energy B_n2. We find a
weakly bound dineutron has little impact but as B_n2 increases its presence
begins to alter the flow of free nucleons to helium-4. Due to this disruption,
and in the absence of changes to other binding energies or fundamental
constants, BBN sets a reliable upper limit of B_n2 <~ 2.5 MeV in order to
maintain the agreement with the observations of the primordial helium-4 mass
fraction and D/H abundance
Protecting the Primordial Baryon Asymmetry From Erasure by Sphalerons
If the baryon asymmetry of the universe was created at the GUT scale,
sphalerons together with exotic sources of -violation could have erased
it, unless the latter satisfy stringent bounds. We elaborate on how the small
Yukawa coupling of the electron drastically weakens previous estimates of these
bounds.Comment: 41 pp., 4 latex figures included and 3 uuencoded or postscript
figures available by request, UMN-TH-1213-9
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