4,171 research outputs found
-odometers and their almost 1-1 extensions
In this paper we recall the concepts of -odometer and -subodometer for
-actions, where is a discrete finitely generated group, which generalize
the notion of odometer in the case G=\ZZ. We characterize the -regularly
recurrent systems as the minimal almost 1-1 extensions of subodometers, from
which we deduce that the family of the -Toeplitz subshifts coincides with
the family of the minimal symbolic almost 1-1 extensions of subodometers.Comment: 18 page
Uniqueness of the Fock quantization of the Gowdy model
After its reduction by a gauge-fixing procedure, the family of linearly
polarized Gowdy cosmologies admit a scalar field description whose
evolution is governed by a Klein-Gordon type equation in a flat background in
1+1 dimensions with the spatial topology of , though in the presence of a
time-dependent potential. The model is still subject to a homogeneous
constraint, which generates -translations. Recently, a Fock quantization
of this scalar field was introduced and shown to be unique under the
requirements of unitarity of the dynamics and invariance under the gauge group
of -translations. In this work, we extend and complete this uniqueness
result by considering other possible scalar field descriptions, resulting from
reasonable field reparameterizations of the induced metric of the reduced
model. In the reduced phase space, these alternate descriptions can be obtained
by means of a time-dependent scaling of the field, the inverse scaling of its
canonical momentum, and the possible addition of a time-dependent, linear
contribution of the field to this momentum. Demanding again unitarity of the
field dynamics and invariance under the gauge group, we prove that the
alternate canonical pairs of fieldlike variables admit a Fock representation if
and only if the scaling of the field is constant in time. In this case, there
exists essentially a unique Fock representation, provided by the quantization
constructed by Corichi, Cortez, and Mena Marugan. In particular, our analysis
shows that the scalar field description proposed by Pierri does not admit a
Fock quantization with the above unitarity and invariance properties.Comment: 14 page
A unique Fock quantization for fields in non-stationary spacetimes
In curved spacetimes, the lack of criteria for the construction of a unique
quantization is a fundamental problem undermining the significance of the
predictions of quantum field theory. Inequivalent quantizations lead to
different physics. Recently, however, some uniqueness results have been
obtained for fields in non-stationary settings. In particular, for vacua that
are invariant under the background symmetries, a unitary implementation of the
classical evolution suffices to pick up a unique Fock quantization in the case
of Klein-Gordon fields with time-dependent mass, propagating in a static
spacetime whose spatial sections are three-spheres. In fact, the field equation
can be reinterpreted as describing the propagation in a
Friedmann-Robertson-Walker spacetime after a suitable scaling of the field by a
function of time. For this class of fields, we prove here an even stronger
result about the Fock quantization: the uniqueness persists when one allows for
linear time-dependent transformations of the field in order to account for a
scaling by background functions. In total, paying attention to the dynamics,
there exists a preferred choice of quantum field, and only one
-invariant Fock representation for it that respects the standard
probabilistic interpretation along the evolution. The result has relevant
implications e.g. in cosmology.Comment: Typos correcte
Quantum unitary dynamics in cosmological spacetimes
We address the question of unitary implementation of the dynamics for scalar
fields in cosmological scenarios. Together with invariance under spatial
isometries, the requirement of a unitary evolution singles out a rescaling of
the scalar field and a unitary equivalence class of Fock representations for
the associated canonical commutation relations. Moreover, this criterion
provides as well a privileged quantization for the unscaled field, even though
the associated dynamics is not unitarily implementable in that case. We discuss
the relation between the initial data that determine the Fock representations
in the rescaled and unscaled descriptions, and clarify that the S-matrix is
well defined in both cases. In our discussion, we also comment on a recently
proposed generalized notion of unitary implementation of the dynamics, making
clear the difference with the standard unitarity criterion and showing that the
two approaches are not equivalent.Comment: 18 page
A uniqueness criterion for the Fock quantization of scalar fields with time dependent mass
A major problem in the quantization of fields in curved spacetimes is the
ambiguity in the choice of a Fock representation for the canonical commutation
relations. There exists an infinite number of choices leading to different
physical predictions. In stationary scenarios, a common strategy is to select a
vacuum (or a family of unitarily equivalent vacua) by requiring invariance
under the spacetime symmetries. When stationarity is lost, a natural
generalization consists in replacing time invariance by unitarity in the
evolution. We prove that, when the spatial sections are compact, the criterion
of a unitary dynamics, together with the invariance under the spatial
isometries, suffices to select a unique family of Fock quantizations for a
scalar field with time dependent mass.Comment: 11 pages, version accepted for publication in Classical and Quantum
Gravit
Quantum Gowdy model: A uniqueness result
Modulo a homogeneous degree of freedom and a global constraint, the linearly
polarised Gowdy cosmologies are equivalent to a free scalar field
propagating in a fixed nonstationary background. Recently, a new field
parameterisation was proposed for the metric of the Gowdy spacetimes such that
the associated scalar field evolves in a flat background in 1+1 dimensions with
the spatial topology of , although subject to a time dependent potential.
Introducing a suitable Fock quantisation for this scalar field, a quantum
theory was constructed for the Gowdy model in which the dynamics is implemented
as a unitary transformation. A question that was left open is whether one might
adopt a different, nonequivalent Fock representation by selecting a distinct
complex structure. The present work proves that the chosen Fock quantisation is
in fact unique (up to unitary equivalence) if one demands unitary
implementation of the dynamics and invariance under the group of constant
translations. These translations are precisely those generated by the global
constraint that remains on the Gowdy model. It is also shown that the proof of
uniqueness in the choice of complex structure can be applied to more general
field dynamics than that corresponding to the Gowdy cosmologies.Comment: 28 pages, minor changes, version accepted for publication in
Classical and Quantum Gravit
Criteria for the determination of time dependent scalings in the Fock quantization of scalar fields with a time dependent mass in ultrastatic spacetimes
For Klein-Gordon fields, it is well known that there exist an infinite number
of nonequivalent Fock representations of the canonical commutation relations
and, therefore, of inequivalent quantum theories. A context in which this kind
of ambiguities arises and prevents the derivation of robust results is, e.g.,
in the quantum analysis of cosmological perturbations. In these situations,
typically, a suitable scaling of the field by a time dependent function leads
to a description in an auxiliary static background, though the nonstationarity
still shows up in a time dependent mass. For such a field description, and
assuming the compactness of the spatial sections, we recently proved in three
or less spatial dimensions that the criteria of a natural implementation of the
spatial symmetries and of a unitary time evolution are able to select a unique
class of unitarily equivalent vacua, and hence of Fock representations. In this
work, we succeed to extend our uniqueness result to the consideration of all
possible field descriptions that can be reached by a time dependent canonical
transformation which, in particular, involves a scaling of the field by a
function of time. This kind of canonical transformations modify the dynamics of
the system and introduce a further ambiguity in its quantum description,
exceeding the choice of a Fock representation. Remarkably, for any compact
spatial manifold in less than four dimensions, we show that our criteria
eliminate any possible nontrivial scaling of the field other than that leading
to the description in an auxiliary static background. Besides, we show that
either no time dependent redefinition of the field momentum is allowed or, if
this may happen, the redefinition does not introduce any Fock representation
that cannot be obtained by a unitary transformation.Comment: 37 pages. Modified title. Improved discussion concerning the spatial
symmetry group. New section (section VI
Massless scalar field in de Sitter spacetime: unitary quantum time evolution
We prove that, under the standard conformal scaling, a massless field in de
Sitter spacetime admits an O(4)-invariant Fock quantization such that time
evolution is unitarily implemented. This result disproves previous claims in
the literature. We discuss the relationship between this quantization with
unitary dynamics and the family of O(4)-invariant Hadamard states given by
Allen and Folacci, as well as with the Bunch-Davies vacuum.Comment: 23 pages. Typos corrected, matches published versio
Florida Reconstruction Impeachments
The impeachment of Governor Harrison Reed not only contributes an interesting chapter to the history of Reconstruction but it also offers a number of novel precedents to the history of American impeachments. Throughout his gubernatorial term (1868-1872), Reed fought a consistent and courageous struggle against carpetbag politicians in Florida. I know of no other state or national civil officer against whom the impeachment remedy was so frequently attempted. On four occasions, he was threatened with legislative removal. Twice the lower house passed impeachment resolutions. Finally, in the last year of his term, his enemies voted a bona fide and legal impeachment against him; reported it to the senate in due form; suspended him from office; and, after all these apparent indices of triumph, they had to return him to power on account of an unusual and embarrassing political situation
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