23 research outputs found
Chaotic inflation limits for non-minimal models with a Starobinsky attractor
We investigate inflationary attractor points by analyzing non-minimally
coupled single field inflation models in two opposite limits; the `flat' limit
in which the first derivative of the conformal factor is small and the `steep'
limit, in which the first derivative of the conformal factor is large. We
consider a subset of models that yield Starobinsky inflation in the steep
conformal factor, strong coupling, limit and demonstrate that they result in
chaotic inflation in the opposite flat, weak coupling, limit. The suppression
of higher order powers of the inflaton field in the potential is shown to be
related to the flatness condition on the conformal factor. We stress that the
chaotic attractor behaviour in the weak coupling limit is of a different, less
universal, character than the Starobinsky attractor. Agreement with the COBE
normalisation cannot be obtained in both attractor limits at the same time and
in the chaotic attractor limit the scale of inflation depends on the details of
the conformal factor, contrary to the strong coupling Starobinsky attractor.Comment: v2: 4 figures added, refs added, minor textual change
Properties of Causal Holographic Information
Causal holographic information [1] is a variant of the Ryu-Takayanagi
proposal for the entanglement entropy of a spatial region in the context of
AdS/CFT, but with the bulk surface defined by causality rather than
extremality. We investigate the properties of causal holographic information,
focusing in particular on the universal coefficient of the logarithmically
divergent term. We find that this coefficient contains a novel conformal
invariant that cannot be written as an integral of local quantities. By
considering higher curvature corrections in the bulk, we identify the
coefficient of the a and c central charges in 4 dimensions. Finally, we
speculate about which CFT quantity could correspond to the causal holographic
information
Vacua and correlators in hyperbolic de Sitter space
We study the power - and bi -spectrum of vacuum fluctuations in a hyperbolic
section of de Sitter space, comparing two states of physical interest: the
Bunch-Davies and hyperbolic vacuum. We introduce a one -parameter family of de
Sitter hyperbolic sections and their natural vacua, and identify a limit in
which it reduces to the planar section and the corresponding Bunch -Davies
vacuum state. Selecting the Bunch -Davies vacuum for a massless scalar field
implies a mixed reduced density matrix in a hyperbolic section of de Sitter
space. We stress that in the Bunch -Davies state the hyperbolic de Sitter
-point correlation functions have to match the planar de Sitter -point
correlation functions. The expressions for the planar and hyperbolic Bunch
-Davies correlation functions only appear different because of the
transformation from planar to hyperbolic coordinates. Initial state induced
deviations from the standard inflationary predictions are instead obtained by
considering the pure hyperbolic vacuum, as we verify explicitly by computing
the power - and bi -spectrum. For the bi -spectrum in the hyperbolic vacuum we
find that the corrections as compared to the standard Bunch -Davies result are
not enhanced in specific momentum configurations and strongly suppressed for
momenta large compared to the hyperbolic curvature scale. We close with some
final remarks, in particular regarding the implications of these results for
more realistic inflationary bubble scenarios.Comment: Added references, removed typos, added author, extensions in first
section and conclusions. 34 pages, 4 figure
Casting Shadows on Holographic Reconstruction
In the context of the AdS/CFT correspondence, we study several holographic
probes that relate information about the bulk spacetime to CFT data. The
best-known example is the relation between minimal surfaces in the bulk and
entanglement entropy of a subregion in the CFT. Building on earlier work, we
identify "shadows" in the bulk: regions that are not illuminated by any of the
bulk probes we consider, in the sense that the bulk surfaces do not pass
through these regions. We quantify the size of the shadow in the near horizon
region of a black hole and in the vicinity of a sufficiently dense star. The
existence of shadows motivates further study of the bulk-boundary dictionary in
order to identify CFT quantities that encode information about the shadow
regions in the bulk. We speculate on the interpretation of our results from a
dual field theory perspective.Comment: 42 pages, 38 figure
Speckle-scale focusing in the diffusive regime with time reversal of variance-encoded light (TROVE)
Focusing of light in the diffusive regime inside scattering media has long been considered impossible. Recently, this limitation has been overcome with time reversal of ultrasound-encoded light (TRUE), but the resolution of this approach is fundamentally limited by the large number of optical modes within the ultrasound focus. Here, we introduce a new approach, time reversal of variance-encoded light (TROVE), which demixes these spatial modes by variance encoding to break the resolution barrier imposed by the ultrasound. By encoding individual spatial modes inside the scattering sample with unique variances, we effectively uncouple the system resolution from the size of the ultrasound focus. This enables us to demonstrate optical focusing and imaging with diffuse light at an unprecedented, speckle-scale lateral resolution of ~5 µm
Translation correlations in anisotropically scattering media
Controlling light propagation across scattering media by wavefront shaping
holds great promise for a wide range of communications and imaging
applications. However, finding the right wavefront to shape is a challenge when
the mapping between input and output scattered wavefronts (i.e. the
transmission matrix) is not known. Correlations in transmission matrices,
especially the so-called memory-effect, have been exploited to address this
limitation. However, the traditional memory-effect applies to thin scattering
layers at a distance from the target, which precludes its use within thick
scattering media, such as fog and biological tissue. Here, we theoretically
predict and experimentally verify new transmission matrix correlations within
thick anisotropically scattering media, with important implications for
biomedical imaging and adaptive optics.Comment: main article (18 pages) and appendices (6 pages