22,565 research outputs found
Geometric Drive of the Universe's Expansion
What if physics is just the way we perceive geometry? That is, what if
geometry and physics will one day become one and the same discipline? I believe
that will mean we will at last really understand physics, without postulates
other than those defining the particular space where the physics play is
performed. In this paper I use 5-dimensional spacetime as a point of departure
and make a very peculiar assignment between coordinates and physical distances
and time. I assume there is an hyperspherical symmetry which is made apparent
by assigning the hypersphere radius to proper time and distances on the
hypersphere to usual 3-dimensional distances. Time, or Compton time to
distinguish from cosmic time is the 0th coordinate and I am able to project
everything into 4-dimensions by imposing a null displacement condition.
Surprisingly nothing else is needed to explain Hubble's expansion law without
any appeal to dark matter; an empty Universe will expand naturally at a flat
rate in this way. I then discuss the perturbative effects of a small mass
density in the expansion rate in a qualitative way; quantitative results call
for the solution of equations that sometimes have not even been clearly
formulated and so are deferred to later work. A brief outlook of the
consequences an hyperspherical symmetry has for galaxy dynamics allows the
derivation of constant rotation velocity curves, again without appealing to
dark matter. An appendix explains how electromagnetism is made consistent with
this geometric approach and justifies the fact that photons must travel on
hypersphere circles, to be normal to proper time.Comment: Presented at the 1st Crisis in Cosmology Confrence, CCC-
Plus-minus construction leads to perfect invisibility
Recent theoretical advances applied to metamaterials have opened new avenues
to design a coating that hides objects from electromagnetic radiation and even
the sight. Here, we propose a new design of cloaking devices that creates
perfect invisibility in isotropic media. A combination of positive and negative
refractive indices, called plus-minus construction, is essential to achieve
perfect invisibility (i.e., no time delay and total absence of reflection).
Contrary to the common understanding that between two isotropic materials
having different refractive indices the electromagnetic reflection is
unavoidable, our method shows that surprisingly the reflection phenomena can be
completely eliminated. The invented method, different from the classical
impedance matching, may also find electromagnetic applications outside of
cloaking devices, wherever distortions are present arising from reflections.Comment: 24 pages, 10 figure
A New Design of Ultra-Flattened Near-zero Dispersion PCF Using Selectively Liquid Infiltration
The paper report new results of chromatic dispersion in Photonic Crystal
Fibers (PCFs) through appropriate designing of index-guiding triangular-lattice
structure devised, with a selective infiltration of only the first air-hole
ring with index-matching liquid. Our proposed structure can be implemented for
both ultra-low and ultra-flattened dispersion over a wide wavelength range. The
dependence of dispersion parameter of the PCF on infiltrating liquid indices,
hole-to-hole distance and air-hole diameter are investigated in details. The
result establishes the design to yield a dispersion of 0+-0.15ps/ (nm.km) in
the communication wavelength band. We propose designs pertaining to
infiltrating practical liquid for near-zero ultra-flat dispersion of
D=0+-0.48ps/ (nm.km) achievable over a bandwidth of 276-492nm in the wavelength
range of 1.26 {\mu}m to 1.80{\mu}m realization.Comment: 6 pages, 13 figures, 1 tabl
Transformation Optics and the Geometry of Light
Metamaterials are beginning to transform optics and microwave technology
thanks to their versatile properties that, in many cases, can be tailored
according to practical needs and desires. Although metamaterials are surely not
the answer to all engineering problems, they have inspired a series of
significant technological developments and also some imaginative research,
because they invite researchers and inventors to dream. Imagine there were no
practical limits on the electromagnetic properties of materials. What is
possible? And what is not? If there are no practical limits, what are the
fundamental limits? Such questions inspire taking a fresh look at the
foundations of optics and at connections between optics and other areas of
physics. In this article we discuss such a connection, the relationship between
optics and general relativity, or, expressed more precisely, between
geometrical ideas normally applied in general relativity and the propagation of
light, or electromagnetic waves in general, in materials. We also discuss how
this connection is applied: in invisibility devices, perfect lenses, the
optical Aharonov-Bohm effect of vortices and in analogues of the event horizon.Comment: 72 pages, 18 figures, preprint with low-resolution images.
Introduction to transformation optics, to appear in Progress in Optics
(edited by Emil Wolf
Blue and violet graviton spectra from a dynamical refractive index
We show that the spectral energy distribution of relic gravitons mildly
increases for frequencies smaller than the Hz and then flattens out
whenever the refractive index of the tensor modes is dynamical during a
quasi-de Sitter stage of expansion. For a conventional thermal history the
high-frequency plateau ranges between the mHz and the audio band but it is
supplemented by a spike in the GHz region if a stiff post-inflationary phase
precedes the standard radiation-dominated epoch. Even though the slope is blue
at intermediate frequencies, it may become violet in the MHz window. For a
variety of post-inflationary histories, including the conventional one, a
dynamical index of refraction leads to a potentially detectable spectral energy
density in the kHz and in the mHz regions while all the relevant
phenomenological constraints are concurrently satisfied.Comment: 10 pages, 2 figure
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