Number 794, October 3, 2006
by Phil Schewe, Ben Stein, and Davide Castelvecchi
Hypersound
Hypersound, acoustic pulsation at 200 gigahertz frequencies, has been
produced in the same kind of resonant multilayered semiconductor
cavity as used in photonics. Physicists at the Institute des
Nanosciences de Paris (France) and the Centro Atomico Bariloche and
Instituto Balseiro (Argentina) generate the high frequency sound
pulses in a solid material made of thin gallium arsenide and aluminum arsenide layers. One
can picture the sound, excited by a femtosecond laser, as being a
short pulse of waves or equivalently as particle-like phonons,
excitations pulsing through the stack of layers. These phonons are
reflected at either end of the device, called a nanocavity, by
further layers with a much different acoustic impedance acting as
mirrors. Acoustic impedance is the acoustic analog of the
refractive index for light.
Bernard Jusserand
(bernard.jusserand@insp.jussieu.fr, 33-1-4427-6980) says that he and
his colleagues hope to reach the terahertz acoustic range. The
wavelength for such "sound" is only nanometers in length. They believe that
a new field, nanophononics, has been inaugurated, and that the
acoustical properties of semiconductor nanodevices will become more
prominent. THz phonons, and more specifically the reported
nanocavities could, for example, be used to modulate the flow of
charges or light at high frequency and in small spaces. THz sound
might also participate in the development of powerful "acoustic
lasers" or in novel forms of tomography for imaging the interior of
opaque solids.
Huynh et al.,
Physical Review Letters, 15 September 2006
Contact Bernard Jusserand
Institute des Nanosciences de Paris
Tel: +33-1-4427-6980
bernard.jusserand@insp.jussieu.fr
Ellipsoidal Universe
A new theoretical assessment of data taken by the Wilkinson Microwave Anisotropy Probe (WMAP)
suggests that the universe -- at least that
part of it that can be observed -- is not spherically symmetric, but more like an ellipsoid.
The WMAP data has served to nail down some of the most important parameters in all of
science, such as the age of the universe since the big bang (13.7 billion years), the
time when the first atoms formed (380,000 years after the big bang), and the fractions of
all available energy vested in the form of ordinary matter, dark matter, and dark energy.
One remaining oddity about the WMAP results, however, concerns the way in which portions of
the sky contribute to the overall map of cosmic microwaves; samples of the sky smaller than
one degree across, or at the degree level, or tens of degrees seem to be contributing
radiation at expected levels. Only the largest possible scale, that on the order of
the whole sky itself (the technical term is quadrupole moment), seems to be under-represented.
Now Leonardo Campanelli of the University of Ferrara and his colleagues Paolo Cea and
Luigi Tedesco at the University of Bari (all in Italy) have studied what happens to the
quadrupole anomaly if one supposes that the shell from which the cosmic microwaves come
toward earth is an ellipsoid and not a sphere. This shell is called surface of last
scattering since it corresponds to that moment in history when photons largely stopped
scattering from charged particles when it became cool enough for many of the particles
to bundle themselves into neutral atoms. If the microwave shell is an ellipsoid with
an eccentricity
(non-sphericity) of about 1 percent, then the WMAP quadrupole is exactly what it should be.
This is not the first time a non-spherical universe has been suggested, but it is the first
time the idea has been applied to the state-of-the-art WMAP data.
Historically an ellipsoidal universe would nicely parallel Johannes Kepler's discovery that
the planetary orbits were ellipses and not circles. This adjustment in astronomical thinking
was just as revolutionary as Copernicus' helio-centric model, and it
helped Newton and others arrive at the idea of an inverse-square law for gravitational attraction.
What could have caused the universe as a whole to be ellipsoidal? Campanelli
(campanelli@fe.infn.it), Cea and Tedesco say that a uniform magnetic field pervading the
cosmos, or a defect in the fabric of spacetime, could bring about a non-zero eccentricity.
Campanelli, Cea, and Tedesco,
Physical Review Letters, 29 September 2006
Contact Leonardo Campanelli
University of Ferrara
campanelli@fe.infn.it
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