PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 794 September 26, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu
HYPERSOUND, ACOUSTIC PULSATION AT 200-GHz 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 GaAs and AlAs 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
([EMAIL PROTECTED], 33-1-4427-6980) says that he and
his colleagues hope to reach the terahertz acoustic range. The
wavelength for such sound is only nm 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)
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 (technical
term: the 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 %, 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 Keplers 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 ([EMAIL PROTECTED]), Cea and Tedesco say that a
uniform magnetic field pervading the cosmos, or a defect in the
fabric of spacetime, could bring about a nonzero eccentricity.
(Campanelli, Cea, Tedesco, Physical Review Letters, 29 September
2006 )
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