Text 731, 99 rader
Skriven 2006-03-17 22:50:14 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 769
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PHYSICS NEWS UPDATE
                                        
The American Institute of Physics Bulletin of Physics News
Number 769   March 17, 2006  by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi

THE INFLATIONARY BIG BANG MODEL has passed a crucial test as
scientists working on the Wilkinson Microwave Anisotropy Probe
released a long-awaited second set of data at a press conference
held March 17.  WMAP was launched in 2001 to map the anisotropies in
the cosmic microwave background (CMB) with far greater precision
than the Cosmic Background Explorer, the predecessor that first
discovered the anisotropies in 1990s.  The earlier release of WMAP
data 3 years ago nailed down several grand features of the universe
that had previously been known only very roughly, including: the
time of recombination (380,000 years after the big bang, when the
first atoms were formed); the age of the universe (13.7 billion
years, plus or minus 200 million years); and the makeup of the
universe (with dark energy accounting for 73% of all energy---see
update 624, http://aip.org/pnu/2003/split/624-1.html).
Since that 2003 announcement WMAP researchers have painstakingly
worked to reduce the uncertainties in their results.  The big new
thing in yesterday's announcement, based on three years of data, was
the release of a map of the sky containing information about the
microwaves' polarization.  The microwaves are partly polarized
(oriented) from the time of their origin (emerging from the so
called sphere of last scattering---see
http://www.aip.org/pnu/2002/split/591-1.html ) and partly polarized
by scattering (later on their journey to Earth) from the pervasive
plasma (mostly ionized hydrogen) created when ultraviolet radiation
from the first generation of stars struck surrounding interstellar
gas.  WMAP now estimates that this reionization, effectively
denoting the era of the first stars, occurred 400 million years
after the big bang, instead of 200 million years as had been
previously thought.
The main step forward is that smaller error bars, courtesy of the
polarization map and the much better temperature map across the sky
(with an uncertainty of only 200 nK), provide a new estimate for the
inhomogeneities in the CMB's temperature. The simplest model, called
Harrison-Zeldovic, posits that the spectrum of inhomogeneities
should be flat; that is, the inhomogeneities should have the same
variation at all scales. Inflation, on the other hand, predicts a
slight deviation from this flatness. The new WMAP data for the first
time measures the spectrum with enough precision to show a
preference for inflation rather than the Harrison-Zeldovic
spectrum---a test that was long-awaited as inflation's smoking gun.
(Papers available at
map.gsfc.nasa.gov/m_mm/pub_papers/threeyear.html )

TWO-DIMENSIONAL CARBON, OR GRAPHENE, has many of the interesting
properties possessed by one-dimensional carbon (in the form of
nanotubes): electrons can move at high speed and suffer little
energy loss.  According to Walt deHeer (Georgia Tech), who spoke at
this week's meeting of the American Physical Society (APS) in
Baltimore, graphene will provide a more controllable platform for
integrated electronics than is possible with nanotubes since
graphene structures can be fabricated lithographically as large
wafers.  Single sheets of graphene were only isolated in 2004 by
Andre Geim (Univ. Manchester).  In graphene, electron velocity is
independent of energy.  That is, electrons move as if they were
light waves; they act as if they were massless particles.  This
extraordinary property was elucidated in November 2005 through
experiments (see background article in the Jan. 2006 Physics Today)
using the quantum Hall effect (QHE), in which electrons, confined to
a plane and subjected to high magnetic fields, execute only
prescribed quantum trajectories.  These tests were conducted by
groups represented at the APS meeting by Geim and Philip Kim
(Columbia Univ.).
The QHE studies also revealed that when an electron completes a full
circular trajectory in the imposed magnetic field, its wavefunction
(encapsulating the electron's quantum wave nature) is shifted by 180
degrees.  This modification, called "Berry's phase," acts to reduce
the propensity for electrons to scatter in the backwards direction;
this in turn helps reduce electron energy loss.  Geim reported a new
twist to this story.  Studying QHE in graphene bilayers he observed
a new version of QHE, featuring a doubled Berry's phase of 360
degrees.  Also, Geim drew a comparison to certain cosmologies in
which multiple universes can co-exist, each with its own set of
physical constants; in graphene, he said, where electrons move in a
light-like way, with a fast speed (but one somewhat less than the
speed of light in vacuum), the parameter which sets the scale of the
electromagnetic force, namely the fine structure constant (defined
as e^2/hc), has a value of roughly 2.0 rather than the customary
1/137.
The goal now is to learn more graphene physics and then worry about
applications.  For example, Walt deHeer reported that a plot of
resistance versus applied magnetic field had a fractal shape.
DeHeer said that has so far has no explanation for this. As for
applications, he said that on an all-graphene chip, linking
components with the usual metallic interconnects (which tends to
disrupt quantum relations) would not be necessary.  Thus the wave
nature of electrons could be more fully exploited for
quantum-information purposes.  De Heer's group so far has been
attempting to build circuitry in this way; they have made graphene
structures (including a graphene transistor) as small as 80 nm and
expect to get down to the 10-nm size.

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 * Origin: Big Bang (1:106/2000.7)