Set-back for Dark Energy

Observational evidences suggest that the rate of expansion of the universe is increasing with time. This goes in contradiction to the expectation of some physicists that the finite energy of expansion would be continuously depleted by the gravitational attraction that holds the universe together. Some cosmologists tried to explain this increasing expansion with “dark energy” which may counteract the force of gravity at relatively short length scales – about 85 micrometres.

In order to explain the observed rate of expansion, dark energy must account for about 70% of all energy in the universe. But physicists still need a direct confirmation of its existence.

(photo of Dan Kapner, lead author of the paper; courtsey: the Eöt-Wash group )

In a recent paper in Physical Review Letters, a team of physicists from the Eöt-Wash group at the Center for Experimental Nuclear Physics and Astrophysics, University of Washington, Seattle reported their measurement of the force of gravity down to 55 micrometres and their conclusion that the inverse-square law remained valid well below 85 micrometres with 95% confidence. In a laboratory set-up, the scientists made very precise measurement of the gravitational attraction between two plates placed upon a torsion pendulum. Although a few other groups in various countries are engaged in such measurements, according to the Eöt-Wash researchers, their experiment offers the highest sensitivity at the length-scale associated with dark energy because it employs more interacting mass at the required separations than other setups.

Those who are familiar with such type of precision measurement will know that this puts a limit on the length-scale of any new type of interaction that can be theoretically predicted to exist. The experiment still does not rule out the existence of dark energy. But the potential implication of this experiment is very significant -- it's indeed a set-back for the theory of dark energy that could explain the increasing expansion of the universe.

Reference:

"Tests of the Gravitational Inverse-Square Law below the Dark-Energy Length Scale"
D. J. Kapner, T. S. Cook, E. G. Adelberger, J. H. Gundlach, B. R. Heckel, C. D. Hoyle, and H. E. Swanson,

Phys. Rev. Lett. 98, 021101 (8th January issue, 2007) Link to Abstract

Visible Light Transmitted Through Nanocable

Boston College physicists (L-R) Krzysztof Kempa, Michael Naughton, Jakub Rybczynski and Zhifeng Ren have transmitted visible light through a "nanocoax" cable they developed that is hundreds of times thinner than a human hair [photo courtsey: Boston College]

A team of physicists from Boston College have created the first nanoscale coaxial cables that can transmit visible light. Operating much like the coaxial cables used to distribute television and radio signals, these cables can transmit light with wavelengths nearly 4 times their 200 nm diameter. This discovery defies a key principle that says light cannot pass through a hole much smaller than its wavelength.

Their coaxial cable is based around a carbon nanotube, which forms the central conductor and is surrounded by a concentric ring of transparent aluminium oxide -- which acts as the dielectric layer – and finally a concentric conducting metal ring that acts as the outer conductor. This structure is able to enclose energy and let the cable transmit electromagnetic signals with wavelengths much larger than the diameter of the cable itself.

This achievement is built upon thir earlier (year 2004) invention of a microscopic antenna that captures visible light in much the same way radio antennae capture radio waves. This time they developed a "nanocoax" -- a carbon nanotube-based coaxial cable with a diameter of about 300 nm (a human hair is several hunderd times wider). The nanocoax is designed in a way such that the center wire protruded at one end, forming a light antenna. The other end was blunt, allowing measurement of the light received by the antenna and transmitted through the medium. The researchers were able to transmit both red and green light into the nanocoax and out the other end, indicating that the cable can carry a broad spectrum of visible light.

The researchers claim that the ability to control light over sub-wavelength distances could lead to better optical microscopes, smaller computer chips and more efficient solar panels. It may open the door to a wide array of new technologies, from high-efficiency, inexpensive solar cells to microscopic light-based switching devices for use in optical computing. The technology could even be used to help some blind people see through the creation of artificial retinas.

Reference: "Subwavelength waveguide for visible light",
Appl. Phys. Lett. 90, 021104 (January 8th issue, 2007) Link to Abstract
Authors: J. Rybczynski, K. Kempa, A. Herczynski, Y. Wang, M. J. Naughton, and Z. F. Ren of Dept of Physics, Boston College, MA; Z. P. Huang and D. Cai of NanoLab Inc., Newton, MA; M. Giersig of Center of Advanced European Studies and Research (CAESAR), Bonn, Germany.

3-D Map of Dark Matter

3-dimensional distribution of dark matter in current universe (image courtsey: HubbleSite.org)

Dark matter is an invisible form of matter that accounts for most of the universe's mass, but that so far has eluded direct detection, or even a definitive explanation for its makeup [See our past posting on the evidence of the existence of dark matter].

Now, an international team of astronomers, using NASA's Hubble Space Telescope, has created a comprehensive 3-dimensional map that offers a first look at the weblike large-scale distribution of dark matter in the universe. The map provides the best evidence yet that normal matter, largely in the form of galaxies, accumulates along the densest concentrations of dark matter. The map reveals a loose network of filaments that grew over time and intersect in massive structures at the locations of clusters of galaxies.

Researchers created the map using Hubble's largest survey of the universe, the Cosmic Evolution Survey ("COSMOS") with an international team of 70 astronomers led by Nick Scoville of California Institute of technology. In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the universe using the Advanced Camera for Surveys' (ACS) Wide Field Camera onboard Hubble. The survey covers a sufficiently wide area of sky allowing for the large-scale filamentary structure of dark matter to be evident. To add 3-D distance information, the Hubble observations were combined with multicolor data from powerful ground-based telescopes.

Almost all current scientific knowledge of the universe is related to only baryonic matter or the normal form of matter that we are familiar with. Now that scientists have begun to map out where dark matter is and how they are distributed alongside the baryonic matter, the next challenge is to determine what it is, and specifically its relationship to normal matter. This 3-D information is thus vital to studying the evolution of the structures of the distribution of matter over cosmic time.

The research results were presented at the 209th meeting of the American Astronomical Society in Seattle, Washington and also appeared online in the journal Nature yesterday.

Upcoming Physics Conferences 2007

Here is a selected list of forthcoming conferences in Physics. You are welcome to freely advertize Physics jobs or conferences in 2Physics by sending an email to 2Physics@gmail.com.

January 8-13: High-energy Quantum ChromoDynamics (QCD) : from RHIC to LHC (Trento, Trentino, Italy)
January 15-19: Higher structures in geometry and physics: Conference in honor of Murray Gerstenhaber's 80th and Jim Stasheff's 70th birthdays (Paris, France)
February 12-23: JIGSAW 2007 -- Joint Indo-German workshop and school on neutrinos in physics, astrophysics and cosmology (Mumbai, India)
February 24- March 3: 45th Schladming winter school on theoretical physics: conceptual and numerical challenges in femto- and peta-scale physics (Schladming, Austria)
March 5-9: Foundations of quantum theory "special focus session" on foundations of quantum theory at the APS March Meeting (Denver, Colorado, USA)
March 12-16: Nano and giga challenges (Tempe, Arizona, USA)
March 16-17: 23rd Pacific Coast Gravity Meeting (Caltech, Pasadena)
March 23-April 3: Quantum Gravity School (Zakopane, Poland)
March 24-29: The origin of galaxies (Otz Valley, nr. Innsbruck, Austria)
March 26-31: X hadron physics (Florianopolis, Brazil)
April 10-13: BICOS 2007 -- Bilbao Encounter On New Standard Cosmology (Bilbao, Spain)
April 23-27: Advanced computing and analysis techniques in physics (Amsterdam, The Netherlands)
June 1-5: Central European workshop on quantum optics, 14th edition (Palermo, Italy)
June 4-7: 6th intl conference on nuclear and radiation physics (Almaty, Kazakhstan)
June 11-22: Summer school on particle physics (Trieste, Italy)
June 11-29: Physics at TeV colliders (Les Houches, France)
June 22-July 3: 19th Petrov school -- summer school-seminar on recent problems in theoretical and mathematical physics (Kazan, Russia)
July 8-14: 7th Edoardo Amaldi Conference on Gravitational waves (Sydney, Australia)
July 26-August 1: 15th intl conference on supersymmetry and the unification of fundamental interactions (Karlsruhe, Germany)
July 30-August 11: Cosmology and particle physics beyond the standard models (Cargese, France)
August 23-29: 13th Lomonosov conferences on elementary particle physics (Moscow, Russia)
September 2-6: Photons, atoms, and qubits (Royal Society, London, UK)
September 3-7: 3rd intl conference on physics and control (Potsdam, Germany)
September 17-21: Quantum Field Theory (Leipzig, Germany)
October 1-5: Planets to Dark Energy (Manchester,UK)
October 11-13: Algebra, geometry, and mathematical physics (Göteborg, Sweden)