Message-ID: <Pine.NEB.4.64.0701221458430.891@panix3.panix.com>
From: Alan Sondheim <sondheim@panix.com>
To: Cyb <cybermind@listserv.aol.com>, Wryting-L <WRYTING-L@listserv.wvu.edu>,
Cyberculture <cyberculture@zacha.org>
Subject: Physics News Update 809 (fwd)
Date: Mon, 22 Jan 2007 14:58:59 -0500 (EST)
---------- Forwarded message ---------- Date: Mon, 22 Jan 2007 14:33:13 -0500 From: physnews@aip.org To: sondheim@PANIX.COM Subject: Physics News Update 809 PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 809 22 January 2007 by Phillip F. Schewe, Ben Stein, Turner Brinton,and Davide Castelvecchi www.aip.org/pnu GRAVITATIONAL WAVE BACKGROUND. In the standard model of cosmology, the early universe underwent a period of fantastic growth. This inflationary phase, after only a trillionth of a second, concluded with a violent conversion of energy into hot matter and radiation. This �reheating� process also resulted in a flood of gravitational waves. (Interestingly, some cosmologists would identify the �big bang�with this moment and not the earlier time=0 moment.) Let�s compare this gravitational wave background (GWB) with the more familiar cosmic microwave background (CMB). The GWB dates from the trillionth-of-a-second mark, while the CMB sets in around 380,000 years later when the first atoms formed. The CMB represents a single splash of photons which were (at that early time) in equilibrium with the surrounding atoms-in-the-making; the microwaves we now see in the sky were (before being redshifted to lower frequencies owing to the universe�s expansion) ultraviolet waves and were suddenly freed to travel unimpeded through space. They are now observed to be mostly at a uniform temperature of about 3 K, but the overall map of the microwave sky does bear the faint imprint of matter inhomogeneities (lumps) existing even then. What, by contrast, does the GWB represent? It stems from three different production processes at work in the inflationary era: waves stemming from the inflationary expansion of space itself; waves from the collision of bubble-like clumps of new matter at reheating after inflation; and waves from the turbulent fluid mixing of the early pools of matter and radiation, before equilibrium among them (known as thermalization) had been achieved. The gravity waves would never have been in equilibrium with the matter (since gravity is such a weak force there wouldn�t be time to mingle adequately); consequently the GWB will not appear to a viewer now to be at a single overall temperature. A new paper by Juan Garcia-Bellido and Daniel Figueroa (Universidad Autonoma de Madrid) explain how these separate processes could be detected and differentiated in modern detectors set up to see gravity waves, such as LIGO, LISA, or BBO (Big Bang Observer). First, the GWB would be redshifted, like the CMB. But because of the GWB�s earlier provenance, the reshifting would be even more dramatic: the energy (and frequency) of the waves would be downshifted by 24 orders of magnitude. Second, the GWB waves would be distinct from gravity waves from point sources (such as the collision of two black holes) since such an encounter would release waves with a sharper spectral signal. By contrast the GWB from reheating after inflation would have a much broader spectrum, centered around 1 Hz to 1 GHz depending on the scale of inflation. Garcia-Bellido (34-91-497-4896, juan.garciabellido@uam.es) suggests that if a detector like the proposed BBO could disentangle the separate signals of the end-of-inflation GWB, then such a signal could be used as a probe of inflation and could help explore some fundamental issues as matter-antimatter asymmetry, the production of topological defects like cosmic strings, primoridal magnetic fields, and possibly superheavy dark matter. (Physical Review Letters, upcoming article; see also http://lattice.ft.uam.es/) TOMOGRAPHY OF PROTONS. In medical imaging, such as MRI, a planar slice of tissue can be imaged in longitudinal space. A three-dimensional image of structure in the body is built up from a composite of planar views. By analogy, physicists at the Jefferson Lab in Virginia are attempting to image the quarks inside protons, one planar slice at a time in momentum space, with the goal being the formation of a three dimensional quark map of the proton. In the case of proton tomography, the �microscope� consists of an intense beam of electrons which strikes a hydrogen target. An electron can scatter from a proton in many ways, but here a single collision is sought, a rather rare event called deeply virtual Compton scattering (DVCS); the incoming electron scatters by sending a virtual photon (a high energy gamma ray) out ahead of it. This scatters not from the proton as a whole, but from one of the elementary quarks that together with the gluons are the building blocks of the proton. The quark re-emits a gamma ray but does not otherwise change its identity. In this way the original target proton retains intact. Thus the overall reaction is as follows: an electron and proton collide and out comes an electron, proton, and gamma ray; the outgoing electron and gamma are detected, and from this a lot about the status of quarks inside the proton can be gleaned. For example, the spatial position of the quark inside the proton (transverse to the direction of the virtual photon) can be related to the angles and energies of the outgoing gamma ray. It�s as if a quark had been removed from one place inside the proton and then returned to another place. In one important sense the Jefferson Lab experiment is not like medical imaging. In conventional microscopy, decreasing the wavelength of the illumination source allows one to see finer details, and this is great when looking at the interior of tumors or cells. But the structures inside a proton, quarks, are pointlike, beyond the resolving power of any probe. Therefore, the structure of protons can be probed but not that of quarks. In proton tomography, the momentum transferred (actually the square of the transfer momentum, or Q^2) from electron to quark in the form of a virtual gamma ray should, up to a point, provide better spatial resolution. Beyond a certain level, however, a larger Q^2 does not get you greater resolving power. What this means is that the gamma is no longer probing the proton as a whole but rather individual quarks. The best one can do is to map out the probabilities for the presence of quarks with a certain momentum to reside at various places inside the proton; this is analogous to the �orbital� clouds used to depict the likely position of electrons in various energy levels inside atoms. Indeed, perhaps the most important thing achieved in the present experiment is to affirm that the scattering becomes independent of Q^2 above a level of about 2 GeV^2. This says that true tomography of the proton is proceeding. DVCS events, which have been seen in other experiments before but never with the exactitude employed here, are rare. Nevertheless, the Jefferson physicists were able to muster a million of them. With a requested upgrade in electron beam energy, the researchers hope to carry their map of the proton to quarks which carry a higher share of the proton�s momentum. This in turn will allow the JLab physicists to explore the origin of proton mass and spin. (Munoz Camacho et al., Physical Review Letters, 31 December 2006; contact Carlos Munoz Camacho, cmunoz@clipper.ens.fr, now also at Los Alamos National Lab, 505-606-6-7 ) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. Physics News Update appears approximately once a week. AUTO-SUBSCRIPTION OR DELETION: By using the expression "subscribe physnews" in your e-mail message, you will have automatically added the address from which your message was sent to the distribution list for Physics News Update. If you use the "signoff physnews" expression in your e-mail message, the address in your message header will be deleted from the distribution list. 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