Extracts from the Internet

Gravitational redshift

In an experiment with ultracold cesium atoms researchers Н. Miiller, A. Peters and S. Chu from the USA and Germany measured the effect of gravitational red shift (the slowing down of time in gravitational field) at an accuracy of 7 × 10^-9 which is the current record. The authors used the data of an earlier experiment on measuring the free fall acceleration. Cesium atoms kicked upwards were placed by a laser pulse into a superposition of two states that corresponded to trajectories which differed in maximum heights by 0.12 mm. The next pulse corrected the trajectories in such a way that they intersected at the lower point while the third laser pulse was used to interference measurement of the phase difference between atomic wave functions on different trajectories. The data obtained in this experiment made it possible to improve the previous result (obtained in 1980 by comparing the readings of the atomic clock on the Earth surface and in a rocket) by four orders of magnitude. High-accuracy measurements of the gravitational red shift are important, among other things, for testing the theory of gravitation. Also, improved accuracy of such measurements may have practical significance for cosmic navigational systems. Source: Nature 463 926 (2010)

Chemical transformations of ultracold molecules

A group of researchers led by R. Grimm (University of Insbruck and Institute of Quantum Optics and Quantum Information in Austria) observed exchange processes in ultracold gas of cesium atoms in magnetic field (at a temperature of 50-100 nK) in the optical trap. Some atoms were transferred to different states of hyperfine splitting A and B by applying microwave pulses. A change in magnetic field (and hence in the molecular bonding energy) in the vicinity of the Feshbach resonance caused exchange processes: replacement of cesium atoms A in a weakly bonded dimer A₂ by cesium atoms B in a different quantum state, A₂ + B → A + AB. The second research team from the Joint Institute of Laboratory Astrophysics (JILA) studied chemical reactions involving ultracold moleculs formed of atoms of potassium and rubidium. This experiment also studied how the spin state of molecules affected the rate of chemical reactions. Sources: Phys. Rev. Lett. 104 053201 (2010) , sciencedaily.com

Phonon “laser”

Two independent groups of researchers created phonon analogs of the optical laser which emit coherent beams of phonons (acoustic oscillations of the crystal lattice). I.S. Grudinin and his coworkers at the California Institute of Technology used two ring microresonators (microtoroids) whose mechanical resonance frequency belonged in the radio frequency range. The energy pumping of the system was created by an optical laser beam through an optical fiber. The microresonators were connected by means of the evanescent field of the beam which resulted in splitting of the energy levels (oscillation modes) of the electromagnetic field. The difference between the energies of sublevels equalled the energy of phonons (at a frequency of about 21 MHz) emitted as a coherent beam when sublevels exchange photons. Earlier experiments already observed stimulated emission of phonons but stimulated emission intensity greater than that of spontaneous emission has not been achieved yet. In the present study this result was obtained owing to high quality of the resonators. R.P. Beardsley and his colleagues at the University of Nottingham created a device that resembles a quantum-cascade laser. Phonons at a frequency of approximately 441 GHz were generated on account of the electron-phonon interaction when electrons tunnel through potential barriers of the semiconductor superlattice which is formed of alternating layers of GaAs and AlAs. The acoustic wave was generated by a powerful laser pulse; new phonons are then generated coherently and amplify the wave. Phonon “laser” may find useful applications both for observing microscopic objects and for a predesigned manner of modifying them. Sources: Phys. Rev. Lett. 104 083901 (2010) , Phys. Rev. Lett. 104 085501 (2010)

Studies of supernovas

Mildly relativistic ejecta. Z. Paragi and his coworkers used EVN (European VLBI network), GBT and WSRT radiotelescopes to observe the SN 2007gr supernova which exploded in the NGC 1058 galaxy at a distance of about 10.6 Mpc from the Earth, and discovered a moderately relativistic motion of matter in the shell shed during the explosion. According to the data of optical observations, the SN 2007gr is an ordinary type-Ic supernova and the speed of expansion of its shell is a mere ≈ 6000 km s^-1. Four Ic supernovas have been recently identified as sources of cosmic gamma-ray bursts which from the theoretical point of view requires that much faster collimated relativistic ejecta (jets) should have been produced; however, no jets have been directly observed. Z. Paragi et al. carried out two radio-interferometric measurements separated by 60 days and obtained a conservative estimate for the ejection velocity of v ≥ 0,6c. A comparison with optical observations implies that only a very small part of matter of the shed shell reached relativistic velocities. An independent team of researchers (A.M. Soderberg et al.) has recently discovered a relativistic jet in another supernova SN 2009bb. Source: Nature 463 516 (2010)

Mechanism of explosion of type-Ia supernovas. M. Gilfanov and A. Bogdan (the RAS Institute for Space Research, Russia and the Max Planck Institute for Astrophysics, Germany) have established by analyzing the data of observation of the space telescopes Chandra, Spitzer and 2MASS that the nearly 95% of explosions of type Ia supernovas should have been caused by collisions of two white dwarfs. An earlier most probable candidate for the cause of thermonuclear explosions of white dwarfs was the increase of the dwarf mass above the Chandrasekhar mass due to the accretion of matter from the companion star in a binary system. M. Gilfanov and A. Bogdan analyzed the available statistics of x-ray sources in several nearby galaxies. The number of such sources was found to be 30-50 times lower than was predicted by the above model of slow accretion in which x-rays are generated over about 10⁷ years before the explosion. On the opposite, the x-ray emission before the explosion in the model of binary merger is very low, which agrees with the absense of x-ray sources. The scenario of merger of two white dwarfs was suggested theoretically by A.V. Tutukov and the dominance of this mechanism was established in theoretical calculations by the method of “population synthesis” (V.M. Lipunov et al., 1997). It should be mentioned, however, that the observation of the type Ia supernova SN 2006X in the M100 galaxy carried out in 2007 favors the alternative accretion scenario of its origin. The SN 2006X supernova may thus belong to the remaining 5% of cases. Since type Ia supernovae serve as “standard candles” to measure cosmic distances, a clarification of the mechanism of their explosions is necessary for evaluating the accuracy of determining cosmological parameters. Source: Nature 463 924 (2010)