On the sound field of a point-shaped sound source in uniform translatory motion

A rigorous analysis presented of the excitation of sound by point sources moving in uniform translatory motion at subsonic or supersonic velocities through a two- or three-dimensional medium at rest. The construction of surfaces of constant phase is based upon Huyghens' principle in such a manner that the propagation in the medium at rest of the elementary waves emanating from the sound source is independent of the momentary state of motion of the sound source. Hence, characteristic traits of the sound propagation may be understood even on the basis of simple geometric constructions

Cosmological model with macroscopic spin fluid

We consider a Friedmann-Robertson-Walker cosmological model with some exotic perfect fluid with spin known as the Weyssenhoff fluid. The possibility that the dark energy may be described in part by the Weyssenhoff fluid is discussed. The observational constraint coming from supernovae type Ia observations is established. This result indicates that, whereas the cosmological constant is still needed to explain current observations, the model with spin fluid is admissible. For high redshifts $z > 1$ the differences between the model with spin fluid and the cold dark matter model with a cosmological constant become detectable observationally for the flat case with $\Omega_{\text{m},0}=0.3$. From the maximum likelihood method we obtain the value of $\Omega_{\text{s},0} = 0.004 \pm 0.016$. This gives us the limit $\Omega_{\text{s},0} > -0.012$ at the $1\sigma$ level. While the model with ``brane effects'' is preferred by the supernovae Ia data, the model with spin fluid is statistically admissible. For comparison, the limit on the spin fluid coming from cosmic microwave background anisotropies is also obtained. The uncertainties in the location of a first peak give the interval $-1.4 \times 10^{-10} < \Omega_{\text{s},0} < -10^{-10}$. From big bang nucleosynthesis we obtain the strongest limit $\Omega_{\text{s},0} \gtrsim -10^{-20}$. The interconnection between the model considered and brane models is also pointed out.Comment: RevTeX4, 15 pages, 10 figures; some minor change

Navigated cup implantation in hip arthroplasty: A meta-analysis

Background and purpose Many studies have suggested that navigation-based implantation can improve cup positioning in total hip arthroplasty (THA). We conducted a systematic review and meta-analysis to compile the best available evidence, and to overcome potential shortcomings because of small sample sizes in individual studies