[en] Cosmological variation of the fine structure constant α due to the evolution of a spatially homogeneous ultralight scalar field (m∼H₀) during the matter and Λ dominated eras is analyzed. Agreement of Δα/α with the value suggested by recent observations of quasar absorption lines is obtained by adjusting a single parameter, the coupling of the scalar field to matter. Asymptotically α(t) in this model goes to a constant value α(bar sign)≅α₀ in the early radiation and the late Λ dominated eras. The coupling of the scalar field to (nonrelativistic) matter drives α slightly away from α(bar sign) in the epochs when the density of matter is important. Simultaneous agreement with the more restrictive bounds on the variation |Δα/α| from the Oklo natural fission reactor and from meteorite samples can be achieved if the mass of the scalar field is on the order of 0.5-0.6 H_Λ, where H_Λ=Ω_Λ^1/2H₀. Depending on the scalar field mass, α may be slightly smaller or larger than α₀ at the times of big bang nucleosynthesis, the emission of the cosmic microwave background, the formation of early solar system meteorites, and the Oklo reactor. The effects on the evolution of α due to nonzero mass for the scalar field are emphasized. An order of magnitude improvement in the laboratory technique could lead to a detection of (α/α)₀

[en] The implications of seven popular models of quintessence based on supergravity or M/string theory for the transition from a decelerating to an accelerating universe are explored. All seven potentials can mimic the ΛCDM model at low redshifts 0=< z=<5. However, for a natural range of initial values of the quintessence field, the SUGRA and Polonyi potentials predict a transition redshift zt∼0.5 for ΩΛ0=0.70, in agreement with the observational value zt∼0.46 and in mild conflict with the ΛCDM value zt=0.67. Tables are given for the quintessence potentials for the recent average w-bar 0 of the equation of state parameter, and for w0 and w1 in the low-z approximation w∼w0+w1z. It is argued that for the exponential potential eλφ to produce a viable present-day cosmology, λ=<2. A robust, scaled numerical method is presented for simulating the cosmological evolution of the scalar field

[en] Numerical simulations of the SVS 13 microjet and bow shock bubble are performed using the WENO method that reproduces the main features and dynamics of data from the Keck Telescope/OSIRIS velocity-resolved integral field spectrograph: an expanding, cooler bow shock bubble, with the bubble center moving at approximately 50 km s⁻¹ with a radial expansion velocity of 11 km s⁻¹, surrounding the fast, hotter jet, which is propagating at 156 km s⁻¹. Contact and bow shock waves are visible in the simulations both from the initial short jet pulse that creates the nearly spherical bow shock bubble and from the fast microjet, while a terminal Mach disk shock is visible near the tip of the continuous microjet, which reduces the velocity of the jet gas down to the flow velocity of the contact discontinuity at the leading edge of the jet. At 21.1 years after the launch of the initial bubble pulse, the jet has caught up with and penetrated almost all the way across the bow shock bubble of the slower initial pulse. At times later than about 22 years, the jet has penetrated through the bubble and thereafter begins to subsume its spherical form. Emission maps from the simulations of the jet—traced by the emission of the shock-excited 1.644 μ m [Fe ii] line—and the bow shock bubble—traced in the lower excitation 2.122 μ m H₂ 1–0 S(1) line—projected onto the plane of the sky are presented, and are in good agreement with the Keck observations.

[en] Recent Hubble Space Telescope (HST) observations of the extragalactic radio source Centaurus A (Cen A) display a young stellar population around the southwest tip of the inner filament 8.5 kpc from the galactic center of Cen A, with ages in the range 1–3 Myr. Crockett et al. have argued that the transverse bow shock of the Cen A jet triggered this star formation as it impacted dense molecular cores of clouds in the filament. To test this hypothesis, we perform three-dimensional numerical simulations of star formation induced by the jet bow shock in the inner filament of Cen A, using a positivity-preserving, weighted, essentially non-oscillatory method to solve the equations of gas dynamics with radiative cooling. We find that star clusters form inside a bow-shocked molecular cloud when the maximum initial density of the cloud is $⩾<!--\; ???\; -->40$ H₂ molecules cm⁻³. In a typical molecular cloud of mass ${10}^6{M}_{\odot <!--\; ???\; -->}$ and diameter 200 pc, approximately 20 star clusters of mass ${10}^4{M}_{\odot <!--\; ???\; -->}$ are formed, matching the HST images.