2005 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/443
Browse
4 results
Search Results
Item Primordial Magnetic Fields in the Post-recombination Era and Early Reionization(2005-01-01) Sethi, Shiv K.; Subramanian, KandaswamyWe explore the ways in which primordial magnetic fields influence the thermal and ion- ization history of the post-recombination universe. After recombination the universe becomes mostly neutral resulting also in a sharp drop in the radiative viscosity. Pri- mordial magnetic fields can then dissipate their energy into the intergalactic medium (IGM) via ambipolar diffusion and, for small enough scales, by generating decaying MHD turbulence. These processes can significantly modify the thermal and ionization history of the post-recombination universe. We show that the dissipation effects of magnetic fields which redshifts to a present value B0 = 3 × 10−9 Gauss smoothed on the magnetic Jeans scale and below, can give rise to Thomson scattering optical depths τ > ∼ 0.1, although not in the range of redshifts needed to explain the recent WMAP polarization observations.We also study the possibility that primordial fields could induce the formation of subgalactic structures for z > ∼ 15. We show that early structure formation induced by nano-Gauss magnetic fields is potentially capable of producing the early re-ionization implied by the WMAP data. Future CMB observations will be very useful to probe the modified ionization histories produItem Physics of CMBR Anisotropies(2005-01-01) Subramanian, KandaswamyThe observed structures in the universe are thought to have arisen from gravitational instability acting on small fluctuations generated in the early universe. These spatial fluctuations are imprinted on the CMBR as angular anisotropies. The physics which connects initial fluctuations in the early universe to the observed anisotropies is fairly well understood, since for most part it involves linear perturbation theory. This makes CMBR anisotropies one of the cleanest probes of the initial fluctuations, various cosmological parameters governing their evolution and also the geometry of the universe. We review here in a fairly pedagogical manner the physics of the CMBR anisotropies and explain the role they play in probing cosmological parameters, especially in the light of the latest observations from the WMAP satelliteItem Minimal tau approximation and simulations of the alpha effect(2005-08-01) Brandenburg, Axel; Subramanian, KandaswamyThe validity of a closure called the minimal tau approximation (MTA), is tested in the context of dynamo theory, wherein triple correlations are assumed to provide relaxation of the turbulent electromotive force. Under MTA, the alpha effect in mean field dynamo theory becomes proportional to a relaxation time scale multiplied by the difference between kinetic and current helicities. It is shown that the value of the relaxation time is positive and, in units of the turnover time at the forcing wavenumber, it is of the order of unity. It is quenched by the magnetic field – roughly independently of the magnetic Reynolds number. However, this independence becomes uncertain at large magnetic Reynolds number. Kinetic and current helicities are shown to be dominated by large scale properties of the flow.Item CMB Anisotropy Due to Tangled magnetic fields in re-ionized models(2005-04-01) Seshadri, T. R.; Subramanian, KandaswamyPrimordial tangled cosmological Magnetic Fields source rotational velocity perturbations of the baryon fluid, even in the post-recombination universe. These vortical modes inturn leave a characteristic imprint on the temperature anisotropy of the Cosmic Microwave Background (CMB), if the CMB photons can be re-scatterred after recombination. Observations from WMAP in- dicate that the Universe underwent a relatively early re-ionization (zri ∼ 15), which does indeed lead to a significant optical depth for re-scattering of CMB photons after the re-ionization epoch. We compute the resulting additional temperature anisotropies, induced by primordial magnetic fields in the postrecombination universe. We show that in models with early re-ionization, a nearly scale-invariant spectrum of tangled magnetic fields which redshift to a present value of B0 ∼ 3 × 10−9 Gauss, produce vector mode perturbations which in turn induce additional temperature anisotropy of about 0.3 to 0.4 µK over very small angular scales, with l upto ∼ 10000 or so.98.62.En, 98.70.Vc, 98.80.Cq, 95.30.Qd