2,111 research outputs found
Do Newton's G and Milgrom's a_0 vary with cosmological epoch ?
In the scalar tensor gravitational theories Newton's constant G_N evolves in
the expanding universe. Likewise, it has been speculated that the acceleration
scale a_0 in Milgrom's modified Newtonian dynamics (MOND) is tied to the scale
of the cosmos, and must thus evolve. With the advent of relativistic
implementations of the modified dynamics, one can address the issue of
variability of the two gravitational ''constants'' with some confidence. Using
TeVeS, the Tensor-Vector-Scalar gravitational theory, as an implementation of
MOND, we calculate the dependence of G_N and a_0 on the TeVeS parameters and
the coeval cosmological value of its scalar field, \phi_c. We find that G_N,
when expressed in atomic units, is strictly nonevolving, a result fully
consistent with recent empirical limits on the variation of G_N. By contrast,
we find that a_0 depends on \phi_c and may thus vary with cosmological epoch.
However, for the brand of TeVeS which seems most promising, a_0 variation
occurs on a timescale much longer than Hubble's, and should be imperceptible
back to redshift unity or even beyond it. This is consistent with emergent data
on the rotation curves of disk galaxies at significants redshifts.Comment: 9 pages, RevTe
A quest for frustration driven distortion in Y2Mo2O7
We investigated the nature of the freezing in the geometrically frustrated
Heisenberg spin-glass Y2Mo2O7 by measuring the temperature dependence of the
static internal magnetic field distribution above the spin-glass temperature,
Tg, using the muSR technique. The evolution of the field distribution cannot be
explained by changes in the spin susceptibility alone and suggests a lattice
deformation. This possibility is addressed by numerical simulations of the
Heisenberg Hamiltonian with magneto-elastic coupling at T>0.Comment: 5 pages 4 figures. Accepted for publication in PR
Kalsifikasi Skeleton Aksial Pada Regenerat Ekor Kadal {Mabouya multifasciata Kuhl)
Kadal {Mabouya multifasciata Kuhl) merupakan salah satu anggota Lacertilia yang terdapat di Indonesia yang dapat mengalami regenerasi setelah hewan tersebut mengalami autotomi ekor. Penelitian ini bertujuan untuk mengetahui kalsifikasi yang terjadi pada skeleton aksial regenerat ekor kadal.
Empat puluhdelapan ekor kadal jantan dewasa berekor asli diautotomi ekornya, dipelihara, dan dibagi menjadi 4 kelompok yang terdiri dari 12 individu ( kelompok I, kelompok II, kelompok III, kelompok IV), masing-masing kelompck berturut-turut mempunyai umur regenerat 4,5,6, dan 12 minggu. Setelah itu dibuat preparat utuh regenerat ekor dengan pewamaan Alizarin RedS-Alcian Blue dan preparat irisan melintang regenerat ekor dengan pewamaan perak nitrat von Kossa.
Hasil penelitian menunjukkan bahwa penulangan secara endokondralis diawali dengan kalsifikasi pada scbagian sisi dalam dan sebagian sisi luar tabung tulang rawan regenerat ekor umur 5 minggu. Pada regenerat ekor berumur 6 dan 12 minggu kalsifikasi sudah meluas ke_seluruh sisi dalam dan sisi luar tabung tulang rawan.
Kata kunci: kalsifikasi, skeleton aksial, regenerat ekor, kada
Abnormal Long-range Spatial Interactions in Amblyopia
AbstractNeural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia. © 1997 Elsevier Science Ltd. All rights reserved
2,3,7,8-TETRAKLORODIBENZO-P-DIOKSIN (TCDD) MEMACU AKTIVITAS BIOSINTESIS PROTEIN DI JARINGAN PALATUM EMBRIO MENCIT
Bahan pencemar dioksin 2,3,7,8-Tetraklorodibenzo-p-dioksin (TCDD) yang diberikan
pada mencit bunting menyebabkan janin yang dihasilkan menderita cacat cleft palate,
dengan ciri bilah palatum kerdil atau bilah palatum tumbuh pendek. Telah dilakukan
penelitian untuk menguji apakah pengurangan biosintesis protein terlibat sebagai
mekanisme yang mendasari hambatan pertumbuhan bilah palatum oleh TCDD. Delapan
ekor mencit bunting dibagi ke dalam empat kelompok, masing-masing diberi TCDD
dosis 0, 5, 10, atau 20 μg/kg berat badan pada hari ke-12 kebuntingan. Besaran aktivitas
biosintesis protein jaringan palatum embrio ditentukan dengan teknik AgNOR pada
sediaan histologis irisan jaringan kraniofasial embrio usia kebuntingan hari ke-15.
Nucleolar Organizing Region (NOR) adalah tempat biogenesis ribosom di inti sel yang
jumlahnya meningkat seiring dengan peningkatan aktivitas biosintesis protein sel.
Rataan jumlah butir AgNOR sel-sel jaringan palatum yang terendah adalah 2,51±0,167
untuk perlakuan 0 μg/kg bb (kontrol), dan berturut-turut 2,87±0,146, 2,87±0,190, dan
2,88±0,160 untuk perlakuan dosis 5, 10, dan 20 μg/kg bb. Jumlah butir AgNOR pada
seluruh kelompok yang diberi TCDD lebih tinggi dibandingkan dengan jumlah butir
AgNOR pada kontrol (P < 0,05). Disimpulkan, pengurangan biosintesis protein
bukanlah mekanisme yang mendasari hambatan pertumbuhan palatum mencit oleh
TCDD. Diduga, peningkatan biosintesis protein oleh TCDD menimbulkan kekacauan
keseimbangan protein-protein faktor tumbuh di jaringan palatum dan mengakibatkan
gangguan pertumbuhan dan diferensias
Choices: The Science of Bela Julesz
Highlights of Bela Julesz's scientific career in visual neuroscienc
Ramsey-like measurement of the decoherence rate between Zeeman sub-levels
Two-photon processes that involve different sub-levels of the ground state of
an atom, are highly sensitive to depopulation and decoherence within the ground
state. For example, the spectral width of electromagnetically induced
transparency resonances in type system, are strongly affected by the
ground state depopulation and decoherence rates. We present a direct
measurement of decay rates between hyperfine and Zeeman sub-levels in the
ground state of Rb vapor. Similar to the relaxation-in-the-dark
technique, pumping lasers are used to pre-align the atomic vapor in a well
defined quantum state. The free propagation of the atomic state is monitored
using a Ramsey-like method. Coherence times in the range 1-10 ms were measured
for room temperature atomic vapor. In the range of the experimental parameters
used in this study, the dominant process inducing Zeeman decoherence is the
spin-exchange collisions between rubidium atoms.Comment: 7 pages, 7 figure
Preferred frame parameters in the tensor-vector-scalar theory of gravity and its generalization
The Tensor-Vector-Scalar theory of gravity, which was designed as a
relativistic implementation to the modified dynamics paradigm, has fared quite
well as an alternative to dark matter, on both galactic and cosmological
scales. However, its performance in the solar system, as embodied in the
post-Newtonian formalism, has not yet been fully investigated. Tamaki has
recently attempted to calculate the preferred frame parameters for TeVeS, but
ignored the cosmological value of the scalar field, thus concluding that the
Newtonian potential must be static in order to be consistent with the vector
equation. We show that when the cosmological value of the scalar field is taken
into account, there is no constraint on the Newtonian potential; however, the
cosmological value of the scalar field is tightly linked to the vector field
coupling constant K, preventing the former from evolving as predicted by its
equation of motion. We then proceed to investigate the post-Newtonian limit of
a generalized version of TeVeS, with {\AE}ther type vector action, and show
that its \beta,\gamma and \xi parameters are as in GR, while solar system
constraints on the preferred frame parameters \alpha_1 and \alpha_2 can be
satisfied within a modest range of small values of the scalar and vector fields
coupling parameters, and for values of the cosmological scalar field consistent
with evolution within the framework of existing models.Comment: 16 pages, 2 figures Figures and corresponding discussion replaced;
added reference
Antigen and Thapsigargin Promote Influx of Ca2+ in Rat Basophilic RBL-2H3 Cells by Ostensibly Similar Mechanisms That Allow Filling of Inositol 1,4,5-Trisphosphate-Sensitive and Mitochondrial Ca2+ Stores
In single, Fura 2-loaded RBL-2H3 cells, antigen and thapsigargin depleted the same intracellular pool of Ca2+ in the absence of external Ca2+; provision of external Ca2+ induced immediate increases in levels of free Ca2+ ([Ca2+](i)). These increases were dependent on the presence of external Ca2+ and, presumably, on influx of Ca2+ across the cell membrane. Both stimulants enhanced intracellular accumulation of 45Ca2+ through ostensibly similar mechanisms because accumulation was blocked to similar extents by various multivalent cations or by depolarization with K+. Because thapsigargin blocked reuptake of Ca2+ into inositol 1,4,5-trisphosphate sensitive stores, uptake occurred independently of the refilling of these stores. Uptake was dependent instead on sequestration of 45Ca2+ in a pool of high capacity that was insensitive to thapsigargin, caffeine, GTP and inositol 1,4,5-trisphosphate but sensitive to ionomycin and mitochondrial inhibitors. The existence of an inositol 1,4,5-trisphosphate-insensitive pool was also apparent in permeabilized cells; at 0.1 μM [Ca2+](i), uptake of 45Ca2+ was largely confined (\u3e 80%) to the inositol 1,4,5-trisphosphate-sensitive pool, but at 2 μM [Ca2+](i) uptake was largely (\u3e 60%) into the inositol 1,4,5-trisphosphate-insensitive pool. Provision of mitochondrial inhibitors along with thapsigargin to block uptake into both pools, did not impair the thapsigargin-induced increase in [Ca2+](i) or influx of Ca2+, as indicated by changes in Fura 2 fluorescence, but did block the intracellular accumulation of 45Ca2+. The studies illustrate the utility of simultaneous measurements of [Ca2+](i) and 45Ca2+ uptake for a full accounting of Ca2+ homoeostasis as exemplified by the ability to distinguish between influx and mitochondrial uptake of Ca2+
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