14 research outputs found
The Use of Cryogenic HEMT Amplifiers in Wide Band Radiometers
Advances in device fabrication, modelling and design techniques have made wide band, low noise cryogenic amplifiers available at frequencies up to 106 GHz. Microwave radiometry applications as used in radio astronomy capitalize on the low noise and large bandwidths of these amplifiers. Radiometers must be carefully designed so as to preclude sensitivity degradations caused by small, low frequency gain fluctuations inherent in these amplifiers
Measurements of the Low Frequency Gain Fluctuations of a 30 GHz High-Electron-Mobility-Transistor Cryogenic Amplifier
Low frequency gain fluctuations of a 30 GHz cryogenic HEMT amplifier have been measured with the input of the amplifier connected to a 15 K load. Effects of fluctuations of other components of the test set-up were eliminated by use of a power-power correlation technique. Strong correlation between output power fluctuations of the amplifier and drain current fluctuations of the transistors comprising the amplifier are observed. The existence of these correlations introduces the possibility of regressing some of the excess noise from the HEMT amplifier's output using the measured drain currents
Particle mesh simulations of the Lyman-alpha forest and the signature of Baryon Acoustic Oscillations in the intergalactic medium
We present a set of ultra-large particle-mesh simulations of the LyA forest
targeted at understanding the imprint of baryon acoustic oscillations (BAO) in
the inter-galactic medium. We use 9 dark matter only simulations which can, for
the first time, simultaneously resolve the Jeans scale of the intergalactic gas
while covering the large volumes required to adequately sample the acoustic
feature. Mock absorption spectra are generated using the fluctuating
Gunn-Peterson approximation which have approximately correct flux probability
density functions (PDFs) and small-scale power spectra. On larger scales there
is clear evidence in the redshift space correlation function for an acoustic
feature, which matches a linear theory template with constant bias. These
spectra, which we make publicly available, can be used to test pipelines, plan
future experiments and model various physical effects. As an illustration we
discuss the basic properties of the acoustic signal in the forest, the scaling
of errors with noise and source number density, modified statistics to treat
mean flux evolution and misestimation, and non-gravitational sources such as
fluctuations in the photo-ionizing background and temperature fluctuations due
to HeII reionization.Comment: 11 pages, 10 figures, minor changes to address referee repor
Dark energy interacting with two fluids
A cosmological model of dark energy interacting with dark matter and another
general component of the universe is investigated. We found general constraints
on these models imposing an accelerated expansion. The same is also studied in
the case for holographic dark energy
Altered neuropathic pain behaviour in a rat model of depression is associated with changes in inflammatory gene expression in the amygdala
The association between chronic pain and depression is widely recognized, the comorbidity of which leads to a heavier disease burden, increased disability and poor treatment response. This study examined nociceptive responding to mechanical and thermal stimuli prior to and following L5-L6 spinal nerve ligation (SNL), a model of neuropathic pain, in the olfactory bulbectomized (OB) rat model of depression. Associated changes in the expression of genes encoding for markers of glial activation and cytokines were subsequently examined in the amygdala, a key brain region for the modulation of emotion and pain. The OB rats exhibited mechanical and cold allodynia, but not heat hyperalgesia, when compared with sham-operated counterparts. Spinal nerve ligation induced characteristic mechanical and cold allodynia in the ipsilateral hindpaw of both sham and OB rats. The OB rats exhibited a reduced latency and number of responses to an innocuous cold stimulus following SNL, an effect positively correlated with interleukin (IL)-6 and IL-10 mRNA expression in the amygdala, respectively. Spinal nerve ligation reduced IL-6 and increased IL-10 expression in the amygdala of sham rats. The expression of CD11b (cluster of differentiation molecule 11b) and GFAP (glial fibrillary acidic protein), indicative of microglial and astrocyte activation, and IL-1 in the amygdala was enhanced in OB animals when compared with sham counterparts, an effect not observed following SNL. This study shows that neuropathic pain-related responding to an innocuous cold stimulus is altered in an animal model of depression, effects accompanied by changes in the expression of neuroinflammatory genes in the amygdala