arXiv subjects

Lloyd Knox

Publications and source records attributed to Lloyd Knox.

99 records · Page 6Linked to original sources

arXiv1998

Forecasting foreground impact on cosmic microwave background measurements

We explore the possible impact of galactic and extragalactic foregrounds on measurements of the cosmic microwave background (CMB). We find that, given our present understanding of the foregrounds, they are unlikely to qualitatively affect the ability of the MAP and Planck satellites to determine the angular power spectrum of the CMB, the key statistic for constraining cosmological parameters. Sufficiently far from the galactic plane, the only foregrounds that will affect power spectrum determination with any significance are the extragalactic ones. For MAP we find the most troublesome foregrounds are radio point sources and the thermal Sunyaev-Zeldovich (SZ) effect. For Planck they are these same radio point sources and the Far Infrared Background. Prior knowledge of the statistics of the SZ component (either via theoretical calculation, or higher frequency observations of just a few percent of the sky, such as will be done by balloon-borne experiments) may significantly improve MAP's determination of the CMB power spectrum. We also explore the foreground impact on MAP and Planck polarization power spectrum measurements.

astro-ph↗

arXiv1998

The Impact of Inhomogeneous Reionization on Cosmic Microwave Background Anisotropy

Hydrogen atoms in the Universe that initially formed at redshift z = 1100 were reionized at 5 < z < 60. It is likely that this transition proceeded through a mixed phase of partial ionization. We develop an analytic approach to calculating the effect on the cosmic microwave background (CMB) of Thomson scattering off this inhomogeneous distribution of free electrons. We study two models of the reionization process, one of which associates ionized patches with overdense regions of the smoothed density field. We find the generated anisotropy to have a power spectrum which peaks at angular scales corresponding to the extent of the ionized regions, and has a width that reflects the correlations between them. There is considerable uncertainty in the amplitude, which depends on how efficiently collapsed objects can reionize the surrounding medium. We calculate the effect of neglecting inhomogeneous reionization in the determination of cosmological parameters from high resolution CMB maps and find that it may be a significant source of systematic error.

astro-ph↗

arXiv1998

Prospects for measuring Inflation parameters with the CMB

We present the status of our study into the feasibility of estimating the parameters of inflation from planned satellite observations of the anisotropy of the cosmic microwave background (CMB).We describe a perturbative procedure for studying inflation models with soft departures from scale free spectra and find that in this case the parameters of inflation are likely to be well determined. We also demonstrate that radical departures from scale free initial perturbations with broad features in $k$-space can be be measured by Planck. However, allowing for very fine features in $k$-space can very strongly compromise the ability to measure cosmological parameters.

astro-ph↗

arXiv1997

CMB Power Spectrum Estimation

We explore power spectrum estimation in the context of a Gaussian approximation to the likelihood function. Using the Saskatoon data, we estimate the power averaged through a set of ten filters designed to make the errors on the power estimates uncorrelated. We also present an improvement to using the window function, $W_l$, for calculating bandpower estimates.

astro-ph↗

arXiv1996

Cosmic Microwave Background Anisotropy Observing Strategy Assessment

I develop a method for assessing the ability of an instrument, coupled with an observing strategy, to measure the angular power spectrum of the cosmic microwave background (CMB). It allows for efficient calculation of expected parameter uncertainties. Related to this method is a means of graphically presenting, via the ``eigenmode window function'', the sensitivity of an observation to different regions of the spectrum, which is a generalization of the traditional practice of presenting the trace of the window function. I apply these techniques to a balloon-borne bolometric instrument to be flown this summer (MSAM2). I find that a smoothly scanning secondary is better than a chopping one and that, in this case, a very simple analytic formula provides a good (40\% or better) approximation to expected power spectrum uncertainties.

astro-ph↗

arXiv1995

Determination of Inflationary Observables by Cosmic Microwave Background Anisotropy Experiments

Inflation produces nearly Harrison-Zel'dovich scalar and tensor perturbation spectra which lead to anisotropy in the cosmic microwave background (CMB). The amplitudes and shapes of these spectra can be parametrized by $Q_S^2$, $r\equiv Q_T^2/Q_S^2$, $n_S$ and $n_T$ where $Q_S^2$ and $Q_T^2$ are the scalar and tensor contributions to the square of the CMB quadrupole and $n_S$ and $n_T$ are the power-lawspectral indices. Even if we restrict ourselves to information from angles greater than one third of a degree, three of these observables can be measured with some precision. The combination $130^{1-n_S}Q_S^2$ can be known to better than $\pm 0.3\%$. The scalar index $n_S$ can be determined to better than $\pm 0.02$. The ratio $r$ can be known to about $\pm 0.1$ for $n_S \simeq 1$ and slightly better for smaller $n_S$. The precision with which $n_T$ can be measured depends weakly on $n_S$ and strongly on $r$. For $n_S \simeq 1$ $n_T$ can be determined with a precision of about $\pm 0.056(1.5+r)/r$. A full-sky experiment with a $20'$beam using technology available today, similar to those being planned by several groups, can achieve the above precision. Good angular resolution is more important than high signal-to-noise ratio; for a given detector sensitivity and observing time a smaller beam provides significantly more information than a larger beam. The uncertainties in $n_S$ and $r$ are roughly proportional to the beam size. We briefly discuss the effects of uncertainty in the Hubble constant, baryon density, cosmological constant and ionization history.

astro-ph↗

arXiv1994

Detectability of Tensor Perturbations Through CBR Anisotropy (final published version)

Detection of the tensor perturbations predicted in inflationary models is important for testing inflation as well as for reconstructing the inflationary potential. We show that because of cosmic variance the tensor contribution to the square of the CBR quadrupole anisotropy must be greater than about 20\% of the scalar contribution to ensure a statistically significant detection of tensor perturbations. This sensitivity could be achieved by full-sky measurements on angular scales of $3^{\circ}$ and $0.5^\circ$.

astro-ph↗

arXiv1994

Testing Inflation with the Cosmic Microwave Background

Cosmic microwave background (CMB) anisotropy may result from both scalar and tensor perturbations. For a sufficiently narrow range of angular scales, CMB perturbations can be characterized by four parameters. Results from the Cosmic Background Explorer fix one combination of the parameters, reducing the parameters to three. If CMB perturbations are from inflation, there is an additional relation, reducing the parameters to two. An appropriate combination of a medium-angle and a small-angle CMB observation can test the inflation hypothesis because inflation cannot explain a high signal in one experiment and a low signal in the other.

astro-ph↗

arXiv1993

Primordial Nucleosynthesis in Conformal Weyl Gravity

Recently conformal Weyl gravity has been considered as a candidate alternative gravity theory. This fourth-order theory is attractive because it is the only metric theory of gravity which is invariant under local conformal transformations of the metric. We calculate the primordial light element abundances in this theory. The major difference {}from the standard cosmology is that the universe expands far more slowly throughout the nucleosynthesis epoch. The production of $^4\rm{He}$ depends strongly on $η$, the ratio of baryons to photons. For $η= 10^{-8}$ the mass fraction of $^4{\rm He}$ is $X_4 \simeq 0.25$ and the number densities relative to hydrogen for $^2{\rm H}$, $^3{\rm He}$ and $^7{\rm Li}$ are $n(^2{\rm H})/n({\rm H}) \simeq 9\times 10^{-20}$, $n(^3{\rm He})/n({\rm H})\simeq 4 \times 10^{-18}$ and $n(^7{\rm Li})/n({\rm H}) \simeq 10^{-13}$. This value of $η$ corresponds to a baryon mass density close to the standard model critical density. However, adjusting $η$ to give a reasonable helium yield forces the deuterium and lithium yields to be small enough that the theory cannot be reconciled with observations.

astro-ph↗