Searcharxiv⌕ Search

arXiv subjects

Douglas Scott

Publications and source records attributed to Douglas Scott.

At least 307 records · Page 17Linked to original sources
arXiv2000

Backreaction and the Parametric Resonance of Cosmological Fluctuations

We study the class of two-field inflationary Universe models λϕ^4/4 + g^2χ^2ϕ^2/2, in which parametric resonance during the initial stages of reheating can lead to an exponential amplification of the amplitude of cosmological fluctuations. Employing both analytical arguments and numerical simulations, we determine the time at which backreaction of fluctuations on the background fields shuts off the exponential growth, making use of the Hartree approximation, and including scalar metric perturbations. For the case g^2/λ=2, we find that the amplitude of fluctuations after preheating will exceed the observational upper bound independent of the value of λ, unless the duration of inflation is very long. Cosmological fluctuations are acceptably small for g^2/λ>=8. We also find that the addition of χ-field self-interaction can limit the growth of fluctuations, and in the negative-coupling case the system can become effectively single-field, removing the resonance.

hep-ph↗
arXiv2000

A search for the submillimetre counterparts to Lyman break galaxies

We have carried out targetted sub-mm observations as part of a programme to explore the connection between the rest-frame UV and far-IR properties of star-forming galaxies at high redshift, which is currently poorly understood. On the one hand the Lyman break technique is very effective at selecting z~3 galaxies. On the other hand `blank field' imaging in the sub-mm seems to turn up sources routinely, amongst which some are star forming galaxies at similar redshifts. Already much work has been done searching for optical identifications of objects detected using the SCUBA instrument. Here we have taken the opposite approach, performing sub-mm photometry for a sample of Lyman break galaxies whose UV properties imply high star formation rates. The total signal from our Lyman break sample is undetected in the sub-mm, at an RMS level of 0.5 mJy, which implies that the population of Lyman break galaxies does not constitute a large part of the recently detected blank-field sub-mm sources. However, our one detection suggests that with reasonable SCUBA integrations we might expect to detect those few LBGs that are far-IR brightest.

astro-ph↗
arXiv2000

Still flat after all these years

The Universe could be spatially flat, positively curved or negatively curved. Each option has been popular at various times, partly affected by an understanding that models tend to evolve away from flatness. The curvature of the Universe is amenable to measurement, through tests such as the determination of the angles of sufficiently large triangles. The angle subtended by the characteristic scale on the Cosmic Microwave sky provides a direct test, which has now been realised through a combination of exquisite results from a number of CMB experiments. After a long and detailed investigation, with many false clues, it seems that the mystery of the curvature of the Universe is now solved. It's an open and shut case: the Universe is flat.

astro-ph↗
arXiv2000

A Characteristic Scale on the Cosmic Microwave Sky

The current suite of results from Cosmic Microwave Background anisotropy experiments is fulfilling the promise of providing extraordinary levels of discrimination between cosmological models. We calculate a binned anisotropy power spectrum, which we tabulate, along with error bars and bin-to-bin correlations, so that it can be easily used for constraining models. The resulting power spectrum is flat at large angles, with a gradual rise to a prominent peak at around 0.5 degrees and a decrease thereafter. This is precisely the shape predicted by inflationary-inspired adiabatic models. Within that class of cosmologies, this characteristic scale imprinted on the CMB sky can be used to infer that the geometry of the Universe is very close to flat. The next wave of CMB results should add fuel to the debate about whether or not the Universe once inflated, as well as beginning in earnest the task of measuring cosmological parameters.

astro-ph↗
arXiv2000

First detections of FIRBACK sources with SCUBA

The FIRBACK (Far InfraRed BACKground) survey represents the deepest extensive 170 micron images obtained by the ISO satellite. The sources detected comprise about 10% of the Cosmic IR Background (CIB) seen by COBE, and, importantly, were observed at a wavelength near the peak of the CIB. Detailed follow-up of these sources should help pin down the redshifts, Hubble types, and other properties of the galaxies which constitute this background. We have used the Submillimetre Common-User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT) to search for sub-mm emission from a sample of 10 galaxies which are bright at 170 microns and for which we had accurate radio positions. Statistically we detect this sample at both 450 microns (~4 sigma) and 850 microns (~ 7 sigma); individual objects are convincingly detected in four cases at 850 microns, and one case at 450 microns. Fits to simple spectral energy distributions suggest a range of low to moderate redshifts, perhaps z=0 to 1.5 for this FIRBACK sub-sample.

astro-ph↗
arXiv2000

Morphing the CMB: a technique for interpolating power spectra

The confrontation of the Cosmic Microwave Background (CMB) theoretical angular power spectrum with available data often requires the calculation of large numbers of power spectra. The standard practice is to use a fast code to compute the CMB power spectra over some large parameter space, in order to estimate likelihoods and constrain these parameters. But as the dimensionality of the space under study increases, then even with relatively fast anisotropy codes, the computation can become prohibitive. This paper describes the employment of a "morphing" strategy to interpolate new power spectra based on previously calculated ones. We simply present the basic idea here, and illustrate with a few examples; optimization of interpolation schemes will depend on the specific application. In addition to facilitating the exploration of large parameter spaces, this morphing technique may be helpful for Fisher matrix calculations involving derivatives.

astro-ph↗
arXiv1999

How exactly did the Universe become neutral?

We present a refined treatment of H, He I, and He II recombination in the early Universe. The difference from previous calculations is that we use multi-level atoms and evolve the population of each level with redshift by including all bound-bound and bound-free transitions. In this framework we follow several hundred atomic energy levels for H, He I, and He II combined. The main improvements of this method over previous recombination calculations are: (1) allowing excited atomic level populations to depart from an equilibrium distribution; (2) replacing the total recombination coefficient with recombination to and photoionization from each level directly at each redshift step; and (3) correct treatment of the He I atom, including the triplet and singlet states. We find that the ionization fraction x_e = n_e/n_H is approximately 10% smaller at redshifts <~800 than in previous calculations, due to the non-equilibrium of the excited states of H, which is caused by the strong but cool radiation field at those redshifts. In addition we find that He I recombination is delayed compared with previous calculations, and occurs only just before H recombination. These changes in turn can affect the predicted power spectrum of microwave anisotropies at the few percent level. Other improvements such as including molecular and ionic species of H, including complete heating and cooling terms for the evolution of the matter temperature, including collisional rates, and including feedback of the secondary spectral distortions on the radiation field, produce negligible change to x_e. The lower x_e at low z found in this work affects the abundances of H molecular and ionic species by 10-25%. However this difference is probably not larger than other uncertainties in the reaction rates.

astro-ph↗
arXiv1999

Cosmic Glows

This is the obligatory Cosmic Microwave Background review. I discuss the current status of CMB anisotropies, together with some points on the related topic of the Far-Infrared Background. We have already learned a number of important things from CMB anisotropies. Models which are in good shape have: approximately flat geometry; cold dark-matter, plus something like a cosmological constant; roughly scale invariant adiabatic fluctuations; and close to Gaussian statistics. The constraints from the CMB are beginning to be comparable to those from other cosmological measurements. With a wealth of new data coming in, it is expected that CMB anisotropies will soon provide the most stringent limits on fundamental cosmological parameters, as well as probing high energy particle physics and the Dark Ages of astrophysics. Nudge, nudge. Wink, wink. Say no more.

astro-ph↗
arXiv1999

New physics from the Cosmic Microwave Background

I review the present status of the Cosmic Microwave Background, with some emphasis on the current and future implications for particle physics. Conclusions are: gravitational instability in a dark matter dominated universe grew today's structure; the Universe remained neutral until z<~50; the CMB power spectrum peaks at 150<~l<~350; the large-scale structure of spacetime appears to be simple; something like inflation is something like proven; we will learn a great deal about cosmology, astrophysics and particle physics from MAP and Planck.

astro-ph↗
arXiv1999

A Preferred-Direction Statistic for Sky Maps

Large patterns could exist on the microwave sky as a result of various non-standard possibilities for the large-scale Universe -- rotation or shear, non-trivial topology, and single topological defects are specific examples. All-sky (or nearly all-sky) CMB data sets allow us, uniquely, to constrain such exotica, and it is therefore worthwhile to explore a wide range of statistical tests. We describe one such statistic here, which is based on determining gradients and is useful for assessing the level of `preferred directionality' or `stripiness' in the map. This method is more general than other techniques for picking out specific patterns on the sky, and it also has the advantage of being easily calculable for the mega-pixel maps which will soon be available. For the purposes of illustration, we apply this statistic to the four-year COBE DMR data. For future CMB maps we expect this to be a useful statistical test of the large scale structure of the Universe. In principle, the same statistic could also be applied to sky maps at other wavelengths, to CMB polarisation maps, and to catalogues of discrete objects. It may also be useful as a means of checking for residual directionality (e.g., from Galactic or ecliptic signals) in maps.

astro-ph↗
arXiv1999

A New Calculation of the Recombination Epoch

We have developed an improved recombination calculation of H, HeI, and HeII in the early Universe which involves a line-by-line treatment of each atomic level. We find two major differences compared with previous calculations. Firstly, the ionization fraction x_e is approximately 10% smaller for redshifts <~800, due to non-equilibrium processes in the excited states of H. Secondly, HeI recombination is much slower than previously thought, and is delayed until just before H recombines. We describe the basic physics behind the new results and present a simple way to reproduce our calculation. This should enable fast computation of the ionization history (and quantities such as the power spectrum of CMB anisotropies which depend on it) for arbitrary cosmologies, without the need to consider the hundreds of atomic levels used in our complete model.

astro-ph↗
arXiv1999

Limits on the gravity wave contribution to microwave anisotropies

We present limits on the fraction of large angle microwave anisotropies which could come from tensor perturbations. We use the COBE results as well as smaller scale CMB observations, measurements of galaxy correlations, abundances of galaxy clusters, and Lyman alpha absorption cloud statistics. Our aim is to provide conservative limits on the tensor-to-scalar ratio for standard inflationary models. For power-law inflation, for example, we find T/S<0.52 at 95% confidence, with a similar constraint for phi^p potentials. However, for models with tensor amplitude unrelated to the scalar spectral index it is still currently possible to have T/S>1.

astro-ph↗
arXiv1999

Faint, moving objects in the Hubble Deep Field: components of the dark halo?

The deepest optical image of the sky, the Hubble Deep Field (HDF), obtained with the Hubble Space Telescope (HST) in December 1995, has been compared to a similar image taken in December 1997. Two very faint, blue, isolated and unresolved objects are found to display a substantial apparent proper motion, 23+/-5 mas/yr and 26+/-5 mas/yr; a further three objects at the detection limit of the second epoch observations may also be moving. Galactic structure models predict a general absence of stars in the color-magnitude range in which these objects are found. However, these observations are consistent with recently-developed models of old white dwarfs with hydrogen atmospheres, whose color, contrary to previous expectations, has been shown to be blue. If these apparently moving objects are indeed old white dwarfs with hydrogen atmospheres and masses near 0.5 M_Sun, they have ages of approximately 12 Gyr, and a local mass density that is sufficient, within the large uncertainties arising from the small size of the sample, to account for the entire missing Galactic dynamical mass.

astro-ph↗
arXiv1999

3D numerical cosmological radiative transfer in an inhomogeneous medium

A numerical scheme is proposed for the solution of the three-dimensional radiative transfer equation with variable optical depth. We show that time-dependent ray tracing is an attractive choice for simulations of astrophysical ionization fronts, particularly when one is interested in covering a wide range of optical depths within a 3D clumpy medium. Our approach combines the explicit advection of radiation variables with the implicit solution of local rate equations given the radiation field at each point. Our scheme is well suited to the solution of problems for which line transfer is not important, and could, in principle, be extended to those situations also. This scheme allows us to calculate the propagation of supersonic ionization fronts into an inhomogeneous medium. The approach can be easily implemented on a single workstation and also should be fully parallelizable.

astro-ph↗
arXiv1999

Gravity waves goodbye

The detection of a stochastic background of long-wavelength gravitational waves (tensors) in the cosmic microwave background (CMB) anisotropy would be an invaluable probe of the high energy physics of the early universe. Unfortunately a combination of factors now makes such a detection seem unlikely: the vast majority of the CMB signal appears to come from density perturbations (scalars) - detailed fits to current observations indicate a tensor-to-scalar quadrupole ratio of T/S < 0.5 for the simplest models; and on the theoretical side the best-motivated inflationary models seem to require very small T/S. Unfortunately CMB temperature anisotropies can only probe a gravity wave signal down to T/S \sim 10% and optimistic assumptions about polarization of the CMB only lower this another order of magnitude.

astro-ph↗
arXiv1999

Implications of SCUBA observations for the Planck Surveyor

We investigate the implications for the Planck Surveyor of the recent sub-millimetre number counts obtained using the SCUBA camera. Since it observes at the same frequency as one of the higher frequency science channels on Planck, SCUBA can provide constraints on the point-source contribution to the CMB angular power spectrum, which require no extrapolation in frequency. We have calculated the two-point function of these sub-millimetre sources, using a Poisson model normalized to the observed counts. While the current data are uncertain, under reasonable assumptions the point-source contribution to the anisotropy is comparable to the noise in the 353GHz channel. The clustering of these sources is currently unknown, however if they cluster like the z~3 Lyman-break galaxies their signal would be larger than the primary anisotropy signal on scales smaller than about 10 arcmin. We expect the intensity of these sources to decrease for wavelengths longward of 850 microns. At the next lowest Planck frequency, 217GHz, the contribution from both the clustered and Poisson terms are dramatically reduced. Hence we do not expect these sources to seriously affect Planck's main science goal, the determination of the primordial anisotropy power spectrum. Rather, the potential determination of the distribution of sub-mm sources is a further piece of cosmology that Planck may be able to tackle.

astro-ph↗
arXiv1999

What have we already learned from the CMB?

The COBE satellite, and the DMR experiment in particular, was extraordinarily successful. However, the DMR results were announced about 7 years ago, during which time a great deal more has been learned about anisotropies in the Cosmic Microwave Background (CMB). The CMB experiments currently being designed and built, including long-duration balloons, interferometers, and two space missions, promise to address several fundamental cosmological issues. We present our evaluation of what we already know, what we are beginning to learn now, and what the future may bring.

astro-ph↗