1,721,608 research outputs found
A fundamental test of the nature of dark matter
No full textDark matter may consist of weakly interacting elementary particles or of macroscopic compact objects. We show that the statistics of the gravitational lensing of high-redshift supernovae strongly discriminate between these two classes of dark matter candidates. We develop a method of calculating the magnification distribution of supernovae, which can be interpreted in terms of the properties of the lensing objects. With simulated data, we show that ≳50 well-measured Type Ia supernovae (Δm ∼ 0.2 mag) at redshifts ∼1 can clearly distinguish macroscopic from microscopic dark matter if Ω0 ≳ 0.2 and all dark matter is in one form or the other
Gravitational magnification of the cosmic microwave background
No full textSome aspects of gravitational lensing by large-scale structure are investigated. We show that lensing causes the damping tail of the cosmic microwave background (CMB) power spectrum to fall less rapidly with decreasing angular scale than previously expected. This is because of a transfer of power from larger to smaller angular scales, which produces a fractional change in power spectrum that increases rapidly beyond l ∼ 2000. We also find that lensing produces a nonzero mean magnification of structures on surfaces of constant redshift if weighted by area on the sky. This is a result of the fact that light rays that are evenly distributed on the sky oversample overdense regions. However, this mean magnification has a negligible affect on the CMB power spectrum. A new expression for the lensed power spectrum is derived, and it is found that future precision observations of the high-l tail of the power spectrum will need to take lensing into account when determining cosmological parameters. © 1997. The American Astronomical Society. All rights reserved
Gravitational microlensing by clustered machos
No full textIt has been proposed that the MACHOs in our Galaxy could be clumped in globular cluster-like associations or RAMBOs (robust associations of massive baryonic objects) (Moore & Silk). Here we investigate the effect such clustering has on the microlensing of stars in the Large Magellanic Cloud. We find that the lensing in a 1 square degree field could be dominated by just a few clusters. As a result the lensing properties vary widely depending on the position and velocity of those clusters which happen to lie between us and the LMC. Moreover, we find a large variance in timescale distributions that suggests that the small-number statistics could easily be dominated by events in the tails of the unclustered distribution (e.g., by long periods). We compare our results with the MACHO collaboration data and find that a "standard" halo made entirely of MACHOs is not strongly disfavored if the clusters have masses of 106 M. For less massive clusters such a halo is not as likely. For 104 M clusters the microlensing statistics are essentially unchanged from the unclustered case. It may be possible to detect very massive clusters from the distribution of events in timescale and space. We provide some examples of timescale distributions. © 1996. The American Astronomical Society. All rights reserved
On breaking cosmic degeneracy
No full textIt has been argued that the power spectrum of the anisotropies in the cosmic microwave background may be effectively degenerate, namely, that the observable spectrum does not determine a unique set of cosmological parameters. We describe the physical origin of this degeneracy and show that at small angular scales it is broken by gravitational lensing: effectively degenerate spectra become distinguishable at l ∼ 3000 because lensing causes their damping tails to fall at different rates with increasing l. This effect also helps in distinguishing nearly degenerate power spectra such as those of mixed dark matter models. Forthcoming interferometer experiments should provide the means of measuring otherwise degenerate parameters at the 5%-25% level. © 1998. The American Astronomical Society. All rights reserved
Silk, J J, 405092
No full textThis record was harvested from a previous catalogue system and will be withdrawn in 2025. Information in this record may be superseded or incomplete. Visit this record in UMA's new catalogue at: https://archives.library.unimelb.edu.au/nodes/view/416841Surname: SILK. Given Name(s) or Initials: J J. Military Service Number or Last Known Location: 405092. Missing, Wounded and Prisoner of War Enquiry Card Index Number: 13689.239182
Item: [2016.0049.49102] "Silk, J J, 405092
Monolithic or hierarchical star formation? A new statistical analysis
Get PDFWe consider an analytic model of cosmic star formation which incorporates supernova feedback, gas accretion and enriched outflows, reproducing the history of cosmic star formation, metallicity, Type II supernova rates and the fraction of baryons allocated to structures. We present a new statistical treatment of the available observational data on the star formation rate and metallicity that accounts for the presence of possible systematics. We then employ a Bayesian Markov Chain Monte Carlo method to compare the predictions of our model with observations and derive constraints on the seven free parameters of the model. We find that the dust-correction scheme one chooses to adopt for the star formation data is critical in determining which scenario is favoured between a hierarchical star formation model, where star formation is prolonged by accretion, infall and merging, and a monolithic scenario, where star formation is rapid and efficient. We distinguish between these modes by defining a characteristic minimum mass, M≳ 1011M⊙, in our fiducial model, for early-type galaxies where star formation occurs efficiently. Our results indicate that the hierarchical star formation model can achieve better agreement with the data, but that this requires a high efficiency of supernova-driven outflows. In a monolithic model, our analysis points to the need for a mechanism that drives metal-poor winds, perhaps in the form of supermassive black hole induced outflows. Furthermore, the relative absence of star formation beyond z∼ 5 in the monolithic scenario requires an alternative mechanism to dwarf galaxies for re-ionizing the universe at z∼ 11, as required by observations of the microwave background. While the monolithic scenario is less favoured in terms of its quality-of-fit, it cannot yet be excluded
Secondary CMB anisotropies from the kinetic SZ effect
Get PDFThe reionization of the universe by stars and quasars is expected to be a highly inhomogeneous process. Moreover, the fluctuations of the matter density field also lead to inhomogeneities of the free electron distribution. These patterns gave rise to secondary CMB anisotropies through Thomson scattering of photons by free electrons. In this article we present an analytic model, based on our previous work which tackled the reionization history of the universe, which allows us to describe the generation of these secondary CMB anisotropies. We take into account the "patchy pattern" of reionization (HII bubbles), the cross-correlations of these ionized regions, the small-scale fluctuations of the matter density field and the contribution from collapsed objects. For an open universe, we find that the angular correlation function C(theta) displays a very slow decline from C(0) similar to 6 10(-13) up to the scale theta similar to 10(-3) rad where it shows a sharp drop. On the other hand, the power-spectrum l(l + 1)C-l/(2 pi) (and the "local average" S-l) exhibits a plateau of height similar to 10(-13) in the range 10(3) < l < 10(6). We find that for large wavenumbers l > 10(4) the signal is dominated by the contribution from collapsed halos while for l > 10(4) it is governed by the large-scale correlations of HII bubbles. This implies that one cannot discriminate reionization by stars from a quasar-driven scenario since the size of ionized regions never dominates the behaviour of the anisotropies. Moreover, the secondary CMB anisotropies arise from a broad range of redshifts (7.5 < z < 10 for the IGM and 0 < z < 7 for galactic halos). Thus, we find that the generation of these anisotropies involves several intricate processes and they are close to the resolution limit of current numerical simulations. The signal expected in our model might bias the cosmological parameter estimation from CMB experiments such as Planck or MAP, and could be detected by future mm-wavelength interferometers (e.g., ALMA)
Helioseismology with long-range dark matter-baryon interactions
Get PDFAssuming the existence of a primordial asymmetry in the dark sector, we study how long-range dark matter (DM)-baryon interactions, induced by the kinetic mixing of a new U(1) gauge boson and a photon, affect the evolution of the Sun and, in turn, the sound speed the profile obtained from helioseismology. Thanks to the explicit dependence on the exchanged momenta in the differential cross section (Rutherford-like scattering), we find that DM particles with a mass of ∼10 GeV, kinetic mixing parameter of the order of 10-9, and a mediator with a mass smaller than a few MeV improve the agreement between the best solar model and the helioseismic data without being excluded by direct detection experiments. In particular, the LUX detector will soon be able to either constrain or confirm our best-fit solar model in the presence of a dark sector with long-range interactions that reconcile helioseismology with thermal neutrino results
Investigating cosmic discordance
Get PDFWe show that a combined analysis of cosmic microwave background anisotropy power spectra obtained by the Planck satellite and luminosity distance data simultaneously excludes a flat universe and a cosmological constant at 99% confidence level. These results hold separately when combining Planck with three different data sets: the two determinations of the Hubble constant from Riess et al. and Freedman et al., and the Pantheon catalog of high-redshift Type Ia supernovae. We conclude that either the Lambda cold dark matter model needs to be replaced by a different paradigm, or else there are significant but still undetected systematics. Our result calls for new observations and stimulates the investigation of alternative theoretical models and solutions
The formation history of the Galactic bulge
No full textThe distributions of the stellar metallicities of K giant stars in several fields of the Galactic bulge, taken from the literature and probing projected Galactocentric distances of similar to500 pc to similar to3 kpc, are compared with a simple model of star formation and chemical evolution. Our model assumes a Schmidt law of star formation and is described by only a few parameters that control the infall and outflow of gas and the star formation efficiency. Exploring a large volume of parameter space, we find that very short infall time-scales are needed (less than or similar to0.5 Gyr), with durations of infall and star formation greater than 1 Gyr being ruled out at the 90 per cent confidence level. The metallicity distributions are compatible with an important amount of gas and metals being ejected in outflows, although a detailed quantification of the ejected gas fraction is strongly dependent on a precise determination of the absolute stellar metallicities. We find a systematic difference between the samples of Ibata & Gilmore, at projected distances of 1-3 kpc, and the sample in Baade's window (Sadler et al.). This could be caused either by a true metallicity gradient in the bulge or by a systematic offset in the calibration of [Fe/H] between these two samples. This offset does not play an important role in the estimate of infall and formation time-scales, which are mostly dependent on the width of the distributions. The recent bulge data from Zoccali et al. are also analysed, and the subsample with subsolar metallicities still rules out infall time-scales greater than or similar to1 Gyr at the 90 per cent confidence level. Hence, the short time-scales we derive based on the observed distribution of metallicities are robust and should be taken as stringent constraints on bulge formation models
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