共查询到20条相似文献，搜索用时 31 毫秒
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Boudewijn F. Roukema 《Monthly notices of the Royal Astronomical Society》2001,325(1):138-142
The inner product provides a conceptually and algorithmically simple method for calculating the comoving distance between two cosmological objects given their redshifts, right ascension and declination, and arbitrary constant curvature. The key to this is that just as a distance between two points 'on' the surface of the ordinary 2-sphere 2 is simply an arc-length (angle multiplied by radius) in ordinary Euclidean 3-space ℰ3 , the distance between two points 'on' a 3-sphere 3 (a 3-hyperboloid ℋ3 ) is simply an 'arc-length' in Euclidean 4-space ℰ4 (Minkowski 4-space ℳ4 ), i.e. an 'hyper-angle' multiplied by the curvature radius of the 3-sphere (3-hyperboloid). 相似文献
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Pier Stefano Corasaniti Marilena LoVerde Arlin Crotts Chris Blake 《Monthly notices of the Royal Astronomical Society》2006,369(2):798-804
The Advanced Liquid-mirror Probe of Asteroids, Cosmology and Astrophysics (ALPACA) is a proposed 8-m liquid-mirror telescope surveying ∼1000 deg2 of the Southern hemisphere sky. It will be a remarkably simple and inexpensive telescope that none the less will deliver a powerful sample of optical data for studying dark energy. The bulk of the cosmological data consist of nightly, high signal-to-noise ratio, multiband light curves of Type Ia supernovae (SNe Ia). At the end of the 3-yr run, ALPACA is expected to collect ≳100 000 SNe Ia up to z ∼ 1 . This will allow us to reduce present systematic uncertainties affecting the standard-candle relation. The survey will also provide several other data sets such as the detection of baryon acoustic oscillations in the matter power spectrum and shear weak-lensing measurements. In this preliminary analysis, we forecast constraints on dark energy parameters from SNe Ia and baryon acoustic oscillations. The combination of these two data sets will provide competitive constraints on the dark energy parameters under minimal prior assumptions. Further studies are needed to address the accuracy of weak-lensing measurements. 相似文献
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Neil Trentham 《Monthly notices of the Royal Astronomical Society》2001,326(4):1328-1332
A major recent development in observational cosmology has been an accurate measurement of the luminosity distance–redshift relation out to redshifts z =0.8 from Type Ia supernova standard candles. The results have been argued as evidence for cosmic acceleration. It is well known that this assertion depends on the assumption that we know the equation of state for all mass–energy other than normal pressureless matter; popular models are based either on the cosmological constant or on the more general quintessence formulation. However, this assertion also depends on a number of other assumptions, implicit in the derivation of the standard cosmological field equations: large-scale isotropy and homogeneity, the flatness of the Universe, and the validity of general relativity on cosmological scales (where it has not been tested). A detailed examination of the effects of these assumptions on the interplay between the luminosity distance–redshift relation and the acceleration of the Universe is not possible unless one can define the precise nature of the failure of any particular assumption. However a simple quantitative investigation is possible and reveals a number of considerations about the relative importance of the different assumptions. In this paper we present such an investigation. We find that the relationship between the distant-redshift relation and the sign of the deceleration parameter is fairly robust and is unaffected if only one of the assumptions that we investigate is invalid so long as the deceleration parameter is not close to zero (it would not be close to zero in the currently favoured ΩΛ =1−Ωmatter =0.7 or 0.8 Universe, for example). Failures of two or more assumptions in concordance may have stronger effects. 相似文献
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A combined sample of 79 high- and low-redshift Type Ia supernovae (SNe) is used to set constraints on the degree of anisotropy in the Universe out to z ≃1. First, we derive the global most probable values of matter density ΩM , the cosmological constant ΩΛ and the Hubble constant H 0 , and find them to be consistent with the published results from the two data sets of Riess et al. and Perlmutter et al. We then examine the Hubble diagram (HD, i.e., the luminosity–redshift relation) in different directions on the sky by utilizing spherical harmonic expansion. In particular, via the analysis of the dipole anisotropy, we divide the sky into the two hemispheres that yield the most discrepant of the three cosmological parameters, and the scatter χ HD 2 in each case. The most discrepant values roughly move along the locus −4ΩM +3ΩΛ =1 (cf. Perlmutter et al.), but by no more than Δ≈2.5 along this line. For a perfect Friedmann–Robertson–Walker universe, Monte Carlo realizations that mimic the current set of SNe yield values higher than the measured Δ in ∼1/5 of the cases (for ΩM ). We discuss implications for the validity of the Cosmological Principle, and possible calibration problems in the SNe data sets. 相似文献
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G. Efstathiou 《Monthly notices of the Royal Astronomical Society》2008,388(3):1314-1320
There has been increasing interest by cosmologists in applying Bayesian techniques, such as Bayesian Evidence, for model selection. A typical example is in assessing whether observational data favour a cosmological constant over evolving dark energy. In this paper, the example of dark energy is used to illustrate limitations in the application of Bayesian Evidence associated with subjective judgements concerning the choice of model and priors. An analysis of recent cosmological data shows a statistically insignificant preference for a cosmological constant over simple dynamical models of dark energy. It is argued that for nested problems, as considered here, Bayesian parameter estimation can be more informative than computing Bayesian Evidence for poorly motivated physical models. 相似文献
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Øystein Elgarøy Tuomas Multamäki 《Monthly notices of the Royal Astronomical Society》2005,356(2):475-479
An alternative to dark energy as an explanation for the present phase of accelerated expansion of the Universe is that the Friedmann equation is modified, e.g. by extra dimensional gravity, on large scales. We explore a natural parametrization of a general modified Friedmann equation, and find that the present supernova Type Ia and cosmic microwave background data prefer a correction of the form 1/ H to the Friedmann equation over a cosmological constant. 相似文献
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M. Viel E. Branchini K. Dolag M. Grossi S. Matarrese L. Moscardini 《Monthly notices of the Royal Astronomical Society》2009,393(3):774-782
We present results from the first high-resolution hydrodynamical simulations of non-Gaussian cosmological models. We focus on the statistical properties of the transmitted Lyman-α flux in the high-redshift intergalactic medium. Imprints of non-Gaussianity are present and are larger at high redshifts. Differences larger than 20 per cent at z > 3 in the flux probability distribution function for high-transmissivity regions (voids) are expected for values of the non-linearity parameter f NL =±100 when compared to a standard Λ cold dark matter cosmology with f NL = 0 . We also investigate the one-dimensional flux bispectrum: at the largest scales (corresponding to tens of Mpc), we expect deviations in the flux bispectrum up to 20 per cent at z ∼ 4 (for f NL =±100 ), significantly larger than deviations of ∼3 per cent in the flux power spectrum. We briefly discuss possible systematic errors that can contaminate the signal. Although challenging, a detection of non-Gaussianities in the interesting regime of scales and redshifts probed by the Lyman-α forest could be possible with future data sets. 相似文献
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We determine cosmological and evolutionary parameters from the 3CR K -band Hubble diagram and K -band number counts, assuming that the galaxies in question undergo pure luminosity evolution. Separately the two data sets are highly degenerate with respect to choice of cosmological and evolutionary parameters, but in combination the degeneracy is resolved. Of models that either are flat or have ΩΛ =0 , the preferred ones are close to the canonical case Ωcold matter =1 , ΩΛ =0 , with luminosity evolution amounting to 1 mag brighter at z =1 . 相似文献
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Phillip Helbig 《Monthly notices of the Royal Astronomical Society》1998,298(2):395-398
Recently, Park &38; Gott claimed that there is a statistically significant, strong, negative correlation between the image separation Δθ and source redshift z s for gravitational lenses. This is somewhat puzzling if one believes in a flat ( k = 0) universe, since in this case the typical image separation is expected to be independent of the source redshift, while one expects a negative correlation in a k = −1 universe and a positive one in a k = +1 universe. Park &38; Gott explored several effects that could cause the observed correlation, but no combination of these can explain the observations with a realistic scenario. Here, I explore this test further in three ways. First, I show that in an inhomogeneous universe a negative correlation is expected regardless of the value of k . Secondly, I test whether the Δθ– z s relation can be used as a test to determine λ0 and Ω0 , rather than just the sign of k . Thirdly, I compare the results of the test from the Park &38; Gott sample with those using other samples of gravitational lenses, which can illuminate (unknown) selection effects and probe the usefulness of the Δθ– z s relation as a cosmological test. 相似文献
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Neil Trentham Ole Möller Enrico Ramirez-Ruiz 《Monthly notices of the Royal Astronomical Society》2001,322(3):658-668
There are a number of theoretical and observational hints that large numbers of low-mass galaxies composed entirely of dark matter exist in the field. The theoretical considerations follow from the prediction of cold dark matter theory that there exist many low-mass galaxies for every massive one. The observational considerations follow from the observed paucity of these low-mass galaxies in the field but not in dense clusters of galaxies; this suggests that the lack of small galaxies in the field is due to the inhibition of star formation in the galaxies as opposed to the fact that their small dark matter haloes do not exist. In this work we outline the likely properties of low-mass dark galaxies, and describe observational strategies for finding them, and where in the sky to search. The results are presented as a function of the global properties of dark matter, in particular the presence or absence of a substantial baryonic dark matter component. If the dark matter is purely cold and has a Navarro, Frenk & White density profile, directly detecting dark galaxies will only be feasible with present technology if the galaxy has a maximum velocity dispersion in excess of 70 km s−1 , in which case the dark galaxies could strongly lens background objects. This is much higher than the maximum velocity dispersions in most dwarf galaxies. If the dark matter in galaxy haloes has a baryonic component close to the cosmic ratio, the possibility of directly detecting dark galaxies is much more realistic; the optimal method of detection will depend on the nature of the dark matter. A number of more indirect methods are also discussed. 相似文献
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Uro&#; Seljak † 《Monthly notices of the Royal Astronomical Society》2001,325(4):1359-1364
We analyse scale dependence of redshift-space bias b and β ≡ Ωm 0.6 / b in the context of the halo model. We show that linear bias is a good approximation only on large scales, for k <0.1 h Mpc−1 . On intermediate scales the virial motions of galaxies cause a suppression of the power spectrum relative to the linear one and the suppression differs from the same effect in dark matter. This can potentially mimic the effect of massive neutrinos, and the degeneracy can only be broken if the power spectrum is measured for k ≪0.1 h Mpc−1 . Different methods to determine β converge for k <0.1 h Mpc−1 , but give drastically different results on smaller scales, which explains some of the trends observed in the real data. We also assess the level of stochasticity by calculating the cross-correlation coefficient between the reconstructed velocity field divergence and the galaxies, and show that the two fields decorrelate for k >0.1 h Mpc−1 . Most problematic are galaxies predominantly found in groups and clusters, such as bright, red or elliptical galaxies, where we find poor convergence to a constant bias or β even on large scales. 相似文献
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Local and global variations of the fine-structure constant 总被引：1，自引：0，他引：1
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Timothy G. Clemson Andrew R. Liddle 《Monthly notices of the Royal Astronomical Society》2009,395(3):1585-1590
We use a dynamical systems approach to study thawing quintessence models, using a multiparameter extension of the exponential potential which can approximate the form of typical thawing potentials. We impose observational constraints using a compilation of current data, and forecast the tightening of constraints expected from future dark energy surveys, as well as discussing the relation of our results to analytical constraints already in the literature. 相似文献