Browsing by Author "Roukema, B. F."

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Cosmological Constant and Quintessence from a Correlation Function Comoving Fine Feature in the 2dF Quasar Redshift Survey
(2001-06-05) Roukema, B. F.; Mamon, G. A.
Local maxima at characteristic comoving scales have previously been claimed to exist in the density perturbation spectrum at the wavenumber k = 2π/LLSS, where LLSS ∼ 100–200 h−1 Mpc (comoving), at low redshift (z < ∼0.4) for several classes of tracer objects, at z ≈ 2 among quasars, and at z ≈ 3 among Lyman break galaxies. Here, this cosmic standard ruler is sought in the “10K” initial release of the 2dF QSO Redshift Survey (2QZ-10K), by estimating the spatial two-point autocorrelation functions ξ(r) of the three-dimensional (comoving, spatial) distribution of the N = 2378 quasars in the most completely observed and “covered” sky regions of the catalogue, over the redshift ranges 0.6 < z < 1.1 (“low-z”), 1.1 < z < 1.6 (“med-z”) and 1.6 < z < 2.2 (“hi-z”). Because of the selection method of the survey and sparsity of the data, the analysis was done conservatively to avoid non-cosmological artefacts. (i) Avoiding a priori estimates of the length scales of features, local maxima in ξ(r) are found in all three diﬀerent redshift ranges. The requirement that a local maximum be present in all three redshift ranges at a ﬁxed comoving length scale implies strong, purely geometric constraints on the local cosmological parameters, in which case the length scale of the local maximum common to the three redshift ranges is 2LLSS = 244±17 h−1 Mpc. (ii) For a standard cosmological constant FLRW model, the matter density and cosmological constant are constrained to Ωm = 0.25 ± 0.10, ΩΛ = 0.65±0.25 (68% conﬁdence), Ωm = 0.25±0.15, ΩΛ = 0.60±0.35 (95% conﬁdence), respectively, from the 2QZ-10K alone. Independently of the type Ia supernovae data, the zero cosmological constant model (ΩΛ = 0) is rejected at the 99.7% conﬁdence level. (iii) For an eﬀective quintessence (wQ) model and zero curvature, wQ < −0.5 (68% conﬁdence), wQ < −0.35 (95% conﬁdence) are found, again from the 2QZ-10K alone. In a diﬀerent analysis of a larger (but less complete) subset of the same 2QZ-10K catalogue, Hoyle et al. (2001) found a local maximum in the power spectrum to exist for widely diﬀering choices of Ωm and ΩΛ, which is diﬃcult to understand for a genuine large scale feature at ﬁxed comoving length scale. A resolution of this problem and deﬁnitive results should come from the full 2QZ, which should clearly provide even more impressive constraints on ﬁne features in density perturbation statistics, and on the local cosmological parameters Ωm, ΩΛ and wQ
Counterexample to Claimed COBE Constraints on Compact Toroidal Universe Models
(2000-03-12) Roukema, B. F.
It has been suggested that if the Universe satisﬁes a ﬂat, multiply connected, perturbed Friedmann- Lemaˆıtre model, then cosmic microwave back- ground data from the COBE satellite implies that the minimum size of the injectivity diameter (shortest closed spatial geodesic) must be larger than about two ﬁfths of the horizon diameter. To show that this claim is misleading, a simple T2×R universe model of injectivity diameter a quarter of this size, i.e. a tenth of the horizon diameter, is shown to be consistent with COBE four year ob- servational maps of the cosmic microwave back- ground. This is done using the identiﬁed circles principle.
Galaxy clustering at Z~2 and halo radii
(2015-03-01) Roukema, B. F.; Valls-Gabaud, D.; Mobasher, B.; Bajtlik, Stanislaw
How to avoid the ambiguity in applying the copernican principle for cosmic topology : Take the observational approach
(2001-03-01) Roukema, B. F.
It is often stated that homogeneity and isotropy of the Universe are assumptions of the almost Friedmann- Lemaˆıtre (FL) model (the hot big bang model), inspired from the Copernican Principle. However, only local homogeneity and isotropy are required by the model: multiply connected almost FL models are locally homogeneous and isotropic, but they can be globally anisotropic and/or globally inhomogeneous. Toy models are used here to show how global anisotropy and/or global inhomogeneity of an almost FL model could be shown directly in observations. This approach may avoid having to make any assumptions regarding global anisotropy and inhomogeneity.
How to distinguish a nearly flat Universe from a flat Universe using the orientation independence of a comoving standard ruler
(2001-01-20) Roukema, B. F.
Several recent observations using standard rulers and standard candles now suggest, either individually or in combination, that the Universe is close to ﬂat, i.e. that the curvature radius is about as large as the horizon radius (∼ 10h−1 Gpc) or larger. Here, a method of distinguishing an almost ﬂat universe from a precisely ﬂat universe using a single observational data set, without using any microwave background information, is presented. The method (i) assumes that a standard ruler should have no preferred orientation (radial versus tangential) to the observer, and (ii) requires that the (comoving) length of the standard ruler be known independently (e.g. from low redshift estimates). The claimed feature at ﬁxed comoving length in the power spectrum of density perturbations, detected among quasars, Lyman break galaxies or other high redshift objects, would provide an adequate standard candle to prove that the Universe is curved, if indeed it is curved. For example, a combined intrinsic and measurement uncertainty of 1% in the length of the standard ruler L applied at a redshift of z = 3 would distinguish an hyperbolic (Ωm = 0.2,ΩΛ = 0.7) or a spherical (Ωm = 0.4, ΩΛ = 0.7) universe from a ﬂat one to 1 − P > 95% conﬁdence
Lifting cosmic degeneracy within a single quasar survey
(2000-12-30) Roukema, B. F.; Mamon, G. A.
In the almost Friedmann-Lemaˆıtre model of the Universe, the density parameter, Ωm, and the cosmo- logical constant, ΩΛ, measure curvature. Several linearly degenerate relations between these two parameters have recently been measured. Here, large scale structure cor- relations at ∼ 100 − 150 h−1 Mpc are found in the co- moving three-dimensional separations of redshift z ≈ 2 quasars. These function as a comoving standard rod of length LLSS ≈ 130 ± 10 h−1 Mpc. A local maximum in the correlation function at ≈ LLSS/2 also appears to be signiﬁcant. By combining separate radial and tangential standard ruler analyses, the lifting of the Ωm − ΩΛ linear degeneracy within a single data set is demonstrated for the ﬁrst time.
Observational Approaches to the topology of the universe
(2000-03-25) Roukema, B. F.
Many diﬀerent and complementary strategies for translating the basic principle of multiple topological imaging into observational analysis are now available, both for three-dimensional and two-dimensional catalogues.
On the comoving distance as arc-length in four dimensions
(2001-08-04) Roukema, B. F.
The inner product provides a conceptually and algorithmically simple method for cal- culating 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 S2 is simply an arc-length (angle multiplied by radius) in ordinary Euclidean 3-space E3, the distance between two points ‘on’ a 3-sphere S3 (a 3-hyperboloid H3) is simply an ‘arc-length’ in Euclidean 4-space E4 (Minkowski 4-spaceM4), i.e. an ‘hyper-angle’ multiplied by the curvature radius of the 3-sphere (3-hyperboloid).
Star formation losses due to tidal debris in `hierarchical' galaxy formation
(2001-07-05) Roukema, B. F.; Ninin, S.; Devriendt, J.; et al.
N-body studies have previously shown that the bottom-up hierarchical formation of dark matter haloes is not as monotonic as implicitly assumed in the Press-Schechter formalism. During and following halo mergers, matter can be ejected into tidal tails, shells or low density ‘atmospheres’ outside of the successor haloes’ viriali- sation radii (or group-ﬁnder outermost radii). The implications that the possible truncation of star formation in these tidal ‘debris’ may have for observational galaxy statistics are examined here using the ArFus N-body plus semi-analytical galaxy modelling software for standard star formation hypotheses. In the N-body simulations studied, the debris typically remain close to the successor halo and fall back into the successor haloes given suﬃcient time. A maximum debris loss of around 16% is found for redshift intervals of around ∆z = 0.4 at z ∼ 1, with little dependence on the matter density parameter Ω0 and the cosmological constant λ0. Upper and lower bounds on stellar losses implied by a given set of N-body simulation output data can be investi- gated by choice of the merging/identity criterion of haloes between successive N-body simulation output times. A median merging/identity criterion is deﬁned and used to deduce an upper estimate of possible star formation and stellar population losses. A largest successor merging/identity criterion is deﬁned to deduce an estimate which minimises stellar losses. The losses for star formation and luminosity functions are strongest for low luminosity galaxies — a likely con- sequence of the fact that the debris fraction is highest for low mass haloes — and at intermediate redshifts (1 < ∼z < ∼3). The losses in both cases are mostly around 10%-30%, have some dependence on Ω0 and negligible dependence on λ0. This upper bound on likely losses in star formation rates and stellar populations is smaller than the uncertainties in estimates of corresponding observational parameters. Hence, it may not be urgent to include a correction for this in Press-Schechter based galaxy formation models, except when statistics regarding dwarf galaxies are under study.
Topology of the Universe
(2000-12-25) Roukema, B. F.
The Hilbert-Einstein equations are insuﬃcient t describe the geometry of the Universe, as they onl constrain a local geometrical property: curvatur A global knowledge of the geometry of space, if pos sible, would require measurement of the topolog of the Universe. Since the subject was discussed i 1900 by Schwarzschild, observational attempts t measure global topology have been rare for most o this century, but have accelerated in the 1990’s du to the rapidly increasing amount of observations o non-negligible fractions of the observational spher A brief review of basic concepts of cosmic topolog and of the rapidly growing gamut of diverse an complementary observational strategies for measu ing the topology of the Universe is provided here.
Topology of the universe: Theory and observation
(2015-03-01) Luminet, Jean-pierre; Roukema, B. F.