Browsing by Author "Mamon, G. A."

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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
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.