133 research outputs found
Transverse Galaxy Velocities from Multiple Topological Images
The study of the kinematics of galaxies within clusters or groups has the
limitation that only one of the three velocity components and only two of the
three spatial components of a galaxy position in six-dimensional phase space
can normally be measured. However, if multiple topological images of a cluster
exist, then the radial positions and sky plane mean velocities of galaxies in
the cluster may also be measurable from photometry of the two cluster images.
The vector arithmetic and principles of the analysis are presented. These are
demonstrated by assuming the suggested topological identification of the
clusters RX J1347.5-1145 and CL 09104+4109 to be correct and deducing the
sky-plane relative velocity component along the axis common to both images of
this would-be single cluster.
Three out of four of the inferred transverse velocities are consistent with
those expected in a rich cluster. A control sample of random `common' sky-plane
axes, independent of the topological hypothesis, implies that this is not
surprising. This shows that while galaxy kinematics are deducible from
knowledge of cosmological topology, it is not easy to use them to refute a
specific candidate manifold.Comment: 13 pages, 7 figures, accepted for MNRA
Galaxy Clustering at z ~ 2 and Halo Radii
The amplitude of the angular two-point galaxy correlation function w(\theta)
for galaxies at z~2 is estimated for galaxies in the Hubble Deep Field by using
a U < 27 complete sub-sample.
(i) It is confirmed that the amplitude of the correlation can be corrected
for the integral constraint without having to make assumptions about the shape
of the correlation function and by avoiding the introduction of linear error
terms. The estimate using this technique is w(5'') = 0.10 \pm 0.09.
(ii) If the biases introduced in faint galaxy selection due to obscuration by
large objects are not corrected for by masking areas around them, then the
estimate would be w(5'') =0.16\pm 0.07.
(iii) The effective (3-D) galaxy pair separation at 5'' and this redshift
range is ~ 25-250 /h kpc, so the correction to the spatial correlation function
\xi(r) due to exclusion of overlapping galaxy dark matter haloes should be
considered. For clustering stable in proper units in an \Omega=1,\lambda=0
universe, our w(5\arcs) estimate (a) implies a present-day correlation length
of r_0 ~ 2.6^{+1.1}_{-1.7}/h Mpc if halo overlapping is ignored, but (b) for a
present-day correlation length of r_0=5.5/h Mpc implies that a typical halo
exclusion radius is r_halo=70^{+420}_{-30}/h kpc.
(iv) The decreasing correlation period (DCP) of a high initial bias in the
spatial correlation function is not detected at this redshift. For an
\Omega=1,\lambda=0 universe and (proper) stable clustering, possible detections
of the DCP in other work would imply that \xi at redshifts greater than z_t =
1.7\pm0.9 would be [(1+z)/(1+z_t)]^{2.1\pm3.6} times higher than at z_t, which
is consistent with our lack of a detection at z ~ 2.Comment: 17 pages, 13 figures, accepted for MNRAS, additional FITS files with
HDF images available at http://www.iap.fr/users/roukema/xi2
The Cosmological Constant and Quintessence from a Correlation Function Comoving Fine Feature in the 2dF Quasar Redshift Survey
Detections of local maxima in the density perturbation spectrum at characteristic comoving scales L~100-200h^{-1}Mpc have previously been claimed. Here, this cosmic standard ruler is sought in the ``10K'' release of the 2dF QSO Redshift Survey (2QZ-10K), by estimating the comoving, spatial correlation functions \xi(r) of the three-dimensional distribution of the N=2378 quasars in the most completely observed and ``covered'' sky regions, over the three redshift ranges 0.6 < z < 1.1, 1.1 < z < 1.6 and 1.6 < z < 2.2. 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 \xi(r) are found in all three 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. The length scale of the local maximum common to the three redshift ranges is 2L= (244\pm17)h^{-1}Mpc. (ii) For a standard FLRW model, the matter density \Omm and cosmological constant \Omega_\Lambda are constrained to \Omm= 0.25\pm0.10, \Omega_\Lambda=0.65\pm0.25 (68% confidence), \Omm= 0.25\pm0.15, \Omega_\Lambda=0.60\pm0.35 (95%), respectively, from the 2QZ-10K alone. Independently of the SNe Ia data, the zero cosmological constant model (\Omega_\Lambda=0) is rejected at the 99.7% confidence level. (iii) For an effective quintessence (w_Q) model and zero curvature, w_Q<-0.5 (68%), w_Q<-0.35 (95%) are found, again from the 2QZ-10K alone
The twin paradox on the photon sphere
We consider a new version of the twin paradox. The twins move along the same
circular free photon path around the Schwarzschild center. In this case,
despite their different velocities, all twins have the same non-zero
acceleration. On the circular photon path, the symmetry between the twins
situations is broken not by acceleration (as it is in the case of the classic
twin paradox), but by the existence of an absolute standard of rest (timelike
Killing vector). The twin with the higher velocity with respect to the standard
of rest is younger on reunion. This closely resembles the case of periodic
motions in compact (non-trivial topology) 3-D space recently considered in the
context of the twin paradox by Barrow and Levin, except that there
accelerations of all twins were equal to zero, and that in the case considered
here, the 3-D space has trivial topology.Comment: Two pages, submitted to Physical Review
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