12 research outputs found
Cosmology under Milne's shadow
Based on the magnitude--redshift diagram for the sample of supernovae Ia
analysed by Perlmutter et al. (1999), Davis & Lineweaver rule out the special
relativistic interpretation of cosmological redshifts at a confidence level of
23 sigma. Here, we critically reassess this result. Special relativity is known
to describe the dynamics of an empty universe, by means of the Milne kinematic
model. Applying only special-relativistic concepts, we derive the angular
diameter distance and the luminosity distance in the Milne model. In
particular, in this model we do not use the underlying metric in its
Robertson-Walker form, so our exposition is useful for readers without any
knowledge of general relativity. We do however, explicitly use the
special-relativistic Doppler formula for redshift. We apply the derived
luminosity distance to the magnitude--redshift diagram for supernovae Ia of
Perlmutter et al. (1999) and show that special relativity fits the data much
better than that claimed by Davis & Lineweaver. Specifically, using these data
alone, the Milne model is ruled out only at a 2 sigma level. Although not a
viable cosmological model, in the context of current research on supernovae Ia
it remains a useful reference model when comparing predictions of various
cosmological models.Comment: 5 pages, 1 figure; a didactic article; matches the version accepted
for publication in PAS
Kurtosis in Large-Scale Structure as a Constraint on Non-Gaussian Initial Conditions
We calculate the kurtosis of a large-scale density field which has undergone
weakly non-linear gravitational evolution from arbitrary non-Gaussian initial
conditions. It is well known that the weakly evolved {\twelveit skewness} is
equal to its initial value plus the term induced by gravity, which scales with
the rms density fluctuation in precisely the same way as for Gaussian initial
conditions. As in the case of skewness, the evolved {\twelveit kurtosis} is
equal to its initial value plus the contribution induced by gravity. The
scaling of this induced contribution, however, turns out to be qualitatively
different for Gaussian versus non-Gaussian initial conditions. Therefore,
measurements of the kurtosis can serve as a powerful discriminating test
between the hypotheses of Gaussian and non-Gaussian nature of primordial
density fluctuations.Comment: uuencoded compressed tar file including postscript text (17 pages)
and 2 postscript figures, submitted to MNRA
Is space really expanding? A counterexample
In all Friedman models, the cosmological redshift is widely interpreted as a
consequence of the general-relativistic phenomenon of EXPANSION OF SPACE. Other
commonly believed consequences of this phenomenon are superluminal recession
velocities of distant galaxies and the distance to the particle horizon greater
than c*t (where t is the age of the Universe), in apparent conflict with
special relativity. Here, we study a particular Friedman model: empty universe.
This model exhibits both cosmological redshift, superluminal velocities and
infinite distance to the horizon. However, we show that the cosmological
redshift is there simply a relativistic Doppler shift. Moreover, apparently
superluminal velocities and `acausal' distance to the horizon are in fact a
direct consequence of special-relativistic phenomenon of time dilation, as well
as of the adopted definition of distance in cosmology. There is no conflict
with special relativity, whatsoever. In particular, INERTIAL recession
velocities are subluminal. Since in the real Universe, sufficiently distant
galaxies recede with relativistic velocities, these special-relativistic
effects must be at least partly responsible for the cosmological redshift and
the aforementioned `superluminalities', commonly attributed to the expansion of
space. Let us finish with a question resembling a Buddhism-Zen `koan': in an
empty universe, what is expanding?Comment: 12 pages, no figures; added Appendix with a calculation of the
cosmological redshift in `private space
The kinematic component of the cosmological redshift
It is widely believed that the cosmological redshift is not a Doppler shift.
However, Bunn & Hogg have recently pointed out that to settle properly this
problem, one has to transport parallelly the velocity four-vector of a distant
galaxy to the observer's position. Performing such a transport along the null
geodesic of photons arriving from the galaxy, they found that the cosmological
redshift is purely kinematic. Here we argue that one should rather transport
the velocity four-vector along the geodesic connecting the points of
intersection of the world-lines of the galaxy and the observer with the
hypersurface of constant cosmic time. We find that the resulting relation
between the transported velocity and the redshift of arriving photons is not
given by a relativistic Doppler formula. Instead, for small redshifts it
coincides with the well known non-relativistic decomposition of the redshift
into a Doppler (kinematic) component and a gravitational one. We perform such a
decomposition for arbitrary large redshifts and derive a formula for the
kinematic component of the cosmological redshift, valid for any FLRW cosmology.
In particular, in a universe with Omega_m = 0.24 and Omega_Lambda = 0.76, a
quasar at a redshift 6, at the time of emission of photons reaching us today
had the recession velocity v = 0.997c. This can be contrasted with v = 0.96c,
had the redshift been entirely kinematic. Thus, for recession velocities of
such high-redshift sources, the effect of deceleration of the early Universe
clearly prevails over the effect of its relatively recent acceleration. Last
but not least, we show that the so-called proper recession velocities of
galaxies, commonly used in cosmology, are in fact radial components of the
galaxies' four-velocity vectors. As such, they can indeed attain superluminal
values, but should not be regarded as real velocities.Comment: 10 pages, 1 figure; matches the version published in MNRA
Towards the optimal window for the 2MASS dipole
A comparison of the 2MASS flux dipole to the CMB dipole can serve as a method
to constrain a combination of the cosmological parameter Omega_m and the
luminosity bias of the 2MASS survey. For this constraint to be as tight as
possible, it is necessary to maximize the correlation between the two dipoles.
This can be achieved by optimizing the survey window through which the flux
dipole is measured. Here we explicitly construct such a window for the 2MASS
survey. The optimization in essence reduces to excluding from the calculation
of the flux dipole galaxies brighter than some limiting magnitude K_min of the
near-infrared K_s band. This exclusion mitigates nonlinear effects and shot
noise from small scales, which decorrelate the 2MASS dipole from the CMB
dipole. Under the assumption of negligible shot noise we find that the optimal
value of K_min is about five. Inclusion of shot noise shifts the optimal K_min
to larger values. We present an analytical formula for shot noise for the 2MASS
flux dipole, to be used in follow-up work with 2MASS data.
The misalignment angle between the two dipoles is a sensitive measure of
their correlation: the higher the correlation, the smaller the expectation
value of the angle. A minimum of the misalignment is thus a sign of the optimal
gravity window. We model analytically the distribution function for the
misalignment angle and show that the misalignment estimated by Maller et al. is
consistent with the assumed underlying model (though it is greater than the
expectation value). We predict with about 90% confidence that the misalignment
will decrease if 2MASS galaxies brighter than K_min = 5 mag are excluded from
the calculation of the flux dipole. This prediction has been indirectly
confirmed by the results of Erdogdu et al. (ABRIDGED)Comment: 14 pages, 3 figures. Significantly expanded version, with added
sections on shot noise and likelihood for beta, as well as an appendix with a
derivation of the distribution for the misalignment angle relaxing the
small-angle assumptio