15 research outputs found
Expanding Space: the Root of all Evil?
While it remains the staple of virtually all cosmological teaching, the
concept of expanding space in explaining the increasing separation of galaxies
has recently come under fire as a dangerous idea whose application leads to the
development of confusion and the establishment of misconceptions. In this
paper, we develop a notion of expanding space that is completely valid as a
framework for the description of the evolution of the universe and whose
application allows an intuitive understanding of the influence of universal
expansion. We also demonstrate how arguments against the concept in general
have failed thus far, as they imbue expanding space with physical properties
not consistent with the expectations of general relativity.Comment: 8 pages, accepted for publication in PAS
The Adventures of the Rocketeer: Accelerated Motion Under the Influence of Expanding Space
It is well known that interstellar travel is bounded by the finite speed of
light, but on very large scales any rocketeer would also need to consider the
influence of cosmological expansion on their journey. This paper examines
accelerated journeys within the framework of Friedmann-
Lemaitre-Robertson-Walker universes, illustrating how the duration of a fixed
acceleration sharply divides exploration over interstellar and intergalactic
distances. Furthermore, we show how the universal expansion increases the
difficulty of intergalactic navigation, with small uncertainties in
cosmological parameters resulting in significantly large deviations. This paper
also shows that, contrary to simplistic ideas, the motion of any rocketeer is
indistinguishable from Newtonian gravity if the acceleration is kept small.Comment: 9 pages, 7 figures, accepted for publication in PAS
Cosmological Radar Ranging in an Expanding Universe
While modern cosmology, founded in the language of general relativity, is
almost a century old, the meaning of the expansion of space is still being
debated. In this paper, the question of radar ranging in an expanding universe
is examined, focusing upon light travel times during the ranging; it has
recently been claimed that this proves that space physically expands. We
generalize the problem into considering the return journey of an accelerating
rocketeer, showing that while this agrees with expectations of special
relativity for an empty universe, distinct differences occur when the universe
contains matter. We conclude that this does not require the expansion of space
to be a physical phenomenon, rather that we cannot neglect the influence of
matter, seen through the laws of general relativity, when considering motions
on cosmic scales.Comment: 6 Pages. To appear in MNRA
Topology of non-linear structure in the 2dF Galaxy Redshift Survey
We study the evolution of non-linear structure as a function of scale in
samples from the 2dF Galaxy Redshift Survey, constituting over 221 000 galaxies
at a median redshift of z=0.11. The two flux-limited galaxy samples, located
near the southern galactic pole and the galactic equator, are smoothed with
Gaussian filters of width ranging from 5 to 8 Mpc/h to produce a continuous
galaxy density field. The topological genus statistic is used to measure the
relative abundance of overdense clusters to void regions at each scale; these
results are compared to the predictions of analytic theory, in the form of the
genus statistic for i) the linear regime case of a Gaussian random field; and
ii) a first-order perturbative expansion of the weakly non-linear evolved
field. The measurements demonstrate a statistically significant detection of an
asymmetry in the genus statistic between regions corresponding to low- and
high-density volumes of the universe. We attribute the asymmetry to the
non-linear effects of gravitational evolution and biased galaxy formation, and
demonstrate that these effects evolve as a function of scale. We find that
neither analytic prescription satisfactorily reproduces the measurements,
though the weakly non-linear theory yields substantially better results in some
cases, and we discuss the potential explanations for this result.Comment: 13 pages, matching proof to be published in MNRAS; new version adds
reference and corrects figure
Active Learning to Overcome Sample Selection Bias: Application to Photometric Variable Star Classification
Despite the great promise of machine-learning algorithms to classify and
predict astrophysical parameters for the vast numbers of astrophysical sources
and transients observed in large-scale surveys, the peculiarities of the
training data often manifest as strongly biased predictions on the data of
interest. Typically, training sets are derived from historical surveys of
brighter, more nearby objects than those from more extensive, deeper surveys
(testing data). This sample selection bias can cause catastrophic errors in
predictions on the testing data because a) standard assumptions for
machine-learned model selection procedures break down and b) dense regions of
testing space might be completely devoid of training data. We explore possible
remedies to sample selection bias, including importance weighting (IW),
co-training (CT), and active learning (AL). We argue that AL---where the data
whose inclusion in the training set would most improve predictions on the
testing set are queried for manual follow-up---is an effective approach and is
appropriate for many astronomical applications. For a variable star
classification problem on a well-studied set of stars from Hipparcos and OGLE,
AL is the optimal method in terms of error rate on the testing data, beating
the off-the-shelf classifier by 3.4% and the other proposed methods by at least
3.0%. To aid with manual labeling of variable stars, we developed a web
interface which allows for easy light curve visualization and querying of
external databases. Finally, we apply active learning to classify variable
stars in the ASAS survey, finding dramatic improvement in our agreement with
the ACVS catalog, from 65.5% to 79.5%, and a significant increase in the
classifier's average confidence for the testing set, from 14.6% to 42.9%, after
a few AL iterations.Comment: 43 pages, 11 figures, submitted to Ap
The WiggleZ Dark Energy Survey: the transition to large-scale cosmic homogeneity
We have made the largest-volume measurement to date of the transition to
large-scale homogeneity in the distribution of galaxies. We use the WiggleZ
survey, a spectroscopic survey of over 200,000 blue galaxies in a cosmic volume
of ~1 (Gpc/h)^3. A new method of defining the 'homogeneity scale' is presented,
which is more robust than methods previously used in the literature, and which
can be easily compared between different surveys. Due to the large cosmic depth
of WiggleZ (up to z=1) we are able to make the first measurement of the
transition to homogeneity over a range of cosmic epochs. The mean number of
galaxies N(<r) in spheres of comoving radius r is proportional to r^3 within
1%, or equivalently the fractal dimension of the sample is within 1% of D_2=3,
at radii larger than 71 \pm 8 Mpc/h at z~0.2, 70 \pm 5 Mpc/h at z~0.4, 81 \pm 5
Mpc/h at z~0.6, and 75 \pm 4 Mpc/h at z~0.8. We demonstrate the robustness of
our results against selection function effects, using a LCDM N-body simulation
and a suite of inhomogeneous fractal distributions. The results are in
excellent agreement with both the LCDM N-body simulation and an analytical LCDM
prediction. We can exclude a fractal distribution with fractal dimension below
D_2=2.97 on scales from ~80 Mpc/h up to the largest scales probed by our
measurement, ~300 Mpc/h, at 99.99% confidence.Comment: 21 pages, 16 figures, accepted for publication in MNRA