167,466 research outputs found
The World is Either Algorithmic or Mostly Random
I will propose the notion that the universe is digital, not as a claim about
what the universe is made of but rather about the way it unfolds. Central to
the argument will be the concepts of symmetry breaking and algorithmic
probability, which will be used as tools to compare the way patterns are
distributed in our world to the way patterns are distributed in a simulated
digital one. These concepts will provide a framework for a discussion of the
informational nature of reality. I will argue that if the universe were analog,
then the world would likely be random, making it largely incomprehensible. The
digital model has, however, an inherent beauty in its imposition of an upper
limit and in the convergence in computational power to a maximal level of
sophistication. Even if deterministic, that it is digital doesn't mean that the
world is trivial or predictable, but rather that it is built up from operations
that at the lowest scale are very simple but that at a higher scale look
complex and even random, though only in appearance.Comment: Third Prize Winning Essay -- 2011 Foundational Questions Institute
(FQXi) Contest "Is Reality Digital or Analog?
SNLS3: Constraints on Dark Energy Combining the Supernova Legacy Survey Three Year Data with Other Probes
We present observational constraints on the nature of dark energy using the
Supernova Legacy Survey three year sample (SNLS3) of Guy et al. (2010) and
Conley et al. (2011). We use the 472 SNe Ia in this sample, accounting for
recently discovered correlations between SN Ia luminosity and host galaxy
properties, and include the effects of all identified systematic uncertainties
directly in the cosmological fits. Combining the SNLS3 data with the full WMAP7
power spectrum, the Sloan Digital Sky Survey luminous red galaxy power
spectrum, and a prior on the Hubble constant H0 from SHOES, in a flat universe
we find omega_m=0.269+/-0.015 and w=-1.061+0.069-0.068 -- a 6.5% measure of the
dark energy equation-of-state parameter w. The statistical and systematic
uncertainties are approximately equal, with the systematic uncertainties
dominated by the photometric calibration of the SN Ia fluxes -- without these
calibration effects, systematics contribute only a ~2% error in w. When
relaxing the assumption of flatness, we find omega_m=0.271+/-0.015,
omega_k=-0.002+/-0.006, and w=-1.069+0.091-0.092. Parameterizing the time
evolution of w as w(a)=w_0+w_a(1-a), gives w_0=-0.905+/-0.196,
w_a=-0.984+1.094-1.097 in a flat universe. All of our results are consistent
with a flat, w=-1 universe. The size of the SNLS3 sample allows various tests
to be performed with the SNe segregated according to their light curve and host
galaxy properties. We find that the cosmological constraints derived from these
different sub-samples are consistent. There is evidence that the coefficient,
beta, relating SN Ia luminosity and color, varies with host parameters at
>4sigma significance (in addition to the known SN luminosity--host relation);
however this has only a small effect on the cosmological results and is
currently a sub-dominant systematic.Comment: Accepted for publication in ApJ. Data available from
https://tspace.library.utoronto.ca/snl
Evidence for dust reddening in DLAs identified through CaII H&K absorption
We present a new sample of 31 CaII(H&K) 3935,3970 absorption line systems
with 0.84<z_abs<1.3 discovered in the spectra of Sloan Digital Sky Survey
(SDSS) Data Release 3 quasars, together with an analysis of their dust content.
The presence of Calcium absorption together with measurements of the MgII 2796,
FeII 2600 and MgI 2853 lines lead to the conclusion that the majority of our
systems are Damped Ly-alpha (DLA) absorbers. The composite spectrum in the rest
frame of the absorber shows clear evidence for reddening. Large and Small
Magellanic Cloud extinction curves provide satisfactory fits, with a best-fit
E(B-V) of 0.06, while the Galactic dust extinction curve provides a poor fit
due to the lack of a strong 2175A feature. A trend of increasing dust content
with equivalent width of CaII is present. Monte Carlo techniques demonstrate
that the detection of reddening is significant at >99.99% confidence. The
discovery of significant amounts of dust in a subsample of DLAs has direct
implications for studies of the metallicity evolution of the universe and the
nature of DLAs in relation to high redshift galaxies. The gas:dust ratio is
discussed. Our results suggest that at least ~40% of the CaII absorption
systems are excluded from the magnitude-limited SDSS quasar sample as a result
of the associated extinction, a fraction similar to the upper limit deduced at
higher redshifts from radio-selected surveys.Comment: 5 pages, 2 figures, accepted MNRAS Letter
The distribution of stellar mass in the low-redshift Universe
We use a complete and uniform sample of almost half a million galaxies from
the Sloan Digital Sky Survey to characterise the distribution of stellar mass
in the low-redshift Universe. Galaxy abundances are well determined over almost
four orders of magnitude in stellar mass, and are reasonably but not perfectly
fit by a Schechter function with characteristic stellar mass m* = 6.7 x 10^10
M_sun and with faint-end slope \alpha = -1.155. For a standard cosmology and a
standard stellar Initial Mass Function, only 3.5% of the baryons in the
low-redshift Universe are locked up in stars. The projected autocorrelation
function of stellar mass is robustly and precisely determined for r_p < 30
Mpc/h. Over the range 10 kpc/kpc < r_p < 10 Mpc/h it is extremely well
represented by a power law. The corresponding three-dimensional autocorrelation
function is \xi*(r) = (r/6.1 Mpc/h)^{-1.84}. Relative to the dark matter, the
bias of the stellar mass distribution is approximately constant on large
scales, but varies by a factor of five for r_p < 1 Mpc/h. This behaviour is
approximately but not perfectly reproduced by current models for galaxy
formation in the concordance LCDM cosmology. Detailed comparison suggests that
a fluctuation amplitude \sigma_8 ~ 0.8 is preferred to the somewhat larger
value adopted in the Millennium Simulation models with which we compare our
data. This comparison also suggests that observations of stellar mass
autocorrelations as a function of redshift might provide a powerful test for
the nature of Dark Energy.Comment: 12 pages, 11 figures, accepted for publication in Monthly Notices,
two appendices added to explore possible systematic biases due to the stellar
mass definition and surface density limit
Observing Long Cosmic Strings Through Gravitational Lensing
We consider the gravitational lensing produced by long cosmic strings formed
in a GUT scale phase transition. We derive a formula for the deflection of
photons which pass near the strings that reduces to an integral over the light
cone projection of the string configuration plus constant terms which are not
important for lensing. Our strings are produced by performing numerical
simulations of cosmic string networks in flat, Minkowski space ignoring the
effects of cosmological expansion. These strings have more small scale
structure than those from an expanding universe simulation - fractal dimension
1.3 for Minkowski versus 1.1 for expanding - but share the same qualitative
features. Lensing simulations show that for both point-like and extended
objects, strings produce patterns unlike more traditional lenses, and, in
particluar, the kinks in strings tend to generate demagnified images which
reside close to the string. Thus lensing acts as a probe of the small scale
structure of a string. Estimates of lensing probablity suggest that for string
energy densities consistant with string seeded structure formation, on the
order of tens of string lenses should be observed in the Sloan Digital Sky
Survey quasar catalog. We propose a search strategy in which string lenses
would be identified in the SDSS quasar survey, and the string nature of the
lens can be confirmed by the observation of nearby high redshift galaxies which
are also be lensed by the string.Comment: 24 pages revtex with 12 postscript firgure
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