134 research outputs found
A Century of Cosmology
In the century since Einstein's anno mirabilis of 1905, our concept of the
Universe has expanded from Kapteyn's flattened disk of stars only 10 kpc across
to an observed horizon about 30 Gpc across that is only a tiny fraction of an
immensely large inflated bubble. The expansion of our knowledge about the
Universe, both in the types of data and the sheer quantity of data, has been
just as dramatic. This talk will summarize this century of progress and our
current understanding of the cosmos.Comment: Talk presented at the "Relativistic Astrophysics and Cosmology -
Einstein's Legacy" meeting in Munich, Nov 2005. Proceedings will be published
in the Springer-Verlag "ESO Astrophysics Symposia" series. 10 pages Latex
with 2 figure
Nucleosynthesis Constraints on Scalar-Tensor Theories of Gravity
We study the cosmological evolution of massless single-field scalar-tensor
theories of gravitation from the time before the onset of annihilation
and nucleosynthesis up to the present. The cosmological evolution together with
the observational bounds on the abundances of the lightest elements (those
mostly produced in the early universe) place constraints on the coefficients of
the Taylor series expansion of , which specifies the coupling of the
scalar field to matter and is the only free function in the theory. In the case
when has a minimum (i.e., when the theory evolves towards general
relativity) these constraints translate into a stronger limit on the
Post-Newtonian parameters and than any other observational
test. Moreover, our bounds imply that, even at the epoch of annihilation and
nucleosynthesis, the evolution of the universe must be very close to that
predicted by general relativity if we do not want to over- or underproduce
He. Thus the amount of scalar field contribution to gravity is very small
even at such an early epoch.Comment: 15 pages, 2 figures, ReVTeX 3.1, submitted to Phys. Rev. D1
The Beginning and Evolution of the Universe
We review the current standard model for the evolution of the Universe from
an early inflationary epoch to the complex hierarchy of structure seen today.
We summarize and provide key references for the following topics: observations
of the expanding Universe; the hot early Universe and nucleosynthesis; theory
and observations of the cosmic microwave background; Big Bang cosmology;
inflation; dark matter and dark energy; theory of structure formation; the cold
dark matter model; galaxy formation; cosmological simulations; observations of
galaxies, clusters, and quasars; statistical measures of large-scale structure;
and measurement of cosmological parameters. We conclude with discussion of some
open questions in cosmology. This review is designed to provide a graduate
student or other new worker in the field an introduction to the cosmological
literature.Comment: 69 pages. Invited review article for Publications of the Astronomical
Society of the Pacific. Supplementary references, tables, and more concise
PDF file at http://www.physics.drexel.edu/univers
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: constraints on the time variation of fundamental constants from the large-scale two-point correlation function
We obtain constraints on the variation of the fundamental constants from the
full shape of the redshift-space correlation function of a sample of luminous
galaxies drawn from the Data Release 9 of the Baryonic Oscillations
Spectroscopic Survey. We combine this information with data from recent CMB,
BAO and H_0 measurements. We focus on possible variations of the fine structure
constant \alpha and the electron mass m_e in the early universe, and study the
degeneracies between these constants and other cosmological parameters, such as
the dark energy equation of state parameter w_DE, the massive neutrinos
fraction f_\nu, the effective number of relativistic species N_eff, and the
primordial helium abundance Y_He. When only one of the fundamental constants is
varied, our final bounds are \alpha / \alpha_0 = 0.9957_{-0.0042}^{+0.0041} and
m_e /(m_e)_0 = 1.006_{-0.013}^{+0.014}. For their joint variation, our results
are \alpha / \alpha_0 = 0.9901_{-0.0054}^{+0.0055} and m_e /(m_e)_0 = 1.028 +/-
0.019. Although when m_e is allowed to vary our constraints on w_DE are
consistent with a cosmological constant, when \alpha is treated as a free
parameter we find w_DE = -1.20 +/- 0.13; more than 1 \sigma away from its
standard value. When f_\nu and \alpha are allowed to vary simultaneously, we
find f_\nu < 0.043 (95% CL), implying a limit of \sum m_\nu < 0.46 eV (95% CL),
while for m_e variation, we obtain f_nu < 0.086 (95% CL), which implies \sum
m_\nu < 1.1 eV (95% CL). When N_eff or Y_He are considered as free parameters,
their simultaneous variation with \alpha provides constraints close to their
standard values (when the H_0 prior is not included in the analysis), while
when m_e is allowed to vary, their preferred values are significantly higher.
In all cases, our results are consistent with no variations of \alpha or m_e at
the 1 or 2 \sigma level.Comment: 18 pages, 16 figures. Submitted to MNRA
Can the Universe Create Itself?
The question of first-cause has troubled philosophers and cosmologists alike.
Now that it is apparent that our universe began in a Big Bang explosion, the
question of what happened before the Big Bang arises. Inflation seems like a
very promising answer, but as Borde and Vilenkin have shown, the inflationary
state preceding the Big Bang must have had a beginning also. Ultimately, the
difficult question seems to be how to make something out of nothing. This paper
explores the idea that this is the wrong question --- that that is not how the
Universe got here. Instead, we explore the idea of whether there is anything in
the laws of physics that would prevent the Universe from creating itself.
Because spacetimes can be curved and multiply connected, general relativity
allows for the possibility of closed timelike curves (CTCs). Thus, tracing
backwards in time through the original inflationary state we may eventually
encounter a region of CTCs giving no first-cause. This region of CTCs, may well
be over by now (being bounded toward the future by a Cauchy horizon). We
illustrate that such models --- with CTCs --- are not necessarily inconsistent
by demonstrating self-consistent vacuums for Misner space and a multiply
connected de Sitter space in which the renormalized energy-momentum tensor does
not diverge as one approaches the Cauchy horizon and solves Einstein's
equations. We show such a Universe can be classically stable and
self-consistent if and only if the potentials are retarded, giving a natural
explanation of the arrow of time. Some specific scenarios (out of many possible
ones) for this type of model are described. For example: an inflationary
universe gives rise to baby universes, one of which turns out to be itself.
Interestingly, the laws of physics may allow the Universe to be its own mother.Comment: 48 pages, 8 figure
Stellar structure and compact objects before 1940: Towards relativistic astrophysics
Since the mid-1920s, different strands of research used stars as "physics
laboratories" for investigating the nature of matter under extreme densities
and pressures, impossible to realize on Earth. To trace this process this paper
is following the evolution of the concept of a dense core in stars, which was
important both for an understanding of stellar evolution and as a testing
ground for the fast-evolving field of nuclear physics. In spite of the divide
between physicists and astrophysicists, some key actors working in the
cross-fertilized soil of overlapping but different scientific cultures
formulated models and tentative theories that gradually evolved into more
realistic and structured astrophysical objects. These investigations culminated
in the first contact with general relativity in 1939, when J. Robert
Oppenheimer and his students George Volkoff and Hartland Snyder systematically
applied the theory to the dense core of a collapsing neutron star. This
pioneering application of Einstein's theory to an astrophysical compact object
can be regarded as a milestone in the path eventually leading to the emergence
of relativistic astrophysics in the early 1960s.Comment: 83 pages, 4 figures, submitted to the European Physical Journal
Primordial Nucleosynthesis for the New Cosmology: Determining Uncertainties and Examining Concordance
Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) have
a long history together in the standard cosmology. The general concordance
between the predicted and observed light element abundances provides a direct
probe of the universal baryon density. Recent CMB anisotropy measurements,
particularly the observations performed by the WMAP satellite, examine this
concordance by independently measuring the cosmic baryon density. Key to this
test of concordance is a quantitative understanding of the uncertainties in the
BBN light element abundance predictions. These uncertainties are dominated by
systematic errors in nuclear cross sections. We critically analyze the cross
section data, producing representations that describe this data and its
uncertainties, taking into account the correlations among data, and explicitly
treating the systematic errors between data sets. Using these updated nuclear
inputs, we compute the new BBN abundance predictions, and quantitatively
examine their concordance with observations. Depending on what deuterium
observations are adopted, one gets the following constraints on the baryon
density: OmegaBh^2=0.0229\pm0.0013 or OmegaBh^2 = 0.0216^{+0.0020}_{-0.0021} at
68% confidence, fixing N_{\nu,eff}=3.0. Concerns over systematics in helium and
lithium observations limit the confidence constraints based on this data
provide. With new nuclear cross section data, light element abundance
observations and the ever increasing resolution of the CMB anisotropy, tighter
constraints can be placed on nuclear and particle astrophysics. ABRIDGEDComment: 54 pages, 20 figures, 5 tables v2: reflects PRD version minor changes
to text and reference
Origin of the Biologically Important Elements
The chemical elements most widely distributed in terrestrial living creatures are the ones (apart from inert helium and neon) that are commonest in the Universe--hydrogen, oxygen, carbon, and nitrogen. A chemically different Universe would clearly have different biology, if any. We explore here the nuclear processes in stars, the early Universe, and elsewhere that have produced these common elements, and, while we are at it, also encounter the production of lithium, gold, uranium, and other elements of sociological, if not biological, importance. The relevant processes are, for the most part, well understood. Much less well understood is the overall history of chemical evolution of the Galaxy, from pure hydrogen and helium to the mix of elements we see today. One implication is that we cannot do a very good job of estimating how many stars and which ones might be orbited by habitable planets
Does Lateral Transmission Obscure Inheritance in Hunter-Gatherer Languages?
In recent years, linguists have begun to increasingly rely on quantitative phylogenetic approaches to examine language evolution. Some linguists have questioned the suitability of phylogenetic approaches on the grounds that linguistic evolution is largely reticulate due to extensive lateral transmission, or borrowing, among languages. The problem may be particularly pronounced in hunter-gatherer languages, where the conventional wisdom among many linguists is that lexical borrowing rates are so high that tree building approaches cannot provide meaningful insights into evolutionary processes. However, this claim has never been systematically evaluated, in large part because suitable data were unavailable. In addition, little is known about the subsistence, demographic, ecological, and social factors that might mediate variation in rates of borrowing among languages. Here, we evaluate these claims with a large sample of hunter-gatherer languages from three regions around the world. In this study, a list of 204 basic vocabulary items was collected for 122 hunter-gatherer and small-scale cultivator languages from three ecologically diverse case study areas: northern Australia, northwest Amazonia, and California and the Great Basin. Words were rigorously coded for etymological (inheritance) status, and loan rates were calculated. Loan rate variability was examined with respect to language area, subsistence mode, and population size, density, and mobility; these results were then compared to the sample of 41 primarily agriculturalist languages in [1]. Though loan levels varied both within and among regions, they were generally low in all regions (mean 5.06%, median 2.49%, and SD 7.56), despite substantial demographic, ecological, and social variation. Amazonian levels were uniformly very low, with no language exhibiting more than 4%. Rates were low but more variable in the other two study regions, in part because of several outlier languages where rates of borrowing were especially high. High mobility, prestige asymmetries, and language shift may contribute to the high rates in these outliers. No support was found for claims that hunter-gatherer languages borrow more than agriculturalist languages. These results debunk the myth of high borrowing in hunter-gatherer languages and suggest that the evolution of these languages is governed by the same type of rules as those operating in large-scale agriculturalist speech communities. The results also show that local factors are likely to be more critical than general processes in determining high (or low) loan rates
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