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Post Info TOPIC: Universe is Bigger
Blobrana


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RE: Universe is Bigger
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Title: Do we Live in a "Small Universe"?
Authors: Ralf Aurich, Holger S. Janzer, Sven Lustig, Frank Steiner
(Version v2)

We compute the effects of a compact flat universe on the angular correlation function, the angular power spectrum, the circles-in-the-sky signature, and the covariance matrix of the spherical harmonics coefficients of the cosmic microwave background radiation using the full Boltzmann physics. Our analysis shows that the Wilkinson Microwave Anisotropy Probe (WMAP) three-year data are well compatible with the possibility that we live in a flat 3-torus with volume ~5x10³ Gpc³.

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Blobrana


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The Hubble constant
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Title: The Hubble constant from galaxy lenses: impacts of triaxiality and model degeneracies
Authors: Virginia L. Corless, Benjamin M. Dobke, Lindsay J. King

The Hubble constant can be constrained using the time delays between multiple images of gravitationally lensed sources. In some notable cases, typical lensing analyses assuming isothermal galaxy density profiles produce low values for the Hubble constant, inconsistent with the result of the HST Key Project (72 ± 8 km/s/Mpc). Possible systematics in the values of the Hubble constant derived from galaxy lensing systems can result from a number of factors, e.g. neglect of environmental effects, assumption of isothermality, or contamination by line-of-sight structures. One additional potentially important factor is the triaxial structure of the lensing galaxy halo; most lens models account for halo shape simply by perturbing the projected spherical lensing potential, an approximation that is often necessary but that is inadequate at the levels of triaxiality predicted in the CDM paradigm. To quantify the potential error introduced by this assumption in estimates of the Hubble parameter, we strongly lens a distant galaxy through a sample of triaxial softened isothermal halos and use an MCMC method to constrain the lensing halo profile and the Hubble parameter from the resulting multiple image systems. We explore the major degeneracies between the Hubble parameter and several parameters of the lensing model, finding that without a way to accurately break these degeneracies accurate estimates of the Hubble parameter are not possible. Crucially, we find that triaxiality does not significantly bias estimates of the Hubble constant, and offer an analytic explanation for this behaviour in the case of isothermal profiles. Neglected triaxial halo shape cannot contribute to the low Hubble constant values derived in a number of galaxy lens systems.

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Blobrana


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Posts: 131433
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RE: Universe is Bigger
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The universe is 13.73 billion years old, give or take 120 million years, astronomers said last week.
That age, based on precision measurements of the oldest light in the universe, agrees with results announced in 2006. Two additional years of data from a NASA satellite known as the Wilkinson Microwave Anisotropy Probe have narrowed the uncertainty by tens of millions of years.

Source

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Blobrana


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Posts: 131433
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Cepheid project
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In the vastness of space, how far is far?
That question has simmered in G. Fritz Benedict's mind since he was 8, when a family friend took him into the backyard of his home and pointed to the constellation Orion.

"Something in my brain went 'snap' ?" - G. Fritz  Benedict, astronomer at the University of Texas.

The experience set him on a lifelong quest to answer one of the most arcane questions in astronomy: How exactly do you measure the universe?

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Blobrana


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Posts: 131433
Date:
Small Universe
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Title: Do we Live in a "Small Universe"?
Authors: Ralf Aurich, Holger S. Janzer, Sven Lustig, Frank Steiner

We compute the effects of a compact flat universe on the angular correlation function, the power spectrum, and the covariance matrix of the spherical harmonics coefficients of the cosmic microwave background radiation using the full Boltzmann physics. Our analysis shows that the Wilkinson Microwave Anisotropy Probe three-year data are well compatible with the possibility that we live in a flat 3-torus with volume ~5x10³ Gpc³.

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Blobrana


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Posts: 131433
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RE: Universe is Bigger
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Blobrana


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Posts: 131433
Date:
Universe’s expansion
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An international team of astronomers led by Fritz Benedict and Barbara McArthur of The University of Texas at Austin has used Hubble Space Telescope to solve one of biggest problems in measuring the universes expansion. The results of their in-depth studies of Cepheid variable stars with HST is published in the April issue of the Astronomical Journal.

We took a classic approach to measuring cosmic distances, made significant improvements, and carried out a successful test. The result is a new, improved distance measuring tool -  Fritz Benedict.

The universes rate of expansion, the Hubble constant, has been hotly debated for decades. To calculate it, astronomers must be able to measure precise distances to galaxies billions of light-years away. That capacity, in turn, is built on a series of measurement techniques in the so-called cosmic distance ladder each of which allow astronomers to measure distances a little farther out into the universe.
One rung in the distance ladder is called a Cepheid variable star. Nearly 100 years ago, astronomers noticed that the light output from intrinsically brighter Cepheids varied more slowly than that from intrinsically fainter Cepheids. But that period-luminosity relationship was not known exactly. Benedicts team set out to precisely determine this relationship for Cepheids in our own galaxy.
To accomplish the calibration, they directly measured the distance to 10 Milky Way Cepheids. They followed these stars for two years, measuring their apparent motion on the sky, called parallax.

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Blobrana


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Posts: 131433
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RE: Universe is Bigger
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Title: How Many Universes Do There Need To Be?
Authors: Douglas Scott, J.P. Zibin
(revised v2)

In the simplest cosmological models consistent with General Relativity, the total volume of the Universe is either finite or infinite, depending on whether or not the spatial curvature is positive. Current data suggest that the curvature is very close to flat, implying that one can place a lower limit on the total volume. In a Universe of finite age, the "particle horizon" defines the patch of the Universe which is observable to us. Based on today's best-fit cosmological parameters it is possible to constrain the number of observable Universe sized patches, N_U. Specifically, using the new WMAP data, we can say that there are at least 21 patches out there the same volume as ours, at 95% confidence. Moreover, even if the precision of our cosmological measurements continues to increase, density perturbations at the particle horizon size limit us to never knowing that there are more than about 10^5 patches out there.

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Blobrana


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Title: Gravitational Lens Time Delays: A Statistical Assessment of Lens Model Dependences and Implications for the Global Hubble Constant
Authors: Masamune Oguri (KIPAC, Stanford)

Time delays between lensed multiple images have been known to provide an interesting probe of the Hubble constant, but such application is often limited by degeneracies with the shape of lens potentials. We propose a new statistical approach to examine the dependence of time delays on the complexity of lens potentials, such as higher-order perturbations, non-isothermality, and substructures. Specifically, we introduce a reduced time delay and explore its behaviour as a function of the image configuration that is characterized by the asymmetry and opening angle of the image pair. In particular we derive a realistic conditional probability distribution for a given image configuration. We find that the probability distribution is sensitive to the image configuration such that more symmetric and/or smaller opening angle image pairs are more easily affected by perturbations on the primary lens potential. On average time delays of double lenses are less scattered than those of quadruple lenses. Furthermore, the realistic conditional distribution allows a new statistical method to constrain the Hubble constant from observed time delays. We find that 15 published time delay quasars constrain the Hubble constant to be H_0=70 ±3 km/s/Mpc. While systematic errors coming from the heterogeneous nature of the quasar sample and the uncertainty of the input distribution of lens potentials should be considered, reasonable agreement with other estimates indicates the usefulness of our new approach as a cosmological and astrophysical probe, particularly in the era of large-scale synoptic surveys.

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Blobrana


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Posts: 131433
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The Hubble Constant
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Title: A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble Constant
Authors: L. M. Macri, K. Z. Stanek, D. Bersier, L. Greenhill, M. Reid

Researchers present initial results from a time-series BVI survey of two fields in NGC 4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of ~3%. The goal of the HST observations is to provide an absolute calibration of the Cepheid Distance Scale and to measure its dependence on chemical abundance (the so-called "metallicity effect").
The researchers carried out observations of two fields at different galactocentric distances with a mean abundance difference of 0.5 dex. They discovered a total of 281 Cepheids with periods ranging from 4 to 45 days (the duration of their observing window). The researchers determine a Cepheid distance modulus for NGC 4258 (relative to the LMC) of 10.88 ± 0.04 (random) ± 0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies \mu (LMC)=18.41 ± 0.10 (r) ± 0.13 (s) mag or D(LMC)= 48.1 ± 2.3 (r) ± 2.9 (s) kpc. We measure a metallicity effect of \gamma=-0.29 ± 0.09 (r) ± 0.05 (s) mag/dex. They see no evidence for a variation in the slope of the Period-Luminosity relation as a function of abundance. The researchers estimate a Hubble Constant of H_0= 74 ± 3 (r) ± 6 (s) km/s Mpc using a recent sample of 4 well-observed type Ia SNe and our new calibration of the Cepheid Distance Scale. It may soon be possible to measure the value of H_0 with a total uncertainty of 5\%, with consequent improvement in the determination of the equation of state of dark energy.

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