2009年11月19日

An upper limit on the stochastic gravitational-wave background of cosmological origin

A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin.

It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standerd astrophysical observations.

Direct measurements of the amplitude of this background are therefore of fun damenral importance for understanding the evolution of the Universe when it was yonger than one minute.

Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a tow-year scibatory (LIGO).

Our result constrains the energy density of the stochastic gravitationa-wave background normalized by the critical energy dendity of the Universe, in the frequency band around 100 Hz, to be < 6.9*10^-6 at 95% confidence.

The data rule out models of early Universe evolution with relatively large equation-of-state parameter, as well as well as cosmic (super) string models with relatively small string tension that are favoured in some string theory models.

This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis and cosmic microwave background at 100 Hz.


### see dada / data bace ###
nature 460,925-1050 20 August 2009 Isse no 7258
Letter p.990 / An upper limit on the stochastic gravitational-wave background of cosmological origin / LIGO & Virgo
News and Views p.964 / Astrophysixs : Garavity ripples chased / Marc Kamionkowski
LIGO in the background

posted by 0≠素子 at 19:10| Comment(0) | cosmos | このブログの読者になる | 更新情報をチェックする

2009年02月27日

Galaxies made simple

At farst glance a galaxy looks pretty complex : each contains bilions of stars subject to many influences including gravity, rotation and radiation.

The hierarchical theory of galaxy formation - currently fashinoable - holds that they are assembled from smaller pieces, through many mergers of cold dark matter, and that the properties of individual galaxies are determinied by six independent parameters.

Now a survey of 200 individual galaxies suggests that things are simpler than that, and that galactic structure is controlled by a single parameter. Though frustratingly, it is not yet clear which parameter that is.

This lebel of organization is at odds with hierarchical galaxy formation and the predictions of cold dark matter cosmology.


### DataBace ###
nature Vol.455 1007-1148 Issue no.7216 23 October 2008
News and Vews p.1049/ ASTROPHYSICS : How do galaxies from? / Sidney van den
Letter p.1082 / Galaxies appear simpler than expected M J Disney et al. (cardiff University)

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posted by 0≠素子 at 23:10| Comment(0) | cosmos | このブログの読者になる | 更新情報をチェックする
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