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11月14日

Gravity ripples chased

Discovering gravitational waves would not only validate Einstein's theory of gravitation but also reveal aspects of the Universe's earliest moments.

The hunt for these elusive ripples is now well under way.


For several decads, physicists have been racing to detect gravitational waves ― tiny disturbances in space-time that propagate at the seed of light.

But the experiments are challenging.

They require detection of minute changes ― a mere fraction of the size of an atomic nucleus ― in kiloletre-scale separations between free-floating masses.

Although gravitational waves have yet to be detected, on page 990 of this issue the LIGO Scientific Collaboration and the Virgo Collaboration report a milestone in this quest.

The authors imorove on a long-standing upper limit to the number of gravitational waves generated in the early Univerce.



### 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 23:44| Comment(0) | astrophysics | このブログの読者になる | 更新情報をチェックする

2009年11月05日

Quantum theory in context

Quantum mechanics has had notable success in the almost 90 years since it was introduced, and its predictions have been confrmed in many experimebts.

Nevertheless, many physicists not content with the axioms of the theory have been searching for an explanation of quantum physical predictions in terms of a classical theory.

An intuitive feature of classical models in non-contextuality : the property that any measurement has a value independent of other compatible measurements being carried out at the same time.

Theory suggests that non-contextuality is in conflict with quantum mechanics, and experiments undertaken with photons and neutrons seem to support this.

However, these tests required the generation of special quantum states and left some loopholes open.

Now Kirchmair et al. have oerformed an exoeriment with trapped ions that overcomes these problems and cannot be explained in non-contextual terms.

Contextuality is therefore a property of nature that does not require the generation of special quantum states or quantum entanglement.

### DataBace ###
nature Vol.460 429-544 Issue no.7254 23 July 2009
Letter p.494
News & Views p.434
posted by 0≠素子 at 18:49| Comment(0) | quantum mechanics | 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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