Archive for the ‘News items’ Category

New distance estimate to the Pleiades

There has been a debate about the distance to the Pleiades cluster that has been going for decades. Although it appears to have been resolved, well this is the claim of a team of radio astronomers in the US, who concluded that the cluster is approximately as far away as originally estimated. This contradicts analyses made form data from the Hipparcos, which suggested that the cluster is 13 parsecs closer than astronomical models predict.

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The Pleiades is the star cluster most obvious to the naked eye in the night sky, and has been known since antiquity. In modern astronomy. It often appears low in the horizon in the Southern hemisphere. The distance to the Pleiades is used to calibrate the cosmic-distance ladder, allowing astronomers to calculate distances to other star clusters and galaxies that are further away. And so, it is important to know the distance to Pleiades precisely, and multiple calculations of it have been made using various methods by various researchers over the decades.

Currently the generally accepted distance was about 134 parsecs (about 437 light-years). Yet in 1999, Floor van Leeuwen of the Institute of Astronomy in Cambridge, UK, used data from the European Space Agency’s Hipparcos satellite to produce what was the most precise calculation to date. This estimate was obtained using trigonometric parallax. Van Leeuven arrived at a distance of about 120 parsecs, and in 2009 he refined his analysis but reached a similar conclusion.

In new research by Carl Melis of the University of California, San Diego and colleagues at several other US institutions, they did their own trigonometric-parallax measurement of five selected stars in the Pleiades cluster using very long baseline radio interferometry. In this technique, measurements are made by linked radio antennas spread across the world, giving the total resolution of a telescope the size of the Earth. The researchers found that the distances of all five stars were in broad agreement with the original figure, with the lowest value being 134.8 parsecs and the highest being 138.4.

However in 2013 ESA launched Gaia, a successor to Hipparcos with much higher specifications, such as higher-sensitivity cameras, that will measure the parallaxes of thousands of stars in the Pleiades cluster. The design principles are conceptually similar, which leads Melis and colleagues to suggest that the unidentified error they believe distorted the Hipparcos measurements of the Pleiades could also affect Gaia. Nevertheless, Melis suspects that “the Gaia measurement is not going to be the same as the Hipparcos measurement. Hopefully then the Hipparcos community is going to have to face the fact that Hipparcos did not produce the correct result.”

See the paper at Science: http://www.sciencemag.org/content/345/6200/1029

Big Bang nucleosynthesis not the origin of lithium-6

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HZDR’s Michael Anders at the LUNA accelerator in Gran Sasso, Italy. (Courtesy: HZDR/M Anders).

 

The only three elements created in the early universe, long before stars and galaxies began to form, were hydrogen, helium and lithium. According to Big Bang nucleosynthesis (BBN) theory, protons and neutrons combined to form these three elements just a few minutes after the Big Bang. While the theory does a good job of predicting the observed abundances of hydrogen and helium isotopes in the universe, it fails miserably when it comes to the two stable lithium isotopes: lithium-6 and lithium-7.

As far as lithium-7 is concerned, numerous observations suggest that there is much less of it in the universe than predicted with BBN, with the theory that underlies the prediction having been confirmed in 2006 by experiments done at LUNA by Daniel Bemmerer of Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany and colleagues. Now, Bemmerer and an international team of physicists have turned their attention to lithium-6, which accounts for about 7% of the lithium here on Earth.

Collisions between hydrogen and helium nuclei deep under a mountain in Italy have confirmed a mystery of cosmic proportions: why the amount of lithium-6 observed in today’s universe is so different from the amount that theory predicts was produced shortly after the Big Bang. Working at the Laboratory for Underground Nuclear Astrophysics (LUNA) at Gran Sasso, an international team of researchers has measured for the first time how fast lithium-6 is produced under conditions similar to those when the universe was a few minutes old. The measured rate suggests that almost all lithium-6 was actually produced well after the Big Bang – something that current theories of nucleosynthesis cannot explain.

The production of lithium-6 by BBN should be dominated by one nuclear reaction, namely the collision and subsequent fusion of deuterium (hydrogen-2) with helium-4 to create lithium-6 and a gamma ray. Bemmerer and colleagues have now used the 400 kV accelerator at LUNA to study this interaction at two collision energies that would have occurred in the early universe. They did this by firing an intense beam of helium-4 nuclei at a target of deuterium gas and monitoring the collisions for the gamma rays associated with the production of lithium-6. The probability that this specific fusion process occurs is very low, and so an important experimental challenge for the physicists was to see the weak gamma-ray signal among all the other radiation produced by the collisions, as well as background signals from naturally occurring radioactive materials and cosmic rays. By going deep underground, LUNA’s researchers were able to reduce the cosmic-ray background, while the effect of naturally occurring radon gas was minimized by flushing the experimental area with nitrogen gas.

After carefully analysing the gamma-ray spectra acquired during two experimental runs, the team calculated the rate at which lithium-6 is produced by fusion – finding it to be more or less as was expected. The team then used BBN to calculate the ratio of lithium-6 to lithium-7 that should have been present in the early universe. The result is of the same order of magnitude as previously calculated, albeit a bit smaller, which makes the observation of high levels of lithium-6 in metal-poor stars even more mysterious.

As for the origin of most of the lithium-6 in the universe, this latest measurement reinforces the argument that it could not have been forged in the early universe. One possibility is that the isotope is produced in stellar flares. A much more radical suggestion is that the excess of lithium-6 was created by hitherto unknown physical processes, making cosmic measurements of the isotope a potential probe of physics beyond the Standard Model of particle physics.

The research is reported in Physical Review Letters. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.042501

The LHC found a boson – but was the Tevatron was there first?

It was a race to find the Higgs Boson – a scientific quest that has been years in the making.

July 2 2012: The Tevatron:

The scientists at the U.S. Department of Energy’s Tevatron collider revealed their latest results on July 2nd, two days before the highly anticipated announcement of the latest Higgs results from CERN and the Large Hadron Collider in Europe. The Tevatron scientists spent 10 years gathering and analyzing data from the CDF and DZero collaborations have and announced that they’d found their strongest indication to date for the much sought after, Higgs boson particle. The team collated the last bit of information out of 500 trillion collisions produced by the Tevatron since March 2001, yet the final analysis of the data did not categorically answer the question of whether the Higgs boson exists, but it stood on the scientific precipice with a fuzzy haze of a result lying in the misty fog of data.

“We achieved a critical step in the search for the Higgs boson,” said Dmitri Denisov, DZero cospokesperson and physicist at Fermilab. “While 5-sigma significance is required for a discovery, it seems unlikely that the Tevatron collisions mimicked a Higgs signal. Nobody expected the Tevatron to get this far when it was built in the 1980s.”

The Tevatron was the world’s second largest proton-antiproton collider. Residing at Fermilab, the Tevatron accelerated and stored beams of protons and antiprotons traveling in opposite directions around an underground ring four miles in circumference at almost the speed of light before colliding them at the centre of two detectors. However, due to funding costs in the US it was shut down on Sept. 29, 2011. And so all data was produced prior to that shut down.

The Tevatron data indicated that the Higgs boson, if exists, should possess a mass between 115 and 135 GeV/c2 ~ 130 times the mass of the proton.

July 4 2012: The LHC:

A few reports were leaked by CERN employees, anomalously of course, states that a discovery announcement was imminent. Then on the 4th July 2012 physicists announced that they have seen a clear signal of a Higgs boson — a key part of the mechanism that gives all particles their masses.

Two independent experiments reported their results, showing convincing evidence of a new boson particle weighing around 125 gGeV/c2, which so far is the best fit for the predictions of the Higgs previously made by theoretical physicists.

Direct at CERN, Rolf Heuer, was reported saying, “we have a discovery,” of a new subatomic particle, a boson, that is “consistent with a Higgs boson.”

The ATLAS experiment has seen a new type of boson decaying into four electrons – a good indicator that it is a Higgs particle.

Boson to be sure but is it the Higgs:

Clearly the result is a boson but the energy-mass level is different than initially predicted – so it’s possible it may be something more exotic – maybe even Dark Matter. In any case it is a huge break through and a great change in about to come to world of particle physics.

SKA Split decision – the best solution!

The selection of the best site for the Square Kilometre Array (SKA), which was between South Africa and a joint Australian/New Zealand bid, surprised most people. But it makes real sense to utilize the preliminary facilities that both nations have started to contribute.

In an earlier article on RelativeCosmos.com I suggested Australia would be best but in light of the split decision, it is a welcome selection!

Australia will launch phase 1 of the plan and get started sooner and South Africa will come in a little later with phase 2. It really is a workable solution and it is coup for both countries’ astronomy communities – many details are yet to be finalised but it’s one of those great times where a turning point makes for great history.

To quote Nature, “But splitting the site does have potential benefits. For one thing, the redundancy created by international collaboration can come in handy. The ISS, for example, continues to be serviced by a slew of vehicles from its different partners, even though the US space shuttle no longer flies there. There is also a perverse financial advantage — multiple partners are less likely to cancel an over-budget project than is a single government. But the greatest benefit is human: a more complex project draws in more people from more places and gives them an opportunity to participate.”

Read more on:  Nature  485, 548 (31 May 2012) doi:10.1038/485548a

This is my new Blog

Welcome to my new Blog – simply a repository for current papers I’ve read really for my purposes of keeping track of all my favourite articles and thinking about Astrophysics: Astronomy and Cosmology.

Others are welcome to use this as a resource and I hope you find it helpful. If you have any comments – please feel free to register and comment on any post.

Cosmology is getting a rough time at the moment, and theoretical physics seems to be somewhat diverse, I’d rather not say lost, and this Blog is my personal attempt to get a lot of relevant information together that I believes is worth looking at more closely.

Who am I? My name is Estelle and I’m an innocent bystander, an enquirer, a visionary, and a futurist. I guess I’m all those things, but more than anything I have a passionate interest in the Universe and what the world is really like.

Additionally, I am a student at the University of Central Lancashire, UK, studying towards a Bachelor of Astronomy with Honours in Relativity. I have a real passion for anything related to Cosmology, Astronomy, and Relativity. I love reading Einstein’s work and feel at home with a telescope – yet my real passion is theoretical.
This is my blog and it is simply a means of keeping all that I read in one place, in the science arena, for future retrospective study and as a publishing aid.