Archive for the ‘Stars in their eyes’ Category

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Why gamma ray bursts are not standard candles

Thursday, June 19th, 2008

GRB noncandles

A revolution is currently underway in our knowledge of gamma ray bursts thanks to NASA’s Swift telescope which has been looking out for them from its perch in orbit since 2004.

But in the wild enthusiasm to embrace the firehose of data that Swift is sending back, it looks as if astronomers have made a basic mistake in their analysis, says Li-Xin Li, a theorist at the Max-Plank Institute for Astrophysics in Garching, Germany.

Gamma ray bursts emit a huge amount of energy in a very short period of time. One of the peculiar discoveries in the Swift data is that the total energy emitted in a burst has a very narrow distribution of around 10^44 Joules.

A number of astrophysicists say this is evidence that GRBs are standard candles. In other words, that GRBs all emit the same amount of energy so measuring their brightness gives you a good idea of their distance.

Cosmological distances are notoriously tricky to measure so finding a new standard candle is a major discovery for astronomers.

Sadly, there is a mistake in this reasoning, say Li. He reckons that astronomers have failed to account for a serious problem in the GRB data known as Malmquist-type selection bias that is well known in other types of astronomical surveys.

The bias comes about because observers see only the brightest GRBs from the most distant parts of the universe because dimmer ones are undetectable at present. So the number of faint GRBs is significantly underestimated.

Re-examine the data with this in mind and the notion of GRBs as standard candles rapidly begins to look untenable.

Still, it was a nice idea while it lasted.

Ref: arxiv.org/abs/0806.2770: Are Gamma-Ray Bursts a Standard Energy Reservoir?

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The mystery of the Plutonic color scheme

Monday, May 26th, 2008

Pluto

Pluto’s three satellites, Hydra, Nix and Charon, are all a similar shade of grey. In fact, Nix and Hydra have exactly the same colour to within our ability to measure it. Pluto, on the other hand, is a beautiful shade of red. How come?

The current thinking is that Charon, Hydra and Nix are a similar colour because they were all formed in the giant impact that created this satellite system.

But today, Alan Stern, former head of NASA’s Planetary Science’s division and principal investigator for the New Horizons mission to Pluto, puts forward an alternative hypothesis.

His idea is that the impact of debris from the Kuiper belt on these bodies could send enough surface material into orbit to coat the satellites nearby. Interesting idea.

Stern calculates that the ejecta velocities on Pluto and Charon would be too low to escape. However, the ejecta from Nix and Hydra could easily escape in enough quantity to cover one another to a depth of tens of metres and to cover Pluto and Charon to a depth of tens of centimetres.

The weather on Pluto generates regular frosts which cover this up as qucikly as it was laid down but no such mechanism operates on the other satellites.

So that might explain the differences and similarities in the Plutonic color scheme. Stern also predicts that if he is right, the colours and albedos of Nix, Hydra and Charon should change slowly as more material is ejected and deposited. So by keeping a sharp eye on them, he can gain further evidence for his theory.

What’s more, he says that this mechanism may be common in the solar sytem wherever small binary systems are found, such as in the asteroid and Kuiper belts. And where this happens, these bodies should have similar colours too.

All we have to do now is to look out for the flurry of papers pointing to evidence that he’s right.
Ref: arxiv.org/abs/0805.3482: Ejecta Exchange, Color Evolution in the Pluto System, and Implications for KBOs and Asteroids with Satellites

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Dark energy linked to supervoids and superclusters

Thursday, May 22nd, 2008

Dark energy and superclusters

“The most profound puzzle of contemporary physics” is how Benjamin Granett and colleagues from the University of Hawaii in Honolulu describe the problem of dark energy. And they’re not kidding.

What we have is the extraordinary observation that type 1a supernovas in the most distant galaxies in the universe are dimmer than they ought to be.

Nobody disputes this evidence; the challenge is to explain it and astronomers have been falling over themselves to construct various fascinating theories.

The astronomers’ darling is that the cosmos is filled with a mysterious “dark energy” that is pushing the universe apart. This causes the most distant galaxies to accelerate away from us faster than they would otherwise do. And since they are further away, the supernovas they contain are dimmer.

Recently, astronomers have suggested that if that is the case, then there ought to be other evidence for dark matter too. In particular, they say that this acceleration should change the way photons are influenced by the gravitational fields associated with superclusters of galaxies and the supervoids between them.

Photons should be “heated” and cooled” as they pass through the crests and troughs of these structures (in other words their energy should depend in a small way on the journey they’ve taken).

So a map of the universe according to photon temperature ought to coincide with the large scale structure of superclusters and supervoids in the universe we see (a phenomena known as the late-time integrated Sachs-Wolfe effect).

It turns out we already have just such a map in the form of the cosmic microwave background data taken by the WMAP spacecraft. But until now the variations of this map have only been weakly linked to the superclusters and supervoids we can see.

Today, Granett and pals have taken this work a step further and presented the first strong statistical link between the structure of the universe as we see it today and photon temperature.

What’s more they say their findings may help to explain a mysterious cold spot in this map that astronomers have been puzzling over for a while now. The cold spot is the result of supervoids they say.

This is interesting work but the question of course is: how robust is their statistical analysis? These kinds of findings are notoriously sensitive to the way in which the data is selected.

Now we see it. But next week, who knows?

Ref: arxiv.org/abs/0805.2974: Dark Energy Detected with Supervoids and Superclusters

Posted in Stars in their eyes | 3 Comments »

How ESA plans to search for other Earth’s

Thursday, May 15th, 2008

Darwin

We’re getting close to the day when we’ll spot an Earth-like planet orbiting another star. Astronomers have already seen a number of superEarth candidates–rocky planets in the habitable zone that are many times larger than Earth. They’ve even begun to analyse the atmosphere of these places and got some idea of what it might be like on their surfaces. Earth-sized planets won’t be far away now.

But if we are to spot the signs of life on these bodies, what should we look for? The European Space Agency has been giving this some serious thought for a mission called Darwin currently pencilled in for launch in 2015. It’s goal is to look for signs of life on Earth-like planets.

Today, the team behind the mission explain some of the reasoning behind their design for the spacecraft. To look for life, they’ve had to make some important assumptions about the form it might take. For example, they’re plumping for carbon-based life forms that rely on water as a solvent. Fair enough but their assumptions go a lot further:

“We assume that extraterrestrial life is similar to life on Earth in its use of the same input and output gases, that it exists out of thermodynamic equilibrium, and that it has analogs to microorganisms on Earth.”

That’s getting pretty specific but they say their hand is forced by the fact that they’ve never seen any other type of life and so can’t possibly know what else to look for.

So Darwin will look for carbon dioxide, ozone and of course water in the atmospheres of these planets as well as methane and ammonia.

Finding those in the right abundances will be good evidence that something interesting is happening on these planets although finding any other gases that are out of geochemical equilibrium will also be an eye-opener.

The trouble is that finding these signatures will by no means be a slam dunk in favour of life.

In one of the classic scientific papers of the 20the century, Carl Sagan and colleagues published their analysis of the data from the Galileo spacecraft’s 1990 flyby of Earth. The spacecraft saw all those gases and more. Their conclusion? “Our results are consistent with the hypothesis that widespread biological activity exists…on Earth”.

Not quite conclusive and that’s from a distance of a thousand kilometres. So it’s hard to imagine that data from Darwin could provide conclusive evidence of life at a distance measured in dozens or hundreds of lightyears. But I guess that’s nothing a good PR team couldn’t solve.

Ref: arxiv.org/abs/0805.1873: Darwin – A Mission to Detect, and Search for Life on, Extrasolar Planets

Posted in Slimey stuff, Stars in their eyes | 3 Comments »

Blind date gives astronomers a new love of the stars

Thursday, May 8th, 2008

Star date

When it comes to studying the night sky, astronomers aren’t short of images. There are huge archives of both amateur and professional images taken in the the age before digital imaging. The Harvard College Observatory Astronomical Plate Stacks contain enough images to cover the entire sky 500 times over.

But although the image quality is excellent, the problem is the indexing. When logs go missing or when data has been badly transcribed, it can be almost impossible to work out exactly what appears in an image or when it was taken.

The error rate in many older collections is high enough to make astronomers think twice about using them. And as astronomy moves towards its goal of creating a Virtual Observatory, in which all images are available online in a kind of giant virtual planetarium, a lack of trust in the data is a serious problem.

If, as an astronomer, you’ve been losing sleep over this issue, you can rest easy. David Hogg at New York University and buddies (including the search giant Google), have solved the problem by reducing it to one of image matching.

They take an astrophotograph of dubious provenance and use a computer program called “Blind Date” to look for asterisms (the shapes that constellations make). When they find a match, this immediately locates the image within a part of the sky. But the really cool part of their technique is based on the fact that stars move over time, albeit by tiny amounts. So any small deviation in the location of stars within an image give an immediate time stamp for when the shot was taken.

The team says this technique works for every science-quality image that it has been tested against and 85 per cent of lower quality images. In some cases, it can date images to within a few months.

The plan is to use Blind Date to produce metadata automatically for every image that is entered into the Virtual Observatory, which should reduce errors substantially and also prevent deliberate spoofing of the project.

That should go a long way to restoring trust.
Ref: arxiv.org/abs/0805.0759: Blind Date: Using Proper Motions to Determine the Ages of Historical Images

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First evidence that water forms in interstellar space

Monday, May 5th, 2008

Star juice

Water is the most abundant solid material in space. Astronomers see it on various planets, on moons, in comets and in interstellar clouds. But how did it get there? Nobody really knows how water could possibly form in the freezing darkness of interstellar space.

At least they didn’t until now. Today, Akira Kouchi and buddies at the Institute of Low Temperature Science at Hokkaido University in Japan say that have created water for the first in conditions similar to those found in interstellar space.

Water forms quite easily when oxygen and atomic hydrogen meet. The problem is that there is not enough of it floating around as gas in interstellar dust clouds. So instead, the thinking is that water must form when atomic hydrogen interacts with frozen solid oxygen on the surface of dust grains in these clouds.

Kouchi and co recreated this process by creating a layer of solid oxygen on an aluminum substrate at 10K and then bombarding it with hydrogen. Sure enough, infrared spectroscopy confirmed the presence of water and hydrogen peroxide, and in the right quantities to explain the abundance of water seen in interstellar clouds.

That’s cool and in more ways than one. All the water in the solar system–in comets, on Mars and in the oceans on Earth–must have formed in exactly this way in the interstellar dustcloud which pre-dated Sol and the planets.

So that’s not just any old water you’re sipping, that’s interstellar star juice.

Ref: arxiv.org/abs/0805.0055: Formation of Hydrogen Peroxide and Water from the Reaction of Cold Hydrogen Atoms with Solid Oxygen at 10 K

Posted in Cold 'n' cool, Stars in their eyes | 11 Comments »

First superheavy element found in nature

Monday, April 28th, 2008

unbibium

The hunt for superheavy elements has focused banging various heavy nuclei together and hoping they’ll stick. In this way, physicists have extended the periodic table by manufacturing elements 111, 112, 114, 116 and 118, albeit for vanishingly small instants. Although none of these elements is particularly long lived, they don’t have progressively shorter lives and this is taken as evidence that islands of nuclear stability exist out there and that someday we’ll find stable superheavy elements.

But if these superheavy nuclei are stable, why don’t we find them already on Earth? Turns out we do; they’ve been here all along. The news today is that a group led by Amnon Marinov at the Hebrew University of Jerusalem has found the first naturally occuring superheavy nuclei by sifting through a large pile of the heavy metal thorium.

What they did was fire one thorium nucleus after another through a mass spectrometer to see how heavy each was. Thorium has an atomic number of 90 and occurs mainly in two isotopes with atomic weights of 230 and 232. All these showed up in the measurements along with a various molecular oxides and hydrides that form for technical reasons.

But something else showed up too. An element with a weight of 292 and an atomic number of around 122. That’s an extraordinary claim and quite rightly the team has been diligent in attempting to exclude alternative explanations such as th epresence of exotic molecules formed from impurities in the thorium sample or from the hydrocarbon in oil used in the vacuum pumping equipment). But these have all been ruled out, say Marinov and his buddies.

What they’re left with is the discovery of the first superheavy element, probably number 122.

What do we know about 122? Marinov and co say it has a half life in excess of 100 million years and occurs with an abundance of between 1 and 10 x10^-12, relative to thorium, which is a fairly common element (about as abundant as lead).

Theorists have mapped out the superheavy periodic table and 122 would be a member of the superheavy actinide group. It even has a name: eka-thorium or unbibium. Welcome to our world!

This may well open the flood gates to other similar discoveries. Uranium is the obvious next place to look for superheavy actinides. I’d bet good money that Marinov and his pals are eyeballing the stuff as I write.

Ref: arxiv.org/abs/0804.3869: Evidence for a Long-lived superheavy Nucleus with Atomic Mass Number A = 292 and Atomic Number Z @ 122 in Natural Th

Posted in Changin' the world, Stars in their eyes | 67 Comments »

ET more likely to pick up radar bursts than radio transmissions

Monday, April 21st, 2008

Radio astronomy

Radar astronomy is a crucial tool in measuring the trajectories of Earth-crossing asteroids. If we’re going to be hit, radar is how we’ll work out when. The technique has also been used to image various bodies such as the asteroid 216 Kleopatra, to measure distances with extreme accuracy and to test relativity by monitoring the orbit of Mercury.

The technique was first used in the 1960s and since then astronomers have broadcast some 1400 radar transmissions towards the heavens, says Alexander Zaitsev at the Institute of Radioengineering and Electronics, Russian Academy of Sciences, in Fryazino near Moscow. These transmissions have lit up  about 2×10^-3 part of the sky.

By contrast, humanity has broadcast only 16 messages intended for extraterretrial consumption. And these have illuminated an area of sky some 2000 times less than the radar astronomy signals.

Since the radar signals were 500 times longer than the ET messages, Zaitsev says that ET is about 1 million times more likely to pick these up first. So first contact is likely to be a little more prosaic than most of us had anticipated.

The purpose of Zaitsev’s calculation is to take on the chicken little’s who say that we should keep our interstellar gobs shut lest some hostile super-civilisation decide to turn us all to astroslavery. He says these people should be more worried about radio astronomy than ET messages and since we can’t stop monitoring Earth-crossing asteroids, the party poopers might as well give up and go home.

It’s all irrelevant anyway. If ET was going to make herself known, she’d have done it by now.

Ref: arxiv.org/abs/0804.2754: Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization

Posted in At the seaside, Fightin', Stars in their eyes | 3 Comments »

Diamonds in the sky: a miner’s guide

Tuesday, April 15th, 2008

Diamonds in the sky

Astronomers have recently wondered whether carbon might form a supercooled liquid under the huge pressures that exist in side carbon-rich white dwarf stars and even inside medium-sized gaseous planets such as neptune and uranus. If that’s the case, then small disturbances in the liquid could trigger the formation of diamonds the size of automobiles.

The trouble is that nobody has been able to create these conditions on Earth so the way in which nucleation might occur is more or less unknown.

Now Daan Frenkel and pals from the University of Amsterdam in The Netherlands say that computer simulations of the behaviour of several thousand carbon atoms under these circumstances have given us the first inkling of how nucleation occur. And the odds are that it’s a tricky process to set in motion.

Applying these results to astrophysical bodies gives two  insights, both of which are bad news for diamond hunters.

First,  nucleation may be so rare that “not a single diamond could have nucleated in a Uranus-sized body during the life of the universe.”

And second, the appropriate conditions may be common in certain white dwarf stars.

Either way, these diamonds are outta bounds for the foreseeable future.

Ref: arxiv.org/abs/0804.1671: State-of-the-art models for the Phase Diagram of Carbon and Diamond Nucleation

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How Hawking radiation may explain dark energy

Tuesday, March 25th, 2008

Dark Hawking Energy

In 1993, the Dutch Nobel prize-winning physicist Gerard t’Hooft suggested that all the information in a region of space can be represented as a hologram, an idea that implies that the laws of physics that govern our universe are somehow encoded on its (higher dimensional) boundary.

This idea, known as the holographic principle, has a certain elegance and so has received widespread attention from some theorists although nobody knowns whether it is a true description of the universe or not.

If it is true, Jae-Weon Lee from the Korea Institute for Advanced Study in Seoul and some pals, say that this boundary should emit Hawking radiation.

Hawking originally dreamt up this radiation idea to describe a process that might occur near the event horizon of a black hole. When pairs of virtual particles pop into existence (as they do all over the universe), they normally annihilate each other and disappear again. But near a black hole, one of these particles can cross the event horizon while the other makes its escape and this gives the impression that the black hole is emiting radiation.

Lee’s team say a similar thing may happen at the holographic boundary and that the energy this creates might be responsible for making the expansion of the universe accelerate.

They also explain why this radiation does not interact with ordinary matter and so is not seen in other ways: it’s wavelength, being universe-sized, is too long.

Seems as good an explanation as any other at this stage, after all the competition is ideas like quintessence, k-essence and quintoms.

Two things though: if this radiation exerts a force, why would it act to accelerate the expansion of the universe and not decelerate it? Lee and co are not convincing on this point. And, I wonder whether t’Hooft, who has some exotic ideas of his own about quantum determinism, would say that this kind of cosmic Hawking radiation is not compatible with the holographic principle and therefore bunkum.

Ref: arxiv.org/abs/0803.1987: Is Dark Energy from Cosmic Hawking radiation?

Posted in At the seaside, Hellraisin', Stars in their eyes | 6 Comments »

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