Galaxies pin down dark energy

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Nov 25, 2010

A new way of measuring the geometry of the universe confirms that dark energy dominates the cosmos and bolsters the idea that this unusual form of energy is described by Einstein's cosmological constant. The technique, developed by physicists in France, involves a relatively easy measurement of the orientation of distant pairs of galaxies.

Over the past decade or so, several kinds of observation, such as measurements of the distances of remote supernovae, have provided strong evidence that the expansion of the universe is accelerating. Cosmologists believe that this expansion is being driven by what is known as dark energy – a substance with negative pressure that opposes the pull of gravity. Unfortunately, however, they have little idea of what dark energy actually is, having been unable to measure its properties well enough to distinguish between rival hypotheses.

The new approach, devised by Christian Marinoni and Adeline Buzzi of the University of Provence in Marseille, should help narrow down the options as well as provide another means of working out the geometry of space. It involves comparing the known shape of very distant objects with the shape of those objects as revealed by astronomers' observations. Astronomers don't measure distances, and hence shapes, directly, but instead measure the extent to which the wavelength of radiation from a distant object has increased – or redshifted. This tells them the speed at which the object and Earth are moving apart.

Hubble's law states that the speed at which objects within the universe move apart from one another is proportional to the distance between them, so knowing the speed of a distant object reveals how far away it is (although this is only approximately true at very great distances). But if the space between that object and the measurer has an unusual geometry or if the expansion of the universe is actually accelerating then the distance measured will not be accurate. So the idea is to vary the quantities that represent the geometry and the strength of dark energy until the distances of interest match up with expectations.

This principle was first proposed by the astronomers George Alcock and Bohdan Paczynski in 1979 but has been difficult to carry out in practice because the redshift due to the local motions of the objects themselves tends to mask that caused by the expansion of the universe. What Marinoni and Buzzi have done is to study a system for which the local motions can be filtered out in quite a straightforward way. They don't measure a shape as such but instead the orientation of pairs of galaxies several billion light years from Earth that are in orbit around one another in binary systems. They reason that such galaxy pairs should be randomly oriented and so a large set of these binary systems should have an even distribution of orientations. Any deviation from that even distribution would reveal the influence of spatial geometry and dark energy, once the local effects have been removed.

To compare their technique against real observations they measured the orientations of galaxy pairs using data from the DEEP2 galaxy redshift survey and then used more local data from the Sloan Digital Sky Survey to calibrate the motion of the galaxies themselves. Their analysis agreed with the standard cosmological model regarding both the geometry of the universe and the abundance of dark energy – confirming that the universe is flat, in other words that it follows the ordinary laws of Euclidean geometry, and that dark energy makes up around 70% of the energy-matter content of the universe.

They also calculated a value for the strength of dark energy that suggests this substance comes in the form of the cosmological constant – a term that Einstein added to (and then removed from) his equations of general relativity. If correct, this means that the repulsive force is constant throughout the evolution of the universe and that it is mathematically is equivalent to the quantum-mechanical energy of the vacuum.

If you keep the technique simple you can avoid biases. Cosmology is a science where systematic errors are just behind the door Christian Marinoni, University of Provence

Marinoni argues that their technique represents a valuable additional approach to understanding dark energy, since, he says, it is "simple, transparent and faithful". In particular, he says, it does not rest on any questionable physical assumptions. "If you keep the technique simple you can avoid biases," he says. "Cosmology is a science where systematic errors are just behind the door."

Alan Heavens of the University of Edinburgh, who wrote a commentary piece to accompany the paper, agrees that the new method is "nice and direct". But he warns that it does contain an assumption that must be tested – that the orbital properties of local galaxy pairs are equal to those of galaxies from 7 billion years ago, when the light left the objects catalogued in the DEEP2 survey.

The research is described in Nature 468 539.

Edwin Cartlidge is a science writer based in Rome

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Do giant spiral galaxies thwart clusters of young stars?

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Nov 2, 2010

Astronomers in Scotland and Germany say simple physics may explain a long-standing paradox: why large clusters of young stars tend to reside in relatively small galaxies and not in giants like the Milky Way. The reason, according to the astronomers, is that giant spiral galaxies, like the Milky Way, spin fast, shearing star clusters before they grow into monsters.

The stunning 30 Doradus complex is the most luminous nursery of young stars in the Local Group – a collection of several dozen nearby galaxies that includes the Milky Way. It stands to reason, therefore, that 30 Doradus would inhabit an equally impressive galaxy, either Andromeda or the Milky Way, the two largest galaxies in the Local Group.

But instead the stunning 30 Doradus complex lies in the Large Magellanic Cloud, a satellite galaxy of our own that emits only one tenth as much light. The newborn stars of 30 Doradus have set gas aglow over an area 700 light-years wide, 30 times the diameter of the well known Orion nebula.

Now Carsten Weidner and Ian Bonnell of the University of St Andrews in Fife and Hans Zinnecker of the Astrophysical Institute of Potsdam have conducted computer simulations that model interstellar clouds of molecular gas which collapse to form star clusters. Says Weidner, "It seems that rotation inhibits the formation of very massive star clusters."

Giant spiral galaxies spin fast. For example, the Milky Way rotates at about 230 kilometres per second, and the even larger Andromeda galaxy spins faster still. By contrast, smaller galaxies, such as the Large Magellanic Cloud, rotate slowly.

Weidner's team ran four computer simulations, each with a different spin speed. "Each model took about a month to compute," Weidner says. In the fast-spinning models, stars and clusters formed over a wide area, because the spin prevented the gas from collapsing into one gigantic cluster. By contrast, in the slowest-spinning model, the gas collapsed and gave birth to a single huge star cluster at the centre. That model might explain why the huge 30 Doradus complex arose in a galaxy much smaller than our own.

This work also applies to colliding galaxies. Says Weidner, "In the collision region, you have less rotational support, so you would also expect more massive clusters." In fact, the famous Antennae galaxies – two large spiral galaxies that are smashing together in the constellation Corvus – have created young star clusters far greater than any young clusters in either the Milky Way or Andromeda.

Bruce Elmegreen, a star-formation expert at the IBM Research Division in Yorktown Heights, New York, says the study is interesting, but he's sceptical of the result. "The connection between galaxy spin and molecular cloud spin is vague," he says. "Does galaxy spin correlate with the spin of molecular clouds? I'm not aware of an answer to that." Weidner responds that fast-spinning galaxies should indeed have faster-spinning clouds, because the clouds interact with one another.

Elmegreen also says that 30 Doradus may owe its great size to factors other than its home galaxy's slow rotation. The Large Magellanic Cloud – which is only 160,000 light-years from Earth – is plowing through the Milky Way's halo. Gas in the halo compresses gas in the Large Magellanic Cloud, a process called "ram pressure" that may have sparked the star formation in 30 Doradus.

Weidner acknowledges that ram pressure may have played a role. "30 Doradus is a complex object," he says, "and we do not claim that we can explain every detail of it. We just say there might be a trend with rotation."

Weidner and his colleagues will publish their work in The Astrophysical Journal and a preprint (arXiv: 1009.1618) is available.

Ken Croswell is the author of eight astronomy books, including Magnificent Universe.

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