Thursday, October 23, 2014

Is Dark Matter Coming From The Sun?

Is Dark Matter Coming From The Sun?:

A huge filament erupts from the Sun in 2012. Credit: NASA Goddard Space Flight Center


A huge filament erupts from the Sun in 2012. Credit: NASA Goddard Space Flight Center
For decades, astronomers and cosmologists have postulated that the Universe is filled with an invisible, mysterious mass known as “dark matter.” For decades, the search for this elusive matter has dominated the field of cosmology. Precise measurements were obtained over 20 years ago when dark matter was first mapped in galaxy halos. Only recently has the existence of dark matter over much larger scales than even galaxy clusters been detected.

Recently, a group of physicists analyzed over 12 years’ worth of telescope data, and have found a signal that some think could be the first detection of a source of dark matter.

And it appears to be coming from … our Sun.

Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. The left panel displays the continuous distribution of dark matter particles, showing the typical wispy structure of the cosmic web, with a network of sheets and filaments, while the right panel highlights the dark matter halos representing the most efficient cosmic sites for the formation of star-bursting galaxies with a minimum dark matter halo mass of 300 billion times that of the Sun. Credit: VIRGO Consortium/Alexandre Amblard/ESA


Distribution of dark matter when the Universe was about 3 billion years old, obtained from a numerical simulation of galaxy formation. Credit: VIRGO Consortium/Alexandre Amblard/ESA
Using information obtained from the European Space Agency observatory, astronomers from the University of Leicester found variations in a stream of X-rays emanating from the Sun that match what would be expected if axions – a hypothetical dark-matter particle – were interacting with Earth’s magnetic field. If confirmed, the axion finding would be a huge discovery.

Axions were originally proposed to explain an anomaly in a different area of physics – the theory of the strong nuclear force, one of the four fundamental forces of nature. These uncharged, very light particles would be created in the Sun’s core and would barely interact with ordinary matter, which would allow them to zip through thousands of kilometers of solar plasma and escape into outer space.

But the axions that interacted with magnetic fields, such as the one that surrounds Earth, would be expected to turn into X-ray photons. Those photons are the particles the researchers say they may have seen.

The team found that as the European space telescope XMM-Newton (also called the X-ray Multi-Mirror Mission) passed through the strong magnetic field on the Sun-side of Earth, it saw a slightly more intense X-ray signal than when it was on the far side of Earth. Discounting known sources of X-rays, the background signal should be the same wherever the spacecraft is, according to the Leicester team.

In their 67-page paper, which was submitted in March of this year and appeared in this month’s issue of Monthly Notices of the Royal Astronomical Society, the researchers did their best to rule out more mundane phenomena – such as interaction between the solar wind and Earth’s magnetic field – before invoking axions as a source.

Axions interacting with Earth's magnetic field to form x-rays. Credit: University of Leicester


Axions interacting with Earth’s magnetic field to form x-rays, as detected by the ESA’s XMM-Newton probe. Credit: University of Leicester
One unusual aspect of this analysis is that it shows XMM-Newton picking up the X-ray photons, even though it is not looking straight into the Sun but at a right angle to it. This is odd because the photons are generally expected to continue in the same direction as the axions they were created from.

But the authors say that the axions could be scattered and end up in the telescope. The authors also show that hints of a similar signal can be found in data produced by NASA’s Chandra X-Ray Observatory, although a formal corroboration will take more data and years of analysis.

The concept of dark matter was first proposed by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way, and then again by Fritz Zwicky in 1933 to account for evidence of “missing mass” in the orbital velocities of galaxies in clusters.

Dark matter has been widely studied, but much like the Higgs Boson, its existence was inferred despite a lack of direct evidence simply because it accounted for discrepancies in the observable data. According to consensus among cosmologists, dark matter could be composed primarily of a not yet characterized type of subatomic particle.

The leader of this most recent study, George Fraser  – an astronomer at the University of Leicester, UK – died just two days after he and his co-authors submitted the paper for publication. According to Andy Lawrence, an astronomer at the Institute for Astronomy in Edinburgh, UK, the study was Fraser’s “most astonishing swan song”.

Mike Watson, another astronomer at the University of Leicester (but who was not involved in the study), says that Fraser was an “exceptional scientist” and the mastermind behind the work. Even so, he expressed some skepticism towards the findings.

“The interpretation is quite appealing,” he said, “and on the human side of this is that we would all like it to be right, as it would be a great tribute to George. But that’s not how you do science.”

In addition, Fraser’s research team are not yet celebrating the publication of their findings, as there appears to be some anomalies in the data that has even them concerned.

“We found an unusual result that we can’t explain by any conventional method, and this axion theory does explain it,” said the study’s co-author Andy Read. “But it is just a hypothesis, and most hypotheses don’t make it.”

Others within the astronomical community are also not convinced that the axion interpretation is correct. Astronomer Peter Coles of the University of Sussex, UK, called the evidence “circumstantial.” In a post on his blog, “In the Dark”, he wrote, “It’s tantalising, but if you want to ask me where I’d put my money I’m afraid I’d probably go for messy local plasma physics rather than anything more fundamental.”

Still, the theory has its share of potential supporters. One such person is Igor Garcia Irastorza, who works on the CERN Axion Solar Telescope (CAST), based at the CERN physics laboratory near Geneva, Switzerland. He expressed that the idea was intriguing, but the kind of axion that would fit such a signal would clash with other astrophysical observations. And, as he said, the particles’ properties would have to be different than what has been theorized for decades.

Corroborating the Leicester findings will take cross-checks from other axion experiments that work in completely different ways to the telescopes, adds Konstantin Zioutas, who leads the CAST experiment. Only time will tell if a source of dark matter has been found, or if this is merely a hiccup in the ongoing search.

Further Reading: MNRAS
University of Leicester press release.



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