Posts Tagged ‘Gravitational waves’

Cold, Dead Stars Could Help Limit Dark Matter

15 Oct

Hunting for cold stellar corpses near the center of the galaxy or in star clusters could put new limits on the properties of dark matter.

“You can exclude a big class of theories that the experiments cannot exclude just by observing the temperature of a neutron star,” said physicist Chris Kouvaris of the University of Southern Denmark, lead author of a paper in the Sept. 28 Physical Review D. “Maybe by observations, which come cheaper than expensive experiments, we might get some clues about dark matter.”

Dark matter is the irritatingly invisible stuff that makes up some 23 percent of the universe, but makes itself known only through its gravitational tug on ordinary matter.

There are several competing theories about what dark matter actually is, but one of the most widely pursued is a hypothetical weakly interacting massive particle (WIMP). Physicists in search of WIMPs have placed experimental detectors deep underground in mines and mountains, and are waiting for a dark matter particle to hit them.

Others have proposed looking for the buildup of dark matter in stars like the sun or white dwarfs. But both subterranean and stellar-detection strategies will light up only for WIMPs larger than a certain size. That size is miniscule — about a trillionth of a quadrillionth of a square centimeter — but dark matter particles could be smaller still.

One way to rule out such diminutive particles is to look to neutron stars, suggest Kouvaris and co-author Peter Tinyakov of the Université Libre de Bruxelles in Belgium.

Neutron stars are the cold, dense remnants of massive stars that died in fiery supernova explosions. They tend to have masses similar to the sun, but in diameter they would barely stretch from one end of Manhattan to the other. This extreme density makes neutron stars exceptionally good nets for dark matter.

“For their size and their temperature, they have the best efficiency in capturing WIMPs,” Kouvaris said. Particles up to 100 times smaller than the ones underground experiments are sensitive to could still make a noticeable difference to neutron stars.

After the fires of their birth, neutron stars slowly cool over millions of years as they radiate photons. But if WIMPs annihilate each other whenever they meet — like a particle of matter meeting a particle of antimatter — as some models suggest they should, dark matter could reheat these cold stars from the inside.

Kouvaris calculated the minimum temperature for a WIMP-burning neutron star, and found it to be about 100,000 kelvins [about 180,000 degrees Fahrenheit]. That’s more than 10 times hotter than the surface of the sun, but more than 100 times cooler than the sun’s fuel-burning interior. It’s also much cooler than any neutron star yet observed.

Dark matter and ordinary matter are thought to clump up in some of the same places, like the center of the galaxy or globular clusters of stars. So Kouvaris and Tinyakov suggest that astronomers try to find a neutron star colder than the minimum temperature in a region with a lot of dark matter floating around.

“If you observe a neutron star with a temp below the one we predict, that excludes a whole class of dark-matter candidates,” Kouvaris said. It could mean the WIMPs are extra-small, or that they don’t annihilate when they meet each other — a property of WIMPs that experiments can’t get at.

“It’s an intriguing idea,” said observational astronomer David Kaplan University of Wisconsin-Milwaukee. “But I’m a little skeptical that it can be done immediately, or even in the near future.”

The center of the galaxy is dusty and difficult to observe, and most globular clusters are so far away that a cold, tiny neutron star hiding inside them would be beyond today’s telescopes. The next generation of ultraviolet telescopes could be up to the task, Kaplan suggests. “But that’s not to say that it will be easy.”

Astronomer Bob Rutledge of McGill University suggests an alternative approach: Rather than squinting for neutron stars’ dim light, astronomers could find them through ripples in space-time called gravitational waves. When two neutron stars merge, they are expected to throw off massive amounts of these waves, and Earth-based detectors like LIGO are already in place to catch them — although no waves have actually shown up yet.

“It would be technically hard, but a sound approach,” Rutledge said. “This sort of thing could become possible in the more distant future.”

Image: Artist’s impression of a neutron star with a powerful magnetic field, called a magnetar. Credit: NASA

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Planets Weighed Using Pulsar Flashes

23 Aug

The rotating corpses of massive stars can help scientists weigh the planets in the solar system. By carefully timing radio blips from spinning stellar leftovers called pulsars, astronomers have measured the masses of all the planets from Mercury to Saturn, plus all their moons and rings.

Until now, the only way to figure out the mass of a planet was to send a spacecraft past it. The spacecraft’s orbit is determined by the gravitational oomph of the planet (plus whatever moons lay within the spacecraft’s orbit), which in turn depends on the planet’s mass. The new method is the first to let astronomers weigh planets from the comfort of Earthbound observatories.

“That’s what’s remarkable about this technique,” said space technologist William Folkner of NASA’s Jet Propulsion Laboratory, a co-author of a study in the upcoming issue of Astrophysical Journal. “I can’t think of any other way to measure masses of planets from the Earth.” 

The new method relies on the clock-like regularity of a class of neutron stars called pulsars, the rapidly spinning remains of massive stars that died in supernova explosions. Pulsars shoot tight beams of radio waves into space that sweep across the sky like a lighthouse, so from Earth they appear to blink or pulse.

Because the Earth is always moving around the sun, the time it takes for these radio blips to reach us is always changing. To get rid of this effect, astronomers calculate when the pulse should reach the solar system’s center of mass, or barycenter — the point around which all the mass in the solar system moves. But because the planets’ arrangement around the sun is constantly changing, the barycenter moves around with respect to the sun, too.

To pin down the center of mass at a given time, astronomers use a special table of where all the planets are, called an ephemeris, plus values for the masses of the planets taken from previous space missions. If the masses are slightly wrong, then a regular, repeating pattern of timing errors appears in the pulsar data. For instance, if Jupiter’s mass is a bit off, then an error appears every twelve years, once for every time Jupiter orbits the sun. Correcting the value for Jupiter’s mass makes the error disappear.

“You can see that 12 year wiggle in timing of neutron stars,” Folkner said. “That tells you how far the sun is from the solar system barycenter, which tells you what the mass of Jupiter is.”

An international team of scientists used three different radio telescopes, the 1000-foot-wide Arecibo telescope in Puerto Rico, the 210-foot Parkes telescope in Australia and the 328-foot Effelsberg telescope in Germany to time the blips from four different pulsars over a period of 5 to 22 years. They then used computer models to use the pulsars’ times to calculate the masses of Mercury, Venus, Mars, Jupiter and Saturn.

The masses the team found are not as accurate as the best measurements from spacecraft flybys, but they’re close. The measurement for Jupiter, for instance, was found to be 0.0009547921 times the mass of the sun. This value is more accurate than the mass determined from the Pioneer and Voyager spacecraft, and less accurate than, but consistent with, the value from the later Galileo spacecraft, which includes more decimal places.

“Our error bars are larger than those of these spacecraft measurements,” said study co-author Andrea Lommen of Franklin & Marshall College. “We are admitting freely that you should still use the mass of Jupiter measured from the spacecraft, but it’s comforting to know that our measurement agrees with that.”

The new method is also the first that can measure the masses of everything in a planetary system, including moons and rings.

“Spacecraft flybys don’t tell us the mass of everything in the Jupiter system, only the parts inside the spacecraft orbit,” Folkner said. “With this pulsar timing mechanism, we’re sensitive to the entire system, including the moons that are outside the orbit of any spacecraft that have flown by.”

The technique is actually a stepping stone to studying something even more exotic: ripples in space-time called gravitational waves that were predicted by Einstein but have never been observed. The timing of pulsar blips should change slightly whenever a gravitational wave goes by, but in order to see these changes, astronomers have to subtract out all the other noise that could alter the pulsar’s clock.

This study is “a graphic demonstration that you really have to understand the solar system really well if you’re going to be able to confidently detect gravitational radiation,” commented astronomer Scott Tremaine of the Institute for Advanced Study in Princeton, New Jersey, who was not involved in the new work. “If they can continue to develop these techniques to the point where they can detect gravitational waves, that will be a dramatically important event.”

Image: The sun, Earth and Jupiter orbit a common center of mass. David Champion, MPIfR

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How do we measure gravitational waves? [Mad Science]

03 Jul
"Gravitational telescopes" let scientists observe fluctuations in spacetime itself. They are, in a word, crazystupidamazing. More »