MAP

Thursday, 3 April 2014

"Moon's Age Revealed, and a Lunar Mystery May Be Solved" - By Charles Q. Choi, Space.com Contributor | April 02, 2014 01:01pm ET

Scientists have pinned down the birth date of the moon to within 100 million years of the birth of the solar system — the best timeline yet for the evolution of our planet's natural satellite.
This new discovery about the origin of the moon may help solve a mystery about why the moon and the Earth appear virtually identical in makeup, investigators added.
Scientists have suggested the moon was formed4.5 billion years ago by a gigantic collision between a Mars-size object named Theiaand Earth, a crash that would have largely melted the  Earth. This model suggested that more than 40 percent of the moon was made up of debris from this impacting body. (Current theory suggests that Earth experienced several giant impacts during its formation, with the moon-forming impact being the last.) [How the Moon Was Born: A Photo Timeline]
However, researchers suspected Theia was chemically different from Earth. In contrast, recent studies revealed that the moon and Earth appear very similarwhen it comes to versions of elements called isotopes — more so than might be suggested by the current impact model. (Isotopes of an element have differing numbers of neutrons from one another.)
"This means that at the atomic level, the Earth and the moon are identical,"study lead author Seth Jacobson, a planetary scientist at the Côte d'Azur Observatory in Nice, France, told Space.com. "This new information challenged the giant impact theory for lunar formation."
How the moon evolved
No one seriously disputed an impact as the most likely scenario for the formation of the moon, Jacobson said. However, a virtually atomically identical moon and Earth threw the exact circumstances of the collision into question, he said.
Now, by pinpointing when the moon formed, Jacobson and his colleagues could help explain why the moon and Earth are mysteriously similar. The scientists detailed their findings in the April 3 issue of the journal Nature. [How the Moon Formed: 5 Wild Lunar Theories]
Efforts to date the moon-forming impact have proposed a range of ages. Some have argued for an early event, about 30 million years after the birth of the solar system, whereas others suggested that it occurred more than 50 million years and possibly as much as 100 million years after the solar system formed.
To help solve this mystery, Jacobson and his colleagues simulated the growth of the solar system's rocky planets — Mercury, Venus, Earth and Mars — from a protoplanetary disk of thousands of planetary building blocks orbiting the sun.
By analyzing how these planets formed and grew from more than 250 computer simulations, the researchers discovered that if the moon-forming impact was early, the amount of material accreted onto Earth afterward was large. If the impact was late, the amount would then be small.
Past research had calculated the amount of material accreted onto Earth after the moon-forming impact. These estimates are based on how on how so-called highly siderophile or "iron-loving" elements such as iridium and platinum show a strong tendency to move into Earth's core. After each giant impact the nascent Earth sustained, these elements would have leached from Earth's mantle and bonded with heavy, iron-rich material destined to sink to Earth's heart.
Moon birth mystery
After the last giant impact that formed the moon, the mantle should have been almost completely stripped of iridium, platinum and their cousins. These elements are still present in the mantle, but only in small amounts, which suggests only a small amount of material accreted onto Earth after the moon-forming impact.
The researchers calculated the moon-forming impact must have occurred about 95 million years after the formation of the solar system, give or take 32 million years.
"A late moon-forming event, as suggested by our work, is very consistent with an identical Earth and moon," Jacobson said.
In addition, recent analyses propose that the impact that created the moon required a faster, more energetic collision than previously suggested. This makes sense if the impact took place relatively late with an older protoplanetary disk, as the new findings suggest.
"Older disks tend to be dynamically more active, since there are fewer bodies left in the disk to distribute energy amongst," Jacobson said.
These new findings raise an interesting new puzzle. While they suggest the moon and the Earth formed together nearly 100 million years after the solar system arose, evidence from meteorites from Mars suggests that the Red Planet formed as little as a few million years after the solar system was born.
"This means that Earth and Mars formed over dramatically different timescales, with Mars forming much faster than the Earth," Jacobson said. "How can this be? Is it just a matter of size? Location? What about Mercury and Venus? Did they grow on similar timescales to the Earth or on timescales more similar to Mars? I think these are some of the really important questions that we, as a community of planetary scientists, will be addressing in the future."

Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

Wednesday, 2 April 2014

Case for Dark Matter Signal Strengthens - By: Natalie Wolchover March 3, 2014 - [simonsfoundation.org/quanta]

Not long after the Fermi Gamma-ray Space Telescope took to the sky in 2008, astrophysicists noticed that it was picking up a steady rain of gamma rays pouring outward from the center of the Milky Way galaxy. This high-energy radiation was consistent with the detritus of annihilating dark matter, the unidentified particles that constitute 84 percent of the matter in the universe and that fizzle upon contact with each other, spewing other particles as they go. If the gamma rays did in fact come from dark matter, they would reveal its identity, resolving one of the biggest mysteries in physics. But some argued that the gamma rays could have originated from another source.
Now a new analysis of the signal claims to rule out all other plausible explanations and makes the case that the gamma rays trace back to a type of particle that has long been considered the leading dark matter candidate — a weakly interacting massive particle, or WIMP. Meanwhile, a more tentative X-ray signal reported in two other new studies suggests the existence of yet another kind of dark matter particle called a sterile neutrino.
In the new gamma-ray analysis, which appeared Feb. 27 on the scientific preprint site arXiv.org, Dan Hooper and his collaborators used more than five years’ worth of the cleanest Fermi data to generate a high-resolution map of the gamma-ray excess extending from the center of the galaxy outward at least 10 angular degrees, or 5,000 light-years, in all directions.
“The results are extremely interesting,” said Kevork Abazajian, an associate professor of physics and astronomy at the University of California, Irvine. “The most remarkable part of the analysis is that the signal follows the shape of the dark matter profile out to 10 degrees,” he said, explaining that it would be “very difficult to impossible” for other sources to mimic this predicted dark matter distribution over such a broad range.
The findings do not constitute a discovery of dark matter, the scientists said, but they prepare the way for an upcoming test described by many researchers as a “smoking gun”: If the gamma-ray excess comes from annihilating WIMPs, and not conventional astrophysical objects, then the signal will also be seen emanating from dwarf galaxies that orbit the Milky Way — diffuse objects that are rich in dark matter but not in other high-energy photon sources such as pulsars, rotating neutron stars that have been floated as alternative explanations for the excess.
“These gamma rays match the predictions of a pretty prototypical WIMP, the kind of thing we were all writing down 10 or 15 years ago,” said Hooper, a theoretical astrophysicist at the Fermi National Accelerator Laboratory and the University of Chicago, and the person who co-discovered the gamma-ray excess with then graduate student Lisa Goodenough in 2009. “That’s where my money is.”
“It’s definitely exciting,” said Neal Weiner, a dark matter specialist at New York University. “I think we’d like to see it somewhere else, like a dwarf galaxy, before getting really excited.”
Preliminary results from the Fermi Collaboration — scientists who process, analyze and release the telescope data — offer hints that there may indeed be a surplus of gamma rays coming from the dwarf galaxies. Although there is currently too little data to determine whether an excess exists, “we are starting to get closer to the range,” said Jennifer Siegal-Gaskins, a physicist at the California Institute of Technology and a member of the Fermi Collaboration. “I would say the next couple of years of data could really be important for testing this excess.”
“If that small excess from the dwarf galaxies turns to a big one, that would convince the whole community,” Hooper said. “That would be game over.”
While most experts agree, some question whether indirect glimpses of dark matter can ever truly constitute a discovery.
Dark matter consists of elementary particles that do not emit or absorb light, because they do not experience the electromagnetic force. These particles are also unaffected by the strong nuclear force, which ensnares many of the known particles into atoms. Cosmologists infer the existence of dark matter, and can model its distribution throughout the cosmos, because it does participate in the force of gravity and therefore plays a leading role in shaping galaxies. If dark matter particles also experience the fourth and final force of nature, called the weak nuclear force, then they are of a type known as a WIMP.
In many theories, pairs of WIMPs can annihilate each other on contact, emitting other particles as they go. If the glimmer of gamma rays from the inner galaxy is the afterglow from such annihilations, then their detected energy levels indicate that they most likely originate from WIMPs with a mass of 35 giga-electron-volts (GeV) annihilating into quarks, or 10-GeV WIMPs annihilating into tau particles.
The 35-GeV WIMP model “fits the data best,” said Tracy Slatyer, an assistant professor of physics at the Massachusetts Institute of Technology and a co-author of the new paper. The fit has greatly improved, she said, since the group’s last analysis of the gamma-ray excess. If the signal wasn’t from dark matter, “it’s not at all clear that going to a better data sample would make the results look better,” she said, “but when I saw the new results, I was amazed.”
WIMPs have not yet shown up in direct detection experiments, which look for spurts of energy coming from their weak interactions with atomic nuclei, usually in detectors placed in mine shafts deep underground to lower the background noise. But this does not mean 35-GeV WIMPs don’t exist, scientists said, because no one knows how frequently they interact. The authors of the new study “could be perfectly right, and we just need detectors two orders of magnitude more sensitive to see the particles,” said Juan Collar, an associate professor of physics at the University of Chicago who helps develop direct detection experiments.
Most of the researchers interviewed for this article said the presence of a gamma-ray excess from the dwarf galaxies would be sufficient proof of WIMPs, but a few said that it might take a direct detection to convince them. “The problem is the universe is a messy place,” said Kathryn Zurek, an associate professor of physics at the University of Michigan. Try as they might to rule out “astrophysics” — shorthand among dark matter researchers for all the conventional stuff in the sky, from pulsars to supernovae to the sun — it is always possible that they have missed something.
The study authors, however, are confident that dark matter is the only plausible source of the gamma rays. “We threw everything including the kitchen sink at the problem,” Hooper said. “My views are on the record.”
Meanwhile, just as Hooper’s group was putting the finishing touches on the new manuscript, two other teams of scientists independently reported the discovery of a different anomaly in the sky: a dash of X-rays emanating from distant galaxies that is consistent with the decay of 7-kilo-electron-volt (keV) sterile neutrinos — heavier and less active cousins of the familiar neutrinos that are also dark matter candidates.
Esra Bulbul, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, and her colleagues spotted the X-rays in data from the Chandra and XMM-Newton space telescopes and published their results Feb. 10. A week later, a group led by Alexey Boyarsky of the University of Leiden in the Netherlands reported the same X-ray excess in telescope observations of the Andromeda galaxy.
“I think we have a very big fish here,” Bulbul said.
Bulbul and colleagues report a statistical significance of between 4 and 5 sigma, meaning the X-ray signal is strong enough that the odds that it is a random fluke are only one in 100,000. However, putative dark matter signals often hover at the 4-sigma brink of statistical significance only to fade into the background when more data is collected. Seasoned veterans of this boom-and-bust cycle are skeptical about the new anomaly, but some have expressed cautious optimism.
“It’s definitely intriguing,” said John Beacom, a theoretical astrophysicist at Ohio State University. “They certainly have tried very hard to eliminate or examine the possibility of an atomic transition being the cause. They’ve also gone to great lengths to eliminate instrumental effects.”
The X-ray bump appeared in all subsets of the data, no matter how Bulbul and colleagues sliced it — a sign that the bump did not come from a bias somewhere in the telescope instrumentation. It was this same omnipresence that convinced particle physicists at the Large Hadron Collider that they had cornered the Higgs boson in 2012 before their signal reached the 5-sigma strength formally needed for a discovery. Further support for the significance of the X-ray excess comes from the Dutch group’s discovery of the same bump at 4.4-sigma strength in a different data set.
If the X-rays come from sterile neutrinos, the existence of these particles would very likely solve a long-standing puzzle about galaxy formation known as the “too big to fail” problem, which asks why objects called dark matter subhalos don’t collapse and form dwarf galaxies. “That’s one of the reasons I’m actually more excited about this result than I would be otherwise,” Abazajian said. The particles also play a role in the seesaw mechanism, the most widely supported explanation for the minuscule mass of regular neutrinos. Decays of sterile neutrinos shortly after the Big Bang could even explain the mysterious dominance of matter over antimatter in the universe today. “Sterile neutrinos get invoked for twenty different reasons,” Beacom said.
Like the gamma-ray signal, the X-ray excess will face a clear-cut test in the near future. The Astro-H telescope, set for launch in 2015, will be sensitive enough to detect the smear of the signal. If the width of the bump is consistent with the expected speed of decaying dark matter particles, “that would be a detection,” Abazajian said.
Both signals are tough to dismiss, raising a strange prospect. “It’s possible they are both dark matter,” Abazajian said. “It would be crazy, but it’s certainly possible.”
The sterile neutrinos associated with the X-rays could account for anywhere from 1 to 100 percent of dark matter, depending on how often they decay. And the WIMPs tied to the gamma rays are almost as flexible. The two could coexist. As Collar put it, “If the matter we know about is so rich in families of particles, what tells you this dark sector we know nothing about is not as rich or richer?”
A theoretical model called “exciting dark matter,” proposed in 2007 by Weiner and Douglas Finkbeiner of Harvard, a co-author of the new gamma-ray paper, even predicts the existence of both a keV-scale dark matter particle and a GeV-scale particle working in tandem. “So, at the moment, I’m quite excited!” Weiner said in an email.
But at least for the next couple of years, another nagging possibility remains.
“It’s like the Monty Python sketch — nobody ever expects the Spanish Inquisition,” Beacom said. “Sometimes in this field nobody expects astrophysics, but it’s almost always astrophysics. All of these groups have tried to be very careful, but it is difficult, and nature may surprise us with astrophysics yet again.”

Correction: This article was revised on March 4, 2014, to reflect that the Fermi Collaboration had exclusive access to only the first years’ worth of telescope data before releasing it to the public. New data is now released weekly.

This article was reprinted on TheGuardian.com.

Monday, 24 February 2014

Could Jupiter become a star? Feb 21, 2014 by Fraser Cain, Universe Today

NASA's Galileo spacecraft arrived at Jupiter on December 7, 1995, and proceeded to study the giant planet for almost 8 years. It sent back a tremendous amount of scientific information that revolutionized our understanding of the Jovian system. By the end of its mission, Galileo was worn down. Instruments were failing and scientists were worried they wouldn't be able to communicate with the spacecraft in the future. If they lost contact, Galileo would continue to orbit the Jupiter and potentially crash into one of its icy moons.
Galileo would certainly have Earth bacteria on board, which might contaminate the pristine environments of the Jovian moons, and so NASA decided it would be best to crash Galileo into Jupiter, removing the risk entirely. Although everyone in the scientific community were certain this was the safe and wise thing to do, there were a small group of people concerned that crashing Galileo into Jupiter, with its Plutonium thermal reactor, might cause a cascade reaction that would ignite Jupiter into a second star in the Solar System.
Hydrogen bombs are ignited by detonating plutonium, and Jupiter's got a lot of hydrogen. Since we don't have a second star, you'll be glad to know this didn't happen. Could it have happened? Could it ever happen? The answer, of course, is a series of nos. No, it couldn't have happened. There's no way it could ever happen… or is there?
Jupiter is mostly made of hydrogen, in order to turn it into a giant fireball you'd need oxygen to burn it. Water tells us what the recipe is. There are two atoms of hydrogen to one atom of oxygen. If you can get the two elements together in those quantities, you get water.
In other words, if you could surround Jupiter with half again more Jupiter's worth of oxygen, you'd get a Jupiter plus a half sized fireball. It would turn into water and release energy. But that much oxygen isn't handy, and even though it's a giant ball of fire, that's still not a star anyway. In fact, stars aren't "burning" at all, at least, not in the combustion sense.
Our Sun produces its energy through fusion. The vast gravity compresses hydrogen down to the point that high pressure and temperatures cram hydrogen atoms into helium. This is a fusion reaction. It generates excess energy, and so the Sun is bright. And the only way you can get a reaction like this is when you bring together a massive amount of hydrogen. In fact… you'd need a star's worth of hydrogen. Jupiter is a thousand times less massive than the Sun. One thousand times less massive. In other words, if you crashed 1000 Jupiters together, then we'd have a second actual Sun in our Solar System.
But the Sun isn't the smallest possible star you can have. In fact, if you have about 7.5% the mass of the Sun's worth of hydrogen collected together, you'll get a red dwarf star. So the smallest red dwarf star is still about 80 times the mass of Jupiter. You know the drill, find 79 more Jupiters, crash them into Jupiter, and we'd have a second star in the Solar System.
There's another object that's less massive than a red dwarf, but it's still sort of star like: a brown dwarf. This is an object which isn't massive enough to ignite in true fusion, but it's still massive enough that deuterium, a variant of hydrogen, will fuse. You can get a brown dwarf with only 13 times the mass of Jupiter. Now that's not so hard, right? Find 13 more Jupiters, crash them into the planet?
As was demonstrated with Galileo, igniting Jupiter or its hydrogen is not a simple matter.
We won't get a second star unless there's a series of catastrophic collisions in the Solar System.

And if that happens… we'll have other problems on our hands.

Saturday, 15 February 2014

"The Fabric of Space and Time is in Turmoil" --More on Stephen Hawking's Black Hole Update, - 14th, Feb.'2014

On January 24, the journal Nature published an article entitled "There are no black holes." In a brief article published on arXiv, a scientific preprint server, Stephen Hawking, currently Director of Research at the Centre for Theoretical Cosmology at the University of Cambridge, proposed a theory of black holes that could reconcile the principles of general relativity and quantum physics.
"According to Einstein's theory of general relativity, a black hole is kind of cosmic central vacuum cleaner that swallows everything in its reach and lets nothing escape. It emits no radiation," says Robert Lamontagne, an astrophysicist at the Department of Physics, Université de Montréal, and Executive Director of the Observatoire du Mont-Mégantic.Since it is not visible and has no boundaries as such, a black hole is classically defined by an area of space called the "event horizon," where nothing can escape. "Beyond this horizon, matter and light flow freely, but as soon as the horizon's intangible boundary is crossed, matter and light become trapped," he says.
However, if we use quantum mechanics to describe a black hole, the laws of thermodynamics must apply. In this description, a black hole emits particles in the form of radiation and, ultimately, evaporates. Hawking himself predicted this in the 1970s.
"Following through with Hawking's argument, we conclude that if there is evaporation there must be a boundary to the event horizon, a place of transition between the inside and outside of the black hole," says Lamontagne. "A high energy envelope, a firewall, which burns up matter, is proposed."
However, this scenario poses a problem: if the firewall exists, we should be able to see it. Furthermore, the existence of a firewall around a black hole is inconsistent with the theory of general relativity.
While the two major theories, that of general relativity (a theory of gravity) and quantum mechanics (a description of the microscopic world), work well in their respective fields, they are not universal: neither can explain alone how black holes work.
"The Holy Grail would be to find THE theory that would unify the other two. And Stephen Hawking has come back with a new proposal," says Lamontagne. Roughly, Hawking suggests that if the firewall is not visible, it is because its position fluctuates constantly and rapidly. "Hawking says, and this is purely hypothetical, that the fabric of space and time is in turmoil and we cannot define its whereabouts."
In short, since we cannot change the principles of either quantum mechanics or general relativity, Hawking proposes to slightly modify the description of black holes. Hence his remark that black holes do not exist the way we thought they did, as we thought we knew them.
In our galaxy, black holes are less numerous than suggested by sci-fi movies. The largest black hole near us is at the center of our galaxy - the Milky Way. It is 30,000 light-years from Earth. Its mass is about one million times that of the Sun, and it occupies a space equivalent to our solar system.
"We cannot see it directly but we have located it because of effects we can observe using various technological methods: it constantly deviates the trajectories of stars in its vicinity," says Lamontagne. Moreover, in 2014, a huge cloud of gas will fall toward this "nearby" black hole. "This is exciting from an astronomical point of view because we will be able to examine the phenomenon for 10 to 20 years to come."
The image at the top of the page shows a rapid X-ray flare that was observed from the direction of the supermassive black hole that resides at the center of our galaxy. This violent flare captured by NASA's Chandra X-ray Observatory has given astronomers an unprecedented view of the energetic processes surrounding this supermassive black hole.
A team of scientists led by Frederick K. Baganoff of MIT detected a sudden X-ray flare while observing Sagiattarius A*, a source of radio emission believed to be associated with the black hole at the center of
our Galaxy.
"This is extremely exciting because it's the first time we have seen our own neighborhood supermassive black hole devour a chunk of material," said Baganoff. "This signal comes from closer to the event horizon of our Galaxy's supermassive black hole than any that we have ever received before. It's as if the material there sent us a postcard before it fell in."
In a just few minutes, Sagittarius A** became 45 times brighter in X-rays, before declining to pre-flare levels a few hours later. At the peak of the flare, the X-ray intensity dramatically dropped by a factor of five within just a 10-minute interval. This constrains the size of the emitting region to be no larger than about 20 times the size of the "event horizon" (the one-way membrane around a black hole) as predicted by Einstein's theory of relativity.
The rapid rise and fall seen by Chandra are also compelling evidence that the X-ray emission is coming from matter falling into a supermassive black hole. This would confirm the Milky Way's supermassive black hole is powered by the same accretion process as quasars and other active galactic nuclei.
Dynamical studies of the central region of our Milky Way Galaxy in infrared and radio wavelengths indicate the presence of a large, dark object, presumably a supermassive black hole having the mass of about 3 million suns. Sagittarius A* is coincident with the location of this object, and is thought to be powered by the infall of matter into the black hole. However, the faintness of Sagittarius A* at all wavelengths, especially in X-rays, has cast some doubt on this model.
The latest precise Chandra observations of the crowded galactic center region have dispelled that doubt, confirming the results of the dynamical studies. Given the extremely accurate position, it is highly unlikely that the flare is due to an unrelated contaminating source such as an X-ray binary system.

"The rapid variations in X-ray intensity indicate that we are observing material that is as close to the black hole as the Earth is to the Sun," said Gordon Garmire of Penn State University, principal investigator of Advanced CCD Imaging Spectrometer (ACIS), which was used in these observations. "It makes Sagittarius A* a uniquely valuable source for studying conditions very near a supermassive black hole."