Universe Today

The Guitar nebula. Image credit: Palomar Observatory. Click to enlarge
Since it makes up a large part of the Universe, you’d think we’d know what dark matter is by now. Sorry, it’s still a mystery, but new theories are coming out all the time. An international team of researchers are now theorizing that dark matter could be a class of particles known as “sterile neutrinos”. These particles, formed right at the Big Bang, could account for the Universe’s missing mass, and would have the handy side effect of speeding up the early formation of stars.

Dark matter may have played a major role in creating stars at the very beginnings of the universe. If that is the case, however, the dark matter must consist of particles called “sterile neutrinos”. Peter Biermann of the Max Planck Institute for Radio Astronomy in Bonn, and Alexander Kusenko, of the University of California, Los Angeles, have shown that when sterile neutrinos decay, it speeds up the creation of molecular hydrogen. This process could have helped light up the first stars only some 20 to 100 million years after the big bang. This first generation of stars then ionised the gas surrounding them, some 150 to 400 million years after the big bang. All of this provides a simple explanation to some rather puzzling observations concerning dark matter, neutron stars, and antimatter.

Scientists discovered that neutrinos have mass through neutrino oscillation experiments. This led to the postulation that “sterile” neutrinos exist – also known as right-handed neutrinos. They do not participate in weak interactions directly, but do interact through their mixing with ordinary neutrinos. The total number of sterile neutrinos in the universe is unclear. If a sterile neutrino only has a mass of a few kiloelectronvolts (1 keV is a millionth of the mass of a hydrogen atom), that would explain the huge, missing mass in the universe, sometimes called “dark matter”. Astrophysical observations support the view that dark matter is likely to consist of these sterile neutrinos.

Biermann and Kusenko’s theory sheds light on a number of still unexplained astronomical puzzles. First of all, during the big bang, the mass of neutrinos created in the Big Bang would equal what is needed to account for dark matter. Second, these particles could be the solution to the long-standing problem of why pulsars move so fast.

Pulsars are neutron stars rotating at a very high velocity. They are created in supernova explosions and normally are ejected in one direction. The explosion gives them a “push”, like a rocket engine. Pulsars can have velocities of hundreds of kilometres per second – or sometimes even thousands. The origin of these velocities remains unknown, but the emission of sterile neutrinos would explain the pulsar kicks.

The Guitar Nebula contains a very fast pulsar. If dark matter is made of particles which reionized the universe – as Biermann and Kusenko suggest – the pulsar’s motion could have created this cosmic guitar.

Third, sterile neutrinos can help explain the absence of antimatter in the universe. In the early universe, sterile neutrinos could have “stolen” what is called the “lepton number” from plasma. At a later time, the lack of lepton number was converted to a non-zero baryon number. The resulting asymmetry between baryons (like protons) and antibaryons (like antiprotons) could be the reason why the universe has no antimatter.

“The formation of central galactic black holes, as well as structure on subgalactic scales, favours sterile neutrinos to account for dark matter. The consensus of several indirect pieces of evidence leads one to believe that the long sought-after dark-matter particle may, indeed, be a sterile neutrino”, says Peter Biermann

Original Source: Max Planck Society

Artist’s illustration of the northern starry river. Image credit: Caltech Click to enlarge
Astronomers have found a narrow stream of stars extending across the sky for about 45 degrees – 90 times the width of the full Moon. The stream emanates from a cluster of 50,000 stars called NGC 5466, and stretches from Ursa Major (or the Big Dipper) to the constellation Bootes. The strength of gravity from the Milky Way is different on opposite sides of the star cluster, which causes it to stretch. Outlying stars are no longer held in the cluster and fall behind, creating the stream.

Astronomers have discovered a narrow stream of stars extending at least 45 degrees across the northern sky. The stream is about 76,000 light-years distant from Earth and forms a giant arc over the disk of the Milky Way galaxy.

In the March issue of the Astrophysical Journal Letters, Carl Grillmair, an associate research scientist at the California Institute of Technology’s Spitzer Science Center, and Roberta Johnson, a graduate student at California State University Long Beach, report on the discovery.

“We were blown away by just how long this thing is,” says Grillmair. “As one end of the stream clears the horizon this evening, the other will already be halfway up the sky.”

The stream begins just south of the bowl of the Big Dipper and continues in an almost straight line to a point about 12 degrees east of the bright star Arcturus in the constellation Bootes. The stream emanates from a cluster of about 50,000 stars known as NGC 5466.

The newly discovered stream extends both ahead and behind NGC 5466 in its orbit around the galaxy. This is due to a process called tidal stripping, which results when the force of the Milky Way’s gravity is markedly different from one side of the cluster to the other. This tends to stretch the cluster, which is normally almost spherical, along a line pointing towards the galactic center.

At some point, particularly when its orbit takes it close to the galactic center, the cluster can no longer hang onto its most outlying stars, and these stars drift off into orbits of their own. The lost stars that find themselves between the cluster and the galactic center begin to move slowly ahead of the cluster in its orbit, while the stars that drift outwards, away from the galactic center, fall slowly behind.

Ocean tides are caused by exactly the same phenomenon, though in this case it’s the difference in the moon’s gravity from one side of Earth to the other that stretches the oceans. If the gravity at the surface of Earth were very much weaker, then the oceans would be pulled from the planet, just like the stars in NGC 5466’s stream.

Despite its size, the stream has never previously been seen because it is so completely overwhelmed by the vast sea of foreground stars that make up the disk of the Milky Way. Grillmair and Johnson found the stream by examining the colors and brightnesses of more than nine million stars in the Sloan Digital Sky Survey public database.

“It turns out that, because they were all born at the same time and are situated at roughly the same distance, the stars in globular clusters have a fairly unique signature when you look at how their colors and brightnesses are distributed,” says Grillmair.

Using a technique called matched filtering, Grillmair and Johnson assigned to each star a probability that it might once have belonged to NGC 5466. By looking at the distribution of these probabilities across the sky, “the stream just sort of reached out and smacked us.

“The new stream may be even longer than we know, as we are limited at the southern end by the extent of the currently available data,” he adds. “Larger surveys in the future should be able to extend the known length of the stream substantially, possibly even right around the whole sky.”

The stars that make up the stream are much too faint to be seen by the unaided human eye. Owing to the vast distances involved, they are about three million times fainter than even the faintest stars that we can see on a clear night.

Grillmair says that such discoveries are important for our understanding of what makes up the Milky Way galaxy. Like earthbound rivers, such tidal streams can tell us which way is “down,” how steep is the slope, and where the mountains and valleys are located.

By measuring the positions and velocities of the stars in these streams, astronomers hope to determine how much Dark Matter the Milky Way contains, and whether the dark matter is distributed smoothly, or in enormous orbiting chunks.

Original Source: Caltech News Release

A new detailed picture of the infant universe, where red indicates warm spots and blue for the cooler areas. Image credit: NASA/WMAP Click to enlarge
Scientists have gathered new evidence that supports the inflationary theory of expansion thanks new data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). The spacecraft has been making continuous observations of the cosmic background radiation; the afterglow of the Big Bang. These latest observations produced a map of the sky so detailed that scientists were able to trace how microscopic fluctuations in the primordial Universe were magnified in a trillionth of a second of rapid expansion to create the stars and galaxies we see today.

Scientists peering back to the oldest light in the universe have new evidence to support the concept of inflation. The concept poses the universe expanded many trillion times its size in less than a trillionth of a second at the outset of the big bang.

This finding, made with NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), is based on three years of continuous observations of the cosmic microwave background (CMB), the afterglow light produced when the universe was less than a million years old.

WMAP polarization data allow scientists to discriminate between competing models of inflation for the first time. This is a milestone in cosmology. “We can now distinguish between different versions of what happened within the first trillionth of a second of the universe,” said WMAP Principal Investigator Charles Bennett of the Johns Hopkins University in Baltimore. “The longer WMAP observes, the more it reveals about how our universe grew from microscopic quantum fluctuations to the vast expanses of stars and galaxies we see today.”

Previous WMAP results focused on the temperature variations of this light, which provided an accurate age of the universe and insights into its geometry and composition. The new WMAP observations give not only a more detailed temperature map, but also the first full-sky map of the polarization of the CMB. This major breakthrough will enable scientists to obtain much deeper insight into what happened within the first trillionth of a second of the universe. The WMAP results have been submitted to the Astrophysical Journal and are posted at

http://wmap.gsfc.nasa.gov/results

Big bang physics describes how matter and energy developed over the last 13.7 billion years. WMAP’s observation of the blanket of cool microwave radiation that permeates the universe shows patterns that mark the seeds of what grew into stars and galaxies. The patterns are tiny temperature differences within this extraordinarily uniform light. WMAP discerns temperature fluctuations at levels finer than a millionth of a degree.

WMAP can resolve features in the cosmic microwave background based on polarization, or the way light is changed by the environment through which it passes. For example, sunlight reflecting off of a shiny object is polarized. Comparing the brightness of broad features to compact features in the microwave background, or afterglow light, helps tell the story of the infant universe. One long-held prediction was the brightness would be the same for features of all sizes. In contrast, the simplest versions of inflation predict the relative brightness decreases as the features get small, a trend seen in the new data.

“This is brand new territory,” said WMAP team member Lyman Page of Princeton University in Princeton, N.J. “The polarization data will become stronger as WMAP continues to observe the microwave background. WMAP’s new results heighten the urgency of seeking out inflation’s gravitational wave sign. If gravitational waves are seen in future measurements, that would be solid evidence for inflation.”

With a richer temperature map and the new polarization map, WMAP data favor the simplest versions of inflation. Generically, inflation posits that, at the outset of the big bang, quantum fluctuations – short-lived bursts of energy at the subatomic level – were converted by the rapid inflationary expansion into fluctuations of matter that ultimately enabled stars and galaxies to form. The simplest versions of inflation predict that the largest-sized fluctuations will also be the strongest. The new results from WMAP favor this signature.

Inflation theory predicts that these same fluctuations also produced primordial gravitational waves whose distortion of space-time leaves a signature in the CMB polarization. This will be an important goal of future CMB measurements which, if found, would provide a stunning confirmation of inflation.

“Inflation was an amazing concept when it was first proposed 25 years ago, and now we can support it with real data,” said WMAP team member Gary Hinshaw of NASA’s Goddard Space Flight Center in Greenbelt, Md.

WMAP, a partnership between Goddard and Princeton, was launched on June 30, 2001. The WMAP team includes researchers in U.S. and Canadian universities and institutes. For images and information on the Web about WMAP, visit:
http://www.nasa.gov/vision/universe/wmap_pol.html

Original Source: NASA News Release