What Can The (Dark) Matter Be?

What better place to look for dark matter than down a mine shaft? A research team from the University of Florida have spent nine years monitoring for any signs of the elusive stuff using germanium and silicon detectors cooled down to a fraction of a degree above absolute zero. And the result? A couple of maybes and a gritty determination to keep looking. 

The case for dark matter can be appreciated by considering the solar system where, to stay in orbit around the Sun, Mercury has to move at 48 kilometers a second, while distant Neptune can move at a leisurely 5 kilometers a second. Surprisingly, this principle doesn’t apply in the Milky Way or in other galaxies we have observed.  Broadly speaking, you can find stuff in the outer parts of a spiral galaxy that is moving just as fast as stuff that is close in to the galactic centre. This is puzzling, particularly since there doesn’t seem to be enough gravity in the system to hold onto the rapidly orbiting stuff in the outer parts – which should just fly off into space. 

So, we need more gravity to explain how galaxies rotate and stay together – which means we need more mass than we can observe – and this is why we invoke dark matter. Invoking dark matter also helps to explain why galaxy clusters stay together and explains large scale gravitational lensing effects, such as can be seen in the Bullet Cluster (pictured above). 

Computer modeling suggests that galaxies may have dark matter halos, but they also have dark matter distributed throughout their structure – and taken together, all this dark matter represents up to 90% of a galaxy’s total mass. 

An artist's impression of dark matter, showing the proportional distribution of baryonic and non-baryonic forms (this joke never gets old).

Current thinking is that a small component of dark matter is baryonic, meaning stuff that is composed of protons and neutrons – in the form of cold gas as well as dense, non-radiant objects such black holes, neutron stars, brown dwarfs and orphaned planets (traditionally known as Massive Astrophysical Compact Halo Objects – or MACHOs). 

But it doesn’t seem that there is nearly enough dark baryonic matter to account for the circumstantial effects of dark matter. Hence the conclusion that most dark matter must be non-baryonic, in the form of Weakly Interacting Massive Particles (or WIMPs). 

By inference, WIMPS are transparent and non-reflective at all wavelengths and probably don’t carry a charge. Neutrinos, which are produced in abundance from the fusion reactions of stars, would fit the bill nicely except they don’t have enough mass. The currently most favored WIMP candidate is a neutralino, a hypothetical particle predicted by supersymmetry theory. 

The second Cryogenic Dark Matter Search Experiment (or CDMS II) runs deep underground in the Soudan iron mine in Minnesota, situated there so it should only intercept particles that can penetrate that deeply underground. The CDMS II solid crystal detectors seek ionization and phonon events which can be used to distinguish between electron interactions – and nuclear interactions. It is assumed that a dark matter WIMP particle will ignore electrons, but potentially interact with (i.e. bounce off) a nucleus. 

Two possible events have been reported  by the University of Florida team, who acknowledge their findings cannot be considered statistically significant, but may at least give some scope and direction to further research.

By indicating just how difficult to directly detect (i.e. just how ‘dark’) WIMPs really are – the CDMS II findings indicate the sensitivity of the detectors needs to bumped up a notch.

Dark Matter in Distant Galaxy Groups Mapped for the First Time

X-ray emission in the COSMOS field (XMM-Newton/ESA)

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Galaxy density in the Cosmic Evolution Survey (COSMOS) field, with colors representing the redshift of the galaxies, ranging from redshift of 0.2 (blue) to 1 (red). Pink x-ray contours show the extended x-ray emission as observed by XMM-Newton.

Dark matter (actually cold, dark – non-baryonic – matter) can be detected only by its gravitational influence. In clusters and groups of galaxies, that influence shows up as weak gravitational lensing, which is difficult to nail down. One way to much more accurately estimate the degree of gravitational lensing – and so the distribution of dark matter – is to use the x-ray emission from the hot intra-cluster plasma to locate the center of mass.

And that’s just what a team of astronomers have recently done … and they have, for the first time, given us a handle on how dark matter has evolved over the last many billion years.

COSMOS is an astronomical survey designed to probe the formation and evolution of galaxies as a function of cosmic time (redshift) and large scale structure environment. The survey covers a 2 square degree equatorial field with imaging by most of the major space-based telescopes (including Hubble and XMM-Newton) and a number of ground-based telescopes.

Understanding the nature of dark matter is one of the key open questions in modern cosmology. In one of the approaches used to address this question astronomers use the relationship between mass and luminosity that has been found for clusters of galaxies which links their x-ray emissions, an indication of the mass of the ordinary (“baryonic”) matter alone (of course, baryonic matter includes electrons, which are leptons!), and their total masses (baryonic plus dark matter) as determined by gravitational lensing.

To date the relationship has only been established for nearby clusters. New work by an international collaboration, including the Max Planck Institute for Extraterrestrial Physics (MPE), the Laboratory of Astrophysics of Marseilles (LAM), and Lawrence Berkeley National Laboratory (Berkeley Lab), has made major progress in extending the relationship to more distant and smaller structures than was previously possible.

To establish the link between x-ray emission and underlying dark matter, the team used one of the largest samples of x-ray-selected groups and clusters of galaxies, produced by the ESA’s x-ray observatory, XMM-Newton.

Groups and clusters of galaxies can be effectively found using their extended x-ray emission on sub-arcminute scales. As a result of its large effective area, XMM-Newton is the only x-ray telescope that can detect the faint level of emission from distant groups and clusters of galaxies.

“The ability of XMM-Newton to provide large catalogues of galaxy groups in deep fields is astonishing,” said Alexis Finoguenov of the MPE and the University of Maryland, a co-author of the recent Astrophysical Journal (ApJ) paper which reported the team’s results.

Since x-rays are the best way to find and characterize clusters, most follow-up studies have until now been limited to relatively nearby groups and clusters of galaxies.

“Given the unprecedented catalogues provided by XMM-Newton, we have been able to extend measurements of mass to much smaller structures, which existed much earlier in the history of the Universe,” says Alexie Leauthaud of Berkeley Lab’s Physics Division, the first author of the ApJ study.

COSMOS-XCL095951+014049 (Subaru/NAOJ, XMM-Newton/ESA)

Gravitational lensing occurs because mass curves the space around it, bending the path of light: the more mass (and the closer it is to the center of mass), the more space bends, and the more the image of a distant object is displaced and distorted. Thus measuring distortion, or ‘shear’, is key to measuring the mass of the lensing object.

In the case of weak gravitational lensing (as used in this study) the shear is too subtle to be seen directly, but faint additional distortions in a collection of distant galaxies can be calculated statistically, and the average shear due to the lensing of some massive object in front of them can be computed. However, in order to calculate the lens’ mass from average shear, one needs to know its center.

“The problem with high-redshift clusters is that it is difficult to determine exactly which galaxy lies at the centre of the cluster,” says Leauthaud. “That’s where x-rays help. The x-ray luminosity from a galaxy cluster can be used to find its centre very accurately.”

Knowing the centers of mass from the analysis of x-ray emission, Leauthaud and colleagues could then use weak lensing to estimate the total mass of the distant groups and clusters with greater accuracy than ever before.

The final step was to determine the x-ray luminosity of each galaxy cluster and plot it against the mass determined from the weak lensing, with the resulting mass-luminosity relation for the new collection of groups and clusters extending previous studies to lower masses and higher redshifts. Within calculable uncertainty, the relation follows the same straight slope from nearby galaxy clusters to distant ones; a simple consistent scaling factor relates the total mass (baryonic plus dark) of a group or cluster to its x-ray brightness, the latter measuring the baryonic mass alone.

“By confirming the mass-luminosity relation and extending it to high redshifts, we have taken a small step in the right direction toward using weak lensing as a powerful tool to measure the evolution of structure,” says Jean-Paul Kneib a co-author of the ApJ paper from LAM and France’s National Center for Scientific Research (CNRS).

The origin of galaxies can be traced back to slight differences in the density of the hot, early Universe; traces of these differences can still be seen as minute temperature differences in the cosmic microwave background (CMB) – hot and cold spots.

“The variations we observe in the ancient microwave sky represent the imprints that developed over time into the cosmic dark-matter scaffolding for the galaxies we see today,” says George Smoot, director of the Berkeley Center for Cosmological Physics (BCCP), a professor of physics at the University of California at Berkeley, and a member of Berkeley Lab’s Physics Division. Smoot shared the 2006 Nobel Prize in Physics for measuring anisotropies in the CMB and is one of the authors of the ApJ paper. “It is very exciting that we can actually measure with gravitational lensing how the dark matter has collapsed and evolved since the beginning.”

One goal in studying the evolution of structure is to understand dark matter itself, and how it interacts with the ordinary matter we can see. Another goal is to learn more about dark energy, the mysterious phenomenon that is pushing matter apart and causing the Universe to expand at an accelerating rate. Many questions remain unanswered: Is dark energy constant, or is it dynamic? Or is it merely an illusion caused by a limitation in Einstein’s General Theory of Relativity?

The tools provided by the extended mass-luminosity relationship will do much to answer these questions about the opposing roles of gravity and dark energy in shaping the Universe, now and in the future.

Sources: ESA, and a paper published in the 20 January, 2010 issue of the Astrophysical Journal (arXiv:0910.5219 is the preprint)

ESA’s Tough Choice: Dark Matter, Sun Close Flyby, Exoplanets (Pick Two)

Thales Alenia Space and EADS Astrium concepts for Euclid (ESA)


Key questions relevant to fundamental physics and cosmology, namely the nature of the mysterious dark energy and dark matter (Euclid); the frequency of exoplanets around other stars, including Earth-analogs (PLATO); take the closest look at our Sun yet possible, approaching to just 62 solar radii (Solar Orbiter) … but only two! What would be your picks?

These three mission concepts have been chosen by the European Space Agency’s Science Programme Committee (SPC) as candidates for two medium-class missions to be launched no earlier than 2017. They now enter the definition phase, the next step required before the final decision is taken as to which missions are implemented.

These three missions are the finalists from 52 proposals that were either made or carried forward in 2007. They were whittled down to just six mission proposals in 2008 and sent for industrial assessment. Now that the reports from those studies are in, the missions have been pared down again. “It was a very difficult selection process. All the missions contained very strong science cases,” says Lennart Nordh, Swedish National Space Board and chair of the SPC.

And the tough decisions are not yet over. Only two missions out of three of them: Euclid, PLATO and Solar Orbiter, can be selected for the M-class launch slots. All three missions present challenges that will have to be resolved at the definition phase. A specific challenge, of which the SPC was conscious, is the ability of these missions to fit within the available budget. The final decision about which missions to implement will be taken after the definition activities are completed, which is foreseen to be in mid-2011.
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Euclid is an ESA mission to map the geometry of the dark Universe. The mission would investigate the distance-redshift relationship and the evolution of cosmic structures. It would achieve this by measuring shapes and redshifts of galaxies and clusters of galaxies out to redshifts ~2, or equivalently to a look-back time of 10 billion years. It would therefore cover the entire period over which dark energy played a significant role in accelerating the expansion.

By approaching as close as 62 solar radii, Solar Orbiter would view the solar atmosphere with high spatial resolution and combine this with measurements made in-situ. Over the extended mission periods Solar Orbiter would deliver images and data that would cover the polar regions and the side of the Sun not visible from Earth. Solar Orbiter would coordinate its scientific mission with NASA’s Solar Probe Plus within the joint HELEX program (Heliophysics Explorers) to maximize their combined science return.

Thales Alenis Space concept, from assessment phase (ESA)

PLATO (PLAnetary Transit and Oscillations of stars) would discover and characterize a large number of close-by exoplanetary systems, with a precision in the determination of mass and radius of 1%.

In addition, the SPC has decided to consider at its next meeting in June, whether to also select a European contribution to the SPICA mission.

SPICA would be an infrared space telescope led by the Japanese Space Agency JAXA. It would provide ‘missing-link’ infrared coverage in the region of the spectrum between that seen by the ESA-NASA Webb telescope and the ground-based ALMA telescope. SPICA would focus on the conditions for planet formation and distant young galaxies.

“These missions continue the European commitment to world-class space science,” says David Southwood, ESA Director of Science and Robotic Exploration, “They demonstrate that ESA’s Cosmic Vision programme is still clearly focused on addressing the most important space science.”

Source: ESA chooses three scientific missions for further study

Dark Matter Detector Heading to the ISS This Summer

AMS-2 during integration activities at CERN facility in Switzerland. Credit: ESA

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The long-awaited experiment that will search for dark matter is getting closer to heading to the International Space Station. The Alpha Magnetic Spectrometer (AMS) is undergoing final testing at ESA’s Test Centre in the Netherlands before being launched on the space shuttle to the ISS, currently scheduled for July, 2010. The AMS will help scientists better understand the fundamental issues on the origin and structure of the Universe by observing dark matter, missing matter and antimatter. As a byproduct, AMS will gather other information from cosmic radiation sources such as stars and galaxies millions of light years from our home galaxy.

ISS officials have been touting that science is now beginning to be done in earnest on the orbiting laboratory. The AMS will be a giant leap in science capability for the ISS. Not only is it the biggest scientific instrument to be installed on the International Space Station (ISS), but also it is the first magnetic spectrometer to be flown in space, and the largest cryogenically cooled superconducting magnet ever used in space. It will be installed on the central truss of the ISS.
Location of where the AMS will be located on the exterior of the ISS. Credits: CERN et Universite de Geneve
AMS had been cut from the ISS program following the 2003 Columbia shuttle accident, but the outcry over the cancellation forced NASA to rethink their decision. Most of AMS’s $1.5-billion costs have been picked up the international partners that NASA wishes to stay on good terms with. 56 institutes from 16 countries have contributed to the AMS project, with Nobel laureate Samuel Ting coordinating the effort.

In an interview with the BBC, Ting said results from AMS may take up to three years to search for antimatter in other galaxies, and dark matter in our own.
The instrument was built at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. The first part of the tests was also conducted at CERN, when the detector was put through its paces using a proton beam from CERN’s Super Proton Synchrotron accelerator to check its momentum resolution and its ability to measure particle curvature and momentum.

AMS’s ability to distinguish electrons from protons was also tested. This is very important for the measurement of cosmic rays, 90% of which are protons and constitute a natural background for other signals that interest scientists. AMS will be looking for an abundance of positrons and electrons from space, one of the possible markers for dark matter.

Once the extensive testing is complete, AMS will leave ESTEC at the end of May on a special US Air Force flight to Kennedy Space Center in Florida. It will be launched to the ISS on the Space Shuttle Endeavour on flight STS-134, now scheduled for July.

Source: ESA

Universe to WMAP: ΛCDM Rules, OK?

Temperature and polarization around hot and cold spots (Credit: NASA / WMAP Science Team)

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The Wilkinson Microwave Anisotropy Probe (WMAP) science team has finished analyzing seven full years’ of data from the little probe that could, and once again it seems we can sum up the universe in six parameters and a model.

Using the seven-year WMAP data, together with recent results on the large-scale distribution of galaxies, and an updated estimate of the Hubble constant, the present-day age of the universe is 13.75 (plus-or-minus 0.11) billion years, dark energy comprises 72.8% (+/- 1.5%) of the universe’s mass-energy, baryons 4.56% (+/- 0.16%), non-baryonic matter (CDM) 22.7% (+/- 1.4%), and the redshift of reionization is 10.4 (+/- 1.2).

In addition, the team report several new cosmological constraints – primordial abundance of helium (this rules out various alternative, ‘cold big bang’ models), and an estimate of a parameter which describes a feature of density fluctuations in the very early universe sufficiently precisely to rule out a whole class of inflation models (the Harrison-Zel’dovich-Peebles spectrum), to take just two – as well as tighter limits on many others (number of neutrino species, mass of the neutrino, parity violations, axion dark matter, …).

The best eye-candy from the team’s six papers are the stacked temperature and polarization maps for hot and cold spots; if these spots are due to sound waves in matter frozen in when radiation (photons) and baryons parted company – the cosmic microwave background (CMB) encodes all the details of this separation – then there should be nicely circular rings, of rather exact sizes, around the spots. Further, the polarization directions should switch from radial to tangential, from the center out (for cold spots; vice versa for hot spots).

And that’s just what the team found!

Concerning Dark Energy. Since the Five-Year WMAP results were published, several independent studies with direct relevance to cosmology have been published. The WMAP team took those from observations of the baryon acoustic oscillations (BAO) in the distribution of galaxies; of Cepheids, supernovae, and a water maser in local galaxies; of time-delay in a lensed quasar system; and of high redshift supernovae, and combined them to reduce the nooks and crannies in parameter space in which non-cosmological constant varieties of dark energy could be hiding. At least some alternative kinds of dark energy may still be possible, but for now Λ, the cosmological constant, rules.

Concerning Inflation. Very, very, very early in the life of the universe – so the theory of cosmic inflation goes – there was a period of dramatic expansion, and the tiny quantum fluctuations before inflation became the giant cosmic structures we see today. “Inflation predicts that the statistical distribution of primordial fluctuations is nearly a Gaussian distribution with random phases. Measuring deviations from a Gaussian distribution,” the team reports, “is a powerful test of inflation, as how precisely the distribution is (non-) Gaussian depends on the detailed physics of inflation.” While the limits on non-Gaussianity (as it is called), from analysis of the WMAP data, only weakly constrain various models of inflation, they do leave almost nowhere for cosmological models without inflation to hide.

Concerning ‘cosmic shadows’ (the Sunyaev-Zel’dovich (SZ) effect). While many researchers have looked for cosmic shadows in WMAP data before – perhaps the best known to the general public is the 2006 Lieu, Mittaz, and Zhang paper (the SZ effect: hot electrons in the plasma which pervades rich clusters of galaxies interact with CMB photons, via inverse Compton scattering) – the WMAP team’s recent analysis is their first to investigate this effect. They detect the SZ effect directly in the nearest rich cluster (Coma; Virgo is behind the Milky Way foreground), and also statistically by correlation with the location of some 700 relatively nearby rich clusters. While the WMAP team’s finding is consistent with data from x-ray observations, it is inconsistent with theoretical models. Back to the drawing board for astrophysicists studying galaxy clusters.

Seven Year Microwave Sky (Credit: NASA/WMAP Science Team)

I’ll wrap up by quoting Komatsu et al. “The standard ΛCDM cosmological model continues to be an exquisite fit to the existing data.”

Primary source: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation (arXiv:1001.4738). The five other Seven-Year WMAP papers are: Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Are There Cosmic Microwave Background Anomalies? (arXiv:1001.4758), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Planets and Celestial Calibration Sources (arXiv:1001.4731), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results (arXiv:1001.4744), Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters (arXiv:1001.4635), and Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Galactic Foreground Emission (arXiv:1001.4555). Also check out the official WMAP website.

Milky Way Has a “Squashed Beachball”-Shaped Dark Matter Halo

This illustration shows the visible Milky Way galaxy surrounded by a "squashed beachball"-shaped dark matter halo. Source: UCLA

This illustration shows the visible Milky Way galaxy surrounded by a “squashed beachball”-shaped dark matter halo. Source: UCLA

Our galaxy is shaped like a flat spiral right? Not if you’re talking about dark matter. Astronomers announced today that the Milky Way’s dark matter halo, which represents about 70% of the galaxy’s mass, is actually shaped like a squashed beachball.

Dark matter is completely invisible, but it still obeys the law of gravity, so the existence of dark matter haloes, and their shape, can be inferred by monitoring the orbits of dwarf galaxies orbiting the much larger Milky Way.

Unfortunately, to determine the orbit of an object, you have to measure its position at several points in that orbit, and dwarf galaxies take about a billion years to go around the Milky Way. Astronomers just haven’t been around long enough to watch even a fraction of a complete orbit. Luckily, they don’t have to.

Dwarf galaxies, just like their full-sized counterparts, and made of billions of stars. When the tidal forces from a big galaxy like the Milky Way act on a dwarf galaxy, the result is a streamer of stars that trace out the dwarf galaxy’s orbit. By using data from huge all-sky surveys, a group of astronomers led by David Law at UCLA were able to reconstruct the orbit of the Sagittarius Dwarf Galaxy. There was just one problem: different parts of the dwarf galaxy had different orbits, which led to wildly different dark matter halo shapes.

Law and his colleagues Steven Majewski (University of Virginia) and Kathryn Johnston (Columbia University) solved this problem by allowing models of the dark matter halo to be “triaxial” – in other words, have different lengths in all three dimensions. The best model solution results in a halo shaped like a beach ball that has been squashed sideways.

“We expected some amount of flattening based on the predictions of the best dark-matter theories,” said Law, “but the extent, and particularly the orientation, of the flattening was quite unexpected. We’re pretty excited about this, because it begs the question of how our galaxy formed in its present orientation.”

Sagittarius is not the only dwarf galaxy orbiting the Milky Way, and Law and his colleagues plan to study the orbits of other dwarf galaxies to refine their model. “It will be important to see if these results hold up as precise orbits are measured for more of these galaxies. In the meantime, such a squashed dark-matter halo is one of the best explanations for the observed data.”

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Click the image for a flyaround view. Source: UCLA

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Source: UCLA

New Results from the CDMS II Experiment

It’s no secret that astronomers claim that most of our universe is made of dark matter that cannot be readily detected. From Fritz Zwicky’s observations of the Coma clusters in the 1920’s which suggested that additional mass would be necessary to hold the cluster together, to the flat rotation curves of galaxies, to lensing in such places as the Bullet Cluster, all signs point to matter that neither emits nor absorbs any form of light we can detect. One possible solution was that this missing matter was ordinary, but cold matter floating around the universe. This form was called Massive astrophysical compact halo objects, or MACHOs, but studies to look for these came up relatively empty. The other option was that this dark matter was not so garden variety. It posed the idea of hypothetical particles which were very massive, but would only rarely interact. These particles were nicknamed WIMPs (for weakly interacting massive particles). But if these particles were so weakly interacting, detecting them would be a challenge.

An ambitious project, known as the Cryogenic Dark Matter Search, has been attempting to detect one of these particles since 2003. Today, they made a major announcement.

The experiment is located a half-mile underground in the Soudan mine in northern Minnesota. The detector is kept here to shield it from cosmic rays. The detectors are made from germanium and silicon which, if struck by a potential WIMP, will become ionized and resonate. The combination of these two features allow for the team to gain some insight as to what sort of particle it was that triggered the event. To further weed out false detections, the detectors are all cooled to just above absolute zero which prevents most of the “noise” caused by the random jittering of atoms thanks to their temperature.

Although the detector had not previously found signs for any dark matter they have provided understanding of the background levels to the degree that the team felt confident that they would be able to begin distinguishing true events. Despite this, false positives from neutron collisions have required the team to “throw out roughly 2/3 of the data that might contain WIMPs, because these data would contain too many background events.”

The most recent review of the data covered the 2007-2008 set. After carefully cleaning the data of as many false events and as much background noise as possible the team discovered that two detection events remained. The significance of these two detections was the result of today’s conference.

Although the presence of these two detections from 8/5 and 10/27 2007, could not be ruled out as genuine dark matter detections, the presence of only two detections was not statistically significant enough to be able to truly stand out from the background noise. As the summary of results from the team described it, “Typically there must be less than one chance in a thousand of the signal being due to background. In this case, a signal of about 5 events would have met those criteria.” As such, there is only a 1:4 probability that this was a true case of a detection of WIMPs.

Astronomer turned writer, Phil Plait put it slightly more succinctly in a tweet; “The CDMS dark matter talk indicates two signals, but they are not statistically strong enough to say “here be dark matter”. Damn.”

For more information:

Collaboration’s Website

Liveblogging of Conference by Cosmic Variance

Did Dark Matter Annihilate Our Early Universe?

A billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions.

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380,000 years after the Big Bang, the Universe cooled from being a hot soup of plasma, to a temperature where protons and electrons could combine to form atoms. This calm period of neutral hydrogen in universal history didn’t last for long however. The neutral hydrogen atoms were ripped apart once more, by a mechanism that would go on to reionize the entire Universe, a process that eventually ended a billion years after the Big Bang.

It is thought the first stars that formed prior to the reionisation epoch probably pumped out some fierce ultraviolet radiation, ionizing the neutral hydrogen, but a new (controversial) theory has been put forward. Did dark matter have a role to play in the reionisation the Universe?

As 85% of the Universe is composed of a type of matter we have yet to fully account for, it seems only natural that scientists would be looking into the possibility that dark matter had a role to play soon after the Big Bang. Although scientists are fairly confident that the reionisation period was driven by the emissions from the very first stars, there are some observational factors that could suggest dark matter annihilation might have had a role to play in the evolution of the Universe.

This is according to Dan Hooper and Alexander Belikov from Fermilab in Batavia, Illinois, in any case. In their theory recently published, the researchers examine the physics behind dark matter annihilation as the mechanism that drove the reionisation epoch.

reion_diagramIn Hooper and Belikov’s work, they focus on dark matter that is theorized to have clumped together under gravitational attraction as the Universe cooled during the neutral hydrogen era (known as the “Dark Ages” – the Universe would have been opaque due to lack of stars and lack of electromagnetic radiation). When the dark matter during this time clumped, it is predicted to annihilate. During dark matter annihilation, high energy gamma-rays are predicted to be generated. Where gamma-radiation goes, ionization of matter is sure to follow.

A single gamma ray might reionise 1000 hydrogen atoms,” says Hooper. “The mechanism could easily have reionised the universe.”

By their reasoning, rather than emissions from stars that may have been forming at the start of the reionisation epoch, a far more potent ionization mechanism could have flooded the Universe. However, some scientists are skeptical of this idea.

We have no evidence yet that any dark matter has ever annihilated,” says Charles Bennett, principal investigator on NASA’s WMAP satellite, which has been studying the reionisation epoch. “I am not saying it is wrong, but it sounds a bit too contrived for me to eagerly accept it.” Bennett sees the dark matter argument as one mystery (reionisation) being explained by another mystery (does dark matter even annihilate?).

For now, the idea that dark matter may have been the underlying mechanism ionizing our Universe remains highly theoretical. But Hooper is eager to study the data from ESA’s upcoming Planck mission as this observatory will be able to study how reionisation proceeded with time. “The time signature of dark matter reionisation will be different from that brought about by stars,” says Hooper.

Source: New Scientist

Dark Matter, Dark Energy; Now There’s “Dark Gulping”

The HST WFPC2 image of gravitational lensing in the galaxy cluster Abell 2218, indicating the presence of large amount of dark matter (credit Andrew Fruchter at STScI).

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For all you dark matter and dark energy fans out there, now there’s another new “dark” to add to the list. It’s called “dark gulping,” and it involves a process which may explain how supermassive black holes were able to form in the early universe. Astronomers from the University College of London (UCL) propose that dark gulping occurred when there were gravitational interactions between the invisible halo of dark matter in a cluster of galaxies and the gas embedded in the dark matter halo. This occurred when the Universe was less than a billion years old. They found that the interactions cause the dark matter to form a compact central mass, which can be gravitationally unstable, and collapse. The fast dynamical collapse is the dark gulping.

Dr. Curtis Saxton and Professor Kinwah Wu, both of UCL’s Mullard Space Science Laboratory, developed a model to study the process. They say that the dark gulping would have happened very rapidly, without a trace of electro-magnetic radiation being emitted.

There are several theories for how supermassive black holes form. One possibility is that a single large gas cloud collapses. Another is that a black hole formed by the collapse of a giant star swallows up enormous amounts of matter. Still another possibility is that a cluster of small black holes merge together. However, all these options take many millions of years and are at odds with recent observations that suggest that black holes were present when the Universe was less than a billion years old. Dark gulping may provide a solution to how the slowness of gas accretion was circumvented, enabling the rapid emergence of giant black holes. The affected dark mass in the compact core is compatible with the scale of supermassive black holes in galaxies today.

Dark matter appears to gravitationally dominate the dynamics of galaxies and galaxy clusters. However, there is still a great deal of conjecture about origin, properties and distribution of dark particles. While it appears that dark matter doesn’t interact with light, it does interacts with ordinary matter via gravity. “Previous studies have ignored the interaction between gas and the dark matter,” said Saxton, “but, by factoring it into our model, we’ve achieved a much more realistic picture that fits better with observations and may also have gained some insight into the presence of early supermassive black holes.”?

According to the model, the development of a compact mass at the core is inevitable. Cooling by the gas causes it to flow gently in towards the center. The gas can be up to 10 million degrees at the outskirts of the halos, which are few million light years in diameter, with a cooler zone towards the core, which surrounds a warmer interior a few thousand light years across. The gas doesn’t cool indefinitely, but reaches a minimum temperature, which fits well with X-ray observations of galaxy clusters.

The model also investigates how many dimensions the dark particles move in, as these determine the rate at which the dark halo expands and absorbs and emits heat, and ultimately affect the distribution of dark mass the system.

“In the context of our model, the observed core sizes of galaxy cluster halos and the observed range of giant black hole masses imply that dark matter particles have between seven and ten degrees of freedom,”?said Saxton. ?”With more than six, the inner region of the dark matter approaches the threshold of gravitational instability, opening up the possibility of dark gulping taking place.?

The findings have been published in the Monthly Notices of the Royal Astronomical Society.

Source: RAS

Astrophysics Satellite Detects Dark Matter Clue?

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An international collaboration of astronomers is reporting an unusual spike of atmospheric particles that could be a long-sought signature of dark matter.

The orbiting PAMELA satellite, an astro physics mission operated by Italy, Russia, Germany and Sweden, has detected a  glut of positrons — antimatter counterparts to electrons — in the energy range theorized to be associated with the decay of dark matter. The results appear in this week’s issue of the journal Nature.

Dark matter is the unseen substance that accounts for most of the mass of our universe, and the presence of which can be inferred from gravitational effects on visible matter. When dark matter particles are annihilated after contact with anti-matter, they should yield a variety of subatomic particles, including electrons and positrons.

Antiparticles account for a small fraction of cosmic rays and are also known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium, which is referred to as a ‘secondary source.” 

Previous statistically limited measurements of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron-positron flux at energies between 300 and 600 GeV. Primary sources could include pulsars, microquasars or dark matter annihilation. 

Lead study author Oscar Adriani, an astrophysics researcher at the University of Florence in Italy, and his colleagues are reporting a positron to electron ratio that systematically increases in a way that could indicate dark matter annihilation.

The new paper reports a measurement of the positron fraction in the energy range 1.5–100GeV.

“We find that the positron fraction increases sharply over much of that range, in a way that appears to be completely inconsistent with secondary sources,” the authors wrote in the Nature paper. “We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.” Another feasible source for the anitmatter particles, besides dark matter annihilation, could be a pulsar, they note.

PAMELA, which stands for a Payload for Antimatter Matter Exploration and Light Nuclei Astrophysics, was launched in June 2006 and initially slated to last three years. Mission scientists now say it will continue to collect data until at least December 2009, which will help pin down whether the positrons are coming from dark matter anihilation or a single, nearby source.

Source: Nature (there is also an arXiv/astro-ph version here)