ESO: New study finds mysterious lack of dark matter

[Doum]

The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

http://www.eso.org/public/news/eso1217/

[Chris Peterson]

The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

Of course, not knowing the properties of dark matter, we don't know if we could ever detect particles directly. But it would be a shame if its dynamics are such that nearly all is in the galactic halo, with little around stars. If it isn't close, we obviously are going to have problems with direct detection. Maybe we'll figure out how to create it in an accelerator. We know about lots of particles that are never observed in the wild.

[neufer]

The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

Of course, not knowing the properties of dark matter, we don't know if we could ever detect particles directly. But it would be a shame if its dynamics are such that nearly all is in the galactic halo, with little around stars. If it isn't close, we obviously are going to have problems with direct detection. Maybe we'll figure out how to create it in an accelerator. We know about lots of particles that are never observed in the wild.

<<If the dark matter within our galaxy is made up of Weakly Interacting Massive Particles (WIMPs), then thousands of WIMPs must pass through every square centimeter of the Earth each second. There are many experiments currently running, or planned, aiming to test this hypothesis by searching for WIMPs. These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of WIMP annihilations.

An alternative approach to the detection of WIMPs in nature is to produce them in the laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect WIMPs produced in collisions of the LHC proton beams. Because a WIMP has negligible interactions with matter, it may be detected indirectly as (large amounts of) missing energy and momentum which escape the LHC detectors, provided all the other (non-negligible) collision products are detected. These experiments could show that WIMPs can be created, but it would still require a direct detection experiment to show that they exist in sufficient numbers in the galaxy to account for dark matter.

Direct detection experiments

Direct detection experiments typically operate in deep underground laboratories to reduce the background from cosmic rays. These include: the Soudan mine; the SNOLAB underground laboratory at Sudbury, Ontario (Canada); the Gran Sasso National Laboratory (Italy); the Canfranc Underground Laboratory (Spain); the Boulby Underground Laboratory (UK); and the Deep Underground Science and Engineering Laboratory, South Dakota (US). The majority of present experiments use one of two detector technologies: cryogenic detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect the flash of scintillation light produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: CDMS, CRESST, EDELWEISS, EURECA. Noble liquid experiments include ZEPLIN, XENON, DEAP, ArDM, WARP and LUX. Both of these detector techniques are capable of distinguishing background particles which scatter off electrons, from dark matter particles which scatter off nuclei. Other experiments include SIMPLE and PICASSO.

The DAMA/NaI, DAMA/LIBRA experiments have detected an annual modulation in the event rate, which they claim is due to dark matter particles. (As the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount depending on the time of year). This claim is so far unconfirmed and difficult to reconcile with the negative results of other experiments assuming that the WIMP scenario is correct.

Directional detection of dark matter is a search strategy based on the motion of the Solar System around the galactic center. By using a low pressure TPC, it is possible to access information on recoiling tracks (3D reconstruction if possible) and to constrain the WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun is travelling (roughly in the direction of the Cygnus constellation) may then be separated from background noise, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

On 17 December 2009 CDMS researchers reported two possible WIMP candidate events. They estimate that the probability that these events are due to a known background (neutrons or misidentified beta or gamma events) is 23%, and conclude "this analysis cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as signal."

More recently, on 4 September 2011, researchers using the CRESST detectors presented evidence of 67 collisions occurring in detector crystals from sub-atomic particles, calculating there is a less than 1 in 10,000 chance that all were caused by known sources of interference or contamination. It is quite possible then that many of these collisions were caused by WIMPs, and/or other unknown particles.

Click to play embedded YouTube video.

Wimps may be vewy vewy heavwy

Indirect detection experiments

Indirect detection experiments search for the products of WIMP annihilation. If WIMPs are Majorana particles (the particle and antiparticle are the same) then two WIMPs colliding could annihilate to produce gamma rays or particle-antiparticle pairs. This could produce a significant number of gamma rays, antiprotons or positrons in the galactic halo. The detection of such a signal is not conclusive evidence for dark matter, as the production of gamma rays from other sources are not fully understood.

The EGRET gamma ray telescope observed more gamma rays than expected from the Milky Way, but scientists concluded that this was most likely due to an error in estimates of the telescope's sensitivity. The Fermi Gamma-ray Space Telescope, launched June 11, 2008, is searching for gamma ray events from dark matter annihilation. At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies and in clusters of galaxies.

The PAMELA experiment (launched 2006) has detected a larger number of positrons than expected. These extra positrons could be produced by dark matter annihilation, but may also come from pulsars. No excess of anti-protons has been observed.

A few of the WIMPs passing through the Sun or Earth may scatter off atoms and lose energy. This way a large population of WIMPs may accumulate at the center of these bodies, increasing the chance that two will collide and annihilate. This could produce a distinctive signal in the form of high-energy neutrinos originating from the center of the Sun or Earth. It is generally considered that the detection of such a signal would be the strongest indirect proof of WIMP dark matter. High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.

WIMP annihilation from the Milky Way Galaxy as a whole may also be detected in the form of various annihilation products. The Galactic center is a particularly good place to look because the density of dark matter may be very high there.>>

[bystander]

Serious Blow to Dark Matter Theories?
Europeans Southern Observatory | 2012 Apr 18

New study finds mysterious lack of dark matter in Sun’s neighbourhood

[imghover=http://www.eso.org/public/archives/imag ... o1217a.jpg]http://www.eso.org/public/archives/imag ... o1217b.jpg[/imghover]Artist’s impression of the expected dark matter distribution around the Milky Way.
(Credit: ESO/L. Calçada)

---

The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.

A team using the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, along with other telescopes, has mapped the motions of more than 400 stars up to 13 000 light-years from the Sun. From this new data they have calculated the mass of material in the vicinity of the Sun, in a volume four times larger than ever considered before.

“The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun,” says team leader Christian Moni Bidin (Departamento de Astronomía, Universidad de Concepción, Chile). “But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!”

Dark matter is a mysterious substance that cannot be seen, but shows itself by its gravitational attraction for the material around it. This extra ingredient in the cosmos was originally suggested to explain why the outer parts of galaxies, including our own Milky Way, rotated so quickly, but dark matter now also forms an essential component of theories of how galaxies formed and evolved.

Today it is widely accepted that this dark component constitutes about the 80% of the mass in the Universe ^[1], despite the fact that it has resisted all attempts to clarify its nature, which remains obscure. All attempts so far to detect dark matter in laboratories on Earth have failed.

By very carefully measuring the motions of many stars, particularly those away from the plane of the Milky Way, the team could work backwards to deduce how much matter is present ^[2]. The motions are a result of the mutual gravitational attraction of all the material, whether normal matter such as stars, or dark matter.

Astronomers’ existing models of how galaxies form and rotate suggest that the Milky Way is surrounded by a halo of dark matter. They are not able to precisely predict what shape this halo takes, but they do expect to find significant amounts in the region around the Sun. But only very unlikely shapes for the dark matter halo — such as a highly elongated form — can explain the lack of dark matter uncovered in the new study ^[3].

The new results also mean that attempts to detect dark matter on Earth by trying to spot the rare interactions between dark matter particles and “normal” matter are unlikely to be successful.

“Despite the new results, the Milky Way certainly rotates much faster than the visible matter alone can account for. So, if dark matter is not present where we expected it, a new solution for the missing mass problem must be found. Our results contradict the currently accepted models. The mystery of dark matter has just become even more mysterious. Future surveys, such as the ESA Gaia mission, will be crucial to move beyond this point.” concludes Christian Moni Bidin.

1. Notes
   
   [*] According to current theories dark matter is estimated to constitute 83% of the matter in the Universe with the remaining 17% in the form of normal matter. A much larger amount of dark energy also seems present in the Universe, but is not expected to affect the motions of the stars within the Milky Way.
   
   [*] The observations were made using the FEROS spectrograph on the MPG/ESO 2.2-metre telescope, the Coralie instrument on the Swiss 1.2-metre Leonhard Euler Telescope, the MIKE instrument on the Magellan II Telescope and the Echelle Spectrograph on the Irene du Pont Telescope. The first two telescopes are located at ESO’s La Silla Observatory and the latter two telescopes are located at the Las Campanas Observatory, both in Chile. A total of more than 400 red giant stars at widely differing heights above the plane of the galaxy in the direction towards the south galactic pole were included in this work.
   
   [*] Theories predict that the average amount of dark matter in the Sun’s part of the galaxy should be in the range 0.4-1.0 kilograms of dark matter in a volume the size of the Earth. The new measurements find 0.00±0.07 kilograms of dark matter in a volume the size of the Earth.[/i]

Kinematical and Chemical Vertical Structure of the Galactic Thick Disk. II. A Lack of Dark Matter in the Solar Neighborhood - C. Moni Bidin et al

• Astrophysical Journal 751(1) 30 (2012 May 20) DOI: 10.1088/0004-637X/751/1/30
  arXiv.org > astro-ph > arXiv:1204.3924 > 17 Apr 2012

Kinematical and Chemical Vertical Structure of the Galactic Thick Disk. I. Thick Disk Kinematics - C. Moni Bidin et al

• Astrophysical Journal 747(2) 101 (2012 Mar 10) DOI: 10.1088/0004-637X/747/2/101
  arXiv.org > astro-ph > arXiv:1202.1799 > 08 Feb 2012

The Case of the Missing Dark Matter
Universe Today | Jason Major | 2012 Apr 18

Has Dark Matter Gone Missing?
Science NOW | Adrian Cho | 2012 Apr 19

[Doum]

Of course dark matter might be present around us. But from that study, there ain't a lot because they dont see any gravitational effect from it. So detection of dark matter around us is still a possibility even if there ain't a lot (or none!!??). Time and study will tell.

[bystander]

Researchers Bring Dark Matter Back
Universe Today | Jason Major | 2012 May 21

[i]The Milky Way and Moon Rise over ESO's Paranal Observatory. (Credit: ESO/H.H. Heyer)[/i] [url=http://www.eso.org/public/images/vlt-mw-potw/][b][i](vlt-mw-potw)[/i][/b][/url]

---

---

Recent reports of dark matter’s demise may be greatly exaggerated, according to a new paper from researchers at the Institute for Advanced Study.

Astronomers with the European Southern Observatory announced in April a surprising lack of dark matter in the galaxy within the vicinity of our solar system.

The ESO team, led by Christian Moni Bidin of the Universidad de Concepción in Chile, mapped over 400 stars near our Sun, spanning a region approximately 13,000 light-years in radius. Their report identified a quantity of material that matched what could be directly observed: stars, gas, and dust… but no dark matter.

“Our calculations show that it should have shown up very clearly in our measurements,” Bidin had stated, “but it was just not there!”

But other scientists were not so sure about some assumptions the ESO team had based their calculations upon.

Researchers Jo Bovy and Scott Tremaine from the Institute for Advanced Study in Princeton, NJ, have submitted a paper claiming that the results reported by Moni Biden et al are “incorrect”, and based on an “invalid assumption” of the motions of stars Biden’s team had used to determine the presence of non-baryonic material — that is, dark matter.

“The main error is that they assume that the mean azimuthal (or rotational) velocity of their tracer population is independent of Galactocentric cylindrical radius at all heights,” Bovy and Tremaine state in their paper. “This assumption is not supported by the data, which instead imply only that the circular speed is independent of radius in the mid-plane.”

In addition, the stars within the local neighborhood move slower than the average velocity assumed by the ESO team, in a behavior called asymmetric drift. This lag varies with a cluster’s position within the galaxy, but, according to Bovy and Tremaine, “this variation cannot be measured for the sample [used by Moni Biden's team] as the data do not span a large enough range.”

[i]Artist's impression of dark matter surrounding the Milky Way. (Credit: ESO/L. Calçada)[/i] [url=http://www.eso.org/public/news/eso1217/][b][i](eso1217)[/i][/b][/url]

---

---

When the IAS researchers took Moni Biden’s observations but replaced the ESO team’s “invalid” assumptions on star movement within and above the galactic plane with their own “data-driven” ones, the dark matter reappeared.

“Our analysis shows that the locally measured density of dark matter is consistent with that extrapolated from halo models constrained at Galactocentric distances,” Bovy and Tremaine report.

As such, the dark matter that was thought to be there, is there. (According to the math, that is.)

And, the two researchers add, it’s not only there but it’s there in denser amounts than average — at least in the area around our Sun.

“The halo density at the Sun, which is the relevant quantity for direct dark matter detection experiments, is likely to be larger because of gravitational focusing by the disk,” Bovy and Tremaine note.

When they factored in their data-driven calculations on stellar velocities and the movement of the halo of non-baryonic material that is thought to envelop the Milky Way, they found that “the dark matter density in the mid-plane is enhanced… by about 20%.”

So rather than a “serious blow” to the existence of dark matter, the findings by Bovy and Tremaine — as well as Moni Biden and his team — may have not only found dark matter, but given us 20% more!

Now that’s a good value.

(Read: Astronomers Witness a Web of Dark Matter) — http://asterisk.apod.com/viewtopic.php?t=26428

On the local dark matter density - Jo Bovy, Scott Tremaine (IAS)

• arXiv.org > astro-ph > arXiv:1205.4033 > 17 May 2012

Dark Matter: Still Existing (One in a Continuing Series)
Discover Blogs | Cosmic Variance | Sean Carroll | 2012 May 23

[bystander]

Plenty of dark matter near the Sun
Royal Astronomical Society | 2012 Aug 09

[attachment=0]mw_hr_00260_disk.jpg[/attachment][/i]

---

Astronomers at the University of Zürich, the ETH Zurich, the University of Leicester and NAOC Beijing have found large amounts of invisible "dark matter" near the Sun. Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive "halo" of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations. The authors also find tantalising hints of a new dark matter component in our Galaxy. The team's results will be published in the journal Monthly Notices of the Royal Astronomical Society.

Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s. He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart. At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone. In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighbourhood has remained more mysterious. For decades after Oort's measurement, studies found 3-6 times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The community was left puzzled, generally believing that the observations and analyses simply weren't sensitive enough to perform a reliable measurement.

In this latest study, the authors are much more confident in their measurement and its uncertainties. This is because they used a state-of-the-art simulation of our Galaxy to test their mass-measuring technique before applying it to real data. This threw up a number of surprises. They found that standard techniques used over the past 20 years were biased, always tending to underestimate the amount of dark matter. They then devised a new unbiased technique that recovered the correct answer from the simulated data. Applying their technique to the positions and velocities of thousands of orange K dwarf stars near the Sun, they obtained a new measure of the local dark matter density.

Lead author Silvia Garbari says: "We are 99% confident that there is dark matter near the Sun. In fact, our favoured dark matter density is a little high. There is a 10% chance that this is merely a statistical fluke. But with 90% confidence, we find more dark matter than expected. If future data confirms this high value, the implications are exciting. It could be the first evidence for a "disc" of dark matter in our Galaxy, as recently predicted by theory and numerical simulations of galaxy formation. Or it could be that the dark matter halo of our Galaxy is squashed, boosting the local dark matter density."

Many physicists are placing their bets on dark matter being a new fundamental particle that interacts only very weakly with normal matter -- but strongly enough to be detected in experiments deep underground where confusing cosmic ray events are screened by over a kilometre of solid rock.

An accurate measure of the local dark matter density is vital for such experiments as co-author Prof. George Lake explains: "If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time your finish reading this article. Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of."

A new determination of the local dark matter density from the kinematics of K dwarfs - Silvia Garbari et al

• arXiv.org > astro-ph > arXiv:1206.0015 > 31 May 2012 (v1), 23 Jul 2012 (v2)

Gamma-Ray Glow Hints at Dark Matter in the Center of Our Galaxy
Science NOW | Adrian Cho | 2012 July 27

[i][b]Leftovers[/b]. Researchers began with a map of the gamma-ray emissions from near the galactic center (left) and subtracted the contributions from known sources (white circles) and other backgrounds to produce a map of emissions that could come from dark matter. (Credit: Kevork Abazajian/University of California, Irvine)[/i]

---

---

The coming decade will be the decade of dark matter, some scientists say, as efforts to detect the mysterious stuff will either pay off or rule out the most promising hypothesis about what it is. But astronomers may have already detected signs of dark matter in the heart of our own Milky Way galaxy, a pair of astrophysicists now says.

Data from NASA's space-borne Fermi Gamma-ray Space Telescope reveal an excess of gamma-rays coming from the galactic center that could be produced as particles of dark matter annihilate one another, Kevork Abazajian and Manoj Kaplinghat of the University of California, Irvine, report in a paper posted to the arXiv preprint server. "There's definitely some source there, and it fits with the dark matter interpretation," Abazajian says. But other researchers say the excess could be an artifact of the way Abazajian and Kaplinghat model the gamma-ray flux, or it could originate from more-mundane sources.

Astronomers have ample evidence that dark matter provides most of the gravity that keeps stars from flying out of the galaxies. And cosmologists have shown that it makes up 85% of all matter in the universe. But physicists don't know what dark matter is.

The leading hypothesis is that dark matter could be made up of weakly interacting massive particles, or WIMPs, which are predicted by some theories. WIMPs would be massive enough to produce lots of gravity but would otherwise interact with ordinary matter only very weakly. Each galaxy would form within a vast cloud of WIMPs.

Physicists are searching for WIMPs in several ways. Some are trying to spot them using exquisitely sensitive underground detectors. Others hope to produce WIMPs at the world's largest atom smasher, the Large Hadron Collider in Switzerland. WIMPs might also annihilate one another when they collide to produce ordinary particles such as gamma rays, and astrophysicists are combing the heavens for signs of such annihilations.

Abazajian and Kaplinghat say that the more than 400 researchers working with the Fermi satellite may have already found that evidence. The two theorists analyzed data collected between August 2008 to June 2012, focusing on a 7-degree-by-7-degree patch of sky around the galactic center. For each of four energy ranges, they mapped the emission across the sky. They fit each map with a "baseline model" that included 17 point-like sources of gamma rays that Fermi had already found in that area, a "diffuse" background that accounts for the general emission from the galactic center, and a spatially uniform background.

They then fit the data with another model that included a contribution from dark matter annihilations, including theoretical estimates of the dark matter's distribution and how the particle annihilations produce gamma rays. Adding the dark matter annihilations greatly improved the fit, they found, suggesting that there is an excess of gamma rays that come from dark matter.

Other researchers, including Daniel Hooper of Fermi National Accelerator Laboratory in Batavia, Illinois, have made similar claims. In fact, Abazajian had previously argued against that interpretation. But the new analysis shows that the dark-matter hypothesis fits the data in three key ways, Abazajian says: It has the right energy distribution, the right spatial distribution, and the right intensity. "When I saw that I was like, 'Holy cow!' " he says. Abazajian cautions, however, that the gamma rays could emanate from a less exotic source, such as previously undetected pulsars.

They might also be explained in an even easier way, says Stefano Profumo, a theoretical astrophysicist at the University of California, Santa Cruz, and a member of the Fermi-satellite team. Abazajian and Hooper's analyses depend critically on the model of the diffuse galactic background, Profumo says. That model had been derived to describe a much bigger area around the galactic center, he says, and is "completely blind to the details at the galactic center." So its use the fits to the data could produce misleading results, he cautions. Still, Profumo agrees that the galactic center is a prime place to look for evidence of dark matter.

Detection of a Gamma-Ray Source in the Galactic Center Consistent with Extended Emission from
Dark Matter Annihilation and Concentrated Astrophysical Emission - Kevork N. Abazajian, Manoj Kaplinghat

• arXiv.org > astro-ph > arXiv:1207.6047 > 25 Jul 2012

On The Origin Of The Gamma Rays From The Galactic Center - Dan Hooper, Tim Linden

• arXiv.org > astro-ph > arXiv:1110.0006 > 30 Sep 2011

The Consistency of Fermi-LAT Observations of the Galactic Center with a
Millisecond Pulsar Population in the Central Stellar Cluster - Kevork N. Abazajian

• arXiv.org > astro-ph > arXiv:1011.4275 > 18 Nov 2010 (v1), 28 Feb 2011 (v3)