Category Archives: Dark Matter

April 11, 2012

Fermi Gamma-ray Telescope Search for Dark Matter

By darkmatterdarkenergy

Dwarf Galaxy in Fornax, Credit: ESO/Digital Sky Survey 2

All of our evidence for dark matter is indirect, that is, we deduce the existence of substantial amounts of dark matter – exceeding the amount of ‘ordinary matter’ by 5 times – from its gravitational effects at large distances, on the scale of galaxies, clusters of galaxies, and even across regions of a billion light-years in size.

The generally favored hypothesis is that dark matter is composed of some sort of new weakly interacting massive particle (WIMP). Such a WIMP would not interact via the electromagnetic force or the strong nuclear force, and thus is very hard to detect directly. There are a number of experiments underway attempting to directly detect the elusive particle. The majority of these are earthbound experiments wherein ordinary matter in crystalline form is used as the detector. Three of these experiments are in fact claiming statistically significant detection rates, the DAMA/Libra experiment in Italy, the CRESST experiment based in Germany, and the COGENT experiment in the U.S.

In this class of experiments one is looking to detect a collision of a dark matter particle with an ordinary matter particle, and if detection is successful, to determine the mass of the dark matter particle, as well as its cross-section for collision with ordinary matter. The cross-section is a way of measuring how close the ordinary matter particle and dark matter particle have to approach each other to have a collision event. A collision event produces decay products (additional particles) that the experiments are then able to detect. The results of the 3 experiments named above are still highly controversial, as a number of other similar experiments are not confirming detection, but collectively they may indicate detection of WIMPs with mass in a range around 5 to 40 times the mass of a proton.

Another approach to more directly detecting dark matter (i.e., not through its gravitational effects) is to look for dark matter particles colliding with one another. The number of such events occurring at the Earth’s surface is expected to be quite low, so one must look into the cosmos. Recently some scientists calculated that about one dark matter particle a month on average strikes an atom inside a human on Earth (but not to worry, other background radiation to which we are exposed is much more significant). But we are in a region of over-concentration of ordinary matter; dark matter is spread out on the largest scales. We need to examine much bigger regions to observe dark matter particles striking one another. But look for what, and how?

NASA’s orbiting Fermi Gamma-ray Telescope provides one way. When two dark matter particles collide, depending on the nature of the dark matter particle (is it its own anti-particle), they can possibly mutually annhilate and produce very energetic gamma-ray photons. Gamma rays are at the most energetic end of the electromagnetic spectrum, which includes X-rays, visible light, and radio waves.

Recently the LAT, or Large Area Telescope, which is the main instrument on board the Fermi Gamma-ray Telescope (in orbit) reported on results based on two years of searching for gamma ray production from dark matter annihilation. The method used was to monitor 10 dwarf galaxies that are gravitationally bound to our Milky Way galaxy.

Take a look at this video from NASA Goddard: No WIMPS in Space?

Dwarf galaxies are thought to be good candidates for this type of dark matter search, as they are the remnants of so-called dark matter halos that may have been the first large-scale gravitationally bound objects to form. Larger galaxies in turn grew from multiple such halos coming together, but today’s remaining dwarf galaxies did not get caught up in significant merger activity. They also have mature stellar populations, so there is not a lot of ongoing activity with respect to supernovae or black holes that would produce gamma rays from these other causes.

The international team using the Fermi telescope looked for gamma rays with energies up to 100 billion electron volts (about 100 times the rest mass energy equivalent of a proton), but did not find any that could be clearly attributed to dark matter. The search will continue, gathering more statistics over time and adding additional target dwarf galaxies to their measurements, in the hopes of either finding dark matter through this method, or putting more constraints on its properties.

References:

http://www.cresst.de/darkmatter.php – CRESST experiment

http://cogent.pnnl.gov/ – COGENT experiment

http://people.roma2.infn.it/~dama/web/home.html – DAMA/LIBRA experiment

http://en.wikipedia.org/wiki/Fermi_Gamma-ray_Space_Telescope

http://en.wikipedia.org/wiki/Dark_matter#Direct_detection_experiments

http://www.nasa.gov/mission_pages/GLAST/news/dark-matter-insights.html – Fermi Gamma-ray telescope

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January 15, 2012

Einstein Rings and Dark Matter

By darkmatterdarkenergy

Horseshoe-shaped Einstein Ring, Image credit: ESA/Hubble and NASA

Just one hundred years ago, in 1912, Albert Einstein predicted that stars would bend the paths of light in their neighborhood. And in 1919 a team lead by Arthur Eddington confirmed this effect with observations made during a solar eclipse expedition, thus providing confirmation of general relativity (which was published by Einstein in 1915). In 1936 Einstein wrote a paper that showed stars could act as lenses through this gravitational bending of light. The basic idea is that you have a foreground star (or it could as well be a galaxy) and a background star (or galaxy) along nearly the same line of sight.

The effect is much easier to observe with galaxies due to their large masses. As the light travels from the background (more distant) galaxy towards the Earth, it passes very near to the foreground (less distant) galaxy, whose gravitational field causes the light path to bend in accordance with the relativistic curvature of space-time in the neighborhood of the foreground galaxy.

The picture from the Hubble telescope is a very beautiful example of an Einstein ring that has a horseshoe shape. It’s nearly a complete ring, because of the very close alignment of the two galaxies along the line of sight. The foreground galaxy is a large red galaxy, with a mass about 10 times that of our own Milky Way galaxy, and the background source galaxy, whose image is distorted into a horseshoe shape surrounding the red galaxy, is blue in color.

The size of the ring depends on the square root of the mass of the foreground galaxy that acts as a lens. This includes both the ordinary matter and the dark matter associated with the galaxy. Einstein rings thus act as important probes of the distribution of dark matter in the universe.

References:
http://en.wikipedia.org/wiki/Einstein_ring

http://www.spacetelescope.org/images/potw1151a/

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June 16, 2011

Do we have a CoGeNT direct detection of Dark Matter?

By darkmatterdarkenergy
CoGeNT detector during installation

CoGeNT detector during installation (Credit: Pacific Northwest National Laboratories)

(cogent = clear, logical, convincing)

The race to demonstrate direct detection of WIMP (weakly interacting massive particle) dark matter is heating up with this month’s release of results from the CoGeNT experiment, located in a mine in northern Minnesota*. They have just published results collected during the first 15 months of data taking. CoGeNT, as the Ge in the name indicates, uses a detector made of germanium.

There are quite a few such experiments that seek to measure the impact of WIMP dark matter as it directly strikes nucleons, that is, protons and neutrons, in some target material. The cross sections expected for such direct impact are extremely low, thus the experiments require relatively large detectors, high sensitivity, and long runs to gather sufficient statistical evidence of impacts and separate good events from background events due to other causes. The most favored candidate is a WIMP with mass somewhat under 10 GeV to perhaps as high as 200 GeV (the proton rest mass is .938 GeV, a GeV is a billion electron volts, and the mass is stated in energy equivalent units).

While XENON, CDMS (located in the same Soudan laboratory in Minnesota) and other direct experiments have not detected dark matter, for a number of years the DAMA/LIBRA project in Italy has been claiming the detection of an annual modulation of a dark matter signal. The modulation is said to be due to movement of the Earth toward and then away from the galactic center as it orbits the Sun each year, with the signal peaking in the second quarter of the year.

The CoGeNT experiment is also now claiming a detection of an annual modulation with about 2.7 or 2.8 sigma (standard deviations) of statistical significance, which is at the margin of a good detection. DAMA/LIBRA, which uses a thallium-doped sodium iodide crystal (salt) detector, claims a very high statistical significance of 8.9 sigma. Generally, 3 sigma of significance is considered sufficient for a good detection and 5 sigma would be considered a solid detection. The DAMA/LIBRA events have until now been unconfirmed, and have appeared to be in conflict with limits from other experiments including XENON and CDMS.

The CoGeNT results are consistent with DAMA/LIBRA in two respects. First, they together imply a relatively low mass of 5 to 12 GeV for the dark matter WIMP. Second, both the CoGeNT and DAMA experiments are consistent with an annual modulation peak occurring sometime between late April and the end of May, as is expected based on the Earth’s orbit combined with the Sun’s movement relative to the galactic center.

While the CDMS results appear to set limits which contradict both the CoGeNT and DAMA results, there are a number of uncertainties in the actual sensitivity of the respective experiments that may allow resolution of the apparent discrepancy.

We eagerly await further results from CoGeNT and from other experiments including CRESST and COUPP that are well suited to measurement of a relatively low mass WIMP particle such as CoGeNT is claiming to have detected.

*The mine is located in a state park, and tours down into the mine run during the summer months. It is also near to the beautiful Boundary Waters Canoe area that crosses into Canada, where I took a ten day canoe excursion as a Boy Scout, decades ago.

References:


http://www.spacedaily.com/reports/New_data_still_have_scientists_in_dark_over_dark_matter_999.html

http://cogent.pnnl.gov/

C. Aalseth et al. 2011, “Search for an Annual Modulation in a P-type Point Contact Germanium Dark Matter Detector” http://arXiv.org/pdf/1106.0650

D. Hooper and C. Kelso 2011 “Implications of CoGeNT’s New Results for Dark Matter” http://arXiv.org/pdf/1106.1066v1

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June 4, 2011

Dark Matter Powered Stars

By darkmatterdarkenergy
Gamma Ray Burster 070125

GRB (gamma ray burster) 070125. Credit: B. Cenko, et al. and the W. M. Keck Observatory.

So what is a “dark star”? It is not a Newtonian black hole as suggested by John Michell in the 18th century who used the term while postulating that gravity could prevent light escaping from a very massive, compact star. It is not a “dark energy star”, which is related to a black hole, but rather than having a singularity at the center, quantum effects cause infalling matter to be converted to vacuum state energy, dark energy. It is not a comic book, science fiction comedy film, or Grateful Dead song.

In this blog entry we are writing about dark matter powered stars. These would be the very first stars, formed within the first few hundred million years of the universe’s existence. The working assumption is that dark matter consists of WIMPs – weakly interacting massive particles, in particular the favored candidate is the neutralino. The neutralino is the lightest particle among the postulated supersymmetric companions to the Standard Model suite of particles. As such it would be stable, would not ordinarily decay and is being searched for with XENON, CDMS, DAMA, AMS-02 and many other experiments.

The first stars are thought by astrophysicists to have been formed from clouds of ordinary hydrogen and helium as well as dark matter, with dark matter accounting for 5/6 of the total mass. These clouds, called “dark matter halos” are considered to have contained from about one million to 100 million times the Sun’s mass. The ordinary matter would settle towards the center as it cooled via radiation processes and the dark matter (which does not radiate) would be more diffuse. The stars forming at the center would be overwhelmingly composed of ordinary matter (hydrogen and helium nuclei and electrons).

Without any dark matter at all, ordinary matter stars up to about 120 to 150 solar masses could form; above this limit they would have very hot surfaces and their own radiation would inhibit further growth by infall of matter from the halo. But if as little as one part in a thousand of the protostar’s mass was in the form of dark matter this limitation goes away. The reason is that the neutralino WIMPs will, from time to time, meet one another inside the star and mutually annihilate since the neutralino is its own anti-particle. The major fraction of the energy produced in the annihilation remains inside the star, but some escapes in the form of neutrinos (not neutralinos).

Annihilation of these neutralinos is a very efficient heating mechanism throughout the volume of the star, creating a great amount of heat and pressure support, basically puffing up the star to a very large size. The stellar surface is, as a result, much cooler than in the no dark matter case, radiation pressure is insignificant, and accretion of significantly more material onto the star can occur. Stars could grow to be 1000 solar masses, or 10,000 solar masses, potentially even up to one million solar masses. Their sizes would be enormous while they were in the dark matter powered phase. Even the relatively small 1000 solar mass star, if placed at the Sun’s location, would extend through much of our Solar System, beyond the orbit of Saturn.

We have mentioned the neutralino meets neutralino annihilation mechanism. A second mechanism for heating the interior of the star would be direct impact of neutralinos onto protons and helium nuclei. This second mechanism could help sustain the duration as a dark matter powered star potentially even beyond a billion years.

Eventually the dark matter fuel would be exhausted, and the heat and pressure support from this source lost. The star would then collapse until the core was hot enough for nuclear fusion burning. Stars of 1000 solar masses would burn hydrogen, and later helium, and evolve extremely rapidly because of the high density and temperature in their cores. After their hydrogen and helium fusion cycles completed there would be no source of sufficient pressure support and they would collapse to black holes (or maybe dark energy stars).

It is calculated with detailed simulations that the dark star mechanism allows for much more massive stars than could be formed otherwise, and this provides a potentially natural explanation for the creation of massive black holes. Our own Milky Way has a black hole around 3 million solar masses at its center, and it appears a majority of galaxies have large black holes. The image at the top of this blog is of a gamma ray burst detection that may have come from a large black hole formation event.

References:

en.wikipedia.org/wiki/Dark_star_(dark_matter)

Freese, K. et al 2008, Dark Stars: the First Stars in the Universe may be powered by Dark Matter Heating, http://arxiv.org/pdf/0812.4844v1

Freese, K. et al 2010, Supermassive Dark Stars, http://arxiv.org/abs/1002.2233

http://news.discovery.com/space/did-dark-stars-spawn-supermassive-black-holes.html

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May 7, 2011

Direct Search for Dark Matter: XENON100

By darkmatterdarkenergy

The direct detection of putative dark matter particles, as opposed to measuring their collective gravitational effects, remains a significant challenge. A number of experiments are actively searching for WIMPs (= Weakly Interacting Massive Particles) as the currently favored candidates for dark matter. Particle physics models with supersymmetric extensions to the Standard Model suggest that the most abundant particle of dark matter would have a mass significantly greater than the proton. The mass is expected to lie somewhere in the range of under 10 times the mass of a proton to possibly as much as 10,000 times the mass of a proton (around 10 to 10,000 GeV/c^2 where GeV is a billion electron volts of energy and we divide by the square of the speed of light to convert to mass). The WIMP name reflects that these particles would only interact with other matter via the weak nuclear force and via gravity. They do not react via either the strong nuclear force or electromagnetism.

It is believed that WIMPs are produced in the Big Bang as a decay mode from the massive release of energy during the inflation phase. The currently most favored candidate WIMP is the proposed least massive supersymmetric particle (LSP), which is expected to be stable. Supersymmetric particles are considered to have large masses and would have the same quantum numbers (properties) as corresponding Standard Model particles, except for their spins, that would differ by 1/2 from their partners. The local density of dark matter is estimated to be about .3 GeV / cc (GeV per cubic centimeter). If the WIMP mass is 100 GeV/ c^2 there would be about 3 particles per liter.

Two major techniques are being employed to search for cosmic WIMPs. One of these is to detect the direct impact of WIMPs with atomic nuclei (via elastic scattering) in underground laboratories here on Earth. These would be very rare events, so large detectors are required and experiments must gather data for a long time. Such an impact leaves products from the interaction and it is these products that are actually detected in an experiment. A second technique is to look for gamma rays, which are produced in the galactic halo of the Milky Way or also the Sun’s interior, when dark matter (WIMP) collisions with ordinary matter occur at those locations. The gamma rays produced in this way can in principle be detected with satellites in Earth orbit.

Beyond these two general techniques to detect WIMPs there is the hope of actually creating these dark matter particles via high energy collisions at the Large Hadron Collider.

One recent set of results is from the XENON collaboration, which is funded by the US government and 6 European nations. The XENON100 experiment is located underground in Italy, in the Gran Sasso National Laboratory. The heart of the detector consists of cooled Xenon of quantity 65 kilograms. The target is in both the liquid and gas phases. When a WIMP strikes a Xenon atom directly, electrons are either knocked out of the Xenon atom or boosted to higher energy orbital levels in the atom. Both scintillation light, due to subsequent decay of the electron orbital, and ionization electrons, are thus generated. The 100 days of exposure of XENON100 analyzed to date have yielded 3 events, but one expects 2 events from background neutrons producing similar signatures, so there is statistically no detection. This result does allow the placement of upper limits on the WIMP cross-section for interaction as a function of mass.

The result appears to be in conflict with another experiment, also located at the same Gran Sasso laboratory, run by the DAMA team. The DAMA/Libra experiment claims a statistically significant detection of an annually modulated “WIMP wind” which reflects the variation in the Earth’s orbital direction with respect to the diffuse background of WIMP particles. The intensity is well above XENON100 limits for certain possible mass ranges of the WIMP major constituent particle.

The race is on to secure the direct detection of dark matter particles, beyond their extensive apparent gravitational effects. Rapid progress in enhancing the sensitivity of detection methods, typically including the use of larger detectors, will increase the probability of better WIMP detection and mass determination in the future.

References:

M. Drees, G. Gerbier and the Particle Data Group, 2010. “Dark Matter” Journal of Physics G37(7A) pp 255-260

J. Feng, 2010. Ann. Rev. Astron. Astrophys. 48: 495, “Dark Matter Candidates from Particle Physics and Methods of Detection” (also available at: arxiv.org/pdf/1003.0904)

S. Perrenod, 2011. Dark Matter, Dark Energy, Dark Gravity, chapter 4, BookBrewer Publishing

http://www.wikipedia.org/wiki/Dark_matter

XENON Dark Matter Project

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April 4, 2011

Dark Matter

By darkmatterdarkenergy

Dark matter is like the hidden part of an iceberg found below the water line. The hidden part is the dominant portion of the mass and supports the structure apparent from the visible portion above. Dark matter couples to ordinary matter through the gravitational force. The ordinary visible matter, which we detect through light from galaxies and stars, is analogous to the portion of an iceberg above the water line.

Why is dark matter important? It dominates the mass-energy density of the universe during the early part of its lifetime. Just after the epoch of the cosmic microwave background (CMB) the universe is composed of mostly dark matter (dark energy comes to dominate much later, during the most recent 5 billion years). But also there is the ordinary matter, which at that time is a highly uniform gas of hydrogen and helium atoms, with slightly overdense and slightly underdense regions.

The existence of a large amount of dark matter promotes much more efficient gravitational collapse of the overdense regions. This is a self-gravitational process in which regions that are slightly denser than the critical mass density (which is also the average mass density of the universe) at a given time will collapse away from the overall expansion that continues around them. Both the dark and ordinary matter in such a region collapse together, but it is the ordinary matter that forms the first stars since it interacts via various physical processes (think friction, radiation, etc.) to a much greater degree allowing for collapse. The dark matter, interacting only via gravity and perhaps the weak nuclear force, but not through electromagnetism, remains more spread out, more diffuse. The dark matter promotes the collapse process though, through increasing the self-gravity of a given region and this results in more efficient formation of stars and galaxies. There are many more stars and galaxies formed at early times than would be the case in the absence of substantial dark matter.

From cosmological observations including the CMB and high redshift (distant) supernovae, we find dark matter is about 1/4 of the mass-energy density of the universe. Dark matter is composed of either faint ordinary matter or, more likely, exotic matter that interacts through the weak and gravitational forces only. Dark matter is clearly detected by its gravitational effects on galaxy rotation curves, and is inferred from the kinematics of clusters of galaxies and the temperature measures of X-ray emission from very hot gas between galaxies in these same clusters. Dark matter is also detected through gravitational lensing effects in our galactic halo and in very large-scale cosmological structures. The abundance of deuterium, which is produced from Big Bang nucleosynthesis, in conjunction with the universe’s now well-known expansion rate, severely constrains the density of baryons (amount of ordinary nucleonic matter, i.e. protons and neutrons) in the universe and leads to the conclusion that over 80% of matter is nonbaryonic.

The MACHO (MAssive Compact Halo Objects) alternative, which refers to potential ordinary matter, is thus limited. The WIMP (Weakly Interacting Massive Particles) contribution is dominant. Hot WIMPS (e.g. neutrinos) are ruled out because they inhibit clumpiness and galaxy formation in the early universe. Cold nonbaryonic dark matter is the best candidate, with the primary candidate being an hypothesized, undiscovered particle. Neutralinos are thought by many particle physicists to be the best candidate for this, and have an expected mass of order 50 to 250 times the mass of the proton. It must be emphasized that no neutralino or similar particles (known as supersymmetric particles) have ever been detected directly. It is hoped that the Large Hadron Collider newly operational at CERN near Geneva may do this.

There may be direct detection of an annual modulation of the WIMP wind in a large scintillation array. There is also a possible indirect detection which manifests as an excess of 1 GeV gamma rays in our galactic halo. Significantly more sensitive detectors are needed to find these elusive particles and to provide a stronger foundation for supersymmetric physics, which postulates many new and heavy particles. An important experiment AMS-02 (Alpha Magnetic Spectrometer, 2nd generation) is scheduled to be carried to the International Space Station on the last Endeavour Shuttle mission scheduled for April 19, 2011. See http://www.ams02.org and http://cosmiclog.msnbc.msn.com/_news/2011/04/04/6403905-will-space-jam-delay-shuttle-launch

The next decade should allow us to shed new light on dark matter. Whatever it is made of, without the existence of substantial amounts of dark matter, there wouldn’t be nearly as many stars and galaxies in the universe, and we very likely wouldn’t be here.

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February 24, 2011

Herschel infrared telescope studies Dark Matter

By darkmatterdarkenergy

The space-borne Herschel infrared telescope, launched by the European Space Agency with NASA participation, is studying galaxy formation in the early universe. It allows astronomers to look back to the first 3 billion years of the universe’s approximately 14 billion year history.

http://www.jpl.nasa.gov/news/news.cfm?release=2011-057

The degree of clustering of these early galaxies is a reflection of the amount of dark matter relative to ordinary matter. The researchers also note that to form large galaxies (such as our Milky Way) that are efficient sites for robust star formation that one needs enough dark matter relative to the ordinary matter in stars and interstellar gas, but not so much that one ends up with many smaller galaxies.

 

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February 22, 2011

Foreword, by Rich Brueckner

By darkmatterdarkenergy

“Through our eyes, the universe is perceiving itself. Through our ears, the universe is listening to its harmonies. We are the witnesses through which the universe becomes conscious of its glory, of its magnificence.”

— Alan Watts

We all know of the Big Bang, how our universe came to be in a massive explosion, seemingly starting from nothingness. And for those who study cosmology, further understanding requires us to define the dark energies that somehow endowed our world with order.

Now, we haven’t observed dark energy, dark matter, and the secrets of dark gravity directly, but we do see their effects. As we learn in this book, without them, the universe would not have formed in a way that could have spawned intelligent life.

As a writer, I am intrigued by these dark energies because they imply a backstory–phenomena that happened first that led to this outcome. So in this way, dark energies seem to me to be metaphors of science. Like the stories of Genesis and Adam and Eve, what they really represent is a deeper truth.

In this book, Dr. Perrenod does a wonderful job of explaining the origins of the universe in way that is accessible to the layman. When you want to understand how the universe came to be, you ask an astrophysicist. But when you really want to know why, I think you have to start by asking yourself some questions. Try a thought experiment.

Put yourself in the place of a Universal Mind before the Big Bang. If you really wanted to understand yourself, you would need to have something intelligent outside of yourself that could experience that which is you. Not to get metaphysical here, but if we were at the scene of a crime, what I’d be suggesting here is motive.

Thanks to modern physics and cosmology, we no longer live in a universe where dark forces lurk far beyond our capacity for comprehension. I believe that, through the works of Stephen Perrenod and others, we will come to that knowing. But even as we look out to the stars, I think it begins with understanding that not only are we within the universe, but the universe is within us.

Rich Brueckner is President of insideHPC.com

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