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Astrophysicist. Cryptocurrency and technology analyst. Macro investor.
October 6, 2012

Dark Energy Survey First Light!

By darkmatterdarkenergy

Last month the Dark Energy Survey project achieved first light from its remote location in Chile’s Atacama Desert. The term first light is used by astronomers to refer to the first observation by a new instrument.

And what an instrument this is! It is in fact the world’s most powerful digital camera. This Dark Energy Camera, or DECam, is a 570 Megapixel optical survey camera with a very wide field of view. The field of view is over 2 degrees, which is rather unusual in optical astronomy. And the camera requires special CCDs that are sensitive in the red and infrared parts of the spectrum. This is because distant galaxies have their light shifted toward the red and the infrared by the cosmological expansion. If the galaxy redshift is one,  the light travels for about 8 billion years and the wavelength of light that the DECam detects is doubled, relative to what it was when it was originally emitted.

Dark Energy Camera

Image: DECam, near center of image, is deployed at the focus of the 4-meter Victor M. Blanco optical telescope in Chile (Credit: Dark Energy Survey Collaboration)

The DECam has been deployed to further our understanding of dark energy through not just one experimental method, but in fact four different methods. That’s how you solve tough problems – by attacking them on multiple fronts.

It’s taken 8 years to get to this point, and there have been some delays, as normal for large projects. But now this new instrument is mounted at the focal plane of the existing 4-meter telescope of the National Science Foundation’s Cerro Tololo Inter-American observatory in Chile. It will begin its program of planned measurements of several hundred million galaxies starting in December after several weeks of testing and calibration. Each image from the camera-telescope combination can capture up to 100,000 galaxies out to distances of up to 8 billion light years. This is over halfway back to the origin of the universe almost 14 billion years ago.

In a previous blog entry I talked about the DES and the 4 methods in some detail. In brief they are based on observations of:

  1. Type 1a supernova (the method used to first detect dark energy)
  2. Very large scale spatial correlations of galaxies separated by 500 million light-years (this experiment is known as Baryon Acoustic Oscillations since the galaxy separations reflect the imprint of sound waves in the very early universe, prior to galaxy formation)
  3. The number of clusters of galaxies as a function of redshift (age of the universe)
  4. Gravitational lensing, i.e. distortion of background images by gravitational effects of foreground clusters in accordance with general relativity

NGC 1365

Image: NGC 1365, a barred spiral galaxy located in the Fornax cluster located 60 million light years from Earth (Credit: Dark Energy Survey Collaboration)

What does the Dark Energy Survey team, which has over 120 members from over 20 countries, hope to learn about dark energy? We already have a good handle on its magnitude, at around 73% presently of the universe’s total mass-energy density.

The big issue is does it behave as a cosmological constant or as something more complex? In other words, how does the dark energy vary over time and is there possibly some spatial variation as well? And what is its equation of state, or relationship between its pressure and density?

With a cosmological constant explanation the relationship is Pressure = – Energy_density, a negative pressure, which is necessary in any model of the dark energy, in order for it to drive the accelerated expansion seen for the universe. Current observations from other experiments, especially those measuring the cosmic microwave background, support an equation of state parameter within around 5% of the value -1, as represented in the equation in the previous sentence. This is consistent with the interpretation as a pressure resulting from the vacuum. Dark energy appears also to have a constant or nearly constant density per unit volume of space. It is unlike ordinary matter and dark matter, that both drop in mass density (and thus energy density) as the volume of the universe grows. Thus dark energy becomes ever more dominant over dark matter and ordinary matter as the universe continues to expand.

We can’t wait to see the first publication of results from research into the nature of dark energy using the DECam.

References:

http://www.noao.edu/news/2012/pr1204.php – Press release from National Optical Astronomical Observatory on DECam first light

www.darkenergysurvey.org

http://www.ctio.noao.edu/noao/ – Cerro Tololo Inter-American Observatory page

http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf – WMAP 7 year results on cosmic microwave background

https://darkmatterdarkenergy.com/2011/03/08/dark-energy-survey/

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August 14, 2012

Dark Matter on Mars?

By darkmatterdarkenergy

Yes, there is most likely dark matter on Mars, and on Earth as well, and throughout our Solar System. The Curiosity rover will not be searching for dark matter, it not only does not have the right instrumentation, but it also remains on the surface, which is not the way to pursue dark matter searches. On Earth, the direct detection experiments searching for dark matter are made primarily by deploying large crystalline detectors in laboratories within deep mines or inside mountains. A lot of shielding is required. There are too many other sources such as cosmic rays and solar wind particles that would interfere with the search.

marscuriosityaug13
Credit: NASA/JPL-Caltech/MSSS  Curiosity Rover

“The expected rate of WIMP interactions is already constrained to be very small (less than one event per kg-year) and the expected nuclear recoil energy is very low (100 keV or less) so background rejection is crucial…  neutrons produce nuclear recoils identical to those from WIMP interactions. To eliminate the fast neutron flux induced by cosmic rays, such experiments must be located deep underground.” – http://astro.fnal.gov/projects/DarkMatter/darkmatter_projects.html

And given the expected interaction rate, one needs detectors with many kilograms of detector volume; a ton or more is desirable. They also need intensive calibration, care and feeding by scientists and technicians.

The dark matter density is expected to be comparable throughout our solar system and in the neighborhood of the Sun. The canonical value that most models use comes from measures of our galaxy’s dynamics and is 0.3 GeV per cubic centimeter (cc). This density is determined by looking at the large-scale gravitational effects of dark matter spread throughout our Milky Way galaxy, including its effect on the rotation rate as a function of distance from the galaxy’s center. It’s important to determine this number to get a handle on the predicted flux of dark matter particles impinging on a detector in one of the labs on Earth.

What does this 0.3 GeV per cc mean in terms of particle density? Well the mass of a proton is about 0.9 GeV where GeV is a billion (giga) electron-Volts and one electron-Volt is the energy of moving a single electron through a one Volt electron potential. This is a convenient unit of measurement for particle physicists. Since GeV is an energy, strictly speaking the mass is in units of GeV/c² (energy divided by the speed of light squared, in accordance with Einstein’s famous equation), “GeV” is used for shorthand. So if the mass of the dark matter particle were equal to the proton, that would imply about 1/3 of a particle per cc. But dark matter particles are heavier than protons according to particle physicists, significantly heavier.

On very large scales, in the early universe, slightly over-dense regions collapsed out of the general Big Bang driven-expansion due to their internal gravity (dominated by the dark matter within) and the ordinary matter in those regions formed galaxies and groups of galaxies, including clusters with up to 1000 or more galaxies. At the cluster of galaxies level the dark matter is dominant, but within an individual galaxy like our Milky Way ordinary matter can dominate due to the high degree of contraction possible with ordinary matter. Dark matter does not clump to the same degree as it can’t “cool off” via radiation.

So while dark matter dominates on the largest scales within the universe, amounting to 5 times as much matter as ordinary matter, within our galaxy the ordinary matter density is larger. Ordinary matter clumps more easily than dark matter, since it interacts with itself and light readily and undergoes cooling via radiative processes. The removal of energy via radiation allows matter to clump into molecular clouds and in turn form into stars and planets from that material.

A recent study by astronomers and astrophysicists associated with research institutes in Switzerland, Germany, the UK and China has used a new method and new data from a large sample of red dwarf stars to measure the dark matter density in the solar neighborhood. In this case what we mean by the neighborhood is up to about 3000 light-years from the Sun, and what is measured is an average number across that large region.

Their method makes fewer assumptions than other methods about the nature of the shape of our galaxy’s halo, i.e. the details of how the density of regular matter falls off as one moves away from the galaxy center. The new result is about 0.9 GeV per cc and comes with a large error bar of +/- 0.5. It does suggest the correct value may be 3 times higher than that previously assumed. Since the proton mass is 0.9 GeV, coincidentally, this would imply the equivalent of around one proton per cc in mass density. The dark matter particle is heavier, so the number of dark matter particles would be lower than one per cc.

Dark matter is thought to be due to a new particle, a WIMP (weakly-interacting massive particle) of some sort such as the lightest supersymmetric particle, which would remain stable against decay over billions of years. No such supersymmetric particle is yet detected, but the LHC (Large Hadron collider outside of Geneva) is working on the supersymmetry problem as well as the Higgs boson. The apparent discovery of the Higgs boson with mass around 125 GeV by the ATLAS and CMS experiments at LHC is consistent with supersymmetry.

While we don’t know the mass for dark matter WIMPs, the range of somewhat less than 10 GeV up to 1000 GeV is generally favored. Using the new value of 0.9 GeV per cc for dark matter density indicates that if the dark matter mass is around 10 GeV then there would be 1/10 of a dark matter particle per cc (or 100 per liter). If the mass of the particle is around 100 GeV then there would be one dark matter particle per 100 cc (which is 10 per liter).

So even with the potential  increase of a factor of 3 in actual density, this is an extremely rare particle and each one has a very low probability of actually interacting with ordinary matter and being detected. Which is why detectors are growing to 1 ton in size and multiple years of very sensitive observations are needed to place limits on the amount of dark matter or, more hopefully, obtain a statistically significant positive detection. A lot of progress is expected in the next couple of years with a new generation of larger detectors coming on line.

Reference:

S. Garbari, C. Liu, J.I. Read, G. Lake 2012, Mon. Not. R. Astron. Soc., submitted; arXiv:1206.0015v2 “A new determination of the local dark matter density from the kinematics of K dwarfs”

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July 19, 2012

Dark Matter Bridge Discovered

By darkmatterdarkenergy

A team of astronomers claims to have detected an enormous bridge or filament of dark matter, with a mass estimated to be of order 100 trillion solar masses, and connecting two clusters of galaxies. The two clusters, known as Abell 222 and Abell 223, are about 2.8 billion light-years away and separated from one another by 400 million light-years. Each cluster has around 150 galaxies; actually one of the pair is itself a double cluster.

Clusters of galaxies are gravitationally bound collections of hundreds to a thousand or more galaxies. Often a cluster will be found in the vicinity of other clusters to which it is also gravitationally bound. The universe as a whole is gravitationally unbound – the matter, including the dark matter – is insufficient to stop the continued expansion, which is driven to acceleration in fact, by dark energy.

Dark matter bridge

Figure: Subaru telescope optical photo with mass density shown in blue and statistical significance contours superimposed. In the filament area found near the center of the image, the contours indicate four standard deviations of significance in the detection of dark matter. The cluster Abell 222 is in the south, and Abell 223 is the double cluster in the north of the image. The distance between the two clusters is about 14 arc-minutes, or about ½ the apparent size of the Moon.

Dark matter was originally called “missing matter”, and was first posited by Fritz Zwicky (http://en.wikipedia.org/wiki/Fritz_Zwicky) in the 1930s because of his studies of the kinematics of galaxies and galaxy clusters. He measured the velocities of galaxies moving around inside a cluster and found they were significantly greater than expected from the amount of ordinary matter seen in the galaxies themselves. This implied there was more matter than seen in galaxies because the velocities of the galaxies would be determined by the total gravitational field in a cluster, and the questions have been where is, and what is, the “missing matter” inferred by the gravitational effects. X-ray emission has been detected from most clusters of galaxies, and this is due to an additional component of matter outside of galaxies, namely hot gas between galaxies. But it is still insufficient to explain the total mass of clusters as revealed by both the galaxy velocities and the temperature of the hot gas itself, since both are a reflection of the gravitational field in the cluster.

Dark matter is ubiquitous, found on all scales and is generally less clumped than ordinary matter, so it is not surprising that significant dark matter would be found between two associated galaxy clusters. In fact the researchers in this study point out that “It is a firm prediction of the concordance Cold Dark Matter cosmological model that galaxy clusters live at the intersection of large-scale structure filaments.”

The technique used to map the dark matter is gravitational lensing, which is a result of general relativity. The gravitational lensing effect is well established; it has been seen in many clusters of galaxies to date. In gravitational lensing, light is deflected away from a straight-line path by matter in its vicinity.

In this case the gravitational field of the dark matter filament and the galaxy clusters deflect light passing nearby. The image of a background galaxy located behind the cluster will be distorted as the light moves through or nearby the foreground cluster. The amount of distortion depends on the mass of the cluster (or dark matter bridge) and how near the line of sight passes to the cluster center.

There is also a well-detected bridge of ordinary matter in the form of hot X-ray emitting gas connecting the two clusters and in the same location as the newly discovered dark matter bridge.  The scientists used observations from the XMM-Newton satellite to map the X-ray emission from the two clusters Abell 222 and Abell 223 and the hot gas bridge connecting them. Because of the strong gravitational fields of galaxy clusters, the gas interior to galaxy clusters (but exterior to individual galaxies within the cluster) is heated to very high temperatures by frictional processes, resulting in thermal X-ray emission from the clusters.

The research team, led by Jörg Dietrich at the University of Michigan, then performed a gravitational lensing analysis, focusing on the location of the bridge as determined from the X-ray observations. The gravitational lensing work is based on optical observations obtained from the Subaru telescope (operated by the Japanese government, but located on the Big Island of Hawaii) to map the total matter density profile around and between the two clusters. This method detects the sum of dark matter and ordinary matter.

They analyzed the detailed orientations and shapes of over forty thousand background galaxies observable behind the two clusters and the bridge. This work allowed them to determine the contours of the dark matter distribution. They state a 98% confidence in the existence of a bridge or filament dominated by dark matter.

The amount of dark matter is shown to be much larger than that of ordinary matter, representing over 90% of the total in the filament region, so the gravitational lensing effects are primarily due to the dark matter. Less than 9% of the mass in the filament is in the form of hot gas (ordinary matter). The estimated total mass in the filament is about 1/3 of the mass of either of the galaxy clusters, each of which is also dominated by dark matter.

Observations of galaxy distributions show that galaxies are found in groups, clusters, and filaments connecting regions of galaxy concentration. Cosmological simulations of the evolution of the universe on supercomputers indicate that the distribution of dark matter should have a filamentary structure as well. So although the result is in many ways not surprising, it represents the first detection of such a structure to date.

 

References:

http://ns.umich.edu/new/releases/20623-dark-matter-scaffolding-of-universe-detected-for-the-first-time – press release from the University of Michigan

http://www.gizmag.com/dark-matter-filaments-found/23281/ “Dark matter filaments detected for the first time”

J. Dietrich et al. 2012 http://arxiv.org/abs/1207.0809 “A filament of dark matter between two clusters of galaxies”

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July 5, 2012

Why the Higgs Boson is not Dark Matter

By darkmatterdarkenergy

The Higgs boson is considered a necessary part of the Standard Model of particle physics. In the Standard Model there are 3 main forces of nature: the electromagnetic force, the weak nuclear force, and the strong nuclear force. The Standard Model does not address gravity and we do not yet have a proven theory for the unification of gravity with the other 3 forces.

On July 4th CERN, the European particle physics lab near Geneva, announced that two experiments using the Large Hadron Collider accelerator, ATLAS and CMS, have both amassed strong statistical evidence (around 5 sigma) for a new particle. This new particle has a mass of about 126 GeV* and “smells” very much like it is the long sought after, and elusive, Higgs boson. The prediction of the Higgs dates from 1964. For comparison, the proton mass is about 0.94 GeV, so the Higgs is around 134 times more massive. Further work is necessary to determine all of its properties, but at this point it looks as if the new particle decays into other particles in the expected manner. It is these decay products that are actually detected.

This decades-long search has proceeded in fits and starts, principally at CERN in Europe and Fermilab in the U.S., with different accelerators and detectors. Over time the experiments were able to exclude possible masses for the Higgs, since the rate of creation of different decay products varies for different putative masses. By the end of 2011 it looked like there was a preliminary signal, not yet of sufficient statistical strength, but that the mass would have to be in the range of about 115 to 130 GeV.

Image

The CMS detector at the Large Hadron Collider. Credit: Mark Thiessen/National Geographic Society/Corbis

One of my professors, Steven Weinberg, won the Nobel Prize in Physics years ago for his work on unifying the electromagnetic force and the weak force. While the Standard Model and the body of work in particle physics provides a theoretical underpinning for all of the particles which we observe, and their quantum properties, and describes a unification of the strong force (which holds together the quarks inside a proton or neutron) with these other two forces, it also requires an additional mechanism to explain why most particles have non-zero masses.

The Higgs mechanism is the favored explanation, and it predicts a particle as the mediator to provide masses to other particles. The Higgs mechanism is theorized as an all-pervasive Higgs field, which slows down particles as they move through it. As you swim through water you feel a drag that slows you down. A fish with a very hydrodynamic design will feel less drag. In the particle world, more massive ones slow down more than the lighter ones, since they interact more strongly with the Higgs field.

The particle corresponding to this mechanism is known as the Higgs boson. Particles can have quantum spin that is a multiple of ½ or an integer multiple. Bosons have integer multiple spins. Actually the spin of the Higgs boson is zero. All of the force mediator particles such as the photon (spin = 1), which mediates electromagnetism, are bosons.

The Large Hadron Collider is in some sense recreating the conditions of the very early universe by smashing particles together at 7000 GeV, or 7 TeV. The Higgs originally would have been created in Nature in the very early part of the Big Bang, around the first one-trillionth of a second. The appearance of the Higgs broke the unification, or symmetry, between the electromagnetic and weak forces that Steven Weinberg demonstrated are one at very high energies. And the Higgs gave mass to particles.

Without the Higgs mechanism, all particles would be massless, and thus travelling at the speed of light, and structure in the universe – stars, planets, galaxies, human beings, would not be possible. Even the existence of the proton itself requires that quarks have mass, although most of the proton mass comes from the energy of the gluons (strong force mediation particle) and ‘virtual’ quark-antiquark pairs found inside it.

The Higgs boson cannot be the explanation for dark matter for a very simple reason. Dark matter must be stable with a very long lifetime, persisting over the universe’s present age of 14 billion years. It mostly sits in space doing nothing except providing additional gravitational interaction with ordinary matter. The favored candidate for dark matter is the least massive supersymmetric particle; being the least massive, it would have nothing to decay into. Supersymmetry is a theoretical extension beyond the Standard Model. No supersymmetric particles are detected as of yet, but the theory has a lot of support and has the benefit of stabilizing the mass of the Higgs itself.

The Higgs boson, on the other hand, decays very rapidly. There are various decay channels, including into quarks, W/Z bosons, leptons or photons, producing these in pairs (two Zs, two top quarks etc.). Sometimes even four particles are produced from a single Higgs decay. It is these decay products that are actually detected in the Large Hadron Collider at CERN.

There are a few experiments that are claiming to have directly detected dark matter. The favored mass range from the COGENT and DAMA/LIBRA experiments is around 10 GeV for dark matter, much more than a proton, but less than 10% of the Higgs’ mass. Now that the Higgs appears to have been found, work will proceed on confirming and elucidating its properties. And the next great hunt for particle physics may be the direct detection of dark matter particles and the beginning of a determination if supersymmetry is real.

* GeV = Giga-electronVolt or 1 billion electron Volts. 1 TeV (Tera-electronVolt) = 1000 GeV

References:

http://en.wikipedia.org/wiki/Higgs_boson

http://en.wikipedia.org/wiki/Standard_Model

http://en.wikipedia.org/wiki/Large_Hadron_Collider

http://www.pbs.org/wgbh/nova/physics/blog/2012/07/higgs-fireworks-on-july-4/

http://www.youtube.com/watch?v=ktEpSvzPROc – Don Lincoln of Fermilab on how we search for the Higgs at particle accelerators

http://www.youtube.com/watch?v=r4-wVzjnQRI&feature=related – BBC documentary

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June 5, 2012

Antimatter is not Dark Matter: Antimatter secrets uncovered with nuclear reactors

By darkmatterdarkenergy

Dark matter is much more abundant than ordinary matter. On the other hand, antimatter is much rarer in the universe than ordinary matter. Antimatter is not dark matter. In general dark matter refers to something other than antimatter, although it would be possible to have dark matter that is anti-dark matter. See this prior post https://darkmatterdarkenergy.com/2011/06/04/dark-matter-powered-stars/

Antimatter refers to matter that is similar to ordinary matter but has the opposite electrical charge from what is seen in regular matter. Electrons have a negative charge of -1, positrons, which are anti-electrons, have a positive charge of +1. Similarly, protons posses a charge of +1, and antiprotons have a charge of -1. Inside protons and neutrons there are quarks. There can also be antiquarks and so on.

When a particle and its associated anti-particle get too close to one another they mutually annihilate and all of their rest mass energy is converted to radiation or other particles, in accordance with E = mc2. For example, the electron has a rest mass of 511 keV (1 keV is one thousand electron-Volts, where the energy of 1 eV is that of moving an electron through a potential of one Volt.) When an electron and positron (anti-electron) annihilate, two gamma rays are produced each with energy around 511 keV. See the figure below, which is the Feynman diagram for the interaction. In the case of electron-positron annihilation, this is the only outcome possible due to the low energy of the two annihilating particles.

Image

Mutual annhilation of an electron and positron yielding two gamma rays at 511 keV each

The big mystery is why there is matter in the universe at all! Why did not the Big Bang produce equal amounts of matter and anti-matter? In such a case the matter and anti-matter mutual annihilation process could have left little or no matter behind, and stars, galaxies, planets and people could not have formed. Cosmologists and particle physicists believe there was some small excess of matter over anti-matter, such that our present amount of matter remained after all the annihilation processes were finished.

This excess of matter over anti-matter is thought to be due to some asymmetry in the laws of physics. In general the laws are highly symmetric. Particle physicists look to understand the degree and nature of any putative asymmetries. One way to do this is by studying neutrinos, very low mass electrically neutral particles which are signatures of the weak nuclear force and products of radioactive decay. The neutrino mass is less than 2 eV, much, much less than the already small electron mass. There are believed to be 3 types of neutrinos – electron neutrinos, muon neutrinos and tau neutrinos – which are in turn associated with the electron, muon and tau particles; the muon and tau are ‘heavy’ members of the electron family.

If the neutrino has non-zero mass, then through a quantum effect known as “neutrino oscillation”, the different types of neutrinos mix together. This is due to the wave nature of all particles in quantum mechanics. Neutrinos have been detected from the Sun for many years, but at a much lower rate than initially expected, which was an outstanding puzzle. The “neutrino oscillation” mechanism resolves the discrepancy. Also, differences in neutrino and antineutrino interactions, which are due to neutrino oscillation, are thought by many particle physics to be related to the excess of matter over antimatter in the universe.

There are 3 parameters of the “neutrino oscillation” theory, which are known as ‘mixing angles’, and two of these, θ12 and θ23, have been reasonably well measured. The third mixing angle, known as θ13, is has not been well measurable until very recently.

Particle physicists working as part of the US-Chinese collaboration at the Daya Bay experiment have announced in March 2012 a positive result for the third mixing angle. It is based on measurements made near two nuclear reactors in China, one at Daya Bay and one at Ling Ao. Nuclear reactors are strong sources of antineutrinos. Another similar experiment, known as RENO, is based at a six-reactor nuclear power site in Korea. As of April 2012 the RENO physicists are also claiming a positive measurement of the θ13 mixing angle parameter, with a similar level of statistical confidence in excluding the zero value hypothesis.

Both experiments are indicating a value of around 0.10 for the mixing angle parameter, satisfying the expression sin2 (2θ13 ) = 0.1.

Other experiments include T2K in Japan, MINOS in the US and the Double Chooz international collaboration based in France. All three are seeing hints of a positive value of θ13 as well, but none have reached the statistical confidence level of the Daya Bay and RENO experiments.

The value being measured is surprisingly large, and thus very supportive of the neutrino oscillation theory for the matter vs. anti-matter discrepancy. These are exciting times for oscillating neutrinos and these experiments are moving us to closer to solving the antimatter quandry!

References:

http://physicsworld.com/cws/article/news/2012/mar/09/daya-bay-nails-neutrino-oscillation

http://www.nu.to.infn.it/exp/all/reno/ – RENO neutrino experiment, Korea

http://theory.fnal.gov/jetp/talks/RENO-results-seminar-new.pdf – Presentation on RENO results

Wikipedia articles on antimatter, annhilation and the neutrion oscillation:

http://en.wikipedia.org/wiki/Antimatter

http://en.wikipedia.org/wiki/Annihilation

http://en.wikipedia.org/wiki/Neutrino_oscillation

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April 23, 2012

Gamma Ray Line at 130 GeV in FERMI LAT Possible Dark Matter Signal?

By darkmatterdarkenergy

In my last blog entry I noted that the Fermi Gamma-ray Telescope collaboration, based on two years of data reduction, was reporting that they had not detected a dark matter signal. They employed a method of looking for gamma rays from nearby dwarf galaxies; these are expected to be good targets due to lack of other gamma ray sources and expected high relative density of dark matter. The team examined gamma ray energies up to 100 GeV and found nothing significant (a proton rest mass energy is just under .94 GeV and 1 GeV is a billion electron volts).

But now just in the past 2 weeks we have an independent author who has examined the publicly available Fermi data for gamma rays emitted from our own Galactic center. He was able to analyze around 3.5 years of data, more than the 2 years’ worth of dwarf galaxy data analyzed. It’s just a little bump in the spectrum (see the central region of the figure below), but he claims to see a positive signal at 130 GeV and with a (marginal) statistical significance just in excess of 3 standard deviations. This is tantalizing, potentially, but not strong enough for a clear detection.

Image

This possible signal is also found at a much higher mass than the neighborhood of 10 GeV where COGENT and DAMA/LIBRA have claimed direct detection in Earth-bound laboratory experiments. The plot thickens and as usual we have to wait for more data from FERMI and other experiments.

http://physicsforme.wordpress.com/2012/04/17/a-tentative-gamma-ray-line-from-dark-matter-annihilation-at-the-fermi-large-area-telescope/

http://arxiv.org/pdf/1204.2797v1.pdf – Preprint by Christoph Weniger, “A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope”

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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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November 24, 2011

Future of Our Runaway Universe (the next Trillion Years)

By darkmatterdarkenergy

Future for our Sun: Ultraviolet image of the planetary nebula NGC 7293 also known as the Helix Nebula. It is the nearest example of what happens to a star, like our own Sun, as it approaches the end of its life when it runs out of fuel, expels gas outward and evolves into a much hotter, smaller and denser white dwarf star. Image Credit: NASA/JPL-Caltech/SSC

In the future, the average density of matter in the universe (both ordinary matter and dark matter) will continue to drop in proportion to the increasing spatial volume as the universe expands ever more rapidly. The dark energy density, however, behaves differently. Dark energy is an irreducible property of even empty space, so as new space is created, the dark energy density remains the same; it is believed to not only take the same value in all portions of space at a given time, but to also have had the same value (per unit volume) for many billions of years.

Since around 5 billion years ago, when the universe was 9 billion years old, the dark energy has dominated over both types of matter (ordinary and dark) and this dominance is only increasing with the universe’s continued expansion. Today it is 73% of the total mass-energy density and it will approach close to 100% in the future. The assumption is made that the cosmological constant or dark energy term that we measure today remains constant into the future. However it cannot be ruled out that it is changing very slowly or might change suddenly at some future date.

In the cosmological constant case, the scale factor for the size of the universe grows exponentially with time. This is known as the de Sitter solution to the equations of general relativity, and it indicates that the expansion of the universe is accelerating into a runaway condition. There is a single parameter, a timescale. Cosmological measurements indicate that the value is such that the size of the universe for each spatial dimension will double and redouble every 11 billion years (the volume will thus grow by 8 times each 11 billion years).

When the universe is 25 billion years old (now it’s 14 billion years old), distant galaxies will be about twice as far away as today (and 4 times fainter). Well before that time we’ll need to evacuate the Earth as the Sun will go into its red giant phase some 5 billion years from now, followed by a white dwarf phase – as shown in the image of the Helix planetary nebula above. When the universe is around 124 billion years old, distant galaxies on average will be 1000 times farther away from us than now. And after 234 billion years they will be an incredible million times farther away than now!

Year                                    Relative Distance                        Relative Brightness

14 billion (Now)                        1                                                1

25 billion                                    2                                                1/4

124 billion                                  1000                                         one-millionth

234 billion                                  1,000,000                               one-trillionth

The distant galaxies that we detect with the Hubble telescope and large Earth-bound telescopes will become invisible since their apparent luminosity will drop as the square of the increasing distance. For example at the time of 124 billion years, they will be 1 million times fainter (1000 squared). At the time of 234 billion years they will be a trillion times fainter (one million squared). Actually it will be worse than this since their light will be redshifted (stretched out by the cosmological expansion) by the same relative distance factor, so light emitted in the visible will be detected in the millimeter radio region when the universe is 100+ billion years old. This is without considering the evolution in their stellar populations, but only their lower mass, fainter stars will survive, further aggravating the situation.

Galaxies themselves are not changing very much in their size or in internal density, rather it is the spacing between galaxies that is on average growing rapidly. Galaxy groups and clusters that are today gravitationally bound will remain bound. Our home, the Milky Way galaxy, and its large neighbor the Andromeda galaxy, will stay together since they are gravitationally bound, and they may very well merge in several billion years due to tidal effects. All of the 40 or so galaxies and dwarf galaxies in our gravitationally bound Local Group may coalesce after 1 trillion years have passed.

Our light cone horizon, which determines which galaxies are even theoretically visible to us, is shrinking in relative terms. Sufficiently distant galaxies are already receding faster than the speed of light from our vantage point and are entirely hidden from us; if the inflationary model is correct as seems to be the case, the universe is immensely larger than what we are able to detect. This is possible and indeed happening because there are no constraints in special relativity or general relativity on the expansion rate of space itself; only the objects within space are constrained to moving at less than the speed of light relative to their local frames of reference.

An intelligent society in the very distant future, possibly our descendants who have moved to a planet in orbit around another star, would observe only one galaxy, namely their own. This would be a larger galaxy formed from the Milky Way and other members of the Local Group. All other galaxies would no longer be visible, first they would become too distant and too faint, and then they would be entirely beyond our light horizon. These descendants or other observers would believe their galaxy to be the only one in the universe, unless they had access to (and a willingness to believe in) very ancient research publications.

We are fortunate to live in this epoch – despite dark matter, dark energy, and dark gravity, the universe is young, and we are immersed in light.

References:

http://spiff.rit.edu/classes/phys240/lectures/future/future.html

The Five Ages of the Universe, Fred Adams and Greg Laughlin, Simon and Schuster, 1999

The Runaway Universe, Donald Goldsmith, Perseus Books, 2000

Dark Matter, Dark Energy, Dark Gravity, Stephen Perrenod, 2011, https://darkmatterdarkenergy.wordpress.com/where-to-find/

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October 5, 2011

2011 Nobel Prize for Dark Energy Discovery

By darkmatterdarkenergy
Measurements of Dark Energy and Matter content of Universe

Dark Energy and Matter content of Universe: The intersection of the supernova (SNe), cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) ellipses indicate a topologically flat universe composed 74% of dark energy (y-axis) and 26% of dark matter plus normal matter (x-axis).

The 2011 Nobel Prize in Physics, the most prestigious award given in the physics field, was announced on October 4. The winners are astronomers and astrophysicists who produced the first clear evidence of an accelerating universe. Not only is our universe as a whole expanding rapidly, it is in fact speeding up! It is not often that astronomers win the Nobel Prize since there is not a separate award for their discipline. The discovery of the acceleration in the universe’s expansion was made more or less simultaneously by two competing teams of astronomers at the end of the 20th century, in 1998, so the leaders of both teams share this Nobel Prize.

The new Nobel laureates, Drs. Saul Perlmutter, Adam Riess, and Brian Schmidt, were the leaders of the two  teams studying distant supernovae, in remote galaxies, as cosmological indicators. Cosmology is the study of the properties of the universe on the largest scales of space and time. Supernovae are exploding stars at the ends of their lives. They only occur about once each fifty to one hundred years or so in a given galaxy, thus one must study a very large number of galaxies in an automated fashion to find a sufficient number to be useful. The two teams introduced new automated search techniques to find enough supernovae and achieve their results.

During a supernova explosion, driven by rapid nuclear fusion of heavy elements, the supernova can temporarily become as bright as the entire galaxy in which it resides. The astrophysicists studied a particular type of supernova known as Type Ia. These are due to white dwarf stellar remnants exceeding a critical mass. Typically these white dwarfs would be found in binary stellar systems with another, more normal, star as a companion. If a white dwarf grabs enough material from the companion via gravitational tidal effects, that matter can “push it over the edge” and cause it to go supernova. Since all occurrences of this type of supernova event have the same mass for the exploding star (about 1.4 times the Sun’s mass), the resultant supernova has a consistent brightness or luminosity from one event to the next.

This makes them very useful as so-called standard candles. We know the absolute brightness, which we can calibrate for this class of supernova, and thus we can calculate the distance (called the luminosity distance) by comparing the observed brightness to the absolute. An alternative measure of the distance can be obtained by measuring the redshift of the companion galaxy. The redshift is due to the overall expansion of the universe, and thus the light from galaxies when it reaches us is stretched out to longer, or “redder” wavelengths. The amount of the shift provides what we call the redshift distance.

Comparing these two different distance techniques provides a cosmological test of the overall properties of the universe: the expansion rate, the shape or topology, and whether the expansion is slowing down, as was expected, or not. The big surprise is that the expansion from the original Big Bang has stopped slowing down due to gravity and has instead been accelerating in recent years! The Nobel winners did not expect such a result, thought they had made errors in their analyses and checked and rechecked. The acceleration did not go away. And when they compared the results between the two teams, they realized they had confirmed each others’ profound discovery of the reality of a dark energy driven acceleration.

The acceleration result is now well founded since it can be seen in the high spatial resolution measurements of the cosmic microwave background radiation as well. This is the radiation left over from the Big Bang event associated with the origin of our universe.

The acceleration is now increasingly important, dominating during the past 5 billion years of the 14 billion year history of the universe. Coincidentally, this is about how long our Earth and Sun have been in existence. The acceleration has to overcome the self-gravitational attraction of all the matter of the universe upon itself, and is believed to be due to a nonzero energy field known as dark energy that pervades all of space. As the universe expands to create more volume, more dark energy is also created! Empty space is not empty, due to the underlying quantum physics realities. The details, and why dark energy has the observed strength, are not yet understood.

Amazingly, Einstein had added a cosmological constant term, which acts as a dark energy, to his equations of General Relativity even before the Big Bang itself was discovered. But he later dropped the term and called it his worst blunder, after the expansion of the universe was first demonstrated by Edwin Hubble over 80 years ago. It turns out Einstein was in fact right; his simple term explains the observed data and the Perlmutter, Riess, and Schmidt measurements indicate that ¾ of the mass-energy content of the universe is found in dark energy, with only ¼ in matter.

Our universe is slated to expand in an exponential fashion for trillions of years and more, unless some other physics that we don’t yet understand kicks in. This is rather like the ever-increasing pace of modern technology and modern life and the continuing inflation of prices.

We honor the achievements of Drs. Perlmutter, Riess, and Schmidt and of their research teams in increasing our understanding of our universe and its underlying physics. Interestingly, only a few weeks ago, a very important supernova in the nearby M101 galaxy was discovered, and it is also a Type 1a. Because it is so close, only 25 million light years away, it is yielding a lot of high quality data. Perhaps this celestial fireworks display was a harbinger of their Nobel Prize?

References:

http://www.nytimes.com/aponline/2011/10/04/science/AP-EU-SCI-Nobel-Physics.html?_r=2&hp

http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html

http://www.nobelprize.org/mediaplayer/index.php?id=1633 (Telephone interview with Adam Reiss)

http://supernova.lbl.gov/ (Supernova Cosmology Project)

https://darkmatterdarkenergy.wordpress.com/2011/08/31/m101-supernova-and-the-cosmic-distance-ladder/

https://darkmatterdarkenergy.wordpress.com/2011/07/04/dark-energy-drives-a-runaway-universe/

 

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