Archivi tag: dark matter

Complementarity of cosmological probes

In the past decade we have been able to determine cosmological parameters with uncertainties smaller than ever before. The availability of many independent observational probes – all pointing in the same direction – suggests that they are also accurate, i.e. not dominated by unknown systematic errors. Remarkably, this wealth of observational data can be explained by a relatively simple cosmological model dominated by dark matter and dark energy and governed by general relativity.

However, many profound questions remain unanswered, including:

  • What is the nature of dark energy ?
  • What is the nature of dark matter ?
  • Is general relativity the correct theory of gravity on the scales of the Universe ?

In the near future, as the precision of independent cosmological probes increases further (for example with the upcoming release of the Planck satellite cosmological results), it will be possible to answer this kind of questions. Alternatively, and more radically, it is also possible that by comparing and combining independent probes of increasing precision and accuracy some fundamental inconsistency in the standard cosmological model will be revealed, leading to the discovery of new physics.

The purpose of the workshop is to bring together in a friendly atmosphere observers and theorists in order to understand the strengths, weaknesses, and complementarity of different cosmological probes, and how best to combine the information and inform models. Pedagogic reviews will introduce each topic, and ample time for discussion will be available during talks and between sessions, and through panel discussions. There will be opportunities for contributed talks and posters. Attendance will be limited to 90 people maximum, on a “first come, first served” basis.

New ‘light’ on dark matter from galaxy clusters

Un gruppo di astronomi della University of Birmingham (UK), Academica Sinica in Taiwan e del Kavli Institute of Physics and Mathematics of the Universe in Japan, hanno trovato una nuova evidenza relativa al comportamento della materia scura, quella enigmatica componente che domina il contenuto di materia dell’Universo. Secondo i loro dati, la materia scura sarebbe di tipo “cold” su cui si basa il modello Lambda-CDM.

At a press conference in Taipei the team of astronomers report their measurements of the density of dark matter in the most massive objects in the universe, namely galaxy clusters.

They found that the density of dark matter decreases gently from the centre of these cosmic giants out to their diffuse outskirts. The fall in dark matter density from the centre to the outskirts agrees very closely with the CDM theory.

Almost eighty years after the first evidence for dark matter emerged from astronomy research, few scientists seriously doubt that it exists. However astronomers cannot see dark matter directly in the night sky, and particle physicists have not yet identified the dark matter particle in their experiments. What is dark matter? Is therefore a big unanswered question facing astronomers and particle physicists, especially because there is strong evidence that 85% of the mass in the universe is invisible dark matter. The team, led by Nobuhiro Okabe (Academia Sinica) and Graham Smith (Birmingham), used the Subaru telescope in Hawaii to investigate the nature of dark matter by measuring its density in fifty galaxy clusters, the most massive objects in the Universe. “A galaxy cluster is like a huge city that you view from above during the night“, explains Smith. “Each bright city light is a galaxy, and the dark areas between the lights that appears to be empty during the night are actually full of dark matter. You can think of the dark matter in a galaxy cluster as being the infrastructure within which the galaxies live. We wanted to know how the density of dark matter changes as you drive from the centre of a these huge cities out to the suburbs“. The density of dark matter depends on the properties of the individual dark matter particles, just like the density of everyday materials depends on what they are made of. CDM, the most successful dark matter theory to date, predicts that dark matter particles only interact with each other and with other matter via the force of gravity, they don’t emit or absorb light. It also predicts that the density of dark matter in the centre of the most massive objects in the Universe, for example the Coma cluster of galaxies, the nearest cosmic giant to Earth, is lower than in less massive objects, including individual galaxies like our own home, the Milky Way. Observing galaxy clusters with the Subaru Prime Focus Camera (Suprime-Cam), the team measured the density of dark matter in galaxy clusters using the effect of gravitational lensing. As predicted by Einstein, gravitational lensing is the change in the direction and shape of a light beam as it travels through the curved space close to a massive object. The apparent shape and position of distant galaxies that astronomers observe are therefore altered by the dark matter in cosmic giants. Lead author Okabe enthused: “The Subaru Telescope is fantastic machine for gravitational lensing. It allows us to measure very precisely how the dark matter in galaxy clusters distorts light from distant galaxies and gauge tiny changes in the appearance of a huge number of faint galaxies”. CDM theory uses two numbers to describe how the density of dark matter in galaxy clusters changes from the dense centre to the low density outskirts. One number is simply the mass of the galaxy cluster, and the second is the so-called concentration parameter, CDM theory predicts that galaxy clusters have a low concentration parameter (less dense central regions), in contrast to individual galaxies that have a high concentration parameter (more dense central regions). The team combined observations of 50 of the most massive known galaxy clusters into a single measurement of the average concentration parameter of massive galaxy clusters. They found that the concentration parameter, that describes how density changes from the center to the outskirts of clusters, matches the CDM theory. In the past, astronomers have studied small handfuls of clusters, finding that they generally have large concentration parameters, suggesting possible problems with CDM theory. Okabe and Smith suspected that if they used a large number of clusters to measure the average concentration parameter, then they might get a different result. “We didn’t know what we would find”, comments Okabe, “we were curious what we would find, if we took a different approach”. After several years of observations, and careful data analysis, the results speak for themselves. The concentration parameter of galaxy clusters in the nearby universe matches CDM theory. Hints that measurements of dark matter in large numbers of galaxy clusters might support CDM theory emerged in 2010 when this team published their preliminary results in the Publications of the Astronomical Society of Japan. They recently won that Society’s 2012 Excellent Paper Award for that work, and are now enjoying the fruits of their hard work to improve their analysis and expand their study in the intervening years. “This is a very satisfying result. Our new results are based on a very careful analysis of the best available data“, comments Okabe. What does the future hold for the team’s continued research on dark matter? Smith noted: “We don’t stop here. For example, we can improve our work by measuring the dark matter density on even smaller scales – right in the centre of these galaxy clusters. Adding measurements on smaller scales will help us to learn more about dark matter in the future”. Team member Masahiro Takada is also excited about the future: “Combining lensing observations of many galaxy clusters into a single measurement like this is a very powerful technique. Japanese astronomers are preparing to use Subaru Telescope’s new Hyper Suprime-Cam (HSC) to conduct one of the biggest surveys of galaxies in human history. Our new results are a beautiful confirmation of our plan to use HSC for gravitational lensing studies“.

University of Birmingham: Cosmic giants shed new light on dark matter

Subaru Telescope: Cosmic Giants Shed New Light on Dark Matter

arXiv: LoCuSS: The Mass Density Profile of Massive Galaxy Clusters at z=0.2

Anapole dark matter?

Comparison of an anapole field with common electric and magnetic dipoles. The anapole field, top, is generated by a toroidal electrical current. As a result, the field is confined within the torus, instead of spreading out like the fields generated by conventional electric and magnetic dipoles. (Michael Smeltzer / Vanderbilt)

La maggior parte della materia presente nell’Universo è composta di particelle che possiedono un campo elettromagnetico insolito, a forma di ciambella o toroidale, chiamato dipolo toroidale o anapolo. Questa idea, che caratterizza le particelle di materia scura con una rara forma di elettromagnetismo, è stata rafforzata da una analisi dettagliata eseguita da una coppia di fisici teorici della Vanderbilt University: Robert Scherrer e Chiu Man Ho i cui risultato sono apparsi nella rivista Physics Letters B.

There are a great many different theories about the nature of dark matter. What I like about this theory is its simplicity, uniqueness and the fact that it can be tested”, said Scherrer. In the article, titled “Anapole Dark Matter”, the physicists propose that dark matter, an invisible form of matter that makes up 85 percent of the all the matter in the universe, may be made out of a type of basic particle called the Majorana fermion. The particle’s existence was predicted in the 1930’s but has stubbornly resisted detection. A number of physicists have suggested that dark matter is made from Majorana particles, but Scherrer and Ho have performed detailed calculations that demonstrate that these particles are uniquely suited to possess a rare, donut-shaped type of electromagnetic field called an anapole. This field gives them properties that differ from those of particles that possess the more common fields possessing two poles (north and south, positive and negative) and explains why they are so difficult to detect. “Most models for dark matter assume that it interacts through exotic forces that we do not encounter in everyday life. Anapole dark matter makes use of ordinary electromagnetism that you learned about in school – the same force that makes magnets stick to your refrigerator or makes a balloon rubbed on your hair stick to the ceiling”, said Scherrer. “Further, the model makes very specific predictions about the rate at which it should show up in the vast dark matter detectors that are buried underground all over the world. These predictions show that soon the existence of anapole dark matter should either be discovered or ruled out by these experiments”. Fermions are particles like the electron and quark, which are the building blocks of matter. Their existence was predicted by Paul Dirac in 1928. Ten years later, shortly before he disappeared mysteriously at sea, Italian physicist Ettore Majorana produced a variation of Dirac’s formulation that predicts the existence of an electrically neutral fermion. Since then, physicists have been searching for Majorana fermions. The primary candidate has been the neutrino, but scientists have been unable to determine the basic nature of this elusive particle. The existence of dark matter was also first proposed in the 1930’s to explain discrepancies in the rotational rate of galactic clusters. Subsequently, astronomers have discovered that the rate that stars rotate around individual galaxies is similarly out of sync. Detailed observations have shown that stars far from the center of galaxies are moving at much higher velocities than can be explained by the amount of visible matter that the galaxies contain. Assuming that they contain a large amount of invisible “dark” matter is the most straightforward way to explain these discrepancies. Scientists hypothesize that dark matter cannot be seen in telescopes because it does not interact very strongly with light and other electromagnetic radiation. In fact, astronomical observations have basically ruled out the possibility that dark matter particles carry electrical charges. More recently, though, several physicists have examined dark matter particles that don’t carry electrical charges, but have electric or magnetic dipoles. The only problem is that even these more complicated models are ruled out for Majorana particles. That is one of the reasons that Ho and Scherrer took a closer look at dark matter with an anapole magnetic moment. “Although Majorana fermions are electrically neutral, fundamental symmetries of nature forbid them from acquiring any electromagnetic properties except the anapole”, Ho said. The existence of a magnetic anapole was predicted by the Soviet physicist Yakov Zel’dovich in 1958. Since then it has been observed in the magnetic structure of the nuclei of cesium-133 and ytterbium-174 atoms. Particles with familiar electrical and magnetic dipoles, interact with electromagnetic fields even when they are stationary. Particles with anapole fields don’t. They must be moving before they interact and the faster they move the stronger the interaction. As a result, anapole particles would have been have been much more interactive during the early days of the universe and would have become less and less interactive as the universe expanded and cooled. The anapole dark matter particles suggested by Ho and Scherrer would annihilate in the early Universe just like other proposed dark matter particles, and the left-over particles from the process would form the dark matter we see today. But because dark matter is moving so much more slowly at the present day, and because the anapole interaction depends on how fast it moves, these particles would have escaped detection so far, but only just barely.

Vanderbilt University: New, simple theory may explain mysterious dark matter
arXiv: Anapole Dark Matter

222° Meeting of the American Astronomical Society

One of the largest astronomy meetings of the year will open to the public for the first time in its history. More than 500 astronomers, journalists and guests will bring their cosmic know-how to Indianapolis next week for the 222nd meeting of the American Astronomical Society (AAS). The conference begins on Sunday (June 2) and runs through June 6 at the Indiana Convention Center; it is the second of two meetings held annually by the AAS. New findings about alien worlds, mysterious dark matter and the Milky Way will be discussed, and this year anyone can take part in the cosmic action. Several presentations on Monday and Tuesday will be geared toward amateurs that decide to pay the fee and attend. The presentations include information about the Hubble Space Telescope, nearby exoplanets, Pluto, and the formation of galaxies in the early universe. In addition to those talks, the society will also hold two free public events during the convention. Throughout the course of the conference, scientists will take part in town hall-style meetings about NASA, the National Science Foundation and other agencies. The latest findings from the badly damaged planet hunting Kepler Space Telescope will be presented as well. Twitter users can follow the conference using the hashtag #AAS222.

A dark-disk Universe

Un gruppo di ricercatori della Harvard University hanno proposto l’esistenza di un diverso tipo di materia scura che non è descritto dai modelli attuali. Nel loro articolo pubblicato su Physical Review Letters, gli scienziati suggeriscono che la materia scura non sia necessariamente ‘fredda’ e che non interagisca con altre particelle.

In the visible Universe, galaxies form into a disk shape, just like the Milky Way. All of its members align roughly along a single plane, this due to the forces of gravity and spin. Objects form into masses which, over time, spread out into a disk shape. Dark matter, on the other hand, appears to hover around galaxies like a halo, at least according to current models (post1; post2). It’s seen as dark, cold and with so little energy that dark matter particles rarely if ever run into one another. The researchers in this new study suggest there may be other types of matter, however, that behaves more like visible matter. And, because of that, they suggest it could bunch up due to dark-matter-type gravity and form disks as well. These disks, which they describe as dark matter component double-disk dark matter, could represent as much as 5 percent of all existing dark matter. For dark matter to clump, it would need to have other properties similar to visible matter as well. For that reason, the researchers suggest it’s possible that there exists dark atoms, dark photons, and likely some form of dark electromagnetic force as well. Research on dark matter over the years has led to a model that describes dark matter as existing in a ball shape—galaxies sit in the middle of the ball, which would mean observers living in a galaxy would “see” it as existing everywhere around them. But it’s possible that other types of shapes exist as well, the researchers suggest, because there are other types of matter in the visible Universe. They note that baryonic matter, that is matter made of strongly acting fermions known as baryons, is believed to make up approximately 5 percent of all matter in the known Universe. For that reason, they conclude that it would appear likely that similar differences in dark matter would occur as well, and perhaps in nearly equal proportions. If true, it would mean there could be whole dark galaxies out there, undetectable, yet as real as those we can see with the naked eye. Much more research will have to be done in this area before adding such types of dark matter to models in general use, of course. Until then, it will remain an abstract theory.

arXiv: A Dark-Disk Universe

STARS 2013 e SMFNS 2013

The events are the second and third in a series of meetings gathering scientists working on astroparticle physics, cosmology, gravitation, nuclear physics, and related fields. As in previous years, the meeting sessions will consist of invited and contributed talks and will cover recent developments in the following topics:
STARS2013 – New phenomena and new states of matter in the Universe, general relativity, gravitation, cosmology, heavy ion collisions and the formation of the quark-gluon plasma, white dwarfs, neutron stars and pulsars, black holes, gamma-ray emission in the Universe, high energy cosmic rays, gravitational waves, dark energy and dark matter, strange matter and strange stars, antimatter in the Universe, and topics related to these.
SMFNS2013 – Strong magnetic fields in the Universe, strong magnetic fields in compact stars and in galaxies, ultra-strong magnetic fields in neutron star mergers, quark stars and magnetars, strong magnetic fields and the cosmic microwave background, and topics related to these.

CDMS-II, evidence of wimpy dark matter?

Physicists operating an experiment located half a mile underground in Minnesota reported this weekend that they have found possible hints of dark-matter particles. The Cryogenic Dark Matter Search experiment has detected three events with the characteristics expected of dark matter particles, MIT graduate student Kevin McCarthy reported at the American Physical Society meeting in Denver on April 13.

There’s a limited amount one can really say from just three events” McCarthy says. “But it certainly warrants some further investigation”. A statistical fluctuation of the experimental background is likely to produce three or more events resembling this result a little over 5 percent of the time. However, all three of these events have energies more like those expected of a low-mass dark-matter particle, something that should happen by chance only 0.19 percent of the time. This consideration brings the result to a higher confidence level, around 3 sigma. These odds might give one a reason to feel optimistic, but they do not pass the criterion physicists use to claim a discovery, so CDMS scientists say they’re staying reserved until they’ve conducted more analysis. CDMS results from 2010 included two potential dark-matter particles in a higher mass range, but physicists ruled out those candidates with further study. The latest result does bring new intrigue to the hunt for dark matter. The best fit with the result would be a dark-matter particle with a mass of 8.6 GeV. This seems to align with some interpretations of recent results from the CoGeNT dark matter experiment and from the Fermi Gamma-Ray Space Telescope. But the CDMS result contradicts indications from the experiment currently leading the field, the XENON experiment. “There’s been an interesting back-and-forth between experiments” says CDMS Spokesperson Blas Cabrera of Stanford University and SLAC National Accelerator Laboratory. “To investigate this hint, we’ll certainly need more data. If a signal persists, it will need to be replicated by other experiments with different technologies before it is accepted by the community”. CDMS collaboration members expect to shed more light on the result themselves later this year with new data they are taking using detectors of a different material, germanium as opposed to silicon. If a dark-matter particle passed through, it could knock against the nucleus of an atom in the detector, releasing a small amount of energy as charge and heat. Scientists keep the detector cold and shielded from cosmic rays underground in order to best watch for such a signal. They study interactions of non-dark-matter particles to rule out background noise. CDMS scientists were surprised this result came from its lighter silicon detectors and not its heavier germanium detectors. When they designed their experiment in the early 2000s, they included the silicon ones only to verify results from the germanium. Scientists expected they would need the bulkier detectors to register the nuclear recoil from massive dark-matter particles, predicted to weigh about 100 times the mass of a proton. However, in the past couple of years, new theories and experimental results have attracted physicists’ attention to the idea of low-mass dark-matter particles—only about 10 times as massive as a proton. The idea for a high-mass dark-matter particle comes mainly from the theory of supersymmetry, which posits that each of the particles we know has a more massive partner particle. The idea of a low-mass dark-matter particle comes mainly from a theory that has only recently begun to gain currency, that of a “dark sector” made up of many types of dark particles and forces. “If there is a dark sector, it could be just as complex as the ordinary matter sector”, says CDMS physicist Bernard Sadoulet of Lawrence Berkeley National Laboratory and the University of California, Berkeley. “Finding low-mass dark-matter particles would not rule out the theory of supersymmetry, but “it’s hard to reconcile it with at least the most vanilla flavors of of the theory”, says CDMS Project Manager Dan Bauer of Fermilab. The year 2013 should be an interesting one in the search for dark matter.

CDMS: Dark Matter Search Results from CDMS-II Silicon Detectors

SLAC: Cryogenic Dark Matter Search Adds New Intrigue with Latest Result
Fermilab:Dark-matter search results from CDMS II silicon detectors
arXiv: Dark Matter Search Results Using the Silicon Detectors of CDMS II
arXiv: Silicon Detector Results from the First Five-Tower Run of CDMS II
E. Figueroa-Feliciano's presentation at Light Dark Matter 2013
K. McCarthy's presentation at APS
B. Cabrera's Panofsky Prize presentation at APS
B. Sadoulet's Panofsky Prize presentation at APS

Planck reveals an almost perfect Universe

Acquired by ESA’s Planck space telescope, the most detailed map ever created of the cosmic microwave background – the relic radiation from the Big Bang – was released today revealing the existence of features that challenge the foundations of our current understanding of the Universe.

The image is based on the initial 15.5 months of data from Planck and is the mission’s first all-sky picture of the oldest light in our Universe, imprinted on the sky when it was just 380 000 years old. At that time, the young Universe was filled with a hot dense soup of interacting protons, electrons and photons at about 2700ºC. When the protons and electrons joined to form hydrogen atoms, the light was set free. As the Universe has expanded, this light today has been stretched out to microwave wavelengths, equivalent to a temperature of just 2.7 degrees above absolute zero. This ‘cosmic microwave background’ – CMB – shows tiny temperature fluctuations that correspond to regions of slightly different densities at very early times, representing the seeds of all future structure: the stars and galaxies of today. According to the standard model of cosmology, the fluctuations arose immediately after the Big Bang and were stretched to cosmologically large scales during a brief period of accelerated expansion known as inflation. Planck was designed to map these fluctuations across the whole sky with greater resolution and sensitivity than ever before. By analysing the nature and distribution of the seeds in Planck’s CMB image, we can determine the composition and evolution of the Universe from its birth to the present day.

Overall, the information extracted from Planck’s new map provides an excellent confirmation of the standard model of cosmology at an unprecedented accuracy, setting a new benchmark in our manifest of the contents of the Universe. But because precision of Planck’s map is so high, it also made it possible to reveal some peculiar unexplained features that may well require new physics to be understood. “The extraordinary quality of Planck’s portrait of the infant Universe allows us to peel back its layers to the very foundations, revealing that our blueprint of the cosmos is far from complete. Such discoveries were made possible by the unique technologies developed for that purpose by European industry,” says Jean-Jacques Dordain, ESA’s Director General. “Since the release of Planck’s first all-sky image in 2010, we have been carefully extracting and analysing all of the foreground emissions that lie between us and the Universe’s first light, revealing the cosmic microwave background in the greatest detail yet,” adds George Efstathiou of the University of Cambridge, UK. One of the most surprising findings is that the fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model – their signals are not as strong as expected from the smaller scale structure revealed by Planck. Another is an asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look. Furthermore, a cold spot extends over a patch of sky that is much larger than expected. The asymmetry and the cold spot had already been hinted at with Planck’s predecessor, NASA’s WMAP mission, but were largely ignored because of lingering doubts about their cosmic origin. “The fact that Planck has made such a significant detection of these anomalies erases any doubts about their reality; it can no longer be said that they are artefacts of the measurements. They are real and we have to look for a credible explanation,” says Paolo Natoli of the University of Ferrara, Italy. “Imagine investigating the foundations of a house and finding that parts of them are weak. You might not know whether the weaknesses will eventually topple the house, but you’d probably start looking for ways to reinforce it pretty quickly all the same,” adds François Bouchet of the Institut d’Astrophysique de Paris. One way to explain the anomalies is to propose that the Universe is in fact not the same in all directions on a larger scale than we can observe. In this scenario, the light rays from the CMB may have taken a more complicated route through the Universe than previously understood, resulting in some of the unusual patterns observed today. “Our ultimate goal would be to construct a new model that predicts the anomalies and links them together. But these are early days; so far, we don’t know whether this is possible and what type of new physics might be needed. And that’s exciting,” says Professor Efstathiou.

New cosmic recipe

Beyond the anomalies, however, the Planck data conform spectacularly well to the expectations of a rather simple model of the Universe, allowing scientists to extract the most refined values yet for its ingredients. Normal matter that makes up stars and galaxies contributes just 4.9% of the mass/energy density of the Universe. Dark matter, which has thus far only been detected indirectly by its gravitational influence, makes up 26.8%, nearly a fifth more than the previous estimate. Conversely, dark energy, a mysterious force thought to be responsible for accelerating the expansion of the Universe, accounts for less than previously thought. Finally, the Planck data also set a new value for the rate at which the Universe is expanding today, known as the Hubble constant. At 67.15 kilometres per second per megaparsec, this is significantly less than the current standard value in astronomy. The data imply that the age of the Universe is 13.82 billion years. “With the most accurate and detailed maps of the microwave sky ever made, Planck is painting a new picture of the Universe that is pushing us to the limits of understanding current cosmological theories,” says Jan Tauber, ESA’s Planck Project Scientist. “We see an almost perfect fit to the standard model of cosmology, but with intriguing features that force us to rethink some of our basic assumptions. “This is the beginning of a new journey and we expect that our continued analysis of Planck data will help shed light on this conundrum.”

A series of scientific papers describing the new results will be published on 22 March.
arXiv: Planck 2013 results. I. Overview of products and scientific results

See also in arXiv: Planck 2013 results (29 papers)

arXiv: Planck 2013 results support the simplest cyclic models

arXiv: Inflationary paradigm in trouble after Planck2013

More info: Planck ESA

48° Rencontres de Moriond: Very High Energy Phenomena in the Universe

48° Rencontres de Moriond: Very High Energy Phenomena in the Universe – The Rencontres de Moriond session on Very High Energy Phenomena in the Universe will review the subject 4 years after the last edition.  Continua a leggere 48° Rencontres de Moriond: Very High Energy Phenomena in the Universe

Cosmology, Large Scale Structure and First Objects

Cosmology, Large Scale Structure and First Objects – This USP Conference shall cover the key issues of Cosmology, from particle physics, fundamental gravitation, cosmic background radiation, dark matter and dark energy, large-scale structure, simulations of the formation of structure in the Universe, as well as comparisons to observations.  Continua a leggere Cosmology, Large Scale Structure and First Objects