No trace of dark matter in gamma-ray background

The data that were analysed in the work described here. Fluctuations in the isotropic gamma-ray background, based on 81 months of data. Emission from our own Galaxy, the Milky Way, is masked in grey. Credit: Mattia Fornasa, UvA/Grappa

Researchers from the University of Amsterdam's (UvA) GRAPPA Center of Excellence have just published the most precise analysis of the fluctuations in the gamma-ray background to date. By making use of more than six years of data gathered by the Fermi Large Area Telescope, the researchers found two different source classes contributing to the gamma-ray background. No traces of a contribution of dark matter particles were found in the analysis. The collaborative study was performed by an international group of researchers and is published in the latest edition of the journal Physical Review D.

Gamma rays are particles of light, or photons, with the highest energy in the universe and are invisible to the human eye. The most common emitters of gamma rays are blazars: supermassive black holes at the centers of galaxies. In smaller numbers, gammy rays are also produced by a certain kind of stars called pulsars and in huge stellar explosions such as supernovae.

In 2008 NASA launched the Fermi satellite to map the gamma-ray universe with extreme accuracy. The Large Area Telescope, mounted on the Fermi satellite, has been taking data ever since. It continuously scans the entire sky every three hours. The majority of the detected gamma rays is produced in our own Galaxy (the Milky Way), but the Fermi telescope also managed to detect more than 3000 extragalactic sources (according to the latest count performed in January 2016). However, these individual sources are not enough to explain the total amount of gamma-ray photons coming from outside our Galaxy. In fact, about 75% of them are unaccounted for.

Isotropic gamma-ray background

As far back as the late 1960s, orbiting observatories found a diffuse background of gamma rays streaming from all directions in the universe. If you had gamma-ray vision, and looked at the sky, there would be no place that would be dark.

The source of this so-called isotropic gamma-ray background has hitherto remained unknown. This radiation could be produced by unresolved blazars, or other sources too faint to be detected with the Fermi telescope. Parts of the gamma-ray background might also hold the fingerprint of the illustrious dark matter particle, a so-far undetected particle held responsible for the missing 80% of the matter in our universe. If two dark matter particles collide, they can annihilate and produce a signature of gamma-ray photons.

This view shows the entire sky in gammy ray radiation, at energies greater than 1 GeV, based on five years of data from the Large Area Telescope instrument on NASA's Fermi Gamma-ray Space Telescope. Brighter colours indicate brighter gamma-ray emission. The large bright band in the middle is the emission from our own Galaxy. Credit: NASA/DOE/Fermi LAT Collaboration

Fluctuations

Together with colleagues, Dr Mattia Fornasa, an astroparticle physicist at the UvA and lead author of the paper, performed an extensive analysis of the gamma-ray background by using 81 months of data gathered by the Fermi Large Area Telescope – much more data and with a larger energy range than in previous studies. By studying the fluctuations in the intensity of the gamma-ray background, the researchers were able to distinguish two different contributions to the gamma-ray background. One class of gamma-ray sources is needed to explain the fluctuations at low energies (below 1 GeV) and another type to generate the fluctuations at higher energy – the signatures of these two contributions is markedly different.

In their paper the researchers suggest that the gamma rays in the high-energy ranges – from a few GeV up – are likely originating from unresolved blazars. Further investigation into these potential sources is currently being carried out by Fornasa, fellow UvA researcher Shin'ichiro Ando and colleagues from the University of Torino, Italy. However, it seems much harder to pinpoint a source for the fluctuations with energies below 1 GeV. None of the known gamma-ray emitters have a behaviour that is consistent with the new data.

Constraints on dark matter

To date, the Fermi telescope has not detected any conclusive indication of gamma-ray emission originating from dark-matter particles. Also, this latest study showed no indication of a signal associated with dark matter. Using their data, Fornasa and colleagues were even able to rule out some models of dark matter that would have produced a detectable signal.

'Our measurement complements other search campaigns that used gamma rays to look for dark matter and it confirms that there is little room left for dark matter induced gamma-ray emission in the isotropic gamma-ray background,' says Fornasa.

ALPHA observes light spectrum of antimatter for first time

In a paper published today in the journal Nature, the ALPHA collaboration reports the first ever measurement on the optical spectrum of an antimatter atom. This achievement features technological developments that open up a completely new era in high-precision antimatter research. It is the result of over 20 years of work by the CERN antimatter community.

"Using a laser to observe a transition in antihydrogen and comparing it to hydrogen to see if they obey the same laws of physics has always been a key goal of antimatter research," said Jeffrey Hangst, Spokesperson of the ALPHA collaboration.

Atoms consist of electrons orbiting a nucleus. When the electrons move from one orbit to another they absorb or emit light at specific wavelengths, forming the atom's spectrum. Each element has a unique spectrum. As a result, spectroscopy is a commonly used tool in many areas of physics, astronomy and chemistry. It helps to characterise atoms and molecules and their internal states. For example, in astrophysics, analysing the light spectrum of remote stars allows scientists to determine their composition.

With its single proton and single electron, hydrogen is the most abundant, simple and well-understood atom in the Universe. Its spectrum has been measured to very high precision. Antihydrogen atoms, on the other hand are poorly understood. Because the universe appears to consist entirely of matter, the constituents of antihydrogen atoms – antiprotons and positrons – have to be produced and assembled into atoms before the antihydrogen spectrum can be measured. It's a painstaking process, but well worth the effort since any measurable difference between the spectra of hydrogen and antihydrogen would break basic principles of physics and possibly help understand the puzzle of the matter-antimatter imbalance in the universe.

Today's ALPHA result is the first observation of a spectral line in an antihydrogen atom, allowing the light spectrum of matter and antimatter to be compared for the first time. Within experimental limits, the result shows no difference compared to the equivalent spectral line in hydrogen. This is consistent with the Standard Model of particle physics, the theory that best describes particles and the forces at work between them, which predicts that hydrogen and antihydrogen should have identical spectroscopic characteristics.

The ALPHA collaboration expects to improve the precision of its measurements in the future. Measuring the antihydrogen spectrum with high-precision offers an extraordinary new tool to test whether matter behaves differently from antimatter and thus to further test the robustness of the Standard Model.

ALPHA is a unique experiment at CERN's Antiproton Decelerator facility, able to produce antihydrogen atoms and hold them in a specially-designed magnetic trap, manipulating antiatoms a few at a time. Trapping antihydrogen atoms allows them to be studied using lasers or other radiation sources.

"Moving and trapping antiprotons or positrons is easy because they are charged particles," said Hangst. "But when you combine the two you get neutral antihydrogen, which is far more difficult to trap, so we have designed a very special magnetic trap that relies on the fact that antihydrogen is a little bit magnetic."

Antihydrogen is made by mixing plasmas of about 90,000 antiprotons from the Antiproton Decelerator with positrons, resulting in the production of about 25,000 antihydrogen atoms per attempt. Antihydrogen atoms can be trapped if they are moving slowly enough when they are created. Using a new technique in which the collaboration stacks anti-atoms resulting from two successive mixing cycles, it is possible to trap on average 14 anti-atoms per trial, compared to just 1.2 with earlier methods. By illuminating the trapped atoms with a laser beam at a precisely tuned frequency, scientists can observe the interaction of the beam with the internal states of antihydrogen. The measurement was done by observing the so-called 1S-2S transition. The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement.

The current result, along with recent limits on the ratio of the antiproton-electron mass established by the ASACUSA collaboration, and antiproton charge-to-mass ratio determined by the BASE collaboration, demonstrate that tests of fundamental symmetries with antimatter at CERN are maturing rapidly.

Optical tractor beam traps bacteria

Picture of the distribution of the genetic information in an Escherichia coli bacterial cell: Physicists at Bielefeld University are the first to photograph this distribution at the highest optical resolution without anchoring the cells on a glass substrate. Credit: Bielefeld University

Until recently, if scientists wanted to study blood cells, algae, or bacteria under the microscope, they had to mount these cells on a substrate such as a glass slide. Physicists at Bielefeld and Frankfurt Universities have developed a method that traps biological cells with a laser beam to study them at very high resolutions. In science fiction books and films, the principle is known as the 'tractor beam.' Using this procedure, the physicists have obtained superresolution images of the DNA in single bacteria. The physicist Robin Diekmann and his colleagues are publishing this new development in the latest issue of the research journal Nature Communications.

One of the problems facing researchers who want to examine biological cells microscopically is that any preparatory treatment will change the cells. Many bacteria prefer to be able to swim freely in solution. Blood cells are similar: They are continuously in rapid flow, and do not remain on surfaces. Indeed, adhering to a surface changes their structure and they die.

'Our new method enables us to take cells that cannot be anchored on surfaces and then use an optical trap to study them at a very high resolution. The cells are held in place by a kind of optical tractor beam. The principle underlying this laser beam is similar to the concept to be found in the television series "Star Trek",' says Professor Dr. Thomas Huser. He is the head of the Biomolecular Photonics Research Group in the Faculty of Physics. 'What's special is that the samples are not only immobilized without a substrate but can also be turned and rotated. The laser beam functions as an extended hand for making microscopically small adjustments.'

The Bielefeld physicists have further developed the procedure for use in superresolution fluorescence microscopy. This is considered to be a key technology in biology and biomedicine because it delivers the first way to study biological processes in living cells at a high scale – something that was previously only possible with electron microscopy. To obtain images with such microscopes, researchers add fluorescent probes to the cells they wish to study, and these will then light up when a laser beam is directed towards them. A sensor can then be used to record this fluorescent radiation so that researchers can even gain three-dimensional images of the cells.

In their new method, the Bielefeld researchers use a second laser beam as an optical trap so that the cells float under the microscope and can be moved at will. 'The laser beam is very intensive but invisible to the naked eye because it uses infrared light,' says Robin Diekmann, a member of the Biomolecular Photonics Research Group. 'When this laser beam is directed towards a cell, forces develop within the cell that hold it within the focus of the beam,' says Diekmann. Using their new method, the Bielefeld physicists have succeeded in holding and rotating bacterial cells in such a way that they can obtain images of the cells from several sides. Thanks to the rotation, the researchers can study the three-dimensional structure of the DNA at a resolution of circa 0.0001 millimetres.

Professor Huser and his team want to further modify the method so that it will enable them to observe the interplay between living cells. They would then be able to study, for example, how germs penetrate cells.

To develop the new methods, the Bielefeld scientists are working together with Prof. Dr. Mike Heilemann and Christoph Spahn from the Johann Wolfgang Goethe University of Frankfurt am Main.

Magnetic mirror could shed new light on gravitational waves and the early universe

Researchers have created a new metamaterial half-wave plate operating at millimeter wavelengths that is less than 1-millimeter thick. When light reflects off the device, the polarization parallel to the wire-grid is reversed in its orientation, whereas the polarization perpendicular to it stays in the same direction. The overall effect is to create a differential phase-shift between orthogonal polarizations equal to 180 degrees. The rotation of the plate causes modulation of the polarization. Credit: Giampaolo Pisano, Cardiff University.

Researchers have created a new magnetic mirror-based device that could one day help cosmologists discover new details about ripples in space-time known as gravitational waves, particularly those emitted when the universe was extremely young.

The new work is part of a multi-institutional collaboration funded by the European Space Agency's (ESA) Technology Research Program to develop technologies necessary for future experiments such as the proposed Cosmic Origins Explorer satellite mission program. This space mission aims to acquire high precision, full-sky maps of the cosmic microwave background – the relic emission that survived since the Big Bang.

Cosmic microwave background has been the subject of intense investigation since its discovery about 50 years ago. Recent years have seen an increased focus on the polarized components of this microwave background – in particular a component called B-mode, which is thought to hold the key to information about primordial gravitational waves and the physical processes that occurred very early in the history of the universe.

In The Optical Society (OSA) journal Applied Optics, the researchers demonstrated a new type of polarization modulator based on a magnetic mirror. The new device could overcome a major challenge to detecting the B-mode polarization—the ability to modulate microwave polarization across a broad frequency range. Broadband operation is necessary to spectrally discriminate the extremely faint B-mode polarization from the foreground radiation of other astrophysical sources.

"We, like others, have been working for over two decades on the development of technologies that would enable the detection of the B-mode polarization," said Giampaolo Pisano, Cardiff University, UK, first author of the paper. "This has proven to be a challenging problem because only a tiny part of the overall signal exhibits this polarization."

Developing the technology

A key component for detecting B-mode radiation is a half-wave plate, a device used to modulate the polarization of electromagnetic radiation. Rotating the half-wave plate causes the polarization of the radiation to also rotate, creating an oscillating pattern that can be distinguished from the constant signal of unpolarized radiation.

Previous implementations of these half wave plates have resulted in inherently narrowband devices due to either the optical properties of available materials or the design used. Operation over a wide range of wavelengths is crucial to distinguishing B-mode polarization originating from the early universe from signals originating from other sources.

"Most of the effort in technology development has been aimed at making optical components that work over larger bandwidths," said Pisano. "A device that covers a wide frequency range would greatly enhance the performance of complex space-borne instrumentation."

In the new work, Pisano and his colleagues tried a completely new approach that uses metamaterials – manmade materials engineered with features not found in natural materials – to create a magnetic mirror that they combined with a polarizing wire grid.

"Metamaterials enabled us to invent a material with the characteristics we needed," said Pisano. "Because the approach we used is new, it allowed us to overcome the frequency range limits that other researchers have faced."

Their new method takes advantage of the fact that the reflection from an artificial magnetic surface will be out of phase from that reflecting from a perfect electric conductor, or metal. Adding the wire grid to the magnetic mirror allows one polarization to "see" the metal grid, while orthogonally polarized radiation reflects off the magnetic mirror. The resulting device can alter polarization over a large microwave frequency range.

The prototype device demonstrated in the paper operates from about 100 to 400 gigahertz with more than 90 percent efficiency, meaning that less than 10 percent of the signal was lost. The researchers say that with some minor adjustments, they expect to achieve even greater bandwidth and higher efficiency.

Getting ready for space

At 20 centimeters across, the prototype device is a miniaturized version of the one that could ultimately be needed for the Cosmic Origins Explorer satellite. The researchers are now working to develop a half-meter version, with the ultimate goal of developing a final device more than a meter in diameter. Making such a large device with the needed precision will require new facilities and new methods for handling the device during the various manufacturing steps, developments that the researchers say will likely be as difficult as developing the initial concept.

"Now that we've demonstrated the concept, we need to perform space qualification tests to demonstrate its ruggedness for a satellite launch," said Pisano. "We also need to deploy it in ground-based B-mode detection instruments to demonstrate its usability in the field."