Time filter

Source Type

Flash Physics is our daily pick of the latest need-to-know developments from the global physics community selected by Physics World's team of editors and reporters The distribution of dark matter in the universe appears to be smoother and more diffuse than previously thought – according to a study of wide-area images of the distant universe. Astronomers at the University of Edinburgh in the UK, Leiden University in the Netherlands, the Argelander Institute for Astronomy in Germany and the Swinburne University of Technology in Australia used the weak gravitational lensing of light from far-off galaxies to map the distribution of dark matter in intervening parts of the universe. The map is at odds with a prediction of dark-matter distribution that is based on the structure of the early universe based on measurements of the cosmic microwave background made by the Planck satellite. "Our findings will help to refine our theoretical model for how the universe has grown since its inception, improving our understanding of the modern-day universe," says Hendrik Hildebrandt of the Argelander Institute. Edinburgh's Catherine Heymans adds: "Unravelling what has happened since the Big Bang is a complex challenge, but by continuing to study the distant skies, we can build a picture of how our modern universe has evolved. The study is described in Monthly Notices of the Royal Astronomical Society. The UK's Engineering and Physical Sciences Research Council (EPSRC) has announced £60m for six new research hubs that aim to transform manufacturing in fields such as composite materials, 3D printing and medicine. The hubs, each receiving £10m, will draw together 17 universities and 200 industrial and academic partners to help turn research into products. The University of Huddersfield will lead a consortium to create a £30m Future Metrology Hub that will be based at the university's Centre for Precision Technologies and will open next year. "Our vision is to develop new technologies and universal methods that will integrate measurement science with design and production processes to improve control, quality and productivity," says physicist Jane Jiang, who will lead the Huddersfield hub. "These will become part of the critical infrastructure for a new generation of digital, high-value manufacturing, the so-called 4th industrial revolution, or Industry 4.0." The other hubs are led by Cardiff University (semiconductors), the universities of Nottingham (composites), Sheffield (advanced powder processes), Strathclyde (advanced crystallisation) and University College London (targeted healthcare). Pakistan will rename a physics research centre in Islamabad after the Nobel laureate Abdus Salam, who died 20 years ago. Born in what is now Pakistan, Salam shared the 1979 Nobel Prize for Physics for his work on unifying the weak and electromagnetic interactions. However, he was never fully celebrated in his native country because he was a member of the Ahmadiyya community. Now, the prime minister Nawaz Sharif has announced that the National Centre for Physics at Quaid-i-Azam University in Islamabad will be called the Professor Abdus Salam Center fo Physics. There will also be five annual fellowships named after Salam, which will be awarded to Pakistani students pursuing PhDs in physics. In addition to his Nobel prize, Salam is remembered for founding the International Centre for Theoretical Physics in Trieste, Italy, in 1964. Now called the Abdus Salam International Centre for Theoretical Physics, the centre fosters the growth of mathematical physics in developing countries.

News Article | December 7, 2016
Site: www.eurekalert.org

Scientists have gained fresh insight into the nature of dark matter, the elusive material that accounts for much of the mass of the Universe. Calculations based on a study of distant galaxies, using a powerful telescope, suggest that dark matter is less dense and more smoothly distributed throughout space than previously thought. The results, from an international team of scientists, will inform efforts to understand how the Universe has evolved in the 14 billion years since the Big Bang, by helping to refine theoretical models of how it developed. Scientists studied wide-area images of the distant universe, taken from the European Southern Observatory in Chile. They applied a technique based on the bending of light by gravity - known as weak gravitational lensing - to map out the distribution of dark matter in the Universe today. Their study represents the largest area of the sky to be mapped using this technique to date. To eliminate bias in their results, scientists carried out three sets of calculations, including two false sets, only revealing to themselves at the outcome which of the sets was real. The new results contradict previous predictions from a survey of the far-off universe, representing a point in time soon after the Big Bang, imaged by the European Space Agency's Planck satellite. This previous study used a theoretical model to project how the Universe should appear today. The disagreement between this prediction and the latest direct measurements suggests that scientists' understanding of the evolving modern day Universe is incomplete and needs more research. The latest study, published in Monthly Notices of the Royal Astronomical Society, was carried out by a team jointly led by the University of Edinburgh, the Argelander Institute for Astronomy in Germany, Leiden University in the Netherlands and Swinburne University of Technology, Australia in an ongoing project called the Kilo Degree Survey, or KiDS. It was supported by the European Research Council. Dr Hendrik Hildebrandt of the Argelander Institute for Astronomy in Germany, who co-led the study, said: "Our findings will help to refine our theoretical model for how the Universe has grown since its inception, improving our understanding of the modern day Universe." Dr Massimo Viola of Leiden University in the Netherlands, who co-led the study, said: "This latest result indicates that the cosmic web dark matter, which accounts for about one-quarter of the Universe, is less clumpy than we previously believed." Professor Catherine Heymans of the University of Edinburgh's School of Physics and Astronomy, who co-led the study, said: "Unravelling what has happened since the Big Bang is a complex challenge, but by continuing to study the distant skies, we can build a picture of how our modern Universe has evolved."

One of the world's largest fully steerable radio telescopes, the Effelsberg 100-m dish, surveyed the entire northern sky in the light of the neutral hydrogen (HI) 21-cm line. This effort, led by Jürgen Kerp (Argelander Institute for Astronomy) and Benjamin Winkel (Max Planck-Institut für Radioastronomie), began in 2008 and has culminated today in the initial data release of the Effelsberg-Bonn HI Survey (EBHIS). Funded by the German Research Foundation (Deutsche Forschungsgemeinschaft - DFG), the EBHIS data base is now freely accessible for all scientists around the world. In addition to the now released Milky Way data, the EBHIS project also includes unique information about HI in external galaxies out to a distance of about 750 million light years from Earth. Hydrogen is THE ELEMENT of the universe. Consisting of a single proton and an electron it is the simplest and most abundant element in space. One could almost consider the universe as a pure hydrogen universe, albeit with some minor "pollution" by heavier elements, among these carbon, the fundamental component of all organisms on Earth. The 21-cm line is a very faint but characteristic emission line of neutral atomic hydrogen (or HI). It is not only feasible to detect the weakest signals from distant galaxies with the 100-m Effelsberg antenna, but also to determine their motion relative to Earth with high precision. A special receiver was required in order to enable the EBHIS project. With seven receiving elements observing the sky independently from each other, it was possible to reduce the necessary observing time from decades to about five years only. Field Programmable Gate Array (FPGA) spectrometers were developed within the course of the EBHIS project, allowing real time processing and storage of about 100 million individual HI spectra with consistently good quality. The individual HI spectra were combined using high-performance computers into a unique map of the entire northern sky and provide unsurpassed richness in detail of the Milky Way Galaxy gas. Astronomy students at Bonn University had unique access to the pre-release EBHIS data. In 2013 the European Space Agency (ESA) signed a memorandum of understanding with the Bonn HI radio astronomers. ESA was granted exclusive access to EBHIS data for their Planck satellite mission and, in return, Bonn students were given unique access to Planck data for their thesis projects. Twelve Bachelor, nine Master, and five Doctoral thesis projects have been successfully completed since 2008. The Square Kilometer Array (SKA), the world's largest future radio astronomical facility, to be constructed in Australia and South Africa, will benefit directly from the EBHIS data. Owing to the construction of SKA as a radio interferometer, it is inherently insensitive to the faint and extended HI emission of the Milky Way and nearby external galaxies. Since the HI gas is measured very well by EBHIS, only combining SKA and EBHIS data will allow one to derive a comprehensive view of the interstellar HI gas. The Effelsberg-Bonn HI Survey will be a rich resource for science in the near and far future. Independent attempts to survey the entire northern sky with a 100-m class telescope are not scheduled. The EBHIS data will thus set the quality standard for the Milky Way Galaxy HI for the next decades. Explore further: An atlas of the Milky Way More information: B. Winkel et al. The Effelsberg-Bonn H i Survey: Milky Way gas, Astronomy & Astrophysics (2015). DOI: 10.1051/0004-6361/201527007

Smolic V.,European Southern Observatory | Smolic V.,Argelander Institute for Astronomy | Riechers D.A.,California Institute of Technology
Astrophysical Journal | Year: 2011

One of the main achievements in modern cosmology is the so-called unified model, which successfully describes most classes of active galactic nuclei (AGNs) within a single physical scheme. However, there is a particular class of radio-luminous AGNs that presently cannot be explained within this framework - the "low-excitation" radio AGN (LERAGN). Recently, a scenario has been put forward which predicts that LERAGNs and their regular "high- excitation" radio AGN (HERAGN) counterparts represent different (red sequence versus green valley) phases of galaxy evolution. These different evolutionary states are also expected to be reflected in their host galaxy properties, in particular their cold gas content. To test this, here we present CO(1→0) observations toward a sample of 11 of these systems conducted with CARMA. Combining our observations with literature data, we derive molecular gas masses (or upper limits) for a complete, representative, sample of 21 z < 0.1 radio AGNs. Our results yield that HERAGNs on average have a factor of 7 higher gas masses than LERAGNs. We also infer younger stellar ages, lower stellar, halo, and central supermassive black masses, as well as higher black hole accretion efficiencies in HERAGNs relative to LERAGNs. These findings support the idea that HERAGNs and LERAGNs form two physically distinct populations of galaxies that reflect different stages of massive galaxy buildup. © 2011. The American Astronomical Society. All rights reserved.

Schneider A.,University of Zürich | Smith R.E.,University of Zürich | Smith R.E.,Argelander Institute for Astronomy | MacCio A.V.,Max Planck Institute for Astronomy | Moore B.,University of Zürich
Monthly Notices of the Royal Astronomical Society | Year: 2012

The dark energy dominated warm dark matter (WDM) model is a promising alternative cosmological scenario. We explore large-scale structure formation in this paradigm. We do this in two different ways: with the halo model approach and with the help of an ensemble of high-resolution N-body simulations. Combining these quasi-independent approaches leads to a physical understanding of the important processes which shape the formation of structures. We take a detailed look at the halo mass function, the concentrations and the linear halo bias of WDM. In all cases we find interesting deviations with respect to cold dark matter (CDM). In particular, the concentration-mass relation displays a turnover for group scale dark matter haloes, for the case of WDM particles with masses of the order of m WDM∼ 0.25keV. This may be interpreted as a hint for top-down structure formation on small scales. We implement our results into the halo model and find much better agreement with simulations. On small scales, the WDM halo model now performs as well as its CDM counterpart. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Smith R.E.,University of Zürich | Smith R.E.,Argelander Institute for Astronomy | Markovic K.,Ludwig Maximilians University of Munich | Markovic K.,Excellence Cluster Universe
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We explore the impact of a ΛWDM cosmological scenario on the clustering properties large-scale structure in the Universe. We do this by extending the halo model. The new development is that we consider two components to the mass density: one arising from mass in collapsed haloes, and the second from a smooth component of uncollapsed mass. Assuming that the nonlinear clustering of dark matter haloes can be understood, then from conservation arguments one can precisely calculate the clustering properties of the smooth component and its cross correlation with haloes. We then explore how the three main ingredients of the halo calculations, the halo mass function, bias and density profiles are affected by warm dark matter (WDM). We show that, relative to cold dark matter (CDM), the halo mass function is suppressed by 50%, for masses ∼100 times the free-streaming mass scale Mfs. Consequently, the bias of low mass haloes can be boosted by as much as ∼20% for 0.25 keV WDM particles. Core densities of haloes will also be suppressed relative to the CDM case. We also examine the impact of relic thermal velocities on the density profiles, and find that these effects are constrained to scales r<1h-1kpc, and hence of little importance for dark matter tests, owing to uncertainties in the baryonic physics. We use our modified halo model to calculate the nonlinear matter power spectrum, and find that there is significant small-scale power in the model. However, relative to the CDM case the power is suppressed. The amount of suppression depends on the mass of the WDM particle, but can be of order 10% at k∼1hMpc-1 for particles of mass 0.25 keV. We then calculate the expected signal and noise that our set of ΛWDM models would give for a future weak lensing mission. We show that the models should in principle be separable at high significance. Finally, using the Fisher matrix formalism we forecast the limit on the WDM particle mass for a future full-sky weak lensing mission like Euclid or the Large Synoptic Survey Telescope. With Planck priors and using only multipoles l<5000, we find that a lower limit of 2.6 keV should be easily achievable. © 2011 American Physical Society.

Smith R.E.,University of Zürich | Smith R.E.,Argelander Institute for Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2012

We investigate the error properties of certain galaxy luminosity function (GLF) estimators. Using a cluster expansion of the density field, we show how, for both volume- and flux-limited samples, the GLF estimates are covariant. The covariance matrix can be decomposed into three pieces: a diagonal term arising from Poisson noise, a sample variance term arising from large-scale structure in the survey volume and an occupancy covariance term arising due to galaxies of different luminosities inhabiting the same cluster. To evaluate the theory one needs the mass function and bias of clusters, and the conditional luminosity function (CLF). We use a semi-analytic model (SAM) galaxy catalogue from the Millennium Run N-body simulation and the CLF of Yang et al. to explore these effects. The GLF estimates from the SAM and the CLF qualitatively reproduce results from the two degree Field Galaxy Redshift Survey (2dFGRS). We also measure the luminosity dependence of clustering in the SAM and find reasonable agreement with 2dFGRS results for bright galaxies. However, for fainter galaxies, L < L *, the SAM overpredicts the relative bias by ∼10-20 per cent. We use the SAM data to estimate the errors in the GLF estimates for a volume-limited survey of volume V ∼ 0.13h -3Gpc 3. We find that different luminosity bins are highly correlated: for L < L * the correlation coefficient is r > 0.5. Our theory is in good agreement with these measurements. These strong correlations can be attributed to sample variance. For a flux-limited survey of similar volume, the estimates are only slightly less correlated. We explore the importance of these effects for GLF model parameter estimation. We show that neglecting to take into account the bin-to-bin covariances, induced by the large-scale structures in the survey, can lead to significant systematic errors in best-fitting parameters. For Schechter function fits, the most strongly affected parameter is the characteristic luminosity L *, which can be significantly underestimated. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Smith R.E.,University of Zürich | Smith R.E.,Argelander Institute for Astronomy | Desjacques V.,University of Zürich | Marian L.,Argelander Institute for Astronomy
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We develop the halo model of large-scale structure as an accurate tool for probing primordial non-Gaussianity. In this study we focus on understanding the matter clustering at several redshifts in the context of primordial non-Gaussianity that is a quadratic correction to the local Gaussian potential, characterized by the parameter fNL. In our formulation of the halo model we pay special attention to the effect of halo exclusion and show that this can potentially solve the long-standing problem of excess power on large scales in this model. The halo model depends on the mass function, clustering of halo centers, and the density profiles. We test these ingredients using a large ensemble of high-resolution Gaussian and non-Gaussian numerical simulations, covering fNL={0,+100,-100}. In particular, we provide a first exploration of how halo density profiles change in the presence of primordial non-Gaussianity. We find that for fNL positive (negative) high-mass haloes have an increased (decreased) core density, so being more (less) concentrated than in the Gaussian case. We also examine the halo bias and show that, if the halo model is correct, then there is a small asymmetry in the scale dependence of the bias on very large scales, which arises because the Gaussian bias must be renormalized. We show that the matter power spectrum is modified by ∼2.5% and ∼3.5% on scales k∼1.0hMpc-1 at z=0 and z=1, respectively. Our halo model calculation reproduces the absolute amplitude to within 10% and the ratio of non-Gaussian to Gaussian spectra to within 1%. We also measure the matter correlation function and find similarly good levels of agreement between the halo model and the data. We anticipate that this modeling will be useful for constraining fNL from measurements of the shear correlation function in future weak lensing surveys such as Euclid. © 2011 American Physical Society.

Smith R.E.,University of Zürich | Smith R.E.,Argelander Institute for Astronomy | Marian L.,Argelander Institute for Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2011

We study the covariance matrix of the cluster mass function in cosmology. We adopt a two-line attack: first, we employ the counts-in-cells framework to derive an analytic expression for the covariance of the mass function. Secondly, we use a large ensemble of N-body simulations in the Λ cold dark matter framework to test this. Our theoretical results show that the covariance can be written as the sum of two terms: a Poisson term, which dominates in the limit of rare clusters; and a sample variance term, which dominates for more abundant clusters. Our expressions are analogous to those of Hu & Kravtsov for multiple cells and a single mass tracer. Calculating the covariance depends on: the mass function and bias of clusters, and the variance of mass fluctuations within the survey volume. The predictions show that there is a strong bin-to-bin covariance between measurements. In terms of the cross-correlation coefficient, we find r≳ 0.5 for haloes with M≲ 3 × 1014h-1M⊙ at z= 0. Comparison of these predictions with estimates from simulations shows excellent agreement. We use the Fisher matrix formalism to explore the cosmological information content of the counts. We compare the Poisson likelihood model, with the more realistic likelihood model of Lima & Hu, and all terms entering the Fisher matrices are evaluated using the simulations. We find that the Poisson approximation should only be used for the rarest objects, M≳ 5 × 1014h-1M⊙, otherwise the information content of a survey of size V∼ 13.5h-3Gpc3 would be overestimated, resulting in errors that are nearly two times smaller. As an auxiliary result, we show that the bias of clusters, obtained from the cluster-mass cross-variance, is linear on scales >50h-1Mpc, whereas that obtained from the auto-variance is non-linear. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

News Article | December 14, 2016
Site: www.nature.com

An analysis of almost 15 million distant galaxies reveals that dark matter may be slightly less dense and more evenly distributed throughout space than was thought. Dark matter makes up one-quarter of the Universe's mass, but is invisible and its presence can only be inferred from its gravitational effects. A team led by Hendrik Hildebrandt of the Argelander Institute for Astronomy in Bonn, Germany, and Massimo Viola of Leiden University in the Netherlands examined galaxy images taken by the European Southern Observatory's VLT Survey Telescope in Chile as part of the Kilo-Degree Survey. The researchers measured cosmic shear: the distortion of the shapes of background galaxies due to light that is warped by the gravitational effects of large-scale structures such as galaxy clusters. The team statistically measured how dark matter subtly distorted the galaxy images, and inferred its density from this. If future measurements confirm this more-even distribution of dark matter, astrophysicists might need to revise their models of how the Universe evolved. Mon. Not. R. Astron. Soc. (in the press); preprint at https://arxiv.org/abs/1606.05338 (2016)

Loading Argelander Institute for Astronomy collaborators
Loading Argelander Institute for Astronomy collaborators