All posts by Serena Nobili

Black holes and the nature of space time

Black holes leave their marks all over the observed universe. They do however also inspire new and exciting ideas about space and time itself, both in the micro-cosmos and on the large scales of the universe.

6b4e4ff3-c6da-4a95-b3c7-4346b8658888This August around 150 researchers from all of the world gathered for a Nordita program and conference devoted to “Black Holes and Emergent Spacetime”, organised in part by fellow OKC-er Larus Thoilacius and myself. In this blog post I shall try to explain part of the excitement and also how some ideas that were discussed address the dark matter and dark energy puzzles directly, as well as the details of the fluctuations in the cosmic microwave background.


The background story starts with the realization by Bekenstein, Hawking and others that black hole behaviour is captured by thermodynamics, when temperature and entropy are identified correctly. Since modern physicists associate thermodynamics to an underlying statistical description of a quantum system, many vague ideas about the nature of such a system has since been proposed.

Finally, in 1997, Juan Maldacena, who was then a first-year postdoc, wrote a paper about black holes that has now gathered more than 12 000 citations. It listed a number of quantum mechanical systems that could precisely and completely describe spacetime and gravitational physics.

Maldacena’s discovery is as close to an experimental breakthrough as theorists get. It changed our perspective on the world in a completely unexpected way, and hordes of theorists have set out to apply it and understand it on a deeper level. Recently, a large community again focuses on black holes and the spacetime as an emergent rather than fundamental concept.

Tasting menu

Before coming to potential applications to observable physics let me highlight some other of the ideas I personally found interesting and fun. They seem distant from observation at present, but can be of conceptual importance.
• Non-singular version of black holes: In higher-dimensional gravity there is a plethora of solutions without the physical singularities that plague the standard rotating Kerr black hole solution. Could it be that actual physical black holes are nonsingular when studied in detail?

• Andy Strominger of Harvard discussed the ideas developed by Hawking, Perry and himself on how information is transported out to arbitrary distances from black holes and could be recovered by detecting so called BMS charges (although in practise many orders of magnitude more work than at
LIGO would be required).

• Nobel laureate and world-renowned particle physicist Gerard ‘t Hooft explained his puzzling recent ideas about the non-classical geometry of black holes. He said that his picture would lead to (in principle) observable correlations between Hawking radiation in opposite directions from a black hole. ‘t Hooft was the first to realise some of the “holographic” properties of gravity that later was concretised in Maldacena’s conjecture.
• So called Higher Spin Black holes were clarified by Juan Jottar from ETH, Zürich. Higher spin symmetries are symmetries that may be an alternative to supersymmetry in regulating short-distance problems of field theory. Another merit of these models is that they permit precise calculations, although technically demanding. Fundamental black hole physics is such a murky subject that explicit constructions are in dire need. The connection gravity and quantum mechanical system promises to be simpler in this case than in most other.
• Jonathan Lindgren, from Brussels, had found exact solutions of particles colliding to form general black holes. Of course, this problem is beyond reach in 3 space dimensions, but his 2-dimensional solution is still interesting.

We had two talks a day in four weeks and 30 talks in the conference, so these examples of talks by a Nobel laureate, a professor, a postdoc and a PhD student cannot do justice to the scope of the program.


inwardboundThere were two talks in the program that focused on cosmology. Both apply the idea of holography to a time evolving universe. These ideas are most naturally applied to universe with an almost constant acceleration of its expansion, i.e. to a quasi-de Sitter universe, although other cases can be described with more effort.

Erik Verlinde described an ambitious project that aims to derive both the effects of dark energy and dark matter as consequences of holography and a spacetime that is emergent rather than fundamental. This is a setting in which spacetime is subject to thermodynamic laws and relations. As I understood it dark energy is proposed to be an elastic medium, and what we interpret as dark matter is then merely the effects of the interactions of ordinary matter with the dark energy medium. I think it would be interesting to find out whether this is really possible. There have been other suggestions on how to avoid dark matter, for example the heavily criticized MOND proposal of a modification of Newton’s laws. Verlinde got the question about how he would explain the apparent separation of dark and luminous matter in the so-called Bullet Cluster, a system of two colliding clusters. His answer was that in contrast to MOND he proposes a general dynamical and relativistic framework that may well lead to such effects, whereas his initial estimates are necessarily crude and rely on thermodynamic equilibrium. Other calculations are needed for time dependent phenomena like the collision in the Bullet Cluster.

Kostas Skenderis presented holographic descriptions of an inflationary era, which permit a direct calculation of the fluctuations of the microwave background. A holographic perspective involves a quantum field theory as in-data to the computation, but in contrast to standard inflation it does not presuppose a geometric description of the inflationary spacetime. This is a definite advantage, since a geometric spacetime is likely to clash with quantum gravity in one form or another. I think it is also exciting to have an entirely new kind of model with a straightforwardly calculable effect on the CMB.

– Bo Sundborg, professor at the Oskar Klein Centre (

Measuring the supernova rate in the early Universe by using galaxy clusters as gravitational telescopes

Supernovae are very rare phenomena in the Universe and their transient nature made them difficult to find for a long time. So, it is not surprising that the discovery rate was around two supernovae per month 30 years ago. Today, we are able to find supernovae daily. For example, the Intermediate Palomar Transient Factory, in which our group at the Oskar Klein Centre is involved, has discovered almost 3000 supernovae in the last few years. However, these supernovae are all relatively nearby, since the survey is not sensitive to the very distant ones.

Supernova rates, particularly at high distances, are important for several reasons. For example, core-collapse supernovae originate from the deaths of massive stars, and their rate can be used to trace the history of star formation in galaxies. Also, supernovae are the one of the major producers of metals in the Universe, so measuring supernova rates informs us about the chemical enrichment of galaxies over time.

However, measuring the supernova rate in the distant Universe is difficult. Even though supernovae are one of the brightest explosions that exist, at distances bigger than four billion light years they are hard to find simply because they become too faint. This has been especially problematic for the study of the rate of core-collapse explosions since they are on average the faintest objects in the supernova family and often embedded in dusty environments. Furthermore, due to the expansion of the Universe, the visual light from all distant objects is shifted to longer wavelengths. From the ground, near-infrared observations are particularly challenging due to the brightness of and variability of the atmosphere at these wavelengths.

Artistic view of the observation of the distant supernovae through a gravitational telescope. The galaxy cluster (in the middle) with its mass, serves as a lens by distorting and magnifying the light coming from the supernova in the galaxy behind the cluster. Credit: Maedeh Mohammadpour Mir.
Artistic view of the observation of the distant supernovae through a gravitational telescope. The galaxy cluster (in the middle) with its mass, serves as a lens by distorting and magnifying the light coming from the supernova in the galaxy behind the cluster. Credit: Maedeh Mohammadpour Mir.

Instead of waiting for more powerful telescopes to come online, we used the existing facilities and the magnification power of galaxy clusters as gravitational telescopes to take a peek at the supernovae in the early Universe. Galaxy clusters are the most massive gravitationally bound objects in the Universe, distorting and magnifying objects behind them. As predicted by Einstein’s general relativity, gravitational lensing magnifies both the area and the flux of background objects, thereby increasing the depth of the survey. In this way, the ability to find very distant supernovae is enhanced. It was Fritz Zwicky who suggested the use of gravitational telescopes nearly 80 years ago, but it is only recently that systematic supernova searches have been performed in background galaxies behind clusters. The supernova group here in Stockholm was the first one to explore this possibility using ground-based facilities in 2003.

As a continuation of this effort, during 2008-2014, we surveyed the galaxy cluster Abell 1689, which is one of the most powerful gravitational telescopes that nature provides. The results are presented in a recent publication  in the journal Astronomy & Astrophysics. We used a near-infrared instrument on the Very Large Telescope in Chile, with obtaining supporting optical data from the Nordic Optical Telescope at La Palma. Our search resulted in the discovery of five very distant and magnified supernovae. Notably, we discovered a supernova located nearly 10 billion lightyears away that was magnified four times by the galaxy cluster, which makes it among the most distant supernovae yet observed. Using these discoveries, we measured the supernova rates up to the time when the Universe was only two billion years old, without requiring any expensive space-based follow-up facilities.

Galaxy cluster Abell 1689 observed through the years 2008-2014 where the position of the discovered supernovae are shown. The red contours are the magnifications of the background sources from the cluster. Prepared by Rahman Amanullah.

Monitoring the foreground galaxy cluster also offers the opportunity to detect supernovae that originate from galaxies which are cluster members. Since clusters are dominated by galaxies where star formation has ceased, these are not core-collapse supernovae, but so-called thermonuclear or supernova Type Ia. We discovered two of this kind in Abell 1689 which allowed us to estimate the rate of supernova explosions in the cluster. Cluster rates are important since they have can be used to study the origin of the mysterious progenitors of supernovae Ia (read also A shocked neighbour) and are essential in understanding the iron abundance in medium between the cluster galaxies.

Another prediction of Einstein’s relativity is that strong gravitational lensing like the one from galaxy clusters, can give multiple images of the same background galaxy. If a supernova explodes in one of these multiply-imaged galaxies, its images will appear with certain time delays relative to each other due to the fact that the light from each image has to travel a different path. Given this event is very very rare and that our observing program was modest, we did not discover such an event. However, in the paper, we estimated the number that can be expected for upcoming transient surveys. We found that LSST , and in particular WFIRST , can be expected to find tens of strongly lensed supernovae that would allow the time delays between the multiple images to be measured, which can be measure the Hubble constant but also other cosmological parameters.

– Tanja Petrushevska (
 and Rahman Amanullah (


Preprint of the manuscript:

Electromagnetic Counterparts to Black Hole Mergers?

The observations of the first gravitational wave by the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured the attention of the world this February, confirming the existence of  gravitational waves as well as further confirming Einstein’s theory of general relativity.
The signature of merging blackholes resulted in a flurry of scientific articles reworking theories casting out models and creating new ones. Other observatories probed the sky looking for electromagnetic signatures across all wavelengths, but nothing was seen. Well almost nothing.

The Fermi Gamma-ray Burst Monitor (GBM) claimed to see an event occurring 0.4 seconds after the LIGO event. It was a very week fluctuation in the data lasting only 1 second. SPI Anti-Coincidence Shield on board The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), with a similar observing range as GBM, could not confirm the signal even though it should have seen the same event. This lead to an interesting conundrum. If GBM really saw gamma-rays from a black hole merger, many existing theories would have to be changed as it is very difficult to have a scenario where such merger can produce gamma-rays.

gamma-rays coming from merging black holes
A cartoon of gamma-rays coming from merging black holes and hitting Fermi GBM. The green histogram shows the result of our analysis. The signal is consistent with background.
Image credit: Composite NASA Goddard Space Flight Center/SXS, the Simulating eXtreme Spacetimes (SXS) project

Therefore, we teamed up with part of the GBM and INTEGRAL teams to do a thorough investigation of the event. Instead of relying on existing analysis tools, we went back to basic statistics and designed new analysis schemes for the data. We applied the tools to the data from the GBM event and could not find a signal above the normal background in the instrument. When we checked with the normal analysis tools, we could see it. Therefore, data from other weak events that were also seen by INTEGRAL were checked with both sets of tools. This was very strange and we needed to find a way to see which analysis was right.

Luckily, INTEGRAL and GBM see a lot of the same gamma-ray events. For events that both INTEGRAL and GBM agree are real events, we can predict what INTEGRAL would see using parameters from GBM data analysis. Using our method and the standard method, we found again that you get two different predictions for what INTEGRAL would see. Our method always predicted correctly the INTEGRAL signal, but the standard analysis over-predicted the strength in INTEGRAL: just like the presumed gravitational-wave counterpart! Thus, we could explain why GBM results were in conflict with INTEGRAL for the gravitational-wave counterpart and prove that the counterpart was merely a background fluctuation in the GBM data.

There will be a lot of exciting science to come from LIGO! GBM is very likely to see a coincident signal when two neutron stars collide as these events should generate copious gamma-rays. When this day comes, we must have the statistical tools to analyze the data ready so that we get the next event right. Once we know we have a signal, we can explore all the exotic theories.

– J. Michael Burgess ( )
Article preprint:
GBM instrument:

Hunting light dark matter with gamma rays

Physicists around the globe are working relentlessly to pin down the nature of dark matter. This enigmatic entity hides itself from our view as it does neither emit nor absorb any radiation. It only reveals itself through its gravitational interaction. With a new analysis of data from NASA’s gamma-ray large area telescope (LAT) on board the Fermi satellite, we have now come closer to test very light dark matter candidates.

Many regard the astrophysical evidence for dark matter as evidence for yet undiscovered fundamental particles. Well-motivated theories suggest that these particles should be quite heavy, probably a couple of dozen times as massive as the proton. If so, either their decay or the annihilation of two such particles could result in energetic light: gamma-ray emission that we could detect with the LAT. Also the LHC and direct detection experiments such as the recently inaugurated Xenon 1T are looking for traces of heavy dark-matter candidates.

However, dark-matter particles could also be extremely light, more than five hundred billion times lighter than the electron. They are often referred to as axion-like particles (ALPs). They are close relatives to the axion which was originally proposed in the late 70s to solve problem in the strong interactions. It was soon realized that this class of particles could also constitute the dark matter. Unfortunately, very light ALPs have lifetimes much longer than the age of the Universe and do not obliterate each other upon meeting. However, there’s a small chance that ALPs transform into ordinary light (and vice versa) in the presence of a magnetic field. This leads to oscillations between light and ALPs very much like neutrino oscillations.

Illustration of the photon-axionlike particle conversion: Light produced in the jet of the galaxy converts into ALPs (dashed lines) and vice-versa when interacting with the magnetic field (wiggled line). Credits: Aurore Simonnet, Sonoma State University and NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring.
Illustration of the photon-axionlike particle conversion: Light produced in the jet of the galaxy converts into ALPs (dashed lines) and vice-versa when interacting with the magnetic field (wiggled line). Credits: Aurore Simonnet, Sonoma State University and NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring.

We have used Fermi-LAT observations of an extremely bright gamma-ray source to search for traces of these oscillations: the central galaxy of the Perseus galaxy cluster. In its center, a spinning super-massive black hole powers a current stream of plasma that emits gamma-ray radiation. When the gamma rays propagate through the magnetic field present in the gas between the galaxies of the cluster, they could transform into ALPs and imprint distortions onto the galaxy’s spectrum.

We did not find any such features above the level of statistical fluctuations which enabled us to rule out ALP models that would contribute sub-dominantly to the overall dark-matter content. These are best constraints in the particular ALP mass region to date. Our limits turned out to be as sensitive as as the future “Any Light Particle Searches” (ALPS) experiment. In the ALPS experiment, it is planned to shoot a powerful laser beam immersed in a magnetic field onto a wall and look for light that might have made it through it in the form of ALPs.

Our analysis only marks the beginning for ALP searches with the Fermi-LAT. With the analysis of more sources we will be sensitive enough to probe ALPs that could make up all the dark matter in the Universe.

– Manuel Meyer (

Preprint of the manuscript:
Fermi satellite:
ALPS II experiment:

Caption for the picture in the cover: Composite image of the central galaxy in the Perseus cluster taken with the Hubble space telescope and the Chandra X-ray satellite.
Credits: X-ray: NASA/CXC/IoA/A.Fabian et al.; Radio: NRAO/VLA/G. Taylor; Optical: NASA/ESA/Hubble Heritage (STScI/AURA) & Univ. of Cambridge/IoA/A. Fabian

ATLAS Thesis Award goes to the Dark Side

Ruth Pöttgen is a postdoctoral researcher in the ATLAS group at Stockholm University. In 2015, she obtained her Ph.D. at the Johannes Gutenberg – University in Mainz, Germany, for her thesis on a “Search for Dark Matter in Events with a highly energetic jet and missing transverse momentum at a centre-of-mass energy of 8 TeV with the ATLAS Detector”. At the ATLAS collaboration meeting in February, Ruth was awarded one out of 4 ATLAS Thesis Awards for outstanding contributions to the ATLAS-Experiment in the context of a Ph.D. thesis; more than 100 theses were eligible.

Ruth Pottgen
Ruth Pottgen

During her Ph.D., Ruth was based at CERN within its doctoral student programme, funded by a Wolfgang-Gentner scholarship. She was active member of the CERN group responsible for the central trigger — a vital component of the ATLAS experiment — and contributed both to its day-to-day operation as well as its upgrade during the long LHC shutdown from 2013-2015.

In 2012, she joined one of her colleagues from the central trigger group in a search for weakly interacting massive particles (WIMPs) — a popular generic class of candidates for Dark Matter particles, for which also various searches are pursued within OKC.

While there is ample compelling evidence for the existence of Dark Matter at various cosmological scales, an unambiguous direct observation of Dark Matter in the laboratory is still pending. The Standard Modell (SM) of particle physics does not provide a viable candidate to account for all of the observed Dark Matter, which is estimated to amount to roughly 80% of the matter content of the universe. WIMPs could, in a natural way, account for the present day abundance of Dark Matter. Such particles feature in many theories for physics beyond the SM, e.g. the lightest supersymmetric particle in R-parity conserving super-symmetry models.

A number of experiments searches for evidence for WIMPs and can be grouped roughly into three different categories. Direct searches look for the nuclear recoil in a target material due to the scattering of WIMPs off nuclei, while indirect searches try to detect the products of WIMP annihilation into SM particles. In recent years, also the search for WIMP pair production at hadron colliders has gathered momentum. It is in many regards complementary to the other search approaches and has become the third pillar for the hunt for Dark Matter.

Like neutrinos, WIMPs do practically not interact with the detector material and are hence themselves ‘invisible’. If they are, however, produced together with some other, ideally highly energetic object, their presence can be inferred from the missing contribution to the momentum balance. One possibility is that the other object is a hadronic jet (a bundle of particles). Such events are commonly referred to as mono-jet events. The signal is then expected to manifest itself as an excess above the Standard Model prediction at large missing transverse energy. While the direct searches are essentially background-free experiments, one of the major challenges in the collider-based search is the careful estimation of irreducible backgrounds, dominated by the production of Z-bosons together with jets, where the former decay in neutrino-antineutrino pairs.

The preliminary ATLAS results based on half of the data set collected at a centre-of-mass energy of 8 TeV in 2012 had been presented by Ruth at the ISAPP summer school on Dark Matter, held in Stockholm and organised by OKC in 2013. The results were discussed with great interest and Ruth was awarded with one of the three poster awards.

For the analysis of the full 2012 proton-proton dataset (20 fb-1) in her thesis, Ruth optimised the event selection with respect to the sensitivity for a WIMP signal. The search was performed in eight signal regions of increasing missing transverse momentum and no significant excess was observed. Model-independent limits on the cross section for new physics as well as bounds on parameters of two different, but related models for Dark Matter were derived.

Together with Ruth, three members of the SU ATLAS group were and are still involved in the mono-jet analysis, but focussing on a different interpretation.

Ruth is currently based at CERN and coordinates the ATLAS search for first and second generation scalar leptoquarks, which is to be published for the Moriond conference in March. Afterwards, she’ll return to Stockholm and is eager to explore possibilities of a collaboration with Dark Matter experts at OKC.

The “Award ceremony” will take place on today, February 25, at CERN.

Gravitational waves finally detected

It seems that nearly exactly 100 years after their prediction by Albert Einstein, Gravitational Waves have finally been directly detected for the first time. Speakers of the LIGO experiment announced yesterday that they have witnessed the final stages of the inspiral and merger of a massive black hole binary system. This marks the beginning of a new type of astronomy with gravitational waves that allows to explore a so-far completely unknown side of the Universe.

Einstein’s Theory of Gravity

In November 1915, nearly exactly 100 years ago, Albert Einstein
presented his new General Theory of Gravity to the Prussian Academy of Sciences. The theory was somewhat perplexing since the effect of gravity was not a force acting on massive bodies, but instead gravity was claimed to warp the four-dimensional space-time we live in.

By now, this theory has been phenomenally successful. It could explain a long-known anomaly of the planet Mercury’s orbit around the Sun, its so-called perihelion shift. The theory also predicted that a gravitational field should deflect the paths of light rays, an effect that was later confirmed experimentally.

According to Einstein’s Special Theory of relativity, however, no information should be able to travel faster than the speed of light. Therefore, if at some location in the Universe a catastrophic event heavily perturbs the space-time, the “news” of this warp can only travel at a finite speed and –according to Einstein’s theory– this must be the speed of light.

One can think of a gravitational wave as a “ripple” travelling across the otherwise smooth space-time. This is similar to throwing a stone into a calm lake: this causes ring-like perturbations that travel away from where the stone hit the water. The physical reality of gravitational waves, however, had been doubted for decades, they were often considered as mere artefacts of Einstein’s theory.

A first glimpse of the elusive waves

This only changed in 1974 when a very “exotic” stellar system was discovered: two neutron stars orbiting around each other at a good fraction of the speed the light. A neutron star is a stellar corpse that emerges when an exploding star compresses its interior to densities that are larger than those in an atomic nucleus. So in a sense, one can think of neutron stars as being gigantic atomic nuclei of about 10 km radius. The newly discovered binary system has turned out to be an excellent laboratory for relativistic gravity, many general relativistic effects predicted by Einstein’s theory could be measured in it to exquisite precision.

Probably the most spectacular effect is that the two neutron stars slowly spiral towards each other, in excellent agreement with the prediction of Einstein’s theory. Within one orbital revolution (which takes less than 8 hours) this is a tiny effect, but since its discovery in 1974 the orbital period has already changed by 40 seconds! This discovery pulverised the doubts about the reality of gravitational waves and the discoverers of the binary system, Russel Hulse and Joseph Taylor, were honoured with the Physics Nobel Prize in 1993.

Listening to the dark side of the Universe

Although convincing, this is only an indirect confirmation of gravitational waves and one would like, of course, to detect them directly. This would mean that one could not only “see” the Universe (via electromagnetic waves) but one could also “listen” to the so far dark side of the Universe by means of gravitational waves. According to all we know, only 4 % of the energy of the Universe is made of matter that we think we understand. This includes all the objects of everyday life that are made of neutrons, protons and electrons.
The remaining 96%, however, may also produce gravitational waves and detecting them directly will open a new window to a completely unknown side of the Universe. Surprises are therefore virtually guaranteed!

In the last 25 years enormous efforts have been undertaken towards a direct detection of gravitational waves. An international detector network has been built up with facilities in Germany, Italy, Japan and the United States. The American “Laser Interferometer Gravitational-wave Observatory” (LIGO) has recently undergone a major upgrade and it has started taking data in its new configuration in September 2015. Once its final design sensitivity has been reached, it will be able to listen to a 1000-times larger volume of the Universe than before.

On 2015 September 14 at 9:50 UTC, during the last stages of “engineering runs” and before the originally planned observation period, LIGO has observed a gravitational wave burst signal from two merging, massive black holes. Both LIGO detectors, separated by 3000 km, saw a so-called “chirp” signal of increasing amplitude and frequency sweeping up frequencies from 35 to 250 Hz. This signal is well explained by the merger of two black holes with 29 and 36 solar masses.If this is the correct interpretation, then 2015 September 14 marks the beginning of the era of gravitational wave astronomy!

The LIGO collaboration consists of about one thousand scientists working in more than fifteen countries. Beyond the collaboration, LIGO’s results will both rely on and inform the observations of dozens of other telescopes and satellite observatories. Researchers would like to observe such extreme events as mergers of black holes and neutron stars with as many instruments as possible, and as soon after the burst as possible. LIGO itself, however, will not be able to localise the direction of bursts in the sky to high accuracy (for the observed event the source position is only known to within 600 square degrees). The first stage of help can come from instruments that monitor large parts or all of the sky continuously. At the Oskar Klein Centre, researchers working on the Fermi Gamma-ray Space Telescope, the Intermediate Palomar Transient Factory (iPTF), and the IceCube Neutrino Observatory were alerted about the September 14 burst, and asked to check whether they had recorded anything unusual around or after the burst time and in the same general direction. No definitive excess has been reported yet from these or other observatories. Seeing no other emission would be consistent with the interpretation of two merging black holes, since it is not obvious how an electromagnetic emission would arise in such a case. On the other hand, if a neutron star were involved, one would expect to see an electromagnetic flash caused by radioactivity from freshly synthesized heavy elements, a so-called “macronova”.

An interesting aspect of the LIGO procedure has been their well-publicised plan to test themselves and their fellow observers with occasional “fake” alerts. Except for a few individuals, no one even in LIGO would know for sure if a given alert was real. For this reason, it was not obvious in the early days after the alert that something real had been observed, let alone the amazing discovery that it turned out to be. It only became gradually, increasingly clear in the last few days before the announcement that this burst, now designated as GW150914, was not only real, but spectacular!

If the black hole merger interpretation is correct, then the lack of definitive detection besides gravitational waves would not be surprising. If the black holes were not surrounded by gas and ordinary matter that could be ejected in the violent aftermath, the merger was likely to be undetectable except by the enormous energy carried away by the gravitational waves themselves (corresponding to about three times the rest mass of our Sun). With LIGO starting to run in its advanced configuration now, and continuing to improve sensitivity, it is likely that more merger events will follow soon. Some of these will involve neutron stars rather than black holes, and these events are expected to leave visible traces.

– Stephan Rosswog ( and Chad Finley (

Stephan Rosswog is Professor of Astronomy at Stockholm University and researches on compact objects such neutron stars and black holes.
Chad Finley is Senior Researcher in Physics at Stockholm University and IceCube coordinator for the joint search with LIGO.

Read also: “High-energy Neutrino follow-up search of Gravitational Wave Event
GW150914 with ANTARES and IceCube”, ANTARES, IceCube, LIGO, and VIRGO collaborations.

The cover photo shows two Black Holes merging into one. This simulation was created by the multi-university SXS (Simulating eXtreme Spacetimes) project. For more information, visit Photo Credit: SXS.

2015 Oskar Klein Medal to be awarded to Rashid Sunyaev

The 2015 Oskar Klein Memorial Lecture will be given by Professor Rashid Sunyaev of the Max Planck Institute for Astrophysics, Garching, who is also the recipient of the Oskar Klein Medal for 2015.

Rashid Sunyaev. Photo credit: Juan Diego Soler
Rashid Sunyaev. Photo credit: Juan Diego Soler
Professor Sunyaev has made groundbreaking contributions to theoretical astrophysics in the areas of cosmology, high-energy astrophysics and X-ray astronomy through his studies of some of the most extreme physical processes in the universe. His theory of the evolution of density fluctuations in the early universe (developed with Zel’dovich) predicted the acoustic peaks that are observed in the cosmic background radiation. He has also made key contributions to the theoretical description of matter accreting onto black holes, predicting a signature for the resulting X-ray emission.

Rashid Alievich Sunyaev received his PhD in astrophysics from Moscow University in 1968 and became a Professor at the Moscow Institute of Physics and Technology in 1974. He is a Director of the Max Planck Institute for Astrophysics in Garching, Germany and has been chief scientist at the Space Research Institute of the Russian Academy of Sciences since 1992.

The Oskar Klein Memorial lecture will take place at 3:15 PM on Thursday, February 4,
in the Oskar Klein Auditorium at the AlbaNova University Centre in Stockholm.

Title: “Unavoidable distortions in the spectrum of CMB and the Blackbody Photosphere
of our Universe”

Abstract: Spectral features in the CMB spectrum contain a wealth of information about physical processes in the early Universe at redshifts z < 2 10^6, i.e. when Universe was older than 2 months. The Cosmic Microwave Background Radiation (CMB) spectral distortions are complementary to other probes of cosmology. In fact, most of the information contained in the CMB spectrum is inaccessible by any other means. This talk outlines the main physics behind the spectral features in the CMB produced throughout the history of the Universe. I will concentrate on the distortions which are inevitable and must be present at the level observable by the next generation of CMB experiments. The spectral distortions considered here include spectral features from cosmological recombination of hydrogen and helium, resonant scattering of CMB by metals during reionization which allows us to measure their abundances, y-type spectral distortions produced during and after reionization and μ - type distortions created at redshifts z > 10^5 due to any significant energy release (for example: due to decay or annihilation of the dark matter particles or due to viscous decay of the primordial sound waves).

Special attention will be given to existence of the Blackbody Photosphere of our Universe at redshift z ~ 2 10^6, behind which Comptonization, double Compton and Bremsstrahlung are able to wash out any spectral distortions arising due to arbitrary strong energy release.

CMB spectral distortions detected on the sky by Planck spacecraft, South Pole Telescope and Atacama Cosmology Telescope permitted us to discover more than thousand unknown before clusters of galaxies (most massive gravitationally bound objects in the Universe, containing thousands of galaxies, hot (kTe > 1 KeV) intracluster gas, gravitational lenses and huge amount of dark matter). These clusters of galaxies are serving today as probes for modern cosmology tracing the growth of the Large Scale Structure with time and containing strong gravitational lenses.

If you want to know more, check also the official webpage for the Oskar Klein Memorial Lectures

Passerby become Supernova hunters in Kungsträdgården

Fysik i Kungsträdgården is one of the major outreach events in Stockholm where scientists have the chance to show their research (read more about the whole event in Fysikum blog). This year the supernova group at the OKC invited the public to look for supernovae with us using the intermediate Palomar (Transient) Factory (iPTF) collaboration telescope in Palomar, California, U.S.A (read more about the iPTF). Due to the time difference between Stockholm and California, we were able to look at live images as they were taken from the telescope, to search for new transient objects such as supernovae. The event was successful, bringing a lot of people from the park in central Stockholm to come and search with us. During this search, we found two supernovae of type Ia with the help of these volunteers, who shared their thoughts with us:

Passerby supernova hunter Manoj Bartakke (in the middle) with Emir (left) and Anders (right)
Passerby supernova hunter Manoj Bartakke (in the middle) with Emir (left) and Anders (right)

“Ahoy! In the midst of bachelor studies, it can sometimes be easy to forget what you are actually studying for, therefore I can only describe my experience of that day as awesome and invaluable. Everyone were very helpful and pedagogical in their explanations of how it all worked. I would rate my experience a 12/10 and I would love to have the chance of doing it again some time.” – Patrik Tegner

Supernova discovery Kungsträdgården
Passerby supernova hunter Patrik Tegner (in the middle) and the supernova group from left to right Tanja, Semeli, Raphael, Anders and Emir

“I was in the Kungsträdgården with my office colleague on 5th September, surfing through stalls from a science exhibition. I came across this ‘supernova’ stall. The two guys there (Emir Karamehmetoglu and Anders Nyholm) , gave us good information of universe and supernova. They also introduced with their research on supernova. It was really interesting and informative. For that duration, I was like in another world. In a practical conducted by them, I was also able to discover one of the new supernova, which they named as ‘iPTF15cpp’ . ( I felt very proud in my internal universe ☺ ). So it was a different and good experience for me.” – Manoj Bartakke

Tanja, Semeli and Emir

The young star cluster perspective of star formation

Star formation is one of the fundamental process contributing to galaxy evolution and therefore in shaping the Universe. Yet it is extremely challenging to build a complete view of this process and its interplay with galactic scale properties. The most challenging aspect is to reconcile physical mechanisms, which operate at the smallest spatial scales (i.e. the size of our solar system) all the way up to galactic scale features such as the large star-forming complexes.

Two teams lead by Angela Adamo have now succeeded in putting forward two different observational projects that aim at understanding the nature of star formation at parsec scales and probe the link between these small scales and the host galactic environment. The projects are based on two recently accepted proposals at two world-class telescopes, the Hubble Space Telescope (HST) by Nasa and ESA, and the Very Large Telescope (VLT) by ESO.

The Hi-PEEC (Hubble imaging probe of extreme environments and clusters) project will use the new observations to look at the closest analogs of the high-redshift starburst galaxies. Galaxies today still carry with them witnesses of those experienced starburst periods, i.e. their globular cluster populations. We want to understand how these ancient populations formed and whether local galaxies can still experience star formation in a similar fashion as at high redshift. The Hi-PEEC team includes 20 astronomers (including OKC members: Göran östlin, Matthew Hayes, Matteo Messa, and Johannes Pushing) from 6 countries.

Fig.1. The Hi-PEEC sample in HST optical archival data. The new observations will  provide ultraviolet and optical information which will allow a detailed study of the young star cluster populations that are forming in these starbursts. The aim is to understand how these clusters form and whether they share properties of the ancient globular cluster populations.
Fig.1. The Hi-PEEC sample in HST optical archival data. The new observations will provide ultraviolet and optical information which will allow a detailed study of the young star cluster populations that are forming in these starbursts. The aim is to understand how these clusters form and whether they share properties of the ancient globular cluster populations.

The second project, based on VLT observations, consists of 21 hours to sample the nearby spiral galaxy Messier 83 (M83) with the integral field spectrograph unit (IFU) MUSE. MUSE is one of ESO’s newest instruments and it is the largest IFU currently available. The observations are organised such to produce a unique mosaic of the M83 spiral system. We will be able to study the effect of stellar cluster feedback on the interstellar medium of the galaxy from the smallest local scales achievable to date with optical spectroscopy to galactic scales. M83 has a very compelling collection of different environments characterised by different star formation properties. The centre of the galaxy is

Fig.2. The contours of the MUSE mosaic are overlaid on a visual band image of the spiral galaxy Messier 83. This unique dataset will be used to understand the interplay between cluster feedback and the ISM conditions.
Fig.2. The contours of the MUSE mosaic are overlaid on a visual band image of the spiral galaxy Messier 83. This unique dataset will be used to understand the interplay between cluster feedback and the ISM conditions.

perturbed by an ongoing starburst. The inter arm regions have very low starformation, conditions which are typical of the lowest efficient star-froming galaxies in the local universe. Finally, the star formation in the arm is typical of local star-forming spirals. The M83 dataset will be a key factor in our understanding of the effect of star formation feedback from local to galactic environments. The team responsible of this dataset counts 18 members (together with OKC members Göran Östlin, Arjan Bik and Matteo Messa) from 11 different institutes in Europe.

– Angela Adamo (

Angela Adamo is a postdoc fellow at the Astronomy department at Stockholm University and a member of the Oscar Klein Centre since 2014. She is part of the Galaxy group, lead by Prof. Göran Östlin. Her main research aims at understanding star formation in the framework of galaxy evolution.

Farewell to Per Olof Hulth, pioneer of Swedish particle astrophysics

Professor emeritus Per Olof Hulth
Professor emeritus Per Olof Hulth
Our colleague and friend Per Olof has passed away after a very brief period of illness. Peo, as we all called him, played a key role in the field of neutrino physics, both nationally and in the international arena. He started his career with six years at CERN (1976-1982), and was coordinator and spokesperson for several neutrino experiments using bubble chambers. In the early 1990’s, he started the experimental particle astrophysics activities in Sweden, first hoping that a high-energy neutrino telescope could be built in the Torneträsk lake of northern Sweden. After realizing that the transparency of the lake water was not adequate for a large Cerenkov telescope, Peo led the Swedish effort in AMANDA, the neutrino telescope buried in the Antartic ice and the predecessor of IceCube. Peo was the first spokesperson of the IceCube collaboration in 2001-2005, at the crucial stage which included the first year of deployment at the Pole. IceCube quickly grew to be a world leading observatory, now using 5160 light sensors deep in the ice to look for the rare signals from astrophysical neutrinos. These were indeed discovered in 2013, a spectacular achievement and a great satisfaction for Peo, who remained a figure in the experiment, also after retirement. Peo’s commitment to Physics and enthusiasm for his work left no room for hesitation. He went to the remote experimental site at South Pole 11 times, and would have gladly gone there again and again. He was a constant source of inspiration for younger colleagues. We are proud to have had him as a colleague and friend. We will miss him dearly.

– Lars Bergström, Chad Finley, Ariel Goobar, Klas Hultqvist and Christian Walck on behalf of the colleagues at OKC.