Category Archives: Talking science

A cold dawn for the first stars

This morning, as I was walking from Tekniska Högskolan metro station to AlbaNova through the Siberian cold which has hit Stockholm, I was thinking about even colder temperatures than the -15 C that I felt on my skin. Did you know that the temperature of Universe was only 3 Kelvin (-270 C) when the first stars were born?! At least that’s what the authors of an article published in ’Nature’ this week claim to have measured and until proven differently they might well be right… Having worked on the Cosmic Dawn (a popular name for the era during which the first stars were born) for many years I was baffled by this news because this temperature is much lower than we thought it was. How could it have been so much colder?

The results in the article were obtained by an experiment called EDGES (Experiment to Detect the Global Epoch of Reionization Signature). The American authors of the paper have over the past decade been trying to measure a signal from hydrogen atoms in the young Universe. When the first stars formed they caused the neutral hydrogen in the Universe to produce this signal. Its strength depends on the temperature of the hydrogen gas and it was produced with a wavelength of 21 cm. However, by the time it reaches us the wavelength has increased to between 1.5 and 5 m (corresponding to frequencies 200 – 60 MHz) since the radiation travelled to us through an expanding Universe. The particular measurement reported in the paper is at a frequency of 78 Mhz, which corresponds to a wavelength of almost 4 m. This means the signal was produced 13.6 billion years ago when the Universe was only 200 million years old!

What makes EDGES special is that it consists of a single antenna which, at least in its original form, fitted in a suitcase. That’s useful because the team of people who worked on the experiment (led by Judd Bowman) is based in Arizona but do their measurements in the desert of Western Australia.Why choose such a remote location? Obviously, not many people live there so there are almost no radio signals made by humans. At other locations, human radio signals can cause big problems; Just think of the FM radio band which runs from 87 to 108 MHz, right in the middle of interesting frequencies for seeing the effect of the first stars. Not picking up these transmissions helps a lot. However, this still doesn’t make it easy to see the far away 21-cm signal from the time of the first stars. The entire sky is filled with bright radio radiation from relativistic electrons and hot plasmas in our own galaxy which is 100,000s, if not million times, stronger than the signal EDGES is trying to see. No Australian desert can help you to avoid the Milky Way!

But similar to people being able to see a small tree on the slopes of a huge mountain, the radio emission from the early Universe can be separated from that of the Milky Way. When you look at different frequencies, the Milky Way radiation shows the same distribution; in other words, it varies in a very regular way with frequency. The 21-cm signal on the other hand is expected to vary significantly from one frequency to another. This is the key to separating them. But since the contrast is quite big, you have to be very sure that your telescope doesn’t artificially produce small variations due to the electronics, interference signals, the Earth’s ionosphere and so on. To get an accuracy of 1 in a 100,000 you need to put in a lot of hard work! In the case of EDGES, they worked their socks off for no less than 10 years out by regularly testing the performance of the antenna in the desert and by frequently testing all the electronic components in the lab.

So after all this hard work, they went out and did the measurements and found a signal! However, something was wrong since the signal was about twice as strong as anything they had expected. Clearly there must be a problem with the antenna. So they went back, changed some things and tried again, only to obtain more or less the same result. They moved the antenna to a different location and measured again; still no change! So after trying many different things and not being able to get anything except a strong signal and not being able to explain it with anything else they decided that it might be real and coming from the time of the first stars.

However, if it is real, it would mean that the temperature of the Universe at the time this signal was produced was at most 3 Kelvin. However, the absolute lowest temperature expected is about 7 Kelvin. Now the difference might not seem much but the problem is that we know of many processes which can increase the temperature but not really of any which can lower it. So, there is a really a problem here, the first stars woke up to a very, very chilly Cosmic Dawn.

However, if we don’t know of any processes which can reduce the temperature then we could try to come up with some. This is exactly what the author of a second paper in Nature did. What is needed is something to cool the early Universe, what could it be? Modern cosmology relies on most of the matter in the Universe to be dark matter, unobservable except through its gravitational effects on normal matter. Now, the type of dark matter which works best in detailed models is known as “Cold Dark Matter” because it’s, well, cold. What if this dark matter would actually interact a little with normal matter? Then the normal matter would lose some of its energy, its heat, to the cold dark matter. It does not need to interact much, just enough to make the temperature drop from 7 to 3 Kelvin. Rennan Barkana, the author of this second paper, worked out the numbers and found this could work. But only if the cold dark matter particles are not too heavy and interact sufficiently with normal matter.

Is this a reasonable explanation? It’s not a type of dark matter particle which is often considered but since we do not know what dark matter is, it’s hard to rule out. Still, it’s an odd result and perhaps the simplest solution is just that despite their best efforts, the EDGES team missed something and the signal is not at all from the time of the first stars but caused by a subtle effect in their equipment. That’s why everyone, including the EDGES team, is hoping another team with a similar radio antenna will confirm their result. Luckily there are several other experiments active which could try this, SARAS, LEDA and possibly NenuFAR, a spin-off from the LOFAR project. However, all of these experiments also need to do the hard work of understanding the tiniest details of their antennae and electronics and how these interact with the radio signals arriving from space and Earth… It may take a while before they are ready.

In the meanwhile I expect there will be a flurry of papers as a result of EDGES result and the proposed cooling by cold dark matter particles. Perhaps the necessary properties for the dark matter particles are already ruled out by some other effects such particles would cause either in a laboratory here on Earth or out in the Universe? Perhaps there are other ‘exotic’ explanations for the detection of such a strong 21-cm signal? We trying to understand a period in the history of the Universe of which we know very little so there is a quite some room for new ideas. There is a lot to look forward to!

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.

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.

Cosmology

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 (bo@fysik.su.se)

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 (stephan.rosswog@astro.su.se) and Chad Finley (cfinley@fysik.su.se)

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. https://dcc.ligo.org/LIGO-P1500271/public

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 http://www.black-holes.org: Photo Credit: SXS.

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 (adamo@astro.su.se)

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.

The 2015 Breakthrough Prize for the accelerated universe

The discovery of the accelerated universe keeps receiving a well deserved attention. On November 9, the Breakthrough Prize Foundation announced the recipients of the 2015 Breakthrough Prize in Fundamental Physics, and all members of the Supernova Cosmology Project and the High-z Supernova Team were awarded the prize “for the most unexpected discovery that the expansion of the universe is accelerating, rather than slowing as had been long assumed.”
Nobel laureates Saul Perlmutter, Brian P. Schmidt, and Adam Riess received the prize in behalf of their collaborations, 3 million US dollars to be shared with 51 team members.

Ariel Goobar
Prof. Ariel Goobar
We gratulate Oskar Klein Centre member Ariel Goobar which is one of the recipient of the prize, and all other team members

Supernova Cosmology Project Team Breakthrough Prize winners: Greg Aldering, Brian J. Boyle, Patricia G. Castro, Warrick J. Couch, Susana Deustua, Richard S. Ellis, Sebastien Fabbro, Alexei V. Filippenko, Andrew S. Fruchter, Ariel Goobar, Donald E. Groom, Isobel M. Hook, Mike Irwin, Alex G. Kim, Matthew Y. Kim, Robert A. Knop, Julia C. Lee, Chris Lidman, Thomas Matheson, Richard G. McMahon, Richard Muller, Heidi J. M. Newberg, Peter Nugent, Nelson J. Nunes, Reynald Pain, Nino Panagia, Carl R. Pennypacker, Robert Quimby, Pilar Ruiz-Lapuente, Bradley E. Schaefer and Nicholas Walton.
High-Z Supernova Search Team Breakthrough Prize winners: Peter Challis, Alejandro Clocchiatti, Alan Diercks, Alexei V. Filippenko, Peter M. Garnavich, Ron L. Gilliland, Craig J. Hogan, Saurabh Jha, Robert P. Kirshner, Bruno Leibundgut, Mark M. Phillips, David Reiss, R. Chris Smith, Jason Spyromilio, Christopher Stubbs, Nicholas B. Suntzeff and John Tonry.

The annual Breakthrough Prizes in fundamental physics, life sciences and mathematics, are sponsored by Google co-founder Sergey Brin and his wife, Anne Wojcicki, a founder of the genetics company 23andMe; Alibaba Group founder Jack Ma and his wife, Cathy Zhang; Russian entrepreneur and venture capitalist Yuri Milner and his wife, Julia; and Facebook founder Mark Zuckerberg and his wife, Priscilla Chan. The goal is to celebrate scientists and generate excitement about the pursuit of science as a career. [1]

The discovery of the acceleration of the universe is an unprecedented breakthrough that marked the direction for all research in modern cosmology, and it was awarded the Nobel Prize in Physics in 2011.

If you want to know more about the work done by the two teams check the -behind the scenes video. Unfortunately the quality is not the best, but it is still interesting to hear all the stories told by both team members while in Stockholm for receiving the Nobel Prize back in 2011.

The Nobel Prize in Physics 2013

Today’s Nobel Prize awarded jointly to François Englert and Peter W. Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”.

Here at OKC we are so delighted to see this prize. It confirms the importance of last year’s discovery of the mechanism and the particle imagined by Englert and Higgs. To tell the truth, although the Higgs particle was only discovered recently it has been part of some of our calculations here at OKC for some time. Some theories of dark matter assume the existence of a Higgs particle. So it was important to confirm this with the ATLAS and CMS experiments, since the discovery we know we are on the right track.
But not until a short time before the discovery annoucement did we really know that the Higgs particle existed. Not so long before the discovery some experimentalists and theorists would get a bit nervous, wondering what would we do if no Higgs particle was found… one would have to start from scratch, change the theory, go back to the drawing board, invent something new but what?

François Englert. Photo: Pnicolet via Wikimedia Commons
Peter W. Higgs. Photo: G-M Greuel via Wikimedia Commons


Thanks to the hard physical discovery of the Higgs particle at CERN we can now move forward, while many other theories without a Higgs particle have faded away into history.
That’s science at work. The Higgs boson is the last missing piece of the so-called standard model of particle physics. Good we got that sorted out!
But we know that the standard model is not the full story, the Higgs particle does not give mass to the neutrinos, nor do we know what is dark matter, as the standard model does not contain any such particles.

The CERN programme with the ATLAS, CMS and LHC experiments is still to provide about 200 times more data than was needed to find the Higgs particle. This is by no mean a guarantee that we will find something new, but it is only by covering new ground with some ingenious new instruments, that there is a chance to learn something new about Nature. The LHC project and the ATLAS and CMS experiments are just fantastic instruments built for that purpose. It is a great privilege to work on the ATLAS experiment and see the Nobel Prize going to particle physics today, after a bit of excitement, here at OKC, we will go back to analysing the data from the ATLAS experiment and see if we can solve another mystery of Nature.

Christophe Clement – researcher at the Oskar Klein Centre

First results of the AMS-02 experiment

The Alpha Magnetic Spectrometer (AMS) collaboration announced its first physics result in Physical Review Letters on 3 April 2013 [AMS Collaboration, Phys. Rev. Lett. 110 (2013)]. This was a long awaited event for the astroparticle physics community. Indeed, this large particle detector was first proposed by Nobel laureate Samuel Ting in 1994, to study primordial cosmic-ray particles in the energy range from 0.5 to 2 TeV. A proof-of-principle spectrometer (AMS-01) flew successfully for 10 days on the space shuttle Discovery during flight STS-91 in June 1998.

AMS-02 is visible at centre of the International Space Station's starboard truss.
The encouraging results have strengthen the undertaking of building the first high-precision astroparticle detector (AMS-02) to be installed on the International Space Station (ISS). Following the loss of the space shuttle Columbia and its crew on 1 February 2003, the space shuttle programme was suspended by NASA, cancelling a number of flights, including the AMS-02 one. On 15 October 2009, hence over six years later, President George W. Bush signs the bill authorising NASA to add another space shuttle to launch AMS-02 on the ISS. On 16 May 2011 the space shuttle Endavour finally takes the AMS detector to space. The crew of the STS-134 mission successfully installed AMS three days later on the ISS S3 truss (see picture), from where AMS is taking continually data ever since.
Continue reading First results of the AMS-02 experiment

Planck data released

Today some of the Oskar Klein Centre’s researchers were viewing the webcast from the data realease of the Planck satellite.

Planck webcast
OKC researchers digesting the new Planck data.

The Planck data is a very impressive set, which will be used extensively in cosmological analyses for many years to come. It will take time to digest all the information in the 29 scientific papers that were also put online today, on Planck Published Papers.

It seems the major features of the LambdaCDM model have been verified, in addition the data can not accomodate a fourth, hypothetical neutrino. The most significant new result is a markedly lower value for the expansion rate (the “Hubble constant”) which in turn influences the estimates of the energy density of the universe in terms of dark matter, ordinary matter, and dark energy, by up to 20 percent.

The only anomaly which was reported during the press conference is a strange undershoot of the angular power spectrum at the 10-degree scale. This seems only to be at the 2.5 – 3 sigma level, and we were a bit surprised that they made such a relatively big deal of it. Of course, it would be very interesting if true (see for example a paper I wrote with Ulf Danielsson, Uppsala University, in 2002 on transplanckian physics that could be at play, paper). However, we should cautiously wait for next year’s added data (including polarization)  to make a safer evaluation of the significance of this anomaly.

Fermi observations proves Supernova Remnants produce Cosmic Rays

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Cyan loops indicate where the remnant is interacting with a dense cloud of interstellar gas. Credit: NASA/DOE/Fermi LAT Collaboration, NOAO/AURA/NSF, JPL-Caltech/UCLA
Particles with TeV energies, like those produced at the LHC, seem exotic. But once outside the protection of our atmosphere, these “cosmic rays” (CR) become exceedingly common. The Fermi Telescope, for example, encounters a hundred thousand CR for every gamma ray it detects. These particles have an impressive scope of local effects, from damaging electronics and inhibiting manned space travel to possibly triggering lightning strikes. And although we have been aware of their existence since the early 1900ʼs, their exact origins remained unclear. Now a study by
the Fermi Collaboration claims to have solved the mystery.

The list of mechanisms capable of accelerating so many particles to high
energies was already a short one. Strong electromagnetic fields, like those surrounding pulsars, could do the job. So could the process for which the gamma-ray telescope got its name – Fermi acceleration. Less intuitive, it amounts to trapping particles in a region where they are repeatedly reflected by inhomogeneous magnetic fields. Each reflection supplies them with more energy, and the series of small enhancements adds up. One place there such conditions exist is at the boundary between slow and fast moving groups of particles, sometimes known as a “shock.” Supernova remnants (SNR), the
aftermath of massive astrophysical explosions, are expected to shock where the expanding sphere of stellar material encounters the surrounding gas.
Continue reading Fermi observations proves Supernova Remnants produce Cosmic Rays

Fermi/Swift GRB Symposium 2012: Polarisation and thermal emission in GRBs

The Fermi/Swift gamma-ray burst Symposium 2012 was held in Munich 7-11 May 2012.

Recent results on the prompt and afterglow emissions in gamma-ray burst were discussed at the Fermi/Swift gamma-ray burst Symposium 2012 which was held in Munich 7-11 May 2012.

Among the most important issues presented was the recent gamma-polarisation measurement with IKAROS-GAP. Significant degrees of polarisation in several bursts have now been detected. In particular, the change in polarisation angle was significantly detected. It was speculated that this is due to variation in emission patches in very narrowly collimated jets.

Another point which gained a lot of attention was on the existence of thermal components in GRB spectra. Previously the leading model for the prompt emission has been optically thin synchrotron emission. However, amounting observational evidence is showing that the photosphere in the relativistic flow is responsible for, at least a part of, the observed emission. Also recent progress in the theory and numerical simulations of relativistic jet was presented, and again thermal emission seems to be unavoidable.

Links:
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Conference website

Gamma ray spectrum of GRB110721A
Gamma-ray spectrum of GRB110721A which will soon be presented in a Fermi publication led by Magnus Axelsson. In addition to the dominant broad component, there is a bump at lower energies which is likely emission from the photosphere. The spectrum cannot be explained by synchrotron emission and thus disproves the long-held view that synchrotron emission alone can explain GRB spectra.