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Comparison of billions of theoretical models with gravitational waves results in the answer to an old riddle
FRANKFURT. How large is a neutron star? Previous estimates varied from eight to sixteen kilometres. Astrophysicists at the Goethe University Frankfurt and the FIAS have now succeeded in determining the size of neutron stars to within 1.5 kilometres by using an elaborate statistical approach supported by data from the measurement of gravitational waves. The researchers’ report appears in the current issue of Physical Review Letters.
Neutron stars are the densest objects in our universe, with a mass larger than that of our sun compacted into a relatively small sphere whose diameter is comparable to that of the city of Frankfurt. This is actually just a rough estimate, however. For more than 40 years, the determination of the size of neutron stars has been a holy grail in nuclear physics whose solution would provide important information on the fundamental behaviour of matter at nuclear densities.
The data from the detection of gravitational waves from merging neutron stars (GW170817) make an important contribution toward solving this puzzle. At the end of 2017, Professor Luciano Rezzolla, Institute for Theoretical Physics at the Goethe University Frankfurt and FIAS, together with his students Elias Most and Lukas Weih already exploited this data to answer a long-standing question about the maximum mass that neutron stars can support before collapsing to a black hole - a result that was also confirmed by various other groups around the world. Following this first important result, the same team, with the help of Professor Juergen Schaffner-Bielich, has worked to set tighter constraints on the size of neutron stars.
The crux of the matter is that the equation of state which describes the matter inside neutron stars is not known. The physicists therefore decided to pursue another path: they selected statistical methods to determine the size of neutron stars within narrow limits. In order to set the new limits, they computed more than two billion theoretical models of neutron stars by solving the Einstein equations describing the equilibrium of these relativistic stars and combined this large dataset with the constraints coming from the GW170817 gravitational wave detection.
“An approach of this type is not unusual in theoretical physics," remarks Rezzolla, adding: "By exploring the results for all possible values of the parameters, we can effectively reduce our uncertainties." As a result, the researchers were able to determine the radius of a typical neutron star within a range of only 1.5 km: it lies between 12 and 13.5 kilometres, a result that can be further refined by future gravitational wave detections.
"However, there is a twist to all this, as neutron stars can have twin solutions," comments Schaffner-Bielich. It is in fact possible that at ultra-high densities, matter drastically changes its properties and undergoes a so-called "phase transition." This is similar to what happens to water when it freezes and transitions from a liquid to a solid state. In the case of neutron stars, this transition is speculated to turn ordinary matter into "quark matter," producing stars that will have the exact same mass as their neutron star "twin," but that will be much smaller and consequently more compact.
While there is no definite proof for their existence, they are plausible solutions and the researchers from Frankfurt have taken this possibility into account, despite the additional complications that twin stars imply. This effort ultimately paid off as their calculations have revealed an unexpected result: twin stars are statistically rare and cannot be deformed very much during the merger of two such stars. This is an important finding as it now allows scientists to potentially rule out the existence of these very compact objects. Future gravitational-wave observations will therefore reveal whether or not neutron stars have exotic twins.
Publication: Elias R. Most, Lukas R. Weih, Luciano Rezzolla, Jürgen Schaffner-Bielich: New constraints on radii and tidal deformabilities of neutron stars from GW170817, Phys. Rev. Lett. 120, 261103. https://doi.org/10.1103/PhysRevLett.120.261103
Picture material can be downloaded under: www.muk.uni-frankfurt.de/72776172?
Figure caption: "Range of the size for a typical neutron star compared to the city of Frankfurt (satellite image: GeoBasis-DE/BKG (2009) Google)".
Further information: Professor Luciano Rezzolla, Frankfurt Institute for Theoretical Physics, Faculty of Physics, and Frankfurt Institute for Advanced Studies, Riedberg Campus, Tel. +49 (0) 69 798-47871, firstname.lastname@example.org.
Economists at Goethe University Frankfurt show that inflation in Europe is more burdensome for low income groups
FRANKFURT. It is quite obvious that inflation may affect individuals differently. However, the extent to which the rate of inflation in the EU’s Member States disadvantages poorer individuals has now been shown by economists Eren Gürer and Professor Alfons Weichenrieder in a recent study.
Essential expenditure, for example for food, rent and electricity, make up a greater percentage of the budget of less well-off families than for more affluent ones. If the prices for such items rise more sharply than for luxury goods, this leads to low-income households having to put up with a higher rate of inflation in their individual shopping baskets. This means that the inflation rate can differ from the general rate of inflation depending on individual consumer habits. Is there a systematic distortion of the individual rate of inflation in the EU to the detriment of lower income brackets? This is a question that economists Eren Gürer and Professor Alfons Weichenrieder of Goethe University Frankfurt have now explored.
The analysis of data from 25 EU Member States from 2001 to 2015 shows that in most countries inflation tends to disadvantage poorer households: The annual inflation rate during this period for the poorest ten percent in a country was on average about 0.7 percent higher than for the richest ten percent. At an average inflation rate of 2.7 percent, this equates to a difference of slightly more than a quarter of the general rate of inflation.
Costs for electricity, rent, private means of transport and food, which have increased at an above-average rate, are above all responsible for this development. These items make up a far greater share in the shopping baskets of lower income groups. However, the effects are not equally pronounced in all countries: Whilst households in Italy and Portugal escaped this “discriminating inflation”, the EU’s Eastern European countries as well as the United Kingdom and Finland were particularly affected.
In Germany, the effect is comparatively moderate. Indeed, the gap here between nominal disposable incomes has by all means widened, as is known from other studies. The influence of inflation on income distribution - a topic neglected in previous studies – is, however, quite modest: It accounts for about one tenth of the increase in inequality already measured in the years under consideration.
This should not, however, blind us to the fact that in the representative German sample the shopping baskets of the lower ten percent became about 4.5 percent more expensive than the shopping baskets of the upper ten percent.
Publication: Eren Gürer and Alfons Weichenrieder, Pro-Rich Inflation in Europe: Implications for the Measurement of Inequality, Goethe University, SAFE Working Paper No. 209, May 2018. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3183723
Further information: Professor Alfons Weichenrieder, Chair of Economics and Public Finance, Theodor-W.-Adorno-Platz 4, Westend Campus, Tel.: +49(0)69-798-34788; email email@example.com
Repeated stimulation enlarges dendritic spines
FRANKFURT. Even in adult brains, new neurons are generated throughout a lifetime. In a publication in the scientific journal PNAS, a research group led by Goethe University describes plastic changes of adult-born neurons in the hippocampus, a critical region for learning: frequent nerve signals enlarge the spines on neuronal dendrites, which in turn enables contact with the existing neural network.
Practise makes perfect, and constant repetition promotes the ability to remember. Researchers have been aware for some time that repeated electrical stimulation strengthens neuron connections (synapses) in the brain. It is similar to the way a frequently used trail gradually widens into a path. Conversely, if rarely used, synapses can also be removed – for example, when the vocabulary of a foreign language is forgotten after leaving school because it is no longer practised. Researchers designate the ability to change interconnections permanently and as needed as the plasticity of the brain.
Plasticity is especially important in the hippocampus, a primary region associated with long-term memory, in which new neurons are formed throughout life. The research groups led by Dr Stephan Schwarzacher (Goethe University), Professor Peter Jedlicka (Goethe University and Justus Liebig University in Gießen) and Dr Hermann Cuntz (FIAS, Frankfurt) therefore studied the long-term plasticity of synapses in new-born hippocampal granule cells. Synaptic interconnections between neurons are predominantly anchored on small thorny protrusions on the dendrites called spines. The dendrites of most neurons are covered with these spines, similar to the thorns on a rose stem.
In their recently published work, the scientists were able to demonstrate for the first time that synaptic plasticity in new-born neurons is connected to long-term structural changes in the dendritic spines: repeated electrical stimulation strengthens the synapses by enlarging their spines. A particularly surprising observation was that the overall size and number of spines did not change: when the stimulation strengthened a group of synapses, and their dendritic spines enlarged, a different group of synapses that were not being stimulated simultaneously became weaker and their dendritic spines shrank.
“This observation was only technically possible because our students Tassilo Jungenitz and Marcel Beining succeeded for the first time in examining plastic changes in stimulated and non-stimulated dendritic spines within individual new-born cells using 2-photon microscopy and viral labelling,” says Stephan Schwarzacher from the Institute for Anatomy at the University Hospital Frankfurt. Peter Jedlicka adds: “The enlargement of stimulated synapses and the shrinking of non-stimulated synapses was at equilibrium. Our computer models predict that this is important for maintaining neuron activity and ensuring their survival.”
The scientists now want to study the impenetrable, spiny forest of new-born neuron dendrites in detail. They hope to better understand how the equilibrated changes in dendritic spines and their synapses contribute the efficient storing of information and consequently to learning processes in the hippocampus.
Publication: Structural homo- and heterosynaptic plasticity in mature and adult new-born rat hippocampal granule cells. DOI: 10.1073/pnas.1801889115 (Jungenitz et al. PNAS, 115:E4670 2018)
Picture material can be downloaded at: www.uni-frankfurt.de/72306770
Caption: The dendrites of newborn neurons (green) are covered with spines, similar to the thorns on a rose stem (Credit: Tassilo Jungenitz).
Further information: Dr Stephan Schwarzacher, Institute for Anatomy I, Faculty of Medicine, Niederrad Campus, Tel.: +49 (0)69 6301-6914, firstname.lastname@example.org