Solving the riddle of neutron stars

March 10, 2015

It has not yet been possible to measure the gravitational waves predicted by Einstein's theory of general relativity. They are so weak that they get lost in the noise of the measurements. But thanks to the latest simulations of the merging of binary neutron star systems, the structure of the sought-after signals is now known. As a team of German and Japanese theoretical astrophysicists reports in the Editor's choice of the current edition of the scientific journal "Physical Review D", gravitational waves have a characteristic spectrum that is similar to the spectral lines of atoms.

Gravitational waves are generated when masses accelerate. The first indirect evidence for their existence was detected in 1974 when the binary pulsar PSR B1913+16 was discovered in the constellation Aquila. The two rapidly rotating neutron stars are drifting towards each other in a spiral shape, which is why, the astrophysicists explain, they are losing energy and emitting gravitational waves. Russell A. Hulse and Joseph H. Taylor received the 1993 Nobel Prize in Physics for this discovery. In the meantime, there are now several large-scale experiments for detecting gravitational waves: the American LIGO experiment, the European Virgo experiment, and the Japanese KAGRA detector. Experts estimate that signals of gravitational waves from merging binary neutron star systems will be detected within the next five years.

"These signals are not easy to detect, because they have an extremely small amplitude." But despite these difficult conditions, it is possible to find them, if you know what to look for in advance," explained Professor Luciano Rezzolla from the Institute for Theoretical Physics at Goethe University. Together with a Japanese colleague from Osaka University, he has studied a number of binary neutron star systems with the help of the latest simulation techniques and has discovered that the merging of the stars generates characteristic gravitational wave spectra. "These spectra correspond, at least logically, to the electromagnetic spectral lines emitted by atoms or molecules. From these we can derive information on the characteristics of the stars," explains Rezzolla.

As the astrophysicists show in two publications with related content in "Physical Review Letters" (published in November 2014) and in the current edition of "Physical Review D," the gravitational waves spectrum is like a fingerprint for the two stars. If scientists learn how to interpret these spectra, they will know what the neutron stars are made of and will be able to determine what is their equation of state, which is so far unknown. Equations of state describe the thermodynamic properties of systems as a function of variables, such as pressure, temperature, volume, or particle number. To this Rezzolla adds: "This is a very exciting possibility, because then we would be able to solve a riddle that has remained unsolved for 40 years: What are neutron stars made of and what is their stellar structure?"

"If the signal is strong and thus the fingerprint is very clear, even a single measurement would be sufficient," Rezzolla predicts. "The prospects of solving the riddle of neutron stars have never been this good. The gravitational waves that we hope to detect in a few years are already on their way from the farthest reaches of the universe."
-end-
You can download images at: http://www.uni-frankfurt.de/54497071

Caption: Four snapshots from the merging of two neutron stars. Convergence to merger takes only a few milliseconds, during which immense masses are accelerated. The signals of the theoretically generated gravitational waves have now been calculated in simulations.

Goethe University Frankfurt

Related Gravitational Waves Articles from Brightsurf:

Weak equivalence principle violated in gravitational waves
New research published in EPJ C proves theoretically that the Weak Equivalence Principle can be violated by quantum particles in gravitational waves - the ripples in spacetime caused by colossal events such as merging black holes.

Remembrance of waves past: memory imprints motion on scattered waves
Now, it appears that between relativity and the classical (stationary) wave regime, there exists another regime of wave phenomena, where memory influences the scattering process.

New populations of black holes revealed by gravitational waves
The gravitational wave detectors LIGO and Virgo have just chalked up their biggest catch yet, a black hole 142 times the mass of the Sun, resulting from the merger of two ''lighter'' black holes.

Tabletop quantum experiment could detect gravitational waves
Tiny diamond crystals could be used as an incredibly sensitive and small gravitational detector capable of measuring gravitational waves, suggests new UCL-led research.

Gravitational waves could prove the existence of the quark-gluon plasma
According to modern particle physics, matter produced when neutron stars merge is so dense that it could exist in a state of dissolved elementary particles.

X-rays and gravitational waves will combine to illuminate massive black hole collisions
A new study by a group of researchers at the University of Birmingham has found that collisions of supermassive black holes may be simultaneously observable in both gravitational waves and X-rays at the beginning of the next decade.

Quantum expander for gravitational-wave observatories
Gravitational-wave detectors use ultra-stable laser light stored in optical cavities to achieve the high sensitivity for detecting gravitational-wave signals from merging binary black holes and neutron stars.

Gravitational lensing provides a new measurement of the expansion of the universe
Amid ongoing uncertainty around the value of the Hubble Constant, uncertainty largely created by issues around measuring distances to objects in the galaxy, scientists who used a new distance technique have derived a different Hubble value, one 'somewhat higher than the standard value,' as Tamara Davis describes it in a related Perspective.

Gravitational waves leave a detectable mark, physicists say
New research shows that gravitational waves leave behind plenty of 'memories' that could help detect them even after they've passed.

DIY gravitational waves with 'BlackHoles@Home'
Researchers hoping to better interpret data from the detection of gravitational waves generated by the collision of binary black holes are turning to the public for help.

Read More: Gravitational Waves News and Gravitational Waves Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.