One hundred years since Einstein proposed gravitational waves as part of his general theory of relativity, an 11-year search performed with CSIRO's Parkes telescope has shown that an expected background of waves is missing, casting doubt on our understanding of galaxies and black holes.

For scientists gravitational waves exert a powerful appeal, as it is believed they carry information allowing us to look back into the very beginnings of the Universe. Although there is strong circumstantial evidence for their existence, they have not yet been directly detected.

The work, led by Dr Ryan Shannon (of CSIRO and the International Centre for Radio Astronomy Research), is published today in the journal Science.

Using Parkes, the scientists expected to detect a background 'rumble' of the waves, coming from the merging galaxies throughout the Universe, but they weren't there. This world-first research has caused scientists to think about the Universe in a different way.

"This is probably the most comprehensive, high precision science that's ever been undertaken in this field of astronomy," Dr Shannon said. "By pushing ourselves to the limits required for this sort of cosmic search we're moving into new frontiers in all areas of physics, forcing ourselves to understand how galaxies and black holes work."

The fact that gravitational waves weren't detected goes against all theoretical calculations and throws our current understanding of black holes into question.

Galaxies grow by merging and every large one is thought to have a supermassive black hole at its heart. When two galaxies unite, the black holes are drawn together and form an orbiting pair. At this point, Einstein's theory is expected to take hold, with the pair predicted to succumb to a death spiral, sending ripples known as gravitational waves through space-time, the very fabric of the Universe.

The image pictured up top is a composite of X-rays from NASA's orbiting Chandra X-ray observatory (blue) and optical data from the Hubble Space Telescope (gold). A close-up of the boxed portion of the image, featuring only X-ray data (pictured here), reveals two distinct black holes at the heart of galaxy NGC3393 that are around 1 million and 30 million times the mass of the Sun, and believed to be involved in what is known as a "minor merger," wherein a galaxy of relatively larger mass "eats" a smaller one.

Although Einstein's general theory of relativity has withstood every test thrown at it by scientists, directly detecting gravitational waves remain the one missing piece of the puzzle.

To look for the waves, Dr Shannon's team used the Parkes telescope to monitor a set of 'millisecond pulsars'. These small stars produce highly regular trains of radio pulses and act like clocks in space. The scientists recorded the arrival times of the pulsar signals to an accuracy of ten billionths of a second.

A gravitational wave passing between Earth and a millisecond pulsar squeezes and stretches space, changing the distance between them by about 10 metres — a tiny fraction of the pulsar's distance from Earth. This changes, very slightly, the time that the pulsar's signals arrive on Earth.

The scientists studied their pulsars for 11 years, which should have been long enough to reveal gravitational waves.

So why haven't they been found? There could be a few reasons, but the scientists suspect it's because black holes merge very fast, spending little time spiralling together and generating gravitational waves.

"There could be gas surrounding the black holes that creates friction and carries away their energy, letting them come to the clinch quite quickly," said team member Dr Paul Lasky, a postdoctoral research fellow at Monash University.

Whatever the explanation, it means that if astronomers want to detect gravitational waves by timing pulsars they'll have to record them for many more years.

"There might also be an advantage in going to a higher frequency," said Dr Lindley Lentati of the University of Cambridge, UK, a member of the research team who specialises in pulsar-timing techniques. Astronomers will also gain an advantage with the highly sensitive Square Kilometre Array telescope, set to start construction in 2018.

Not finding gravitational waves through pulsar timing has no implications for ground-based gravitational wave detectors such as Advanced LIGO (the Laser Interferometer Gravitational-Wave Observatory), which began its own observations of the Universe last week.

"Ground-based detectors are looking for higher-frequency gravitational waves generated by other sources, such as coalescing neutron stars," said Dr Vikram Ravi, a member of the research team from Swinburne University (now at Caltech, in Pasadena, California).

The International Centre for Radio Astronomy Research (ICRAR) is a joint venture between Curtin University and The University of Western Australia with support and funding from the State Government of Western Australia.

The Daily Galaxy via CSIRO

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Scientists working with ALICE (A Large Ion Collider Experiment), a heavy-ion detector on the Large Hadron Collider (LHC) ring, have made precise measurements of particle mass and electric charge that confirm the existence of a fundamental symmetry in nature. The investigators include Brazilian researchers affiliated with the University of São Paulo (USP) and the University of Campinas (UNICAMP).

"After the Big Bang, for every particle of matter an antiparticle was created. In particle physics, a very important question is whether all the laws of physics display a specific kind of symmetry known as CPT, and these measurements suggest that there is indeed a fundamental symmetry between nuclei and antinuclei," said Marcelo Gameiro Munhoz, a professor at USP's Physics Institute (IF) and a member of the Brazilian team working on ALICE.

 

 

The findings, reported in a paper published online in Nature Physics on August 17, led the researchers to confirm a fundamental symmetry between the nuclei of the particles and their antiparticles in terms of charge, parity and time (CPT).

These measurements of particles produced in high-energy collisions of heavy ions in the LHC were made possible by the ALICE experiment's high-precision tracking and identification capabilities, as part of an investigation designed to detect subtle differences between the ways in which protons and neutrons join in nuclei while their antiparticles form antinuclei.

Munhoz is the principal investigator for the research project "High-energy nuclear physics at RHIC and LHC", supported by São Paulo Research Foundation (FAPESP). The project--a collaboration between the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the United States and ALICE at the LHC, operated by the European Organization for Nuclear Research (CERN) in Switzerland--consists of experimental activities relating to the study of relativistic heavy-ion collisions.

Among other objectives, the Brazilian researchers involved with ALICE seek to understand the production of heavy quarks (charm and bottom quarks) based on the measurement of electrons using an electromagnetic calorimeter and, more recently, Sampa, a microchip developed in Brazil to study rarer phenomena arising from heavy-ion collisions in the LHC.

According to Munhoz, the measurements of mass and charge performed in the symmetry experiment, combined with other studies, will help physicists to determine which of the many theories on the fundamental laws of the universe is most plausible.

"These laws describe the nature of all matter interactions," he said, "so it's important to know that physical interactions aren't changed by particle charge reversal, parity transformation, reflections of spatial coordinates and time inversion. The key question is whether the laws of physics remain the same under such conditions."

In particular, the researchers measured the mass-over-charge ratio differences for deuterons, consisting of a proton and a neutron, and antideuterons, as well as for nuclei of helium-3, comprising two protons and one neutron, and antihelium-3. Recent measurements at CERN compared the same properties of protons and antiprotons at high resolution.

The ALICE experiment records high-energy collisions of lead ions at the LHC, enabling the study of matter at extremely high temperatures and densities.

The lead-ion collisions provide an abundant source of particles and antiparticles, producing nuclei and the corresponding antinuclei at nearly equal rates. This allows ALICE to make a detailed comparison of the properties of the nuclei and antinuclei that are most copiously produced.

The experiment makes precise measurements of both the curvature of particle tracks in the detector's magnetic field and the particles' time of flight and uses this information to determine the mass-to-charge ratios for nuclei and antinuclei.

The high precision of the time-of-flight detector, which determines the arrival time of particles and antiparticles with a resolution of 80 picoseconds and is associated with the energy-loss measurement provided by the time-projection chamber, allows the scientists involved to measure a clear signal for deuterons/antideuterons and helium-3/antihelium-3, the particles studied in the similarity experiment.

The image at the top of the page is an artist's conception that illustrates the history of the cosmos, from the Big Bang and the recombination epoch that created the microwave background, through the formation of galactic superclusters and galaxies themselves. The dramatic flaring at right emphasizes that the universe's expansion currently is speeding up.

The Daily Galaxy via Fundação de Amparo à Pesquisa do Estado de São Paulo

Image credit: cfa.harvard.edu and http://alicematters.web.cern.ch

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