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Is gravity wrong?

NGC 1300

Comparison of galaxies' mass estimates and their spin rates pose challenges for the standard theory of gravity. A newer theory called MOND aims to solve the riddle.

A STUDY OF THE SPIN RATES of gas-rich galaxies supports an alternate theory of gravity known as MOND, according to work by University of Maryland Astronomy Professor Stacy McGaugh.

This latest of several successful MOND predictions, raises new questions about the accuracy of the reigning cosmological model of the universe, writes McGaugh in a paper to be published in March in the journal Physical Review Letters.

Modern cosmology says that for the universe to behave as it does, the mass-energy balance of the universe must be dominated by dark matter and dark energy. However, direct evidence for the existence of these invisible components remains lacking.

An alternate, though unpopular, possibility is that the current theory of gravity does not adequately describe the dynamics of cosmic systems.

Enter MOND

A few concepts that would modify our understanding of gravity have been proposed. One of these is Modified Newtonian Dynamics (MOND), which was hypothesised in 1983 by Moti Milgrom, a physicist at the Weizmann Institute of Science in Rehovot, Israel.

One of MOND’s predictions specifies the relationship between the overall mass of a galaxy and its rotation velocity. However, uncertainties in estimating the masses of star-dominated spiral galaxies (such as our own Milky Way) previously had prevented a definitive test.

Galaxies UGC 2885 and F549-1

The star-dominated spiral galaxy UGC 2885 (left) and the gas-rich galaxy F549-1 (right). A new study of the spin rates of gas-rich galaxies supports an alternate theory of gravity. Images by Zagursky and McGaugh.

To overcome this problem, McGaugh instead examined gas-rich galaxies, which have fewer stars and a lot more mass in the form of interstellar gas. He says it is easier to gauge the mass of gas than of stars.

McGaugh compiled a sample of 47 gas-rich galaxies and compared each one’s mass and rotation velocity with the relationship expected by MOND.

All 47 galaxies fell on or very close to the MOND prediction.

By comparison, no dark matter model performed as well.

MOND vs the rest

Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by the dark matter-dark energy theory. However, according to McGaugh, it does not account well for what happens at the scales of individual galaxies and smaller.

“MOND is just the opposite,” he said. “It accounts well for the ‘small’ scale of individual galaxies, but MOND doesn’t tell you much about the larger universe.”

“If we’re right about dark matter, why does MOND work at all?” asks McGaugh. “Ultimately, the correct theory—be it dark matter or a modification of gravity—needs to explain this.”

Adapted from information issued by University of Maryland. Galaxy image courtesy NASA / ESA / HHT / STScI / AURA / Hubble Collaboration.

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A new way to weigh planets

Diagram showing the Sun, Earth and Jupiter orbiting a common barycentre

Measurements of signals from pulsars (purple lines) are affected by Earth's movement in its orbit around the Solar System's centre of mass, the barycentre. The barycentre moves too, due to the gravitational influence of the other planets. By working backwards from the pulsar signals, scientists can work out the gravitational pull of the planets, and from that deduce their masses.

  • Radio waves from pulsars affected by Solar System’s gravity
  • Adjusting the pulsar wave measurements gives gravity readings
  • From the gravity readings, the planet’s masses can be found

Astronomers from Australia, Germany, the UK, Canada and the USA have come up with a new way to weigh the planets in our Solar System, using radio signals from pulsars.

“This is first time anyone has weighed entire planetary systems—planets with their moons and rings,” said team leader Dr. David Champion of the Max-Planck-Institut fuer Radioastronomie in Bonn, Germany.

“And we’ve provided an independent check on previous results, which is great for planetary science.”

Measurements of planet masses made this new way could feed into data needed for future space missions.

Until now, astronomers have weighed planets by measuring the orbits of their moons or the trajectories of spacecraft flying past them. That’s because mass produces gravity, and a planet’s gravitational pull determines the orbit of anything that goes around it—both the size of the orbit and how long it takes to complete.

The new method is based on adjustments astronomers have to make to signals from pulsars…small spinning stars that deliver regular ‘blips’ of radio waves.

The Earth is travelling around the Sun, and this movement affects exactly when pulsar signals arrive here. To remove this effect, astronomers calculate when the pulses would have arrived at the Solar System’s exact centre of mass, or barycentre, around which all the planets orbit.

Because the arrangement of the planets around the Sun changes all the time, the barycentre moves around too.

To work out its position, astronomers use both a table (called an ephemeris) of where all the planets are at a given time, and the values for their masses that have already been measured.

If these figures are slightly wrong, and the position of the barycentre is slightly wrong, then a regular, repeating pattern of timing errors appears in the pulsar data.

“For instance, if the mass of Jupiter and its moons is wrong, we see a pattern of timing errors that repeats over 12 years, the time Jupiter takes to orbit the Sun,” said Dr Dick Manchester of CSIRO Astronomy and Space Science.

The CSIRO's Parkes radio telescope

The CSIRO's Parkes radio telescope made most of the pulsar signal measurements.

But if the mass of Jupiter and its moons is corrected, the timing errors disappear. This is the feedback process that the astronomers have used to determine the planets’ masses.

Better measurements of planet masses

Data from a set of four pulsars have been used to weigh Mercury, Venus, Mars, Jupiter and Saturn with their moons and rings.

Most of these data were recorded with CSIRO’s Parkes radio telescope in eastern Australia, with some contributed by the Arecibo telescope in Puerto Rico and the Effelsberg telescope in Germany.

The masses were consistent with those measured by spacecraft. The mass of the Jovian system (Jupiter and its moons)—0.0009547921 times the mass of the Sun—is significantly more accurate than the mass determined from the Pioneer and Voyager spacecraft, and consistent with, but less accurate than, the value from the Galileo spacecraft.

The new measurement technique is sensitive to a mass difference of 200,000 million million tonnes—just 0.003% of the mass of the Earth, and one ten-millionth of Jupiter’s mass.

“In the short term, spacecraft will continue to make the most accurate measurements for individual planets, but the pulsar technique will be the best for planets not being visited by spacecraft, and for measuring the combined masses of planets and their moons,” said CSIRO’s Dr George Hobbs, another member of the research team.

Repeating the measurements would improve the values even more. If astronomers observed a set of 20 pulsars over seven years they’d weigh Jupiter more accurately than spacecraft have. Doing the same for Saturn would take 13 years.

“Astronomers need this accurate timing because they’re using pulsars to hunt for gravitational waves predicted by Einstein’s general theory of relativity”, said Professor Michael Kramer, head of the ‘Fundamental Physics in Radio Astronomy’ research group at the Max-Planck-Institut fuer Radioastronomie.

“Finding these waves depends on spotting minute changes in the timing of pulsar signals, and so all other sources of timing error must be accounted for, including the [influences] of Solar System planets.”

Adapted from information issued by CSIRO / D. Champion, MPIfR.

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Gravity mission down under

Artist's impression of the twin GRACE satellites

Artist's impression of the twin GRACE satellites, which measure Earth's gravity field.

  • Funding for Australia to develop laser system
  • For use on the GRACE Follow On science satellite
  • GRACE measures Earth’s gravity field

The Australian National University has welcomed the announcement of $4.7 million in funding for the next stage in the Gravity Recovery and Climate Experiment (GRACE) satellite program, which will include laser testing and results analysis at the University.

GRACE measurements reveal melting of the polar ice caps and are used to monitor changes in ground water.

The announcement was made by Senator the Hon Kim Carr, Minister for Innovation, Industry, Science and Research, as part of a suite of funding announcements from the Australian Space Research Program (ASRP).

Dr Paul Tregoning

Dr Paul Tregoning's team will analyse the data from the new GRACE mission.

The project, led by The Australian National University, will bring together expertise from NASA’s Jet Propulsion Laboratory, EOS Space Systems, the CSIRO’s Australian Centre for Precision Optics, the National Measurement Institute and Germany’s Albert Einstein Institute.

Researchers will develop prototype hardware for a laser ranging system to fly on NASA’s GRACE Follow On mission. GRACE is a satellite mission that has provided new and unexpected insights into the natural processes of Earth.

The laser system will be developed by researchers from the ANU Centre for Gravitational Physics led by Dr Daniel Shaddock, while a team led by Dr Paul Tregoning from the ANU Research School of Earth Sciences will analyse the data from the new mission.

“This new laser system for GRACE Follow On will improve the measurement by a factor of 25 compared to the original GRACE mission,” said Dr Shaddock.

“Australian researchers will partner closely with NASA and German scientists to ensure that our system will perform in the harsh space environment.”

“This funding will make Australia a partner in a space mission of global importance,” added Dr Tregoning.

Gravity mapping satellite

The joint NASA-German Aerospace Centre GRACE project is a five-year mission to precisely measure Earth’s shifting water masses and map their effects on Earth’s gravity field.

It measures Earth’s gravity field by measuring the separation between Dr Paul Tregoning’s with an accuracy of one millionth of a metre (less than 1/10th the width of a human hair).

GRACE geoid

GRACE map showing how the Earth's gravity field is not smooth, but bumpy due to uneven distribution of mass.

Launched March 17, 2002, GRACE senses minute variations in Earth’s surface mass and corresponding variations in Earth’s gravitational pull.

The monthly gravity maps generated by Grace will be up to 1,000 times more accurate than current maps, substantially improving the accuracy of many techniques used by oceanographers, hydrologists, glaciologists, geologists and other scientists to study phenomena that influence climate.

Environment studies

“The GRACE mission has already provided significant and valuable data to researchers, including allowing us—for the very first time—to see just how much water has been lost from the Murray Darling Basin as a result of drought,” said Dr Tregoning.

“The GRACE data showed us just how serious that problem was, and that we had lost some 200 cubic kilometres of water over six years; that’s the equivalent of 400 Sydney Harbours.”

“Obviously, information like this is essential for policy makers to plan for a healthy and prosperous future for the country,” said Dr Tregoning.

“But the funding also allows for fantastic opportunities for researchers to analyse this data and work on an international space project. It will allow local academics to show that Australian research is competitive in the international space arena,” he said.

Adapted from information issued by ANU. Photos by Cole Bennets / NASA.

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