General relativity passes cosmic test Einsteins theory holds in extreme gravitational fields

Chalk up another win for "orthodox" scientific theories. General relativity has again passed stringent tests.

General relativity survives gruelling pulsar test — Einstein at least 99.95 percent right

An international research team led by Prof. Michael Kramer of the University of Manchesters Jodrell Bank Observatory, UK, has used three years of observations of the "double pulsar", a unique pair of natural stellar clocks which they discovered in 2003, to prove that Einsteins theory of general relativity - the theory of gravity that displaced Newtons - is correct to within a staggering 0.05%. Their results are published on the 14th September in the journal Science and are based on measurements of an effect called the Shapiro Delay.

The double pulsar system, PSR J0737-3039A and B, is 2000 light-years away in the direction of the constellation Puppis. It consists of two massive, highly compact neutron stars, each weighing more than our own Sun but only about 20 km across, orbiting each other every 2.4 hours at speeds of a million kilometres per hour. Separated by a distance of just a million kilometres, both neutron stars emit lighthouse-like beams of radio waves that are seen as radio "pulses" every time the beams sweep past the Earth. It is the only known system of two detectable radio pulsars orbiting each other. Due to the large masses of the system, they provide an ideal opportunity to test aspects of General Relativity.

The large mass of the pulsars and their proximity to each other is the key thing, resulting in a very strong gravitational field. The binary pulsar system provides an opportunity to check the validity of general relativity under conditions that are more extreme than any studied before.

Shapiro delay can be described as an apparent change in the speed of light in a strong gravitational field. It occurs because spacetime itself is warped in the field, which effectively forces light to travel a larger distance. Since the radio-frequency beam from each pulsar sweeps across Earth at a very precise frequency (22.8 milliseconds for one, 2.8 seconds for the other), like an exceptionally accurate clock, it is possible to predict exactly when the beam should be seen. Any departure from this prediction would be due to the Shapiro delay. The orbital period of the two pulsars around each other is about 2.4 hours. During this period, the distance between the pulsars varies, so the mutual gravitational fields vary correspondingly. This allows the theoretical delay time to be calculated, and the observations match the prediction very well.

This effect is distinct from the time dilation which occurs in a large gravitational field. The dilation causes time intervals to appear to lengthen. So the period of rotation of each pulsar appears to change, and the spectrum of radio waves from each object is redshifted, as the pulsars experience a change in the gravitational field. Here again, the observations match the predictions of general relativity very well.

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Additional information:

Pulsars Gyrations Confirm Einsteins Theory

News article about the research from Science. (Subscription required for full access.)

Tests of General Relativity from Timing the Double Pulsar

The actual research paper from Science. (Subscription required for full access.)

Millisecond Pulsars as Tools of Fundamental Physics

A review paper by Kramer posted to the arXiv in May 2004. It explains the underlying physics in some detail. It also describes the binary pulsar system, which had only recently been discovered.

The Confrontation between General Relativity and Experiment

An expository paper by Clifford M. Will that reviews the status of experimental tests of general relativity and of theoretical frameworks for analyzing them.

General relativity passes cosmic test - Einsteins theory holds in extreme gravitational fields.

News article at Nature.com news. (Subscription required)

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Tags: relativity, general relativity, pulsars, astrophysics

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Big Bang Theory Saved

From the title you might think that the theory was in serious peril. But it wasnt, really.

Big Bang Theory Saved

An apparent discrepancy in the Big Bang theory of the universes evolution has been reconciled by astrophysicists examining the movement of gases in stars.

Professor John Lattanzio from Monashs School of Mathematical Sciences and Director of the Centre for Stellar and Planetary Astrophysics said the confusion surrounding the Big Bang revolved around the amount of the gas Helium 3 in the universe.

The issue arises from what is actually one of the greatest successes of the big bang theory – quantitative calculations of the relative abundances of a few light element produced within the first 5-10 minutes of the universe by the process of "big bang" nucleosynthesis.

Detailed calculations depend on vaious factors, such as the temperature and rate of expansion of the universe during the time in question, as well as the densities and relative abundances of the "raw materials" (mostly protons, neutrons, photons, and neutrinos) when nucleosynthesis begins. The calculations are complex due to the dependence on so many factors, but theyre no real sweat with modern computers. Even before modern computers, the first physicists to make the computations (George Gamow and associates) in the late 1940s were able to come up with surprisingly good results, all things considered. (They predicted a mass fraction of about 50% helium-4, when the correct figure is more like 25%.)

The objective is to compute the abundances of the light elements deuterium (hydrogen-2), helium-3, helium-4, lithium-6, and lithium-7 (relative to protons – ordinary hydrogen). In order to check the correctness of the calculations, the numbers have to be compared with actual measurements of the relative abundances of these elements, either at the present time, or at some known time in the past.

And thats where the difficulties lie. In the first place, its necessary to be sure the relative abundances can be measured accurately. This is nontrivial, since, in the most extreme case, the predicted abundance of lithium-7 is a minuscule mass ratio on the order of 10^-10. (Lithium-6 is an even smaller ratio, too small to measure.)

The second problem is that theres no way to observe the relative abundances right after nucleosynthesis is complete. At best, the measurement could be made a billion years or so after the big bang. And in practice, the best measurements are made on nearby objects, corresponding to more than 13 billion years after the big bang. So you have to make allowances for any changes that could have occurred in that time span, due to incremental production or destruction of isotopes (in stars, for example).

The most problematic case has been with helium-3. Until the research just reported, there has been much less helium-3 detected than should be expected, because this isotope can be produced in low mass star like our sun. But the discrepancy can now be accounted for, since this helium-3 should be destroyed near the end of a stars life:

Near the end of a stars life there is a core flash and it was at around this time that the computer models revealed a small instability in the movement of the gases in the star. "When we looked at this in 3D we found this hydrodynamic instability caused mixing and destroyed the helium 3 so that none was released into space," Professor Lattanzio said.

Additional information:

Deep Mixing of ³He: Reconciling Big Bang and Stellar Nucleosynthesis – original research report in Science (subscription rqd for full access)

On the case of the "missing" helium – PhysicsWeb

Scientists crack open stellar evolution – Lawrence Livermore National Laboratory

Nucleosynthesis


Tags: astrophysics, big bang, nucleosynthesis

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