Special issue of General Relativity and Gravitation on dark energy

Speaking of astrophysical theories that still have many skeptics, how about dark energy? Whatever your opinion of the theory, here's a real treasure trove of information – a whole issue of the journal General Relativity and Gravitation, and all the articles are available for free. But for how long I don't know, so better go get it now:

General Relativity and Gravitation: Special issue on dark energy

From the introduction:

General Relativity and Gravitation has put together a special issue on “dark energy” in cosmology, because it is a major challenge to gravitational physics and actually to all of theoretical physics. We look at the observational side (the astrophysical data for dark energy and alternative explanations of that data), phenomenological models for dark energy and possible tests of these models, and the quantum gravity side (why do we expect a very large cosmological constant? what are the possible explanations of why it is small?). The idea is not to pursue one particular approach to this important problem, but rather to produce a survey of significant approaches that have been taken as regards each of its aspects.

Tags: dark energy, general relativity

Black holes exist

It may come as a surprise to lay readers to learn that even as recently as 10 years ago there were prominent physicists who still doubted the existence of black holes.

For example, there's Sir John Maddox, a trained chemist and physicist who was editor of Nature for 22 years. In a book published in 1998 (What Remains to be Discovered), he wrote, "The concept of black holes raises serious difficulties of a philosophical character." (p. 43) (Remember what C. F. Gauss said about philosophers.) And, "The habit of others in referring to black holes as "putative" seems to imply a collective uneasiness about the concept." (p. 112)

But recent skeptics of black holes are in good company. Einstein, for one, vigorously objected to the idea. Around 1935 Subrahmanyan Chandrasekhar, later a Nobel Prize winner and now celebrated, but only in his mid-20s at the time, had the audacity to argue that black holes might form from collapsed stars only 1.44 times as heavy as the Sun – and nearly had his career wrecked from Sir Arthur Eddington's sharp criticism.

Even very recently one still sees theoretical studies – such as reported here, here, here, here, here, here – that offer alternatives to the standard relativisic model of black holes.

Nevertheless, to the consternation of skeptics, the evidence for the correctness of the standard model of black holes just continues to pile up:

Quasar tests general relativity to the limit

[T]eam leader Mauri Valtonen of Tuorla Observatory in Finland claims the work provides the first hard evidence for black holes, which are so massive that space–time is predicted to completely curve in on itself: "People refer to the concept of black holes all the time, but strictly speaking one first has to prove that general relativity holds in strong gravitational fields before we can be sure that black holes exist," he told physicsworld.com.

And actually, the more significant part of that research is its validation of general relativity in very strong gravitational fields:

Astronomers have obtained the most compelling evidence yet that massive objects dramatically warp space–time, as predicted by Einstein's general theory of relativity. Although the geometric nature of gravity was first demonstrated in 1919, when Arthur Eddington famously detected the subtle warping effect of the Sun on the light from distant stars, the new results provide the first test of Einstein's theory in much stronger gravitational fields.

Tags: black holes, general relativity

General relativity passes cosmic test - Einstein's 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 Manchester's 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 Einstein's theory of general relativity - the theory of gravity that displaced Newton's - 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 Einstein's 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 - Einstein's theory holds in extreme gravitational fields.

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

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Tags: relativity, general relativity, pulsars, astrophysics

It's all relative

In the previous message we mentioned an interesting article on relativity: Relativity at the centenary, which pointed out that gravitational physics has become an experimental science. There's a lot going on with studies of black holes, the search for gravitational waves, detailed tests of special and general relativity. And on the theoretical side, there's a lot of activity in the (as yet still unsuccessful) quest for a theory of quantum gravity.

Of course, the theory of relativity is intimidating to a lot of people, but it doesn't need to be. Special relativity actually involves little more that basic physics (ideas like mass, velocity, force, energy) and high school algebra with rudimentary calculus. The mathematics needed for general relativity is somewhat more sophisticated. But the basic concept described by the math is pretty simple: concentrations of matter cause space to curve, and the motion of two (or more) massive objects that interact gravitationally can be understood as "straight lines" in the curved space.

Once you've read some overviews, like the Wikipedia articles just referred to, you will have the basic ideas needed to learn more about relativity. Fortunately, there's an excellent online reference with a large number of review articles that explain in more detail many of the most interesting topics in the contemporary theory of relativity. It's called Living Reviews in Relativity -- and it's all free.

The journal, which covers both theory and experiment, is now in its 9^th year. Some of the articles are valuable for understanding basic topics in cosmology. Here are some of the more generally accessible articles:

• Loop Quantum Gravity (1998)
  
• Stationary Black Holes: Uniqueness and Beyond (1998)
  
• The Cosmic Microwave Background (1998)
  
• Gravitational Wave Detection by Interferometry (2000)
  
• The Cosmological Constant (2001)
  
• Computational Cosmology: From the Early Universe to the Large Scale Structure (2001)
  
• The Thermodynamics of Black Holes (2001)
  
• Experimental Searches for Dark Matter (2002)
  
• Gravitational Waves from Gravitational Collapse (2003)
  
• Testing General Relativity with Pulsar Timing (2003)
  
• On the History of Unified Field Theories (2004)
  
• Quantum Gravity in Everyday Life: General Relativity as an Effective Field Theory (2004)
  
• Brane-World Gravity (2004)
  
• Measuring our Universe from Galaxy Redshift Surveys (2004)
  
• Quantum Gravity in 2+1 Dimensions: The Case of a Closed Universe (2005)
  
• Modern Tests of Lorentz Invariance (2005)
  
• Binary and Millisecond Pulsars (2005)
  
• Loop Quantum Cosmology (2005)
  
• The Confrontation between General Relativity and Experiment (2006)
  

Note that there have been updated versions of some articles. In each case, only the most recent update (as of this writing) and its corresponding date are listed.

Tags: relativity, special relativity, general relativity, black holes, quantum gravity, pulsars