Stars say relativity still works
Einstein defibrillated, CERN silicon questioned
The Special Theory of Relativity may be under re-evaluation following CERN’s astonishing neutrino observations, but over in the world of astronomy, general relativity has had another reconfirmation from the Neils Bohr Institute at the University of Copenhagen.
Radek Wojtak, Steen Hansen and Jens Hjorth have published in Nature the results of observations of distant galactic clusters. The 8,000 distant clusters sampled in the experiment have such immense gravity that general relativity predicts it will affect light emerging from them.
In particular, the gravity red-shifts the light emerging from the cluster. Since the speed of light is fixed (at least for light; neutrinos might be another matter), gravity can’t slow it down, so the impact of very high gravity on light is to soak up some of its energy, moving it towards the red end of the visible light spectrum.
In their observations, the three researchers compared the amount of red shift experienced by light emerging from the centre of the galactic clusters to that observed for light emitted from near the edge of the cluster.
"We could measure small differences in the redshift of the galaxies and see that the light from galaxies in the middle of a cluster had to 'crawl' out through the gravitational field, while it was easier for the light from the outlying galaxies to emerge", said Radek Wojtak.
While this is “only” a reconfirmation of what we already knew about light, the new research is believed to be the first time the theory has been applied on such a large scale. With a measurement of the cluster’s total mass (El Reg would like to know how that’s accomplished, commenters), the researchers were able to predict the difference in red shift between the centre and the periphery, and say that their observations were “in complete agreement” with the prediction.
In addition, they say such research can help increase our knowledge of the “dark universe”, since Wojtak said the result is “a strong indication for the presence of dark energy”.
Meanwhile, here’s an interesting discussion of the CERN results: could they have been affected by the behavior of the field-programmable gate arrays (FPGAs) used in CERN’s equipment?
High-performance computing enthusiast Marc Bevand speculates that the neutrino observations may have affected by FPGAs in the data acquisition system.
The Register isn’t prepared to make a call one way or the other. It is, however, an interesting discussion and underlines the importance of CERN’s decision to release its data for all and sundry to analyse. ®
Mea Culpa Thanks to the readers that pointed out my error. I gave a distance when I should have referred to the number of galaxies sampled. This is now corrected.
Calculating cluster weight
The weight of the individual Galaxies in the cluster is known as it's printed on the side of each bar.
The Virial Theorem
Initial estimates of galactic mass are based on the Virial Theorem.
Assuming that the only force in play is gravity and assuming that stellar velocities have more or less evened out ('virialised') from the galaxy's initial formation, then the Virial Theorem shows that the total kinetic energy is equal to half the potential energy. Total kinetic energy is proportional to total mass. Total potential energy (in a given configuration) varies as square of mass. So if the relative motion of the stars about the galaxy centre of mass is known - and information about their velocities is available from spectroscopic observations - by making reasonable assumptions about their distribution within the galaxy it is possible to estimate the total mass.
Unfortunately, observations of the variation of stellar velocities with distance from the centre are not in accord with this simple model. The prediction is that outside the main concentration of stars, velocities should fall off with the inverse square of distance. In fact, velocities deduced from spectographic observations appear pretty much constant to a considerable distance from the centre. Equally puzzling is the observations that stellar (linear) velocities are in the range of 150 to 350 km/S irrespective of large differences in the size of the galaxy they inhabit.
It is the discrepancy between the 'Keplerian model' (stars in a galaxy behaving in a similar way to planets around a star under the influence of inverse square law gravity only) and the observation of more or less constant velocity which gave rise to the idea of dark matter. This dark matter is assumed, by some miracle or other, to sit in the shape of a halo which is in precisely ihe right place to cause the observed pattern of stellar velocities.
A current rationalisation of the dilemma which some cosmologists favour comes from MOND - Modified Newtonian Dynamics. Here it is assumed that the inverse square law does not hold at very large distances.
For reasons which largely escape me, the alternative explanation proposed half a century ago by Hannes Alfvèn that electromagnetic processes are in play is generally treated as heresy.
Reply to Full Mental Jacket
Reddening due to absorption by galactic dust is not the same as redshift. Reddening is a filtering effect - blue light is more likely to scatter, red light less so. More of the original red light reaches us than of the original blue light. In this case, information is lost because light of specific wavelengths never reaches us.
But redshift changes the colour of the light, by lengthening its wavelength. The original information is not lost, merely shifted down the spectrum.