* Astronomy

Blobrana · L

RE: Modified Newtonian dynamics (MOND)

Blobrana · L

MOND predicts dwarf galaxy feature prior to observations

A modified law of gravity correctly predicted, in advance of the observations, the velocity dispersion - the average speed of stars within a galaxy relative to each other - in 10 dwarf satellite galaxies of the Milky Way's giant neighbour Andromeda.
The relatively large velocity dispersions observed in these types of dwarf galaxies is usually attributed to dark matter. Yet predictions made using the alternative hypothesis Modified Newtonian Dynamics (MOND) succeeded in anticipating the observations.
Read more

Blobrana · L

Title: Hubble Constant, Lensing, and Time Delay in TeVeS
Authors: Yong Tian, Chung-Ming Ko, Mu-Chen Chiu

Hubble constant can be determined by the time delay of gravitational lensing systems. As data on time delay observation accumulates, it is time to revisit this approach. As in other dynamical phenomena in scales of galaxy and cluster of galaxies, gravitational lensing in these scales is also plagued by the problem of excess acceleration or gravity (a.k.a. missing mass problem). There are always some accelerations unaccounted for by luminous matter. Usually dark matter is introduced to interpret the discrepancy. However, MOdified Newtonian Dynamics (MOND) is more successful in explaining the excess accelerations in galaxy scale. We adopt TeVeS as the relativistic version of MOND to study gravitational lensing phenomena, and we can evaluate the Hubble constant from the derived time-delay formula. To apply our method, we rely on the CASTLE quasar lensing survey and the Sloan Digital Sky Survey (SDSS). Four samples are suitable for our study. Using only the luminous part of the lensing galaxies, the average of the derived Hubble constant is 68.5 km s^-1Mpc^-1

Read more (88kb, PDF)

Blobrana · L

Dark matter theory challenged by gassy galaxies result

A controversial theory that challenges the existence of dark matter has been buoyed by studies of gas-rich galaxies.
Instead of invoking dark matter, the Modified Newtonian Dynamics theory says that the effects of gravity change in places where its pull is very low.
Read more

Blobrana · L

Gas Rich Galaxies Confirm Prediction of Modified Gravity Theory

Recent data for gas rich galaxies precisely match predictions of a modified theory of gravity know as MOND according to a new analysis by University of Maryland Astronomy Professor Stacy McGaugh. This -- the latest of several successful MOND predictions -- raises new questions about accuracy of the reigning cosmological model of the universe, writes McGaugh in a paper to be published in March in Physical Review Letters.
Read more

Blobrana · L

Title: A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies
Authors: Stacy S. McGaugh

The current cosmological paradigm, LCDM, requires that the mass-energy of the universe be dominated by invisible components: dark matter and dark energy. An alternative to these dark components is that the law of gravity be modified on the relevant scales. A test of these ideas is provided by the Baryonic Tully-Fisher Relation (BTFR), an empirical relation between the observed mass of a galaxy and its rotation velocity. Here I report a test using gas rich galaxies for which both axes of the BTFR can be measured independently of the theories being tested and without the systematic uncertainty in stellar mass that affects the same test with star dominated spirals. The data fall precisely where predicted a priori by the modified Newtonian dynamics (MOND). The scatter in the BTFR is attributable entirely to observational uncertainty. This is consistent with the action of a single effective force law but poses a serious fine-tuning problem for LCDM.

Read more (33kb, PDF)

Blobrana · L

If, for some reason, you managed to make an egg stand on its end during the recent equinox, experimental physicists could do with your help. An exquisitely sensitive experiment that can only be performed during equinoxes could test some of the theories that offer an alternative to dark matter.
Stars on the outskirts of galaxies are moving much faster than can be explained by the gravity of visible matter. To account for the extra gravity, astronomers have proposed the existence of dark matter. In the 1980s, Mordechai Milgrom, then at Princeton University, suggested that the observations could also be explained by tweaking Newton's law of gravity. Milgrom's theory of modified Newtonian dynamics (MOND) has since evolved into two forms: gravitational MOND, which modifies the inverse-square law of gravity, and inertial MOND, which modifies Newton's second law of motion.
In both cases, when objects are moving with an acceleration above a certain threshold, a0, the well-known laws hold true. When the acceleration falls below a0, the laws are modified slightly to explain, for example, the speed of stars at the edges of galaxies. The value of a0 is about a billionth of the acceleration due to Earth's gravity, so the effects of MOND cannot ordinarily be seen on Earth. This means astronomers have had to rely on observations of galaxies and galaxy clusters to test MOND. Now, Alex Ignatiev, at the Theoretical Physics Research Institute in Melbourne, Australia, is proposing a way to test inertial MOND on Earth.

Source