The rotation curve of a galaxy indicates the orbital velocity of material depending on the distance from the center of the galaxy. In the 1970s, astronomer Vera Rubin measured the rotation curves of several of the nearest spiral galaxies . To his amazement, the curves were flat over the edge of the stellar disk of the galaxy [see here for more information.] Under the standard theory of gravity implies that the mass in the outskirts of these systems is much greater than that of gas and stars detected in them. Today the flatness of rotation curves of spiral galaxies remains key evidence dark matter, which in the broadest sense is a matter that obeys the law of gravity, but does not emit any light.
Dark matter is an integral component of the current most favored cosmological model, in which 90% of matter in the universe is composed of massive particles ("cold"), which interact only through gravity. The model is called Lambda-Cold Dark Matter (ΛCDM ), with Λ denoting cosmological constant (sometimes called "dark energy ") that accelerates the expansion in recent cosmological times. An impressive set of observations are consistent with this picture, since the spectrum fluctuations in the cosmic microwave background (the first issue of free photons in the Universe, for example, see [ here ]), the luminosity of the distant supernovae [more here and here ]. The community of astrophysicists has adopted ΛMDL as the standard cosmological model, which makes the nature of matter Dark is one of the most important outstanding issues in the field of astrophysics.
But not all cosmologists are happy with this image. Some are uncomfortable with the unknown components in the standard model, and they worry that the cosmology in the last 30 years has been downgraded to "name unknown" (eg, [see here ], but see also [ here]). In spiral galaxies, at least, an alternative to invoking dark matter is to modify the theory of gravity in regions where there is a discrepancy between the mass inferred from the dynamics (using gravity normal) and baryonic mass (ie, the census of normal material found in stars and gas). The most successful of these ideas is MOND (Modified Newtonian Dynamics [see here ]), which modifies gravity at small accelerations to produce flat galactic rotation curves. For gravitational acceleration ( a) less than a critical value to 0 , the usual relationship of Newton F = m to is modified F = m to 2 / to 0 and therefore the circular speed around a mass M asymptotically approaches a constant value v = f ( G M to 0 ) 1 / 4 . Once the universal constant to 0 fixed observationally, MOND explains the way the rotation curve of galaxies with baryonic mass alone, while the correlation is observed between their rates of rotation and luminosity.
By construction, MOND requires the baryonic mass of a galaxy is proportional to the fourth power of the circular speed is M b α v f 4 . Independent verification of this relationship is impractical in typical spiral galaxies in which stars dominate the baryonic mass, it is difficult to reliably infer total stellar mass M * . Although most of the light in a galaxy comes from massive stars, most of the stellar mass in the galaxy lies in the faint, low mass stars. To estimate the stellar mass of a galaxy from the amount of light emitted, we need a model that combines the well-understood stellar physics with estimates of the distribution of mass and age of stars in the galaxy. These models exist (for example, [see here ]), but suffer from significant systematic uncertainties that can be correlated with the mass. To avoid these uncertainties, one is forced to rely on to measure MOND M * from rotation curves of galaxies, which imposes the relation M b α v f 4 to the data. At this stage the relationship is not a prediction of MOND, but rather a consequence of the imposition of MOND for M b from the beginning.
composite image of spiral galaxy IC gas-rich dwarf 2574.El atomic gas is represented in blue, orange stars are old and young stars are shown in púrpura.La horizontal green bar shows a scale physical than 15000 light years this corresponds approximately half of the distance between the sun and the center of the Milky Láctea.IC 2574 not part of the study McGaugh but it illustrates how the baryonic mass of some galaxies is completely dominated by hydrogen atómico.Credito: Courtesy of the Things survey team.
Writing in Physical Review Letters [see here ] Stacy McGaugh at the University of Maryland, USA, suggests that for a class of galaxies with stellar masses are compensated by their masses atomic gas (eg, see image), the relationship M b α v f 4 can be used to test the validity of MOND. Unlike the stellar masses, the masses of atomic gas are easily measured using the line "21 cm", an emission line at a rest wavelength of 21 cm (observable with radio telescopes), which is produced by the hyperfine transition spin-flip of the ground state atomic hydrogen. As the atomic hydrogen gas the main contributor to M b in these galaxies, the systematic uncertainties in the stellar masses irrelevant. M b and v f can therefore be measured independently of any cosmological theory or gravity and used to test MOND. McGaugh
literature collected in a sample of 47 gas-rich galaxies, for which recent observations of the spectral line of 21 cm provide reliable estimates of both the atomic gas mass (which is combined with models of stellar population masses produce M b) and asymptotically flat rotation speed vf. These data show a striking similarity to the prediction of MOND of M b α v f 4 , and also agrees with the acceleration parameter to 0 needed to bring the curves rotation of galaxies dominated by stars. McGaugh, also states that the data have no dispersion on the prediction of MOND beyond measurement errors. This statement seems to be premature, due to incomplete statistics, large distance uncertainties (many of the masses of galaxies are based on estimated distances only), and other realities coverage do not seem to be taken into account. This introduces biases in the observed scale and has been shown to reduce the observed dispersion (eg, see [ here]). These biases will most likely not significantly change the observed correlation, but make sure the exact details, in particular the interpretation of dispersion. The relationship
M b α v f 4 can be interpreted by the model Λ CDM , only require a particular level, among the masses subject baryonic dark of a galaxy (in particular that the detectable fraction of baryons is proportional to the speed of rotation). There is a well known astrophysical processes such as energy feedback from star formation that produces the correct qualitative trend, but it takes some adjustment to match the details. I find that MOND explains the observed relationship is a new triumph for the model in the context of these gas-rich galaxies. McGaugh
results add a new facet to the argument that it is better to explain MOND galaxies standard cosmology. However, as admitted McGaugh, MOND can not compete with Λ CDM as a complete cosmological theory. MOND attempts to generalize in a relativistic theory of gravity fully abound, but even more promising as ( Teves or tensor-vector-scalar [see here ]) struggles to interpret the combination of observations scale model of the universe that Λ CDM explained very well. The adjustment required in MOND, especially to explain the dynamics of galaxy clusters, is more severe than that suffered by the Λ model CDM to match the rotation curves of galaxies. We know that standard but poorly baryonic physics understood, plays an important role in shaping the properties of galaxies in the CDM Λ . The reconciliation of MOND with clusters of galaxies, by contrast, requires the invocation of significant amounts of missing matter, which MOND was designed to avoid, in the first place.
McGaugh The proposed test is a critical MOND, and applying it to the gas-rich galaxies observed recently is an important proof of concept. An ideal sample for this test should soon be available from studies with the next generation of radio telescopes that will produce large catalogs spatially resolved gas-rich galaxies. If MOND will or not other tests in the future as McGaugh found in this work will, it is clear that any viable cosmological scenario is complete unless you can explain the remarkable success of MOND phenomenology in spiral galaxies.
more information
here, here and here
source of information:
http://physics.aps.org/articles / v4/23
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