Astronews a cura di Massimiliano Razzano A Case of MOND Over Dark Matter According to Newton'sSecond Law of Dynamics,they have lower velocities than objects near the center.But observations confirm that galaxies rotatewitha uniform velocity. Some astronomers believe theorbital behavior of galaxies can be explained moreaccurately with Modified Newtonian Dynamics (MOND)a modified version of Newton's Second Law - than bythe rival, but more widely accepted, theory of darkmatter.The dark matter theory assumes that a haloof dark matter surrounds each galaxy, providing enoughmatter (and gravity) that all the stars in a galaxy disc orbit with the same velocity. MOND, however uses adifferent explanation, and a recent study of eight dwarfgalaxies that orbit the Milky Way seems to favor theMOND approach over the dark matter theory."MOND was first suggested to account for things that wesee in the distant universe," said Garry Angus, of the University of St Andrews. "This is the first detailed studyin which we've been able to test out the theory on something close to home. The MOND calculations and the observationsappear to agree amazingly well."Usually the equation F=ma (force = mass X acceleration)solves your basic acceleration problems. But it doesn'texplain the observed rotation of galaxies. MOND suggeststhat at low values of acceleration, the acceleration of aparticle is not linearly proportional to the force. Accordingto Angus, MOND adds a new constant of nature (a0) tophysics, besides the speed of light and Planck's constant.Above the constant, accelerations are exactly as predictedby Newton's second law (F=ma). Below it, gravity decayswith distance from a mass, rather than distance squared.This constant is so small that it goes unnoticed with thelarge accelerations that we experience in everyday life.For instance, when we drop a ball the gravity is 100 billiontimes stronger than a0 and the accelerated motion of theEarth round the Sun is 50 million times stronger.However, when objects are accelerating extremely slowly,as we observe in galaxies or clusters of galaxies, then theconstant makes a significant difference to the resultinggravitational forces.When MOND is applied to nearby dwarf galaxies, oneeffect is that tidal forces from the Milky Way, which havea negligible effect in classical Newtonian Mechanics, canactually make a big difference. This is particularly significantfor the dwarfs orbiting close to our Galaxy."In these dwarf galaxies, the internal gravity is very weak.Compared to the gravity of the Milky Way," said Angus,"MOND suggests that the Milky Way is a bit like a bankthat loans out gravity to nearby dwarf galaxies to makethem more stable. However, there are conditions on theloan: if the dwarf galaxies start to approach the bank, theloan is gradually reduced or even cancelled and the dwarfsmust pay it back. In two galaxies, we've seen what couldbe signs that they've come too close too quickly and areunable to repay the loan fast enough. This appears to havecaused disruption to their equilibrium."Angus used MOND to calculate the ratio of mass to amountof light emitted by the stars in the dwarf galaxies from theobserved random velocities of the stars collected independently.He also calculated the orbital paths of the stars in the dwarfgalaxies. In all eight cases, the MOND calculations for theorbits were within predictions. For six of the eight galaxies,the calculations were also a good match to expected valuesfor mass-to-light ratios; however for two galaxies, Sextansand Draco, the ratios were very high, which could well suggesttidal effects. The value for Sextans could also be due to poorquality measurements of the galaxy's luminosity, which Angussaid are improving all the time for these ultra dim objects."These tidal effects can be tested by updating the 13 year oldluminosity of Sextans and making accurate observations of theorbits of Draco and Sextans around the Milky Way. We alsoneed to carry out some detailed simulations to understand theexact mechanisms of the tidal heating," said Angus.If Newton's gravity holds true, the dark matter needed in thedwarf galaxies has constant density in the center which iscontrary to theoretical predictions, which suggest densityshould rise to the center."Even without direct detection, the dark matter theory isdifficult to prove or refute and although we may not be ableto prove whether MOND is correct, by carrying out thesekind of tests we can see if it continues to hold up or if it isdefinitely ruled out," said Angus.Original News Source: Royal Astronomy Society's NationalAstronomy Meeting
Dalle galassie.....
Astronews a cura di Massimiliano Razzano A Case of MOND Over Dark Matter According to Newton'sSecond Law of Dynamics,they have lower velocities than objects near the center.But observations confirm that galaxies rotatewitha uniform velocity. Some astronomers believe theorbital behavior of galaxies can be explained moreaccurately with Modified Newtonian Dynamics (MOND)a modified version of Newton's Second Law - than bythe rival, but more widely accepted, theory of darkmatter.The dark matter theory assumes that a haloof dark matter surrounds each galaxy, providing enoughmatter (and gravity) that all the stars in a galaxy disc orbit with the same velocity. MOND, however uses adifferent explanation, and a recent study of eight dwarfgalaxies that orbit the Milky Way seems to favor theMOND approach over the dark matter theory."MOND was first suggested to account for things that wesee in the distant universe," said Garry Angus, of the University of St Andrews. "This is the first detailed studyin which we've been able to test out the theory on something close to home. The MOND calculations and the observationsappear to agree amazingly well."Usually the equation F=ma (force = mass X acceleration)solves your basic acceleration problems. But it doesn'texplain the observed rotation of galaxies. MOND suggeststhat at low values of acceleration, the acceleration of aparticle is not linearly proportional to the force. Accordingto Angus, MOND adds a new constant of nature (a0) tophysics, besides the speed of light and Planck's constant.Above the constant, accelerations are exactly as predictedby Newton's second law (F=ma). Below it, gravity decayswith distance from a mass, rather than distance squared.This constant is so small that it goes unnoticed with thelarge accelerations that we experience in everyday life.For instance, when we drop a ball the gravity is 100 billiontimes stronger than a0 and the accelerated motion of theEarth round the Sun is 50 million times stronger.However, when objects are accelerating extremely slowly,as we observe in galaxies or clusters of galaxies, then theconstant makes a significant difference to the resultinggravitational forces.When MOND is applied to nearby dwarf galaxies, oneeffect is that tidal forces from the Milky Way, which havea negligible effect in classical Newtonian Mechanics, canactually make a big difference. This is particularly significantfor the dwarfs orbiting close to our Galaxy."In these dwarf galaxies, the internal gravity is very weak.Compared to the gravity of the Milky Way," said Angus,"MOND suggests that the Milky Way is a bit like a bankthat loans out gravity to nearby dwarf galaxies to makethem more stable. However, there are conditions on theloan: if the dwarf galaxies start to approach the bank, theloan is gradually reduced or even cancelled and the dwarfsmust pay it back. In two galaxies, we've seen what couldbe signs that they've come too close too quickly and areunable to repay the loan fast enough. This appears to havecaused disruption to their equilibrium."Angus used MOND to calculate the ratio of mass to amountof light emitted by the stars in the dwarf galaxies from theobserved random velocities of the stars collected independently.He also calculated the orbital paths of the stars in the dwarfgalaxies. In all eight cases, the MOND calculations for theorbits were within predictions. For six of the eight galaxies,the calculations were also a good match to expected valuesfor mass-to-light ratios; however for two galaxies, Sextansand Draco, the ratios were very high, which could well suggesttidal effects. The value for Sextans could also be due to poorquality measurements of the galaxy's luminosity, which Angussaid are improving all the time for these ultra dim objects."These tidal effects can be tested by updating the 13 year oldluminosity of Sextans and making accurate observations of theorbits of Draco and Sextans around the Milky Way. We alsoneed to carry out some detailed simulations to understand theexact mechanisms of the tidal heating," said Angus.If Newton's gravity holds true, the dark matter needed in thedwarf galaxies has constant density in the center which iscontrary to theoretical predictions, which suggest densityshould rise to the center."Even without direct detection, the dark matter theory isdifficult to prove or refute and although we may not be ableto prove whether MOND is correct, by carrying out thesekind of tests we can see if it continues to hold up or if it isdefinitely ruled out," said Angus.Original News Source: Royal Astronomy Society's NationalAstronomy Meeting