blogtecaolivelli

Dalle galassie.....

Astronews a cura di Massimiliano Razzano

 A Case of MOND Over Dark Matter According to Newton's

Second Law of Dynamics,

they have lower velocities than objects near the center.

But observations confirm that galaxies rotate

witha uniform velocity. Some astronomers believe the

orbital behavior of galaxies can be explained more

accurately with Modified Newtonian Dynamics (MOND)

a modified version of Newton's Second Law - than by

the rival, but more widely accepted, theory of darkmatter.

The dark matter theory assumes that a halo

of dark matter surrounds each galaxy, providing enough

matter (and gravity) that all the stars in a galaxy disc 

orbit with the same velocity. MOND, however uses a

different explanation, and a recent study of eight dwarf

galaxies that orbit the Milky Way seems to favor the

MOND approach over the dark matter theory.

"MOND was first suggested to account for things that we

see in the distant universe," said Garry Angus, of the

 University of St Andrews. "This is the first detailed study

in which we've been able to test out the theory on something

 close to home. The MOND calculations and the observations

appear to agree amazingly well."

Usually the equation F=ma (force = mass X acceleration)

solves your basic acceleration problems. But it doesn't

explain the observed rotation of galaxies. MOND suggests

that at low values of acceleration, the acceleration of a

particle is not linearly proportional to the force. According

to Angus, MOND adds a new constant of nature (a0) to

physics, besides the speed of light and Planck's constant.

Above the constant, accelerations are exactly as predicted

by Newton's second law (F=ma). Below it, gravity decays

with distance from a mass, rather than distance squared.

This constant is so small that it goes unnoticed with the

large accelerations that we experience in everyday life.

For instance, when we drop a ball the gravity is 100 billion

times stronger than a0 and the accelerated motion of the

Earth 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 the

constant makes a significant difference to the resulting

gravitational forces.

When MOND is applied to nearby dwarf galaxies, one

effect is that tidal forces from the Milky Way, which have

a negligible effect in classical Newtonian Mechanics, can

actually make a big difference. This is particularly significant

for 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 bank

that loans out gravity to nearby dwarf galaxies to make

them more stable. However, there are conditions on the

loan: if the dwarf galaxies start to approach the bank, the

loan is gradually reduced or even cancelled and the dwarfs

must pay it back. In two galaxies, we've seen what could

be signs that they've come too close too quickly and are

unable to repay the loan fast enough. This appears to have

caused disruption to their equilibrium."

Angus used MOND to calculate the ratio of mass to amount

of light emitted by the stars in the dwarf galaxies from the

observed random velocities of the stars collected independently.

He also calculated the orbital paths of the stars in the dwarf

galaxies. In all eight cases, the MOND calculations for the

orbits were within predictions. For six of the eight galaxies,

the calculations were also a good match to expected values

for mass-to-light ratios; however for two galaxies, Sextans

and Draco, the ratios were very high, which could well suggest

tidal effects. The value for Sextans could also be due to poor

quality measurements of the galaxy's luminosity, which Angus

said are improving all the time for these ultra dim objects.

"These tidal effects can be tested by updating the 13 year old

luminosity of Sextans and making accurate observations of the

orbits of Draco and Sextans around the Milky Way. We also

need to carry out some detailed simulations to understand the

exact mechanisms of the tidal heating," said Angus.

If Newton's gravity holds true, the dark matter needed in the

dwarf galaxies has constant density in the center which is

contrary to theoretical predictions, which suggest density

should rise to the center.

"Even without direct detection, the dark matter theory is

difficult to prove or refute and although we may not be able

to prove whether MOND is correct, by carrying out these

kind of tests we can see if it continues to hold up or if it is

definitely ruled out," said Angus.

Original News Source: Royal Astronomy Society's National

Astronomy Meeting