Skip to comments.Do dwarf galaxies favour MOND over dark matter?
Posted on 04/03/2008 8:16:25 PM PDT by LibWhacker
A detailed analysis of eight dwarf galaxies that orbit the Milky Way indicates that their orbital behaviour can be explained more accurately with Modified Newtonian Dynamics (MOND) than by the rival, but more widely accepted, theory of dark matter. The results were presented by Garry Angus, of the University of St Andrews, at the RAS National Astronomy Meeting in Belfast on Wednesday 2nd April.
'MOND was first suggested to account for things that we see in the distant universe. 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. We've also found some exciting tidal effects predicted by MOND that we should be able to test through future observations and simulations,' said Angus.
Essentially, MOND adds a new constant of nature (a0) to physics, besides the speed of light, Planck's constant among others. Above it, accelerations are exactly as predicted by Newton's second law, which says that a force equals an object's mass times its acceleration. 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 day-to-day 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 the population of 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 our Galaxy most closely.
'In these dwarf galaxies, the internal gravity is very weak compared to the gravity of the Milky Way. 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,' said Angus.
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 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 dwarfs has constant density in the centre which is contrary to theoretical predictions, which suggest density should rise to the centre.
'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.
Source: Royal Astronomical Society
I heard somewhere yesterday that “micro galaxies” are in the process of colliding with the Milky Way. (That ought to do wonders for property values!)
Anyway, I wonder if the dwarf galaxies referred to in this article are the same galaxies that are bumpin’ stars with the Milky Way.
Yes, because dark matter is the 20th century’s luminiferous ether
what do the polls say?
So comparing weight, if I were to throw a baseball 90mph, I could throw a red giant at around the speed of light?
I shall reserve my opinion until a scientific consensus is reached. ;-)
Geez - next they’ll tell us we’re living in only four dimensions instead of the 2,376 or whatever it is they’ve been theorizing about - score one for the good old law of parsimony.......
The latest Smithsonian has an excellent writeup on the evolving proof that at the hear of every Galaxy is a black hole, and the methods being used to prove that beyond a doubt.
Considering that the earth is "dark matter" I disagree.
15 years ago extrasolar planets were solely the province of science fiction. Now we have detected over 200, by three separate techniques.
The only reason we consider "light matter" -- stars -- normal is that we have been seeing them since the beginning of man.
I am willing to believe there are objects in the universe we know nothing about, and which could be fairly common, just dark.
This sounds a bit like the hypothesis of continuous creation.
If you can throw a baseball 90 mph why aren’t you in uniform and helping the Rangers out?
Heresey! Dark Matter is the CONSENSUS! Why even look at data that could contradict it? The science is SETTLED! And it’s all GW Bush’s and Big Oil’s fault! We need new laws to ban Dark Matter — it’s DANGEROUS! We need new taxes on evil companies who are responsible for Dark Matter, and new government programs to study it, as well as new government programs to ease the impact of Dark Matter on the disadvantaged. What, you oppose this? Then you’re a flat earther! You should be JAILED! Dang capitalism and conservatives, it’s ALL YOUR FAULT!
Only Hope(tm) and Change(tm) can save us!!!
"Dark matter" refers to a fundamentally different type of matter. OK explanation here: http://en.wikipedia.org/wiki/Dark_matter
Even your reference agrees that dark matter includes planets. I stand by my assertion that the earth is "dark matter".
As I see it, cosmologists, and I have known more than a few, have no really good idea how many "dwarf stars", or "rogue planets" really exist. If all it takes to account for observed effects is postulating many more ordinary objects, which are known and fit within our current understanding of the universe, that is a more reasonable explanation than postulating enormous amounts of exotic new material which has not bene observed anywhere.
I live in CT.
But I can’t throw that fast anyway. If I could, it wouldn’t be very accurate.
what about dark-matter credits?