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The cuspy halo problem arises from cosmological simulations that seem to indicate cold dark matter (CDM) would form cuspy distributions — that is, increasing sharply to a high value at a central point — in the most dense areas of the universe. This would imply that the center of the Milky Way, for example, should exhibit a higher dark-matter density than other areas. However, it seems rather that the centers of these galaxies likely have no cusp in the dark-matter distribution at all.
This remains an intractable problem. Speculation that the distribution of baryonic matter may somehow displace cold dark matter in the dense cores of spiral galaxies has not been substantiated by any plausible explanation or computer simulation.
Cuspy halo problem - Wikipedia
"Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy," explained Peñarrubia. "That makes it much more challenging to determine the distribution of dark matter."
Their data showed that in both cases, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across. This contradicts the prediction that the density of dark matter should increase sharply toward the centers of these galaxies.
"If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter 'pit' at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches," said Peñarrubia.
Dark matter mystery deepens
The dwarf galaxy problem, also known as the missing satellites problem, arises from numerical cosmological simulations that predict the evolution of the distribution of matter in the universe. Dark matter seems to cluster hierarchically and in ever increasing number counts for smaller-and-smaller-sized halos. However, although there seems to be enough observed normal-sized galaxies to account for this distribution, the number of dwarf galaxies is orders of magnitude lower than expected from simulation.  For comparison, there were observed to be around 38 dwarf galaxies in the Local Group, and only around 11 orbiting the Milky Way, yet one dark matter simulation predicted around 500 Milky Way dwarf satellites.
Dwarf galaxy problem - Wikipedia
The standard model, also called the "lambda cold dark matter model," says that satellite dwarf galaxies in the Milky Way and Andromeda are expected to behave a certain way: The galaxies would form in halos of dark matter, be widely distributed and would have to move in random directions, said Marcel Pawlowski, a postdoctoral researcher in the astronomy department at Case Western Reserve University and lead author of the new study.
"But what astronomers see is different," Pawlowski said. "We see the satellite galaxies are in a huge disk and moving in the same direction within this disk, like the planets in our solar system moving in a thin plane in one direction around the sun. That's unexpected and could be a real problem."
Dwarf galaxies don't fit standard model
A team using the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, along with other telescopes, has mapped the motions of more than 400 stars up to 13 000 light-years from the Sun. From this new data they have calculated the mass of material in the vicinity of the Sun, in a volume four times larger than ever considered before.
“The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun,” says team leader Christian Moni Bidin (Departamento de Astronomía, Universidad de Concepción, Chile). “But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!”
Serious Blow to Dark Matter Theories?
The team, led by C. Moni Bidin used ~300 red giant stars in the Milky Way‘s thick disk to map the mass distribution of the region. To eliminate any contamination from the thin disc component, the team limited their selections to stars over 2 kiloparsecs from the galactic midplane and velocities characteristic of such stars to avoid contamination from halo stars. Once stars were selected, the team analyzed the overall velocity of the stars as a function of distance from the galactic center which would give an understanding of the mass interior to their orbits.
Using estimations on the mass from the visible stars and the interstellar medium, the team compared this visible mass to the solution for mass from the observations of the kinematics to search for a discrepancy indicative of dark matter. When the comparison was made, the team discovered that, “[t]he agreement between the visible mass and our dynamical solution is striking, and there is no need to invoke any dark component.”
Missing Milky Way dark matter
Dr. Benoit Famaey (Universities of Bonn and Strasbourg) explains: "The dark matter seems to 'know' how the visible matter is distributed. They seem to conspire with each other such that the gravity of the visible matter at the characteristic radius of the dark halo is always the same. This is extremely surprising since one would rather expect the balance between visible and dark matter to strongly depend on the individual history of each galaxy."
Dr. Zhao at the SUPA Centre of Gravity notes, "The pattern that the data reveal is extremely odd. It's like finding a zoo of animals of all ages and sizes miraculously having identical, say, weight in their backbones or something. It is possible that a non-gravitational fifth force is ruling the dark matter with an invisible hand, leaving the same fingerprints on all galaxies, irrespective of their ages, shapes and sizes."
Is Unknown Force In Universe Acting On Dark Matter?
The new, relatively high-resolution WSRT measurements suggest that VIRGOHI21 is indeed a single object, ruling out previous suggestions that its rotation was an illusion caused by two passing gas clouds.
But they do confirm a mystery raised by previous studies. The object's normal matter weighs a few hundred million times the mass of the Sun. But its dark matter - inferred by studying the rotation speed of the cloud - appears to weigh at least 100 times as much.
That ratio is much higher than expected - in all other galaxies, dark matter outweighs normal matter by a factor of only 10. "Even if this is a dark galaxy, it is not what you expect to find. The number of baryons is too low," says Michael Merrifield of the University of Nottingham in the UK, who was not on Minchin's team.
'Dark galaxy' continues to puzzle astronomers
originally posted by: intrptr
So the "Missing Matter" is associated with Galaxies right? Centers of gravity are centers around something, I think they haven't a clue how massive the singularities at the center of most galaxies truly are. We can't even observe them directly, how can they tell how much mass is in there?
Wouldn't it then be logical to assume that perhaps this dark matter and gravity are one and the same...a graviton perhaps...or as in dark matter is the "particle" and gravity the energetic effect of the particle as it relates to other particles both dark and visible...?
originally posted by: Aleister
Possibly galaxies are born with such a massive rotating momentum that it will take billions of more years before the gravitational effect of the center mass can grab the outer areas enough to cause them to slow. How many times in its lifetime has our own galaxy made a round-trip?
Even if the galactic cores were much larger than we think they are it still wouldn't explain the flat rotation curve, it would still have to drop off with distance from the core.
originally posted by: ChaoticOrder
a reply to: Mastronaut
I assume you haven't read the first part of this thread yet. There are many good reasons we believe that some type of invisible mass is surrounding most or all galaxies. The MOND theory simply cannot explain the range of observations which support the existence of this invisible mass. The question is not whether dark matter exists, the question is what exactly is dark matter, is it some type of particle or something even stranger.
If convenience is the only reason to believe a theory then inflation MUST have existed despite is the most controversial part of the BB theory.
We know GR can't explain everything, and we don't have a working GUT theory, but we can't accept that they aren't "right" or we would taint the history of physics.
MOND is just a general therm to define theories that modify Newton's laws, many of them explain galaxy rotation curves quite well, but where is it stated that the same theory must also account for gravitational lensing?
Also, is this lensing really "gravitational" given that we can't see any matter where we find these distortions?
But how much research time has been devoted to DM and how much to MOND-like theories?
originally posted by: intrptr
a reply to: ChaoticOrder
"As above so below"? The rings of Saturn come to mind… how do we account for their flat, well defined plane at the equator?
The outer part of the rings rotate slower than the inner parts, which is what we normally expect to see. The same thing happens with the moons of Jupiter, or even our entire solar system (outer planets orbit more slowly).
The strange thing about the galaxy is that is doesn't follow that rule, the outer parts basically orbit at the same velocity as the inner parts.
A figure skater spins up the closer in she pulls her arms… so I expect that to be similar.
"Basically"… our one galaxy? I'd read any link you have making that claim, if you would be so kind.
A general feature of the galaxy rotation curves that have been obtained through measurement to date is that the orbital speed of stars and gas is rising or almost constant as far from the galactic centre as it can be measured: that is, stars are observed to revolve around the centre of the galaxy at increasing or the same speed over a large range of distances from the centre of the galaxy. If disc galaxies have mass distributions similar to the observed distributions of stars and gas then, the orbital speed would always decline at increasing distances in the same way as do other systems with most of their mass in the centre, such as the Solar System or the moons of Jupiter.
Galaxy rotation curve - Wikipedia