MOND vs Darkness

[CraigS]

Following on from the recent thread about Hershel finding less dark matter but more stars, we have this one …

Gas rich galaxies confirm prediction of modified gravity theory

Modern cosmology says that for the universe to behave as it does, the mass-energy of the universe must be dominated by dark matter and dark energy. However, direct evidence for the existence of these invisible components remains lacking. An alternate, though unpopular, possibility is that the current theory of gravity does not suffice to describe the dynamics of cosmic systems.

.. so, enter Modified Newtonian Dynamics (MOND);

Using recently published work that he and other scientists had done to determine both the mass and flat rotation velocity of many gas rich galaxies, McGaugh compiled a sample of 47 of these and compared each galaxy's mass AND rotation velocity with the relationship expected by MOND. All 47 galaxies fell on or very close to the MOND prediction. No dark matter model performed as well.
…
Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter - dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.

"MOND is just the opposite," he said. "It accounts well for the 'small' scale of individual galaxies, but MOND doesn't tell you much about the larger universe.
..
Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter - dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.
..
"If we're right about dark matter, why does MOND work at all?" asks McGaugh. "Ultimately, the correct theory - be it dark matter or a modification of gravity - needs to explain this."

As he says, sounds a bit like playing around with models to fit the 'sweet spot' of observations. His last question (underlined) is a good one, though .. but there again, Newtonian Gravitation has always had sweet spots anyway.

The 'Darkness' remains for the moment, but MOND may be a possibility for ultimately superseding it.

Cheers

[renormalised (Carl)]

Too early in the morning to discuss this!!!

Both hypotheses have a long way to go before anything definitive can be said, either way.

(Notice I didn't call them theories)

[CraigS]

Originally Posted by renormalised

Too early in the morning to discuss this!!!

Both hypotheses have a long way to go before anything definitive can be said, either way.

(Notice I didn't call them theories)

Hmm .. (I've had a few coffees now) ..

I think its fair to say that DM is a hypothesis supported by indirect observational evidence (a bit like the Oort Cloud) and it may also be fair to say, it is also inferred from other theoretical models and predictions.

But MOND is a direct variant of Newton's Law of Universal Gravitation. Newtonian gravitation is an approximation for GR Theory, and GR Theory, will/has been used in the establishment of a Unified Field Theory.

Law=> GR Theory => Grand Unified Theory. (I guess 'the apple', was an initial observation upon which an hypothesis was formed leading ultimately, to a Law .. it may also have been a myth )

What these guys are claiming is that MOND Theory has now predicted the mass and rotation velocities of a sample of 47 galaxies and found a good 'fit' or 'agreement' between the observations and the predictions. But DM cosmology works well on the scales of galaxy cluster and up - which is where the proportion of DM in the obs. universe 'swamps' ordinary matter.
So long as this remains the case, DM is here to stay, (firmly, as a hypothesis).. yet again.

Comments welcome.

Cheers

[marki]

To me MOND sounds far more plausible then DM but I guess we will just have to wait and see. We probably cant see the forrest for the trees on this one me thinks.

Mark

[CraigS]

So, a week has gone by since the original MOND article was published and now, we have the counter arguments popping up, which basically insist on the reality of dark matter.

The best is probably from Sean Carroll's blogsite. (He's a real life cosmologist). He shows why the MOND idea doesn't work and all paths still lead back to the need for dark matter:

MOND is ugly:
"It’s an ungodly concatenation of random fields interacting in highly-specific but seemingly arbitrary ways".

MOND doesn’t fit clusters:
It works for some galaxies but its not even close for clusters.

Even with MOND, you still need dark matter:
Even MOND’s biggest supporters admit that you need dark matter to explain galaxies.

MOND doesn’t even fit all galaxies:
MOND fails for dwarf spheroids.

Gravity doesn’t always point in the direction of where the ordinary matter is:
See the Bullet cluster - "gravitational lensing is absolutely unambiguous: to fit the data, you need to do better than just modifying the strength of Newtonian gravity".

MOND doesn’t fit the cosmic microwave background:
"In English: dark matter pushes up the first and third peak in the <CMBR Power Spectrum>, while suppressing the second and fourth peak. That would be extremely hard to mimic in a theory without dark matter".

He concludes with:

Originally Posted by Sean Carroll

Personally, I think it would be much cooler if gravity were modified, compared to the idea that it’s just some dumb new particle out there. I’ve put some thought into the prospect myself, which helped lead to some productive research ideas. But ultimately the universe doesn’t care what I prefer. Dark matter is real - gravity could also be modified, but there’s no reasonable doubt about the dark matter. So let’s try to figure it out.

It seems Phil Plait also agrees with him. He concludes with:

Originally Posted by Phil Plait

few astronomers seriously doubt the existence of dark matter.

So, for all those out there who firmly insist that the credibility of dark matter is a problem for Astronomers and present day Cosmology, I am yet to find a legitimate scientist who would openly call it such. I will keep looking.

Comments welcome.

Cheers

[renormalised (Carl)]

"Dark Matter" is nothing more than a hypothesis concocted to try and explain a few anomalies with gravity and the structure of the Universe. They have no viable candidate for what it might be and they really still haven't much of a clue as to the physical dynamics of the stuff (except in very broad terms). So, to say it's real is stretching credulity a little too far, yet.

The best they can say is there's something there they can't detect and it's having a profound effect on the the structure of the galaxies in particular and the Universe as a whole.

[CraigS]

G'Day Carl;
Gotta rush … but did you see the comments about the Bullet Cluster here. (Scroll down to the image).
Its pretty interesting observational 'evidence', to say the least.
Its the first time I've looked at this image, in this particular way.

Cheers

[renormalised (Carl)]

Those comments really mean jack....they still don't know what it is and what's to say something else couldn't mimic those results. They're putting all their eggs in one theoretical basket and that's dangerous, theoretically.

Here's the clanger....if dark matter interacts with both itself and normal matter via gravitational influences, then eventually both the dark matter and the normal matter will fall to the centre of the cluster...whether they pass through one another like "ghosts" or not. Since the dark matter is the most massive, because there's supposedly more of it, it will be right at the center, with the ordinary matter on the outside, for the most part.

[CraigS]

Hmm .. there must be a few assumptions in what you say …

For example, don't they say that DM can be clumpy (albeit, on the larger scales) ? So, if it is clumpy, then perhaps its interaction with normal matter doesn't necessarily result in it falling to the centre of the cluster ?


Cheers

[renormalised (Carl)]

Originally Posted by CraigS

Hmm .. there must be a few assumptions in what you say …

For example, don't they say that DM can be clumpy (albeit, on the larger scales) ? So, if it is clumpy, then perhaps its interaction with normal matter doesn't necessarily result in it falling to the centre of the cluster ?


Cheers

Whether it's clumpy or not, if it is the majority of the mass in a gravitational system, it will fall to the centre of that system, especially if it doesn't interact in any other way with matter (or even itself). The only thing which will keep it from falling to that centre is some sort of internal pressure, or pressure like mechanism, which means it must be interacting in some fashion...with something. That's why I said earlier they barely know or even understand anything about this. They've made their own assumptions, haven't found any evidence (except circumstantial) for its existence and because no one has come up with a truly solid alternative, they've just run with the baton and embellished the reality of it.

[CraigS]

Here's another thought ..

The main thing which differentiates normal matter from dark matter is electromagnetic interaction, yes ?

So why couldn't the added effects of the Electroweak and Electrostrong forces result in normal matter coming together more quickly than the DM clumps ?

Once the normal matter starts forming its 'aggregates', they then slowly begin to interact (gravitationally .. as the aggregates become significant), with the DM, which then passes straight through the aggregates on its way to another clump of DM ? (The process goes on over and over …??…).

What we see in the Bullet Cluster is that there is a difference in dynamic behaviours when comparing DM and normal matter.

Ah dunno … … I just made all that up !! …

Mind you, there's a lot of research going into the behaviours of dusty matter and how it comes together .. and I think EM forces play a big role in the very early stages of it all (??)

Cheers

[CraigS]

Originally Posted by renormalised

That's why I said earlier they barely know or even understand anything about this. They've made their own assumptions, haven't found any evidence (except circumstantial) for its existence and because no one has come up with a truly solid alternative, they've just run with the baton and embellished the reality of it.

Yes .. that's Ok, though .. its only a hypothesis which can be made to work with most available observations … and most theories.

What is the alternative ?

Cheers

[renormalised (Carl)]

Best way to tackle this is to break this problem down into its constituent parts.....

1. Apparent amount of dark matter vs normal matter
2. Distribution of both dark and normal matter w.r.t. one another
3. How are both interacting with one another
4. If dark matter only interacts with normal matter and/or itself gravitationally, how does that explain (2) and (3)
5. If doesn't explain, then what is happening?? What other process/es is/are going on.

Then we work from there.

[renormalised (Carl)]

Originally Posted by CraigS

Yes .. that's Ok, though .. its only a hypothesis which can be made to work with most available observations … and most theories.

What is the alternative ?

Cheers

Apart from MOND and it's variations...and possibly a few other theories (which I can't remember offhand), nothing substantial.

That's why they're running with it. But it maybe at the expense of them looking like idiots in the future.

[CraigS]

Originally Posted by renormalised

That's why they're running with it. But it maybe at the expense of them looking like idiots in the future.

Does that ever happen ?

Using Bert's analogy (hope he doesn't mind me using it, credits to Bert):

Originally Posted by avandonk

You only have to look at some of the ancient Greek 'laws' of science.

For a constant velocity a constant force needs to be applied.

Experimental proof. When the force is removed the object stops.
There was an anomaly with arrows, spears and rocks. But it was proposed that force supplied by the air from the thrower kept the object in flight.
Sound familiar as a fudge factor?
…
If ancient Greeks needed to invoke some sort of mythical air propulsion against their known theory of mechanics it only showed they did not have a fully developed theory.

We find ourselves in the same conundrum with dark energy and dark matter.

.. but does anyone look back at the ancient Greeks as 'idiots' ?

hmmm

Cheers

[renormalised (Carl)]

Originally Posted by CraigS

Does that ever happen ?

Using Bert's analogy (hope he doesn't mind me using it, credits to Bert):


.. but does anyone look back at the ancient Greeks as 'idiots' ?

hmmm

Cheers

They can if they persist in believing in something when there's been another answer come up at a later stage, or when they've known the other answer for some time and they just don't want to part with the old cherished notions.

[CraigS]

Originally Posted by renormalised

They can if they persist in believing in something when there's been another answer come up at a later stage, or when they've known the other answer for some time and they just don't want to part with the old cherished notions.

Agreed … 'dogmatic' behaviour demonstrates that its slipped into metaphysics or pseudoscience.

Cheers

[OzRob (Rob)]

There are problems with MOND. Larger galaxy clusters appear to have more mass than we can observe. Even when MOND is taken into consideration there is still not enough luminous matter to produce what we observe. Detailed studies of the cosmic microwave background agree with the presence of dark matter. The current MOND theory is complicated and requires some dark matter to work.

Ref: Dark Cosmos, Hooper.

[renormalised (Carl)]

Originally Posted by CraigS

Agreed … 'dogmatic' behaviour demonstrates that its slipped into metaphysics or pseudoscience.

Cheers

More like the "R" word

[renormalised (Carl)]

Originally Posted by OzRob

There are problems with MOND. Larger galaxy clusters appear to have more mass than we can observe. Even when MOND is taken into consideration there is still not enough luminous matter to produce what we observe. Detailed studies of the cosmic microwave background agree with the presence of dark matter. The current MOND theory is complicated and requires some dark matter to work.

Ref: Dark Cosmos, Hooper.

Yes, there is....however that doesn't mean what they're finding is matter, dark or otherwise. They don't know what it is they're looking at. "Dark Matter" is just a label for our ignorance. If they knew, or had a good idea of what it may be, then they wouldn't be having all these discussions about it. They'd be doing all the empirical studies needed to quantify and qualify what it is.