Why is the scientific community divided on the existence of dark matter?

[us40]

Hello,

I found same number of arguments and observation regarding dwarf galaxies which supports mond (modification of Newtonian dynamics) and generally same number of arguments and observational support for dark matter so why large number of physicist community is in favor of dark matter.I means it is easy to understand that when people found that the orbit of mercury is not as predicted by Newton law and no "inner" planet was there and after some time general relativity gave solution to that problem.So in the case of why stars do not move slowly from center of galaxy is it not right approach that we give larger time to mond and not dark matter.

 

[Drakkith]

The issue is that dark matter accounts for more than just galaxy rotation curves. For example, it accounts for the gravitational lensing from areas where there isn't enough normal matter to explain it. Dark matter just happens to best explain our observations at the moment.

 

[Chronos]

Dark matter is the preferred explanation for large scale structures, in fact, it is the only serious contended on this level. Where it falters is on small scales, like dwarf galaxies in the MW halo. This discrepancy appears to be disappearing as more and better observational data becomes available. See, for example, http://arxiv.org/abs/1306.0913 - Cold dark matter: controversies on small scales.

 

[megacal]

April 2014 issue of Astronomy Magazine has an article by Bob Berman entitled, "Finding the Missing Universe" which gives an even-handed review of DM & MOND without favoring or dismissing either.

This was an eye-opener for me, as I didn't even realize there was any alternative theory to explain the mass discrepancy in galaxies besides DM, and only saw a reference to MOND about a month ago here without realizing its meaning.

Since reading the article in Astronomy, I've searched here for more information on MOND, and
found several threads here at the forums going back to at least 2005. I'm still taking in the
history of the competing theories.

I'm not championing MOND, but for the benefit of those who are new like myself,
here is a good resource to see what it's about.

http://www.astro.umd.edu/~ssm/mond/

Berman also referred to the "food fight" between the majority who support DM, and the minority
who are investigating MOND and other possible explanations for the mass discrepancy in in galaxies.

Marcus's thoughtful thread on "What and Why Falsifiability?" should be a sticky, and taken to heart by all.

Either way, DM or MOND, the conversation should be kept respectful, cheering one another on,
with a common purpose, i.e. to find the truth. Friendly competition is healthy. Ridicule is unproductive and mean spirited, and unworthy of an objective scientist.

"As iron sharpens iron..." Proverbs 27:17

 

[Drakkith]

Who are you directing the last paragraph in your post to?

 

[megacal]

To whomever it applies on either side.

 

[phinds]

To whomever it applies on either side.

Since it clearly doesn't apply to anyone in this thread, I think Drakkith's question was valid and you have not answered it. Why do you think it DOES apply? Why did you say it?

 

[megacal]

Sorry...could not get back sooner.

No offense meant. It was not directed towards anyone in this thread, but to those in general who
deride the research of others, e.g. who are investigating MOND in this case.

In fact, I've been impressed with the level of respect given MOND here, even though
most seem to think DM is the answer.

Note: Mod, if my post was out of line, please edit or delete.

I don't think the original question was adequately addressed as it only supported DM, and totally
ignored MOND's correlation with the rotation curves, as though the matter is already settled.

In spite of the circumstantial evidence for DM, if we are immersed in it (?), then why can't we
measure it in the lab? How do you reconcile that? Why not look at other possible explanations,
e.g. MOND, especially if it correlates, even if not in all cases.

Is DM a perfect fit?

BTW, this article is a year old, but may be at least be an answer for us40.

It's reasonable and respectful, which should be expected.

 

[Drakkith]

I don't think the original question was adequately addressed as it only supported DM, and totally
ignored MOND's correlation with the rotation curves, as though the matter is already settled.

The first two replies addressed it. Dark matter adequately explains more than just galaxy rotation curves, which MOND doesn't.

In spite of the circumstantial evidence for DM, if we are immersed in it (?), then why can't we
measure it in the lab? How do you reconcile that?

Dark matter is thought to interact only through gravity. As such, it is VERY difficult to measure here on Earth.
Plus, who says we're immersed in it?

Why not look at other possible explanations,
e.g. MOND, especially if it correlates, even if not in all cases.

We are. Or rather astronomers are. New observations are constantly being taken and compared against the multiple theories and models we have, including MOND, the various forms of Dark Matter, and other less well known theories.

Is DM a perfect fit?

Not yet.

BTW, this article is a year old, but may be at least be an answer for us40.

It's reasonable and respectful, which should be expected.

I'd say that article sums the issue up fairly well. It seems to be a little geared towards defending MOND, but that's okay.

 

[Jonathan Scott]

I'm not keen on MOND as a theory (I don't think I've yet seen a variant I would classify as even self-consistent, let alone physically plausible), but as an empirical rule for predicting galaxy rotation curves it is so successful that I feel it demands an explanation.

It may be that the explanation is that DM happens to behave in such a way as to reproduce the MOND predictions. Personally I find that even less plausible than the current MOND theories.

 

[megacal]

The last 1/3 of the article by Scott Dodelson is the most important from my point of view:

The communities now are quite disparate and find it difficult to engage one another. Is the MG vs. dark matter dispute identical to the disagreements between people from different religions, say, virtually impossible to resolve because the two sides cannot communicate? Certainly not. We are scientists, and facts will change our minds. Some examples of things the vast majority of the MG community accepts or will accept:

MG is not theoretically favored over dark matter because "dark matter is something new". Both approaches are changing the fundamental lagrangian of nature by adding new terms and new degrees of freedom.
The fact that Xenon100 or Fermi (or perhaps AMS in a few days) has not seen dark matter does not mean the theory is excluded. There is plenty of room in theories like supersymmetry and even more in other more generic models.
If dark matter is detected unambiguously via direct and/or indirect detection, then MG would indeed fall outside the realm of reasonable scientific investigation.
On the other hand, our dispute does share similarities with those that divide adherents of religion. We are passionate, we come at things from different directions with different preconceptions, so it is sometimes difficult to speak the same language, to focus on a single question. At the end of the day, just like the devout in different religious traditions, we are all after the same goal, in our case, trying to understand nature. It is premature to state that our way is the only way.

I just think it's best to keep one's mind open to more than one possibility, even while focusing on
one avenue of research. Maybe the two (or more) will converge at some point, or at least make some contribution and/or discovery along the way that will benefit all.

More is to be gained for the advancement of knowledge by cooperation and communication, rather than by taking hard stances, becoming entrenched, and lobbing stones at the other side.

BTW, Scott Dodelson was asked to be the guest contributor to the blog because he is in the DM camp. He sounds like someone who is well respected all around. I appreciate his willingness to address the issues, i.e. DM vs MOND, and the conflict between the two sides.

Per the blog host:

Scott is a very well-respected cosmologist. He is a scientist at Fermilab and a professor in the Department of Astronomy and Astrophysics and the Kavli Institute for Cosmological Physics at the University of Chicago. His research focuses on the largest and smallest scales of the universe: the interplay of cosmology and particle physics. He investigates the nature of dark matter and dark energy, works on the cosmic microwave background and is also interested in modified gravity theories. In addition to his many papers, he has written the textbook “Modern Cosmology”.