Wtf is dark matter?

[jamest]

Sounds like a desperate attempt to make our physical models of the universe/galaxies consistent with observation. Is there reasonable evidence to suggest it actually exists? If so, wtf is it?

[Xamonas Chegwé]

You're UK based, aren't you, James? Did you happen to watch the Horizon program about DM a few nights back? If not, catch it on iPlayer. It presents a lot of the evidence in layman's terms.

And fitting ones theories to observed facts about the universe is not "desperate". It's the very basis of science. It's why Einstein's theory of gravity supplanted Newton's. Why we no longer think that the planets circle the Earth on mysterious "spheres" that cause their erratic movement. And why we no longer sacrifice virgins to volcano gods.

[jamest]

You're UK based, aren't you, James? Did you happen to watch the Horizon program about DM a few nights back? If not, catch it on iPlayer. It presents a lot of the evidence in layman's terms.

I caught about half+ of it, and it stayed in my mind... hence this thread. Very interesting, but I couldn't help but think that we were "making stuff up" just to sustain our existing model(s). From what I saw, we still have no conclusive evidence that it exists. And I'm completely in the dark regards to what it might actually be [like].

And fitting ones theories to observed facts about the universe is not "desperate". It's the very basis of science. It's why Einstein's theory of gravity supplanted Newton's.

Point taken.

[hackenslash]

Dark matter is a placeholder. It's the name we give to an effect that doesn't match our expectations regarding the orbits of stars in the outer edges of galaxies, especially small galaxies that rotate quickly, because given the amount of matter we can actually detect, these stars are travelling too quickly to be gravitationally bound to their galaxies. That means that either there is something wrong with our picture of gravity, or there's something there that we can't see that's exerting sufficient gravitational influence to keep those stars in their orbits. Whatever the solution is to these anomalous observations, the effect, which has been observed, is called dark matter. The term itself is a matter of historical contingency, arising from confidence that our model of gravity is largely correct, stemming from the fact that it's withstood huge amounts of testing. The status of dark energy is similar. It's real, we just don't know what it is yet. The proposed solutions for dark matter are many and varied, including WIMPs, MACHOs, an incorrect understanding of gravity, and ordinary matter residing on an adjacent brane, and ALL of them are called dark matter, because dark matter is simply what we call the observed effect.

[hackenslash]

fitting ones theories to observed facts about the universe is not "desperate". It's the very basis of science. It's why Einstein's theory of gravity supplanted Newton's. Why we no longer think that the planets circle the Earth on mysterious "spheres" that cause their erratic movement. And why we no longer sacrifice virgins to volcano gods.

Eminently quotable.

[jamest]

It's the name we give to an effect that doesn't match our expectations regarding the orbits of stars in the outer edges of galaxies, especially small galaxies that rotate quickly, because given the amount of matter we can actually detect, these stars are travelling too quickly to be gravitationally bound to their galaxies.

Yes, this was explained in the documentary. One question which arose in my mind, was why dark matter doesn't affect the slower orbital speeds of planets on the outer edges of the solar system in the same manner in which it supposedly affects the orbital speeds of stars in our galaxy?

[jamest]

fitting ones theories to observed facts about the universe is not "desperate". It's the very basis of science. It's why Einstein's theory of gravity supplanted Newton's. Why we no longer think that the planets circle the Earth on mysterious "spheres" that cause their erratic movement. And why we no longer sacrifice virgins to volcano gods.

Eminently quotable.

Except that some/many scientific theories didn't arise merely as a consequence of having to sustain an existing model which had come into question, in the manner in which theories about dark matter have emerged.

[Xamonas Chegwé]

Except that some/many scientific theories didn't arise merely as a consequence of having to sustain an existing model which had come into question, in the manner in which theories about dark matter have emerged.

You have it backwards. Science does not try to preserve existing models - not at all! It adapts existing models to accommodate new evidence. And, if those models can't cope with the new evidence, it abandons them like victorian orphans.

[jamest]

Except that some/many scientific theories didn't arise merely as a consequence of having to sustain an existing model which had come into question, in the manner in which theories about dark matter have emerged.

You have it backwards. Science does not try to preserve existing models - not at all! It adapts existing models to accommodate new evidence. And, if those models can't cope with the new evidence, it abandons them like victorian orphans.

I would argue that it does [sometimes] try to preserve existing models. For instance, the new theory about dark matter has arose because science has 'faith' in its models of gravity.

[Xamonas Chegwé]

Except that some/many scientific theories didn't arise merely as a consequence of having to sustain an existing model which had come into question, in the manner in which theories about dark matter have emerged.

You have it backwards. Science does not try to preserve existing models - not at all! It adapts existing models to accommodate new evidence. And, if those models can't cope with the new evidence, it abandons them like victorian orphans.

I would argue that it does [sometimes] try to preserve existing models. For instance, the new theory about dark matter has arose because science has 'faith' in its models of gravity.

No. There is no faith involved. There is trust, certainly. But that trust is based upon models that have made testable predictions and passed those tests with flying colours! Faith requires belief without such tests - that's why it's called "faith" and not "common sense"!

[hackenslash]

One question which arose in my mind, was why dark matter doesn't affect the slower orbital speeds of planets on the outer edges of the solar system in the same manner in which it supposedly affects the orbital speeds of stars in our galaxy?

Classical orbital mechanics tells us that, the further you get from the barycentre (point about which all bodies orbit; in the case of our solar system, this is inside the sun for every body in orbit) of any given system, the slower the orbital velocity must be for any body to remain gravitationally bound (i.e., if faster, it will escape). In the case of stars in the outer edges of galaxies, especially smaller galaxies, we observe that their orbital velocities are similar to those of stars closer to the barycentre of the galaxy.

As a result, most prospective solutions are rooted in the proposal that dark matter dominates in the halos of galaxies. Some have dark matter suffusing all of space, but still dominating in the halo (WIMPs), and some have very small but massive bodies, also largely dominating in the halo (MACHOs).

[Rum]

I am pretty well up on this stuff too, albeit on layman terms and I have to agree at least in part with Jamest suspicions. There do seem to be rather a lot of newish findings that make the standard model begin to look a bit shaky. The increasing speed of expansion along with dark matter and now dark energy. Sure such theories tend to be incremental and new findings offer new hypotheses, but the last decade or two seem to deepen the mysteries rather than shed light on them to my way of thinking.

So be it of course. Not blaming scientificists or ought!

[jamest]

Except that some/many scientific theories didn't arise merely as a consequence of having to sustain an existing model which had come into question, in the manner in which theories about dark matter have emerged.

You have it backwards. Science does not try to preserve existing models - not at all! It adapts existing models to accommodate new evidence. And, if those models can't cope with the new evidence, it abandons them like victorian orphans.

I would argue that it does [sometimes] try to preserve existing models. For instance, the new theory about dark matter has arose because science has 'faith' in its models of gravity.

No. There is no faith involved. There is trust, certainly. But that trust is based upon models that have made testable predictions and passed those tests with flying colours! Faith requires belief without such tests - that's why it's called "faith" and not "common sense"!

Whatever. The fact is that sometimes science goes out of its way to preserve existing models... which is the point I was making.

[hackenslash]

Except that some/many scientific theories didn't arise merely as a consequence of having to sustain an existing model which had come into question, in the manner in which theories about dark matter have emerged.

That's really not what happened. It's well understood that dark matter is a serious issue for general relativity. You'll note above that one of the things I listed as a candidate solution for dark matter is 'our understanding of gravity is incorrect'.

We do know that, in all other scenarios we've been able to measure, GR has performed flawlessly to the limit of our ability to measure. When we see an anomaly, we investigate ALL candidate solutions, not least because the proposer of a verified solution that overturns GR will go down as the supreme physicist of the century, as Einstein did. The idea that it's wrong is being studied in every one of the candidate solutions.

We call it science.

[jamest]

but the last decade or two seem to deepen the mysteries rather than shed light on them to my way of thinking.

The last decade or two? The last century, I would say, given QM.