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Comment by Tycho | Hacker News Reader

This observation notwithstanding, if dark matter cannot be observed, isolated or detected, then why should we treat it as anything more than an arbitrary hypothesis? Why not dismiss it similar to how we dismiss the idea that God created the universe, or indeed that God is holding the cosmos together?

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It can be both observed and detected, it can be mapped even. The detection is a pretty indirect one right now though.

There's the usual stuff about galaxy rotation curves, there's the http://en.wikipedia.org/wiki/Bullet_Cluster which people consider to be more of a smoking gun, but then there's also evidence from http://en.wikipedia.org/wiki/Baryon_acoustic_oscillations visible in the CMB. That last one's slightly over my head, but apparently cosmologists find it extremely convincing, especially when combined with the other evidence. The wiki page talks about dark energy, but the subject has significance for dark matter too. Let me see if I can find the relevant plot...

At the end of the day it's just another weakly interacting particle. Last time we discovered one of those was the neutrino in the 50s, it's not an outlandish thing to think about.

EDIT: here's the BAO stuff: http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto6.h...

Well, the thing is that "dark matter" doesn't necessarily have to all be super-exotic unknown particles. It's just that the behavior we see in the universe suggests that galaxies, clusters of galaxies, clusters of clusters, etc. contain more matter than we are able to detect, since if they contained only the matter visible to us their gravitational behavior would be different.

Of course, quite a bit of it might turn out to be exotic new types of particles, but that's a question for theory and experiment.

The precedent here is quite strong, incidentally, on a few counts.

We have high confidence in our understanding of gravity on the macro scale, and that understanding has survived some crises. For example, the orbit of the planet Uranus, as originally calculated, was "wrong" -- it did not appear to be conforming to the understood behavior of gravity. One theory which was developed to explain this, and which could at the time have been criticized in the style of your comment, was that there must be another planet further out, whose gravity was perturbing Uranus' orbit. Calculating backwards from Uranus' behavior, this theory predicted the location in the sky of the additional planet... which was then promptly observed via telescope (this was the discovery of Neptune).

Similarly, we have high confidence that weakly-interacting particles -- undiscovered variants of which are a popular candidate for examples of dark matter -- exist. The neutrino is the classic example: a particle which was posited by theory as "this is the only thing that makes sense given what we see", but which would necessarily be incredibly difficult to detect on account of barely if ever interacting with anything (and thus, again, would be open to your criticism). Of course, the neutrino was eventually detected and its existence confirmed.

So there is nothing wrong or arbitrary or unusual, in terms of the history of physics, in positing something like dark matter.

>> It's just that the behavior we see in the universe suggests that galaxies, clusters of galaxies, clusters of clusters, etc. contain more matter than we are able to detect, since if they contained only the matter visible to us their gravitational behavior would be different.

I've often wondered about this(and don't know any physicists so would really love some insight on this), why do we assume that there's additional matter rather than question whether our models are correct at that scale?

>, why do we assume that there's additional matter rather than question whether our models are correct at that scale?

There are also physicists that question the models. Some scientists follow a path of inquiry based on assumptions that Einstein's theory is mostly correct -- hence a postulation of "dark matter" which leads to searching for the existence of it(1). But there are other physicists that assume there's something missing in the accepted equations that would explain the galaxy rotation speeds without any need for "dark matter".

Each competing theory tries to accumulate more and more evidence for their case until one "wins" (scientific consensus)(2). Right now, the "dark matter" line of inquiry has more scientists, more papers, and more press coverage.

(1)direct instead of indirect observation

(2)Some might say Dark Matter theory has already "won" in the marketplace of ideas

Some do: http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

The reason that we are talking about dark matter is that it is detectable by its gravitational effects on ordinary matter. Our ways of looking at it are still incredibly blunt but they're much better than what we had 20 years ago. We can make hypotheses about dark matter and conceive observations to test these hypotheses.

As for dismissing God, I don't see why, we just need to come up with some signature that would differ between a designed universe and an accidental one and apply for observation time on a suitable instrument.

Dark matter has a different cognitive status than God, because there is some evidence, albeit inconclusive, that suggests dark matter (from what I understand; IANAP). In contrast, there is no evidence that suggests God, so a claim that God exists would be arbitrary and is therefore rightfully dismissed.

I would say that God is exactly what you are studying when you are looking at dark matter. God is a metaphor for the unknowable ground of being, the mystery that holds us all. The wisdom traditions have been about understanding the patterns of the Universe, in all senses and to provide a mechanism and context to dissolve the idea that something exists as separate from anything else. Theists and Atheists both seem to get it wrong when they argue over the "existence" of God, as Joe Campbell said, it's about understanding how to read a metaphor and experiencing the direct internal subjective experience of reconnection.

Isn't it the other way about? The reason we are hypothesising dark matter is because we can't explain some gravitational measurements. Did someone postulate dark matter particles, then later say 'well if these exist, they must cause some strange gravitational patterns.' ?

Yes, dark matter is of course an explanation invented because of observations. But it would just be a just-so story if you couldn't probe further consequences of that explanation by making new targeted observations.

Dark matter was proposed as an explanation for observations, e.g. the galaxy rotation problem. [1] Assuming the hypothesis is correct, then we have therefore already observed it (albeit indirectly, but all observations are indirect).

[1] http://en.wikipedia.org/wiki/Galaxy_rotation_curve

Isn't that putting the cart before the horse a bit? You need to explain galaxy rotation, fine, but I could invent any arbitrary hypothesis to say why the galaxies are rotating too fast, then cite the rotation as evidence for the hypothesis. This doesn't strike me as very good science. I would want to capture some dark matter in a lab and examine its properties (or something equivalent) and then conclude that this might explain our astronomical observations.

Sometimes you need a lab so big that you need to observe astronomical phenomena before your time runs out.

Many scientific theories have a history of the following sort:

1. Huh, those are some weird observations. What could explain it? 2. Maybe it's X? That could explain it. 3. So could Y? 4. Here's an experiment we could do to rule out Y! 5. Hmm, actually the result of that experiment was consistent with Y, but constrains some of its free parameters. And Y could still be false.

And so on.

Science doesn't require that you have something in a lab, just that you make hypotheses which are testable by some sort of observation. Otherwise we'd have to throw out nearly all of cosmology.

There are competing hypotheses still with respect to dark matter (e.g. that the force of gravity works differently than we thought over long distances), precisely because we haven't yet figured out how to do the experiments to rule out all but one hypothesis. But that's why it's called a hypothesis. We haven't yet figured out the answer! That's the beauty of science.

Dark matter explains two things that we currently don't know how else to explain: how the galaxies are rotating so fast without flying apart and why we see gravitational lensing from seemingly otherwise empty space.

I have never heard about detecting gravitational lensing coming from empty space. Do you have any info/references on that?

Here are the first links that came up when Googling it:

http://www.lsst.org/lsst/science/scientist_dark_matter

http://scienceblogs.com/startswithabang/2011/04/20/how-gravi...

http://www.ifa.hawaii.edu/~ger/ASTRO-110_sp08/Lecture28_Dark...

When two galaxy clusters collide, the ordinary matter can get slowed down by non-gravitational interactions, leaving the dark matter to continue on inertia and gravity alone.

Closest references I could find: http://phys.org/news/2013-02-dark-galaxies-ways-collision-he...

"In the Bullet Cluster, a collision between two galaxy clusters appears to have caused a separation of dark matter and baryonic matter." - https://en.wikipedia.org/wiki/Dark_Matter

Because there are numerous calculations and observations indicating that it is there. In other words, the hypothesis is supported by evidence. That cannot be said for certain supernatural beings.

Because dark matter is the only theory that fits all of the observational evidence. There have been many theories put forward to explain the phenomenon which has been labeled "dark matter" and the only one that has withstood the test of time is "cold dark matter" in the form of "weakly interactive massive particles" (or WIMPs). Perhaps that theory is wrong, but so far it's held up remarkably well while other theories have crumbled.

Is it? Cosmologists constantly keep adding Dark Matter, Dark Energy and yet they can't explain things like the results of the experiment in this article. Could it be that the solution is much simpler? Could it be that we're simply misinterpreting or miscalculating some of the data?

Very unlikely. There have been a huge number of observations and a lot of eyes on the data. The evidence for something that behaves as if dark matter exists is overwhelming.

Could it be that your understanding of the issues is simplistic, and scientists are actually asking all the relevant questions and dark matter is still the best answer they can come up with?

I'm going to second what gnaritas has said and suggest that perhaps you just have an insufficient understanding of the evidence and the current state of cosmological theory.

Also, despite the name "dark energy" has no relation to dark matter and the evidence for either does not overlap. Moreover, the inability of one theory to explain everything else in the universe does not invalidate that theory. The theory of biological evolution fails to explain the existence of neutron stars, yet it is still a valid theory.

There are observations which we can't explain with current scientific theories, and Dark Matter is a theory that seems to explain many of these observations. Either we'll find irrefutable evidence for it, or we'll ditch it, and find another theory to explain the observations. Most science is formed by testing theories, even if those theories are wrong and you end up making different theories to explain the observation.

IMHO our current conception of dark matter/energy is just a modern application of 'aether'[1]. It basically means we're not sure what it is yet, but it's there.

[1] http://en.wikipedia.org/wiki/Aether_(classical_element)

I'm a layman, but my guess is that as physicists know that there isn't enough matter and energy in the universe to account for its mass, they really want to know why.

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