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Comment by inclemnet | Hacker News Reader

> If they exist, WIMPs would have a mass between one and 1000 times that of a proton. They would interact only through the feeble weak nuclear force

And gravity. The article even mentions this earlier, but seems to have forgotten about it by the second paragraph. Honestly, I think that a lot of dark matter's popular science reputation as misguided and made up is that articles like this seem so eager to ignore the relatively large amount of information we have about its gravitational behaviour.

(Not that it is necessarily WIMPs, but I think people would be a lot less eager to dismiss this explanation as 'aether-like' if they had a better idea of why WIMPs are proposed in the first place)

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So this is what I don't understand: there is supposedly 5x as much dark matter as matter. With that much of it floating around, how is it possible that we ever convinced ourselves that general relativity is true? Dark matter is supposedly embedded inside galaxies to the point where stars within those galaxies move faster than they otherwise would. Well, shouldn't the same be true of our galaxy? In fact, shouldn't it be true of our solar system? I just don't see how it's possible that we ever collected data in support of GR only to be surprised later by an anomaly nearly an order of magnitude bigger than the base data. If someone who understands cosmology could please explain this to me I would be most grateful.

Space is largely empty; this is a crucial point I will come back to.

Assume a spherical distribution of (dark) matter, with density small enough that we can ignore relativistic effects (which is almost always the case). If you are inside the spherical distribution at a distance R from the center of that distribution, the only effect you will feel is that of the mass inside a sphere of radius R, centered on that distribution. (This is well known for any inverse square law; I can explain more if required.)

The average density of normal/dark matter is very, very small. Inside our solar system, the dominant gravitational effect is that of the Sun: any dark matter having the expected density (from other measurements) would contribute only a tiny amount, so as to be essentially undetectable.

On much, much larger scales, such as the scale of galaxies, far enough from the center (and assuming the dark matter is essentially a spherical distribution coinciding with the galaxy), the cumulative amount of the additional gravitational pull is enough to be detected. Remember that the distance to the nearest stars is huge. Yet, our Sun is within a galaxy. So, as I wrote above, the average density of normal/dark matter is very, very small. However, over such large distances as those of the size of galaxies, the cumulative effects can be observed. However, distances involved within our solar system are such that no additional pull from dark matter (comparing the motion of say Mercury to that of Jupiter) is expected to have a visible effect.

is it possible that dark matter is regular matter, just so diffuse that we don't notice/see it? could it be hydrogen atoms just floating in empty space, at such low densities we would never notice but in the grand scheme of things add up? Edit: I dont mean to propose a potential solution here, it was more a question of why it must be unknown matter, why can't it be normal matter.

No, it is not possible. Cosmological models are actually tightly constrained by data. Even in the late 80s, when we did not have such good data, I remember being amazed at the fact that primordial nucleosynthesis calculations together with observational data were enough to conclude that the number of light "normal" neutrino species was 3, while there were still speculations in Particle Physics that we could have more than 3 generations of particles. The data accumulated over the last 30 years has constrained things even more.

If some other explanation than invoking some new type of matter that can not be seen was possible to explain the observational data, don't you think that Physicists would use that instead of coming up with postulating that the Universe is dominated by "dark matter" and "dark energy" which we have not seen yet?

Looking back at what I wrote I can understand your reaction - im wasnt trying to propose a new solution that someone hadn't considered. It was more a question of why couldn't it be that, which you answered quite well, thank you.

Neutral hydrogen in space is actually quite readily observed via radio telescopes as the famous spin-flip 21cm line. This signal has been very useful in mapping the distribution of neutral hydrogen within our own galaxy and those of our galactic neighbors.

In addition to this, there exist many arguments in favor of classifying normal matter and dark matter separately.

Thanks for that explanation. Two followup questions if you don't mind:

1) Why doesn't dark mater clump together to form dark galaxies?

2) Do we see dark matter's influence in the motions of the stars in the milky way?

> Why doesn't dark mater clump together to form dark galaxies?

Because of the weak nature of its interactions - it barely interacts with itself, never mind binding into local blobs. You can imagine starting with a big sphere of dark matter; under gravity it will collapse inwards towards its centre, but although it falls to a small region it just passes through itself and goes right out the other side again...oscillating right back to something close to the initial sphere. This is in contrast to 'normal' matter, which strongly interacts with itself and so will not pass through itself - instead it can bind and (through a range of physical effects) become planets, stars, or other things. Or at least heat up and redistribute energy in more interesting ways.

> Do we see dark matter's influence in the motions of the stars in the milky way?

Yes, you can see e.g. https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar... for some information.

Thanks!

I want to start off by saying I dont actually know the answer to your question - but I believe it is because gravity is weird all the way around. It is basically imperceptible at the micro level but is dominant at the macro. Its by far weaker than all of the other forces (38 orders of magnitude!). Even at solar system levels, gravity is strong enough to keep the planets in orbit but the solar tide is minimal. My sense is that while dark matter is abundant, even if our solar system was full of it you probably wouldn't notice. I don't think we did notice it until we started to look at things on a galactic scale. (if any of this is wrong, and it very well might be, id love to be corrected)

To start off, nothing about dark matter contradicts GR. All contemporary cosmological models of the Universe, which are constructed from GR, include dark matter.

The "surprise" that is attributed to the discovery of dark matter involves estimates of the mass of a galaxy due to contributions from stars, gas, etc., that fall woefully short of mass estimates obtained from observations of galaxy rotation velocities (kinematics). That is, we know how much mass there is there because of our understanding of gravity, but other methods of measuring the mass don't come close to this figure.

At galactic size scales GR adds virtually nothing to the Newtonian kinematics, the general relativistic corrections are substantial only when the gravitational field is very strong (e.g., binary neutron stars/black holes, very very near super massive black holes). GR is used in cosmology to model the universe on very large scales; scales so large that the "clumpy" universe that we see comprised of stars, galaxies, galaxy clusters, actually starts to look very, very smooth. Again, even within galaxy clusters, modeling the evolution of the mass in a cluster via GR doesn't change much compared with a model using Newtonian gravity.

There currently exists no experiments which contradict GR, and a great amount of data (including observations of stars near the Milky Way's supermassive black hole) which directly supports it. Most people cite special relativity as the correction method behind time-dilation effects for satellites in orbit (e.g., GPS) but strictly speaking, special relativity is only completely accurate in the absence of a gravitational field.

For GPS, in addition to the time-dilation effect due to relative speed of the satellites with respect to observers on Earth, there is a second, smaller (iirc) effect to take into account: that of the different position in a gravitational well, which is a purely GR effect.

I thought gravitational lensing was a GR effect?

It definitely is, and is a powerful tool in the determination of mass for very massive galaxies and galactic clusters. That being said, I don't think this contradicts anything I mentioned in my post -- gravitational lensing doesn't affect galactic kinematics.

OK, that makes sense. Thanks!

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