Could dark matter not exist? • Hacker News Board

Re: Could dark matter not exist?

#61

Post by smeyer » Tue, Jun 30, 2015, 3:34 AM UTC

themartorana wrote: post #59

bdamm wrote: post #48

Earlier quoted context omitted.

Perhaps it could be some form of friction we don't yet understand? I mean a form of gravitational friction? The nice part of being an armchair physicist is spouting off nonsense - I suppose if I really wanted to find out, I could make up a mathematical model of my theory and then find out if the evidence fits. Sadly, I really have no idea how to do that!

Ditto. Out of curiosity, is dark matter spread unevenly throughout the universe, or is it directly inverse to matter? Like, when we expect dark matter causes an effect, is it because we expect there to be a massive dark-matter "object" acting upon matter, or can we attribute it to denser dark matter as space between matter increases? Edit: would viewing dark matter provide a negative image of the universe? I'm asking…

No, dark matter is definitely not the inverse of regular matter. Dark matter is spread unevenly through the universe, and it's spread in a pretty similar (but still notably different) way to the spread of regular matter. For example, where you have galaxies with lots of regular matter there is also lots of dark matter, but it has a slightly different distribution. You seem to be imagining something cool, but not something that bears much resemblance to our universe.

Re: Could dark matter not exist?

#62

Post by vorg » Tue, Jun 30, 2015, 4:25 AM UTC

tjradcliffe wrote: post #40

mixmastamyk wrote: post #7

Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way. This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dar…

There's nothing magical about the required properties, so I'm not sure why anyone would describe them that way. They are no different than the properties imputed to neutrinos or the aether: http://www.tjradcliffe.com/?p=1801 The important thing about extra-galactic dark matter is there is too much of it to be made of ordinary particles: protons and neutrons and electrons. We know how many ordinary nucleons there are…

> Dark matter may or may not exist but there is absolutely nothing out of the ordinary about it

18% of matter is "regular" matter that interact with each other via bosons, and the other 82% is "dark" matter that doesn't interact with the known bosons. Could dark matter have its own "bosons"? Even more interesting, could dark matter be partitioned into various categories based on the "boson" they interact with? If so, perhaps the categories would have random but regularly distributed size ratios, for example:

* 25% of matter is zyzotic interacting via yetions

* 18% of matter is hadronic (i.e. quarks, leptons) interacting via bosons (i.e. photons, gluons, W, Z)

* 13% of matter is xenatic interacting via winnions

* 7% is vivacions interacting via ululons

* smaller partitions at 5%, 4%, 2%, 1.9%, 1.8%, 1.8%, 1.7%, 1.7%, 1.7%, 1.7%, etc etc etc

* there'd be millions of partitions overall

* the smallest partitions would consist of only 1 particle each (and perhaps its anti-particle) in the entire Universe

This would put various properties such as mass, etc of the hadronic particles as being randomly determined, perhaps quantumly, a split instance after the Big Bang.

Re: Could dark matter not exist?

#63

Post by ngoldbaum » Tue, Jun 30, 2015, 4:48 AM UTC

orbifold wrote: post #47

zardo wrote: post #44

Earlier quoted context omitted.

In geometric algebra it's a single equation. ∇E=∂B/∂t

Actually you can unify E and B into a closed two-form F and get in vacuum dF = 0 d★F = ★j Where j = rho dt + jx dx + jy dy + jz dz is the charge and current density, ★ is the Hodgestar operator and d is the exterior derivative. The equations in a medium are slighly less elegant.

And if you write the electromagnetic field in terms of the 4-vector potential, it's just \Box{A_μ} = 0 (where Box is the D'Alembertian operator). Then again, this doesn't really mean anything without the context of what the notation means...

Feynman has a discussion about this in Volume 2 of the Feynman Lectures: http://www.feynmanlectures.caltech.edu/II_25.html

(I still can't get over how awesome it is that I can deep-link into the Feynman Lectures!!)

Re: Could dark matter not exist?

#64

Post by AdieuToLogic » Tue, Jun 30, 2015, 4:54 AM UTC

> Could dark matter not exist?

Sure.

Re: Could dark matter not exist?

#65

Post by bzbarsky » Tue, Jun 30, 2015, 5:18 AM UTC

AnimalMuppet wrote: post #23

rcxdude wrote: post #6

Earlier quoted context omitted.

Yeah, but AFAIK all the explanations based around adjusting our theory of gravity have failed to match the results, and tend to be pretty complicated and thus prone to overfitting.

Whereas, since you can adjust the hypothetical distribution of dark matter to be whatever you need it to be to fit the data, you cannot falsify dark matter this way, since proving that no possible distribution of dark matter could fit the evidence is pretty much impossible. Still, to me there seems to be some level of equal implausibility in saying "our theory of gravity is wrong" and in saying "our theory of matter…

From my limited understanding, what's interesting about the dark matter approach is that if you pick the distribution of dark matter to fit the rotational velocity profiles of galaxies that also just happens to be the distribution that has a good fit to various other unrelated data sets (cosmic microwave background fluctuations, say).

Now maybe there's a deep underlying reason we don't know about that makes those unrelated data sets in fact related to each other. Or maybe it just happens that the dark matter actually exists. But the point is that the dark matter theories we have were absolutely falsifiable, as the article points out. They made predictions that were then tested and so far the predictions have been correct.

Re: Could dark matter not exist?

#66

Post by geon » Tue, Jun 30, 2015, 5:21 AM UTC

InclinedPlane wrote: post #56

pavel_lishin wrote: post #52

Earlier quoted context omitted.

Nitpick: I don't believe neutrinos are classified as WIMPs, since they are either massless, or have an incredibly small mass.

Neutrinos conclusively have mass since they experience flavor oscillations (if they were massless that would be impossible). Current estimates are that the total mass of all flavors of neutrinos is a fraction of 1 eV (less than a billionth the mass of a proton). Anyway, neutrinos aren't generally called "WIMPs" because they already have a name, the term "WIMP" is generally reserved for new particles that have yet to…

Does the "massive" in WIMP mean "have a mass" or "higher mass than other particles"?

Re: Could dark matter not exist?

#67

Post by ianamartin » Tue, Jun 30, 2015, 6:13 AM UTC

Of course dark matter doesn't exist.

Hello, people. Dark matter is the eflourescent ether theory of the 21st century.

There are things that we don't know how to explain right now.

We have some ideas about how they can be explained. But they are really really bad.

We will be laughing at dark matter theory in a few years.

Re: Could dark matter not exist?

#68

Post by InclinedPlane » Tue, Jun 30, 2015, 6:45 AM UTC

geon wrote: post #66

InclinedPlane wrote: post #56

Earlier quoted context omitted.

Neutrinos conclusively have mass since they experience flavor oscillations (if they were massless that would be impossible). Current estimates are that the total mass of all flavors of neutrinos is a fraction of 1 eV (less than a billionth the mass of a proton). Anyway, neutrinos aren't generally called "WIMPs" because they already have a name, the term "WIMP" is generally reserved for new particles that have yet to…

Does the "massive" in WIMP mean "have a mass" or "higher mass than other particles"?

It means it has non-zero mass. There are some theories of dark matter which would involve particles having thousands of times less mass than even a neutrino.

Re: Could dark matter not exist?

#69

Post by xioxox » Tue, Jun 30, 2015, 6:59 AM UTC

mixmastamyk wrote: post #7

Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way. This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dar…

The constraints from big bang nucleosynthesis (see e.g. http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto4.h...) say normal (baryonic) matter only makes up a small fraction of the mass in the universe. Therefore the remaining matter has to be non-baryonic (not normal gas or dust).

Re: Could dark matter not exist?

#70

Post by mixmastamyk » Tue, Jun 30, 2015, 7:15 AM UTC

tjradcliffe wrote: post #40

mixmastamyk wrote: post #7

Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way. This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dar…

There's nothing magical about the required properties, so I'm not sure why anyone would describe them that way. They are no different than the properties imputed to neutrinos or the aether: http://www.tjradcliffe.com/?p=1801 The important thing about extra-galactic dark matter is there is too much of it to be made of ordinary particles: protons and neutrons and electrons. We know how many ordinary nucleons there are…

I'll try to explain what I meant about magic. Your piece was interesting on how building mental models is an important part of science which opened my eyes a bit. However, in terms of this discussion I was thinking there might be lower-hanging fruit that makes dark matter appear farther-fetched.

For example, we have calculated the mass of a galaxy, but it doesn't spin right at that mass. So, instead of:

1. Assuming our calculations are wrong, or

2. There is more interstellar dust/gas/solar-wind mass than we realize, or

3. Heavier Black hole at center, or spinning speed/time dilation occurring?

4. Effects of gravity are weaker at very long distances

a different theory is presented instead.... that 2/3 of the universe is made of a brand-new undetectable substance.

As you mentioned it may or may not be true, and I've learned a lot about why it is convincing in this larger thread. Still, as Sagan said, "extraordinary claims require extraordinary evidence."

It's probably a flaw of the layman that ideas that have yet to be confirmed tend to appear magical.