Is Dark Matter Challenging Our Understanding of the Universe?

[LAP3141]

I understand that the evidence for dark matter, although indirect, is quite strong. Yet there are a few things that puzzle me about the dark matter idea.

If dark matter is the predominant form of matter in the universe, why does it have no effect on solar system dynamics? The solar system is quite stable and there cannot be any significant dark matter near the solar system or else planetary motions would be disturbed.

If the distribution of dark matter is localized to certain regions only, then how does big bang cosmology account for this peculiar distribution?

Also, the standard model of physics does not seem to anticipate dark matter. Is this truly the case? Do we then have to scrap the standard model and big bang cosmology because neither can include dark matter?

It seems that the dark matter idea threatens to invalidate all the modern physical theories.

 

[jedishrfu]

These are all good questions and of course science has no answers to them. Dark Matter was suggested to explain an anomaly in our calculations for galactic spin. The anomaly was that based on the estimated mass of the galaxy they should be spinning apart and yet no such thing is happening. By postulating this extra mass in the form of matter that we can't see the calculations begin to work.

The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.[2] Other lines of evidence include observations in gravitational lensing,[3] from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisions,[4] and from the motion of galaxies within galaxy clusters. In the standard Lambda-CDMmodel of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter and energy, 26.8% dark matter and 68.3% of an unknown form of energy known as dark energy.[5][6][7][8] Thus, dark matter constitutes 84.5%[note 2] of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content.[9][10][11][12]

So now we search for the elusive dark matter. We don't know if its atomic in size ala some unknown particles that have yet to be discovered or some massive cloud that is invisible to our telescopes and the frequencies of light that we can see and we have yet to image it.

Because dark matter has not yet been observed directly, it must barely interact with ordinary baryonic matter and radiation. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly-interacting massive particles (WIMPs), or https://en.wikipedia.org/wiki/Gravitationally-interacting_massive_particles (GIMPs).[13] Many experiments to directly detect and study dark matter particles are being actively undertaken, but none has yet succeeded.[14] Dark matter is classified as cold, warm, or hot according to its velocity (more precisely, its free streaming length). Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.

If it turns out to be a particle then the standard model must be extended or scrapped for a better model otherwise something in physics may be broke and in need of fixing. We just don't know yet.

Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists,[15] intrigued by certain observations that do not fit the dark matter theory,[16] argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, or entropic gravity. These models attempt to account for all observations without invoking supplemental non-baryonic matter.[17]

You can read more at Wikipedia:

https://en.wikipedia.org/wiki/Dark_matter

 

[Bandersnatch]

If dark matter is the predominant form of matter in the universe, why does it have no effect on solar system dynamics?

Because - as it is modeled - it doesn't interact strongly in any way apart from gravitationally, hence it doesn't form compact objects, unlike with regular matter. As a result, the distribution is very smooth, and there's very few of it - mass-wise - in any small volume like the solar system. It's only when you compare the mass of an entire average galaxy to its DM halo that you see the ~5:1 ratio.
I don't remember the estimates of local density off the top of my head, but IIRC it was on the order of a few tonnes within the entire volume of the Sun, or a small asteroid within 1 AU-radius sphere. That's not enough to produce measurable perturbations on the motion of other bodies in the solar system.

If the distribution of dark matter is localized to certain regions only, then how does big bang cosmology account for this peculiar distribution?

I don't think there's any tension with BB vis-a-vis DM clumping gravitationally around galaxies. Some initial inhomogeneities are expected, which then lead to density fluctuations, which in an expanding universe lead to DM filamentation and clustering. It might be those DM overdensities around which regular-matter galaxies first nucleate.
You may be interested in the 'Illustris' simulation, showing this process in action.

Also, the standard model of physics does not seem to anticipate dark matter. Is this truly the case?

The standard model has been showing some deficienciences for a while (there's an entire forum section called 'Beyond the standard model'), so it wouldn't be that surprising that it doesn't accommodate DM. I believe there are still some options left for SM-particles to fill the role of DM, but I'll let somebody more knowledgeable in this area to respond.

 

[Janus]

I understand that the evidence for dark matter, although indirect, is quite strong. Yet there are a few things that puzzle me about the dark matter idea.

If dark matter is the predominant form of matter in the universe, why does it have no effect on solar system dynamics? The solar system is quite stable and there cannot be any significant dark matter near the solar system or else planetary motions would be disturbed.

If the distribution of dark matter is localized to certain regions only, then how does big bang cosmology account for this peculiar distribution?

In terms of overall density, the solar system is actually a very dense pocket of matter when compared to our local stellar neighborhood or the galaxy as a whole.
It is estimated that the total amount of dark matter in the Solar system is about the same as a small asteroid, and has a density of ~6e-13 kg per cubic kilometer. The sun is about 8000 parsecs from the galactic center, which works out to be ~2.5e17 km. If dark matter was equally spread out evenly then the amount of dark matter that would effect the Sun's orbit around the center of the Galaxy would that amount contained within the spherical volume closer to the center than the Sun is or a volume of 6.3e52 cubic meters. At the same density as that of DM in the solar system, this works out to be 3.8 e40 kg or or the equivalent of ~19 billion solar masses. This is a significant amount of mass compared to the baryonic matter closer to to the center of the galaxy than the Sun. So the same density of dark matter that has an insignificant effect on the Solar system because of the relatively high density of baryonic matter in the solar system, would have a significant effect on the galactic scale.

 

[pdxautodidact]

I understand that the evidence for dark matter, although indirect, is quite strong. Yet there are a few things that puzzle me about the dark matter idea.

If dark matter is the predominant form of matter in the universe, why does it have no effect on solar system dynamics? The solar system is quite stable and there cannot be any significant dark matter near the solar system or else planetary motions would be disturbed.

If the distribution of dark matter is localized to certain regions only, then how does big bang cosmology account for this peculiar distribution?

Also, the standard model of physics does not seem to anticipate dark matter. Is this truly the case? Do we then have to scrap the standard model and big bang cosmology because neither can include dark matter?

It seems that the dark matter idea threatens to invalidate all the modern physical theories.

Generally dark matter is described as a halo, which would be a sphere around the galaxy.. If the majority of the dark matter around the Milky Way is present at its boundary, then its much, much farther than the distance to the sun (LY vs. AU).

The standard cosmological model is lambda CDM, which considers cold (non-relativistic) dark matter with a cosmological constant representing dark energy. I haven't done the simulations, but I don't see how to two are incompatible. One would assume that since dark matter is the majority of the matter content of the universe, I would consider a condensate with density anisotropies provided the initial gravitational seeds for baryonic matter to start to form galaxies.

The standard model is several theories that have all been meshed together hodge podge to explain what we see. The standard model doesn't explain anything at all, it just shows how things work. There are problems with the Standard Model. It doesn't attribute a mass to neutrinos naturally. It predict inflation. Most conspicuously, it doesn't describe gravity (since dark energy and dark matter are gravitational phenomena, it's a pretty good hint that the standard model is incomplete). It's not going to be invalidated--things on Earth are going to continue to be governed by it like they are now.

The key thing to remember is that dark matter is an observation. We don't know what it is. All theories are made to fit observations. If they don't, then they're useless. We have LOTS of theories regarding dark matter, dark energy, and extensions of the standard model. Don't fret; it'll all work out.

 

[Chronos]

To expand just a bit, dark matter does not clump. The evidence suggests it is pretty much uniformly dispersed in regions of space where it collects [mainly where galaxies reside] so its gravitational influence is also smoothly dispersed. If a relatively large amount was in the solar system it would tend to even out planetary orbital velocities much like it evens out the orbital velocity of stars around the milky way. This is not in evidence as planetary orbital velocities are inversely proportionate to their distance from the sun.

 

[stefan r]

...dark matter does not clump. The evidence suggests it is pretty much uniformly dispersed in regions of space where it collects...

Which evidence? Hypothetically suppose that some points in the solar system sometimes have 10,000x concentrations of dark matter. How could that be detected?

Particles usually scatter chaotically unless there is something that prevents that. I think it is a very reasonable assumption that dark matter will scatter and disperse on hyperbolic paths. That is not the same as actually getting the evidence.

A particle following an elliptical path according to Kepler's laws does not spent the same amount of time at each stretch of the ellipse. We could have a highway where 10 cars cross a suspension bridge each minute. There is more weight on a given span of the bridge if the cars are moving at 10 kilometers per hour than if the cars are moving at 100 kilometers per hour. Why would dark matter have the same density everywhere around the Milky Way?

 

[Janus]

Which evidence? Hypothetically suppose that some points in the solar system sometimes have 10,000x concentrations of dark matter. How could that be detected?

Particles usually scatter chaotically unless there is something that prevents that. I think it is a very reasonable assumption that dark matter will scatter and disperse on hyperbolic paths. That is not the same as actually getting the evidence.

A particle following an elliptical path according to Kepler's laws does not spent the same amount of time at each stretch of the ellipse. We could have a highway where 10 cars cross a suspension bridge each minute. There is more weight on a given span of the bridge if the cars are moving at 10 kilometers per hour than if the cars are moving at 100 kilometers per hour. Why would dark matter have the same density everywhere around the Milky Way?

DM is not expected to be uniformly dense throughout the galaxy, For example, the density of Dark matter at the center of the Milky way is estimated to be ~150 times that in the region of the Solar system. This still isn't very dense. A good illustration I ran across calculated that a single grain of birch pollen in a cubic km of space, would be 20 times more than the amount of dark matter contained in that same volume of space in the Solar system. At the center of the Galaxy you would need ~8 grains of pollen per KM³ to equal the dark matter density there. At your suggested 10,000x concentration, you are looking at 500 grains per cubic km, or still only ~1/10 the density of the interplanetary medium.

The overall distribution of DM would be expected to be statistical. While it doesn't interact with itself or other matter by other ways, its does do so by gravity, and these interactions would tend to cause it to be more dense at the center of galaxy. It isn't that DM doesn't clump, it is just that the gravitational interaction it is restricted to makes it a slow process, so it has only had time to form large loose structures rather than small compact ones.

 

[Ilythiiri]

In terms of overall density, the solar system is actually a very dense pocket of matter when compared to our local stellar neighborhood or the galaxy as a whole.

Nobody mentioned the idea that baryonic matter, being dense and all that with gravity well, may act as a lens for moving DM particle flow. Implications.

Quick google-glance: https://arstechnica.com/science/201...-of-dark-matter-may-lie-just-beyond-the-moon/

So, halo shape might be a crude oversimplification of DM: https://phys.org/news/2018-03-unprecedentedly-wide-sharp-dark.html

 

[nikkkom]

I understand that the evidence for dark matter, although indirect, is quite strong. Yet there are a few things that puzzle me about the dark matter idea.

If dark matter is the predominant form of matter in the universe, why does it have no effect on solar system dynamics? The solar system is quite stable and there cannot be any significant dark matter near the solar system or else planetary motions would be disturbed.

There can be a lot of DM in and around Solar System with practically no observable effect on its orbits, if its density is uniform. That's precisely what DM theory postulates - that DM's density varies only on the galaxy size scale.

Also, the standard model of physics does not seem to anticipate dark matter. Is this truly the case? Do we then have to scrap the standard model and big bang cosmology because neither can include dark matter?

The SM does not have massive neutrinos either. But they _do_ have nonzero mass. Therefore, there should be either right-handed sterile neutrinos (if SM neutrinos get their mass the same way other fermions do), or some other extension/revision of SM should be used. Bingo. Now you have your candidate DM particles within this extension of SM.

 

[KurtLudwig]

From what I have read, dark matter particles have not been found. DM does not interact with photons or electrons. It does not form large clumps, similar to stars. But it does interact with baryonic matter through gravity. Maybe DM exists in another parallel universe that flows through ours, with its gravity penetrating into our universe. Maybe "entangled gravitational strings" from their CDM are reaching our matter. While riding in a bumpy trolley in Vienna, Austria, at the turn of 1900, Ernst Mach remarked that the bumpy feeling is due our body being connected to the distant stars. Extending this thought further: Is there a physical law that contradicts the extension of gravity's effect between universes? Since electrons and photons cannot interact with matter in another universe, this would help to explain why dark matter has not been detected. Kurt Ludwig

 

[stefan r]

... in another parallel universe that flows through ours...

If something is "flowing through" then it could not be "parallel". It could be perpendicular, or angled. Neither a parallel nor a skew universe would ever flow through our universe.

 

[anorlunda]

Which evidence? Hypothetically suppose that some points in the solar system sometimes have 10,000x concentrations of dark matter. How could that be detected?

Particles usually scatter chaotically unless there is something that prevents that. I think it is a very reasonable assumption that dark matter will scatter and disperse on hyperbolic paths. That is not the same as actually getting the evidence.

I may be wrong, but I think the issue is not scattering, but gravitational collapse. Stars form from the collapse of dust clouds. Galaxies collapse from gas or dust clouds, and they have more density at the center. But gravitational collapse can't happen unless the gravitational energy is radiated away. If dark matter radiated away its energy, it wouldn't be dark; would it?

The picture above is from a very entertaining paper, The Potato Radius: a Lower Minimum Size for Dwarf Planets

 

[KurtLudwig]

How can gravitational energy be radiated away? Wouldn't it just heat up the matter of the star or planet?
Kurt Ludwig

 

[anorlunda]

How can gravitational energy be radiated away? Wouldn't it just heat up the matter of the star or planet?
Kurt Ludwig

Yes, and then the hot object emits light. Unless it is dark matter, because if it emitted light it wouldn't be dark.

 

[KurtLudwig]

I apologize for not immediately realizing that a hot object emits light.
Gravity results from the curvature of space-time. At a point (event) in space-time, there is information about the vector at that time. Does this information contain energy? What actually is this information? Is gravity an entanglement as the theory on Emerging Gravity claims? Kurt Ludwig

 

[anorlunda]

Gravity results from the curvature of space-time. At a point (event) in space-time, there is information about the vector at that time. Does this information contain energy? What actually is this information? Is gravity an entanglement as the theory on Emerging Gravity claims? Kurt Ludwig

I think all that is off-topic for this thread. Nobody mentioned information. But you may be interested in this PF Insights article.
https://www.physicsforums.com/insights/how-to-better-define-information-in-physics/

 

[KurtLudwig]

Does black matter have momentum and inertia? Is it a liquid or a gas? Is it predominantly located at the center of galaxies? Kurt Ludwig

 

[phinds]

Does black matter have momentum and inertia?

First, it's DARK matter, not black matter and second, it has mass so what do you think?

Is it a liquid or a gas?

Find a definitive answer to that and there's a Nobel Prize in your future

Is it predominantly located at the center of galaxies?

No, it is spread throughout the entire halo.

 

[KurtLudwig]

Sorry for the slip on dark matter.
Coming back to my first post #11 on Sept. 19, is it possible that dark matter is actually located in another universe and only its "gravitational strings" are reaching into our universe? It would help explain why it cannot be seen or detected. Its distribution and its inability to clump are very strange indeed. If it were in another universe, there are other physical laws and its properties maybe permitted. A somewhat similar situation exists in black holes. The matter cannot be seen, yet gravity escapes and helps in the formation galaxies. (My education stopped at undergraduate physics in 1961 and I am trying to educate myself in todays physics.) Thanks to another mentor for referring me to an article on what is meant by information in physics.
What do the members of the physics forums think?

 

[phinds]

... is it possible that dark matter is actually located in another universe

There is no belief that even if "other universes" exist, that anything could "spill over" into our universe since they would be causally disconnected. We get this question a couple of times a year at least and the consensus is, no. Basically, It's a pop-science BS claim.

 

[Orodruin]

is it possible that dark matter is actually located in another universe and only its "gravitational strings" are reaching into our universe?

That is just word sallad unless you can give a suitable reference to a reputable source.

 

[Janus]

Sorry for the slip on dark matter.
Coming back to my first post #11 on Sept. 19, is it possible that dark matter is actually located in another universe and only its "gravitational strings" are reaching into our universe? It would help explain why it cannot be seen or detected. Its distribution and its inability to clump are very strange indeed. If it were in another universe, there are other physical laws and its properties maybe permitted. A somewhat similar situation exists in black holes. The matter cannot be seen, yet gravity escapes and helps in the formation galaxies. (My education stopped at undergraduate physics in 1961 and I am trying to educate myself in todays physics.) Thanks to another mentor for referring me to an article on what is meant by information in physics.
What do the members of the physics forums think?

How is this a more reasonable explanation than the idea that Dark Matter is simply a type of matter that does not interact electromagnetically? (Especially when we already an example of such a particle in the neutrino).
It seems that a lot of people go to far extremes to avoid accepting that electromagnetically interacting matter might not be in the majority in our universe.