questions about dark matter

[boo]

is dark matter nothing more than a wildcard thrown in to a mathematical equation to make the equation work?

what evidence do we have for it other than that?

will we ever be able to see it? 

what do you ( or scientists) think it is exacly?

Edited September 18, 2019 by boo

[Strange]

20 minutes ago, boo said:

what evidence do we have for it other than that?

There are multiple lines of evidence for the existence of effects caused by something we label "dark matter". These include the orbital speeds within galaxies, the speeds of orbits with galaxy clusters, gravitational lensing, the spectrum of the CMB, the formation of large scale structures in the universe, and probably more.

Nearly all of this evidence points to dark matter being a form of matter which does not interact electromagnetically. So, similar to neutrinos but it must be more massive because the distribution shows it to be moving more slowly.

24 minutes ago, boo said:

will we ever be able to see it? 

Not "see" it because it doesn't;t interact with light. I guess the question means will we ever have a more "direct" detection of the particles that make it up? Probably.

It took over a decade to detect neutrinos "directly" before they were first detected. Obviously, it is harder to detect dark matter particles (otherwise we would have known what they were, perhaps even before observing the effects). It is rather inevitable that each new type of particle is going to be harder to detect.

28 minutes ago, boo said:

is dark matter nothing more than a wildcard thrown in to a mathematical equation to make the equation work?

I don't know what that means. Neptune was a "wildcard thrown in to a mathematical equation to make the equation work". The same could be said of electrons, photons, gravity, energy ...

Physics is described in terms of equations. When we discover new things, they are are included in those equations.

That would be true whether dark matter is a modification to gravity or some form of matter. So the question doesn't really make sense.

[boo]

34 minutes ago, Strange said:

There are multiple lines of evidence for the existence of effects caused by something we label "dark matter". These include the orbital speeds within galaxies, the speeds of orbits with galaxy clusters, gravitational lensing, the spectrum of the CMB, the formation of large scale structures in the universe, and probably more.

Nearly all of this evidence points to dark matter being a form of matter which does not interact electromagnetically. So, similar to neutrinos but it must be more massive because the distribution shows it to be moving more slowly.

Not "see" it because it doesn't;t interact with light. I guess the question means will we ever have a more "direct" detection of the particles that make it up? Probably.

It took over a decade to detect neutrinos "directly" before they were first detected. Obviously, it is harder to detect dark matter particles (otherwise we would have known what they were, perhaps even before observing the effects). It is rather inevitable that each new type of particle is going to be harder to detect.

I don't know what that means. Neptune was a "wildcard thrown in to a mathematical equation to make the equation work". The same could be said of electrons, photons, gravity, energy ...

Physics is described in terms of equations. When we discover new things, they are are included in those equations.

That would be true whether dark matter is a modification to gravity or some form of matter. So the question doesn't really make sense.

thanks for your response

ok, very layman question here,  (and i do apologise if these questions appear nonsensical to those of you who study the subject), but it would clear a lot up for me.

if you bumped into dark matter would you feel it?  or would you pass through it without feeling anything?

and one more,  

if there was dark matter in front of regular matter, would it obscure the light coming from it?  or would we see right through the dark matter as tough it was invisible?

Edited September 18, 2019 by boo

[Mordred]

Well being weakly interactive it would behave much like neutrinos in so far as a neutrino can pass through a light year of solid lead without an interaction.

Assuming DM is right hand neutrinos 

[Strange]

1 hour ago, boo said:

if you bumped into dark matter would you feel it?  or would you pass through it without feeling anything?

No you would not. In fact dark matter is passing through you all the time (although the density of dark matter around the Earth is very low). Just like neutrinos.

1 hour ago, boo said:

if there was dark matter in front of regular matter, would it obscure the light coming from it?  or would we see right through the dark matter as tough it was invisible?

It would be invisible, because it doesn't interact with light.

[Janus]

1 hour ago, boo said:

thanks for your response

ok, very layman question here,  (and i do apologise if these questions appear nonsensical to those of you who study the subject), but it would clear a lot up for me.

if you bumped into dark matter would you feel it?  or would you pass through it without feeling anything?

and one more,  

if there was dark matter in front of regular matter, would it obscure the light coming from it?  or would we see right through the dark matter as tough it was invisible?

First of all, you have to consider what happens when you "bump into" everyday matter.  The interaction between you and that matter is all done via electromagnetic fields.  The electromagnetic fields in the matter interact with the electromagnetic fields of the matter which you are made of.   So the "solidness" any object is just due to this electromagnetic interaction.   This electromagnetic interaction is also responsible for the objects interaction with light or any other frequency of the electromagnetic spectrum.  They are reason they glow when hot or absorb, reflect, or scatter electromagnetic radiation. 

Dark matter, by its very nature, does not interact with light in this manner.  It is completely lacking in electromagnetic interaction ( like the aforementioned neutrino).  Thus it also would not participate in the type of "bumping into" interaction everyday matter does, and thus passes right through you pretty much like you weren't there.

 

[Strange]

Good article on everything we know and don’t know about dark matter here: https://www.forbes.com/sites/startswithabang/2019/09/19/was-dark-matter-really-created-before-the-big-bang/

[mistermack]

Could it be that dark matter is moving so slowly that we can't register any evidence of collisions? 

If so, maybe they could look for it by looking at particles in the Large Hadron Collider, as they travel, rather than when they hit a target.

[Strange]

6 minutes ago, mistermack said:

Could it be that dark matter is moving so slowly that we can't register any evidence of collisions? 

The observed distribution requires them to be moving reasonably fast. Also, I'm not sure why the speed would affect how easy they were to detect. But maybe that depends on what properties they have.

9 minutes ago, mistermack said:

If so, maybe they could look for it by looking at particles in the Large Hadron Collider, as they travel, rather than when they hit a target.

They have been looking for any unexpected behaviour that could be evidence of dark matter. (Not sure what "as they travel means"; particles can, in general, only be detected when they hit a detector. There are some exceptions like Cherenkov detectors and cloud chambers.)

[mistermack]

2 minutes ago, Strange said:

I'm not sure why the speed would affect how easy they were to detect

In the same way that you can feel the wind more, the faster it's blowing. If it's still, you can't detect it. 

 

4 minutes ago, Strange said:

what "as they travel means";

Just that if the dark matter were to be hardly moving, then maybe you can maybe create an energetic collision by having the target moving at near the speed of light.

[Strange]

3 minutes ago, mistermack said:

In the same way that you can feel the wind more, the faster it's blowing. If it's still, you can't detect it. 

That probably depends on the type of interaction. Some might require the dark matter particle to have a certain kinetic energy. This is why we can't currently detect neutrinos below a certain energy (ie. speed).

5 minutes ago, mistermack said:

Just that if the dark matter were to be hardly moving, then maybe you can maybe create an energetic collision by having the target moving at near the speed of light.

Ah, I see what you mean: detecting dark matter particles interacting with the proton beam? I think that is pretty unlikely because the density of dark matter is very low.

They are hoping that some unknown particle interactions might be seen as a result of the protons collisions.

[mistermack]

2 minutes ago, Strange said:

This is why we can't currently detect neutrinos below a certain energy (ie. speed).

Ah, that's what I was getting at. If dark matter is so unlikely to interact with other matter, maybe there is no mechanism available that can speed it up to a speed that it can be detected at. 

[Strange]

2 minutes ago, mistermack said:

Ah, that's what I was getting at. If dark matter is so unlikely to interact with other matter, maybe there is no mechanism available that can speed it up to a speed that it can be detected at. 

Current attempts to detect it are based on assumptions about the mass, speed, types of interactions, etc. depending on the dark matter model used. No doubt there are whole ranges of models that would render it undetectable. At the extreme, that it just doesn't interact with normal matter at all, under any conditions. Which would make it impossible to detect (unless it decays or interacts with itself).

[mistermack]

I was just musing on from this paragraph in the article that you linked :

" It must have been born cold (i.e., it was moving slowly compared to the speed of light even at early times), and it must not collide or interact (above a certain constrained threshold) with itself or any of the Standard Model particles. "

If it started out moving slowly, and hasn't interacted since, it seemed like it's highly probable that it's still moving slowly. 

I'm sure that all of this has been thoroughly gone into, I'm just mulling over what I've read.      

[Strange]

8 hours ago, mistermack said:

I was just musing on from this paragraph in the article that you linked :

" It must have been born cold (i.e., it was moving slowly compared to the speed of light even at early times), and it must not collide or interact (above a certain constrained threshold) with itself or any of the Standard Model particles. "

If it started out moving slowly, and hasn't interacted since, it seemed like it's highly probable that it's still moving slowly. 

Indeed. But the reason to say it must have been "born cold" is because we know it is moving (relatively) slowly now.

("Relatively slowly" means not at a significant percentage of the speed of light. I don't know what kind of range of speeds this allows.)

[mistermack]

If there is six or seven times as much dark matter as observable matter in the Universe, it must surely have an effect on black holes. 

If they are sucking in six tons of dark matter, for every ton of the usual stuff, it's going to affect the calculation for whether a black hole is liable to be shrinking or growing. 

According to wikipedia :  " A black hole of one solar mass (M_☉) has a temperature of only 60 nanokelvins (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5×10²² kg (about the mass of the Moon, or about 133 μm across) would be in equilibrium at 2.7 K, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb and thereby lose mass." 

I wonder if the calculation of that allows for the constant flow of dark matter into the black hole? Maybe you could detect the dark matter by how black holes behave. 

[Strange]

9 minutes ago, mistermack said:

If there is six or seven times as much dark matter as observable matter in the Universe, it must surely have an effect on black holes. If they are sucking in six tons of dark matter, for every ton of the usual stuff, it's going to affect the calculation for whether a black hole is liable to be shrinking or growing. 

They will absorb a lot less dark matter than normal matter for the same reason that dark matter is distributed evenly around the galaxy, rather than forming structures like stars and planets. Because it doesn't interact electromagnetically, it has no way of slowing and forming clumps in the same way that matter does. So while matter will form accretion disks around black holes (either formed from material pulled from stars or passing clouds of gas) the density of dark matter around a black hole will not be significantly different from anywhere else.

Quote

According to wikipedia :  " A black hole of one solar mass (M_☉) has a temperature of only 60 nanokelvins (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5×10²² kg (about the mass of the Moon, or about 133 μm across) would be in equilibrium at 2.7 K, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb and thereby lose mass." 

I wonder if the calculation of that allows for the constant flow of dark matter into the black hole? Maybe you could detect the dark matter by how black holes behave. 

That calculation is only considering radiation. Even a black hole that is completely isolated will still absorb some of the interstellar gas around it. In that case, it will probably absorb more dark matter than matter.

But we can't (yet) measure minute changes like that in black holes. We can only observe the effects when they absorb large amounts of matter and become "active" (or, indirectly, from the stars orbiting them).

[mistermack]

So an accretion disk acts like a spider's web, capturing matter that would otherwise float by? I can picture that, but I'm not sure it would increase the intake of the black hole substantially. Once stuff gets orbiting, it can stay there for billions of years, unless the density of objects is high. 

If you regard the Milky Way as an accretion disk, around the Supermassive black hole at the centre, it's taken fourteen billion years and we're still not sucked in.

If ordinary matter is moving at a reasonable velocity relative to a massive body like the Earth or Sun or a black hole, it can easily whizz by like a comet round the Sun. But if dark matter is moving slowly, it wouldn't have that escape mechanism. 

Which prompts an interesting question. Does the dark matter of the Milky Way rotate around the centre with the normal matter? Or is that an unknown as well?

[Strange]

39 minutes ago, mistermack said:

So an accretion disk acts like a spider's web, capturing matter that would otherwise float by?

Not really, it is just matter that is in orbit around the black hole (rather like the rings of Saturn, or the planet around the Sun).

39 minutes ago, mistermack said:

I can picture that, but I'm not sure it would increase the intake of the black hole substantially. Once stuff gets orbiting, it can stay there for billions of years, unless the density of objects is high. 

Stuff is in the accretion disk because it has been pulled there by the black hole and will end up in the black hole (the clue is in the word "accretion"  )

39 minutes ago, mistermack said:

If you regard the Milky Way as an accretion disk, around the Supermassive black hole at the centre

Absolutely not. Apart from anything else, the mass of the black hole is insignificant on these scales.

39 minutes ago, mistermack said:

If ordinary matter is moving at a reasonable velocity relative to a massive body like the Earth or Sun or a black hole, it can easily whizz by like a comet round the Sun. But if dark matter is moving slowly, it wouldn't have that escape mechanism. 

As far as I know, there is no reason to think that dark matter is moving significantly slower than normal matter. (On average.)

39 minutes ago, mistermack said:

Which prompts an interesting question. Does the dark matter of the Milky Way rotate around the centre with the normal matter? Or is that an unknown as well?

Good question. Probably not quite the same, because it doesn't interact so will presumably have a larger range of velocities than normal matter.

 

39 minutes ago, mistermack said:

Which prompts an interesting question. Does the dark matter of the Milky Way rotate around the centre with the normal matter? Or is that an unknown as well?

Apparently, it does: 

Quote

Using a computer simulation called Eris, which replicates the physics of our galaxy and the dark matter in it using supercomputers, the researchers discovered that the velocity of dark matter lined up perfectly with the velocity of stars that contained both heavy metals and lighter elements.

https://www.outerplaces.com/science/item/17611-scientists-say-track-speed-dark-matter-using-old-stars

https://astrobites.org/2017/12/13/how-fast-is-dark-matter/

[Janus]

On 9/21/2019 at 3:20 AM, mistermack said:

If there is six or seven times as much dark matter as observable matter in the Universe, it must surely have an effect on black holes. 

If they are sucking in six tons of dark matter, for every ton of the usual stuff, it's going to affect the calculation for whether a black hole is liable to be shrinking or growing. 

According to wikipedia :  " A black hole of one solar mass (M_☉) has a temperature of only 60 nanokelvins (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5×10²² kg (about the mass of the Moon, or about 133 μm across) would be in equilibrium at 2.7 K, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb and thereby lose mass." 

I wonder if the calculation of that allows for the constant flow of dark matter into the black hole? Maybe you could detect the dark matter by how black holes behave. 

While there is more total dark matter in the universe, it is spread out much more thinly.  Black holes tend to form in the same places as stars do. In the galactic disk, for example.  The average density of interstellar material is ~ 1 proton mass/cc,  The estimated dark matter density in the region of the solar system is 1 proton mass/3cc or 1/3 the interstellar medium.

The average interplanetary medium in the solar system at Earth orbit distance is 5 proton masses/cc, (and can range as high a 100 proton masses/cc)

So just by relative density alone, you would not expect the in fall of dark matter to make up a majority of the mass falling into a black hole.  Now add to this the fact that visible matter, on its fall inward collides with other visible matter.  This results in the production of radiation that comes at the expense of the KE of the matter involved. Thus a bit of matter originally on a course that would take it on a hyperbolic path around the BH with a periapis outside of the event horizon can lose energy in such a collision and have its trajectory crossing the EH. Or it could be put into an closed orbit around the BH. In this case, it is subject to collisions with other orbiting particles, further robbing it of KE until it does fall in past the EH.  DM, on the other hand doesn't collide, has no such mechanism for shedding energy, and thus will only cross the EH if its original trajectory takes it there.

The end result is that the black hole is effectively a larger target for visible matter than it is for dark matter, which is compounded by the fact that in the region surrounding black holes, you are going to have a higher density of visible matter than dark matter to start with.

 

Edited September 22, 2019 by Janus
typos

[LaurieAG]

On 9/18/2019 at 11:18 PM, boo said:

what do you ( or scientists) think it is exacly?

The ratio of calculated universal matter (per the LambdaCDM model) verses visible universal matter (from WMAP or PLANCK data) equals 2*Pi +/- 1.1% (for both).

[mistermack]

I find the concept of dark matter quite fascinating. It prompts so many questions. 

If it only reacts with visible matter through gravity, that kicks up some questions for me. One question is how much dark matter is sitting inside the Sun, unaffected by all the heat and turmoil, just held in place by gravity. 

If you look up the composition of the Sun, it's given as Hydrogen 75% , Helium 24% and Oxygen 1% approximately. So, after four and a half billion years, it's accumulated zero mass of dark matter. Common sense says that there must be some lurking in there, but maybe it's such a tiny amount that it doesn't show up in measurements. 

[Janus]

1 hour ago, mistermack said:

I find the concept of dark matter quite fascinating. It prompts so many questions. 

If it only reacts with visible matter through gravity, that kicks up some questions for me. One question is how much dark matter is sitting inside the Sun, unaffected by all the heat and turmoil, just held in place by gravity. 

If you look up the composition of the Sun, it's given as Hydrogen 75% , Helium 24% and Oxygen 1% approximately. So, after four and a half billion years, it's accumulated zero mass of dark matter. Common sense says that there must be some lurking in there, but maybe it's such a tiny amount that it doesn't show up in measurements. 

The total amount of dark matter in the entire solar system is estimated to be about equal in mass to a small asteroid spread out evenly throughout the solar system.  The amount inside the Sun would be expected to be around the same density as it is everywhere else in the solar system.  If you take the extent of the solar system as being out to a distance of Neptune's orbit, the total volume of the Sun is 1/3e11 that volume and thus you would expect to find about that fraction of a small asteroid's mass on DM inside of it.

DM would not tend to collect inside the Sun. If you start with a particle of DM falling towards the Sun, it will just fall faster and faster as it gets nearer.   When it reaches the surface, it just keeps falling, picking up more speed.  It passes the center and begins to climb out, back past the surface on the other side and back out into space again. The most the Sun's gravity does is bend that trajectory, because there is nothing that will slow it down so that it can be captured by the Sun.   This is different than visible matter, which once it hits the surface is slowed down by the Sun so that it loses the speed it needs to climb back out again and it is captured.

[mistermack]

Thanks. That's a pretty stunning picture, even if it is the logical consequence of not interacting. 

Presumably the same can't happen with a black hole, or can it? Once inside the event horizon there is no path out so I'm guessing it would be trapped inside. 

[Mordred]

With a BH there is no spacetime path for DM to escape the event horizon.