Dark matter and gravitational lending

[Michael Kieth Adams]

I’ve got two ideas, first, could gravitational lending be used to determine the actual shape of galaxies including dark matter?  The other is that dark matter is gravitational, it has gravity and is affected by gravity, what if all our measures are off by 95%?There must be dark matter in our sun and in the core of the earth which means our measurements almost have to be off.  What if dark matter exists at an angle (wrong term, I know) such as to make it only measured by gravity?

[andrew s]

1) Yes lensing is used to compute the distribution of matter (normal and dark) in galaxies. 

2) If our measures are that far out I think our predictions on binary stars motion would be way off but they seem ok. Look here for a bigger example.

3) the mass of dark matter in the solar system is estimated to be about 1/9 the mass of Ceres.

I have no idea what your last sentence means. We currently don't know what dark matter is.

Regards Andrew 

Edited September 8, 2023 by andrew s

[Michael Kieth Adams]

I have no idea what that last bit might mean either ( pardon my typing mistakes, cataracts make it difficult for me to read).   I’m surprised by one ninth the mass of ceres though.   Wouldn’t that indicate that gravity does not have the same effect on dark matter as it does on regular matter?  I would think that dark matter would follow he same paths as normal matter if gravity acts on it as it does for visible matter.   If our measurements were all skewed by the presence of undetected ( except by mass) dark matter, how would we know?

 

If dark matter had some quality that made it only detectable by gravity, couldn’t it be hiding everywhere?  What would the super colliders results look like if we could examine its results using gravity.  We believe in our guts that if we can’t see it some way it can’t be true, but we also know that that is not the way things are.

 

[andrew s]

You miss two key points.

The average density of matter is very, very low. We only notice it when it's concentrated into stars, planets and clouds etc.

Which brings me to my second point. While dark and normal matter are effected by gravity in the say way dark matter can't concentrate as normal matter does. To concentrate matter needs to lose angular momentum and kinetic energy. Normal matter does this by emitting electromagnetic radiation giving us some of the most spectacular sights e.g. accretion discs. Dark matter can't do this so remains diffuse.

Regards Andrew 

Edited September 9, 2023 by andrew s

[Michael Kieth Adams]

Andrew, thanks for paying attention to a curious layman.  If I remember correctly electromagnetism is expressed by an electric wave and a magnetic one at 90 degrees to each other.  What if dark matter emitted something equivalent at different angles.  Would we be able to detect it?  95% of the mass of the entire universe sounds like gravity has to have ( I know, physics does not like he words has to) behave in a similar manner as visible matter.  Does gravity lensing occur where no visible matter exists?  If not there should be some connection between visible matter and dark matter.  I’m thinking that it may be that visible matter follows dark matter rather than the reverse.  I would expect there to be a difference between spiral galaxies and more diffuse ones with dark matter following the shapes of the galaxies it is close to.  If dark matter follows the contours of visible matter shouldn’t it be giving off something even if we can’t detect it.  I hope I have said something that makes sense.  Thanks.  Mike

[robin_astro]

17 hours ago, andrew s said:

While dark and normal matter are effected by gravity in the say way dark matter can't concentrate as normal matter does. To concentrate matter needs to lose angular momentum and kinetic energy. Normal matter does this by emitting electromagnetic radiation giving us some of the most spectacular sights e.g. accretion discs. Dark matter can't do this so remains diffuse.

 

Yes that is what I understood but was then surprised to find that physics does not preclude the formation of supermassive dark matter stars, in the early universe at least

https://www.scientificamerican.com/article/jwst-might-have-spotted-the-first-dark-matter-stars/

[Michael Kieth Adams]

Again I missed your main points.  Sorry.  Dark matter does  accumulate in or near galaxies doesn’t it?  If I remember correctly, the Milky Way does not have enough matter to hold together, hence dark matter.  Gravity is a weak force, maybe it does not have the same effect on dark matter as visible matter but instead a similar one.  If dark matter had a gravitic effect on itself there should be gravity Lensing effects all over the place with no visible causes.   We have demonstrated that two particle can be linked in such a way that what happens to one happens to the other.  Doesn’t this imply faster than light?  The universe is wonderful and weird.  I’m starting to ramble.  Sorry and thank you.

[andrew s]

Yes it can accumulate in galaxies but it is still very dilute compared to stars and planets etc. As I said about 1/9 Ceres mass in the solar system. 

While personally fun wild speculation won't actually give you much insight. 

Regards Andrew 

[andrew s]

28 minutes ago, robin_astro said:

Yes that is what I understood but was then surprised to find that physics does not preclude the formation of supermassive dark matter stars, in the early universe at least

https://www.scientificamerican.com/article/jwst-might-have-spotted-the-first-dark-matter-stars/

Interesting with lots of maybe and ifs and buts. If darkmatter is a particle and if it is its own anti particle then in the much denser early universe it could have got dense enough for there to be a significant number of collision of dark matter particles to annihilate and power a dark star.

If such a process was going on now in our galaxies dark matter halo we should see a background glow. I am not aware of any such detection so it seems to be too diffuse. 

Regards Andrew 

[Michael Kieth Adams]

I just read that a group has discovered clumps of dark matter using lensing effects.  Implications I know norguess not.  If I understand what I read yesterday,  dark matter does accumulate or appears to do so.  Wouldn’t that mean that it must be giving off something?  To accumulate it must lose  angular momentum and kinetic energy and produce what?  Dark matter and dark energy are a huge part of our world about which we know very little.  We do know how visible matter acts there must be a way to deduce more about  our invisible universe.

[andrew s]

It accumulates at galactic scales not stellar or planetary.  On galactic scales it doesn't need to lose angular momentum and energy as it would to form stars or planets.

Trying to understand dark matter and dark energy is an  ongoing research effort with considerable effort going in.

Regards Andrew 

Edited September 12, 2023 by andrew s

[Gfamily]

One difference between ordinary matter (OM) and dark matter (DM) is the way that OM can aggregate together.

If you consider a piece of OM on the edge of the Milky Way or the Solar System. The material at the outside has a lot of gravitational potential, and when it falls in it converts the gravitational potential energy into kinetic energy (it speeds up) and after passing through the inner Solar System It heads back out and slows down. 

However, if it hits something in the middle, it'll give up some of its kinetic energy as heat and the light that hot objects give off. This means that after the collision, the OM won't go as far out and thus the Ordinary Matter collects in the centre of the Solar System.

The means of aggregation is the conversion of kinetic energy into light (electromagnetic energy) which is then lost from the system.

Because Dark Matter does not participate in any process involving electromagnetic energy, it doesn't have a way of losing energy, so any DM that falls towards the centre, simply passes through it and back out again with no means of aggregating there in the same way that OM can.

This means that the average density of DM in the inner Solar System remains very low (similarly in the inner reaches of the Milky Way there's no particular concentration). 

Interestingly, it seems that any DM that falls directly into the Supermassive Black Hole at the centre of a galaxy will be captured and will not escape - as far as I can tell, this is the only way of constraining DM. This last point is not one I've seen referenced anywhere, so it is my personal contribution to cosmology. If I was a better astronomer I'd have a clue how to apply this hypothesis to see if it's useful. 🙂

Edited September 12, 2023 by Gfamily

[Michael Kieth Adams]

Thank you for your explanation but I keep hearing the phrase that we know of in my head.  We have recently discovered that apparently space can expand faster than the speed of light and that two linked particles can somehow communicate instantly.  Just thos e two things must  clear a lot of bets from the table.  Maybe dark matter accumulates around black holes.  Maybe there are dark matter fairies who sweep it into piles for later use.  Probably not, but the universe is  bigger and stranger than we know.  Mike

[andrew s]

28 minutes ago, Gfamily said:

Interestingly, it seems that any DM that falls directly into the Supermassive Black Hole at the centre of a galaxy will be captured and will not escape - as far as I can tell, this is the only way of constraining DM. This last point is not one I've seen referenced anywhere, so it is my personal contribution to cosmology. If I was a better astronomer I'd have a clue how to apply this hypothesis to see if it's useful.

Yes it has to directly "impact" the BH event horizon.  As you point out it can't join the accretion disc. Not sure it's a unique insight though.

Regards Andrew 

[andrew s]

14 minutes ago, Michael Kieth Adams said:

We have recently discovered that apparently space can expand faster than the speed of light and that two linked particles can somehow communicate instantly.  Just thos e two things must  clear a lot of bets from the table

Well it depends on your definition of recent. Hubble's law was established in 1929.  Quantum entanglement was proposed by Einstein, Podolsky and Rosen in 1935.

While pop science likes to sensationalise faster than light metric expansion and spooky action at a distance these are well known to working scientists as part of their day to day work.

What you may not realise is that our classical world emerges from entanglement with the environment at large. You and I have our classical form due to our interaction with for example the CMB photons, the molecules of the atmosphere and light from the Sun.

Regards Andrew 

Edited September 12, 2023 by andrew s

[Gfamily]

9 hours ago, andrew s said:

Yes it has to directly "impact" the BH event horizon.  As you point out it can't join the accretion disc. Not sure it's a unique insight though.

Regards Andrew 

Yes, the DM needs to impact the event horizon,  but a significant difference with a BH is that the 'cross section area' of the black hole is proportional to the square of the mass, (rather than the ⅔ power or less for stellar/planetary bodies), and also that any infalling DM will have no obstructions to impacting the event horizon, in contrast to OM which will generally have to negotiate the accretion disc and potentially high magnetic fields in the vicinity of the BH. Given that we often detect BHs via their emission of radio and X rays from jets, much infalling OM fails to make it to the event horizon.

So primordial supermassive BHs could potentially accrete DM significantly faster than ordinary matter.

[andrew s]

@Gfamily if you Google black holes, dark matter,  primordial you will find lots of papers both on accretion rates and if primordial black hole were dark matter.

Regards Andrew 

[Michael Kieth Adams]

I have heard estimates of 90% and 80% for dark matters mass in the universe.  That seems to be quite a lot of stuff.  Does a diffuse cloud of dark matter around the Milky Way constitute enough to hold things together?  If all this mass lies ( mostly) outside the visible milky way why doesn’t the milky way get pulled apart?  Again, dart matter does accumulate in clumps.  Doesn’t that mean that it has lost momentum and should be giving off something.  We already know that what we see is not all there is.  Could that be true of  electromagnetic radiation as well?  I enjoy this a lot.  Thank you all.  Mike

[andrew s]

The idea that dark matter is in a halo around a spiral galaxy is misleading.  Its distribution is similar to the of normal matter see here . As an example for NGC 3198.

 

The bottom figure shows the density of normal matter (b) and darkmatter (h).

So they have an additive gravitational effect holding the galaxy together while flattening the rotation curve.

Regards Andrew 

Edited September 18, 2023 by andrew s

[Michael Kieth Adams]

That’s what I thought.  Dark matter clumps which means it has to lose momentum and should mean that it gives something up.  Have Imissunderstood something?     Mike

[andrew s]

48 minutes ago, Michael Kieth Adams said:

That’s what I thought.  Dark matter clumps which means it has to lose momentum and should mean that it gives something up.  Have Imissunderstood something?     Mike

It does not need to "lose" momentum to "clump" on galactic scales. Dark matter, via the gravitational interaction, just converts its initial momentum into orbital momentum just as ordinary matter would.

It's only when normal matter gets close enough for friction to come into play e.g. star, planet formation and accretion disks that energy and momentum are lost via EM radiation. 

Regards Andrew 

 

 

Edited September 19, 2023 by andrew s

[Michael Kieth Adams]

Andrew, again thank you but dark matter in clumps, however they formed, almost certainly collides with itself.  Probably doesn’t happen much but it should happen some, and it should give off something.  I know that even the dark matter is not very dense but collisions should occur sometimes, however rarely.  Would there be a difference between a collision with other dark matter and a collision with visible matter?  I would think that regions near black holes would be good places to look.  I would also look at clumps of dark matter away from galaxies.  I’ve got a feeling they form around black holes which we also can’t see.  Hope I have made some sense.  Mike

[andrew s]

Simple answer is we don't know enough about dark matter to know what happens if and when it collides with itself or normal matter. As @robin_astro pointed out one idea is that it is it's own anti particle in which case we would expect it to give off its energy as radiation - presumably electromagnetic. 

There are lots of good ideas but no experimental results to sort them out.

Regards Andrew 

[Michael Kieth Adams]

The idea that isolated clumps of dark matter are around black holes is probably not always true.  There should be a lot of objects with a lot of gravity that do not give off light, but there should be some that do form around black holes.  It should be possible to predict what might happen and get some results.  Of course we are talking about studying objects that cannot be seen but only inferred. Seems like it would be easier if we had a closer clump to study.  Now that we know that they exist, maybe we can find one.  Mike