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Dark matter


Kevin_Hall
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2 hours ago, Genady said:

The acceleration is relative to an observer. If an observer is attached to the mass, it does not accelerate anywhere. To any other observer, it accelerates - free falls - toward them with G*M/(R^2), where R is the distance from the observer and M is the mass of the stuff inside a ball of radius R.

What if there are two observers, at rest with respect to each other? 

Do they each see acceleration toward them? 

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18 minutes ago, Genady said:

The answer depends on your frame of reference, i.e. an observer. In the first scenario, as we consider an orbit around the Sun, yes it will get tighter. In the second scenario, wherever you put an observer, the Earth will be free falling toward it (plus the initial velocity.) There is no an absolute frame of reference to give an absolute answer.

I try to put all observers down to Earth :)

No, I don't see how the Earth's orbit will get tighter if I add uniform mass density to the whole Universe.

It would get tighter if I spread the added mass in a large sphere centered on the Sun - and only the part of the mass inside Earth's orbit would affect the Earth, exactly as you said. But I didn't do that - Instead I distributed the mass uniformly over the whole universe (I didn't center it at the Sun). 

In my opinion, adding uniform mass density (positive or negative) to the whole Universe, does not change local trajectories of gravitationally bound objects. It might affect, as you said, the expansion of the Universe as a whole, but this is an even darker topic.

It seems to me, there is still enough wiggle space in our theories and observations that we cannot say for sure there is no additional (positive or negative) offset to the observed distribution of dark matter density.

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More specific situation, for example: if a satellite is held at rest with respect to Earth, it will certainly experience acceleration. While if it free falls toward Earth, the Earth free falls toward it, and both feel nothing.

10 minutes ago, Danijel Gorupec said:

In my opinion, adding uniform mass density (positive or negative) to the whole Universe, does not change local trajectories of gravitationally bound objects. It might affect, as you said, the expansion of the Universe as a whole, but this is an even darker topic.

I'm sure your opinion will change after learning GR.

Here is how Penrose - who knows something about gravity :) - describes the situation:

 

Previously, an inertial motion was distinguished as the kind of motion that occurs when a particle is subject to a zero total external force. But with gravity we have a difficulty. Because of the principle of equivalence, there is no local way of telling whether a gravitational force is acting or whether what ‘feels’ like a gravitational force may just be the effect of an acceleration. Moreover, as with our insect on Galileo’s rock or our astronaut in orbit, the gravitational force can be eliminated by simply falling freely with it. And since we can eliminate the gravitational force this way, we must take a different attitude to it. This was Einstein’s profoundly novel view: regard the inertial motions as being those motions that particles take when the total of non-gravitational forces acting upon them is zero, so they must be falling freely with the gravitational field (so the effective gravitational force is also reduced to zero). Thus, our insect’s falling trajectory and our astronauts’ motion in orbit about the Earth must both count as inertial motions. On the other hand, someone just standing on the ground is not executing an inertial motion, in the Einsteinian scheme, because standing still in a gravitational field is not a free-fall motion. To Newton, that would have counted as inertial, because ‘the state of rest’ must always count as ‘inertial’ in the Newtonian scheme. The gravitational force acting on the person is compensated by the upward force exerted by the ground, but they are not separately zero as Einstein requires. On the other hand, the Einstein inertial motions of the insect or astronaut are, according to Newton, not inertial.

Penrose, Roger. The Road to Reality (pp. 393-394). Knopf Doubleday Publishing Group.

Edited by Genady
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On 12/22/2021 at 8:32 PM, swansont said:

Careful - they are not thought to interact via the weak interaction, but via some new method on the scale of, or weaker than, the weak interaction

Are they thought not necessarily to interact via the weak interaction, or certainly not to interact via the weak interaction?

Edited by Genady
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If there is five times as much dark matter as ordinary matter in the Universe, does that mean that black holes have five times as much content from dark matter as ordinary matter? 

And in that case, there would be five times as much dark matter passing the event horizon, at any instant as ordinary matter. And could that be detected as it flows into the BH ?

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3 hours ago, mistermack said:

If there is five times as much dark matter as ordinary matter in the Universe, does that mean that black holes have five times as much content from dark matter as ordinary matter? 

And in that case, there would be five times as much dark matter passing the event horizon, at any instant as ordinary matter. And could that be detected as it flows into the BH ?

Still the nature of the BH would be the same?

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2 hours ago, beecee said:

Still the nature of the BH would be the same?

I mean if it is a BH that has formed entirely from DM, or a BH from baryonic matter, or a mixture of both, there is no way to tell the difference.

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1 hour ago, beecee said:

I mean if it is a BH that has formed entirely from DM, or a BH from baryonic matter, or a mixture of both, there is no way to tell the difference.

That's correct. There is a theorem that any BH is entirely defined by these three parameters: its mass, charge, and angular momentum.

7 hours ago, mistermack said:

If there is five times as much dark matter as ordinary matter in the Universe, does that mean that black holes have five times as much content from dark matter as ordinary matter? 

And in that case, there would be five times as much dark matter passing the event horizon, at any instant as ordinary matter. And could that be detected as it flows into the BH ?

Dark matter falling into a BH would increase the BH mass, but would not be detected otherwise as it doesn't radiate.

BTW, we can talk about "flowing into" BH, but not about "passing the event horizon", because nothing ever can be observed passing the event horizon, by an external observer.

Edited by Genady
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9 hours ago, Genady said:

Are they thought not necessarily to interact via the weak interaction, or certainly not to interact via the weak interaction?

All science is provisional. There is no data to support this; the weak interaction is very short-ranged and is not responsible for the gravitational interaction exhibited by dark matter. 

WIMPs are particles not in the standard model, and interact via mechanism(s) not in the standard model.

Saying it interacts weakly is ambiguous, since there is something called the weak interaction, and there is also the notion of not having a robust coupling. Physicists kinda painted ourselves into a corner a little bit with this

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On 12/26/2021 at 8:44 PM, Genady said:

I'm sure your opinion will change after learning GR.

Sadly, I might never arrive to learn GR... But if we limit ourselves to the old Newton space, we can luckily analyze certain questions from this thread by simple geometry.

First, can mass evenly distributed outside Earth's orbit have any gravitational effect on the Earth? Down there is a picture of 2D newtonian-like universe (linear, infinite):

mass1.png.19fbc24e700e561a560afe0165336124.png

In the center there is Sun. The Earth is orbiting it - Earth's position at some instant is shown as point 'E''... I added some mass density (hatched) everywhere except for one circular Sun-centered 'bubble. The 'bubble' is slightly larger than the Earth's orbit. So, can this distributed mass (hatched) have any gravitational influence on Earth?

Easy; due to symmetry all the gravitational effect of distributed mass on Earth that is outside of the Earth-centerd green circle will cancel out. Only the mass within green circle (the crescent-moon-shaped region 'A') will provide net gravitational influence to the Earth. So, It looks quite obvious to me that the distributed mass will indeed affect the Earth (it pulls the Earth in the positive direction of the vertical axis).

 

The same approach... Down there I depicted the same case, but now the 'bubble' is exactly the size of Earth's orbit.

mass2.png.37d0a87a3c99b0d17cb99d9d67fa4b25.png

Again due to symmetry, I can actually ignore all the distributed mass except for the region 'a' inside the green circle... Because the region 'a' is circular, it is very easy to calculate its mass and therefore its influence on the Earth. In fact, the region 'a' is the same size as the region of the 'bubble'.... [This means that if we fill the 'bubble' also with the same mass density as the rest of space, there will be no net gravitational effect on the Earth... Something that was just obvious to me even without this complex discussion.]

If you ask for more realistic space (GR, limited observable space, limited universe age, finite gravity propagation speed etc.) I cannot analyze this.

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Sorry, but this is one of the specific cases where Newton gravity just can't answer the question, but GR can: gravitational effects of an infinite mass distribution. What it means is, that applying Newton in such cases can give a variety of contradictory answers, depending on how you want to calculate a diverging integral.

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9 minutes ago, Genady said:

Sorry, but this is one of the specific cases where Newton gravity just can't answer the question, but GR can: gravitational effects of an infinite mass distribution. What it means is, that applying Newton in such cases can give a variety of contradictory answers, depending on how you want to calculate a diverging integral.

So, you think that when GR is accounted for, then it will turn out that only the mass within Earth's orbit will affect Earth's orbit - by making the orbit tighter - even if the whole Universe is uniformly filled with certain mass density?

And, consequently, by examining revolution speed of stars in a galaxy, scientists were able (and actually did) calculate that there is no significant uniformly-distributed component of dark matter density?

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6 minutes ago, Danijel Gorupec said:

So, you think that when GR is accounted for, then it will turn out that only the mass within Earth's orbit will affect Earth's orbit - by making the orbit tighter - even if the whole Universe is uniformly filled with certain mass density?

Yes.

6 minutes ago, Danijel Gorupec said:

And, consequently, by examining revolution speed of stars in a galaxy, scientists were able (and actually did) calculate that there is no significant uniformly-distributed component of dark matter density?

Yes. (They did calculate the amount of uniformly distributed dark energy using GR, as you know.)

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  • 4 weeks later...

A bit of a leap, but...

Might gravity work convectively? Like, in a water vortex, just with another dimension, through which the backdraft "flows"? Thus causing intergalactic gravity being slightly less than zero? Or is that disproved / excluded by existing TheoPhys?

Did anybody try a model / do the numbers based on that?

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44 minutes ago, Godot said:

A bit of a leap, but...

Might gravity work convectively? Like, in a water vortex, just with another dimension, through which the backdraft "flows"? Thus causing intergalactic gravity being slightly less than zero? Or is that disproved / excluded by existing TheoPhys?

Did anybody try a model / do the numbers based on that?

Is this question referring to some other theory of gravity, different from GR?

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To search for dark matter, scientists have proposed using a detector consisting of single-stranded DNA.
The detector should work as follows. First, a WIMP, a hypothetical dark matter particle, knocks out a metal core from a gold plate, which falls into a "forest" of DNA molecules. On the way from the plate to the opposite polymer substrate, the nucleus manages to break a number of nucleic acid molecules, the fragments of which are separated from the plate and carefully assembled. After amplification during PCR, the sequence of damaged fragments is determined, which makes it possible to establish damage points in molecules and, accordingly, the trajectory of the movement of the gold nucleus. Thus, it will be possible to establish the trajectory of the movement of the nucleus with an accuracy of several nanometers, which means that the energy of the particles can be accurately determined.
The existence of dark matter is supposed to be determined by comparing the results of the search for WIMPs during the day and at night - when, due to the rotation of the Earth, the detectors will be located at different angles to the constellation Cygnus. Based on existing theories of dark matter and the direction of movement of our galaxy, it is believed that the Earth passes through dark matter moving from this constellation.
Yes, molecular biology is involved in the work on fundamental physics.

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That second link has the DM breaking the DNA strand, which is not what was posted.

"When a dark matter particle enters the detector, it slices through any DNA strands in its path

...

physicists can reconstruct the track of the dark matter particle through the machine."

This suggests they think that a DM particle will break multiple strands. I wish they would have gone into detail as to why they think DM would interact in this fashion.

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On 1/21/2022 at 7:53 PM, Genady said:

Perhaps that post referred to this, much older (IMHO, equally useless) suggestion: Hunting dark matter with DNA | Science News

Ummm...   ...my turf! (I used to be a molecular bologist)

This is a nice and elegant proposal to build a novel detector with superb spatial resolution - though it's actually more like 3nm, not the single one the paper peddles. Still, better than other currently used detection systems.

But... ...neither this article nor the quoted primary source https://arxiv.org/pdf/1206.6809.pdf give even the slightest hint as to how they'd manage to differentiate between WIMPs and other particles. *shrug*

 

["convective gravity"]

On 1/20/2022 at 3:13 PM, Genady said:

Is this question referring to some other theory of gravity, different from GR?

None I know of, that's why I'm asking f there is any. It might even be compatible with GR, as the assumption would be that 0g (not the free fall type, but the absolute) might not be the bottom of the scale. After all, we're way deep in the gravity well ot the milky way / local group / ... 

Only hand-waving from me, no data or such. Just wondering whether anybody did such a theory / the math based on that assumption.

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