Do we still need Dark Matter to make things work?

[Beagleboy]

So there I was, sitting in front of my Playstation 4, flying my wee Diamondback through the outer edges of the MilkyWay on Elite Dangerous (I live a very exciting life). Anyway, I was flitting from solar system to solar system, marvelling at the depth of the game. The number and variety of different planets, their moons and asteroid belts is quite astonishing and great fun to explore. However, all good things come to pass, and Mrs Beagleboy eventually shouted at me that it was past my bedtime. Could I sleep though? Nae chance! Firstly because I lay in bed doing the maths and realised the journey I'd embarked on in the game is going to take me about 40-50 hours....real hours...to complete, but also because I started thinking about Dark Matter.

Now, as I understand it, current models can't keep galaxies from flying apart. This is down to there not being enough detectable mass to produce a strong enough gravitational attraction to keep things together. Have I got the right end of the stick there? As technology is improving though, we're seeing more and more real matter being detected. First it was all those massive, hot Jupiters whizzing around next to their parent stars, and now as detection methods become more sophisticated, we're starting to detect earth sized planets. We can only assume that as technology progresses, detection levels will increase as well and more and more planets will be detected, not forgetting their associated moons, asteroid belts, comets and Oort clouds. 

So where does that leave us with our reliance on Dark Matter, this undefinable, undetectable stuff that I like to think of as 'The Force'? Again, if I'm reading and understanding things correctly, there's already discontent growing over the reliance on 'Dark Energy' to explain why the Universe's expansion is accelerating. Some of the constants used in these calculations possible aren't that constant after all. I'm just wondering how much of a shortfall there is between the amount of Dark Matter required to keep us from zipping off into the wilderness and the amount of extra mass we'd need to find in the Universe to cover that shortfall with 'real' matter.

Forgive me if my science is laughably bad, I never progressed beyond O-Grade physics! I'd just like a decent nights sleep...oh and a massively upgraded Hyperdrive so I can cut down the journey time to the Bubble Nebula.

Cheers! 

 

[andrew s]

Sleep easy. Our best current model of the Universe is still the Lamda Cold Dark Matter model using Einstein's General Relativity. This requires dark matter to explain many observed features of night sky including the formation of the galxies and cluster of galaxies through to the rotation curves of individual galaxies. It is also required for the results of gravitational lensing.

Dark energy ( in the form of Einstein's cosmological constant) is the best fit we have to explain the acceleration the the scale factor of the expansion of the universe.

What scientists do or don't like about these models is irrelevant until they can find a better model that better fits the facts and makes new predictions.

So far they don't have one.

Regards Andrew

[vlaiv]

It is not just matter (a bit of pun intended? ) of galaxies flying apart. There are bunch of "places" where dark matter plays a role - like rotational speed of galaxies.

Currently our understanding is that ratio of "regular" to dark matter is something like 1:4 - we need 4 times as much dark matter as regular one to keep things in check.

Sun, according to our estimates accounts for around 99.86% of total mass of solar system. There are much more massive stars out there ....

It is safe to say that more than 99% of all regular matter in usual galaxy comes in form of luminous matter - ie stars. All other things, like planets, asteroids, gas clouds, whatever, that we consider regular matter and can be detected (interacts with electromagnetic radiation in some form) adds up to less than one percent. So we have 399% more to go - not sure that advances in technology are going to help us discover it is "hidden" regular mass. We need a "break-thru in technology" (to use some of gaming terminology ) in order to detect dark matter. Or alternatively - we need our theories and understanding modified.

Now, since you are interested in all of this, here is a bit of food for thought:

- Can EM radiation be responsible for Dark Matter? According to GR, any energy present will bend space-time, and there is a lot of electro-magnetic radiation bouncing off around the galaxy - each star is emitting plenty, all over the spectrum (hint: you can estimate total star output by using simple e=mc2 and judge relation between total energy radiated by star and its mass)

- Can EM radiation be responsible for Dark Energy? If you think about it, two stars that are emitting EM radiation will repel each other due to photon pressure (some of the photons from one star will hit other star and vice versa, transferring some of momentum). Our universe is filled with stars. Do they all push off each other by shining EM radiation, and what would be estimate of intensity of such interaction (hint: star size, flux distance relation due to spherical propagation of EM from star)

 

[andrew s]

28 minutes ago, vlaiv said:

It is safe to say that more than 99% of all regular matter in usual galaxy comes in form of luminous matter - ie stars.

Not sure this is true. Do you have a reference?

According to to Sandres in "The Dark Matter Problem" CUP  on page 91  "... in the rich cluster [of galaxies] the total mass of the hot X-ray emitting gases ranges from 10^13 solar mass to more than 10^14 solar mass. This typically exceeds the mass of the stars in galaxies by a factor of three or four."

The discovery of this gas reduced the missing mass from a factor of 100 found by Zwicky to about a factor of 6 but still not enough to remove the need for dark matter.

Regards Andrew

PS I can recommend the book as it has little mathematics and describes the issues well.

[andrew s]

47 minutes ago, vlaiv said:

Now, since you are interested in all of this, here is a bit of food for thought:

- Can EM radiation be responsible for Dark Matter? According to GR, any energy present will bend space-time, and there is a lot of electro-magnetic radiation bouncing off around the galaxy - each star is emitting plenty, all over the spectrum (hint: you can estimate total star output by using simple e=mc2 and judge relation between total energy radiated by star and its mass)

- Can EM radiation be responsible for Dark Energy? If you think about it, two stars that are emitting EM radiation will repel each other due to photon pressure (some of the photons from one star will hit other star and vice versa, transferring some of momentum). Our universe is filled with stars. Do they all push off each other by shining EM radiation, and what would be estimate of intensity of such interaction (hint: star size, flux distance relation due to spherical propagation of EM from star)

Personally I don't see how these speculations as help full. With O grade physics I doubt they could be meaning fully addressed.

The simple answer to both is no. Don't miss any sleep pondering them.

Regard Andrew

[Beagleboy]

Aha! That's kind of what I was wondering about Andrew. I appreciate that the Dark Matter model is still the best fit scenario as we understand things at the moment. I was just curious as to how much of a gap there was between the Dark Matter model and the 'we just haven't spotted all the stuff out there model'.

 

[andrew s]

1 minute ago, Beagleboy said:

I was just curious as to how much of a gap there was between the Dark Matter model and the 'we just haven't spotted all the stuff out there model'.

 

I am sure there is more to find but we now have telescopes that cover the full electromagnetic spectrum so there are not many places for normal matter to hide. The X-ray emitting gas seems to be the last big chunk. 

Regards Andrew 

[vlaiv]

22 minutes ago, andrew s said:

Not sure this is true. Do you have a reference?

According to to Sandres in "The Dark Matter Problem" CUP  on page 91  "... in the rich cluster [of galaxies] the total mass of the hot X-ray emitting gases ranges from 10^13 solar mass to more than 10^14 solar mass. This typically exceeds the mass of the stars in galaxies by a factor of three or four."

The discovery of this gas reduced the missing mass from a factor of 100 found by Zwicky to about a factor of 6 but still not enough to remove the need for dark matter.

Regards Andrew

PS I can recommend the book as it has little mathematics and describes the issues well.

I was referring to "ordinary" galaxy and mass distribution within, but point well taken, quite rich source of regular matter in gasses in intergalactic space.

17 minutes ago, andrew s said:

Personally I don't see how these speculations as help full. With O grade physics I doubt they could be meaning fully addressed.

The simple answer to both is no. Don't miss any sleep pondering them.

Regard Andrew

Well, indeed not helpful to topic, but as a food for thought I believe interesting. I was trying to further spark interest in subject and general discussion on topic. Not limited to O grade or any other level of physics knowledge (I consider my self to have none in such scheme of things, level I mean).

First one indicates that there is more than mass that produces gravity, such as temperature of object, its rotational speed, any radiation present, and it is interesting to address those and see how and to which level they impact gravity and after all of them are accounted for why we still need and consider dark matter.

Second one is intended to spark interest in "larger" picture - cosmological models and how "dark" things (meaning energy and matter) fit together.

[andrew s]

Ok taking your speculations at face value I would comment as follows.

Radiation, matter (ordinary and dark) and dark energy have different impacts on the expansion scale factor under the LCDM model.

After the big bag and up to about 47,000 years relativistic particles (photon and neutrinos) dominated and the scale factor a(t) scales as  t^1/2. Dominated means had the highest mass/energy density.

From 47,000 to 9.8 billion years matter dominated and a(t) scales as t^2/3 

From 9.8 billion years on a(t) scales as exp(Ht) and dark energy dominates.

These are different due to the different "equations of state" of radiation, matter and dark energy if dark matter and dark energy had their origins in EM radiation this would not be the case.

Regards Andrew

 

PS for completeness the equation of state is characterised by w in cosmological models. For relativistic particles w =1/3, non relativistic matter w = 0

and dark energy w = -1.  -1 is the best current measure of w from the CMB. Pointing to the cosmological constant being the source of dark energy.

[George Jones]

6 hours ago, Beagleboy said:

Aha! That's kind of what I was wondering about Andrew. I appreciate that the Dark Matter model is still the best fit scenario as we understand things at the moment. I was just curious as to how much of a gap there was between the Dark Matter model and the 'we just haven't spotted all the stuff out there model'.

 

In addition to the good points that Andrew has made, there are other lines of evidence for dark matter.

First, a digression that does not involve dark matter. Consider gas infalling on a star, neutron star, or black hole. As it spirals in, the gas heats up why? Friction caused by electric/electromagnetic forces.

Now consider the hot, dense soup of stuff in the very early universe. Some parts of this primordial soup are slightly more/less dense than other parts. These inhomogeneities set up gravity/pressure oscillations. In an overdensity, the region collapses because of gravity. As collapse proceeds, matter heats up until pressure becomes large to counteract gravity, after which the regions starts to expands. as it expands, it cools, pressures drops, gravity wins, and the region starts to collapse. And so on. Both normal matter and dark matter contribute to the gravity part, but only normal matter contributes to the pressure part, since dark matter is electromagnetically inert.

The effects of these oscillations are imprinted on the the observed Cosmic Microwave Background radiation as very small temperature variations. Different amounts of the various components of the universe will affect these oscillations. Statistical analysis of the observed angular size of these temperature variations in the CMB give

68% dark energy, 26% normal matter, and 5% normal matter.

Note that this argument is independent of the amount of normal matter that remains unseen by our present instruments.

[Stub Mandrel]

I've just read 'A Brief History of Time' - it felt awfully out of date, and his prediction that a Grand Unified Theory to unite gravity with the other three forces would be found by 2000 was a bit off...

Amazing how things have moved on and around, though.

[vlaiv]

8 hours ago, andrew s said:

Ok taking your speculations at face value I would comment as follows.

Radiation, matter (ordinary and dark) and dark energy have different impacts on the expansion scale factor under the LCDM model.

After the big bag and up to about 47,000 years relativistic particles (photon and neutrinos) dominated and the scale factor a(t) scales as  t^1/2. Dominated means had the highest mass/energy density.

From 47,000 to 9.8 billion years matter dominated and a(t) scales as t^2/3 

From 9.8 billion years on a(t) scales as exp(Ht) and dark energy dominates.

These are different due to the different "equations of state" of radiation, matter and dark energy if dark matter and dark energy had their origins in EM radiation this would not be the case.

Regards Andrew

 

PS for completeness the equation of state is characterised by w in cosmological models. For relativistic particles w =1/3, non relativistic matter w = 0

and dark energy w = -1.  -1 is the best current measure of w from the CMB. Pointing to the cosmological constant being the source of dark energy.

In terms of cosmological models how would you explain energy conservation (it is integral part of equations)?

For example, what happens to the energy of red shifted EM? Where does the energy needed to overcome gravity potential energy come from?

[andrew s]

1 hour ago, vlaiv said:

In terms of cosmological models how would you explain energy conservation (it is integral part of equations)?

The current cosmological models are based on General Relativity and in general energy is not conserved. Energy conservation is a consequence of time reversal symmetry and this is not the case in a curved space-time.

1 hour ago, vlaiv said:

Where does the energy needed to overcome gravity potential energy come from?

It was thought that gravity would eventually halt the expansion and the expansion would reverse. However, dark energy is currently the driver of the expansion. If dark energy is due to a cosmological constant (best current view) then the energy density is constant per unit volume and so increases with the expansion - an example of the non-conservation of energy in GR.

Regards Andrew

PS GR has many non-intuitive features especially when applied over the vast distances of the cosmos and as with QM a lot of what we thought we understood does not simply apply, even if that understanding included SR.

[vlaiv]

1 hour ago, andrew s said:

The current cosmological models are based on General Relativity and in general energy is not conserved. Energy conservation is a consequence of time reversal symmetry and this is not the case in a curved space-time.

Time reversal or time translation?

I found a reference to this question with a bit of explanation, where it is stated that indeed there is energy conservation regardless of curved space-time.

https://physics.stackexchange.com/questions/259759/conservation-of-energy-vs-expansion-of-space

 

[Ruud]

Dark matter explains so much, and we need it just about everywhere,
but we can't find it here! To me that is the strangest thing about it.

It's avoiding us. It is very shy.

[andrew s]

12 hours ago, vlaiv said:

Time reversal or time translation?

I found a reference to this question with a bit of explanation, where it is stated that indeed there is energy conservation regardless of curved space-time.

https://physics.stackexchange.com/questions/259759/conservation-of-energy-vs-expansion-of-space

 

It is normally referred to as time reversal symmetry. Translational symmetry (in space) leads to the conservation of momentum.

I agree it can be argued either way as in the stackexchange reference. This link is to a more complete discussion http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

As you will see it is not a simple issue.  

Curved space time leads to, in general, multiple answers to the same question.

As an example consider two observers A and B on opposite sides of the equator of a sphere both traveling North at x km/s what is their relative velocity?

To do the sums you translate the vectors, putting them head to toe, flip one and look at the resultant vector.

So far so good.

Case 1. So take A's vector and translate it around the equator (keeping it pointing North) until it gets to B. They are both pointing N of length x km/s so inverting one and subtracting gives 0 km/s as their relative velocity.

Case 2. Take A's vector and this time translate it via the great circle through North keeping it pointed along the great circle. This time when you get to B it will be pointing south! So when you do the vector sum you get a relative velocity of 2x km/s.

Now from our extrinsic view we can see what is going on but if you were embedded in the curved spacetime your intrinsic view would be that vector addition is path dependent. 

Regards Andrew

 

[andrew s]

11 hours ago, Ruud said:

Dark matter explains so much, and we need it just about everywhere,
but we can't find it here! To me that is the strangest thing about it.

It's avoiding us. It is very shy.

So it seems. One problem is that the expected amounts in the solar system are quite small ( see here https://darkmatterdarkenergy.com/2013/08/30/dark-matter-in-the-solar-system-does-it-matter/ ) and so make it's detection on earth very difficult.

Regards Andrew

[Stub Mandrel]

For me, the question is:

Quantum variations in the pre-inflationary universe resulted in a very uneven distribution of 'normal' matter, stars, galaxies, etc. etc.

While dark matter appears to be concentrated in galaxies, from its effects it appears not to be concentrated into smaller units.

Why are there not the equivalent of stars, planets and black holes made of dark matter?

 

Presumably something (dark energy?) can interact with dark matter and oppose gravity to stop it collapsing into massive objects.

 

To me this suggests a fifth fundamental force with a strength between gravity and the other three known forces and a similarly arranged force distance relationship (perhaps cube of distance?)

However this is based on intuition not calculation so i am sure it is deeply wrong...

[andrew s]

2 hours ago, Stub Mandrel said:

Why are there not the equivalent of stars, planets and black holes made of dark matter?

 

George Jones answered this above. To form compact objects like stars matter needs to lose energy and momentum and ordinary matter does this via the electromagnetic interaction (it radiates).

As dark matter can't do this by definition it can't form compact objects in the same way as ordinary matter. It's not that it can resist gravity it just just moves on by. 

Regards Andrew

[Stub Mandrel]

40 minutes ago, andrew s said:

George Jones answered this above. To form compact objects like stars matter needs to lose energy and momentum and ordinary matter does this via the electromagnetic interaction (it radiates).

As dark matter can't do this by definition it can't form compact objects in the same way as ordinary matter. It's not that it can resist gravity it just just moves on by. 

Regards Andrew

So two dark particles would attract each other, orbiting each other without radiating anything away from them, faster and faster and smaller and smaller until they form a micro-black hole.

Lots of such particles should clump to form rotating black holes.

[andrew s]

2 minutes ago, Stub Mandrel said:

So two dark particles would attract each other, orbiting each other without radiating anything away from them, faster and faster and smaller and smaller until they form a micro-black hole.

Lots of such particles should clump to form rotating black holes.

No they would either pass each other or form a stable orbit depending on the exact dynamics.

Regards Andrew 

[Stub Mandrel]

1 hour ago, andrew s said:

No they would either pass each other or form a stable orbit depending on the exact dynamics.

Regards Andrew 

Exactly - why don't increasing numbers form stable orbits about each other, attracting more dark matter, further collapsing until... black hole? 

The answers I find seem to boil down to 'they don't because they don't'.

Would dark matter black holes emit Hawking radiation?

Not emitting EM radiation wouldn't stop dark matter from being trapped in a Black hole if it fell in, and if it did it would contribute to the black hole's mass.

[andrew s]

1 hour ago, Stub Mandrel said:

Exactly - why don't increasing numbers form stable orbits about each other, attracting more dark matter, further collapsing until... black hole? 

The answers I find seem to boil down to 'they don't because they don't'.

Would dark matter black holes emit Hawking radiation?

Not emitting EM radiation wouldn't stop dark matter from being trapped in a Black hole if it fell in, and if it did it would contribute to the black hole's mass.

Neil, you will have to do some research to answer these questions. These are my opinions based to no additional research.

The density of dark matter is low and although cold dark matter is moving at non relativistic speeds the chance of a third particle adding to an orbiting pair is low as it is just as likely to disrupt them. It is hard to get normal matter to aggregate due to the conservation of angular momentum even with radiative cooling.

For dark matter to fall into a black hole it would have to be a direct hit on the event horizon. It can't spiral in as normal matter will do due to viscosity and radiative cooling. If it did fall in yes it would add to it's mass.

If dark matter black holes exist then I don't see why they should not emit Hawking radiation if the prediction is correct. 

Regards Andrew

 

[laowhoo]

4 hours ago, andrew s said:

No they would either pass each other or form a stable orbit depending on the exact dynamics.

Regards Andrew 

 

The Bullet Cluster?

https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoking-gun-for-dark-matter/

 

[Stub Mandrel]

It must have some ability to clump or it wouldn't be associated with galaxies like that...