Are there multiple theories for the origin of dark matter and dark energy?

[Mike2]

thanks Nereid! this link you gave
http://www.astro.umd.edu/~ssm/mond/mondvsDM.html

is also very helpful. I had not realized how successful MOND was in predicting other things. It is even more impressive than I thought. So I agree all the more with your "certainly interesting" assessment.

However am still leery because can't imagine any underlying theory

I have a sneaking suspicion that MOND is due to an as yet unknown affects of GR. This could be gravitational redshifting of rest mass, or perhaps gravitational redshifting of gravitons, or perhaps the affect of the gravitational effects of the universe as a whole as the universe becomes less dense.

 

[turbo]

MOND is pretty interesting, and it points out that we may not understand gravity as well as we had hoped. One consequence is that we may once again have to regard gravitation as a force acting over a distance through a medium. GR says that mass curves space-time, and orbits are merely paths along momentum-conserving geodesics, which may be true, but we may at some point have to reconsider that concept, if dark matter remains as elusive as ever and MOND continues to make accurate predictions regarding the behavior of galaxies. If MOND survives the next few years, we may also have to reconsider whether gravitational mass and inertial mass are truly equivalent. Exciting times.

 

[marcus]

MOND is pretty interesting, and it points out that we may not understand gravity as well as we had hoped. One consequence is that we may once again have to regard gravitation as a force acting over a distance through a medium. GR says that mass curves space-time, and orbits are merely paths along momentum-conserving geodesics, which may be true, but we may at some point have to reconsider that concept, if dark matter remains as elusive as ever and MOND continues to make accurate predictions regarding the behavior of galaxies. If MOND survives the next few years, we may also have to reconsider whether gravitational mass and inertial mass are truly equivalent. Exciting times.

yes turbo exciting times
let's be cautious about giving up on GR (and falling into the arms of action at a distance) because it could be that a small modification of the geometric approach will achieve the results of mond.

you might look at the second half of page 15 of
http://arxiv.org/gr-qc/0406100

the game is, can you get mond-like effects by bending the tangent space (where energy and momentum and acceleration are born) a little.
well that is putting it too vaguely, but you can see what Florian Girelli says in the paper.

 

[Nereid]

The August 2002 issue of Scientific American has an article by Milgrom ("Does Dark Matter Really Exist?"). Here's what he had to say about GR and cosmology in that article (extracts only):

"Successful as it may be, MOND is, at the moment, a limited phenomenological theory. [...] And MOND is limited because it cannot yet be applied to all the relevant phenomena at hand. The main reason is that MOND has not been incorporated into a theory that obeys the principles of relativity, either special or general. [...] The phenomena that fall outside the present purview of MOND are those that involve, on the one hand, accelerations smaller than a₀ (so that MOND plays a role) and, on the other, extreme speeds or extremely strong gravity (so that relativity is called for). [...] Light propogating in the gravitational fields of galactic systems [satisfies] both criteria. MOND cannot properly treat this motion, which pertains to gravitational lensing. [...] A second system which requires MOND and relativity is the universe at large. It follows that cosmology cannot be treated in MOND.[/color]"

 

[marcus]

The August 2002 issue of Scientific American has an article by Milgrom ("Does Dark Matter Really Exist?"). Here's what he had to say about GR and cosmology in that article (extracts only):

"Successful as it may be, MOND is, at the moment, a limited phenomenological theory. [...] And MOND is limited because it cannot yet be applied to all the relevant phenomena at hand. The main reason is that MOND has not been incorporated into a theory that obeys the principles of relativity, either special or general. [...] The phenomena that fall outside the present purview of MOND are those that involve, on the one hand, accelerations smaller than a₀ (so that MOND plays a role) and, on the other, extreme speeds or extremely strong gravity (so that relativity is called for). [...] Light propogating in the gravitational fields of galactic systems [satisfies] both criteria. MOND cannot properly treat this motion, which pertains to gravitational lensing. [...] A second system which requires MOND and relativity is the universe at large. It follows that cosmology cannot be treated in MOND.[/color]"

The main reason is that MOND has not been incorporated into a theory that obeys the principles of relativity[/color]

this is a useful perspective (from the standpoint of August 2002) because it highlights what is new in the following 2004 papers:

http://arxiv.org/gr-qc/0406100

The prospect of building mond-like effects into a GR-type theory
is what they are talking about on the second half of page 15 and in the last 3 or 4 lines of the conclusions paragraph at the end.

See also another 2004 link I just gave a few posts back.
http://ws2004.ift.uni.wroc.pl/html.html

or pages 3 and 4 of
http://arxiv.org/hep-th/0406276
where mond is again approached from a cosmological constant direction.

At this point we are, i believe, merely talking about some coherent lines of investigation being taken by a few people---the 3 authors of one paper and the 2 authors of the other and possibly others I don't know of. Oh yes a couple of physicists at Monpellier too. Just a handful. And this line of investigation could not pan out! If-you-want-to-be-sure-of-results-go-somewhere-else-sort of thing.

The significance is what your August 2002 quote points out, the need to build mond into a GR-type theory, which is precisely what these people see as a promising line of research.

 

[Nereid]

Thanks marcus, I should have acknowledged your posts (and the papers); perhaps I was too focussed on passing on what the founder of MOND had to say about these two aspects. Good to see that much has happened in only two years!

 

[marcus]

Thanks marcus, I should have acknowledged your posts (and the papers); perhaps I was too focussed on passing on what the founder of MOND had to say about these two aspects. Good to see that much has happened in only two years!

Dear Nereid, you are generous. I wish that much had happened in this department, and we would both be very pleased. But IMO the most that can be said is that this is a line of investigation which appears to a handful of people to show promise. As far as I know there is little else to report. Perhaps if you check the links out yourself you may see something substantial which I missed.

The quote from Milgrom, the urMONDer, is just what we need to establish a kind of baseline. They must incorporate mondy effects in a (nice) theory, which they have not done yet, and we have Milgrom's word for it.

 

[Chronos]

For the record, I despise dark matter and dark energy. I am very resistant to the idea the universe is mostly composed of 'invisible stuff'. And no, I don't have a better theory. MOND is ad hoc and I have a visceral distrust of ad hoc theories. Err, did I mention I think string theory is the most contrived waste of time in the history of science? Beat me with a cricket bat till I come to my senses.

 

[kurious]

Kurious:

MOND cannot explain gravitational lensing according to Milgrom.
So whatever causes MOND interacts weakly with photons?
Also Greywolf has said on these forums that if electromagnetism is taken into
account, the anaomalously high velocities in galaxies can be accounted for.

 

[marcus]

Kurious:

1.MOND cannot explain gravitational lensing according to Milgrom.

2.So whatever causes MOND interacts weakly with photons?

3.Also Greywolf has said on these forums that if electromagnetism is taken into account, the anaomalously high velocities in galaxies can be accounted for.
-----end quote----

1. Do you have something recent from Milgrom? All I have seen is something dated August 2002. Should we write Mordehai Milgrom email and ask him his current opinion? Also I don't know whether to consider him the authority on MOND (no matter what he says in 2004) since he is primarily known for having been the first to postulate it, quite some time ago. Is he still at the forefront? Personally I have no idea.

2. Whatever causes MOND interacts weakly with photons? "Whatever causes" would be some underlying physics incorporating the observed MOND effect, still to be discovered. Whatever underlying physics, if it and the effect are real, would probably fit the observed gravitational lensing as well.

3.

------------------

Dear Kurious, I urge you to have a look at Smolin's lecture slides on this.
I've given the link several times.
http://ws2004.ift.uni.wroc.pl/html.html
Go to "Lectures" and click on Smolin's third talk.

either mond is real and has some nice underlying physics, or not. we should be finding that out and hopefully soon.
there is a strange coincidence that points to the possibility of some nice physics, and it might be purely coincidence or it might not.

you ought to be aware of that coincidence, perhaps you are.

the coincidence is between the cosmological constant Lambda that governs the accelerating expansion
and the acceleration threshold of the MOND effect

if someone can either explain that coincidence, or show that it is purely coincidence and therefore insignificant, it would be a bit of progress

 

[Mike2]

The significance is what your August 2002 quote points out, the need to build mond into a GR-type theory, which is precisely what these people see as a promising line of research.

Do these "MOND"ifications you speak of mean altering the equations of GR a little with some more fudge factors added to Einstein's field equations? Or do they mean to apply the present GR equations in a different way?

It occurs to me that if the frequency changes of string vibrations alters mass due to gravitational effects, then you would see a breaking in symmetry between inertial mass and gravitational mass, as is done in MOND. The gravitational affects on mass would change, but it would seem that in the local frame of reference the inertial mass would not be affected by the change due to the distance to other massive objects.

 

[turbo]

Googling on gravity and pendulum, I found this paper regarding an interesting pendulum experiment. Does anyone here know if these results have been replicated anywhere?

From the paper below: "The results of Fourier analysis show, that the transferred first harmonic gravitational energy is much lower compared to the theoretical calculation that was based on Newtonian theory. The reason of this discrepancy is due to the significantly decreasing interaction force between the equal masses, which was proven by a proper phase analysis of the excitation. From an evaluation of the measured data, we have concluded that the energy transfer between equal masses (both being 24 kg) was less then seven percent of the theoretical value based on the Newtonian gravity model."

http://www.journaloftheoretics.com/Articles/3-6/Grav-pub.htm

 

[kurious]

Marcus:
the coincidence is between the cosmological constant Lambda that governs the accelerating expansion
and the acceleration threshold of the MOND effect

Kurious:
Milgrom pointed out originally that if
you divide the speed of light by the age of the universe
in seconds you get 10^-10 - the MOND threshold.
I think it is a good idea to contact Milgrom and see what he
has to say about MOND.


"proponents of MOND point out that some of the candidates for dark
matter such as WIMPs are as astonishing as asserting that gravity behaves differently over long distances than we normally think"

from an interesting overview of MOND at:
http://encyclopedia.thefreedictionary.com/Modified Newtonian dynamics

 

[marcus]

Kurious:
Milgrom pointed out originally that if
you divide the speed of light by the age of the universe
in seconds you get 10^-10 - the MOND threshold.
I think it is a good idea to contact Milgrom and see what he
has to say about MOND.

Marcus: yes but don't you think that is rather lame?
why would the age of the universe have anything to do with it?

the galaxies we see (and measure rotation curves for) come from
all different times in the history of the universe

they all have different "ages of the universe" so should they all
have different MOND acceleration threshholds? But no.

Please consider other possibilities. By now one would expect that in the ordinary course of events other people would have done more constructive things with Milgrom's suggestion than Milgrom did himself originally. If a suggestion is any good, people take it and go with it.

 

[marcus]

Dear Kurious,

the age of the universe is not a fundamental physical constant.

there are some fundamental physical constants, like the mass of the electron, and fine structure alpha, and Newton G, and the speed of light.

they are nice.

if there is a nice physical theory underlying the observed MOND effect then the approximate threshold will probably not be sharp, it will most likely be an acceleration marking the rough location where a smooth curve has a smooth change in behavior

that place, that threshhold, will be calculable from fundamental physical constants-----or it will itself be a fundamental constant----this is the way things have turned out in the past. It is the way physics is organized and how it has grown up.

When Milgrom saw this acceleration threshold he didnt have Lambda, so he punted.
Lambda the CC contains the length 9.5 billion lightyears----call it L.
If Lambda is a fundamental constant (to be one day listed with the others at the NIST website) then so is L.

If Milgrom had known about L in the 1980s, he would not have had to punt and make a wild seemingly rather shallow guess. he would have recognized, or someone would have told him,
that the acceleration is c²/L

then the age of the universe would not have been mentioned.

unfortunately nobody had a figure for the CC until 1998
that was the year cosmology totally changed.

anyway that's my personal take on it. I'd be interested to know what Milgrom says now, and i hope you write him email, but i would not
automatically take it as authoritative about MOND (who is an authority?
MOND is a fascinating flukey business)

 

[kurious]

This distance L is about 10^25 - 10^26 metres.
The Newtonian calculation I did earlier in this thread
gives a decceleration due to gravity of 10^-11 m/s^2
at this distance range.
This is of a similar order of magnitude to the acceleration
of supernovae due to dark energy at the same distance.
It is as though gravity has changed signs.This may be a trivial point or it may not.

 

[Nereid]

This distance L is about 10^25 - 10^26 metres.
The Newtonian calculation I did earlier in this thread
gives a decceleration due to gravity of 10^-11 m/s^2
at this distance range.
This is of a similar order of magnitude to the acceleration
of supernovae due to dark energy at the same distance.
It is as though gravity has changed signs.This may be a trivial point or it may not.

How would you go about determining whether it's trivial or not? What experiments or observations would you suggest that might help (in principle ones are perfectly OK)?

 

[marcus]

This distance L is about 10^25 - 10^26 metres.
The Newtonian calculation I did earlier in this thread
gives a decceleration due to gravity of 10^-11 m/s^2
at this distance range.
.

Hi Kurious, it is great to be doing calculations with you! Let me check
your figures. The relevant thing one wants to do, assuming both L and c are fundamental constants, is calculate the only acceleration which it is possible to calculate directly from a speed and a length, namely
c²/L

I assume this is what you calculated, so I will see if I get the same thing!

A lightyear is 9.46E15 meters and L is 9.5E9 lightyears
so L is 8.99E25 meters, but let us be relaxed and call it 9E25 meters.

Now squaring the speed of light gives a number like 9E16 and we have to divide that by the number 9E25 we got earlier for L. So we get

E-9 meter per second per second.

10^-9 meter per second per second

One nanometer per second per second.

But wait Kurious! this does not seem to be the same as what you got.

Have I made a mistake?

Or did you use some other method for calculating an acceleration?

did you by any chance use the Hubble parameter? Remember that this is not a constant and has changed radically in the course of a few billion years---one cannot use it as a constant for this type of thing.

Anyway please clue me in how you got something two orders of magnitude different.

 

[marcus]

How would you go about determining whether it's trivial or not? What experiments or observations would you suggest that might help (in principle ones are perfectly OK)?

Would it be OK, Nereid, to focus first of all on seeing how Kurious got the number 10^-11 before we talk about designing experiments?
the number itself strikes me as two orders of magnitude too small and may indicate some misunderstanding which we might clear up

 

[Nereid]

Would it be OK, Nereid, to focus first of all on seeing how Kurious got the number 10^-11 before we talk about designing experiments?
the number itself strikes me as two orders of magnitude too small and may indicate some misunderstanding which we might clear up

Well, kurious is free to answer either your post or mine, or Chronos' ... or free to ignore all of us

Personally, I like to see if folk who have questions and ideas can think through the implications of these for themselves (you're all totally shocked, right? Not in a million years did you think that Nereid had such thoughts! the shock! the horror!) ... and some may think I have a particular interest in doing experiments and making observations (I can't for the life of me think how anyone could form such an opinion, but, it's a free world).

 

[kurious]

The gravitational force acting on a particle of mass m, on the
surface of a sphere of radius 10^26 metres and with a mass of 10^52
kg is given by
G x10^52 m / (10^26) ^ 2

The acceleration is given by G x10^52 / (10^26) ^ 2 = 10^ - 11 m/ s^2
This calculation is valid because r > 10^25 metres so General Relativity won't give a significantly different value ( I checked with Ted Bunn moderator on sci.physics.research).
I have not done this with a calculator so the acceleration could be one order of magnitude bigger or smaller.
Now, Dark energy is at least 14 times more abundant than baryonic matter.
So if we did a similar Newtonian calculation for dark energy (assuming this is valid and dark energy is more normal than we think it is) we would expect
the magnitude of acceleration to be ten times larger
i.e 10^-11 x 10 = 10^-10 m/s^2 , the MOND threshold value.
The important questions here are:
why are the attractive and repulsive accelerations so similar?
why are they close to the MOND threshold value?

I have also noticed the following (could be coincidence -:

If the universe oscillates between a Big Bang and a Big crunch-
can two particles at opposite ends of it, be considered to be
undergoing simple harmonic oscillation?
If the potential energy of the oscillator is given by G m1 m2 /r and
m1 is the mass of the universe,10^52 kg,r = 10^26 metres - the current
size of the universe -then since the PE of a simple harmonic
oscillator is given by
PE = 1/2 k x^2, the force constant k becomes 10 ^ -37 m2.
using frequency of oscillator = ( k / m2 ) ^1/2,
frequency = ( 10^ -37m2 / m2 )^ 1/2 = 10^ - 18.5 per second.
In other words the universe oscillates every 10 ^ 18.5 seconds - about
its current age!
m2 must be a mass of a certain size - I have never calculated it.
Presumably the gravitational effects of dark energy and baryonic matter
would play a big role in generating the force constant of such an oscillator.

 

[Mike2]

It occurs to me that if the frequency changes of string vibrations alters mass due to gravitational effects, then you would see a breaking in symmetry between inertial mass and gravitational mass, as is done in MOND. The gravitational affects on mass would change, but it would seem that in the local frame of reference the inertial mass would not be affected by the change due to the distance to other massive objects.

Wait... just a moment... processing... Doesn't time slow down near heavy objects. So wouldn't a vibrating object appear to be vibrating slower near more dense matter of a galactic core relative to the lesser dense edges? So would it not be the case that strings would appear to be vibrating more quickly, and thus have more mass, the farther one gets from heavy object, such as the core of a galaxy? So then,... wait a moment... processing... wouldn't matter appear heavier at further distances from a galactic core? Or alternatively, wouldn't the inertial mass appear lighter than the gravitational mass (wrt core) so that it took more velocity to get the kinetic energy required to balance the gravitational energy?

 

[marcus]

The gravitational force acting on a particle of mass m, on the surface of a sphere of radius 10^26 metres and with a mass of 10^52
kg is given by G x10^52 m / (10^26) ^ 2

The acceleration is given by G x10^52 / (10^26) ^ 2 = 10^ - 11 m/ s^2
This calculation is valid because r > 10^25 metres so General Relativity won't give a significantly different value ( I checked with Ted Bunn moderator on sci.physics.research).
I have not done this with a calculator so the acceleration could be one order of magnitude bigger or smaller.
Now, Dark energy is at least 14 times more abundant than baryonic matter...

thanks, kurious. I see better now where you are coming from and why you got E-11 instead of E-9
or as you say 10^-11 instead of 10^-9

Now I hope you will address Nereid's question about designing an experiement to explore the implications of your calculation, since she so wishes. Sorry to take everyone's time----the two orders of magnitude troubled me.

Please note that the mass of 10^52 kilograms is somewhat arbitrary since the mass inside of any sphere of fixed size will change. Also the ratio of dark energy (if it exists) to baryonic energy is subject to change. for whatever reason you have chosen to calculate using quantities which are not generally considered fundamental constants---and are not assumed constant thru time AFAIK.

and why not nobody really knows what's constant and what isnt.

I had something different in mind, namely just using Lambda and c.
(dont even need G or hbar, just those two constants the cosmological and the speed of light). So we get different answers, as might be expected

 

[Chronos]

I am still curious about where the 10^52m came from. It still looks like numerology to me.

 

[kurious]

Note that the value of 10^26 metres in the oscillator would be the extension
from equilibrium.So if the period is 10^18.5 seconds, then the oscillator is nearly fully extended now and can go 2 x 10^26 metres in the other direction.Since the universe is currently thought to be about 3 x 10^26 metres in radius this is possible.
The implications of the oscillator would be that the universe never gets smaller than 10^25-10^26 metres.This is bad news from the point of view of it seeming to be a reasonable idea because it is in disagreement with inflation theory.One second after the big bang the universe had a temperature of 10^10 K according to Steven Weinberg. Since temperature is proportional to energy density,and the volume
of the universe depends on r^3 (and therefore temperature on 1/r^3), then if the temperature of the universe depends on microwave photons predominantly,the universe would have been about 10^24 metres in radius one second after the big bang
(I have allowed for the fact that the microwave photons had one thousand times more energy at the time of the Big Bang).I am not very sure of this calculation because
I don't really understand how general relativity affects cmbr photon energy and temperature when the universe gets smaller - I am using Newtonian and classical ideas to try and reach some reasonable conclusions.
In response to Nereid's questions, I would say this:
If the acceleration of supernovae due to dark energy is related to the decceleration due to Newtonian gravity, then there is a way forward if dark energy is considered to be repulsive gravity.A gravitational force carrier (if one exists) could be changing from one physical state to another and when it does this it could change from causing attraction to causing repulsion.
Low energy Higgs particles exert a negative pressure like dark energy but I don't know if this could be used to justify saying,for example,that a gravitational force carrier with a low energy could be repulsive and a high energy carrier could cause attraction.Themain problem with attributing gravitational repuslion to force carriers is that they are not supposed to have mass according to quantum mechanics and that would then raise the question of how the universe has enough mass to be geometrically flat which is what obsevation says it is.
I used the value of 10^52 kg in my last post because that seems to be the currently accepted baryonic rest mass of the universe.

 

[kurious]

MIKE2:
So would it not be the case that strings would appear to be vibrating more quickly, and thus have more mass, the farther one gets from heavy object, such as the core of a galaxy?

KURIOUS:

Wouldn't gravitons vibrate quicker (assuming gravitons can interact with gravitons )
and get bigger masses at the edge of a galaxy and so be able to cause a bigger gravitational force than expected?

 

[marcus]

Could anybody list (and write a few explanatory words about) the various theories which explain the origin of the dark matter and the dark energy?

Thanks!

this is the post that started this thread

it seems like a constructive request.

I expect Nereid, among other people, has the basic info on tap
to list the various offered explanations of dark energy/matter.

It would be nice to have a list---with a little thumbnail sketch of each proffered explanation----as salsero requests.

Can anyone help salsero out?

 

[marcus]

Personally, in my participation in this thread, I've been trying to get some consideration of one direction that "explaining" dark energy and matter has taken recently.

It may be that there are no dark energy and matter but, instead, there is a new fundamental constant (like Planck's constant, or the Newton G constant)

I have given links to some papers by various authors, a halfdozen or so,
modifications of SR to serve as flat limit of QG, subsuming mond and cosmological constant (and possibly pioneer anomaly)

recently gathered some links like this also (including a mond link from nereid) in Marlon's LQG thread

It seems to me that this possible explanation, which gets rid of Dark, is very high risk---but it is attracting people to work on it. So it is interesting, I find, to watch although unsettling like watching a highwire act at the circus.

In 1899 Planck just barely had the idea that his constant existed. he had not even published the 1900 paper about the radiation law! Alejandro has given a link to an 1899 paper of Planck containing the suspicion of a basic constant. this is how it is (as I see it) now.

there is a length. It might be 9.5 billion lightyears. Prodded by kurious, I have calculated that this is the same as 9E25 meters. this length is not to be confused with the Hubble length or the "radius of the observed universe" or any of that stuff, which is very different.
this length may be conjectured to be a fundamental constant in cosmology.
as if space were pre-stressed concrete and some one had measured the
inherent stress in it and this length was an indicator of the stress (I know that sounds fantastic or dumb, I want to convey the idea of a fundamental constant---one of the deep proportions intrinsic in nature)

If there is this length and it really is a fundamental constant, and it actually participates in certain laws, then (goes this highly risky speculation) there may not need to be any dark matter or dark energy.

nobody should feel threatened by this and feel they have to tear it to shreds and not let the children hear, or whatever, because it is merely a very very remote possibility that some (I think rather courageous) people are working on. If I hear any more i will keep you posted.

meanwhile, since Salsero asked, what are the various proposed explanations of what Dark Matter and Dark Energy (assume they exist) might be?

 

[Mike2]

MIKE2:
So would it not be the case that strings would appear to be vibrating more quickly, and thus have more mass, the farther one gets from heavy object, such as the core of a galaxy?

KURIOUS:

Wouldn't gravitons vibrate quicker (assuming gravitons can interact with gravitons )
and get bigger masses at the edge of a galaxy and so be able to cause a bigger gravitational force than expected?

Yes, I've thought about that as well, and now you've given me an opportunity to express those concerns too.

As I understand it, both gravitons and photons can have any energy level (the energy is not quantized) but mass is the result of quantized states of vibrations. This would "seem" to mean that photons and gravitons are subject to instantaneous corrections in energy to comply with local conservation of energy. They crawl a bit out of a well, and their energy decrease a bit to match that differential climb against gravity. But mass is quantized, so it doesn't respond to differential changes is energy (its energy is quantized). It must, however, respond to changes in geometry. Everything changes when geometry changes. And as I understand it, time is distorted by gravity so that frequencies (clocks) slow down near heavy object and speed up away from them. This would make the frequency of vibrating strings speed up away from mass. And if mass is a result of the frequency of vibrating strings, then mass would appear to be more massive when more distant from heavy objects.

Yet, I am not entirely convinced of anything at this point. I'm not sure how accompanying space distortions would contribute or contradict this scenario. So if any of you geniuses would like to work out the details, by all means, be my guest. Perhaps these things have already been worked out in some reference book somewhere. It's probably in the book I'm reading right now, but it will take me a month to get that far.

 

[Nereid]

this is the post that started this thread

it seems like a constructive request.

I expect Nereid, among other people, has the basic info on tap
to list the various offered explanations of dark energy/matter.

It would be nice to have a list---with a little thumbnail sketch of each proffered explanation----as salsero requests.

Can anyone help salsero out?

Good timely reminder marcus, thanks.

IIRC, I wrote a somewhat lengthy and tedious post (or posts) on what observations have lead to the consensus view that the universe really does contain an awful lot of 'dark matter'; as salsero asked about theories which 'explain' the origin of DM (and DE), I won't go into that here.

First just a fine point on terminology - I personally don't think that any theory in science 'explains' anything! Sure, it's often used as a shorthand, but I feel we should never forget what science is and what its limits are; a good scientific theory is one which is consistent with good observations and the results of good experiments, and makes specific, concrete predictions which, when tested, match what is observed. 'Explanations' are how we may choose to interpret successful theories. (note that there's much more to science than my few words; I just want to highlight one aspect).

So, what is 'dark matter'? Some astronomers feel there's no need for the concept at all - all the good data can be shown to be consistent with good theories that we already have.

MOND is such a theory - the observations are accounted for modifications to Newtonian dynamics.

Our own PF member Garth has his SCC theory in which dark matter is just ordinary baryonic matter; the observations which others feel indicate the existence of dark matter are accounted for in SCC by modifications to GR.

Other astronomers consider the observations to clearly indicate a lot of mass in a form that is non-luminous, but feel there's no need to introduce a new form of non-baryonic matter to account for the observations - sand, pebbles, rocks, highly ionised gas, boulders, planetismals, etc may be enough.

Finally, there are some who feel the universe is not expanding; they have radically different cosmological models from the concordance one; I have no idea how these folk account for the observations of dark matter.

So, what is this 'dark matter' for those who consider the observations point to lots of non-baryonic mass? Well, the observations don't constrain things much - beyond saying it is collisionless and massive. Some particle physicists are happy to tweak their post-Standard Model theories and suggest all manner of particles as the components of DM - axions, LSSPs, and more. In these models, the dark matter would have 'frozen out' of the expanding universe very early, and interacted only gravitationally ever since. Others have suggested exotica such as primordial black hole pairs (the mass of atoms?), even 'particles' that are millions of light-years in size.

AFAIK, there are only two sets of observations which lead some to postulate 'dark energy', the light curves of distant supernovae and combined cosmological observations (CMBR, large-scale structure, primordial nuclide abundances, and maybe more) - these last lead to 'dark energy' only through models (basically, you need dark energy in the models for them to match observations).

As with dark matter, there are some (including me) who feel that the data aren't well enough established yet to make a compelling case for DE; in particular, the SN data needs another 5+ years of analysis for some of us to be comfortable (oh, and a lot more SN data would help a great deal too ).

For theories, well, take your pick - cosmological constant, quintessence, and probably much more (IMHO, when data don't constrain theories much, creative theoreticians quickly come up with half a dozen good theories ... and that's just before breakfast!) marcus probably has a much better picture of this veritable zoo than I do. (Please don't misunderstand me; these theories are to be welcomed, they are an essential part of science).