Dark matter, dark energy

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.

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Mike2

Mike2 said:
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?

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marcus

Gold Member
Dearly Missed
kurious said:
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

Gold Member
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.

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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

Gold Member
Dearly Missed
salsero said:
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

Gold Member
Dearly Missed
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

kurious said:
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

Staff Emeritus
Gold Member
marcus said:
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).

marcus

Gold Member
Dearly Missed
In that case Nereid, if all the ideas of Dark (energy and matter) are so speculative that one may "take one's pick". Then I shall provisionally choose to consider the one that gets rid of Dark altogether.

One supposes that there a fundamental length constant (analogous to the fundamental speed constant c and the fundamental angular momentum hbar) and that by current estimates this constant is 9E25 meters----or if you prefer 9.5 billion light years. And that this constant affects the curvature of space and the action of gravity in the weak acceration limit.
this is one of the scariest intellectual steps I have ever considered taking.

marcus said:
...
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...
the point is that if L is 9E25 meters
then the Lamda they found from the supernova observations is
Lambda = 1/L2

and the mond threshhold acceleration found from galaxy rotation curves
is given by
c2/6L

and some rather brave people, I think, have recently said that this makes glimmers of sense to them because of a way that Special Rel can be deformed to have more than one invariant scale

and in doing the deformation, one generates predictions which may be testable by GLAST

so there is a deadline. these ideas must be worked out in time to make a prediction before GLAST goes into orbit.

Oh and the pioneer anomaly if there really was one would be covered by
that c2/6L too. Of course all this may be merely some coincidences, however eerie.

Nereid

Staff Emeritus
Gold Member
Nereid said:
kurious said:
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)?
kurious said:
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.
First, a small clarification: the observed acceleration isn't "due to supernovae", we simply use distant supernovae as a means of observing the distant universe; those observations suggest that the rate of expansion of the universe - and that includes the galaxies in which the SN live(d) - is increasing, and has been for the past several billion years.

Can you convert your word picture ("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") into numbers? e.g. at what rate would the carrier changes have to take place in order to make the observational data?

Without working out the details of some new theory of carriers (or anything else), what experiment or observation could you do - even in principle - that would tell you whether 'gravity had changed signs', or the accelerations are of the same magnitude (but opposite signs)?

Nereid

Staff Emeritus
Gold Member
marcus said:
In that case Nereid, if all the ideas of Dark (energy and matter) are so speculative that one may "take one's pick". Then I shall provisionally choose to consider the one that gets rid of Dark altogether.

One supposes that there a fundamental length constant (analogous to the fundamental speed constant c and the fundamental angular momentum hbar) and that by current estimates this constant is 9E25 meters----or if you prefer 9.5 billion light years. And that this constant affects the curvature of space and the action of gravity in the weak acceration limit.
this is one of the scariest intellectual steps I have ever considered taking.

the point is that if L is 9E25 meters
then the Lamda they found from the supernova observations is
Lambda = 1/L2

and the mond threshhold acceleration found from galaxy rotation curves
is given by
c2/6L

and some rather brave people, I think, have recently said that this makes glimmers of sense to them because of a way that Special Rel can be deformed to have more than one invariant scale

and in doing the deformation, one generates predictions which may be testable by GLAST

so there is a deadline. these ideas must be worked out in time to make a prediction before GLAST goes into orbit.

Oh and the pioneer anomaly if there really was one would be covered by
that c2/6L too. Of course all this may be merely some coincidences, however eerie.
:rofl:

It really depends on how far from the mainstream you wish to go!

And it's not just GLAST that will test many things; Gravity ProbeB will test Garth's SCC (and much else), LISA will test the Pioneer anomaly (and much else besides), SNAP will measure the acceleration of the rate of expansion to much higher accuracy (and much else), and so on.

To be sure we all understand here - the 'concordance model' is pretty good - it is consistent with lots of cosmologically relevant data (some say there is essentially no data which isn't, within experimental error); there are a number of quite independent sets of observations of dark matter - not even MOND accounts for all these (and it's the best alternative IMHO). wrt 'dark energy', I just don't see that the data are sound enough yet - from dust, to compositionally different behaviour of SNs, to systematic effects of many kinds ... so many things to tidy up first. While not out of the mainstream, the view that the SN data aren't firm enough yet is certainly a minority view!

kurious

The most frustrating thing about dark energy and dark matter is that they could be all around us here on Earth and yet we can't detect them!
Steven Weinberg has said "dark energy is the bone in the throat" for particle physicists
and cosmologists alike.But whatever it is, if we are to understand it, then it must have at least some physical propeties in common with all the other forms of matter we have an understanding of.At one time neutrinos were suspected of being dark matter but I think observation has ruled this out now.

salazar18

Dar matter and antimatter are real. Anti-atoms have been created/ dark matter. Read Hawking's book (History of Time) for possible solutions. I have a different theory which I will make abalable later.

kurious

NEREID:
Can you convert your word picture ("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") into numbers? e.g. at what rate would the carrier changes have to take place in order to make the observational data?

Without working out the details of some new theory of carriers (or anything else), what experiment or observation could you do - even in principle - that would tell you whether 'gravity had changed signs', or the accelerations are of the same magnitude (but opposite signs)?

KURIOUS:

If the force carriers for repulsion are increasing in number at the expense of the number of carriers for attraction, then stars should start to move away from the galactic centre and the Milky way, for example, should get bigger.Perhaps the Sun would become less luminous.Paul Dirac once suggested gravity was stronger in the past but the Sun would have burnt up too much fuel by now for this to be true.However this might not be a problem in the repulsive gravity scenario outlined above.

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kurious

One last thought on attractive gravitational force carriers becoming repulsive carriers:
if attractive carriers are bosons and they turn into fermions (in analogy to photons becoming positrons and electrons) then because fermions can't be in the same region of space,space must expand.However this would require bosons to collide with baryonic matter to conserve four-momentum and it would require that the particle-antiparticle pair does not become a boson again.Quantum mechanics says that the force carrier for gravity is spin 2 even if that carrier does not turn out to be a graviton.
This spin 2 could yield 4 spin 1/2 particles but I think someone on sci.physics.research said that spin doesn't necessarily have to be conserved for this kind of transformation,
so more particles may form.When the gravitational force carriers have all become fermions the universe would not need to expand anymore and fermions could come together to form bosons and the radius of the universe could decrease.The fermions would have to be moving close to the speed of light so that their energy density is similar to their pressure - dark energy is
expected to have this characteristic.Bosons moving at the speed of light and colliding with baryonic matter normally yield particle-antiparticles moving
close to light speed.
My idea of spin should not be confused with spintessence theory for dark energy which is different altogether.

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