Question about dark matter gravity and the expanding universe.

[YoungDreamer]

My question/thought is this...

We know gravity is related to matter, could dark matter have "anti-gravity"

Just like magnets have different polarities, one to attract and one to repel, could gravity have similar properties that we are unable to identify, perhaps related to types of matter or energy that we cannot identify, yet?

This is a question I thought of while pondering the mysteries of the expanding universe and I thought that gravity is the only force that can affect things on such a huge scale, but since it is expanding and not contracting I thought maybe there could be a type of gravity with an opposite "charge".

I also wondered if gravity could be the contributor so I will also ask could there have been other big bangs all throughout an infinite universe and our observable universe is being pulled by those other hyperclusters of galaxies, I don't know what else to call the sum of matter created by a big bang, and they are too far away for their light to reach us?

And if the gravity from something is reaching another object does that mean its light should reach it as well?
Meaning that gravity works at the speed of light?

 

[Janus]

My question/thought is this...

We know gravity is related to matter, could dark matter have "anti-gravity"

That would defeat the whole purpose that dark matter is supposed to serve. Dark matter provides the extra gravity needed to explain the rotation curves of galaxies. If it had anti-gravity it would have exactly the opposite effect.

 

[Cosmo Novice]

Just like magnets have different polarities, one to attract and one to repel, could gravity have similar properties that we are unable to identify, perhaps related to types of matter or energy that we cannot identify, yet?

So you are asking if gravity has currently undetectable effects on currently undetectable matter? I don't think you will get very far with that vein of questioning so you may want to reconstruct your question.

This is a question I thought of while pondering the mysteries of the expanding universe and I thought that gravity is the only force that can affect things on such a huge scale, but since it is expanding and not contracting I thought maybe there could be a type of gravity with an opposite "charge".

What do you mean by this? Remember that gravity is one of the weaker electromagnetic forces, gravity also doesn't have a "charge". While I understand what you mean, essentially repellant, gravity in GR is the curvature in space time and there is a clear distinction between gravity in GR and Newtonian gravity.

I also wondered if gravity could be the contributor so I will also ask could there have been other big bangs all throughout an infinite universe and our observable universe is being pulled by those other hyperclusters of galaxies, I don't know what else to call the sum of matter created by a big bang, and they are too far away for their light to reach us?

This can be ruled out by current observations and CMB uniformity. Essentially CMB is so uniform it is likely this is not the case, the idea of an infinite space to expand into is one explored in a Milne topological Universe and one that has a lot of observational evidence against it.

And if the gravity from something is reaching another object does that mean its light should reach it as well?
Meaning that gravity works at the speed of light?

Newtonian gravity assumes instantaneous gravitational "force", this can be seen using Newtonian mechanics on calculations of solar orbits - where without instantaneous propogation orbital mechanics fail. However in GR gravity is not a "force", in GR, the curvature in space/time created by gravity propogates at the speed of light. These may seem to contradict each other but they do not, in GR, the gravitational center is not central to the center of mass but takes into account velocity and position. Essentially the compounded result is that the propogation of gravity at light speed is almost canceled resulting in the same results as Newtonian mechanics.

Anyone that can expand on this would be greatly appreciated.

 

[longshot_nl]

I think the universe expands increasingly fast becouse on the outside of our universe is less matter, so time moves faster there.

And i think in contrary to popular thought that time and space and matter begun at a big bang, only time and matter begun at the big bang, space was already there.
Before the big bang, time went infinetely fast becouse there was no matter.
I see matter as a disturbance in space. If you think about it, what we percieve as 'nothing' has more complexity in it than matter. All kinds of waves go through it and once it hits matter the waves materialize like we see in the dual slit experiments.
Its like the 'emtyness' part of the universe is 1 membrane and the matter side of the universe is another and they bumped into each other in a higher dimension. The only things the two share is energy and time.. The time in the one domain of waves moves at a constant speed of light, the time in the domain of matter is constant.

I think dark matter is a calculating error. Sceintists wanted to account for 'missing matter'. There is no missing matter. I saw once on discovery channel that it was a calculating error and after that never more. But i believe that is the truth.

I am sure many won't agree with me but this is my view.

 

[Cosmo Novice]

I think the universe expands increasingly fast becouse on the outside of our universe is less matter, so time moves faster there.

And i think in contrary to popular thought that time and space and matter begun at a big bang, only time and matter begun at the big bang, space was already there.
Before the big bang, time went infinetely fast becouse there was no matter.
I see matter as a disturbance in space. If you think about it, what we percieve as 'nothing' has more complexity in it than matter. All kinds of waves go through it and once it hits matter the waves materialize like we see in the dual slit experiments.
Its like the 'emtyness' part of the universe is 1 membrane and the matter side of the universe is another and they bumped into each other in a higher dimension. The only things the two share is energy and time.. The time in the one domain of waves moves at a constant speed of light, the time in the domain of matter is constant.

I think dark matter is a calculating error. Sceintists wanted to account for 'missing matter'. There is no missing matter. I saw once on discovery channel that it was a calculating error and after that never more. But i believe that is the truth.

I am sure many won't agree with me but this is my view.

Many of the points you made are without foundation (at least not a referenced one you have provided) - in fact some of these statements are a little ridiculous.

If you can strengthen the points you made with accompanyting scientific papers - and articulate your points more clearly and concisely, making a clear distinction between ideas then you will receive better feedback. Essentially you have just flown in the face of accepted science without any credible argument, "I believe" does not count unfortunately.

Please do not take this as harsh criticism, as if you approach the site in the correct way there are many things you can learn and great discussions to be had. However you must consider the PF rules and promotion of personal theory is a breach of these.

Cosmo

 

[bapowell]

I am sure many won't agree with me but this is my view.

You should allow your views to be shaped by evidence, rather than prejudice. Ignoring empirical evidence in favor of preconceived ideas about nature is dangerous, and in my opinion, a terrible waste of a functioning intellect.

 

[longshot_nl]

Many of the points you made are without foundation (at least not a referenced one you have provided) - in fact some of these statements are a little ridiculous.

If you can strengthen the points you made with accompanyting scientific papers - and articulate your points more clearly and concisely, making a clear distinction between ideas then you will receive better feedback. Essentially you have just flown in the face of accepted science without any credible argument, "I believe" does not count unfortunately.

Please do not take this as harsh criticism, as if you approach the site in the correct way there are many things you can learn and great discussions to be had. However you must consider the PF rules and promotion of personal theory is a breach of these.

Cosmo

Thank you for your oppinion on my views.
Although my words seem like logic to me it didn't occur to me people like you want me to back up my words by other peoples sceintific papers. Or maybe you want me to make a sceintific paper for you?
If sceintists thought the same, nothing new would ever be discovered or developed.
I have done a great deal of thought about the subject and have done my reseach.
On the same note i haven't seen any link to sceintific papers here yet. But then i am here since yesterday.
Also i haven't seen many TV programs or youtube movies referring to sceintific papers.
In the future i will expain more clearly why i think the way i think it.
But i don't obey the rule 'shut up if you don't think the same as i'.

Einsteins theory of relativity states that time goes slower on places where there is more matter (energy). On the outside of the universe is less matter so time goes faster there.

 

[Ryan_m_b]

Thank you for your oppinion on my views.
Although my words seem like logic to me it didn't occur to me people like you want me to back up my words by other peoples sceintific papers. Or maybe you want me to make a sceintific paper for you?
If sceintists thought the same, nothing new would ever be discovered or developed.
I have done a great deal of thought about the subject and have done my reseach.
On the same note i haven't seen any link to sceintific papers here yet. But then i am here since yesterday.
Also i haven't seen many TV programs or youtube movies referring to sceintific papers.
In the future i will expain more clearly why i think the way i think it.
But i don't obey the rule 'shut up if you don't think the same as i'.

Einsteins theory of relativity states that time goes slower on places where there is more matter (energy). On the outside of the universe is less matter so time goes faster there.

There is no such rule, the way that science works is that people learn the subject first (this way they can demonstrate a good enough understanding of the topic for people to take them seriously as well as allowing them to come up with good hypotheses), develop a hypothesis about some observed phenomenon, test their hypothesis with experiments, publish the data.

It is not enough to misunderstand the current science, say "I think" and then complain when people point out your flaws.

P.S This site is packed full of links to scientific papers. Many of us here are scientists. I would advise you to check your facts before making claims, in all walks of life.

 

[longshot_nl]

There is no such rule, the way that science works is that people learn the subject first (this way they can demonstrate a good enough understanding of the topic for people to take them seriously as well as allowing them to come up with good hypotheses), develop a hypothesis about some observed phenomenon, test their hypothesis with experiments, publish the data.

It is not enough to misunderstand the current science, say "I think" and then complain when people point out your flaws.

P.S This site is packed full of links to scientific papers. Many of us here are scientists. I would advise you to check your facts before making claims, in all walks of life.

Ok i will not use the words 'I believe' or 'I think'.
And i will refer more to established sceince.

In all walks of life huh?
Where did you get evidence that i don't do that already in other walks of life.?
It must be hard to have a conversation if you have to check your facts every sentence and while you are double checking it, double check it elsewhere too (you said all walks of life).

Point taken: Will refer more to established sceince.
Point rejected: What i said is just a little ridiculous becouse i didn't refer to established science / papers.

But this thread isn't about me. go back to topic please.

 

[Ryan_m_b]

Ok i will not use the words 'I believe' or 'I think'.
And i will refer more to established sceince.

In all walks of life huh?
Where did you get evidence that i don't do that already in other walks of life.?
It must be hard to have a conversation if you have to check your facts every sentence (you said all walks of life).

Point taken: Will refer more to established sceince.
Point rejected: What i said is just a little ridiculous becouse i didn't refer to established sceince.

Yes in all walks of life it is best to have empirical evidence before making a decision, it is definitely best to say something only when you know there is evidence for it.

What you said was not just ridiculous because you did not show any evidence to support your non-mainstream ideas but because much of it conflicts with what has already been established by mainstream physics.

Science does not work by someone coming up with random ideas off of the top of their heads (even if their ideas do have internal consistency), it works off of what we already have established.

I think the universe expands increasingly fast becouse on the outside of our universe is less matter, so time moves faster there.

What does "outside of the universe" even mean? Current understanding in physics is that there is no such thing.

And i think in contrary to popular thought that time and space and matter begun at a big bang, only time and matter begun at the big bang, space was already there.

Time and space are one (hence "spacetime"), you cannot have one without the other. Physics has nothing to say about what occurred "before" the big bang or even if that makes any sense. If you are going to suggest something you are going to need evidence for it.

Before the big bang, time went infinetely fast becouse there was no matter.

Time does not "go" at all.

I see matter as a disturbance in space. If you think about it, what we percieve as 'nothing' has more complexity in it than matter. All kinds of waves go through it and once it hits matter the waves materialize like we see in the dual slit experiments.
Its like the 'emtyness' part of the universe is 1 membrane and the matter side of the universe is another and they bumped into each other in a higher dimension. The only things the two share is energy and time.. The time in the one domain of waves moves at a constant speed of light, the time in the domain of matter is constant.

This doesn't even make much sense. If you are proposing the existence of other universes you are going to need evidence that such things even exist before trying to discuss their properties.

I think dark matter is a calculating error. Sceintists wanted to account for 'missing matter'. There is no missing matter. I saw once on discovery channel that it was a calculating error and after that never more. But i believe that is the truth.

To back up that statement you need evidence from peer-reviewed literature. It is not enough to claim something without evidence.

 

[bapowell]

Point taken: Will refer more to established sceince.
Point rejected: What i said is just a little ridiculous becouse i didn't refer to established science / papers.

You have made several assertions that have no basis in scientific fact in a science forum. Seriously...you don't see the issue?

 

[longshot_nl]

Yes in all walks of life it is best to have empirical evidence before making a decision, it is definitely best to say something only when you know there is evidence for it.

As far as i know there is evidence. And i don't see anyone referring to papers proving the opposite. So its a bit silly people say how to do it and at the same time don't do it themselves. This is based on hard evidence in this thread.

What you said was not just ridiculous because you did not show any evidence to support your non-mainstream ideas but because much of it conflicts with what has already been established by mainstream physics.

Well, i am against establishment. I am all for evolving. Humans are evolving creatures not established and so should be their idears.

Science does not work by someone coming up with random ideas off of the top of their heads (even if their ideas do have internal consistency), it works off of what we already have established.

Yes it does. All evolution start with random events. Evolution of humans and other life forms by random changes to the DNA, evolution of universes and stars by random pull of matter into stars and planets and even science have evolved from Newton to einstein by random brainwaves and observations and will keep evolving. The events that don't work are rejected but that doesn't mean they should not have happened.
But no one here tells me exactly why it cannot be, referring to papers. But rather say i shouldn't have the idear at all and attack it on the words i am using and the fact that it isn't supported by established papers.
It seems there are two types of people, those who want to discover and evolve and those that want to keep the established established.

Please go back to topic.

 

[Ryan_m_b]

As far as i know there is evidence. And i don't see anyone referring to papers proving the opposite. So its a bit silly people say how to do it and at the same time don't do it themselves. This is based on hard evidence in this thread.

I refer you to the burden of proof, specifically the fact that the onus is on the claimant.

Well, i am against establishment. I am all for evolving. Humans are evolving creatures not established and so should be their idears.

Now you are being very, very ignorant. Everything around you is the product of the scientific method, it works by intelligent people taking what we know and pushing it further. This is done through research, observation, hypothesis, testing, conclusion. It is not accomplished by ignoring what scientists already have to say and spouting inconsistent, illogical ideas that go against mainstream science.

Yes it does. All evolution start with random events. Evolution of humans and other life forms by random changes to the DNA, evolution of universes and stars by random pull of matter into stars and planets and even science have evolved from Newton to einstein by random brainwaves and observations and will keep evolving. The events that don't work are rejected but that doesn't mean they should not have happened.
But no one here tells me exactly why it cannot be, referring to papers. But rather say i shouldn't have the idear at all and attack it on the words i am using and the fact that it isn't supported by established papers.
It seems there are two types of people, those who want to discover and evolve and those that want to keep the established established.

Cosmological development and biological evolution are nothing to do with the gradual improvement in our understanding of the world around us. As I have said the scientific method is based on intelligent exploration of the world through well performed, methodical experimentation. Not random musings or "brain waves", if you were trying to "discover" you would have read physics books on the subject and proposed ways of testing your hypothesis.

Some reading for you;

http://en.wikipedia.org/wiki/Time_dilation
http://en.wikipedia.org/wiki/Shape_of_the_Universe
http://en.wikipedia.org/wiki/Time#Time_and_the_Big_Bang

 

[WannabeNewton]

I think the universe expands increasingly fast becouse on the outside of our universe is less matter, so time moves faster there.

And i think in contrary to popular thought that time and space and matter begun at a big bang, only time and matter begun at the big bang, space was already there.
Before the big bang, time went infinetely fast becouse there was no matter.
I see matter as a disturbance in space. If you think about it, what we percieve as 'nothing' has more complexity in it than matter. All kinds of waves go through it and once it hits matter the waves materialize like we see in the dual slit experiments.
Its like the 'emtyness' part of the universe is 1 membrane and the matter side of the universe is another and they bumped into each other in a higher dimension. The only things the two share is energy and time.. The time in the one domain of waves moves at a constant speed of light, the time in the domain of matter is constant.

I think dark matter is a calculating error. Sceintists wanted to account for 'missing matter'. There is no missing matter. I saw once on discovery channel that it was a calculating error and after that never more. But i believe that is the truth.

I am sure many won't agree with me but this is my view.

My problem with this (and maybe others), aside from lacking evidence to back up the claims, is that you can't just put together physics terms from different frameworks and hope to come up with something coherent. If you can bring mathematics to the table then you can be taken more seriously. Statements like "The time in the one domain of waves moves at a constant speed of light" can't be plucked from thin air - there has to be some mathematical formulation of it; there is a reason why mathematics is the language of physics. Otherwise, such statements will come off as absurd as many have pointed out.

 

[bcrowell]

I think the universe expands increasingly fast becouse on the outside of our universe is less matter, so time moves faster there.

Please see the following FAQ: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN , "What is the Universe expanding into?" There is no "outside."

And i think in contrary to popular thought that time and space and matter begun at a big bang, only time and matter begun at the big bang, space was already there.
Before the big bang, time went infinetely fast becouse there was no matter.
I see matter as a disturbance in space. If you think about it, what we percieve as 'nothing' has more complexity in it than matter. All kinds of waves go through it and once it hits matter the waves materialize like we see in the dual slit experiments.
Its like the 'emtyness' part of the universe is 1 membrane and the matter side of the universe is another and they bumped into each other in a higher dimension. The only things the two share is energy and time.. The time in the one domain of waves moves at a constant speed of light, the time in the domain of matter is constant.

I think dark matter is a calculating error. Sceintists wanted to account for 'missing matter'. There is no missing matter. I saw once on discovery channel that it was a calculating error and after that never more. But i believe that is the truth.

I am sure many won't agree with me but this is my view.

The rest of this material is overly speculative.

One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. However, it is against our posting guidelines to discuss new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional scientific discussion.

The following are some online forums that specialize in discussing independent research, highly speculative ideas, or ideas that run counter to mainstream science:

http://groups.google.com/group/alt.sci.physics
http://www.bautforum.com/showthread...ATM-Idea-Advocates-Read-before-posting-in-ATM
http://groups.google.com/group/sci.physics.research -- Read this first: http://math.ucr.edu/home/baez/spr.html
http://www.sciforums.com
http://groups.google.com/group/rec.arts.sf.science

 

[zonde]

My question/thought is this...

We know gravity is related to matter, could dark matter have "anti-gravity"

Just like magnets have different polarities, one to attract and one to repel, could gravity have similar properties that we are unable to identify, perhaps related to types of matter or energy that we cannot identify, yet?

This is a question I thought of while pondering the mysteries of the expanding universe and I thought that gravity is the only force that can affect things on such a huge scale, but since it is expanding and not contracting I thought maybe there could be a type of gravity with an opposite "charge".

If you intended to ask question about expanding universe then it seems that you have mixed up dark matter and dark energy.
So if you would ask if dark energy could have "anti-gravity" then the answer would be that dark energy basically is "anti-gravity".

 

[zonde]

That would defeat the whole purpose that dark matter is supposed to serve. Dark matter provides the extra gravity needed to explain the rotation curves of galaxies. If it had anti-gravity it would have exactly the opposite effect.

That's not so. At least until you have tried to model dark matter this way and proved that this gives poor match for actual observations.

The thing is that it depends how you distribute sources of anti-gravity. If you place source of gravity and source of anti-gravity at opposite sides from point in question you would get increased gradient of field.

 

[Tanelorn]

Zonde, the observed, higher than predicted, rotational velocities of stars on the outsides of galaxies somehow requires a greater gravitational force towards the center of the galaxies otherwise they would move out to higher orbits or even be ejected altogether. So it seems to me that having a negative gravitational field outside the galaxy pushing these stars back in, might produce a similar effect as additional mass, but I am only a layman. I just scrapped a BSc in Physics and Electronics a long time ago, didnt even study GR at all as I recall. :)


http://en.wikipedia.org/wiki/Galaxy_rotation_curve

 

[YoungDreamer]

I tried to phrase my question without sounding like I was proposing unfounded ideas, but someone else came along and did it anyways.
With that being said I'm glad that we got back to my question, I could've phrased it better so that everyone would have seen what I was trying to say, but it seems like the last two posts are moving toward the direction I intended.

 

[alex_j]

Hello, I'm new here and a layman by any terms of the word with a very general knowledge of mathematics.

I have a few questions I was hoping some of you could help to answer. They are regarding dark matter and here they are together with some assumptions I have made that may not be correct.

As I understand it, dark matter is a name we gave to "something" that seems to be adding gravity to the galaxies and causing them to remain together in a higher rate of spin as they would have otherwise.

My first question is if this dark matter or "something" is in any way found to be somehow linked to gravity, by proportion or otherwise. As I understand it, this dark matter is a massless particle that adds gravity (?). I put this as a question because as I understand it, all the mass in the galaxy can be explained but what cannot be explained is the gravity associated with the galaxy so I assume that it is mass-less. If this is so, doesn't this seem a lot like the definition of a graviton?
How absurd would it be to imagine that this dark matter has something to do with the messenger that pertains to gravity?

My last question regarding dark matter is how large of a scale do we need to use before we can measure its effects. We already know that it can be measured at the scale of galaxies but can it be measured in smaller scales like individual stars? planets? moons? molecules? quantum level particles? More clearly, how large a scale do we need to go to before we see an anomaly in F=Gm1m2/r2?

Well that's about it, I hope I haven't embarrassed myself too much and thanks for any answers.

 

[Nabeshin]

As I understand it, this dark matter is a massless particle that adds gravity (?).

This is false. Dark matter is most certainly something with some mass (or at least energy). Now, whether it's WIMPs, MACHOs, or some other strange particle, all of these have mass.

My last question regarding dark matter is how large of a scale do we need to use before we can measure its effects. We already know that it can be measured at the scale of galaxies but can it be measured in smaller scales like individual stars? planets? moons? molecules? quantum level particles? More clearly, how large a scale do we need to go to before we see an anomaly in F=Gm1m2/r2?

Well that's about it, I hope I haven't embarrassed myself too much and thanks for any answers.

This depends on how well we can measure things! Theoretically, if we could chart the motion of all bodies larger than ~10^19kg in our solar system to the precision that including them in calculations actually yielded measurable changes, then we should be able to detect DM at a solar-system level. Of course, we are nowhere near being able to do this (and likely never will be).

Indirectly, one can probe DM by examining how stars form, but this isn't really a direct test. For direct, observable effects, you really need to go to star cluster scales at the smallest.

 

[alex_j]

Thank you very much for taking the time to reply Nabeshin, I appreciate it.

You have answered my question about size and measurements perfectly so that I understand it. But there is still one nagging question that remains regarding the first part of my question about dark matter and mass.

Please excuse my ignorance and simplicity but,,, if dark matter has mass, shouldn't it interact with other particles of mass?

 

[Nabeshin]

Please excuse my ignorance and simplicity but,,, if dark matter has mass, shouldn't it interact with other particles of mass?

Why would it imply this?

How do particles interact? With one of the four fundamental forces: gravity, weak, strong, and electromagnetic forces. We know it interacts gravitationally, that's how we found it. We know it DOESN'T interact electromagnetically (or if it does, so weak that we cannot detect it), this is why it's called dark! The other two forces, the strong and the weak, have much more to do with the particle physics involved in what it turns out to be. If a dark matter particle interacts only via the weak force, well then we rightly expect its interaction with matter to be very weak!

The thing to answer your second post is to just realize that dark matter, for all intents and purposes, couples only to normal matter (and itself, for that matter) through its gravitational interaction. That is, if I threw two blobs of dark matter at each other, they would pass right through each other. Or a blob of dark matter right through a normal wall. So, when two galaxies which have large halos of dark matter collide, the dark matter does just that: the halos pass right through each other. The galaxies, on the other hand, mesh together and experience a frictional force which tends to bring them together.

Recall now that the only link between DM and normal mass is gravity. So the normal matter tends to 'fall into' the gravity well created by dark matter (theoretically the dark matter could fall into a gravity well created by normal matter, but observationally we know there is much more dark matter so the first scenario is more likely).

The answer to basically any question regarding dark matter dynamics is found within the principles I've tried (hopefully with some measure of success!) to explain above.

Have I done a decent job at explaining? :)

 

[DavidMcC]

Nabeshin, the most confusing point about dark matter in the context of galaxy cluster collisions such as the Bullet is the stateent that DM "does not interact with itself". I have always taken this to mean that it does not interact gravitationally with itself. If this is the case, then surely both whimps and machos are falsified, as particles with mass cannot fail to be influenced by gravity. MOND is also falsified, by the gravitational lensing observations from other galaxy cluster collisions, since MOND, as a modification to Newton's 2nd law, does not generate its own gravity, AFAIK.

 

[bcrowell]

Nabeshin, the most confusing point about dark matter in the context of galaxy cluster collisions such as the Bullet is the stateent that DM "does not interact with itself". I have always taken this to mean that it does not interact gravitationally with itself.

No, there is no such thing as matter that doesn't interact gravitationally with itself. Anything with rest mass or energy contributes to the stress-energy tensor, which influences the curvature of spacetime. That curvature then influences the motion of any particle.

 

[Wim Nobel]

My question/thought is this...

We know gravity is related to matter, could dark matter have "anti-gravity"

Just like magnets have different polarities, one to attract and one to repel, could gravity have similar properties that we are unable to identify, perhaps related to types of matter or energy that we cannot identify, yet?

This is a question I thought of while pondering the mysteries of the expanding universe and I thought that gravity is the only force that can affect things on such a huge scale, but since it is expanding and not contracting I thought maybe there could be a type of gravity with an opposite "charge".

I also wondered if gravity could be the contributor so I will also ask could there have been other big bangs all throughout an infinite universe and our observable universe is being pulled by those other hyperclusters of galaxies, I don't know what else to call the sum of matter created by a big bang, and they are too far away for their light to reach us?

And if the gravity from something is reaching another object does that mean its light should reach it as well?
Meaning that gravity works at the speed of light?

I would like to reply to the original question.
It has already correctly remarked that dark matter still has normal positive mass, and can therefore not be a source of "anti-gravity". However, dark energy might have that property.
Although the formulas for electromagnetism and gravity are similar (proportional to strength of force source, inverse proportional to square of distance), the difference is that charge comes in distinct quanta, positive or negative, while mass is as far as we know only positive and no mass-quantum has been conjectured.

Before dark energy was discovered, the established model of the universe would be that it would either stop expanding and start contracting in the future (if there were enough mass to produce the necessary gravity), or continue braking without ever stopping to expand (if there were not enough mass). However observations indicate that the expansion is accellerating, something that can't be understood if gravity between ordinary (light or dark) matter is the only force influencing the large scale dynamics, as this would always be an attracting (braking) force.

Currently, dark energy is supposed to be the source of the acceleration, which means that it is causing a repulsion. But is this a repulsing variant of gravity? Since the gravity field of a massive object is given by GM/r^2, and G (gravitational constant) and r (modulus of the distance vector) are positive by definition, the only way to get a negative (repulsing) field is to enter a negative M (mass). But I'm not sure it works that way. Maybe the repulsion of dark energy should be calculated in an entirely different way and is not connected with gravity at all. Maybe it should be considered as an entirely different fundamantal force (can anybody please comment on this?).

Anyway, the phrase in the original message "but since it is expanding and not contracting" should be replaced by "since it is accelerating and not braking", because expansion by itself does not require a repulsive force.

I don't think hypothetical hyperclusters of galaxies could solve this problem, because they would cause a local acceleration in their vicinity, burt still contibute to the braking of the universe as a whole.

Finally, the propagation speed of a gravity field is certainly equal to that of an electromagnetic field, i.e. the speed of light. Although gravity can be seen as a distortion of the curvature of space, there is an alternative description that makes use of gravitons, particles that carry the force just like photons carry the electromagnetic force. I don't know why but experts seem to be pretty confident that these particles are massless, and therefore must travel with speed of light.

 

[JonDE]

I'd also like to expand on what the above poster has asked, maybe to help clear up some of the confusion (for myself and others). I have heard dark energy referred to in different ways, some as those it is actually energy of a type, such as I have heard that dark energy accounts for 73% of the energy-mass in the universe. But then I have heard it refer to as more of a law of physics, but when they do this generally they start calling it a cosmological constant, kinda like they are saying the universe is trying to expand at a certain rate, but gravity is holding it back.
If anyone could help me clarify my confusion on this as I don't see how the two can be the same.

 

[bapowell]

Currently, dark energy is supposed to be the source of the acceleration, which means that it is causing a repulsion. But is this a repulsing variant of gravity? Since the gravity field of a massive object is given by GM/r^2, and G (gravitational constant) and r (modulus of the distance vector) are positive by definition, the only way to get a negative (repulsing) field is to enter a negative M (mass). But I'm not sure it works that way. Maybe the repulsion of dark energy should be calculated in an entirely different way and is not connected with gravity at all. Maybe it should be considered as an entirely different fundamantal force (can anybody please comment on this?).

In the weak field limit of General Relativity in the presence of a cosmological constant (or dark energy), $\Lambda$, one has for the Newtonian potential $\Phi$:

$$\nabla^2 \Phi = -\nabla g = 4\pi G\rho - \Lambda$$
where g is the gravitational acceleration. This gives
$$g=-\frac{GM}{r^2} + \frac{\Lambda r}{3}$$
and you can see the $\Lambda$ is a repulsive contribution.

 

[bapowell]

I'd also like to expand on what the above poster has asked, maybe to help clear up some of the confusion (for myself and others). I have heard dark energy referred to in different ways, some as those it is actually energy of a type, such as I have heard that dark energy accounts for 73% of the energy-mass in the universe. But then I have heard it refer to as more of a law of physics, but when they do this generally they start calling it a cosmological constant, kinda like they are saying the universe is trying to expand at a certain rate, but gravity is holding it back.
If anyone could help me clarify my confusion on this as I don't see how the two can be the same.

Dark energy is the name of the stress-energy that causes accelerated expansion: $\ddot{a}>0$. Dark energy must satisfy $p < -\rho/3$, i.e. have a sufficiently negative pressure. The cosmological constant, $\Lambda$, satisfies $p = -\rho$, and so can be thought of as a special case of dark energy. It has constant energy density $\dot{\rho} = 0$.

 

[Tanelorn]

Can dark energy be viewed as a fundermental characteristic of space itself which causes space to expand at a certain rate? Perhaps in a similar way to space having certain characteristics which causes light to travel at the speed of light?

 

[bapowell]

Can dark energy be viewed as a fundermental characteristic of space itself which causes space to expand at a certain rate? Perhaps in a similar way to space having certain characteristics which causes light to travel at the speed of light?

I would hesitate to describe it that way. Dark energy, as a source of stress-energy, is no different than radiation or dust, in that it is a density that occupies space, i.e. it occupies the RHS of Einstein's Equations. If the origin of dark energy is indeed quantum vacuum energy, then it is in some sense a fundamental ingredient, but I still wouldn't qualify it as a fundamental characteristic of spacetime itself.

 

[Wim Nobel]

$$g=-\frac{GM}{r^2} + \frac{\Lambda r}{3}$$

Thanks for clarifying this. I knew the cosmological constant had something to do with it, but had no idea the formula was as simpe as that. I guess those papers are so full of formulas that I couldn't grasp the essence.

However, two questions remain unsolved for me.

1. Why do we regard the gravitational attraction and the cosmological repulsion as aspects of one and the same force (field)? Or, to put it otherwise, would it be alright if I would say that there is a fundamental force called gravitation that is always attracting, proportional to mass and inversely proportional to distance squared, and that there is another fundamental force called the cosmological force that is always repulsive and proportional to the distance? If not, then why not add the formula for electrical attraction and repulsion (and those for strong and weak forces) as well to this sum?

2. What does it mean that the universe consists of 27% (light and dark) matter and 73% vacuum energy? From the formula, a comparison between the two terms only makes sense if a particular value of r is chosen. And the only reasonable value I can think of to choose would be the radius of the visible universe. But still, how do we know these proportions? We can measure (very unaccurately) the acceleration in the distant universe and derive a value for lambda from there. And we might estimate all gravitating matter by observing the dynamics of clusters etc. Is this the way this calculation is done? I can't imagine we can acquire an accurate result from this.

 

[bcrowell]

Finally, the propagation speed of a gravity field is certainly equal to that of an electromagnetic field, i.e. the speed of light. Although gravity can be seen as a distortion of the curvature of space, there is an alternative description that makes use of gravitons, particles that carry the force just like photons carry the electromagnetic force. I don't know why but experts seem to be pretty confident that these particles are massless, and therefore must travel with speed of light.

You don't have to resort to quantum gravity to understand why small-amplitude gravitational disturbances propagate at c. (We don't have a working theory of quantum gravity, and there are fundamental reasons to believe that gravitons can never be directly detected, even by fairly godlike means.) The Einstein field equations are a classical field equation, and they predict that small-amplitude disturbances propagate at c.

The reason we're confident that the graviton, if it exists, must be massless is because it would be the quantization of a classical field which is already known for classical reasons to have waves that travel at c.

I would hesitate to describe it that way. Dark energy, as a source of stress-energy, is no different than radiation or dust, in that it is a density that occupies space, i.e. it occupies the RHS of Einstein's Equations. If the origin of dark energy is indeed quantum vacuum energy, then it is in some sense a fundamental ingredient, but I still wouldn't qualify it as a fundamental characteristic of spacetime itself.

I could be wrong, but I believe the following is correct. Einstein originally thought of the cosmological constant as being a fixed property of space, not just another matter field. When people today use the term "dark energy," is has connotations that they're thinking of it as a matter field. It would be a completely meaningless semantic distinction except that if it's a matter field, then it could have dynamics of its own. When people talk about a cosmological constant, they're thinking of it as a constant, so it can't have its own dynamics.

 

[Tanelorn]

Thanks Ben and bapowell for your replies regarding my question about the characteristics of space.

I have been listening to an audio version of "Ether and the Theory of Relativity" an address delivered on May 5th, 1920, in the University of Leyden
by Albert Einstein. Is the following about the existence of some kind of ether still believed correct?

"Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."

 

[bcrowell]

Thanks Ben and bapowell for your replies regarding my question about the characteristics of space.

I have been listening to an audio version of "Ether and the Theory of Relativity" an address delivered on May 5th, 1920, in the University of Leyden
by Albert Einstein. Is the following about the existence of some kind of ether still believed correct?

"Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."

We've drifted onto a topic that's unrelated to the original topic of this thread, so if you want to discuss this further, I'd suggest starting a new thread.

FAQ: Didn't Einstein say that general relativity was an aether theory? Is general relativity compatible with an aether?

No, Einstein didn't say that general relativity was an aether theory. Einstein wrote a 1924 paper in which he made the philosophical point that although relativity killed off the luminiferous aether as the supposed medium of electromagnetic vibrations, it still imbued the vacuum with specific physical characteristics, such as curvature and energy. The basic point of the paper is that we can't decide, purely based on philophical ideas like Mach's principle, whether the vacuum has its own properties; we actually have to go through the usual scientific cycle of theory and experiment in order to find out the answer. Internet kooks love to misinterpret and overinterpret this paper, or to misrepresent it by saying that Einstein referred to GR in general, throughout his career, as an aether theory.

A more subtle question is what kinds of aether theories can be constructed, and how they relate to (or don't relate to) general relativity. Philosophers and historians of scientists have debated whether any real aether theory ever actually existed, and what that would mean. Earman (1989) investigates earlier work by Trautman (1966), and concludes: "[A]bsolute space in the sense of a distinguished reference frame is a suspect notion, not because armchair philosophical reflections reveal that it is somehow metaphysically absurd, but because it has no unproblematic instantiations in examples that are physically interesting and that conform even approximately to historical reality." Debate on this philosophical and historical issue continues,[Rynasiewicz 2003] but one should keep in mind that this discussion is all about theories that have been falsified by observation since the Michelson-Morley experiment. Jacobson (2008) has investigated a theory in which Lorentz invariance is approximate, and is broken in the gravity sector at large Lorentz boost velocities. This theory includes phenomena like aether dust settling onto a planet and giving it an aether charge. Jacobson's model has two adjustable parameters which, if nonzero, differentiate it from general relativity, and which are constrained by astrophysical observations. It is important to note that the model is not compatible with Galilean relativity, and it predicts all the same counterintuitive phenomena as standard relativity, including, e.g., the twin paradox, length contraction, and black holes.

A. Einstein, "Über den Äther," Schweizerische naturforschende Gesellschaft 105 (1924) 85

original text - http://www.wikilivres.info/wiki/Über_den_Äther

English translation of [Einstein 1924]- http://www.oe.eclipse.co.uk/nom/aether.htm

commentary by John Baez on [Einstein 1924] - http://web.archive.org/web/20070204022629/http://math.ucr.edu/home/baez/RelWWW/wrong.html

A. Trautman, in B. Hoffmann (editor) Perspectives in Geometry and Relativity, Bloomington, 1966, p. 413.

J. Earman, World Enough and Space-Time, Absolute versus Relational Theories of Space and Time. Cambridge, 1989, MIT.

Rynasiewicz, "Field Unification in the Maxwell-Lorentz Theory with Absolute Space," Philosophy of Science 70 (2003) 1063, available at http://philsci-archive.pitt.edu/archive/00001096/ . Rynasiewicz starts by summarizing two important earlier papers that are now difficult to obtain: Trautman 1966 and Earman 1989.

Ted Jacobson, "Einstein-aether gravity: a status report," 2008, http://arxiv.org/abs/0801.1547