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Dark Matter/Energy

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Dark Matter/Energy was created by Son Goku

I've been reading over old posts and it seems that quite a few people disagree with the ideas of Dark Matter and Energy in modern cosmology.

What do people find dubious about it?
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16 years 9 months ago #26792

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

In a previous thread it was described as another "luminiferous ether".
I can understand to some extent peoples reservations on this topic. Again it is probably down to lack of observation, but that has been changing in recent times. You can't yet go into Lidl and buy a bag of dark matter, though I believe they're selling positrons for €25 a gram.
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16 years 9 months ago #26813

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Replied by voyager on topic Re: Dark Matter/Energy

Personally I have no problems with Dark Matter, there is ample evidence that there is mass galaxies that we cannot see. It may proove to be Nutrions since it is looking like they are not QUITE massless or if could be some cool non-barionic matter. Either way I'm happy to accept that there is matter that does not emit light in galaxies.

Dark energy on the other hand seems to be going out on a bit of a limb to me. The experimental data is not as clear cut and it just strikes me as a desperate attempt to hold on to old theories we like despite observations not really agreeing with the old theories.

I'm open to a more enlightened and educated perspective on this lot though.

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16 years 9 months ago #26816

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Replied by Son Goku on topic Re: Dark Matter/Energy

Dark energy on the other hand seems to be going out on a bit of a limb to me. The experimental data is not as clear cut and it just strikes me as a desperate attempt to hold on to old theories we like despite observations not really agreeing with the old theories.


Funnily enough most people on the net have more of a problem with Dark Matter, even though, as you said, it is the one with more experimental evidence.

For anybody whose is interested, the motivation for Dark Energy is as follows:
General Relativity is described by the following equation:

It doesn't matter what the terms mean, just the lambda term, which is the cosmological constant.
In the early years of General Relativity, most relativists set this to zero since it makes General Relativity much easier to solve.
(Although even then GR is pretty much impossible to solve.)
At one point Einstein, as I'm sure you've all heard, assumed it had a small value which turned out to be incorrect.

However in recent years it has been seen that the universe is expanding, so Cosmologists went back to General Relativity to see if this matched what it predicted.
It turns out General Relativity does predict it, when you have a very small lambda.

The problem is General Relativity doesn't say were lambda comes from.
So now we're in the unusual position of having something which matches our theory, but as no explanation.

In recent years, some people have proposed that lambda is caused by Dark Energy.
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16 years 9 months ago #26823

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

Also, the value of lambda has probably changed over the evolution of the universe. It is not necessarily a constant, and I believe Einstein must have been very frustrated at having to introduce a constant with an unknown value, that will likely have different values at different points in time.

In recent years, some people have proposed that lambda is caused by Dark Energy.

i.e. that an increase in Dark Energy will cause greater expansion of the Universe - if I'm reading that correctly.
This then starts a chain of questions like:
where is the 'new' dark energy coming from?
for a given amount of dark energy we get a period of expansion which must eventually equalize.?
Maybe we need a "new" equation:
Dark E = Dark m (c*c)
Therefore dark energy and dark matter are interchangeable.
The question I would have is - is dark matter and dark energy interchangeable with energy and matter as we currently know it - probably not as it would be a lot easier to detect etc.

Speaking of detection - there are currently surveys going on into the distribution of dark matter in our own Galaxy. I don't know how they are doing this, but I would presume that they look at the angular momentum of stallar objects in say clusters, the total mass and then ask the question "can we see enough mass to justify the angular momentum?", if not then just add quantity x amount of dark matter to that area of the galaxy and hunky dory everything is now balanced. Please tell me I'm wrong - that there is a neater way of detecting dark energy.
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16 years 9 months ago #26840

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Replied by dave_lillis on topic Re: Dark Matter/Energy

What do people think of the theory regarding the source of "dark matter" or more accurately its gravitational effect, in that this excess gravity is actually normal gravity leaking from other parallel universes into ours.
The theory suggests that matter seems to be unable to crossover, except gravitons which supposedly can. This explains the extra gravity that
"dark matter" supplies.
Given that dark matter cannot emit nor reflect light and seems to be completely undetectable so far except through gavity, is this theory totally off the wall, has it been debunked/surpassed or is there serious work going on in this.
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16 years 9 months ago #26851

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

Dave,

I mentioned this theory previously on this thread. This is currently a serious theory being followed by - oh I forget her name. Personally along with Gravitons I think it's bunkum. There is a lot about string theory which is beautiful and elegant, but this is not one of them.
There seems to be some "serious" work going on with respect to this theory, I recently watched a documentary where this theory seemed to get pride of place in the recent development category. If gravitons don't exist then this theory goes down the pan, and I'm not a fan of gravitons.
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16 years 9 months ago #26857

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Replied by dave_lillis on topic Re: Dark Matter/Energy

Do you mean on a previous thread, I dont see it above?
Anyway, what is it about gravitons that you dont like? or to put it better why dont you think they exist.
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16 years 9 months ago #26862

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

Do you mean on a previous thread, I dont see it above?
Anyway, what is it about gravitons that you dont like? or to put it better why dont you think they exist.


Sorry your right, I'm getting my threads tangled. Here is the thread I meant. www.irishastronomy.org/boards/viewtopic.php?t=4050
Sorry about that both themes seem to be converging.

Ok why don't I believe gravitons exist?
For electrical energy we have the corresponding particle the electron.
For light we have the photon.
And for gravity we have the graviton (or do we?).
GR says nothing about the need for a graviton and no-one is looking for it apart from some string theorists who need it to unify a theory of everything to include gravity.
Also gravity per se is not truly a force (in the sense that electrical force is)see the thread mentioned above for the arguments.

Also if you think of in these terms - Matter curves spacetime to create what we call gravity, it's not a force it's simply curved spacetime. Why does matter then also need to create another particle just because spacetime has been curved? It just doesn't add up.
Ask what are the predictions for a graviton - does it have mass? a charge? a spin? Do a search on google and you'll come up with some very indeterministic predictions - "The graviton is the exchange particle for the gravity force. Although it has not been directly observed, a number of its properties can be implied from the nature of the force. Since gravity is an inverse square force of apparently infinite range, it can be implied that the rest mass of the graviton is zero." - and also it has no charge is long range and apparently can travel much faster than the speed of light (all this is very convenient 'cause there is then no way to ever detect the'particle'). -Bunkum
The main reason there is no particle is that Gravity is not a force in the true sense.
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16 years 9 months ago #26867

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Replied by dave_lillis on topic Re: Dark Matter/Energy

I would 100% agree with you except that we have an example of a dual property "particle" already.
Light behaves as a wave and a particle, I do realise that light is an electromagnetic phenomena, and that gravity is not.
I do tend to agree with your above points, just dont be completely surprised if someone somehow manages to prove that the graviton has a mass, does spin and all that, nature has a quirky way of throwing many spanners in the works. Surely we dont have enough info yet to come to a definitive conclusion, or do we??
For instance, many years ago who would have thought a black hole could evaporate.
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16 years 9 months ago #26872

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Replied by Son Goku on topic Re: Dark Matter/Energy

Ok why don't I believe gravitons exist?
For electrical energy we have the corresponding particle the electron.
For light we have the photon.
And for gravity we have the graviton (or do we?).
GR says nothing about the need for a graviton and no-one is looking for it apart from some string theorists who need it to unify a theory of everything to include gravity.
Also gravity per se is not truly a force (in the sense that electrical force is)see the thread mentioned above for the arguments.

The graviton is pretty much the hope of String Theory and has quite a lot of History behind it.

For the past century there have been two major schools of theoretical physics, "relativists" and "particle physicists".
The first is based on General Relativity and the later based on Quantum Field Theory.

In 1970 when Particle Physics managed to explain the three non-gravitational forces, Particle Physicists attempted to tackle gravity.

They presumed the same treatment would work that they used on the other forces:
Get the classical field, turn it into a giant Quantum Field Theoretic Integral and solve this integral using Feynman Diagrams.

For gravity this did not work and could not work, however most Particle Physicists trained in the 70s and 80s (which include today's String Theorists) were not educated that well in General Relativity.

The problem was that Quantum Fields live in flat spacetime, but in General Relativity, gravity isn't another field in flat spacetime, it is the curvature of spacetime, so we can't quantise it.
(At least we can't make a Quantum Field Theory out of it)

For a graviton to exist gravity would have to not be the warping of spacetime, but just another field in flat spacetime that looks like the warping of spacetime at the classical level.

The String Theory community grew out the belief that Quantum Field Theory is right and gravity is just another force in flat spacetime.
The Loop Quantum Gravity community grew out of the belief that gravity isn't a force in spacetime, so we need a different way to do things.

(There is also a third school of thought which thinks we aren't ready to make turn Gravity quantum just yet)
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16 years 9 months ago #26896

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

The Loop Quantum Gravity community grew out of the belief that gravity isn't a force in spacetime, so we need a different way to do things.


Personally I think the Loop Quatum Gravity theory is more to my taste, though still not entirely, it's still got some "hairys" I don't like, but I do think it's a more accurate model.

Getting back to GR. It still baffles me how many physicists have still not grasped it's significance/importance - they still try to marry classical and GR physics - which seems an impossibility. Again, I say that that once a student reaches third level that all notions of newtonian physics (apart from obviously the working ones) be thrown out, and that the midset of GR be instilled. Else, we will continue to have ridiculous theories on gravity leaking thro' dimensions, when gravity iteself is only an observabale result of spacetime curvature. What physicists should be concentrating on is why matter causes spacetime to curve. ( I have some ideas on the subject, but until I have I have it fully worked out in my own spacetime I won't elaborate, and I certainly won't be showing the math).
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16 years 9 months ago #26982

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Replied by voyager on topic Re: Dark Matter/Energy

The Loop Quantum Gravity community grew out of the belief that gravity isn't a force in spacetime, so we need a different way to do things.


Personally I think the Loop Quatum Gravity theory is more to my taste, though still not entirely, it's still got some "hairys" I don't like, but I do think it's a more accurate model.

Getting back to GR. It still baffles me how many physicists have still not grasped it's significance/importance - they still try to marry classical and GR physics - which seems an impossibility. Again, I say that that once a student reaches third level that all notions of newtonian physics (apart from obviously the working ones) be thrown out, and that the midset of GR be instilled. Else, we will continue to have ridiculous theories on gravity leaking thro' dimensions, when gravity iteself is only an observabale result of spacetime curvature. What physicists should be concentrating on is why matter causes spacetime to curve. ( I have some ideas on the subject, but until I have I have it fully worked out in my own spacetime I won't elaborate, and I certainly won't be showing the math).


You're gonna be horrified by this but I have an honours physics degree and I NEVER did GR as part of my degree. That was reserved for the single honours people, us double honours people were obviously not worthy!

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16 years 9 months ago #26997

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

I have an honours physics degree and I NEVER did GR as part of my degree.


There you go - just proves my point. GR is not generally something you read and understand first off. Secondary schools is where we should start teaching this stuff, not leaving it up to students to read and interpret/misinterpret themselves. It can take a while to get your head around GR, a lot of it is mindset. If you're coming from a long term classical physics background it's very hard to park Newton and accept GR, the temptation is to try to meld the two and this usually ends in confusion and failure. I don't have a physics degree - just Electronics - so I have had to find my own way by self education, and extensive and cross discipline reading. Maybe that's why I'm more questioning of some of the above theories.
Physicists need to get out more - what I mean by that is read up on whats going on in other disciplines and fields as diverse as Biology. Too much of physics nowadays is specialised in small compartments and nobody is sticking their heads out of their own box, this will ultimately lead to an ingenuity gap.
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16 years 9 months ago #27001

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Replied by voyager on topic Re: Dark Matter/Energy

I have an honours physics degree and I NEVER did GR as part of my degree.


There you go - just proves my point. GR is not generally something you read and understand first off. Secondary schools is where we should start teaching this stuff, not leaving it up to students to read and interpret/misinterpret themselves. It can take a while to get your head around GR, a lot of it is mindset. If you're coming from a long term classical physics background it's very hard to park Newton and accept GR, the temptation is to try to meld the two and this usually ends in confusion and failure. I don't have a physics degree - just Electronics - so I have had to find my own way by self education, and extensive and cross discipline reading. Maybe that's why I'm more questioning of some of the above theories.
Physicists need to get out more - what I mean by that is read up on whats going on in other disciplines and fields as diverse as Biology. Too much of physics nowadays is specialised in small compartments and nobody is sticking their heads out of their own box, this will ultimately lead to an ingenuity gap.


I agree with you. I have a pretty balanced and rounded education because most of it I picked up myself. If I only knew what I was tought in my degree I'd be a rubbish computer scientist and a hopless physicist!

As for the compartmentalization that definitly exists in physics, it is FAR broader than that. It is a feature of science as a whole these days. If you don't believe me, try working accademically in a cross disciplinary area, you'll soon find that you fall between all the stools and find it very hard to get funded because science has been broken up in to separate pigeonholes and funding is so specific that if you don't fit into a slot you are in trouble. My PhD is on e-learning and has a very distinct physics and more precicely Astronomy slant. The Comptuer science funding agencies see it as being too education related so they won't fund, the education agencies see it as too computer sciency so they won't fund and the physics agencies see no hard physics there so they too are not interested. This is the closed, compartmentalised attitude you described realised in practice and it's not at all a healthy thing for science.

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16 years 9 months ago #27002

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

This is the closed, compartmentalised attitude you described realised in practice and it's not at all a healthy thing for science.


I agree totally - Science does not benefit from compartmentalisation. And I'm sorry to hear about your funding problem but that's probably par for the course anyway - people run a mile whe you look for funding and generally try to blame it on someone else, but thats not science - it's politics and administration.
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16 years 9 months ago #27003

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Replied by voyager on topic Re: Dark Matter/Energy

This is the closed, compartmentalised attitude you described realised in practice and it's not at all a healthy thing for science.


I agree totally - Science does not benefit from compartmentalisation. And I'm sorry to hear about your funding problem but that's probably par for the course anyway - people run a mile whe you look for funding and generally try to blame it on someone else, but thats not science - it's politics and administration.


I did eventually manage ot get some funding but 3 out of hte 5 years of my PhD were un-funded, which makes life very difficult. Mind you funding is a major problem here. Regardless of how great the research you are doing is you will always have to work hard for funding but if you don't fit nicely into a compartment then you are at a dissadvantage straight away.
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16 years 9 months ago #27007

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Replied by JohnMurphy on topic Re: Dark Matter/Energy

if you don't fit nicely into a compartment then you are at a dissadvantage straight away


Ever decreasing circles - and we know how that ends - a black hole!
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16 years 9 months ago #27008

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Replied by albertw on topic Re: Dark Matter/Energy

You're gonna be horrified by this but I have an honours physics degree and I NEVER did GR as part of my degree. That was reserved for the single honours people, us double honours people were obviously not worthy!


The basics were covered in first year in UCD Physics. SR was dealt with in more detail. In 1st year Maths Physics SR was done briefly; then again SR is in the Leaving Cert Applied Maths course. While Leaving Cert doesnt explicitly cover any relativity they do now do a decent bit of modern particle physics at higher level.

The one nice thing about having a BSc. in computer science from UCD is that I got to spend 3 years doing various bits of physics as part of the course.
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16 years 9 months ago #27010

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Replied by Son Goku on topic Re: Dark Matter/Energy

The newest Chandra results make a pretty harsh case against alternate theories of gravity.
Dark Matter seems to have been observed multiple times and it will be difficult for MOND and other such theories to explain these results.

Here are the provisional abstracts for the results:
Abstract 1
Abstract 2
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16 years 5 months ago #31841

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Replied by DaveGrennan on topic Re: Dark Matter/Energy

Just saw this!

news.bbc.co.uk/2/hi/science/nature/5272226.stm

If this turns out to be anything more than hype, then it may well be the biggest news in cosmolgy/physics ever??
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16 years 5 months ago #31921

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Replied by Son Goku on topic Re: Dark Matter/Energy

Just saw this!

news.bbc.co.uk/2/hi/science/nature/5272226.stm

If this turns out to be anything more than hype, then it may well be the biggest news in cosmolgy/physics ever??

It's definitely not hype, the study is actually more rigorous than usual. And simulations confirm that only General Relativity can reproduce these results.
Here is the real paper:
arxiv.org/abs/astro-ph/0608407
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16 years 5 months ago #31922

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Replied by voyager on topic Re: Dark Matter/Energy

That's pretty cool ... good to see us getting more of a handel on this Dark Matter stuff .... will shut up some of the anti-big-bangers who argue that Dark Matter is just a conspiracy to make the big bang stand up.
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16 years 5 months ago #31925

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Replied by ftodonoghue on topic Re: Dark Matter/Energy

Just saw this!

news.bbc.co.uk/2/hi/science/nature/5272226.stm

If this turns out to be anything more than hype, then it may well be the biggest news in cosmolgy/physics ever??



Surely this discovery is just another (albeit more direct ) confirmation of dark matter. I thought it was pretty much established science that dark matter existed. :?:
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16 years 5 months ago #31926

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Replied by Maddad on topic Re: Dark Matter/Energy

Personally I have no problems with Dark Matter, there is ample evidence that there is mass galaxies that we cannot see.

We use dark matter to explain motions. Right now we only have Keplerian explanations, but we do not know for certain that other rules do not affect objects, especially on large scales. If we assume that no other laws other than Kepler's control these motions, then yes indeed, there is ample evidence for dark matter.
16 years 4 months ago #32786

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