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Dark Matter/Energy

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tetsabb
55737.  Mon Feb 27, 2006 10:45 am Reply with quote

Hmmmm. Was this published on April 1st perhaps?

 
grizzly
55742.  Mon Feb 27, 2006 11:00 am Reply with quote

No, last week. Feedback invites the readers to write in with such stories. The feedback section is more the comedy section of the magazine with plenty of sarcasm.

 
grizzly
56602.  Thu Mar 02, 2006 1:02 pm Reply with quote

More research goes into the debate:

Quote:
DARK energy, the mysterious stuff invoked to explain why the expansion of the universe is accelerating, could have a simple explanation. The energy may be coming from neutrinos that were created in copious quantities just after the big bang.

The leading candidate for dark energy is Einstein's "cosmological constant", which proposes that the vacuum of space has an inherent energy that counters gravity. But if you calculate the density of this energy using quantum theory, it works out at nearly 120 orders of magnitude greater than what would fit with cosmological observations. So physicists have proposed ever more exotic explanations for dark energy that require, for instance, the existence of extra dimensions.

Now a team of Italian physicists says the answer has been under our noses all along. Antonio Capolupo and Giuseppe Vitiello of the University of Salerno and Salvatore Capozziello at the University of Naples claim that dark energy can be explained by neutrinos, particles that have no charge and little or no mass. Vast numbers were created just after the big bang, and many remain today because they barely interact with matter.

Neutrinos come in three "flavours", and recent experiments have confirmed they can switch flavours. According to the researchers, this neutrino "mixing" contributes just the right amount of energy to the vacuum of space. "Neutrino mixing may solve the problem of dark energy," says Capolupo.

Several years ago, the Italian trio, along with Massimo Blasone of the University of Salerno, developed a model to better explain neutrino mixing. Recently, the trio used the model to calculate how much energy the neutrinos contribute to the vacuum of space. The result fitted very well with the observational values for dark energy - assuming that they are the only significant contributors to the energy of vacuum (www.arxiv.org/astro-ph/0602467).

Intriguingly, there have been indications that every cubic centimetre of space contains about 300 neutrinos, and that the energy density of these particles is roughly equal to the energy density required to drive the acceleration caused by dark energy. However, no one could explain how the energy of neutrinos translates into the vacuum energy of space. The Italians' model of neutrino mixing now provides a mechanism. The vacuum is thought to be a cauldron of particles that pop in and out of existence and the neutrinos are transmuted into energy by their interactions with the vacuum. "It reveals an elegant, deep connection between particle physics and cosmology," says Capolupo.

Other cosmologists are cautious. "We are far from a complete understanding of both dark energy and the problem of [neutrino] mixing," says Nikolaos Mavromatos at King's College London. Scott Dodelson at particle research lab Fermilab in Chicago agrees. "If their explanation works theoretically, it is extremely interesting," he says. "Their model in principle is testable by looking at how dark energy evolves over time."

Once the team works out exactly how dark energy changes with time, projects such as the Supernova Cosmology Project can test the predictions. The model also predicts how neutrinos change flavours, which could be tested by experiments such as at the Sudbury Neutrino Observatory in Ontario, Canada.

From issue 2541 of New Scientist magazine, 04 March 2006, page 14

 
tetsabb
56607.  Thu Mar 02, 2006 2:21 pm Reply with quote

Quote:
From issue 2541 of New Scientist magazine, 04 March 2006, page 14


My God, an article from the future... wrong thread, surely?

 
metamorphiccat
56613.  Thu Mar 02, 2006 2:47 pm Reply with quote

Interesting to read the thoughts here on whether dark matter and dark energy are the key to our understanding of the Universe. Ptolemy's geocentric model of the Universe raises an interesting thought about this for me. I have no idea whether non-physicists are reading this, so experts please forgive me for going over things you already know and please elaborate on anything you think I've over-simplified. In Ptolemy's time, most people were convinced that the Earth was the centre of the Universe, and ever more sophisticated and intricate modifications (planets moving on epicycles on epicycles with eccentricities in the centre of their rotation) were made to his model of the Universe in order to make it fit observational data. Looking back, it is obvious to us that people were so fixated on accepted theory that they had great difficulty looking beyond it, and came up with ideas that have no basis in reality simply because the model was made to fit observational data so astonishingly closely.

At the moment, many people are firmly convinced of the validity of our current understanding of gravity and the Universe, and we bring in ideas like dark matter, dark energy and additional spatial dimensions in order to make the data we have fit with our theories. Our current theory of gravitation fits the data as closely as Ptolemy's model of the Universe fitted data on planetary motions.

I wonder. Will future generations look back at dark energy and dark matter as the epicycles of our generation? Is it time for another scientific revolution? I have to admit that I hope so, because I think we need the odd revolution every now and again to shake things up a bit. Preferably without any beheadings or other such revolution related activity.

Becky

 
Gray
56630.  Thu Mar 02, 2006 5:06 pm Reply with quote

Yes, I think Dark Matter and quantum theory are due to bite the dust fairly soon as 'mechanistic theories'.

There was a report recently about a re-calculation of galactic revolution that took into account reletavistic measurements (the ones that were originally Newtonian being those that required Dark Matter to account for the rotation speed).

The new calculations didn't require any extra mass at all, therefore doing away with the need for DM as far as galactic revolution is concerned (though not universal expansion) but the theory doesn't seem to have caught on for some reason...

 
HarryAlffa
56633.  Thu Mar 02, 2006 5:21 pm Reply with quote

metamorphiccat -
Quote:
I wonder. Will future generations look back at dark energy and dark matter as the epicycles of our generation?

Very good point well made.
I'm very suspicious of this type of thinking.


However.

I have a new theory myself, which explains the accelerated expansion of the universe.
But first; dark matter was introduced to explain why stars in the outer reaches of the Galaxies where orbiting at such a speed that they should fly off into deep space. The observed matter in a Galaxy didn't provide enough gravity to keep these stars within the Galaxy.

An analogy would be favourite here. The old lead weights on a rubber sheet explains gravity by saying "look at the indentations, this is how gravity works".
If we stretch the rubber sheet, the indentations become less deep. Is then the force of gravity becoming less as the Universe expands? I don't know.

I vote we throw in the rubber towel on the rubber sheet model because it mixes 2D and 3D images.
Instead, imagine a big bath sponge that you've compressed in your hands and then let go. The expanding sponge represents the expansion of spacetime, but not the matter it contains. Lets put little discrete drops of water inside the sponge and let it go again. The drops of matter are flying farther and farther apart.
Now lets imagine that these drops "repell" the sponge of spacetime and that where a lot of matter sits, the sponge is thinner, until we get to the stage where there is so much matter that it makes a sphereical bubble where no sponge exists; a black hole.
So matter can at least displace spacetime, like a submarine in the sea.
Imagine also that close to matter the sponge of spacetime is thinner ie. stretched.
Gravity is now explained by matter rolling toward thinner spacetime and away from thicker spacetime.

So my theory is simple - imagine matter "repelled" spacetime.

Galaxies thin out the spacetime they occupy and outside them are "compression zones" of thicker spacetime. The repulsive force of spacetime is enough to account for the stars in the outer reaches of Galaxies being kept in orbit around the Galaxies centres.
It also explains the accelerating expansion of the universe, all these Galaxies and Clusters pushing against spacetime.

That's my theory, and I'm not sticking to it. It's just a theory.

 
dr.bob
56660.  Fri Mar 03, 2006 5:24 am Reply with quote

metamorphiccat wrote:
please elaborate on anything you think I've over-simplified.


OK, well you did ask :)

metamorphiccat wrote:
Our current theory of gravitation fits the data as closely as Ptolemy's model of the Universe fitted data on planetary motions.


This is not an over-simplification, this is just incorrect. Our current theory of gravitation fits the observed data a lot more closely than Ptolemy's model of the Universe fitted data on planetary motions. Also, we now have a lot more data to fit to the model, so I don't think it's fair to draw a direct comparison between today's theories and Ptolemy's ideas.

However, the main conclusion of your post is bang on the money:

metamorphiccat wrote:
Will future generations look back at dark energy and dark matter as the epicycles of our generation? Is it time for another scientific revolution?


I would say "almost certainly" and "it's hard to tell".

The history of science generally follows a simple pattern. Someone comes up with a theory which explains the observed data. This theory is then tested with more and more observations. Eventually some data will appear which seems to contradict this theory. However, since a convincing, consistant theory is pretty hard to come up with, scientists will tend to stick to the original theory because it does, after all, still explain most things and allow them to do their calculations. This theory will then be modified and stretched to try and explain the new data.

Eventually a time will come where there is too much data which doesn't fit and the theory is being stretched too far. At this point a scientific revolution comes about where a wholly new theory is proposed which explains all that the old theory explained plus the new data which doesn't fit in. This new theory is then tested to make sure it makes sense and, if it does, adopted by the scientific community. Then the whole cycle starts again.

Gray wrote:
There was a report recently about a re-calculation of galactic revolution that took into account reletavistic measurements. The new calculations didn't require any extra mass at all


Do you have a link for that? I've not heard of it and it sounds intriguing.

 
Gray
56664.  Fri Mar 03, 2006 5:37 am Reply with quote

Yes, it's here:

http://xxx.lanl.gov/abs/astro-ph/0602519

This abstract only talks about a 30% reduction in the amount of dark matter required, but I'm sure I remember seeing a 'total reduction' statement in the news report I read. I'll try to find it.

There is lots of previously invisible baryonic matter being found by the Chandra X-ray telescope, so it's piling up...

http://www.wired.com/news/space/0,2697,66487,00.html

 
grizzly
56665.  Fri Mar 03, 2006 5:47 am Reply with quote

tetsabb wrote:
Quote:
From issue 2541 of New Scientist magazine, 04 March 2006, page 14


My God, an article from the future... wrong thread, surely?


The magazine doesn't go on general sale until that date. Us subsribers get it a couple of days early.

 
dr.bob
56731.  Fri Mar 03, 2006 10:07 am Reply with quote

Gray wrote:
Yes, it's here:


Wow, thanks for that Gray. That was Really Interesting (hmm, does that mean it doesn't belong on this site?)

Unfortunately I don't understand GR well enough to know whether or not they're making unreasonable assumptions, but it's interesting nevertheless.

Gray wrote:
This abstract only talks about a 30% reduction in the amount of dark matter required, but I'm sure I remember seeing a 'total reduction' statement in the news report I read. I'll try to find it.


Well, on page two of the article they mention:

Quote:
Quite recently there was an attempt to use GR rather than Newtonian physics to describe the internal dynamics of galaxies: a perturbative solution has been found which was claimed to solve the problem of flat rotation curves without invoking exotic dark matter. However, in addition to the galactic perfect fluid it exhibits matter sources localized in the galactic disk, which were later shown to be exotic ones. Even some arguments have been put forward that such an approach can never explain away dark matter.


Perhaps some journo just read the first sentence and thought he'd sex-up the article a bit.

The article also points out that there is other evidence for dark matter (such as microlensing) quite seperate from the flat rotation curves of galaxies, so it sounds unlikely that you'll be able to get rid of dark matter that easily :)

 
metamorphiccat
56781.  Fri Mar 03, 2006 1:26 pm Reply with quote

Quote:
This is not an over-simplification, this is just incorrect. Our current theory of gravitation fits the observed data a lot more closely than Ptolemy's model of the Universe fitted data on planetary motions.


Do you have any references for that? It's just that everything I've read so far suggests that isn't the case at all. Perhaps I was a little thrown by the material I've seen suggesting that some of the observations made would require a huge proportion of the Universe to be dark matter for gravitation to work the way we think it does. I think one report said 95%, which I think is a fair size for an error.

Or perhaps I should stop reading all this rubbish in New Scientist.

 
dr.bob
57348.  Mon Mar 06, 2006 6:58 am Reply with quote

metamorphiccat wrote:
Do you have any references for that?


No, I'm afraid not. I'm not sure there is a reference which directly compares Ptolemy's models with modern ideas and concludes what proportion of the data is explained by each. However, Ptolemy's model was beset with problems from the very start and never really explained planetary motion satisfactorially except in very vague terms.

metamorphiccat wrote:
It's just that everything I've read so far suggests that isn't the case at all. Perhaps I was a little thrown by the material I've seen suggesting that some of the observations made would require a huge proportion of the Universe to be dark matter for gravitation to work the way we think it does.


But that's just one aspect of gravitation. What about the precise way that we can predict the motions in the solar system that allows us to send probes like Voyager on missions lasting many years past three other planets and still arrive at Neptune a matter of a few seconds later than predicted? Or the way we can predict precisely how much light will be deflected from a straight line when passing close to a massive body?

The theory of gravity covers just about everything in the universe. Yes there are definitely problems with some large scale observations, but in the vast majority of cases the current theory works extremely well. I don't think that could ever be said about Ptolemy's model, even with the crude measurements available at the time.

 
samivel
57466.  Mon Mar 06, 2006 11:36 am Reply with quote

dr.bob wrote:
What about the precise way that we can predict the motions in the solar system that allows us to send probes like Voyager on missions lasting many years past three other planets and still arrive at Neptune a matter of a few seconds later than predicted?


I don't suppose there's any chance NASA could be given a rail franchise, is there?

 
Tas
57473.  Mon Mar 06, 2006 11:52 am Reply with quote

Quote:
Certainly it's possible that Newton was wrong. Indeed, some of his theories have already been replaced by newer theories such as relativity. However, his laws of gravitation have been shown to work in almost every observation that we've made so far all around the universe. To come up with a theory that not only works that well but works a bit better is really hard.


Doesn't this show that Newton was wrong? Or at least only partially right?

:-)

Tas

 

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