Dark Energy

[Anonymous]

Rynner - could the Big Bang have pushed everything out so quickly that the stuff has not stopped moving yet?

I suppose I should have asked that before.

 

[rynner2]

Rynner - could the Big Bang have pushed everything out so quickly that the stuff has not stopped moving yet?

Yep, that's the general idea!

Here are a few thousand websites with more information!

(And if you still have queries after that lot, I don't know the answer either! )

 

[QuaziWashboard]

The example of an object in orbit is a demonstration of why centrifugal force doesn't really exist.

Huh? so what makes my cloths stick to the inside of the washing machine drum when it's going through spin cycle? What keeps water in a bucket if I hold it by the handle and spin my arm around quickly? What Keeps people in their seats on a rollercoster when it does a loop? Why would NASA have plans for space ships that spin to create an artificial gravity using centrifugal force if it doesn't exist?

The object concerned would move in a straight line through space if it were not acted upon by the force of gravity of (say for instance a planet); the gravity is your centripetal force, moving the object constantly toward the centre of gravity of that planet.

Right...with you so far.....

But the object cotinues to move ever forward, along a curved line which encompasses the planet in an orbit.

Which is exactly how centrifugal force works isn't it? If the object didn't have something to anchor it in a circular motion, (in this case gravity) it would fly off in a straight line.

 

[Xanatico]

What he means is that centrifugal force isn´t real, it is an illusion created by other forces. Newtons 2nd law I believe, about how moving objects keep going unless acted upon by another force. If you swing something around and let go, I believe it doesn´t go in a straight line out from you, but rather sideways due the fact that this is the direction it was actually moving at the time of release.

 

[OneWingedBird]

If that was the case it might explain the dark aspect of the matter.

but if it's travelling so fast the light can't reach us, it's also travelling so fast the gravity can't reach us either...

Why would NASA have plans for space ships that spin to create an artificial gravity using centrifugal force if it doesn't exist?

i think it's the same principle as the passenger in the car example that's explained in the wiki link... the car turns, and you're pressed to the side because your body is trying to keep moving in the same direction

which is why rotating part of a spacecraft to create 'artificial gravity' only works while you're touching part of the craft that's rotating. if you were floating in the middle of it when the rotation began, you'd be unaffected and would stay floating there while the craft rotated around you

(iirc this was used as a premise in an episode of babylon 5, where a character has to throw himself from a monorail car that runs through the centre of the huge space station, and almost gets splatted by the rotational velocity of the outer part/'ground'. though i'm not quite sure it would have looked like how they did it...)

 

[crunchy5]

but if it's travelling so fast the light can't reach us, it's also travelling so fast the gravity can't reach us either...

Iirc there is a school of thought that gravity might be a wave that is faster than light possibly from a Horizon prog about string theory, again iirc the nothing can go faster than light rule only applies to things with mass, the quantum issue where parts of an atom can be in more than one place at a time I think are an example of this. :? :? :? 8)

 

[crunchy5]

Just found this.

http://www.physorg.com/news85310822.html

Lately, some scientists have turned the question around, from “is dark matter correct?” to “is our standard theory of gravity correct?” Most recently, Fermilab scientists Scott Dodelson and former Brinson Fellow Michele Liguori demonstrated one of the first pieces of theoretical evidence that an alternative theory of gravity can explain the large scale structure of the universe.

“To definitively claim that dark matter is the answer, we need to find it,” Dodelson explained to PhysOrg.com. “We can do this in one of three ways: produce it in the lab (which might happen at Fermilab, the soon-to-start LHC, or ultimately the International Linear Collider), see a pair of dark matter particles annihilate and produce high energy photons (there are about a half dozen experiments designed to look for this), or see a dark matter particle bump a nucleus in a large underground detector (again, about 10 experiments are looking for this). Until one or more of these things happen, skeptics are still allowed. … After they happen, skeptics will become crackpots.”

Although cosmologists have never directly observed dark matter, they have many good reasons for not giving up hope. The ways that galaxies rotate and starlight bends (gravitational lensing) stray from predictions based on visible matter. Further, the formation of large cosmic structures (such as galaxies and galaxy clusters) would have required significantly large matter perturbations when the Universe was less than a million years old that simply don’t exist in a theory of general relativity before “tacking on” dark matter.

“It is extremely important to see how well a no-dark-matter cosmology does,” said Dodelson. “[In the standard theory,] we are asking the community to believe in the existence of a particle that has never been seen. We have to be damned sure that you can't explain the universe without this huge leap. Our Figure 1 [see citation below] illustrates that, in standard gravity, a no-dark-matter model does not do well at all.”

While altering the theory of gravity may seem like pulling the rug out from under a century of observations and pain-staking calculations, an alternative theory may simply be “more correct” than today’s standard theory. Just as Einstein’s theory was “more correct” than Newton’s because it improved upon the older one by noticing more specific details (e.g. extraordinary masses and speeds), a new alternative theory may only drastically change gravity at certain scales.

“Perhaps a fundamental theory of gravity which differs from general relativity on large scales can explain the observations without recourse to new, unobserved particles,” wrote Dodelson and Liguori in their study published in Physical Review Letters. “Now more than ever before, there are very good reasons to explore this idea of modifying gravity. For, the case of dark energy also hinges on the assumption that general relativity describes gravity on larges scales. Dark energy is even more difficult to explain than dark matter, so it seems almost natural to look at gravity as the culprit in both cases.”

The new theory (or groundwork for it) under investigation would be Jacob Bekenstein’s relativistic covariant theory of gravity (TeVeS), published in 2004. Bekenstein based his theory on a modified version of Newtonian theory from the early ‘80s, dependent on gravitational acceleration and called modified Newtonian dynamics (MOND) by its founder, Mordecai Milgrom.

“MOND, the original theory on which TeVeS is based, was already quite successful at explaining galactic dynamics (even better, in some cases, than the dark matter paradigm), but it failed completely at explaining other observations—gravitational lensing in particular,” explained Liguori. “For this reason, it couldn't be considered a real alternative to dark matter. Bekenstein’s theory, by generalizing MOND, retains its good features while overcoming its main problems at the same time. This makes TeVeS a much more interesting theory than MOND. It is then worthwhile (and necessary) to test TeVeS’ predictions in detail and compare them to the standard dark matter paradigm to see if TeVeS can be a viable alternative.”

Dodelson and Liguori find Bekenstein’s theory intriguing in this context because, for one, the gravitational acceleration scale in the theory is very close to that required for the observed acceleration of the Universe. The scale is also very similar to that proposed in “post hoc” theories such as dark energy. Even more interesting is the fact that the origins of Bekenstein’s theory had nothing to do with cosmic acceleration.

But the feature of Bekenstein’s theory that Dodelson and Liguori focus on most is that the theory—unlike standard general relativity—allows for fast growth of density perturbations arising from small inhomogeneities during recombination. Building on this finding from scientists Skordis et al. earlier this year, Dodelson and Liguori have found which aspect of the theory actually causes the enhanced growth—the part that may solve the cosmological structure problem.

The pair has discovered that, while Bekenstein’s theory has three functions which characterize space-time—a tensor, vector and scalar (TeVeS)—it’s the perturbations in the vector field that are key to the enhanced growth. General relativity describes space-time with only a tensor (the metric), so it does not include these vector perturbations.

“The vector field solves only the enhanced growth problem,” said Dodelson. “It does so by exploiting a little-known fact about gravity. In our solar system or galaxy, when we attack the problem of gravity, we solve the equation for the Newtonian potential. Actually, there are two potentials that characterize gravity: the one usually called the Newtonian potential and the perturbation to the curvature of space. These two potentials are almost always very nearly equal to one another, so it is not usually necessary to distinguish them.

“In the case of TeVeS, the vector field sources the difference between the two,” he continued. “As it begins to grow, the difference between the two potentials grows as well. This is ultimately what drives the overdense regions to accrete more matter than in standard general relativity. The quite remarkable thing about this growth is that Bekenstein introduced the vector field for his own completely independent reasons. As he remarked to me, ‘Sometimes theories are smarter than their creators.’"

Dodelson and Liguori see this solution to large structure formation as an important step for a gravity theory based on baryon-only matter. Other problems that their theory (or any alternative theory) will have to confront include accounting for the mismatch in galaxy clusters between mass and light. Also, the theory must conform to at least two observations: the galaxy power spectrum on large scales, and the cosmic microwave background fluctuations, which correspond to baby galaxies and galaxy clusters.

“As Scott says, until dark matter will be observed, skeptics will be allowed,” said Liguori. “Despite the many and impressive successes of the dark matter paradigm, which make it very likely to be correct, we still don't have any final and definitive answer. In light of this, it is important to keep an eye open for possible alternative explanations. Even when, after the analysis, alternative theories turn out to be wrong, the result is still important, as it strengthen the evidence for dark matter as the only possible explanation of observations.”

 

[rynner2]

Er, as you were... (for now)

Dark energy may not exist in space, scientists claim
Dark matter and energy, the mysterious forces thought to make up 96 per cent of the universe, may not exist according to a groundbreaking study.
By Heidi Blake
Published: 7:30AM BST 15 Jun 2010

British scientists have claimed that the method used to calculate the make-up of the universe may be wrong.

The universe as we know it – formed of recognisable components such as planets, stars, asteroids and gas - accounts for just four per cent of the cosmos, according to the three-decades old Standard Model of Cosmology.

The rest is thought to be made up of mysterious dark matter and dark energy. This permeates space and powers the expansion of the universe.

But physicists at Durham University now claim the calculations on which the Standard Model is based could be fatally flawed.

This raises the possibility that the “dark side” of the cosmos does not exist, which in turn could mean that the universe is expanding less quickly than previously thought.

Dr Robert Massey of the Royal Astronomical Society, which published the findings, said: “This would challenge greatly our assumptions about the long term future of the universe, because the assumption at the moment is that the universe is expanding and if it isn’t that would be a huge shock.

“It could even mean that the expansion of the universe is slowing down and could grind to a halt.”

A new analysis of measurements taken by NASA of Big Bang heat radiation in 2001 showed that the heat waves may be far smaller than previously thought.

When the measurements were first taken in 2001 the size of the ripples in the Cosmic Microwave Background radiation led scientists to conclude that the cosmos is made up of four percent "normal" matter, 22 percent "dark" or invisible matter and 74 percent "dark" energy.

But scientists now claim that the waves of radiation which were previously measured at about twice the size of the full moon may in fact be less than half that size.

Professor Tom Shanks, who led the research, said: “CMB observations are a powerful tool for cosmology and it is vital to check for systematic effects. If our results prove correct then it will become less likely that dark energy and exotic matter particles dominate the universe. So the evidence that the universe has a dark side will weaken.”

http://www.telegraph.co.uk/science/spac ... claim.html

 

[rynner2]

Cosmologists seem to change their minds more often than I change my socks..

Dark energy and flat Universe exposed by simple method

Researchers have developed a simple technique that adds evidence to the theory that the Universe is flat.

Moreover, the method - developed by revisiting a 30-year-old idea - confirms that "dark energy" makes up nearly three-quarters of the Universe.

The research, published in Nature, uses existing data and relies on fewer assumptions than current approaches.

Author Christian Marinoni says the idea turns estimating the Universe's shape into "primary school" geometry.

While the idea of the Earth being flat preoccupied explorers centuries ago, the question of whether the Universe itself is flat remains a debatable topic.

The degree to which the Universe is curved has an effect on what astronomers see when they look into the cosmos.

A telescope on or near Earth may see an image of a celestial object differently from how the object actually looks, because the very fabric of space and time bends the light coming from it.

Christian Marinoni and Adeline Buzzi of the University of Provence have made use of this phenomenon in their technique.

The current model of cosmology holds that only 4% of what makes up our Universe is normal matter - the stuff of stars and planets with which we are familiar, and that astronomers can see directly.

The overwhelming majority of the Universe, the theory holds, is composed of dark matter and dark energy. They are "dark" because they evidently do not absorb, emit and reflect light like normal matter, making direct views impossible.

Dark energy - purported to make up 73% of the known Universe - was proposed as the source of the ongoing expansion of everything in the cosmos. Astronomers have also observed that this expansion of the Universe seems to be accelerating.

Even though gravity holds that everything should attract everything else, in every direction astronomers look there is evidence that things are in fact moving apart - with those objects further away moving faster.

Dark energy is believed to pervade the essence of space and time, forcing a kind of "anti-gravity" that fits cosmologists' equations but that is otherwise a mysterious quantity.

"The problem is that we do not see dark energy because it doesn't emit light, so we cannot measure it by designing a new machine, a new telescope," explained Professor Marinoni.
"What we have to do is to devise a new mathematical framework that allows us to dig into this mystery," he told BBC News.

The technique used in this study was first proposed in 1979 by researchers at the universities of Princeton and Berkeley in the US.

It relies on measuring the degree to which images of far-flung astronomical objects are a distortion of their real appearance. The authors originally suggested a spherical object would work.

The way the image is distorted should shed light on both the curvature of the Universe and the recipe of matter, dark matter and dark energy it is composed of.

The problem until now has been to choose an object whose real, local appearance can be known with certainty.

Professor Marinoni and Dr Buzzi's idea was to use a number of binary galaxies - pairs of galaxies that orbit each other.
Since nature shows no preference for the direction these galaxies would be orbiting one another, a look across the whole sky should spot the full spectrum of orbital planes - up, down, left, right, side-on and so on.
Put all of them together and they should approximate a sphere.

The idea was checked using data from the Sloan Digital Sky Survey The team formed a kind of average of all of those binary galaxies, and corrected for the varying speeds at which the galaxies might be orbiting each other.

The calculation also takes into account the relative proportion of dark energy in the Universe.

The equation was then juggled until the collection of binaries did indeed look like a uniform mix of directions.
The results suggest that the Universe is made up of about 70% dark energy.

"In general relativity, there is a direct connection between geometry and dynamics," Professor Marinoni explained, "so that once you measure the abundance of matter and energy in the Universe, you have direct information on its geometry; you can do geometry as we learn in primary school."

The team's conclusions suggest the Universe is indeed flat - an assumption first put forth by Albert Einstein and seemingly confirmed by more recent observations but that remains one of the most difficult ideas to put on solid theoretical footing.

Alan Heavens, a theoretical astrophysicist at the University of Edinburgh, said that the strength of the result lies in that it requires few assumptions about the nature of the cosmos.
"The problem that Marinoni and Buzzi have attacked is to see if we can get another, rather clean way of working out what the geometry of the Universe is without going through some fairly indirect reasoning, which is what we do at the moment," Professor Heavens told BBC News.
"They get complete consistency with [results from] existing methods, so there's nothing suprising coming out - thankfully - but it's a neat idea because it really goes rather directly from observations to conclusions."

However, while the abundance of dark energy seems on an ever-firmer footing, its nature remains a mystery.
"I don't think it can tell us in a lot of detail what the dark energy is," Professor Heavens said. "I think it's probably not precise enough - certainly not yet."

http://www.bbc.co.uk/news/science-environment-11810553

 

[TinFinger_]

iirc the reason currently though to explain why gravity is so weak( it covers multiple dimensions)could explain the missing mass

we cant tell what the other dimentions might contain on a galactic scale and there for cant say for sure how its infuence might be altered(gravity)
over all our dimension might be virtually empty as compared to other dimesions

 

[EnolaGaia]

Here's an odd and interesting new interpretation of the dark energy conundrum. Newly published research suggests the motive force accelerating cosmic expansion is pressure from nothingness - i.e., the massive cosmic voids themselves.

Giant voids of nothingness may be flinging the universe apart

Dark energy could be caused by pressure from giant voids of nothingness that may be flinging the universe apart.

Gigantic deserts of almost complete nothingness that make up most of the universe may be causing the expansion of the universe to speed up, new research suggests. That means these vast tracts of nothingness could explain dark energy, the mysterious force that seems to be flinging the universe apart. ...

FULL STORY: https://www.livescience.com/dark-energy-may-cause-voids