There are various cosmological phenomena which depend on the amount of dark matter or dark energy or sometimes fractions like how much energy in the universe is dark energy compared to anything else.
It's possible to compute how much dark matter there is because we can observe it's effects on visible matter. Galaxies come out being more massive than can be accounted for from their visible material. Turns out they are a number of times more massive.
Working out dark energy amounts is a little tricker. The equations of motion for the expansion of the universe depend on the value of dark energy in the universe (see
). Since we can measure things like the acceleration of the expansion and curvature, we're able to come up with values for the amount of dark energy in any given volume of space. Since we have values for the number density of galaxies in huge regions of space, we can then compare how much matter we can see to how much matter we can't see to how much energy we can't see. Hence the 4:25:71 (or so) ratios given for visible matter:dark matter:dark energy.
I'm not a cosmologist and I skipped the lectures on it provided at a recent summer school I went to, so I'm having to remember this from a few years back so the specific details are a little shaky but hopefully you get the drift. Ask if there's anything specific you're interested in.
why couldnt the dark energy turn itself into dark matter, and visa versa?
They are entirely different phenomena. The prefix 'dark' doesn't imply they are intertwined, but that they are something we don't know much about. We can infer their existence but not a lot else at present. This is an example of physicists admitting ignorance (hey, we don't have all the answers!) but putting it forth as an open problem to be worked on.
'Dark' was choosen because we cannot see the material. Normal matter interacts with light. We see the Earth, the Moon and the Sun because light bounces off them or is emitted from them. Dark matter is like the heavier, chunkier big brother of neutrinos. Neutrinos don't interact with light because they don't have electromagnetic charge. However, they are so light, they do not have much impact on large scale cosmology (though they play a critical role in supernovae). Dark matter is thought to be matter which is just as weakly interacting but billions
of times more massive than neutrinos, particle for particle. Many supersymmetry particle physicists have put forward various theorised particles which match such properties, the prime candidate being the lightest supersymmetric particle
, which has a good chance of being stable. The LHC will be searching for supersymmetry in the coming years and if it's found it'll be a huge boost to models of dark matter.
17th October 2007 - 11:16 AM
Cliff: Apologies if this isn't quite what you asked, but I hope it's of some help:
Dark energy is driving the expansion of the universe. This certainly seems to have taken place, and appears to be taking place at an increasing rate. Take a look at Dark Energy on wikipedia, and follow the link to the "equation of state". http://en.wikipedia.org/wiki/Dark_energyhttp://en.wikipedia.org/wiki/Equation_of_S...%28Cosmology%29
I can’t find something that shows the calculation clearly, but google on “How much dark energy” and perhaps you’ll find something you like:http://cfa-www.harvard.edu/seuforum/de_speedingup.htm
Dark matter was originally proposed as the reason behind what is called "flat galactic rotation curves". Galaxies often take a spiral configuration, exhibiting rotation. When you compare this rotation with the planets in our solar system, there is a marked difference. A planet like Neptune trundles around its orbit at a much lower velocity than the earth. http://www.astro-tom.com/getting_started/p...ssification.htm
But a star near the rim of a galaxy trundles around at much the same speed as a star closer to the centre. To explain this, Zwicky and others proposed that there’s a lot of hidden matter exerting a gravitational influence. http://en.wikipedia.org/wiki/Dark_matter
Again I suggest you google on “How much dark matter” to find something that shows a calculation or estimate that you like. http://www.faqs.org/faqs/astronomy/faq/part8/section-9.html
Dark matter is also used to explain unexpected observations of non-rotational galactic motion and gravitational lensing, see for example the bullet cluster
. However a number of other people dispute the existence of dark matter, and propose that it is explained by some other factor. The first person to do this was Mordehai Milgrom with his MOND theory. There are some issues with MOND, and there are variants. http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Can I chip in to say personally I favour a MOND-like explanation: the dark energy that has caused the expansion of space has caused more expansion of intergalactic space than of the space within galaxies. Thus space is not uniform. This means gravity does not follow the inverse square law exactly, and appears to reach out further than you would normally expect. Thus there is no need for dark matter. A similar effect might also apply to Pioneer.
17th October 2007 - 11:47 AM
Proof of dark matter is usually shown as a tiny blob of size where our whole solar system would be less than a pixel and saying that dark matter must exist there as we cannot explain it any other way. Alternatively christians will point to it and say God must have made it since it cannot have existed any other way.
Our theory of gravity is essentially that what works in our solar system works exactly the same across the whole universe on all scales. We have no evidence of that.
Dark energy is just a silly fudge to try and keep the discredited big bang idea afloat.
17th October 2007 - 01:36 PM
kaneda: I know what Dark energy is, and it isn't a silly fudge. Cross my heart and hope to die. Dark matter perhaps, but not Dark energy.
17th October 2007 - 08:25 PM
"Can I chip in to say personally I favour a MOND-like explanation: the dark energy that has caused the expansion of space has caused more expansion of intergalactic space than of the space within galaxies. Thus space is not uniform. This means gravity does not follow the inverse square law exactly, and appears to reach out further than you would normally expect. Thus there is no need for dark matter. A similar effect might also apply to Pioneer."
The total amount of gravity should not increase depend on the distance. In order to get "1/r" in the galaxy disk, the gravity should be unevenly distributed. And I am afraid that MoND theory can not explain the dynamics of the barred spiral galaxy. The bar is solidly rotating. http://www.geocities.jp/imyfujita/galaxy/galaxy01.html
17th October 2007 - 09:04 PM
Dark Energy is quite destinguishable from Dark Matter. They aren't interchangable, in the sense of time reversed.
Dark energy is a hypothetical form of energy with a negative pressure that is thought to permeate spacetime. According to Einstien’s theory of relativity, the effect of negative pressure is in opposition to gravitational force on massive and strong scales. It is believed by some that such a force maybe responsible for the universes evident accelerated expansion; which is ultimately indicating that our universe is dying!
Dark energy would make up the lions share of missing matter, making up about 74% of all matter in the vast universe. Two forms of dark energy are proposed as the 'cosmological constant,' a constant energy density filling spacetime homogeneously and there is 'quintessence', a dynamic field which has an energy density that can vary in space and time. It requires high-precision measurements to determine the exact speed of expansion.
During the 1990's, observations of type 'la supernovae' by the 'Supernova Cosmology Project' and the 'High-Z Supernova Search Team' have provided the suggestion that the universe is indeed accelerating - being backed up with several pieces of observable evidences which include measurements of the 'background microwaves', 'gravitational lensing' and improved measurements on observable supernovae have been consistent with something called the 'Lambda-Chromodynamics Model,' (but i will not explain this as it is not germane to the argument.)
The most direct evidence we have for dark energy comes from the la supernovae - the Hubble red-shift, a way of measuring 'time warps' from the light of receding objects - measuring how distant a thing is, is more difficult to say the least. Thus, it is necessary to find 'standard candles' - these universal candles actually help us in determining certain distances and magnitudes with good accuracy. Without standard candles, it is relatively impossible for us to measure red-shift distances without us making a wild guess, which is really not acceptable in the realms of science. Type la supernovae are the best known candles in our cosmos for our observations - simply because they are extremely bright and thus visible at distances of billions of light years.
The la supernovae is explained by the favored example of an old 'white dwarf,' which gains mass from a counterpart star and grows until it reaches the 'Chandrasekhar limit' - at this limit, the white dwarf is unstable to thermonuclear runaway and the result is that it explodes with a tremendous brightness, flaring ever outward with energy. It is this brightness that makes observing the Hubble red-shift possible - and it is our observations that indicate the universe isn't decelerating, but is in fact accelerating, at an alarming rate... and it is these observations that the acceleration in universal expansion might be caused by dark energy with negative pressure.
The presence of dark energy is really needed in physics, to reconcile with the measured geometry of spacetime with the total matter in our universe. Measurements of the cosmic background radiation made recently the 'Wilkinson Microwave Anisotropy Probe,' indicates that the universe is nearly flat - a big leap i must say from Einstien’s highly curved universe. For a universe to be flat, the mass-energy density of the universe must be proportional to the 'critical density.'
'Baryons' and 'dark matter' is thought to cover only about 30% of the universes critical density matter (more on dark matter later.) Thus, it implies the existence of an additional form of energy to account for the missing 70%. Before we continue, baryons are in the family of 'hadrons.' We have seen two types of baryons in this book - they being protons and neutrons that make up the nucleus of atoms.
The very nature of dark energy is a matter of speculation. It is thought to be extremely homogeneous, not very dense and doesn't interact with electromagnetic, strong or weak forces; only gravity. Since it is not very dense, around 10^-29 grams per cubic centimeter, it is very hard to comprehend dark energy being detected in the laboratory - with it making at least 70-74% of universal matter would have as you can imagine a drastic impact on expansion. Two leading models as we have seen, concerning dark energy are quintessence and the cosmological constant.
The simplest explanation for dark matter is that it is the 'cost of having space.' Thus a volume of spacetime has some kind of intrinsic, fundamental energy. This is the cosmological constant. It is sometimes called 'Lambda' after the Greek letter, which is used as a mathematical symbol to represent this quantity. The cosmological constant is estimated to be on the order of 10^-29g/cm3 or 10^120 in reduced 'Plank Units.'
The cosmological constant might be well known by the reader. It was first proposed by Einstein himself as a mechanism to proclaim a solution to the gravitational field equation, that would lead to Einstien’s strange dream; a static universe. Though, his calculations proved to be quite unstable, and that a static universe would be unpredictable due to local inhomogeneties which would result in a universe that contracts or accelerates - this means that the universe, if it expands only a tiny fraction will continue to expand due to a release of energy from the vacuum, and if it decelerates a tiny fraction, again it will continue to contract. Thus, we now know that the universe is accelerating, and Hubble red shift proved this fact - Einstien’s mistake went past history, as nothing but a curious flaw, as he himself said, it was his biggest blunder.
Dark energy might arise from the particle-like excitations in a dynamical field called quintessence. Quintessence can vary in spacetime, so it differs from the cosmological constant. Some scientists believe quintessence might be found in violation of Einstien’s 'Equivalence Principle.'
The 'cosmic coincidence,' theory asks the problem to why the universe accelerated when it did. If acceleration began earlier than it did, planets stars and galaxies would not have had time to form life - we ultimately would not have been here if it had started earlier. Promoters of the 'anthropic principle,' use this to support evidence of the universes means to create us - possibly by superintelligence, like God.
However, the 'tracker behavior' of quintessence solves this paradox. The tracker behavior model of quintessence has a density which closely tracks (but less than) the radiation density until the matter-radiation is equal, which ultimately triggers the 'quintessence field' to act as dark energy to dominate the universe. One case of quintessence which is thought to exist is called 'phantom energy,' where the energy density can increase in time. As weird and wonderful phantom energy sounds, it might actually result the universe in a 'Big Rip,' as we shall see later. I truly hope not.
'Exotic matter,' is a hypothetical form of matter that has both a negative energy density and a negative pressure - it is thus, an antigravity substance. All states of known matter have a positive energy density and pressure that are always less than the energy density in magnitude. A good example of energy density you may have heard of is in a stretched rubber band; when a rubber band is stretched, its energy density is about 100 trillion times greater than the pressure. If such matter could be created, it might help us to manage faster-than-light travel, or using it to grow wormholes big enough for human transportation back in time, as we saw in part titled 'time.'
Exotic matter is non-baryonic; and one form of exotic matter is called 'dark matter' - called dark matter, not because it is a darkly colored mass, but because scientists have no idea what it is. (Most) dark matter in the universe will be non-baryonic - and scientists are confident 20-25% of all matter in the universe is made up of the hypothetical and mysterious dark matter.
At the birth of our universe, 15 billion years ago, all that existed was a hot primordial soup of erratic particles. As the universe began to cool down, ordinary particles such as neutrons, protons and electrons started to join together to form stable atoms, forming all the elements we see today - which was predominantly helium and hydrogen atoms.
The theory of element-making in the first few minutes of the universe was called, 'the big bang nucleosynthesis,' and is recorded in the 'standard model'. The theory was good in predicting hydrogen and helium to pervade our part of the universe. However, the theory, it turns out, relied rather sensitively on the amount of baryonic matter that the universe had available - and the big bang nucleosynthesis predicts the right ratio's for the produce of elements in our universe today - even if 15% of the critical mass of the universe was suffice in stopping cosmic expansion (big crunch).
Of course, it may turn out that our predictions are flawed. However, because the prevailing view that the standard model predicts at least more than 15% of all matter is made up of dark matter, we can be sure that most of all dark matter are not made up of baryons - and since we are not made up of dark matter, it must be another type of matter that accounts for the dark matter. We are made up of protons, neutrons, gluons, electrons, quarks ect. - Just to mention a few.
Some dark matter might be well-known. Some scientists think that the 'neutrino' particle is in the family of dark matter. Billions upon billions of neutrino's pass through our body every second, spurted out by the sun. For a while, scientists thought that the neutrino might not contain mass, and moved like a luxen particle; like a photon, and traveled at the speed of light. Yet, this hypothesis was proven to be wrong, as it was shown to have a small mass after all.
Another hopeful candidate for non-baryonic dark matter goes by the acronym of 'WIMPS' - Weakly Interacting Massive Particles, that belong to a class of hypothetical heavy elements that hardly interact at all with common matter - hence the fact we have not discovered any as yet - heavy elements does not need to mean anything special - one heavy element we know of here on earth is iron but of a different class.
Some think that the WIMPS do not exist - basing this on the evidence that we have never seen one of these hypothetical particles, like the 'axion'. German scientists are planning an experiment that is essentially designed to 'tease' out dark matter in the form of the exotic particles that can travel through hard physical substances, like a mountain - just like the axion particle.
Dr. Andres Ringwald of DESY laboratory hopes to use a magnetic field to transform a laser beam of photons into axion-like particles. He say's, 'the idea is to send a laser beam along a transverse magnetic field, a fraction of the laser photons will transform into the new particles and travel freely through a wall without being absorbed. Finally, another magnetic field located on the other side of the wall can transform back some of these particles into photons - apparently from nothing.'
The discovery of exotic matter, if found through these experiments will extend the standard model of particle physics; some results that may even seem science-fiction-like, just as the good Dr. informs us, 'suppose that photon regeneration does work - you could set up an axion beam radio. Place the first part of the experiment somewhere to emit the beam and the second part far away, but in line with the first. You could then transport photons - and thus signals - over long distances through materials that normally absorb photons, such as rock and sea water.'
Scientists are sure these Wimps particles exist, and are in close rival of 'MACHOS' - Massive Compact Halo Objects. According to the theory of MACHOS, galaxies like our own are cocooned by exotic systems of dark matter haloes, which are populated by luminous objects, such as 'brown dwarfs', which is the remnant of a Red Giant.
We can use specialized lenses, that can focus and bend light rays from a source behind the observable objects, like a shadow play. This is called 'microlensing,' and has had some success - some scientists think we might be able to detect the MACHOS using this technique - using the same technique, they have discovered the presence of planets previously not detected.
However, not enough MACHOS have been found to account for a fifth of all dark matter - however, as you can guess, because Macho's are made out of baryons, they will be restricted by the big bang nucleosynthesis predictions, since baryonic matter makes up a fraction of all the matter in the universe. This must mean, that our calculations say that MACHOS are ruled out for having the bigger slice of dark matter pie. Another way to catalogue dark matter is to say whether it is of the 'cold' or 'hot' varieties. Very light dark matter that moves a fraction shy of 'c' - the speed of light - is called hot dark matter. Cold dark matter is accounted for by WIMPS. There is, as scientists suspect, more cold dark matter than the hot varieties - just like we have more Bradyons than Luxens in our part of the universe.
We also know that, whatever dark matter is made out of, it will be a major gravitational producer in the universe. Thus, scientists suspect that the missing dark matter most probably helped in the formation of galaxies. In fact, there will be entire galaxies made up of dark matter. Most of these galaxies will look like our own - others like colorful clouds, set out in all their array, as if it where on a magicians darkened stage.
18th October 2007 - 09:00 AM
Iori Fujita: I did say there are some issues with MOND. That's why I said "MOND-like". See figure 33 on page 35 of the pdf file here: http://www.relativityplus.info
for a simple illustration of why gravity appears to go from 1/r² to 1/r. See the area at the bottom right of the picture.
18th October 2007 - 01:53 PM
Farsight. I would feel happier with DE and DM if people had a clue as to what they are rather than just being the explanation of something unknown.
18th October 2007 - 02:21 PM
"why gravity appears to go from 1/r² to 1/r"
"1/r²" is for the three dimensional sphere, and "1/r" is for the two dimensional cylinder.
These are different with each other.
18th October 2007 - 05:28 PM
Kaneda: I feel happy with Dark Energy, and I think I know what it is. But because of this I'm not happy with Dark Matter at all.