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View Full Version : Dark Matter - as difficult to detect as the Oort cloud.



Harry Fryer
2007-Nov-26, 08:44 AM
I was watching a programme last night about comets.
It was explained that short range comets could originate from the Kuiper belt (just past Neptune) and long range comets could orginate from the Oort cloud (outside our solar system).
The problem with confirming this is that comets and stuff in the Oort cloud/Kuiper belt is very dark. It only reflects 4% of the suns light. Bear in mind that a lump of coal reflects 8% of sun light.
After years of searching many Kuiper belt objects have been found.

This lead me to suppose that the elusive dark matter could be made of the same matter as in the Oort cloud/kuiper belt.
Furthermore, if the matter in the Oort cloud/kuiper belt is the remnants of the matter that made up the planets/satellites in the solar system, then all planetary systems could have their own Oort clouds. As we are finding more and more planet-like objects then there must be more and more Oort cloud matter.

undidly
2007-Nov-26, 09:01 AM
Comets are objects from the Oort cloud/kuiper belt and are made from normal matter.What is the atomic weight of dark matter?.Is it made of atoms?,is it matter at all?,is it real?.
I think not .It is just an explanation for what looks like unexplained mass.
The real reason is that G does not fall off inversely as the square root of distance.Newton got it wrong,but only by a microscopic amount.

Harry Fryer
2007-Nov-26, 10:36 AM
I understood dark matter to be thought of as having the same gravitational properties of matter but just very difficult to detect.

Laguna
2007-Nov-26, 10:39 AM
I understood dark matter to be thought of as having the same gravitational properties of matter but just very difficult to detect.
We have no idea what dark matter actually is.
All we now AFAIK is that it has mass and that it does not emit any radiation, like "bright matter" does.
Dark Matter is a synonym for something fundamentally different than bright matter.

Iori Fujita
2007-Nov-26, 11:55 AM
In 19th century, all people talked about "Aether" which A. Einstein gave the final sentence. But even now some believe in the existance of Aether. Then this Aether has come back to life again like "zombie" in the universe of the 21st century in a form of "dark matter" or "dark energy".
"Aether" and "dark matter" have a common point, which is "unseen".

http://www.geocities.jp/imyfujita/galaxy/galaxy01.html

I think Newton is right and that Einsein is right. But something is stiil unseen. So I try to add one thing. "Light bends Gravity."

elliptical galaxy e.g. NGC4881 Three Dimension GM(<r)m/r*r = mv*v/r
spiral galaxy e.g. NGC4414 Two Dimension G'M(<r)m/r = mv*v/r
barred spiral galaxy e.g. NGC1300 One Dimension G"M(<r)m = mv*v/r
Emissions like strong lights will distort the gravity.

Iori Fujita

Harry Fryer
2007-Nov-26, 12:13 PM
what makes the matter in the Oort cloud different from Dark Matter?

As far as I understand it, Oort cloud matter does not give off radiation, otherwise it's existence would have been much easier to prove.

Laguna
2007-Nov-26, 12:42 PM
what makes the matter in the Oort cloud different from Dark Matter?

As far as I understand it, Oort cloud matter does not give off radiation, otherwise it's existence would have been much easier to prove.
Thee Oort Cloud consists of dust, ice and rock. Those are made from protons neutrons and electrons like you and me and the rock you are standing on. Dark matter is not.

Do not confuse matter that is too cold to radiate enough energy for us to detect with dark matter.

Harry Fryer
2007-Nov-26, 01:20 PM
Thanks Laguna2,

My problem with this is that we do not know what dark matter is. We find it difficult (although not impossible) to detect its effects.
Dark matter is considered to be matter due to the discovery that there is not enough conventional matter in galaxies to explain their evolution and the gravitational effect that dark matter has on the rest of the 'visible' objects make it appear to be matter or matter-based.

Oort cloud matter reflects very little light, and little if no radiation. They are also difficult to detect even though we can see the gravitational effect they have on other objects. I understand that the reason the Oort cloud gives off little radiation is because it is at the extremity of the solar system and receives very little heat from the Sun. Do other 'cold'/distant objects also not give off radiation? How do we detect those?

antoniseb
2007-Nov-26, 06:11 PM
Oort cloud matter reflects very little light, and little if no radiation. They are also difficult to detect even though we can see the gravitational effect they have on other objects.
We do detect the Oort cloud in the form of a steady stream of long period comets coming back to perihelion. This gives us some idea of the total population and mass of the Oort cloud. What would you guess is the mass of the Oort cloud? What would you guess is the appropriate amount of dark matter for the roughly one light year radius that the Oort cloud has?

There probably is something like the Oort cloud around most stars. The matter in these clouds do not provide a significant digit in the sum of all the mass in the galaxy. So yes, in English parlance, the Oort cloud is pretty dark, and most of the matter in it is hard to detect, but it is not what we call dark matter... If it were, it is possible that the dinosaurs would not have died off 65 million years ago, as that comet clearly interacted with the matter on the surface of the Earth.

Harry Fryer
2007-Nov-27, 08:21 AM
Thanks for all your replies.

I kinda like the idea that there is a 'soup' of comet-like matter outside of all celestial objects.

grant hutchison
2007-Nov-27, 12:11 PM
My problem with this is that we do not know what dark matter is.We do know it's not conventional (baryonic) matter, though.
We have an idea of the density of baryons (protons and neutrons) in the early universe, because that density would affect how much fusion went on in the aftermath of the big bang, which would affect the relative abundance of light elements we see today. The ratios we see in the present universe indicate that there just weren't enough baryons around to fully account for the total mass we observe in the universe today: there has to be some component that isn't conventional (made of baryons) matter. And that is what people refer to as "dark matter".

Grant Hutchison

nutant gene 71
2007-Nov-28, 09:29 PM
How can we be certain 'Dark Matter' is not baryonic matter merely exhibiting what it shows, more gravity? Hence, we can use 'dark galaxies' for gravity lensing, but we cannot see them for lack of luminosity. Why not 'non-luminous matter' instead of exotic dark matter, but which simply exhibits more gravity than what we know of local baryonic matter? Is there a specific reason why this is not possible? :think:

BaDboD
2007-Nov-29, 07:02 PM
Newton got it wrong,but only by a microscopic amount.

Newton got it perfectly correct, for earth and the distances involved therin, at that scale. It is the changing relationship between masses, distances and gravity and, with at the time unrealised, scales; which is wrong. Sweetser was nearly there. I'm not here to explain the simple. Needless to say the missing element is elementary, yet denied.

cmsavage
2007-Dec-03, 08:20 PM
How can we be certain 'Dark Matter' is not baryonic matter merely exhibiting what it shows, more gravity? Hence, we can use 'dark galaxies' for gravity lensing, but we cannot see them for lack of luminosity. Why not 'non-luminous matter' instead of exotic dark matter, but which simply exhibits more gravity than what we know of local baryonic matter? Is there a specific reason why this is not possible? :think:
There are a few reasons why most dark matter cannot be baryonic. Actually, though, when dark matter was first proposed to explain the motions of galaxies and their rotations, stellar remnants (neutron stars, black holes, etc.), planets, comets and other condensed forms of baryons that do not emit significant radiation (i.e. they were dark) were obvious candidates. These are typically referred to as MACHOs (Massive Compact Halo Objects). Over time, MACHOs lost favor.

When a MACHO passes in front of a distant star, their mass causes light to bend and the star will appear to brighten (called gravitational microlensing). Microlensing should occur often enough for us to have noticed if these things are abundant enough to account for all the dark matter. Current limits suggest MACHOs can make up at most about 10% of the dark matter, with the caveat that the microlensing results do not apply to things about the size of comets or smaller.

However, comets and asteroids are mostly (by mass) made up of carbon, nitrogen, oxygen, or heavier elements. Hydrogen and helium do not condense except at extremely low temperatures and comet sized objects composed mainly of these two elements would very quickly evaporate and return to gasses or ions that are very easy for us to detect. Only large objects such as large planets or stars have enough gravity to keep these light elements from escaping back out into the galaxy, but those are the objects that have been ruled out by microlensing results. So the only way for MACHOs to be the dark matter is if they are all in small objects like comets or asteroids that are composed of heavier elements like carbon, oxygen, etc. The problem is that, if the universe is 90% carbon or heavier, there would be very obvious differences in stars, for one. Almost all stars are composed of ~99% hydrogen and helium (that can be measured by looking at their spectra). It is hard to explain (a) why only the hydrogen and helium initially collapse into stars when most matter is in the form of other elements and (b) why these comets/asteroids do not fall into the stars at the rate that would be expected and change the stars' composition well past the point that they can be detected?

One MACHO that is still a potential candidate is primordial black holes. These are black holes that were created sometime around the Big Bang, but still must have typical masses equivalent to that of comets or smaller to avoid the microlensing limits. The problem with this is that we don't know how many such black holes could have been created that early in the universe and only at such small sizes. But then again, black holes are not baryonic (they may have been created from baryons, but only extremely early in the universe).

Finally, measurements of the CMB and nucleosynthesis show that there must have been non-baryonic dark matter in the early universe. During the Big Bang, everything was very hot and mainly uniform. Initially, all matter was in the form of a dense sea of energetic particles (basically a plasma). At one stage, baryons were all in the form of protons and neutrons: heavier elements could not form because energetic ions would hit them and cause them to disintegrate. As the universe expanded and cooled, however, the protons and neutrons began to combine without being destroyed since ions were no longer energetic enough to knock them apart again. But there is a catch: a neutron is unstable and decays to a proton, electron, and anti-neutrino with a half-life of only a few minutes. When the universe was hot, energetic collisions caused the reverse process to occur, turning protons into neutrons, so neutrons were initially available. But once the temp cooled and the neutrons start decaying, the formation of heavier elements could no longer continue. So there was only a short period where heavier elements/isotopes were created. Mostly, only deuterium and helium were produced, with very tiny amounts of Lithium and a couple other elements/isotopes. The relative abundances are very sensitive to how the universe evolved during that short period of creation (called Big Bang Nucleosynthesis) and the presence of dark matter (remember baryons are only in the form of protons and neutrons at this point) has a very specific affect on how the universe evolves. Measurements of those abundances indicate that there was a significant amount of dark matter around at that point, which was necessarily non-baryonic.

The CMB anisotropies (measured by WMAP and other experiments) also showed that, during the Big Bang, the majority of matter was not interacting with the plasma like protons and neutrons do, so there is another indication that dark matter is predominantly non-baryonic. That is a longer explanation, so I will not give it here (unless someone really wants it).

torque of the town
2007-Dec-03, 08:33 PM
I was watching a programme last night about comets.
.



Your a Sky at Night fan then?

nutant gene 71
2007-Dec-04, 09:42 PM
There are a few reasons why most dark matter cannot be baryonic. Actually, though, when dark matter was first proposed to explain the motions of galaxies and their rotations, stellar remnants (neutron stars, black holes, etc.), planets, comets and other condensed forms of baryons that do not emit significant radiation (i.e. they were dark) were obvious candidates. These are typically referred to as MACHOs (Massive Compact Halo Objects). Over time, MACHOs lost favor.

When a MACHO passes in front of a distant star, their mass causes light to bend and the star will appear to brighten (called gravitational microlensing). Microlensing should occur often enough for us to have noticed if these things are abundant enough to account for all the dark matter. Current limits suggest MACHOs can make up at most about 10% of the dark matter, with the caveat that the microlensing results do not apply to things about the size of comets or smaller. …snip…
Thanks CM, I copy that, great explanation.

My curiosity was triggered by MACHO-like matter where we found evidence of ‘dark galaxies’ resulting in microlensing, i.e.: First Invisible Galaxy Discovered in Cosmology Breakthrough (http://www.space.com/scienceastronomy/050223_dark_galaxy.html). In a more recent article, Invisible Matter Loses Cosmic Battle (http://www.space.com/scienceastronomy/071203-mm-dark-galaxies.html), where it says: ” Astronomers have long tried to explain theoretical models that predict there should be much more dark matter in the central regions of dwarf galaxies than observations suggest is the case.” The suggestion seems to be (through computer simulation) that the early universe had some sort of ‘feedback’ which increased matter (gas) density in the central regions of dwarf galaxies. However, whether such high densities still exist, outside the central core black hole of a spiral galaxy, for example, is not clear. I suspect that remnants of the original galaxy formations had undergone sufficient evolution that whatever existed in their early origin may have been so modified by later galaxy-energy dynamics as to make it difficult to observe today, though we know from galaxy rotation curves that there is something ‘dark’ and ‘heavy’ to account for non-Newtonian dynamics. Smaller objects, such as comets or asteroids, even if they were detectable at extreme distances from the Sun may not be massive enough to cause microlensing, so they could be disqualified as material evidence. But ‘dark galaxies’, if they were made up of ordinary baryonic matter (mostly gas and dust) as to cause gravitational lensing but not visible to ordinary observation at any e.m. wavelength, might point in the direction of matter, dark and invisible, as having properties of much higher gravitational value (i.e., higher Newton’s G factor) than ordinary matter that is visible (because it glows), which would be an event worth pursuing.

However, since at this time we have no such direct evidence, only anecdotal evidence, that perhaps Newton’s G is not a universal constant (far away from Earth’s observation capability – we can’t go out there to check), then this remains a moot point. The Pioneer Anomaly might be some indication of this possibility, that cold dark matter is higher G matter, but until we can confirm it from other sources (not just MOND), then we for the present must rely on the model such as you presented, which is supported by Big Bang cosmology and the CMB measurements. The MACHOs might not really be in the running, but perhaps still insufficient evidence to write them off entirely? Not totally sure we need 'exotic' dark matter, when ordinary matter can do it, if just higher gravity G. That said, it would require a complete rewrite of known gravity physics, since this would of necessity mean the universe is 'isotropic and homogenous' at some (much higher) value of G other than that computed in current models as being universal to account for all that CDM. :eek:

I think that was the gist of my original question: Do we know for certain that CDM is positively NOT ordinary baryonic non-luminous matter far out there in intergalactic space, or even within galaxy space, that exhibits mighty G to account for the (other 80%) MACHO factor?

Perhaps, unless there is a very strong reason this cannot be true, I would think, so MACHO-like matter gets a second wind. In effect, we would have to reverse the normal order of things, that in some regions of space (for reason now not known or understood) matter in 'cool' stars, like white and brown dwarfs, or space dust and gas, like 'dark galaxies', may exhibit something unknown to us here, which is a much higher G factor for ordinary matter. And if this were true, could it account for the non-decay of neutrons in neutron stars, for example, where the normal neutron to proton decay is arrested by the much higher G factor? As for the Oort Cloud, we know it's there, but I think it is too far away and not luminous enough to really observe what it is really made of. Can it be ordinary matter?

grant hutchison
2007-Dec-04, 09:57 PM
As for the Oort Cloud, we know it's there, but I think it is too far away and not luminous enough to really observe what it is really made of. Can it be ordinary matter?We get to observe samples of the Oort regularly, because that's where the long-period comets come from. (In fact, we infer the existence of the Oort by calculating the orbits of these comets as they pass through the inner system.) By looking at the spectra of Oort comets, we see that they are composed of ordinary matter containing identifiable atoms and molecules.

Grant Hutchison

cmsavage
2007-Dec-05, 05:19 PM
I think that was the gist of my original question: Do we know for certain that CDM is positively NOT ordinary baryonic non-luminous matter far out there in intergalactic space, or even within galaxy space, that exhibits mighty G to account for the (other 80%) MACHO factor?

Perhaps, unless there is a very strong reason this cannot be true, I would think, so MACHO-like matter gets a second wind. In effect, we would have to reverse the normal order of things, that in some regions of space (for reason now not known or understood) matter in 'cool' stars, like white and brown dwarfs, or space dust and gas, like 'dark galaxies', may exhibit something unknown to us here, which is a much higher G factor for ordinary matter.
From direct observations, we know:

Dark matter in the early universe was not baryonic (from CMB and nucleosynthesis).
The majority of dark matter in the universe today is not in the form of MACHOs greater than about 0.0000001 times the mass of the Sun (from microlensing).


We infer based upon theoretical reasons (or, perhaps, prejudices) that the dark matter in the early universe and the dark matter today are the same, in which case the current dark matter must be non-baryonic (since that is what it was early on). There is nothing that says those two have to be the same and, if they are not, today's "dark matter" could be explained by a modified gravity (e.g. MOND) or very small MACHOs. But the early universe dark matter still needs to be non-baryonic and has not been shown to be explained by a modified gravity. So you probably still need exotic dark matter and now you need for that early dark matter to disappear at some point so that small MACHOs (or MOND) can take over. The theory becomes much more complicated this way. Rather, assuming that the early and current dark matter are the same seems like a very reasonable assumption.

If present day dark matter is in the form of small baryonic MACHOs like comets or asteroids, we just cannot understand how such objects could have been created and exist in sufficient numbers without be noticed or significantly altering the state of the universe. There should be billions of these things in the space between our Sun and the nearest stars and some should be passing through our solar system regularly. It is very difficult to explain why they don't.

These are the reasons why MACHOs are generally discounted as the current dark matter. It is not proven they are not, but theories have to become fairly contrived and complicated for that to be the case.


And if this were true, could it account for the non-decay of neutrons in neutron stars, for example, where the normal neutron to proton decay is arrested by the much higher G factor?
Actually, the presence of neutrons in neutrons stars rather than protons and electrons is explainable with current physics. In free space, neutron -> proton + electron (ignoring neutrinos here) because energy is released in this way. However, under the extremely high pressure in a collapsed star core, the opposite happens: proton + electron -> neutron allows energy to be released because it allows the matter to become more densely packed (protons repel each other, neutrons do not).


As for the Oort Cloud, we know it's there, but I think it is too far away and not luminous enough to really observe what it is really made of. Can it be ordinary matter?
Yes. The Oort Cloud is expected to be icy bodies like comets through Pluto sized planets. Several such planets have now been observed. But the total mass of the Oort cloud is expected to be much smaller than that of Jupiter and is a negligible part of the solar system's mass.