Missing Matter?

bdewoody · May 30, 2007

There's been a lot written about the universe's "missing matter" and what it might be. I think a better term would be undiscovered matter. Like all these small galaxies they have just announced were found. It seems that sooner or later when our detection methods improve we will discover what's really out there. <div class="Discussion_UserSignature"> Bob DeWoody </div>

SpeedFreek · May 30, 2007

Well yes, but of course the "missing matter" isn't simply other stars and galaxies that we haven't found yet. It is the "missing matter" in each galaxy that we are searching for. Simply put, galaxies seem to exert more gravitational influence on the stars within them, than can be accounted for by the number of stars within them. It seems like there must be a lot more mass in each galaxy than we can see. According to current cosmology, only around 4% of the universe is made from "hot" visible matter (baryonic matter), around 21% is cold dark matter and the other 75% is dark energy. So if matter (seen and unseen) adds up to around 25% of the universe it is this matter we should see in each galaxy, but we only see the 4% and cannot find the "missing" 21%. One of the reasons we think that matter is missing is our observations of the motions of stars within galaxies. According to classical theory, the stars at the edges of galaxies should be moving a lot slower than the stars closer to the centre. But the stars at the edge are moving far faster than classical theory predicts, and should fly out of the galaxy due to their velocities. The fact that these stars don't fly out of the galaxy means something is missing from our understanding. At first, it seemed like maybe our understanding of gravity at the galactic scales may be wrong, but recently more and more evidence from different sources leads us to the conclusion that our understanding of galactic motion is indeed correct and something else must be stopping the outer stars from escaping their galaxies. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

jaxtraw · May 30, 2007

Here's some wildly cranky thoughts- Space is expanding. This is almost universally accepted. It is also pretty much accepted by everybody that space only expands in areas which are not gravitationally bound. It doesn't expand inside galaxies, we're told, or inside clusters, or even inside superclusters. It seems space is only allowed to expand in the voids between the galaxy clusters. This, if correct, tells us something very interesting- mass, or rather the gravitational field it creates, has a direct effect on the expansive urge of space. I've never seen it quantified just *how* gravitationally bound a region needs to be to stop expansion, rather there seem to be just general statements that space either expands (in unbound areas) or it doesn't (in bound areas) but it seems unlikely that there is this hard Boolean choice and we must therefore assume that expansion is proportional to... well, to the local gravitational field, somehow. We might say that at the centre of a cluster it's not expanding at all, and at the edge it's expanding quite a lot, and neither half way up nor halfway down the hill it's expanding a bit, or some amount. I daresay the reason I've never seen this amount quantified is I simply dont' read at a high enough academic level (i.e. at all) but it would be nice to find out how much gravity it takes to slow the Hubble constant to, say, one half. I'd sleep easier at night and I could impress women with my cosmological knowledge, and become a kind of "babe magnet" perhaps. Anyway, we can draw some amazing cranky, kind of flaky conclusions from these basic facts. Taking into account that the universe conserves mass, pretty much, except when it loses some converting it into energy, we can presume that in the past, the same mass being crammed into a smaller volume, there were less intergalacticcluster voids, or rather they were smaller, so therefore there was less space available to expand. So we should expect, as the voids

MeteorWayne · May 30, 2007

jaxtraw: "Space is expanding. This is almost universally accepted. It is also pretty much accepted by everybody that space only expands in areas which are not gravitationally bound. It doesn't expand inside galaxies, we're told, or inside clusters, or even inside superclusters. It seems space is only allowed to expand in the voids between the galaxy clusters. This, if correct, tells us something very interesting- mass, or rather the gravitational field it creates, has a direct effect on the expansive urge of space. " As I understand it, that is not correct. Space expands everywhere, the motion of objects within that space is controlled by gravity. I could be wrong. <div class="Discussion_UserSignature"> But the Krell forgot one thing John. Monsters. Monsters from the Id. I really, really, really, really miss the "first unread post" function </div>

SpeedFreek · May 30, 2007

Yes, that is my understanding too. The metric expansion of space is an incredibly weak force when compared to gravity, so gravity overrides it. Wherever gravity binds objects, the distance in between those objects does not increase - they are held together by gravity whilst space expands around them. One might think of the space in between them as expanding past them, if it helps. In the case where objects (or groups of objects) are not gravitationally bound, the distance in between those objects or groups will grow due to the metric expansion of the space in between them. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

jaxtraw · May 30, 2007

I've tried to get a clear answer on this and the consensus one always seems to be that space is not expanding in gravity controlled areas. If it were, if you think about it, then space would actually be "rushing past us", an effect that would be non-negligible on cosmic scales. Even on galactic scales, expanding space would try to carry stars out of the galaxy, so they'd have to orbit *slower* to compensate. "Space expands between the galactic clusters" seems to be the standard answer I find. It always seems a bit ad hoc anyway- the expansion is nowhere predicted by theory (relativity has nothing whatsoever to say about it). It's an observed effect, not a predicted one. Nobody knows why it's expanding. It just is. As such, I feel it's my right as a Crank to suggest any ad hoc hypothesis, preferably devoid of any supporting evidence, I care to, and then declare that the evil Einsteinist Anti-Plutonist "professional" astronomical establishment are suppressing my groundbreaking work, out of sheer jealousy. <img src="/images/icons/smile.gif" />

origin · May 30, 2007

I've tried to get a clear answer on this and the consensus one always seems to be that space is not expanding in gravity controlled areas. The consensus of physicists, is that space is expanding everwhere even within you. <div class="Discussion_UserSignature"> </div>

jaxtraw · May 30, 2007

Ah, but there's the nub. How would one experimentally verify that? Is that not just an assumption?

SpeedFreek · May 30, 2007

If it were, if you think about it, then space would actually be "rushing past us", an effect that would be non-negligible on cosmic scales. Even on galactic scales, expanding space would try to carry stars out of the galaxy, so they'd have to orbit *slower* to compensate. But the expansion is so slow that space would not be rushing past us at all. The expansion would be so small as to be unmeasurable in the local area. The rate of expansion is estimated to be around 75m/s per megaparsec. A megaparsec is approximately 3,262,000 light years. So that means an object 3.2 million light years away is receding at 75 meters a second. As the expansion is metric (the metric that defines distance changes over time), this means over a distance of 320,000 light years, the expansion rate is 7.5 meters per second, over 32,000 light years it's 0.75 m/s, or 1.6 miles per hour! There, an object 32,000 light years away would be receding from us at a heady 1.6 miles per hour if it were not gravitationally bound by being inside our own galaxy, which is over 100,000 light years across. That puts the force trying to push the stars apart at opposite sides of our galaxy at something around 5 miles per hour! Locally, gravity always wins. So the expansion is many magnitudes slower than the scenario you are describing, and gravity can easily dominate it. Actually, the scenario you presented is more reminiscent of the possible "heat death" at the end of the universe. If the expansion continues to accelerate there will come a time, many aeons in the future, when the expansion rate will overcome gravity, and indeed it might one day overcome the bonds within atoms themselves. But that's a long way off. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

SpeedFreek · May 30, 2007

How would one experimentally verify that? Is that not just an assumption? One could not experimentally verify it at such a small scale, we can only observationally verify it at distances over billions of light years. So yes, it is an assumption. Or perhaps it is a principle, eh? You seem to be treating space as a substance, streaming past galaxies and pushing them apart, but it is just a dimension. The metric that describes the dimension is changing. That dimension is space, a multi-dimensional nothingness. It is a property of this universe that distances increase between objects when no gravity binds them. Maybe it is better to think of it as an expansion of distance. If you are an object bound by gravity within your local group, as a star in a galaxy, a planet around your sun, a body upon your planet or within your own body, an increase in the distance of "space" is irrelevent to you, as nothing changes for you. It simply means objects not gravitationally bound to your system are becoming more distant. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

jaxtraw · May 30, 2007

speedfreek- "The rate of expansion is estimated to be around 75m/s per megaparsec" The Hubble Constant is around 77km/s/Mpc. You're out by a factor of 1000 <img src="/images/icons/wink.gif" /> That equates to, say, around 50km/s between us and the Andromeda spiral, which is mildly significant. It depends where you're measuring the expansion from, anyway. If the centre of gravity of your local cluster is some distance away, i.e. you're an outlying galaxy, you may find space whizzing past you much faster than that. Approximately, it's about 2.5cm/s per light year, so that's about 25km/s per million light years. If you were, say, 10M light years from the centre of the cluster, that's 250km/s. Not enormous, but quite big really. Bigger than very small, any way. <img src="/images/icons/smile.gif" />

jaxtraw · May 30, 2007

"You seem to be treating space as a substance, streaming past galaxies and pushing them apart, but it is just a dimension. The metric that describes the dimension is changing. That dimension is space, a multi-dimensional nothingness. It is a property of this universe that distances increase between objects when no gravity binds them." I find this attitude curious. We all sit around talking about the manifold properties of space; its shape, its permeability, its polarisation (in terms of the virtual particle swarm within), and then we say "and it's nothing". It sure sounds like a something to me. A metric is useful, very useful in math. But a metric isn't a thing. If you project a grid onto something you're measuring, and change the size of the grid, nothing happens, because your metric is just a notional quantity; a fiction; a construction for your own purposes. It has no indpendent existence. Outside of mathematics, it's the *metric* that is a nothing. Everyone's terrified of being associated with an ether. But the ether was a hypothetical magic substance that filled space. It's an entirely different thing to say that space *is* a something, which it clearly is. You can't bend a nothing. Nothings can't expand. Nothings aren't created, nihil ex nihilo. You can call the something a nothing if you like, but it's still a something.

SpeedFreek · May 30, 2007

Whoops! Yes, you are correct of course, the thing is I actually checked, but misread it! But the correct figure still equates to a very small increase in the distance of space at small scales. Even a factor of 1000 doesn't mean that much in this context. You keep saying that space would be whizzing past you but this is still clearly untrue. The space within a metre of you would be moving at 77km/s per Megaparsec. I will do the maths properly this time! 77(km/s)/Mpc = 2.772(mm/hour)/197,517,600 miles. <img src="/images/icons/smile.gif" /> That really doesn't constitute any form of "whizzing" I know of! Imagine the speed of space a meter from you - it is infinitesimally small. You say it all depends where you are measuring the expansion from, but this has absolutely no effect on your measurements. You use the example of an outlying galaxy, relative to the centre of gravity of the cluster. But the expansion is always at it's smallest, at it's closest distance, wherever you are! That's the whole point. The metric that defines distance is changing, over time. I have found that a good illustration of this effect would be to fill a space with ping-pong balls. All the balls are expanding at the same rate, at the same time. Now, wherever you are, the centres of the balls next to you are moving away very slowly, but the further you look, the faster the centres of the balls are moving away from you. Put yourself in a different place within this space and the effect would be exactly the same. <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

kelvinzero · May 30, 2007

I think Im going to side with jaxtraw, space is not expanding in areas where something is orbiting. Im also walking on pretty shakey territory though. My reasons for suspecting this are: (1) In general relativity gravity is not a force operating across space, it is the effect of objects travelling along straight lines through a curved spacetime. So if gravity counteracts the tendency for objects to recede from each other, it has done this by counteracting the effects on space, not by throwing a lifeline across space to hapless bodies being dragged away by the current. (2) All the discussions by physicists originally seemed to relate the expansion of the universe to the question of how much mass there is. If the universe had had enough mass to be a black hole, they would have kept showing us all those diagrams where the universe was a sphere and talked about all space and time crunching in on itself. Presumably this still happens in black holes anyway, if not everywhere. I could also be entirely wrong. I cant take this any deeper than the readers digest level <img src="/images/icons/wink.gif" />

jaxtraw · May 30, 2007

"I have found that a good illustration of this effect would be to fill a space with ping-pong balls. All the balls are expanding at the same rate, at the same time. Now, wherever you are, the centres of the balls next to you are moving away very slowly, but the further you look, the faster the centres of the balls are moving away from you. Put yourself in a different place within this space and the effect would be exactly the same." I disagree in this case; I agree of course in general. The expansion of space means that wherever you are is "stationary" and everybody else is moving away from you- that's something everybody knows (except the 99% who don't lol). However, this is only true if you are very small compared to the universe. Suppose I use the famous "surface of a balloon" analogy. We stick some little cardboard disks, to be galaxies, on the balloon, and we inflate it. Now we agree the disks (the galaxies) do not change size, because gravity holds them together. If you are in the centre of the galaxy (where it is stuck to the balloon) you see no movement of space (the balloon surface) beneath you. Now suppose you are on the edge of the carboard disk. Now you do see the balloon surface moving beneath you, as it expands. Not very much, but a little. The larger the disk of cardboard is, the faster you will see rubber running beneath your feet (er, you are an ant, perhaps, sitting on the perimeter of the cardboard disk!). A galaxy cluster is large, and its "carboard disk" is large. We can presume approximately that the centre of the cluster is roughly stationary relative to space- it has little proper motion. But towards the edge of the cluster, where the galaxies are bound, space must be expanding "beneath them". Imagine at some point in time the sphere of space which encapsulates the cluster. Some time later, that sphere has got bigger. The cluster hasn't. So, space must have "gone past" the outlier galaxies. What do

jaxtraw · May 30, 2007

KelvinZero- "2) All the discussions by physicists originally seemed to relate the expansion of the universe to the question of how much mass there is. If the universe had had enough mass to be a black hole, they would have kept showing us all those diagrams where the universe was a sphere and talked about all space and time crunching in on itself." Indeed! If gravity doesn't slow/stop the expansion of space, how indeed could there ever be a big crunch, and how could gravity slow the expansion of the universe (even if there's insufficient mass to stop it)?

SpeedFreek · May 31, 2007

This is a very interesting discussion, for sure! In the early universe it is thought that gravity slowed the expansion. This would be because all the objects in the universe were closer together, and gravity was binding them together against the expansion, slowing it down. The expansion was still happening, objects were receding from each other, but the pace of recession was being slowed by the gravitational attraction between them. But as the expansion slowed, the objects were still receding from each other, and thus the gravitational attraction still lessened until there came a point where the gravitational attraction between them was so small that the expansion "won the battle". Once the battle was won, space would expand at a constant rate, rather than a decelerating rate. This was the picture we had until a few years ago, where we observed that the expansion actually seemed to be accelerating again. The question of how the expansion might affect the objects themselves has been addressed in the past. One scenario has the expansion having a very small effect on the objects within the system - that equilibrium is reached at a slightly different size than it otherwise would be. This applies to atoms, galaxies, globular clusters, solar systems, everything! So we have a scenario here where the expansion of space causes all matter in the universe to be a little "larger" than it would be were there no expansion. The forces within atoms win out over the expansion, but they might settle into equilibrium in a different way to how they would settle without expansion. So it is indeed possible that space is expanding everywhere, including inside the atoms within our own bodies, but the forces that hold those atoms together win out and hold them together in a slightly larger state than they would be without expansion. But seeing as the expansion is happening everywhere, all matter would be affec <div class="Discussion_UserSignature"> _______________________________________________ SpeedFreek </div>

vandivx · May 31, 2007

"Everyone's terrified of being associated with an ether. But the ether was a hypothetical magic substance that filled space. It's an entirely different thing to say that space *is* a something, which it clearly is." --------------------- yes, you say 'everyone's terified' and evidently that includes you too ether wasn't 'magical' or at least wasn't meant to be that but if you should insist, why your *something* that you claim the space is is not magical in the same way but you're right that there is something everywhere, metric has to be metric of something, of itself the metric is just an abstraction ether is just a generic term for 'something' that is everywhere and permeates everything, that's all, and it doesn't matter if it was described wrongly in the past as to its specifics, no need to distance oneself from it or to be terrified of it <img src="/images/icons/wink.gif" /> vanDivX <div class="Discussion_UserSignature"> </div>

kelvinzero · May 31, 2007

I wouldnt mind if pysicists started calling space ether. We just dont have the particular sort of ether disproved by the Michaelson Morley experiments. There was nothing wrongheaded about them looking for it, and their negative discovery was an amazing event.

emperor_of_localgroup · May 31, 2007

jaxtraw: please stay with these SDC forums. SDC readers may accidentally and reluctantly learn something new from you. You may help them start thinking along a new 'dimension', which I failed. You will get my Nobel Prize vote, hahahaha. <div class="Discussion_UserSignature"> Earth is Boring </div>

jaxtraw · Jun 1, 2007

Speedfreak, I've been musing over how to answer your points and every time I seem to contemplate writing a book, heh. I'll try to be concise <img src="/images/icons/smile.gif" /> It seems to me that we agree that gravity opposes the expansion of space. Let's go back to your ping pong ball model, with a little qualification- let's imagine that your balls represent "gravity controlled areas". What I mean by this is that each ball contains mass which is gravitating. We can say that each of your balls defines an area in which gravity overcomes the cosmic expansion force (I avoid the term cosmological constant for a reason!). Let's call it Q. So, imagine two of your balls. When your balls are close together, there is little space between them which can expand. So, they move apart, but only slowly. As they get further apart, there is more space between them which can expand, so they diverge more quickly. Now at first sight, over the universe as a whole, this should look like the old understanding. But your balls take up a volume. Imagine now many balls (a whole universe full of balls in fact). As your balls diverge (and remember, being gravity controlled, your balls don't expand!) the proportion of space taken up by balls reduces. The ratio of expanding/not expanding space increases. This is what I'm suggesting leads to an observed increase in the rate of expansion. IOW, if your balls are crammed together, there's hardly any expanding space between them pushing them apart. Once your balls are far apart, more space opens up to expand, and the expansion increases in speed. This would lead to an illusion that Q is increasing, when it's not, merely the proportion of the universe which Q can push apart increases. It leaves us with a curious paradox though. In the distant past, there was a time when your balls were crammed tightly together- even overlapping- so there was no space at all available to expand. G />Q throughout the

jaxtraw · Jun 1, 2007

I'm not terrified of ether, but I don't think it makes much sense either. The ether was considered to be a medium that filled space- people looked for it dragging and swirling and all sorts. But that still leaves us with space to explain, and the ether to explain- which is kind of magical since it's supposed to be everywhere but undetectable, which is more like god than science. I see spacetime itself as the medium, rather than an ether which fills spacetime.

vandivx · Jun 3, 2007

The ether was considered to be a medium that filled space- people looked for it dragging and swirling and all sorts. But that still leaves us with space to explain, and the ether to explain- which is kind of magical since it's supposed to be everywhere but undetectable, which is more like god than science. you know if fish could talk it would say the same about water what you just said of ether - that its supposed to be everywhere but nobody has seen or detected it one major manifestation of ether is space, ie, space is an attribute of ether, without ether we couldn't move or look around (assuming we would be around at all which we wouldn't) so ether doesn't fill space(time), that's an old and mistaken view, too bad people can only keep repeating what the old guys once said centuries ago and bring nothing new to the table 'spacetime' is an abstraction that has no physical substance as it is used in physics today but if you want to take it as a medium that is everywhere, why then that's ether under different name and why play word games, important is that there is something out there everywhere and why not call it ether, at least then everybody knows what we talk about, using the term spacetime will only cause confusion among physicists one day this talk about undetectability of ether will be seen as that old man who looked for his glasses while he had them on his nose I'd rather say it is today's physics in many areas that seems 'more like god than science' in consequence of taking space as nothing and it will be so untill it wakes up from couple hundred years of mistakes launched by the two bunglers - the Michelson and Morley with their erroneous interpretation of their otherwise fine experiment vanDivX <div class="Discussion_UserSignature"> </div>

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