[Bobboau]

dark energy, the suposed force that pushes galaxies apart, violates the law of conservation of energy.

if you were to fix an infinitely long string to object A who's far enough away from object B as to be repulsed via dark energy, and you ran that sting around an axle which was attached to a generator on object B, you'd have perpetual energy. because dark energy will perpetually accelerate the two objects away from each other, and the farther they get the more they will be accelerated.

so either the theory of conservation of energy is wrong (doubt it), or the dark matter energy hypothesis is wrong.

[watsisname]

Eh, but dark energy is (supposedly) expanding the space itself, instead of moving the objects directly.  Not sure if Conservation of Energy is being violated in that case.

[Goober5000]

Mmm, I don't think so.  In that scenario, the objects would be moving from a high energy state to a low energy state, converting "dark matter potential energy" to kinetic energy.  That doesn't violate the law of conservation of energy any more than a brick sliding down a ramp pulling a string violates it.

Good idea though.  This is the kind of thread we need more of in this forum.

[Bobboau]

well the thing is you will never EVER run out of dark energy, the system would never break down, entropy in the universe would decrease, dogs and cats would sit down and solve there differences diplomatically. it's as if there is some infinite wellspring of energy that can be tapped, and this is imposable, energy cannot be created. it's a brick moveing down a ramp that is constantly being move up, there is no ground state.

[Flipside]

So the next time we encounter a subspace string, it's Bobbs fault?

[Admiral Nelson]

Not really.  If you has two stars next to one another, over time the one without the string attached would gradually outpace the one with it attached.  There is no reason to think that the string would not eventually cause the star to halt its motion away from the other object to which it is attached.  Eventually the star would stop or the string would break.

[Bobboau]

we are assuming a string here of infinite length and strength, to represent some hypothetical means of converting the energy of motion of the two objects away from each other into something more practical, but on a basic level I'm attacking the concept of dark energy from the fact that it will cause objects to accelerate infinitely, and thus gain infinite, or unlimited at least, energy. the only problem is from a practical perspective there is no way for these objects to interact, but this shouldn't matter the energy of the system is going up, or I suppose if you look at it another way it's going down, but the point is it's changein which is the violation.

the thing with dark energy is that the further you get from an object the stronger it gets, so it the energy being extracted by the string is less than what is needed to move the two objects the amount of dark energy effecting the two objects will only get stronger, the stars or what ever would just keep moving faster and faster.

[aldo_14]

dark energy, the suposed force that pushes galaxies apart, violates the law of conservation of energy.

if you were to fix an infinitely long string to object A who's far enough away from object B as to be repulsed via dark energy, and you ran that sting around an axle which was attached to a generator on object B, you'd have perpetual energy. because dark energy will perpetually accelerate the two objects away from each other, and the farther they get the more they will be accelerated.

so either the theory of conservation of energy is wrong (doubt it), or the dark matter hypothesis is wrong.

Aren't dark matter and dark energy different theories, anyways?

[Bobboau]

yes I mis-typed that last bit, sorry

[perihelion]

Completely different.  I doubt I'm going to beat Herra to the punch by more than a few minutes, so I'll let him explain the details.  However, the gist of it is that dark energy is what (theoretically) is responsible for accelerating the expansion of the universe.  Everywhere you look, the further away an object is from you, the faster it is receding from you (ie, the greater its cosmological redshift).  Dark energy is one possible mechanism to explain why that is happening.  Dark energy would be causing the "fabric of space" to expand dragging matter trapped within that fabric along with it.

Dark matter is much more like "normal" baryonic matter.  However, it only interacts with normal matter through gravity.

I was reading a Scientific American article a few days ago dealing with dark matter and dark energy, and one thing struck me that I haven't been able to puzzle out.  If dark matter only interacts through gravity (no electroweak, no strong nuclear) why is it forming into "halos" and not dark matter black holes?  I mean, if gravity is the only force it experiences, what would stop it from collapsing in on itself?  Especially just after the Big Bang when the average density of the universe was much higher?  The only reason all stars don't collapse into black holes is that strong nuclear interactions create radiation pressure to balance gravitational pressure.  Does dark matter have its own set of nuclear and electromagnetic forces, it just doesn't interact with ours (baryonic matter's)?

Heh.  Have at it, Herra.

[Admiral Nelson]

This seems to assume that an infinitely long object or connection can be created without energy.  An infinitely long object would have infinite mass and thus require an infinite amount of energy to create.  It is thus you connection which provides the infinite energy.  Since you assume that a connection of infinite length can be used to generate energy, it isn't surprising that the amount of energy generated would itself be infinite .  Therefore it is the hypothetical connection which violates the law of conservation of energy, not the dark energy theory itself.

[Herra Tohtori]

I'd say it's just the universe screwing our simian-eukleidean-newtonian world perspective.

I mean, we're used to saying that if something is getting further from us faster and faster, it is "accelerating" and we also naturally assume that there needs to be a force causing that acceleration. The problem in this is that it works exellently in a place where there's three linear stable space dimensions and one linear time dimension, which is a often a good approximation of the universe in small scale but it stops being accurate in many cases.

In this case, if there's more space being generated everywhere, universally, then there is no need for a force to exist to accelerate objects away. Which is, of course, the case here... and a reverted case of this can be seen when you pick an object into your hand, and let it go. It'll fall... and on our simian context it seems to be accelerating towards the center of Earth's gravity. But if we put ourselves into the object's reference frame, the force suddenly doesn't seem to exist any more - in fact it appears that something forces Earth to accelerate towards us.

Gravity, like the expanding factor of universe, is an apparent force, a phenomenon that affects matter through the shape (and amount) of space in between the objects. It's not a direct force like between two electric charges... It's just space tossing things around in geodesic trajectories that just appear curved to our perspective.

Not to mention that no one actually knows why the most fundamental law of physics, conservation of momentum (ie. energy) actually works. No one knows for sure why inertia exists, or why inertial mass is exactly the same as gravitational mass. We just know that this stuff seems to work... as long as the space stays pleasantly eucleidean. If it doesn't, we start having anomalies like the perihelion of Mercury, guidance satellites throw us dozen kilometres off-target etc. etc.

So, if you want to think the universe as a big box full of stuff expanding from the center... then you're right, conservation of momentum (energy) doesn't apply in itself. But thinking out of the box... every point is the center of expansion, experiencing no changes of momentum and hence there is no net increase of (kinetic) energy in the universe. The generation of space is uniform phenomenon in the universe, although gravity tends to negate it's effects (which, of course, makes complete sense in a way).

No one also knows if energy is needed to create space. Of course the easiest way to increase the local amount of space is to concentrate a whole lot of stuff into one point, creating a distortion in time/space, which curves the space locally and stretches the space, increasing the amount of it. For example, if you take a hermetically sealed cube with static temperature, fill it with 1 atm pressure gas and take the cube into space far from heavy objects such as stars, planets and stuff... you should notice a small increase in the pressure of the gas inside the cube, because the amount of space limited by the cube's dimensions will decrease as the curving of space decreases.

It is, in fact, possible that bit by bit, the mass of the universe itself is slowly causing the curvature of the universe to change, which causes stretching, which we see as increase of space between objects, ie. expansion.

But I wouldn't know for sure. 


EDIT:

->perihelion: Got me.

I don't know much about dark matter, but the thing to remember with it is that it seems to be affected by gravity and weak nuclear interaction. That also explains why it doesn't collapse to itself... It just keeps going through itself. Kinda like superfluid - are you familiar with the term? For example, you can pump superfluid in a pipe to both directions at once, they just pass through each other.

The reason for this is that touch is interaction dealt purely with electrodynamics. Electrons bumping from each other. Dark matter can't be touched and it can't touch itself. Kinda like cosmological King Midas there. So consequently, dark matter cannot form any points of concentration. It can orbit one point or several points in fact, but it doesn't hit itself so it isn't likely to form energy density high enough to form event horizon (ie. black hole). Even if you make two "hunks" or clouds of dark matter fall directly towards each other, they will pass through the center of gravity or close by, pass each other, and start resonating around the center of gravity.

[Bobboau]

but on a basic sense, if I had a 50 million light year long string and tied it around a moon in another galixy wouldn't the effects of dark energy cause the string to move on my end? (ignoring the fact that the two points of interest are both orbiting different points of rotations and ignoring all other matter in the universe). this expansion isn't causing matter to expand with it, otherwise there would be no detectable change, so the string, would have to get draged along with the moon.

[Herra Tohtori]

but on a basic sense, if I had a 50 million light year long string and tied it around a moon in another galixy wouldn't the effects of dark energy cause the string to move on my end? (ignoring the fact that the two points of interest are both orbiting different points of rotations and ignoring all other matter in the universe). this expansion isn't causing matter to expand with it, otherwise there would be no detectable change, so the string, would have to get draged along with the moon.

Well, you don't have a 50 million light year long strings, do you?

However, you do have much longer strings of fotons. They stretch on the way. Kinda...

Anyway, let's make things a bit simpler. Assuming you had this very light string made of rigid material and you would position it between two observers, one megaparsec away from each other, and waited a second... then the distance between observers would increase by about 71 kilometres. Which means that the center of gravity for the string would stay at the same place locally, both ends of the string would distance from the observers by half that amount.

However, if the string's other end was fixed to other observer... then there would be two objects, Observer A + string, and Observer B. Assuming the objects would be rigid, the expansion of space would effect between the centers of gravity for these new objects. For the sake of simplicity, let's assume the observers are both guys, keeps the pronouns in check easier. Plus I doubt many women would bother to do this experiment...

Assuming that the string is really really light and only shifts the center of gravity a bit, keeping it inside Observer A, then yes, it would indeed move away from Observer B. However, Observer B would see that as Observer A pulling the string away while moving away, and Observer A would have to exert force to the string to keep it attached to himself.

Obviously, though, signal speed becomes a problem here. A rigid string is an impossibility. The expansion from T(0) to T(1 second) would affect the distance between the observers instantaneously during the second, but both observers would only see the change after 3,262,000 years (Megaparsec is about that many light years). Hence, the string would invariably stretch. In this phase, things start getting really really confusing, what with observers being on different time cones and stuff, and common sense simply leaves us stranded. The observers don't even have the same present. Both observers observe each other from 3,262,000 years in the past. How fracked up is that... You simply couldn't arrange your string experiment. Even if you manage to do it as a thought experiment, it's because you're bypassing things that actually would render the test obsolete by several millions of years.

[Bobboau]

so, the force on the string from OB would not transmit instantly, but it would propagate through the string untill it gets to OA, once that force gets to that point on the string it would start to move twards OA. now this is all just a round about way of saying that the energy between them is changing with no outside input to the system, this is the point and debating the technical challenge of making this string and araingeing it is irrelevant, if in that araingement you could convert dark energy into some other form of energy, then dark energy is a form of energy and is being created every second, and conservation is not held. that is the key point to, well, my point.

it wouldn't be suprizeing if dark energy turned out to not be what we thought it was, given how new the observations are, we've only had a few years sence we first detected it.

[Admiral Nelson]

The real problem with the original statement is that it postulates an infinitely long connection between objects.  This connection, be it a string or anything else, would itself require an infinite amount of energy to create.  Otherwise you have a device that can create energy from nothing -- a perpetual motion machine.  It is actually dark energy that isn't really relevant here.  The original statement boils down to "Perpetual motion can exist if one assumes that a perpetual motion machine exists".

[Herra Tohtori]

Perhaps the distance between galactic supergroups doesn't change after all.

Perhaps the time just passes faster and faster in the empty parts of space, far from gravitational interference which makes time pass at this speed where we are. Although I must say that I personally think that expanding space is more believable than constricting time... but I wouldn't know for sure. They would have much the same effect I'm afraid, so there's really no telling after all.


By the way, you could kinda simulate what would happen if you had two space ships, a long long cable and a black hole.

Simply fly the two ships on tangential trajectories, linked with the cable, and make them pass the black hole so that the cable swoops closer to the event horizon than the ships. That should theoretically be essentially the same thing as we're talking about here, at least to the point where the distance from event horizon starts to increase again; as the cable gets closer to the horizon, the volume of space increases as its curvature increases. That should mean that both ships further from the horizon should experience a tugging force towards the centerpoint of the cable; if the cable was on coils on both ends, the coils should spin to release more cable to fill the increased amount of space between the ships.

Obviously, the energy produced this way is of the same nature as when you tie a rope into a stone and coil it onto a drum, then drop the stone from a high place. It will release potential energy, but as there are no infinite gravity wells, the energy stored and released is not infinite.

But as to what comes to dark energy being the same thing as regular energy (or matter for that part), I tend to agree. It's most likely just a currently unknown part of how energy and space interact. Possibly something like what I wrote previously: It is, in fact, possible that bit by bit, the mass/energy of the universe itself is slowly causing the curvature of the universe to change, which causes stretching of space, which we see as increase of space between objects, ie. expansion. Perhaps this is what we have named "dark energy".

In which case, of course, you wouldn't be creating energy. You would just transfer it from... somewhere else. But that in itself is obviously an intriguing idea... subspace reactors anyone?

[Mika]

Regarding dark matter and dark energies, I find they are strangely similar to ye olde aether. In ways there must be superheavy exotic particles somewhere to make galaxies rotate "correctly" but in similar fashion most of the universe should be made of this stuff and yet we cannot find a single particle here. I recall we had this same thing in the beginning of the 1900s.

Anyways, the experimentalist's insight is that if you can't measure it you can't really base any assumptions on it - and you can't really build much on top of it.

But since there seem to be people who are more familiar with the astronomical things, I have been wondering for some time how has the cosmic background radiation been measured and how uniform it turned out to be?

Mika

[Flipside]

Oddly enough Aether or Ether was one of the first things that came to mind when I started hearing about Dark Matter.

I remember reading an article about threading Dark Matter down a wormhole to stabilise it or something, but one thing I have certainly learned about AstroPhysics is that by about the 4th paragraph, I understand about every 3rd word

[Herra Tohtori]

Regarding dark matter and dark energies, I find they are strangely similar to ye olde aether. In ways there must be superheavy exotic particles somewhere to make galaxies rotate "correctly" but in similar fashion most of the universe should be made of this stuff and yet we cannot find a single particle here. I recall we had this same thing in the beginning of the 1900s.

I've been thinking about it too, but in a bit different sense... Aether is actually closer to Higgs field, which supposedly is a quantum field filling every place... it is supposedly source of inertia, mass would interact through it using particle called "Higgs' boson" if I've understood if correctly, but it's just a hypothesis since there's no record of such particle. Dark matter, though, is more like when Wolfgang Pauli postulated that according to quantum mechanics, there should be this very difficult-to-observe particle released in nuclear reactions... which we know now as "neutrino". He based his postulate on undirect observations, just like the assumption of dark matter is made from galactic rotation.

More like 1900's is the fact that we tend to think we've got most things figured out, just like smost physicist tended to believe that klassical mechanics explained everything along with Maxwell's electrondynamics... there were small anomalies that were thought to be explained by further research but basically based on existing theories. Then came two gentlemen called Einstein and Planck and laid basis to quantum mechanics and theories of relativity. Applied to this day, we think that quantum electrodynamics, quantum chromodynamics and general relativity explain everything, when sufficiently improved... while in fact there could be some unexplained observation that is explained much better with some new ideas that possibly are much different from GR or quantum mechanics, but so that quantum mechanics and GR can be derived from them. But I digress, again...

So, about the rotation of galaxies... The problem seems to be that current gravity theories applied to galactic movements don't predict the movements accurately. Thus there are two possibilities.

1. The gravity model (general relativity) is wrong
2. The gravity model is right, which means that an indirect observation of something, later named "dark matter" is made.

Option 1 is certainly possible, but the thing is that general theory of relativity is very accurate model of gravity, according to all field experiments  - other than galactic movements, that is. That means that GR [general relativity] would be accurate only on "medium" scale and inaccurate on both quantum scale and galactic scale. Which leaves us wondering, why would it be so fricking accurate in between. It is a possibility, but it feels so unlikely that the more believable option is the existence of unknown and difficult to observe substance - dark matter.

Anyways, the experimentalist's insight is that if you can't measure it you can't really base any assumptions on it - and you can't really build much on top of it.

Empiristic science is based on observations. And observations of galactic rotation movement seem to suggest that there's literally more to galaxies than it looks. That is, assuming that GR is reasonably accurate theory of gravitation. Which assumption seems to be supported by multiple experiments. It's like this: you have two die. The maximum result of one throw is 12, minimum is 2. Statistically most common result would be seven. Now, you do an experiment a thousand times and look at the results, and none of the throws shows more then twelve or less than two, and the most commonly observed value will be seven. So, when two thousand die are cast at once, the results should be between 2000 and 12000, and most likely the result should be somewhere around 7000.

Now you look at distant place, where someone seemingly throws two thousand die onto a table at once and sends you the result of all the eyes counted together. And what is the most immediate assumption if the result happens to be more than 12,000?

This is kinda what the observations tell us about the galaxies... the analogy is not perfect, but will do for now. There are three chances here:

1. Someone is sending the wrong results (observations are misinterpreted) - unlikely, but possible.

2. The die used are different from the ones used in the experiments - equivalent to general relativity becoming unreliable at extreme distances. Possible, but why the hell would someone use differently eyed die when they only have six sides... It is possible, but makes no sense at all. Although that's what universe is like most of the time anyways.

3. There are invisible or hidden die in addition to the die that we see. If we assume that the die are similar (GR applies) and the results are correct (observations are not flawed), then this is the option that is preferable.

But since there seem to be people who are more familiar with the astronomical things, I have been wondering for some time how has the cosmic background radiation been measured and how uniform it turned out to be?

As to how it has been measured... the answer is, with microwave telescopes. The cosmic background radiation is, in general, similar in spectrum distribution of a very cold black body radiation - approximately 2.725 K - but the expansion of universe has stretched the infra-red radiation all the way to micro-wave radio spectrum. The temperature distribution of the background sky has small, really small differences above and below said 2.725 K radiation, so it's not uniform... Although the differences are (AFAIK) only about 1/100,000 compared to the average temperature... But those small differences in the beginning eventually ensured that the enthropy level of the universe was sufficient to last billions of years until thermal death.

The first satellite to research cosmic background radiation was named COBE (Cosmic Background Explorer) and it gave this kind of results:



The last and most accurate observations were made by WMAP (Wilkinson Microwave Anisotropy Probe), and it recorded fluctuations with much greater resolution and accuracy:



Again, the difference between coldest and hottest spots is exaggerated on the image. It's only fractions of a degree between the "hottest" and "coolest" place in the map.

In near future, the Planck Surveyor will hopefully improve the WMAP results further.


Long message again. Sorry about that. Hope you could get through it...