Hard Light Productions Forums
Off-Topic Discussion => General Discussion => Topic started by: Bobboau on March 14, 2007, 12:53:43 pm
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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.
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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.
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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. :)
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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.
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So the next time we encounter a subspace string, it's Bobbs fault? ;)
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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.
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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.
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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?
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yes I mis-typed that last bit, sorry
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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.
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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.
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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. :p
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.
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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.
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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? :p
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. :rolleyes:
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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.
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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".
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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. :drevil: 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? :drevil:
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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
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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 ;)
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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:
(http://upload.wikimedia.org/wikipedia/en/thumb/1/1d/COBE_cmb_fluctuations.gif/800px-COBE_cmb_fluctuations.gif)
The last and most accurate observations were made by WMAP (Wilkinson Microwave Anisotropy Probe), and it recorded fluctuations with much greater resolution and accuracy:
(http://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/WMAP.jpg/800px-WMAP.jpg)
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. :cool:
Long message again. Sorry about that. Hope you could get through it... ;)
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someone should do an experement to determine the mass of the Higgs' boson... :nervous:
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someone should do an experement to determine the mass of the Higgs' boson... :nervous:
Am I the only one who first read that as Higgs' bosom?
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I'm waiting for someone to get the reference...
"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."
you could say the same thing about Newtonian mechanics, quantum theory explains the super small and relativity explains the super big, but why does Newtonian theory work so well in the middle space. the thing is we KNOW that all three theories are wrong, they just happen to be good in a certain range of variables, quantum mechanics is good for things to small to see, relativity is good for things to large or fast to comprehend, and Newtonian model is good for things we deal with in every day life. why would it seem so hard to believe that there is an upper boundary on relativity just like there is on Newtonian physics? have you come into some sort of mindset were newton was fundamentally flawed in some special way that the other two models were not? no, the other two models are flawed just as the newton model is, it wouldn't suprize me in the least to find there is an upper boundary for relativity or a lower boundary for quantum mechanics. I find distasteful the modern physics community's attitude that invisible untouchable particles that float through everything is not only a more plausible explaination for new observations on previously investigated scales than "we might be wrong" but that they seemingly didn't even consider it until many years later and still the people who try to determine if there are flaws typically get ridiculed and laughed out of there positions. I don't know about you but these magical forces and particles we invent to fit our anomalous observations into our existing theories, that doesn't seem right.
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Actually, alternative gravity models are serious science... There was an article in a Finnish astronomy magazine (Tähdet ja Avaruus) about a dude who's doing just that, trying to figure out alternative gravity models.
Anyway, the main problem with gravity is not with scale but with accuracy. Newtonian model is a rough approximation that works surprizingly well, but errors cumulate. General relativity fixes those issues very well, but in micro scale it has problems reaching sufficient accuracy to predict gravitational effects between particles. And quantum gravitation doesn't work either, it gives infinite forces which don't exist in reality etc. etc.
It is possible that general relativity is not applicable to large enough scale, but it's equally possible that it is accurate and dark matter exists. There's no way of knowing for sure, but those two are the only two options... And while we are waiting for the next-gen gravity theory to be composed - something that retains the accuracy of general relativity or improves upon it, while explaining the galactic rotation without dark matter - we're better off interpreting the observation data using the best known theories.
The thing is, in addition to galactic rotation there are other observations supporting the existence of dark matter (and dark energy)... namely the cosmic background radiation of the universe. I don't know the details, but somehow the data apparently tells that
* The universe is 13.7 billion ± 200 million years old [3].[1]
* The universe is composed of:
o 4% ordinary baryonic matter
o 22% an unknown type of dark matter, which does not emit or absorb light.
o 74% a mysterious dark energy, which acts to accelerate expansion.
* The cosmological scenarios of cosmic inflation are in better agreement with the three-year data, although there is still an unexplained anomaly on the largest angular measurement of the quadrupole moment.
* The Hubble constant is 70 (km/s)/Mpc, +2.4/-3.2
* The data are consistent with a flat geometry.
* CMB polarization results provide experimental confirmation of cosmic inflation favoring the simplest versions of the theory.
Don't ask me how these conclusions are achieved, I have no way to perform any kind of source critique here... I don't know the maths and only have a shoddy grasp of the physics in question at my best days. At the moment, that is... :p
...
By the way, considering Higgs' boson: CERN's Large Hadron Collider is AFAIK supposed to confirm the existence or nonexistence of Higgs' boson. If it's found, it'll likely be a major confirmation of string theory. If it isn't found... Well, someone will have to start developing thong theory instead, I guess. :nervous:
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http://uncyclopedia.org/wiki/Dark_energy (http://uncyclopedia.org/wiki/Dark_energy)
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"Emotional argument: It just can't be that way!"
:)
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Physicists also know the gravitational power of dark energy, which is currently equivalent to 1.337*10-27 grams per cubic centimeter.
you have got to be ****ting me... dark energy is leet.
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As for why something can be way off for large and small scales but incredibly accurate in between... a twelfth-order polynomial approximation of the sine function is really accurate for a few cycles near x=0, but diverges rapidly thereafter in both + and - directions.
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Isn't there another theory going around that the effect of dark energy is really just that gravity may work differently over extremely long ranges than it does over short ones?
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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. :p
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.
(http://img245.imageshack.us/img245/9389/meananything4ti9.jpg) (http://imageshack.us)
There still needs to be a force that generates the space, unless you want to take "that" perspective and say the universe violates what we term "causality". I don't know what your point is with the reference frames. Two things observe different things. Are you trying to say that space is really not expanding, it's everything that's getting smaller?
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.
The perihelion of Mercury is hardly an anomaly--it's explained fully by special relativity. Same goes for guidance satellites. The GPS is adjusted to compensate for the effects of special relativity's predictions. Also, it's kind of a moot question why the laws of the universe are so, because they would be incredibly hard to discover and have very little bearing on our world.
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.
I'm actually not sure on this myself, but I believe that's the whole idea, except that part with the stretching. Everything I've read says that mass in the universe counteracts expansion, because of its gravity.
Your explanation of dark matter makes no sense. Dark matter is simply matter we cannot see because it does not emit what we use to see it. It may consist of particles we do not know of yet, but it is otherwise ordinary matter. Even if, as you say, it does not physically interact with most matter (like neutrinos don't) they would, according to my admittedly limited worldview, still collapse to a point. If they are affected by gravity, then they will eventually "hit" each other and eventually cease movement, because even if they keep passing through one another they lose energy each time. If dark matter does not collide with other matter including itself, then it will form a point of very little volume indeed, whose infinitesimality is limited only by the Pauli exclusion principle.
NOTE:I may indeed have misused several terms and principles in my rebuttal. If this is the case then please inform me at once with verification from the literature. I criticize only so that I may be enlightened.
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Isn't there another theory going around that the effect of dark energy is really just that gravity may work differently over extremely long ranges than it does over short ones?
Yes, there is. Search for "MOND," which stands for MOdified Newtonian Dynamics. Mathematically, it works out fairly well, but physically it makes no real sense to me. The last time I read up much about it, I thought the basic idea was interesting as a thought experiment, but it was not really addressing root causes.
http://www.astro.umd.edu/~ssm/mond/ (http://www.astro.umd.edu/~ssm/mond/)
@ Herra, I have to agree with Agent Koopa for the moment. If dark matter does not interact with itself or baryonic matter through strong nuclear or electromagnetic forces, there'd be even less to stop dark matter from collapsing into black holes than baryonic matter.
However, I would have to caution you (Agent Koopa) from assuming that the Pauli Exclusion Principle applied at all to Dark Matter. There are plenty of subatomic particles which could care less about sharing a common quantum state. The Pauli principle only applies to fermions, not bosons. Normal matter has lots of both. Wiki does a decent job introducing the topic. http://en.wikipedia.org/wiki/Boson (http://en.wikipedia.org/wiki/Boson)
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 ;)
That wouldn't be dark matter. Whenever I have read about something like that, it's been referred to as "negative matter" or more frequently "exotic matter." Such matter would have a "negative gravitational charge" for want of a better term. It would interact and create gravitational fields in the exact opposite manner to normal matter. You'd need something like that to "hold open a wormhole mouth" or make a warp drive remotely possible. As you can probably tell, this is much more the realm of science fiction than astrophysics at this point. As far as I know, there's no theoretical basis for such matter actually existing. But man, it would be really cool if there was!
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Thank you about the cosmic background radiation data, but that actually did not answer the question. The point was how was this data exactly measured? If I remember correctly, it had something to do with a satellite based measurement, on which the measurement data was averaged over 2 or 3 years to reduce the noise level accordingly. However, I think this process needs some explanation, since according to my understanding satellite is moving along with everything else on the space. Now, I would think that this should actually average (read: distort) the measurement result itself also - and considering the time it is taken, quite significantly also. Would you happen to have a link on the mission itself so that I could find out how it was really done?
Considering measurement results, that is the only way you really find out anything. Any scientific theory should have empirical evidence to support it, but unfortunately, this doesn't really happen with dark matters and string theories. What we have is actually a collection of measurements and observations that don't fit to the dogma, but we don't have any direct measurement of dark matter, even though by the definition of dark matter itself it should be easily measurable and detectable (or at least its properties would lead me to think so). String theory itself is a misnomer as there is no empirical evidence that would support it, so it cannot be called a theory.
There are many interesting phenomena happening in every day life that don't have a sound physical explanation yet and still researchers insist going in to depths of the universe without any real way to verify their logical conclusions. I don't have anything against space research, but thinking about the structure of universe some Mparsecs away reminds me more of guess work rather than science.
As an example of everyday things that begs to be explained is sonoluminesence, where acoustic wave is changed to electromagnetic wave. However, no one has been able to give an explanation what causes it. More so, this would be important since it is easily verifiable, almost everyone can construct a setup by themselves and it doesn't require expensive systems to detect it. Further, the spectrum of the EM radiation pulse shows that the equivalent black body temperature might get close to 60 000 K in some reported cases. Also, this would probably be the best radiation point source known since the data about the emitting area would suggest that the radiation is coming from a sphere of approximative size of 1 µm. Even stranger, adding some gas in the fluid where this is happening might increase the emitted intensity over three decades. This kind of light source, if intense enough, would have practical uses in many different things.
Mika
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However, I would have to caution you (Agent Koopa) from assuming that the Pauli Exclusion Principle applied at all to Dark Matter. There are plenty of subatomic particles which could care less about sharing a common quantum state. The Pauli principle only applies to fermions, not bosons. Normal matter has lots of both. Wiki does a decent job introducing the topic. http://en.wikipedia.org/wiki/Boson (http://en.wikipedia.org/wiki/Boson)
Thanks a lot! I was under the impression that the Exclusion Principle applied to all matter, probably because atoms include electrons and such. I typed my post while my internet appeared to be temporarily down and only restored itself when I had finished, (convenient, hm?) so I didn't get a chance to check Wikipedia. Since your post I've been skimming the related articles, but I have to say they're not very good, because I understood barely anything I haven't picked up from less-technical books! :lol: I'm learning to accept that there are things that you need to take a class in before understanding. :rolleyes:
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-Koopa: :lol:@ the comic...
The perihelion of Mercury is hardly an anomaly--it's explained fully by special relativity. Same goes for guidance satellites. The GPS is adjusted to compensate for the effects of special relativity's predictions. Also, it's kind of a moot question why the laws of the universe are so, because they would be incredibly hard to discover and have very little bearing on our world.
I know that general relativity (not special btw; special relativity doesn't actually say anything about gravity) explains the perihelion of mercury.
It is an anomaly to classical, Newtonian physics, which works fine at low speeds and requires space to be euclidian and not curved like it really is. Which is what I tried to say, but the internal references in that particular sentence were a bit mixed up:
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.
"This stuff" was supposed to refer to Newtonian physics, but looking back to the text I can see that I wouldn't understand it that way even myself. My bad.
Your explanation of dark matter makes no sense. Dark matter is simply matter we cannot see because it does not emit what we use to see it. It may consist of particles we do not know of yet, but it is otherwise ordinary matter. Even if, as you say, it does not physically interact with most matter (like neutrinos don't) they would, according to my admittedly limited worldview, still collapse to a point. If they are affected by gravity, then they will eventually "hit" each other and eventually cease movement, because even if they keep passing through one another they lose energy each time. If dark matter does not collide with other matter including itself, then it will form a point of very little volume indeed, whose infinitesimality is limited only by the Pauli exclusion principle.
Herra, I have to agree with Agent Koopa for the moment. If dark matter does not interact with itself or baryonic matter through strong nuclear or electromagnetic forces, there'd be even less to stop dark matter from collapsing into black holes than baryonic matter.
Well, weakly interactive massive particles (WIMPs, and I didn't make this up!) are just one possible candidate for dark matter. But if we consider how a cloud of WIMPs would behave, you must remember that it only interacts through weak interaction and gravity. That means that the most profound thing we tend to associate with matter is not there: you cannot touch dark matter. Touch is dealt solely via electromagnetic interaction, and if dark matter lacks it, it will be totally different from matter as we know it.
For example, if dark matter really consists of WIMPs, it really wouldn't concentrate on any kinds of blobs. The formation of mass concentrations requires that the particles can hit each other. Normal matter works like this - if you put a gas or dust cloud into space, it will start falling towards common center of gravity, and eventually particles start hitting a surface and larger and larger sphere forms etc. etc.
But what would happen if the particles can't touch each other? The answer is that they would simply fall through the center of gravity - not necessarily at the same time, mind you - and start oscillating at complex pattern around the cloud's center of gravity. There's nothing to stop them at the center of gravity. Of course it is possible for these particles to form an event horizon (black hole), but it's not as likely as you might think. It would only happen if sufficient amount of particles happened to fall into small enough area at the same time.
You can ask why won't neutrinos collapse into black holes. The universe is full of them and they do have a small mass. The answer is, there's no concentrations to collapse to, since they can't affect each other via electromagnetic interaction like most particles.
Anyway, weakly interacting massive particles are just one hypothetical solution to the problem of dark matter. Wiki has quite intereesting article about Dark Matter and it says about what I could say about the subject, so I'm going to quote a part of the possible explanations here:
(...) to explain structure in the universe, it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter.
At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos). None of these are part of the standard model of particle physics, but they can arise in extensions to the standard model. Many supersymmetric models naturally give rise to stable WIMPs in the form of neutralinos. Heavy, sterile neutrinos exist in extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.
Pauli's exclusion principle doesn't really concern sizes of particle concentrations, it just states that two fermions with same quantum properties cannot share the same space. But if there's different quantum properties involved, you can easily pack several particles to occupy same space. Simplest example of this is the electron shells of atoms. The electrons in the shell occupy the same space, but have slightly different quantum properties. Not that Pauli's exclusion principle would have anything to do with whether or not weakly interactive massive particles will/can form black holes. They can, but they won't do that automatically.
->Mika: Sorry, I don't really know that much of the actual methods of background radiation research. But the thing with background radiation is that it appears the same everywhere as far as I know. What little changes perhaps are caused by the movement of the probe during the measurements, they would most likely be insignificant compared to the diameter of the observable universe.
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IIRC (I probably don't) the Pauli Exclusion Principle has to do with the summing of superimposed quantum states. Two superimposed fermions sum to zero, meaning that the probability of them occupying the same space and time is zero. Two or more superimposed bosons sum to a nonzero number.
Or something like that. It's been a while since I read up on this stuff.
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The point about the background radiation is that it behaves as if it is from a source so far away that it is observed as being exactly the same from points that are huge distances apart (in other words, it shows no parallax), so that even if measurements are taken many years apart, the movement of the Earth (both around the Sun, and the movement due to the Sun's movement) has no effects on what is seen.
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Aha, It seems that I have forgotten parallax. For some reason I was thinking that it would actually show, but wait, it indeed does if there is a star which actually has a parallax within the field of view of the instrument. How is the effect of these objects negated? I would guess that the star would have a maximum of emission somewhere within the visible or UV region and this could be filtered out but still the wavelengths of the backround radiation would enter the instrument. But as Herra Tohtori [I could add here a seemingly smart comment about his nick but this could not be understood correctly at all if one is not familiar with the culture here, so I leave it out :D] pointed out, this might explain some of the non-uniformities.
My next question would be that given that there is a large galactic halo which extends over the region we are located in, how can we be sure that we are actually measuring free space (pun intended) background radiation and not the galactic background radiation?
In case some one was wondering, yes I did skip basic astronomical courses in the first year - but I suspect I wouldn't remember them even if I had :D
Mika
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Am I the only one lost after 4 posts?...
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Am I the only one lost after 4 posts?...
Not at all :P
But it's a good read anyway..
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Today I read something about two colliding galaxies, known as bullet galaxy or something like that, this seems to be the first direct confirmation of the existence of dark matter. To make long story short, in these photographs (not in visible range) the center of gravity of the galaxy seems to continue unaffected even though visible matter is slowed down. This result was from August 2006, and I have to say this is quite impressive display. Currently I cannot figure out any other reason what would cause this behavior, than dark matter.
However, I would still wait for confirmation in the laboratory, I still think it should be fairly easy to detect these particles in the experimental setups like Super Kamiokande that detect solar neutrinos. As far as I understand, the scattering cross section of the heavier dark matter particle should be larger than neutrino's (if understood correctly, they both feel the same forces), so we should be able to see them more.
Saturday night and I'm writing about this? Well at least I'm drunken... which might be even more pathetic.
Mika
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Well that would be the case if dark matter consists of WIMPs.
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anyway, this thread is about dark energy, not dark matter.
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Well that would be the case if dark matter consists of WIMPs.
...And it probably does, at least according to WMAP observations, which tend to block out the possibility that the dark matter would consists of MACHO's. That is, it the interpretation of the measurements is correct...
By the way, am I only one who finds it profoundly hilarious that a bunch of scientists names the candidates for dark matter MACHO's and WIMP's - and even more, that WIMP's are the likely winner for the job? :lol:
Seriously, those abbreviations are like directly from The Junior Woodchucks titles...
anyway, this thread is about dark energy, not dark matter.
Well, the way I see it is that energy is by definition a potential to do work. Hence, because space expands at accelerating speed and it is assumed that the accelerating expansion demands energy, it is also assumed that there is some form of energy potential that provides the energy for the expansion. It's dark only because we don't know what that energy source is.
It's kinda similar definition as matter has. If energy has gravitational mass, it is matter. If it doesn't have it, it's just energy. Relativistic energy forms don't (as far as I know) have effect on gravitation; these energy forms include kinetic energy, electromagnetic radiation and stuff like that. They are relative to observer. Gravitational mass, however, has an energy potential on its own inertial frame (ie. at rest). That's what separates matter from random energy. But that also means that dark matter is just a form of matter because it has gravitational mass; it's just our definition to call if "dark" because we don't know what it is.
The same thing as with X-rays, by the way. They were named X-rays because the founder of them had no fricking idea what they were, and only later it was found out that it's actually same stuff as light, only sligtly tuned-up version of it... but I digress.
The same thing is with dark energy. It's just a name. It'll stop being dark as soon as it's figured out. But the main thing is that it wouldn't be energy at all if it couldn't do work. Also, as it's obviously doing work (accelerating the expansion) that means that at some point the energy source will be depleted. That will most likely occur some time billions of years after the thermal death of the universe.
Unless the conservation laws are completely b0rked, which I don't think is the case.
So. Perhaps it would indeed be possible to power up a generator attached to this mega-string. It would obviously be practically impossible, but maybe not theoretically. Could be interesting to find out a way to harness this energy source. Subspace power generator, anyone...? :drevil:
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"at some point the energy source will be depleted. That will most likely occur some time billions of years after the thermal death of the universe.
Unless the conservation laws are completely b0rked, which I don't think is the case."
which was my initial point, if dark energy works like we think it does then conservation of energy does not hold.
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Not necessarily, if the [dark] energy ends some day (ie. is entirely converted to work as expansion of space). Then the acceleration of expansion will halt, and what happens after that is not really our headache.
The net amount of energy in universe (known matter+dark matter+known energy+dark energy) will remain the same, regardless of where the dark energy comes from or what it is. In the process of expanding (according to our assumption that energy is required to accelerate the expansion of space) the space, the amount of dark energy is converted into more familiar forms of energy - mostly seen as kinetic energy of fastly escaping distant galaxies (although again, there can be several interpretations whether or not their kinetic energy actually increases or not, or does it just look like that to us).
Eventually, as the thermal energy differences of universe are depleted and maximum entropy is slowly creeping closer and closer, probably some day also the source of dark energy will be depleted, at which point there is no dark energy any more.
The assumption that the accelerated expansion will go on for infinite time is just as ridiculous as assumption of string of infinite length. Whatever is accelerating universe (if energy is required to that - I have my doubts) - will not last forever, because there is no infinite energy in the universe.
What is interesting is that in the inflation of universe the expansion was first accelerated insanely much, but then it's speed decreased substantially, and now the speed is again increasing. So if we look at the acceleration curve of the speed of expansion, it starts at zero at t=0, and goes almost instantly way up and stays there for a while. Then it comes down below zero while the speed of expansion decreases. Now, it's above zero again, increasing the speed of universe.
You know what it sounds like to me? Damped harmonic oscillation. Which is not surprizing, considering that harmonic oscillation is rather common phenomenon in universe. Though I have no idea on how many periods have there been already, or what the frequency is, or what the initial conditions were and how the oscillation is dampening, it certainly seems to have at least some characteristic properties of oscillation. Itäs also probable that this has been thought before, but it would feel somehow intuitive to me.
Then again, intuitive is not always what physics is... :cool:
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but as far as I know our current understanding of dark energy is that it will not run out some day.
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...current understanding of dark energy...
...is modest at best. We know something is there accelerating the expansion. What we do not know are (to mention a few things):
-Why did the expansion accelerate very fast at the inflation?
-What made the inflation speed decrease so much after the initial inflation phase?
-What caused the acceleration speed to rise to current level again?
The acceleration speed hasn't been a constant thorough the history of universe. We don't know how it will proceed in the future. These are only few things we don't know about the nature of dark energy.
I don't personally know any reason why the dark energy couldn't run out.
Then again, we actually know that conservation of energy is not entirely valid principle considering cosmology. We know that things (a whole universe of them) can literally pop out of nothingness.
We just have this assumption that in current universe, energy cannot be born. It's a good principle but not necessarily always applicable, as our existence proves. So perhaps more energy is constantly generated into the universe from... nothing. Perhaps the birth of our universe is a continuous process. I don't know. But the idea of energy coming out of nothing is rather interesting.
Find a way to manipulate the speed at which the energy is created, and we're in the stars. :pimp:
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Then again, we actually know that conservation of energy is not entirely valid principle considering cosmology. We know that things (a whole universe of them) can literally pop out of nothingness.
We just have this assumption that in current universe, energy cannot be born. It's a good principle but not necessarily always applicable, as our existence proves. So perhaps more energy is constantly generated into the universe from... nothing. Perhaps the birth of our universe is a continuous process. I don't know. But the idea of energy coming out of nothing is rather interesting.
Find a way to manipulate the speed at which the energy is created, and we're in the stars. :pimp:
The energy of the universe is (according modern understanding) constant. At the big bang energy gave birth to matter and anti-matter but for some reason matter didn't decay as fast as anti-matter and then when the universe was calm enough to have matter/anti-matter reactions there was an imbalance in the ratio that allowed some matter continue to exist.
According to this theory energy is constant, but where the "inicial" energy came from is anyone's guess. Most people don't question what came before so...
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We just have this assumption that in current universe, energy cannot be born. It's a good principle but not necessarily always applicable, as our existence proves. So perhaps more energy is constantly generated into the universe from... nothing. Perhaps the birth of our universe is a continuous process. I don't know. But the idea of energy coming out of nothing is rather interesting.
Which sounds a bit like steady state theory.
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Not quite.
Steady state assumes that the universe remains similar, ie. either the universe doesn't expand or more mass is continuously generated to fill the gaps in the expanding universe. Neither has been observed, but instead there is actual proof of a "static" energy level of the vacuum of space itself. Its existence can be proved by Casimir-effect, for example, but as it's just a static, and for all practical purposes uniform, energy level, it cannot be used as energy source, as that requires differences in energy potential.
But the point is that vacuum's quantum fluctuation keeps the space itself at certain energy level. And there's a lot of space. And as energy has a tendency to dissipate and spread, but in this case it is bound to vacuum... perhaps that causes a "pressure" that expands the space itself and the energy yield of quantum fluctuation along with it.
So perhaps it is the energy of those fluctations that is accelerating the expansion. That would certainly explain the inflation of the universe - as the universe itself is likely result of a gargantual quantum fluctuation, the enery yield of vacuum in the beginning would have likely been much bigger than it is now. Perhaps. If this is indeed the case here, then the acceleration of the expansion will continue, approaching zero but never reaching it. That will result in universe that forever expands with accelerating rate, but the acceleration itself would decrease as time passes.
That in itself doesn't explain the changes in the speed of expansion during inflation. But this all is just speculation, of course.
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Then again, we actually know that conservation of energy is not entirely valid principle considering cosmology. We know that things (a whole universe of them) can literally pop out of nothingness.
Well, "nothingness" is relative. It is so impossible to think about what came before the universe existed that we shouldn't try. And, besides, conservation of energy only holds for an isolated system, so it hardly works with the creation of the universe. In addition to that, particles pop in and out of existence all the time, an existence so brief as to escape the jurisdiction of the principle of conservation of energy.
Find a way to manipulate the speed at which the energy is created, and we're in the stars. :pimp:
That, or *BOOM*! :lol:
The energy of the universe is (according modern understanding) constant. At the big bang energy gave birth to matter and anti-matter but for some reason matter didn't decay as fast as anti-matter and then when the universe was calm enough to have matter/anti-matter reactions there was an imbalance in the ratio that allowed some matter continue to exist.
According to this theory energy is constant, but where the "inicial" energy came from is anyone's guess. Most people don't question what came before so...
That's because it's almost impossible to question. Outside our space-time, time has no meaning and space has no meaning. This obviously makes it rather hard to define.