My Ramblings On Dark Energy And Space-Time

[Sylandro]

Anti-energy? We already have the concept of anti-matter after all' date=' so I would imagine it wouldn't be that much of a leap to suggest the same concept for energy. Matter and antimatter annihilate each other, may be the opposing forms of energy repel one another (perhaps one more so than the other, see below) like opposing poles of a magnet.

It actually makes some sense thinking of it as a magnetic-like effect. We already have the normal energy from the big bang going in all directions, so as it encounters anti-energy it accelerates as anti-energy repels it. This of course begs the question of why they don't eventually cancel each other out as the repelling forces between both types of energy equal out at some point in the universe. I would imagine this would be because normal energy may not be able to affect anti-energy in the same way the latter affects the former.

I always do wonder why the phenomenon's of the universe particularly care that something or someone is observing them.[/quote']

Well, our universe is 73% dark energy...

[K^2]

Though last time I heard they didnt know if anti matter emmited normaly gravity or not did that change?

Your information is outdated by many decades. Accelerator experiments confirm that anti-particles and anti-matter have exactly the same energy as normal particles and matter. So while we don't have experimental measurements of gravitational forces due to anti-matter, that's really not necessary. It is well established that the "charge" of the gravitational force is the energy-stress tensor, which is equivalent for matter and anti-matter. So as far as gravity goes, matter and anti-matter are absolutely the same.

I always do wonder why the phenomenon's of the universe particularly care that something or someone is observing them.

They don't. The only things affected by observation are measurements. Which is quite intuitive. You can't have a measurement without an observer of some kind' date=' even if it's just measuring equipment that's going to play a role of observer.

What's interesting is that choice of observer makes a difference on observation. But that's no different than choice of coordinate system making a difference in observations in Relativity. The actual physics does not change. QM works the same way. Unfortunately, Copenhagen Interpretation is absolutely terrible at treating a system with multiple observers, which lead to a lot of people claiming that QM actually has some sort of "magic" properties, where observer influences the universe. That is not the case. Many Worlds Interpreation is much better at this. It allows you to treat a system with multiple observers same as you would treat a system from multiple reference frames in relativity. This is why I frequently point people to MWI whenever they get confused about QM. It's the same physics, but under MWI, it's way more intuitive.

It actually makes some sense thinking of it as a magnetic-like effect. We already have the normal energy from the big bang going in all directions, so as it encounters anti-energy it accelerates as anti-energy repels it.

Sorry, this would take too long to explain, so I hope you'll just take my word for it. It simply doesn't work like this. While stress-energy is a kind of conserved charge, there are no matter states corresponding to negative amounts of it. It's a broken symmetry, if you will. Anything with negative energy must very rapidly decay. Another way to think about it, it would be like having pockets of vacuum in a room filled with air. Yeah, you can come up with something like that, but these pockets will instantly collapse. There is just no way to maintain large quantities of negative energy without doing something really tricky. It's something that'd be very difficult to do artificially, let alone be something that occurs naturally.

Dark energy, in contrast, isn't negative energy. It's more like pressure. Normal matter energy has no pressure. If you have something like very energetic gas, it will have pressure. Both in normal sense, and in energy-stress sense. But for ordinary matter, regardless of what you do with it, the pulling strength of gravity is going to be greater than pushing of stress-energy-pressure. Dark energy, however, has to be something with a lot of pressure compared to how much energy it has. So it pushes instead of pulling.

I suppose, from layman's perspective, it's the same thing. If it pushes things apart, it must be anti-gravity with negative mass. But from perspective of theory, it's actually very different. We expect it to have positive mass, but causing space-time around it to expand anyways. Which is really, really weird, and we all want to study the hell out of it, but we have no idea where to get any.

[Stargate525]

They don't actually; what actually happens AFAIK is that various states are indeterminate (certain states are still likelier than others, though) until interacted with. It doesn't matter if you are watching it; it doesn't even matter if anyone is watching it - the actual events happen because of the interactions of particles. This still leads to some counter-intuitive effects, but we can predict most of what goes on.

nope; quantum particles are, for all intents and purposes, in both places until someone looks at it. A single photon, given a chance to go through one of two slits, but not watched which one it goes through, will go through BOTH, interact with ITSELF, and form a wave-pattern on the card behind those slits. There are also some electronic gates, or something, that utilize the fact that a particle can be in two places at once.

And by 'not make sense,' I mean that they are far from intuitive conclusions about how the world works, and are in many cases very, very convoluted.

[Dominatus]

And what I mean by that is' date=' what if in the universe there is some minuscule amount of "gravity" affecting the content of the universe, which would be localized (ie, strongest) wherever the universe originates (For the big bang to have happened then there had to have been one location where everything expanded out from, as otherwise the idea of the entirety of the universe being a singularity before the big bang doesn't make sense, because then the singularity would occupy the same amount of space as the universe that is about to expand from that singularity) [/quote']

... Gravity isn't simply a localized event, though. I remember my physics teacher saying something about how no matter where you are in the universe, an objects gravitational pull will still be on you. Like if I go to the edge of the known universe, earth's gravity will still have an effect on me, albeit one so low it could be immeasurable. I'm sorry, but these ideas are more along the lines of science fiction, if not outright fantasy.

[K^2]

nope; quantum particles are, for all intents and purposes, in both places until someone looks at it. A single photon, given a chance to go through one of two slits, but not watched which one it goes through, will go through BOTH, interact with ITSELF, and form a wave-pattern on the card behind those slits. There are also some electronic gates, or something, that utilize the fact that a particle can be in two places at once.

TheDarkStar is right, though, in that it still requires an interaction. "Watching" which slit a particle goes through involves interaction with that particle. Observation isn't a passive thing.

Though, things do get way more complicated when you start considering experiments like Quantum Eraser, but that's a bit beyond the scope of this discussion.

[ArmchairGravy]

Germane to the topic: http://www.bbc.co.uk/news/science-environment-25809967

[Linear]

Such poor science.

[K^2]

Germane to the topic: http://www.bbc.co.uk/news/science-environment-25809967

It's related, but one should keep in mind that this was quite expected, and dark energy has nothing to do with dark matter, other than both being so far undetected. (Hence, "dark".) The observation of the structure is nice, but it doesn't help us understand dark matter or dark energy.

[J.Random]

dark energy has nothing to do with dark matter

Profane question: can't they both be explained in terms of brane multiverse as interaction between two adjacent branes (some sort of graviton "leak" between them)? Because from commoner's point of view, both phenomenons are kinda relative to each other: it's either not enough gravity where it should be (dark energy) or too much gravity where it shouldn't (dark matter).

[K^2]

Profane question: can't they both be explained in terms of brane multiverse as interaction between two adjacent branes (some sort of graviton "leak" between them)? Because from commoner's point of view, both phenomenons are kinda relative to each other: it's either not enough gravity where it should be (dark energy) or too much gravity where it shouldn't (dark matter).

The problem isn't just that there isn't enough gravity. It's that there is an actual large scale repulsive effect. As I've pointed out, it's more of a pressure than anti-gravity.

Leaky branes could help explain dark matter. Not as an intersection, however, but by having multiple branes running almost parallel to each other, all causing distortions in the bulk. This would effectively increase the mass of something like a galaxy or larger, because locations of galaxies in neighboring branes would correlate strongly, but not affect gravity on the local scale, because locations of stars would be pretty random.

There are some problems with it as well. First, the elephant in the room. If the above is true, there should be locations in our space which behave as if there is a star there, but without there actually being one. To generate as much "dark matter" effect as we observe, the leak has to be strong enough to keep entire planetary systems by a star from another brane. Now, that'd be pretty rare, and we might simply not have detected something like this, but the universe would have to teeming with "phantom" objects ranging from gas clouds, to asteroids, to stars and black holes. They should outnumber normal matter. But we see no evidence of this.

There are more subtle, but deeper issues as well. For example, curvature of the brane itself would generate its own gravity. But it'd have restrictions due to the effective manifold being embedded. Gravity goes from problem of curvature of a non-embedded manifold to one that's embedded in another curved manifold. Curvature in the bulk would then have to work according to General Relativity, due to symmetries, while curvature in the brane would follow more complicated equations. This would result in significant corrections to General Relativity, and again, we don't see this. With some gravitational effects measured to 12 orders of magnitude in precision, we ought to have noticed something.

Finally, there is the question of dark energy. As I've pointed out, it corresponds to pressure in energy-stress. The interesting thing about that is that it does not depend on larger scale makeup of the verse. It's like stress in the real structure. Doesn't matter what's causing it. Stress is a real measurable thing. And so it is with the dark energy. Even if the actual cause of the inflation is the brane being inflated like a soap bubble by who knows what, the dark energy would be an actual energy in the brane. And we ought to be able to detect it and measure it.

For the moment, I'd say that we should be looking at it as a problem in standard 3+1 dimensional field theory. Whatever else is going on, either the dark matter and dark energy should have corresponding measurable energy in space accessible to it, and we should find it and study it, or we are missing a big piece of the puzzle, like another field, and until we fix that, trying to guess the bigger picture is blind guessing.

Once we do find the actual sources of these effects, then we can start talking about their nature. And if we'll need extra dimensions then, then we'll use them.

[J.Random]

Thank you. Of course, I'll have to find something to read on lambda-cdm to understand it, but before that, another profane question, if I may: shouldn't the value of this negative pressure decrease over time? I mean, if we're talking about stress in the structure, our "structure" (space-time) is flexible - it expands under this negative pressure. So, eventually, shouldn't expansion stop at some equilibrium point?

[K^2]

No, actually, because gravitational attraction drops with expansion, it's predicted that we won't have an equilibrium again. I'm saying again, because measurements suggest that there was a phase of deceleration following the initial rapid expansion. Between that and current phase of accelerated expansion, we must have passed an equilibrium point. So the universe is heading for a cold death, by the looks of it.

Edit: I don't know all the details, but there seem to be 3 factors. Radiation pressure of the big bang, which dropped off quickly as the universe cooled, gravitational attraction that tries to pull everything together, and the dark energy pressure which seems to win this tug-of-war once the universe is large enough.

Edited January 24, 2014 by K^2

[J.Random]

Although there's something intuitively wrong here, I'll guess I just don't understand it well enough. Thanks.

Edit:

Wait! I knew I forgot something - Andromeda! I'm kinda lost again. Is there any estimate at which distance this negative pressure should overtake gravitational pull? Obviously, it's not on solar system scale, or even galaxy scale: afaik, Milky Way doesn't expand. Moreso, even if Hubble's research told us that everything is moving away from us, we will collide (well, mix) with Andromeda galaxy some billions years in the future, won't we? So, this pressure isn't high (or low) enough on galaxy group scale either? What about cluster? Or those supercluster-thingies-which-i-don't-remember-the-term-for? Giant nodes in the sponge-like universe structure?

Edited January 25, 2014 by J.Random