Neutrino beam reveals 'new aspect' of cosmos

[Fizyk]

No, I mean flat as in no acceleration of the expansion and thus no change in the energy of the universe.

Well, actually the supernovae observations indicate that the expansion accelerates, but that doesn't matter in this case. Still expansion = no time-translation symmetry = no energy conservation.

Non-zero matter density curves space, but not space-time. Important difference (Because the curvature effect comes from the time-like dimension).

Umm, unfortunately no and no. Let me put it this way: general relativity doesn't at any point separate space from time, also Einstein equations are about the curvature of space-time, not space (because there is no "space"). You can introduce some coordinates and call one of them "time", and the rest "space", but it's not something implied by the theory, it's just something that is convenient to humans (because we are used to time being something different from space).

What you are probably referring to is that the effect of bodies falling on each other comes from the things happening to time - yes, it is true to some extent. But still space-time is curved as a whole

BTW, curved space implies curved space-time (assuming what you call "space" isn't some random spatial slice, but really has the meaning of space in some frame of reference).

 

[Urwumpe]

No, the expansion of space comes already from the definition of space-time actually. Spacetime is constant, but space expands, the time-like dimension is reduced (which is actually a measure of energy density). If spacetime would expand, we would get a accelerating expansion of space.

And of course you can separate them. They are just dimensions. They just don't have a proper meaning alone. Space itself is of little use for predictions without time.

 

[Fizyk]

No, the expansion of space comes already from the definition of space-time actually.

No, for example Schwarzschild space-time is a good space-time, but there is no expansion there.

Spacetime is constant

Yeah, because by definition it includes time, so it has very limited possibilities when it comes to changing (There would have to be another time in which space-time would be changing).

but space expands, the time-like dimension is reduced (which is actually a measure of energy density).

Space expands - yeah, with this I can agree. I'm not sure what you mean by the second part, though.

And of course you can separate them. They are just dimensions. They just don't have a proper meaning alone. Space itself is of little use for predictions without time.

Well, can you separate the vertical direction from horizontal, especially in the interstellar space where there is no obvious "down"? They are just dimensions, too, but very tightly connected - just like space and time.

 

[Urwumpe]

No, for example Schwarzschild space-time is a good space-time, but there is no expansion there.

Yes, but that doesn't matter at all: The Schwarzschild metric is just a physical simplification that doesn't exist in reality.

Well, can you separate the vertical direction from horizontal, especially in the interstellar space where there is no obvious "down"? They are just dimensions, too, but very tightly connected - just like space and time.

connected or not doesn't really matter, depending on which aspects you are interested in. You can talk about space without caring for the fourth dimension. You just can't make accurate predictions on a relativistic scale then. But mathematically, all is possible. :lol:

 

[Fizyk]

Yes, but that doesn't matter at all: The Schwarzschild metric is just a physical simplification that doesn't exist in reality.

My point is: the definition of space-time doesn't imply expansion

You can talk about space without caring for the fourth dimension. You just can't make accurate predictions on a relativistic scale then. But mathematically, all is possible.

Ok, but the whole universe is a relativistic scale It's not only about big velocities or densities, big distances in curved space-time also require you to look at the space-time as a whole.

 

[Urwumpe]

My point is: the definition of space-time doesn't imply expansion

It does, if you have a non-zero cosmological constant. The real universe is non-zero, Schwarzschild assumes it to be zero - and reduces also the world to a single source of gravity.

 

[Fizyk]

It does, if you have a non-zero cosmological constant. The real universe is non-zero, Schwarzschild assumes it to be zero - and reduces also the world to a single source of gravity.

In most cases, yes. Though if you fine-tune the constant, you get a static universe, which was the very reason for Einstein to introduce it.

 

[Urwumpe]

In most cases, yes. Though if you fine-tune the constant, you get a static universe, which was the very reason for Einstein to introduce it.

Yes, but the static universe idea pretty much backfired. :lol:

We don't even know if there is a constant or a function for this. Einstein just introduced this factor as his biggest blunder, but it remained in active service, because it is terribly useful.

 

[Fizyk]

Yes, but the static universe idea pretty much backfired.

True
But still, the point is - a space-time is just a 4-dimensional space with a lorentzian metric. No expansion involved in the definition It is only a consequence of the laws of physics. That's not really relevant to the rest of the discussion, though

 

[T.Neo]

This is all a very interesting scientific discussion, but I must ask an engineer's question:

Can we build a spaceship with it?

 

[Fizyk]

This is all a very interesting scientific discussion, but I must ask an engineer's question:

Can we build a spaceship with it?

With neutrinos or Big Bang - no, I don't think so

 

[xx_mortekai_xx]

Yeah, but that is actually the good question, with how much energy the universe really started. We only know that the space time and thus the total energy in the observable universe is likely constant.

From what we can tell, the universe has zero total energy, and therefore needed not contain or be anything "before" it existed. (i put before in quotes because its pretty nonsensical to talk about before time. Before and after are necessarily reliant on time existing.)

---------- Post added at 03:21 PM ---------- Previous post was at 03:19 PM ----------

No, I mean flat as in no acceleration of the expansion and thus no change in the energy of the universe.

Non-zero matter density curves space, but not space-time. Important difference (Because the curvature effect comes from the time-like dimension). A curved space-time, additionally to the curved space, is an much more complex topic, because the matter gets a much stronger or weaker effect, and energy is transfered into or from the universe.

Considering the time dilation that occurs close to a massive object, I think it might be accurate to describe non-zero matter densities as curving space-time.

 

[Urwumpe]

From what we can tell, the universe has zero total energy

Not correct, since the universe expands (without energy, there would be no expansion, how fast it expands though, has a strong relation to its total energy), has a mass (E=mc²), and also has non-zero field energy in various different fields.

---------- Post added at 05:23 PM ---------- Previous post was at 05:22 PM ----------

Considering the time dilation that occurs close to a massive object, I think it might be accurate to describe non-zero matter densities as curving space-time.

That is acceptable. :lol:

 

[xx_mortekai_xx]

Not correct, since the universe expands (without energy, there would be no expansion, how fast it expands though, has a strong relation to its total energy), has a mass (E=mc²), and also has non-zero field energy in various different fields.

Yes, but could the expansion of the universe and the seemingly overabundance of energy simply be the result of antimatter annihilation at the very beginning of the universe? Energy is convertable, for lack of a better description, from one form to another.

Think Hawking radiation. Virtual particles are said to be created just outside of the event horizon. When those are created, they are created with zero total energy, and always create pairs of particles that usually annihilate, and could be said to not actually exist, except that one falls into the black hole and the other escapes, carrying with it energy from the black hole, hence black holes that evaporate.

Could a similar mechanism not also be responsible for the supposed imbalance?

 

[Urwumpe]

Yes, but could the expansion of the universe and the seemingly overabundance of energy simply be the result of antimatter annihilation at the very beginning of the universe? Energy is convertable, for lack of a better description, from one form to another.

Antimatter is not negative energy. That is a wrong concept there, exotic matter would be the proper term for that kind of phenomena. Remember: When you annihilate particles and their antiparticles, the energy is not simply lost in nothing (= 0 Joule energy), but actually you get the equivalent of the annihilated masses in form of other particles.

Also, the concept of an initially symmetric universe has already taken so many hits, that it is likely wrong. There is the final real evidence missing, but many particles are their own anti-particle, which already means that depending on how matter was created, in which order the particles appeared, the universe could have had more or less dedicated anti-particles.

(Also of course, there is not THE anti-matter. Every anti-particle is the opposite to one particle. Colliding anti-protons with neutrons wouldn't result in just one electron being left of the neutron)

 

[Fizyk]

Considering the time dilation that occurs close to a massive object, I think it might be accurate to describe non-zero matter densities as curving space-time.

It is, if you believe in what general relativity says, and for now it seems to work quite well

many particles are their own anti-particle

Not that many, actually. Among the Standard Model elementary particles, it's just photon, Z0 boson, maybe gluons (I'm not sure) and still hypothetical Higgs boson and graviton. If you count hadrons, there are some neutral mesons which are their own antiparticles (like pi0), but they are still vastly outnumbered by other hadrons

Back to the total energy of the universe - one more argument why it's not well-defined. Energy is a quantity which only makes sense when it's defined relative to something (just like velocity or momentum). When you say that total energy of the universe is equal to something, a question arises - energy relative to what?

 

[Urwumpe]

Back to the total energy of the universe - one more argument why it's not well-defined. Energy is a quantity which only makes sense when it's defined relative to something (just like velocity or momentum). When you say that total energy of the universe is equal to something, a question arises - energy relative to what?

The answer would be quite simple: Does it matter?

If you are in different inertial frames in the universe, and observe the universe, do you arrive at different estimates of energy or the same estimate everywhere?

 

[Fizyk]

The answer would be quite simple: Does it matter?

If you are in different inertial frames in the universe, and observe the universe, do you arrive at different estimates of energy or the same estimate everywhere?

Well, if the notion of the total energy of the universe makes sense (and I'd still argue that it doesn't ) - it should matter (though, to be honest, I'm not 100% sure about that - see below). Imagine the most extreme case of you moving almost at light speed relative to the Earth - all of the matter in the universe starts to move almost at light speed realtive to you, thus having a lot more energy than relative to the Earth.

What I'm not sure about is if the far objects in front of you don't cancel the effect - as they appear to rapidly escape from us now, if you were to chase them, they would be slower realtive to you and have less energy. I don't think that would be enough, though.