B Does red-shifted light cause expansion of space?

[Addez123]

Light can get red-shifted due to expansion of space, this leads to a loss of energy.
Would it be possible that this energy is what's causing the universe to expand in the first place?

 

[Drakkith]

Would it be possible that this energy is what's causing the universe to expand in the first place?

It is not. The high energy density of the very early universe worked to stop expansion, slowing it down. The reduction of this energy density allows the expansion to accelerate (since normal energy density counteracts dark energy), but it is not its cause.

 

[Addez123]

As I understand it: The high energy density holds the universe from expanding. We've crossed a line at which the density is not enough to hold the universe from expanding and thus it expands and cause the energy density to become even less. Correct?

 

[phinds]

Light can get red-shifted due to expansion of space, this leads to a loss of energy.
Would it be possible that this energy is what's causing the universe to expand in the first place?

It is possible that you are laboring under the misconception that because the light, as it redshifts, loses energy, this energy has to GO somewhere due to "conservation of energy". The reason this is a misconception is that there there IS no "conservation of energy" on cosmological scales. This seems counter-intuitive when you first encounter it but it is a fundamental part of cosmology.

 

[Addez123]

It is possible that you are laboring under the misconception that because the light, as it redshifts, loses energy, this energy has to GO somewhere due to "conservation of energy". The reason this is a misconception is that there there IS no "conservation of energy" on cosmological scales. This seems counter-intuitive when you first encounter it but it is a fundamental part of cosmology.

Is it really proven? I mean, how would you know?

 

[phinds]

Is it really proven? I mean, how would you know?

I don't think "proven" even applies here. How would you "prove" a negative? Energy on cosmological scales is frame dependent. That means that an increase in energy in one frame could be seen as a decrease of energy in another frame. What would "conservation" even mean in those circumstances?

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

 

[Drakkith]

As I understand it: The high energy density holds the universe from expanding. We've crossed a line at which the density is not enough to hold the universe from expanding and thus it expands and cause the energy density to become even less. Correct?

The universe is already expanding and has been for 13 billion years. What I think you're talking about is whether or not the universe will continue to expand forever, or whether it will eventually collapse. Evidence currently points towards the former.

 

[phinds]

@Addez123 you clearly have many misconceptions about cosmology. That's perfectly normal but the best way to fix that is to get a beginner's book on basic cosmology and read it. Asking random questions on an internet forum is a poor way to learn what needs to be approached systematically.

 

[Addez123]

@Addez123 you clearly have many misconceptions about cosmology. That's perfectly normal but the best way to fix that is to get a beginner's book on basic cosmology and read it. Asking random questions on an internet forum is a poor way to learn what needs to be approached systematically.

I would if I had time. I find these things interessting but I don't have time to study it atm. Top prio of studying, outside school, would be quantum physics tho.

 

[PAllen]

Is it really proven? I mean, how would you know?

In some sense, the situation is even worse. For cosmology, you cannot even consistently define total energy. In GR, you need asymptotic flatness for a global definition of total energy, and cosmological solutions do not have this property. Thus, you don't even get to ask about conservation, because you can't define total energy in the first place.

 

[PeterDonis]

In GR, you need asymptotic flatness for a global definition of total energy, and cosmological solutions do not have this property

Technically, asymptotic flatness is what you need to define the ADM energy or the Bondi energy. There is also the Komar energy, which requires only that the spacetime be stationary. But cosmological solutions don't have that property either.

 

[Orodruin]

In some sense, the situation is even worse. For cosmology, you cannot even consistently define total energy. In GR, you need asymptotic flatness for a global definition of total energy, and cosmological solutions do not have this property. Thus, you don't even get to ask about conservation, because you can't define total energy in the first place.

Technically, asymptotic flatness is what you need to define the ADM energy or the Bondi energy. There is also the Komar energy, which requires only that the spacetime be stationary. But cosmological solutions don't have that property either.

Translation to B-level: Spacetime has some properties that make it difficult to even define what "total energy" means. There are several options that all place different requirements on the spacetime in order for the "total energy" to be defined. The current model that we have for cosmology does not satisfy any of those requirements.

 

[Addez123]

Translation to B-level: Spacetime has some properties that make it difficult to even define what "total energy" means. There are several options that all place different requirements on the spacetime in order for the "total energy" to be defined. The current model that we have for cosmology does not satisfy any of those requirements.

E = mc^2 and E = hc/λ isn't valid in cosmology?

 

[Drakkith]

E = mc^2 and E = hc/λ isn't valid in cosmology?

Neither of those give you the total energy of the entire universe.

 

[Vanadium 50]

I would if I had time.

So your position is that because you don't want to put in the time to read a book, we have to more of spend out time correcting your misconceptions rewriting what is in those books? Just how much less valuable do you think our time is than yours? People have been a lot more polite to you than I would have expected, given this lack of respect for their time.

Phinds gave you some very good advice. You will learn more by taking it than ignoring it.

 

[Nugatory]

E = mc^2 and E = hc/λ isn't valid in cosmology?

They are both valid in cosmology.

Much of this is explained in the link that Drakkith provided in post #7 of this thread.

 

[Drakkith]

They are both valid in cosmology.

Much of this is explained in the link that Drakkith provided in post #7 of this thread.

You mean the link phinds provided in post #6?

 

[phinds]

You mean the link phinds provided in post #6?

What a horrible thought that people can't tell us apart

 

[Nugatory]

What a horrible thought that people can't tell us apart

:) Sorry about that

 

[Addez123]

One last question then.
Red-shifting occurs when universe expand, this leads to "loss" of energy. But if the universe were to contract the light would be blue-shifted and no energy be lost. So isn't it obeying the conseration of energy then?

 

[Drakkith]

One last question then.
Red-shifting occurs when universe expand, this leads to "loss" of energy. But if the universe were to contract the light would be blue-shifted and no energy be lost. So isn't it obeying the conseration of energy then?

No, then we'd be gaining energy.

 

[Orodruin]

One last question then.
Red-shifting occurs when universe expand, this leads to "loss" of energy. But if the universe were to contract the light would be blue-shifted and no energy be lost. So isn't it obeying the conseration of energy then?

No. You would destroy energy and then create energy. Conservation means it is the same at all times.

 

[phinds]

One last question then.
Red-shifting occurs when universe expand, this leads to "loss" of energy. But if the universe were to contract the light would be blue-shifted and no energy be lost. So isn't it obeying the conseration of energy then?

No. "Conservation of energy" has to be current to be meaningful. You can't lose energy and then gain it back later and call that "conservation". Besides which, the universe will never, as far as we can tell, contract.

 

[Battlemage!]

I don't think "proven" even applies here. How would you "prove" a negative? Energy on cosmological scales is frame dependent. That means that an increase in energy in one frame could be seen as a decrease of energy in another frame. What would "conservation" even mean in those circumstances?

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

Is this analogous to a simple concept like kinetic energy of a thrown ball depending on your frame?

EDIT- wait, I'm guessing the difficulties associated with defining speed in non-flat spacetime rear their head here too?

 

[phinds]

Is this analogous to a simple concept like kinetic energy of a thrown ball depending on your frame?

yes, but it's more complicate than that, as I understand it, because of the expansion of space.

EDIT- wait, I'm guessing the difficulties associated with defining speed in non-flat spacetime rear their head here too?

I'll have to leave that one to one of our more knowledgeable members

 

[stevmg]

Red shift is a result of the expanding universe, not the cause of it. Basic Doppler wave mechanics. Must keep the cart and horse in the right order. Question is did the original singularity have infinite energy? This is possible mathematically (1/zero). If it did, the universe will never cease to expand.

 

[phinds]

Red shift is a result of the expanding universe, not the cause of it. Basic Doppler wave mechanics. Must keep the cart and horse in the right order. Question is did the original singularity have infinite energy? This is possible mathematically (1/zero). If it did, the universe will never cease to expand.

That is not a necessary condition. Given that "dark energy" is constantly being created, it will continue to expand regardless of the initial conditions.

 

[stevmg]

@phinds - Never thought of that.

 

[Orodruin]

If inflation is correct there might never have been an initial singularity.

 

[PeterDonis]

Basic Doppler wave mechanics.

Which assumes flat spacetime, but the spacetime of the universe as a whole is not flat. So your assertion does not explain why, as you put it, "redshift is the result of an expanding universe".

did the original singularity have infinite energy? This is possible mathematically (1/zero). If it did, the universe will never cease to expand

This is not correct as an assertion about possible mathematical models that are consistent with the Einstein Field Equation. One such model is a closed universe that recollapses; it has both an initial and a final singularity, and both have "infinite energy" by your definition.

As far as our actual universe is concerned, AFAIK nobody really believes there was an "initial singularity" with infinite energy; what is believed is that the classical GR model breaks down when spacetime curvature becomes large enough, and we need some other theory (the most likely candidate is a theory of quantum gravity) to tell us what happens in that regime. So the "initial singularity" in the mathematical models based on classical GR is an artifact of the model, not a claim about our actual universe.