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a reply to: Arbitrageur

Thank you for the explanations - I need some time to review the links.

In the meantime, I have a quick question: If dark energy exists, all theories regarding dark energy must include a model for origins. The origin of dark energy has to fit with the first law or else everything we think we know falls apart. If dark energy obeys the first law and still expands, wouldn't the outward pressure against large objects decrease over time?
The basic question is - do theoreticians assume that dark energy obeys the first law when they design their models?

I just came across this paper today which I need to study - but it basically describes using the entropy of dark matter as a measurement tool (I think).

Cosmographic Thermodynamics of Dark Energy
pdfs.semanticscholar.org...

originally posted by: Phantom423
The origin of dark energy has to fit with the first law or else everything we think we know falls apart.

Everything we think we know doesn't fall apart just because we don't know one thing like an origin. For example, in the Big Bang Theory, the model is based on evidence that at some time in the past the universe was very hot and very dense. That's it. We don't know if the origin was a "singularity" or something else, but lacking that knowledge doesn't invalidate our observations and models of what happened after the origin, whatever the origin was.

The second thing I would note is that vacuum energy is one of the least understood things in physics, theoretically. We have dark energy observations suggesting a certain value for vacuum energy, and vacuum energy is the most popular guess about the cause of dark energy, but we have no valid theoretical prediction for vacuum energy. Some naive predictions using existing theory result in an answer so wrong it's called "the vacuum catastrophe". So for now we can assume observations are probably correct and assume our model is missing a piece of the puzzle which is why we can't make predictions to match observation. Nima Arkani Hamed spent the better part of a decade trying to solve that problem and didn't, and nobody has been recognized as solving it, though Delbert Larson tried to contact Dr. Hamed with his proposed solution, and got no reply. I told Delbert Larson I have no experience that comes close to comparing with Dr. Hamed's years of work on the problem to evaluate his proposed solution, so I really can't say if he's solved it, but I am aware that others also say they have solved the unsolved problem and so far no consensus solution has been accepted so it's still considered an unsolved problem.

The basic question is - do theoreticians assume that dark energy obeys the first law when they design their models?

The first law is basically an energy conservation law which applies to closed systems. The observable universe is not a closed system, so technically it doesn't apply.

You can still ask if energy is conserved when we see space expanding with all this additional energy in the newly created space. Where is all that energy coming from, isn't it appearing out of nowhere and violating conservation of energy? Whatever the origin of dark energy, we can say that the added dark energy is balanced by the added gravitational potential energy, so under this idea energy is conserved and energy in the observable universe, and possibly in the universe as a whole can remain zero. This theoretical physicist explains and lists the applicable equations at the link:

Energy conservation of the universe

Philip Gibbs, PhD theoretical physics.

The overall average energy density of the universe is in fact zero and this does not change as it expands. It is a sum of different energy contributions which behave differently.

The most familiar contribution is from the mass of matter around us (including dark matter) which has an energy equivalent according to [math]E = mc^2[/math]. The density of this energy decreases as the volume expands just as you would expect.

Another form of energy is dark energy and this contribution is actually larger on a cosmic scale (about four times larger at present) The density of dark energy remains constant as the universe expands so the amount of energy in an expanding volume is increasing.

The reason that this does not lead to a violation of energy conservation is that there is also an energy contribution from the gravitational field due to the expansion of the universe itself. This is on average the negative of the other contributions so that the total is zero on average. It's magnitude increases because the expansion rate is increasing due to the dark energy.

If you want the equations which describe all this they are as follows

See the link for the equations.

I understand this explanation, though I am not sure if this explanation is universally accepted and I didn't understand the paper you cited well enough to tell if it's consistent with this explanation or not (I didn't spend much time reviewing it). When you deal with anything that has "dark" in the name you should presume that means it's not well-understood and whatever ideas we have can be and probably will be refined with additional observational data and better models to fit them.

a reply to: Arbitrageur

Why isn't the vacuum energy, or zero-point energy, accounted for in this equation - isn't the vacuum energy separate from dark energy?

a(t) is the universal expansion factor as a function of time normalised to 1 at the current epoch. It started as zero and increases with time as the universe gets bigger.

What does that really mean? Exactly what started at zero? How is that factor calculated i.e. from what data?

originally posted by: Phantom423
a reply to: Arbitrageur

Why isn't the vacuum energy, or zero-point energy, accounted for in this equation - isn't the vacuum energy separate from dark energy?

From my last post:

originally posted by: Arbitrageur
vacuum energy is the most popular guess about the cause of dark energy.

I thought this was clear enough but it means we are not 100% sure that dark energy is caused by vacuum energy and only vacuum energy. I've seen numerous physicists say that dark energy and vacuum energy are one in the same, but other more cautious physicists say they aren't so sure, for example even if Vacuum energy is one cause of the accelerationg expansion, it might not be so simple that it's the only cause, there could be other causes. So once again the caveat about dealing with anything named "dark" applies, that we don't really claim that this topic is well understood, just that we have these observations and this ΛCDM is our best attempt to explain them but it has some tension so as with all models I don't assume it's perfect and some physicists have proposed extended or alternative models.

"a(t) is the universal expansion factor as a function of time normalised to 1 at the current epoch. It started as zero and increases with time as the universe gets bigger."

What does that really mean? Exactly what started at zero? How is that factor calculated i.e. from what data?

The primary data I'm aware of that relate to dark energy are type 1a supernovae observations, a type of "standard candle" as it's called which brightness can be determined and therefore we have some idea of how far away each supernova happened. We can also measure the redshift which is represented by z. So, by measuring the type 1a supernovae redshift and comparing that with how bright the type 1a supernovae appear to us on earth versus their assumed "standard candle" brightness we can ascertain the expansion rate from different type 1a supernovae at various distances. The first data came from two papers in 1998, here's a link to one of them:

Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant

We present observations of 10 type Ia supernovae (SNe Ia) between 0.16 < z < 0.62. With previous data from our High-Z Supernova Search Team, this expanded set of 16 high-redshift supernovae and 34 nearby supernovae are used to place constraints on the Hubble constant (H_0), the mass density (Omega_M), the cosmological constant (Omega_Lambda)

and they concluded that the cosmological constant (Omega_Lambda) was not zero so the expansion of the universe is accelerating and the idea of dark energy was invoked. They mention vacuum energy as a pre-eminent possibility, but don't come out and say they have any certainty that's the cause, here's how they worded it:

The time evolution of the cosmic scale factor depends on the composition of mass-energy in the Universe. While the Universe is known to contain a significant amount of ordinary matter, Ω M , which decelerates the expansion, its dynamics may also be significantly affected by more exotic forms of energy. Pre-eminent among these is a possible energy of the vacuum (Ω Λ ), Einstein’s “cosmological constant,” whose negative pressure would do work to accelerate the expansion

Of course there are more recent papers with more recent data you can find in a search but I haven't kept up with all of them. For further explanation of the dimensionless scale factor a, see the ΛCDM model in the "Cosmic expansion history" section, and if that's not self-explanatory, feel free to cite any passages from there which don't make sense and maybe we can try to clarify them.

a reply to: Arbitrageur

I thought that vacuum energy would be separate from dark energy based on the description here:

Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. This behavior is codified in Heisenberg's energy–time uncertainty principle. Still, the exact effect of such fleeting bits of energy is difficult to quantify. The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum.[1]

en.wikipedia.org...

If dark enery/vacuum energy are the same, then Gibbs' equation is correct. But if vacuum energy is another energy somehow intertwined with dark energy, then doesn't it have to be factored into that equation? Gibbs says:

Another form of energy is dark energy and this contribution is actually larger on a cosmic scale (about four times larger at present) The density of dark energy remains constant as the universe expands so the amount of energy in an expanding volume is increasing. The reason that this does not lead to a violation of energy conservation is that there is also an energy contribution from the gravitational field due to the expansion of the universe itself. This is on average the negative of the other contributions so that the total is zero on average. It's magnitude increases because the expansion rate is increasing due to the dark energy.

Wouldn't the contribution from the gravitational field be insufficient if there are two types of energy present - dark energy and vacuum energy. Vacuum energy can be observed in the Casimir effect and the Lamb shift. Is the quantum vacuum state something that can't or shouldn't be factored into Gibbs' equation?

a reply to: Phantom423
As I said, some physicists will tell you dark energy is vacuum energy, but the reason the authors of that paper discovering dark matter and others don't put it that way and instead say things like it's a "preeminent possibility" because our model for vacuum energy is an unsolved problem in physics leading to the "vacuum catastrophe". This is what Matt O'Dowd calls it:



If we had a good model for vacuum energy prediction consistent with the observed value of accelerating expansion of the universe, then I think we could say dark energy is vacuum energy; if that was the case maybe they would have announced the find as vacuum energy and the term "dark energy" might not have been coined.

So you're trying to get me to say they are they same thing and I'm saying they probably are, however good scientists are skeptical of everything until they have good evidence, which is lacking in this case to say dark energy is vacuum energy and only vacuum energy due to our inability to make quantitative predictions about vacuum energy.

If dark energy/vacuum energy are the same, then Gibbs' equation is correct. But if vacuum energy is another energy somehow intertwined with dark energy, then doesn't it have to be factored into that equation?

I think even if the accelerating expansion of the universe is more complicated than just the result of vacuum energy, his conclusion that the net energy remains zero after the expansion will remain true. However you have a point that if dark energy may have more than one component, say maybe vacuum energy and something else, then of course more accurate equations incorporating each of the proposed contributions might describe that more accurately.

Here's an example of an alternate idea for dark energy besides simply calling it vacuum energy:

Dark energy could be the offspring of the Higgs boson
That wouldn't be my first guess, but as long as dark energy is not well understood, it's probably a good idea to explore different ideas, which you can find happening in the research.

Maybe watching this video will help you understand that vacuum energy is not well understood but is still an unsolved problem and thus we should hesitate about making definitive statements about it which are unsupported, like Vacuum energy and dark energy are the same thing. That may in fact be a true statement, but until the evidence supporting that is better, I think the authors of the 1998 paper stating that vacuum energy is a pre-eminent possibility for explaining the "dark energy" observations is much more accurate than saying they are sure that vacuum energy explains observations.

I think you can handle watching this but I don't recommend it for Daniel Koenig, I'm afraid his head might explode since it contradicts his notions that "something is something and nothing is nothing and something can't be nothing and nothing can't be something"x1000. Anyway it's a fairly good overview that's not too technical.

a reply to: Arbitrageur

So you're trying to get me to say they are they same thing and I'm saying they probably are, however good scientists are skeptical of everything until they have good evidence, which is lacking in this case to say dark energy is vacuum energy and only vacuum energy due to our inability to make quantitative predictions about vacuum energy.

I agree 100% - good science requires good evidence, no matter what the field. What's interesting is that DE doesn't emit radiation or absorb radiation. All our detection technology is based on various wavelengths of energy like light and radio waves - hard to analyze something you can't see.

Thanks for the link - will watch later today.

originally posted by: Phantom423
What's interesting is that DE doesn't emit radiation or absorb radiation. All our detection technology is based on various wavelengths of energy like light and radio waves - hard to analyze something you can't see.

We can say most dark matter doesn't seem to interact with electromagnetic radiation, but the picture of the curved galaxy I previously posted shows we can observe the effects on light. For dark energy or vacuum energy I think the bigger issue with a theory that predicts large amounts of vacuum energy is that would affect spacetime in general relativity, maybe giving the universe a spherical geometry instead of appearing flat. That's mentioned in the Vacuum catastrophe video.

Thanks for the link - will watch later today.

You're welcome. You might wonder how someone can talk about nothing for 10 minutes, but it's such an interesting topic that he made two more 10 minute videos also talking about "nothing", aka the vacuum catastrophe and zero point energy, so you might be interested in these too. I mentioned the vacuum catastrophe but he goes into more depth with facts and figures.

The Vacuum Catastrophe

Key point, predicted vacuum energy is 120 orders of magnitude larger than observed value, thus the catastrophe.

Zero-Point Energy Demystified

Key point, the schemes to extract energy from zero point are scams because even if there was a lot of zero point energy, it's inaccessible.

a reply to: Arbitrageur

Zero point energy is accessible in certain conditions, but may not be entirely safe to extract

I came across this article today which was in the news - I think they got this relationship wrong - if the wavelength of a photon increases, its energy decreases - E = hc/wl - energy and wavelength are inversely related. Did I miss something in the article?

Boomerang photons already move at the speed of light, so they don't pick up any speed from their trips around black holes. But they do pick up energy. That energy takes the form of increased wavelength of the light, and the individual photon "packets" carry more energy than they had when they entered the mirror.

www.nbcnews.com...

a reply to: Phantom423
You are correct,the article is incorrect. The article should have read as shown in this correction:

"they do pick up energy. That energy takes the form of increased wavelength frequency of the light"

The energy of a photon is just a constant multiplied by the frequency, so as you said, the energy increases as the frequency increases (as the wavelength decreases).



It's an interesting idea to steal energy from a black hole for propulsion, because we really need drives which need no fuel to accelerate, like the one discussed, to travel the galaxy. Unfortunately it is unlikely to help with our first foray to the stars since I don't know of any nearby black holes, which seems like a good thing that none are known to be too close to us.

a reply to: Arbitrageur

How can you be so sure known forces are what we think they are?

originally posted by: InTheLight
a reply to: Arbitrageur

How can you be so sure known forces are what we think they are?

I never said I was sure about that. The whole basis for science is that it's falsifiable, meaning if someone can collect enough data and present enough evidence to show what we now think is wrong and something else is right, then science is supposed to be open to considering and accepting the new data.

The requirements for the new evidence that would cause us to change our models are substantial, because we already have a lot of evidence that our current models are consistent with how nature behaves in many respects. So on the one hand scientific models are subject to revision based on new data, but on the other hand, many non-scientists seem to be unaware of how much data and analysis supports the current models, and how hard it is to overturn all of that.

Your question reminded me of this Feynman video where he says he has different degrees of certainty about different things but he is not absolutely sure of anything which seems like a sound scientific viewpoint to me.

Great Minds: Richard Feynman - The Uncertainty Of Knowledge

a reply to: Arbitrageur

Thank you, I will continue my reality search.

a reply to: InTheLight
We may not be sure what gravity really is, but we can predict its effects well, so it's possible to be a little too skeptical about forces we don't understand fully, like this:

How Science Works


If you find something in your search for reality, how do you know if it's really true or not? Testing the idea with science seems to be a good way to find a truth that different people can agree on. As Feynman said if you're trying to choose a religion or similar belief system like that, it's hard to figure out which one is right because they don't seem to be based on science or things different people can agree on. In my own search for reality, I was disappointed to have failed on all of my telekinesis experiments at the age of 10, what a disappointment. But at least we have a lot of really cool technology now to make up for that, so I got over my disappointment.

Planck constant needs fixing. It is constant for particular time flow. If time flow changes planck constant needs to change. Energy of the photon is also dependent on intensity or amplitude.
a reply to: Phantom423

a reply to: Arbitrageur

The rules seem to work pretty well.

Who cares why?

Ques:
If an ATC is transmitting at say, 118 MHZ to an aircraft at 30000ft. At what exact freq would the aircraft receive this transmission?

a reply to: Hyperboles

2.54061 meters.
www.everythingrf.com...

Or are you talking about red/blue shift due to gravity gradient?

originally posted by: Phage
a reply to: [post=24259720]Hyperboles[/pos



Or are you talking about red/blue shift due to gravity gradient?

Yes