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Dark Energy Superfluous?

Posted August 04, 2010 11:00 PM by Jorrie
Pathfinder Tags: cosmology dark energy

Taiwanese scientist Wun-Yi Shu made some waves last month with a mathematical model that challenges the dark energy driven model of cosmology. Here's a quote from the Technology Review blog:1

"Shu's idea is that time and space are not independent entities but can be converted back and forth between each other. In his formulation of the geometry of spacetime, the speed of light is simply the conversion factor between the two. Similarly, mass and length are interchangeable in a relationship in which the conversion factor depends on both the gravitational constant G and the speed of light, neither of which need be constant.

So as the Universe expands, mass and time are converted to length and space and vice versa as it contracts."

The first paragraph says nothing new, but the second may have some interesting implications. He uses this idea (in a somewhat controversial manner) to predict redshifts that closely match those observed in the late 1990's, as shown in the graphic (from the paper). The data represent the effective peak magnitude of Type Ia supernovae in the B-band spectrum (688 nm) against redshift.

Shu claims that, if correct, his model eliminates the need for dark energy. There is a heavy price to be paid, though - both the universal constants c and G must be variables, i.e. they must be changing over cosmological time. It did not take long for other scientists to find holes in Shu's theory. The most readable critique that I found was on Sarah Kavassalis' Blog "This Week in the Universe".2

As far as I can see, her main point is that changing c and G requires a complete redo of Einstein's field equations (EFEs). The EFEs require those values to be what their names say: constants. According to Kavassalis, Shu changed the EFEs in an arbitrary manner, yielding them invalid and in a way meaningless.

She also claims that such a good fit to the observed data can be achieved without doing proper science. Her final words in the blog article are revealing: "Yes, if you pick and choose what physics to ignore you can arrive at meaningless equations."

I am less dismissive, because I like alternatives that show at least some promise. Maybe one of them will one day be refined to properly explain redshift observations in a more palatable way than dark energy.

What do you think?

Jorrie

1. Big Bang Abandoned in New Model of the Universe

2. "Cosmological Models with No Big Bang" review

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#1

Re: Dark Energy Superfluous?

08/05/2010 9:31 AM

I appreciate Kavassalis's comment, "Yes, if you pick and choose what physics to ignore you can arrive at meaningless equations." Whether Shu has done this or not, I really don't know, but I do know it can be done. In a senior biology lab, I used software to fit a 9th order polynomial to the data recording bacteria colony growth over time. To me it was a joke. The periodic measurements indicated that colony grew quickly then asymptotically leveled out at a maximum for the petri dish. To my astonishment, the professor saw the high R-squared value for the 9th order polynomial fit and was temporarily excited about the possibility that I had discovered a new model for bacteria growth. The long and the short of it is: don't be convinced that a curve which neatly fits dots necessarily corresponds to anything in the real world.

Topic shift... Also, if I recall, Shu's model does not account for cosmic background radiation. Big Bang cosmology needs "dark energy" to account for the accelerating expansion of the universe. Would Shu's model require a dark cosmic microwave oven to account for the background radiation?

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#2
In reply to #1

Re: Dark Energy Superfluous?

08/05/2010 11:53 AM

Hi GKC, you asked: "Would Shu's model require a dark cosmic microwave oven to account for the background radiation?"

AFAIK, models that predict present cosmic expansion (with dark energy or not) must have a form of background radiation, because the cosmos must have been much denser and hotter in the distant past. Shu did not quite say what his model predicts for the distant past, but I presume it is similar to the BB (whether his model makes sense or not).

It is also good to take note that BB cosmology does not necessarily require dark energy. There may still be other observational explanations for the apparent increase in expansion rate.

-J

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#3
In reply to #1

Re: Dark Energy Superfluous?

08/06/2010 1:33 AM

Dark Energy is just one more of those "quasi facts" proposed by our scientific community, being just one of many. over the last century.

The scientific community have been starved of the truth for so long...that, they now subscribe to the notion.... why spoil a good story with the truth.?

The most central factor to the source of all those eneregies to found in the Universe is still largely over looked, with little or no discussion on this vital matter.

The Aether ( vacuum of space) Where else could you hide an infinite quantity of energy..with out bumping into it.?

Distance is the greatest liar in the Universe.

waro.

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#4

Re: Dark Energy Superfluous?

08/06/2010 5:10 AM

May be we find what suits us now? Our time scale, a pin prick
in the total time, has almost inevitably(?) provided "constants" for us.

i.e. Einstein could be both right, and wrong. Right, in / for "our time"
millions of years; yet also wrong, within the total time of the universe.
Without a beginning or end, what we "know" now, only applies to now.

My learned colleagues here know far more about this than I ever will.
However, the mystery (to me) of a "forever" even with our created laws,
patently cannot be; (because: whatever anything "is" in our limited view,
needs to be bounded by "something.") and this currently seems insolvable.

As they say, it's a job in progress; I'm sure you will let me know the answer?

jt.

They said.. "go for it" - but when I went; it had gone!

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#5

Re: Dark Energy Superfluous?

08/06/2010 11:30 AM

He may or may not be onto something, but there is a human tendency to not want to throw away something that mostly works for something that is unproven but might work better. therefore physicists and cosmologists are comfortable with General relativity, even if there are clear and glaring problems with it, so they are reluctant to accept anything that is heretical. Such marriage to a flawed theory tends to blind you to other possibilities.

I was watching a show on the discovery channel last week on the subject of dark matter and dark energy and there was an observation of two galaxies colliding that had been imaged by Hubble and then processed to show where the dark matter had to be based on observed gravitational effects of the "real matter". And it appeared that the dark matter simply passed right through not only the existing "real" matter but other dark matter as if nothing was there. I find it very hard to wrap my mind around the concept of matter that has mass but does not interact with other masses except through gravity. almost as if they were free Higgs Bosons!

Further the show indicated that for every pound of "real" matter, there were 5 pounds of "dark matter" and further, that between the two, they only accounted for about 25% of all the forces acting on the universe. 75% of the forces were attributed to "dark energy" which physicists know even LESS about.

It is almost as if there is/are a parallel universe(s) overlaying our own with gravitational and cosmological constants very very close but not quite the same as ours and the effects and forces are bleeding across the barriers and interacting with matter in the other universe and vice versa, but the resultant forces are very very weak and only appear when observed over very large areas and very long time frames. I know there are multiple theories about parallel universes or multi-dimensional universes (string and brane theories tend to go this direction) but I've never seen quite this idea proposed before, at least not in this form, or if it was it was never explained in quite that manner. Hell at this point, this idea is no more testable than any of the others so we all have an equal chance at being right at this point.

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#6
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Re: Dark Energy Superfluous?

08/07/2010 3:50 PM

I myself am very much in similar thought as Rorschach.

What is possible? Those things that are possible are. What is weak here, is strong there?

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#7
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Re: Dark Energy Superfluous?

08/08/2010 2:16 AM

Hi Rorschach, you wrote: "...therefore physicists and cosmologists are comfortable with General relativity, even if there are clear and glaring problems with it, so they are reluctant to accept anything that is heretical."

I can't agree that this is the state of science at the moment. Firstly, there is no 'clear and glaring problems' with GR. All they know is that it will not work at microscopic levels (where quantum theory works), but at the level of the solar system and even the galaxy, they find no errors in it. There are several attempts going on to unify quantum theory and GR, but so far they were unsuccessful. At cosmic scales, the picture may be different, but there exists no theory that better fits all observations than GR, with its cosmological constant.

Not that physicists are not trying to find errors in GR - it is constantly being tested at every available opportunity and it is every scientist's dream to be the one that "proved Einstein wrong" at the macroscopic or even cosmic level. So, far all attempts failed, but one day it may happen...

No alternative theory to GR has ever survived honest scrutiny against experiment, except if it makes exactly the same predictions for all experiments than what GR does. Then it is just a different philosophical base, i.e., a different interpretation of GR, not a different theory.

-J

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#8

Re: Dark Energy Superfluous?

08/08/2010 4:41 AM

In my opinion.....

There is no need to use mathematics to dismantle the theory of "dark energy".......commonsense will do that for us.

The unlimited potential energies that are available to the visible and measurable Universe, cannot reveal themselves together, at the same momment in time, in the Universe as we know and see it.

One is derived from the other, as all our visible and measurable energies are extracted simultaneously, from the unseen potential source that exists with in the Universe.

The so called "dark energy" is used to distract those folk who reason that the "big Bang" is a nonsense,

Modern university manufactured physicists tremble at the notion, that there exists in the Universe phenomena which operate with out velocity irrespective of distance, or those classified as immutable, what is more they go on to label phenomena, for which there is at present no scientific evidence.... as "supernatural"

How can we ever learn the truth about the physics of all that which exists in this Universe if we continue to promote unproved theories as a substitute for the facts.

An expert is a person who possess ALL of the facts about a subject.

I ask, during the last 100 years how many theories have been resolved to become Facts..? further more, how many of those theories, still remain theories.

A theory is little more than an idea..and it's excellence is often a matter of opinion....not a matter of fact.

There is so much important information about our Sun which we cannot explain, yet our astronomers claim to know so much more about similar objects, thousands of light years`from Earth.

We are learning more and more ...about less and less

waro

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#9
In reply to #7

Re: Dark Energy Superfluous?

08/08/2010 10:10 AM

Jorrie, you prove my point while trying to disprove it. The glaring problems are just the ones you glossed over plus one you ignored completely which is the entire basis of this post to begin with. the two major problems are:

Inability to comport with quantum gravity.

And then there is the issue of the "cosmological constant" aka dark energy.

Yes the theory has worked very very well, but there ARE problems with it, and your willingness to gloss over that is proof on my point. Humans are loathe to throw away something that they are comfortable with and that works for most things.

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#10
In reply to #9

Re: Dark Energy Superfluous?

08/09/2010 2:07 AM

Rorschach, for the record, your two 'glaring problems' are also just 'glossed over' in your replies...

In short:

"Inability to comport with quantum gravity."

Quantum gravity is not a worked-out theory - quantum physics (microscopic) and GR (macroscopic) are worked out theories and neither works in the sphere of applicability of the other. The unification of the two may be what is today termed 'quantum gravity theory', but it is not workable yet. This is a known problem and not quite glossed over.

"And then there is the issue of the "cosmological constant" aka dark energy."

The cosmological constant or vacuum energy (Λ) is not exactly the same as 'dark energy', but just a subset of it. Λ is part and parcel of modern relativity theory - in fact GR does not properly represent the real, observed world without Λ. The ΛCDM cosmic model conforms to GR and it fits all cosmic observations so well that it is the best candidate model by a long way. It seems to confirm (but never prove) GR. IMO cosmologists wish they can find a 'glaring problem' there - it will make their lives much more interesting...

The term 'dark energy' refers to a wider class of phenomena, all attempts to try and understand what the cause of the apparent accelerated expansion is, if it actually IS real acceleration. This is a problem of observation and deduction, not of GR. As it happens, ΛCDM provides better correlation with observations than other forms of dark energy, but this does not rule out other mechanisms.

I can't respond in more detail to broad statements, but if you have specific points that you would like more clarity on, I'm happy to tell you what I know and where you might find more professional input.

-J

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#11
In reply to #10

Re: Dark Energy Superfluous?

08/10/2010 1:12 AM

Jorrie, a couple of questions.

"Λ is part and parcel of modern relativity theory". If I recall right, Einstein considered it his "biggest blunder" when he learned about Hubble's finding that the universe expands (I think Einstein used it to make the universe static). So why do you claim it now part of relativity?

"As it happens, ΛCDM provides better correlation with observations than other forms of dark energy". Again I recall that vacuum energy is found to be some 1060 times too strong to work as dark energy. Your statement seems false to me.

Sorry to be attacking your view, but these things puzzle me and I would like to hear your reasons.

SL

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#12
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Re: Dark Energy Superfluous?

08/10/2010 6:15 AM

Hi Guest, I must say that it was a nice friendly "attack"!

Q1: "If I recall right, Einstein considered it his 'biggest blunder'".

Yes, Einstein was never happy with Λ in his field equations and was very happy to drop it completely. Later analysis, first during the "Relativity Renaissance (the 1960s, after Einstein's death) and later in the 1980s, showed that there is a proper place for Λ in the cosmological solution to the field equations. Due to virtual particle formation, the vacuum could (and perhaps should) have net energy. Cosmic observations of the time did however show that Λ must be very close to zero, so it was set to zero in the equations and left at that.

During the late 1990s, the 'bomb shell' of accelerated expansion popped out of supernova observations. The obvious thing to turn to was a small, non-zero Λ and it seemed to work perfectly at the time (and still works nicely today). 'Occam's razor' requires that we accept the simplest model, until it is falsified...

Q2: "Again I recall that vacuum energy is found to be some 1060 times too strong to work as dark energy".

Neither general relativity, nor the ΛCDM model make any prediction for the value of Λ, just as they make no prediction for any other energy density in the ΛCDM model. The model is a perfect solution the Einstein's field equations and the parameter values are determined by observation. It is just like the mass density of Earth, or any other body for that matter, is not predicted by any theory that I know of - it is determined by observation.

Quantum physics, on the other hand, does predict the value of vacuum energy and it gets a value of the order 10120 larger than what is found by experiment (even worse than the 1060 that you mentioned). This is an embarrassment for physics in general, not really for GR or the ΛCDM model. There are arguments for why vacuum energy is canceled out by some or other mechanism[1], but I think we just do not understand what is going on there (yet).

Hope this helps :-)

-J

[1]. The quantum physics value of vacuum energy density is apparently a whopping 1094 kg/m3, while the observed value is around 10-26 kg/m3. One gets the feeling the 'quantum mechanics' have it wrong on their value...

Prof Ned Write posted an excellent discussion of vacuum energy here.

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#13
In reply to #12

Re: Dark Energy Superfluous?

08/10/2010 9:26 AM

It is Prof. Ned Wright who wrote the article, not Write, right?

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#14
In reply to #12

Re: Dark Energy Superfluous?

08/10/2010 1:54 PM

Thanks Jorrie.

I hope dark/vacuum energy will be refuted soon, because it is just so uncomfortable and confusing to me. :(

I wish Wun-Yi Shu good luck...

SL

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#15
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Re: Dark Energy Superfluous?

08/10/2010 2:28 PM

And it is entirely possible that it is Quantum mechanics that has the problem, not GR, but again, it works so well that everyone is willing to overlook it's failure to comport with GR. There is also the bit about the Higgs which still has not been found even though the LHC should probably have found one by now. And without the Higgs, the whole theory gets a little shaky. After all if there is no Higgs, what is mass? So again nobody is willing to toss the theory on the scrap heap because it works so well, but it may very well be blinding people to a better approach that works better.

My beef isn't with the theories themselves Jorrie, I know they all have their flaws. The beef is with the human tendency to wear blinders, and to attack apostasy en masse when someone does come up with another way of looking at things. People are essentially social herd animals and it is hard to break from the herd. It goes against human evolution. It is the rare individual that doesn't care what others think of his ideas. and that fear of rejection can and does influence how people look at problems.

Since they both do work so well within their frames of reference, it is entirely possible that all that is needed is another way to see the problem in order to find the solution. But to do that, the physics community needs to work to put the blinders aside.

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#16
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Re: Dark Energy Superfluous?

08/11/2010 12:22 AM
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#17
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Re: Dark Energy Superfluous?

08/11/2010 9:07 AM

Very intriguing, I had heard of this theory before but the description was very superficial and it didn't impart a lot of confidence in the theory (again perception drives acceptance.). this description seems much more hopeful.

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#18
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Re: Dark Energy Superfluous?

08/12/2010 10:48 AM

I have read some of the (many) papers around Horˇava's theory and I'm left with the impression that it suffers from the 'generic' problem of all quantum gravity (QG) theories so far - not compatible with GR in the sense that it requires some from of preferred frame of reference or 'background frame'.[1] In more technical terms, it is not Lorentz invariant at all distances.

Quantum gravity does not fall inside my level of expertise, so I cannot absorb all the stuff that's been written, but my overall impression is that Horˇava's is a promising theory, provided its inconsistencies can be solved.

-J

[1] See e.g. http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.3046v2.pdf

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#19

Re: Dark Energy Superfluous?

08/25/2010 4:08 AM

Hi Jorrie. The "dark energy" (DE) concept, always, bothers me. I'd prefer not to... exist but it does. [Btw, the adjective "dark" is given to everything that is still unexplained (e.g. "Dark Matter", "Dark Energy", "Dark flow").] DE is just the "mysterious" name of the cause of the accelerated expansion of the universe. We are not even sure that it is a kind of energy. The main consideration is that the DE is, essentially, the quantic energy of the empty space, i.e. the energy of the vacuum. As the universe is expanded the "quantity" of vacuum is increased, hence the overall vacuum energy is increased, applying more pressure on the universe itself, overcoming gravity and leading on its accelerated expansion over time.

What bothers me, though, is the "violation" of the principle of the energy conservation: Considering the universe as a closed system, it should have a fixed amount of total energy. Hence the increment of the DE (hence the increment of its total energy) seems to be a paradox.

Is there any chance that the accelerated expansion of the universe could be arised by the properties (i.e. geometry) of the space itself???

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#20
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Re: Dark Energy Superfluous?

08/25/2010 5:18 AM

Hi George, your description of vacuum energy is very good.

You also wrote: "Hence the increment of the DE (hence the increment of its total energy) seems to be a paradox."

If the 'dark energy' is indeed vacuum energy (Ωv or also termed ΩΛ), the total energy in the cosmos remains constant, despite the expansion being accelerated!

Look at this equation from my chapter 14:

If you square both sides, the left side is proportional to kinetic energy of expansion and the right side proportional to cosmic potential energy. Hence kinetic energy plus the negative potential energy equals zero.1 The trick is that the vacuum energy 'adds' to the negative potential energy as much as it adds to the kinetic expansion energy.

"Is there any chance that the accelerated expansion of the universe could be arised by the properties (i.e. geometry) of the space itself???"

In a way, that is catered for by eq. 14.9 as well. The first term under the root represents space curvature and if Ω ≠ 1, it does contribute to the energy density and hence to both the potential and kinetic energy. In such a case, one may say that the accelerated expansion is partially caused by the increasing space curvature, but it cannot do it on its own, AFAIK. In any case, observations pin Ω to just about unity, so any curvature component must be very small.

We obviously do not know if our ΛCDM model is correct, but at the moment it is the best we have, fitting all the observations within experimental error bands.

-J

Note 1: The total energy does not necessarily have to be zero. The zero-point energy could be anything, but the main thing is that it remains constant.

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#21
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Re: Dark Energy Superfluous?

08/25/2010 1:50 PM

Hi George.

At a second thought, my: "In such a case, one may say that the accelerated expansion is partially caused by the increasing space curvature..." is 'inverted'. The 'increasing' should rather be 'decreasing'.

The confusion comes from the conventional ways of defining curvature, curvature energy and Ω. When Ω < 1, then the curvature energy term [(1-Ω)/a2] is positive, representing positive potential energy, but decreasing spatial curvature. It means that the expansion is faster than what the energy densities can restrain, causing the universe to go more 'open'.

Somewhat confusing, but this does not quite make the expansion accelerate, just decreasing slower. So, I do not think that the accelerated expansion can be caused by the geometry of space.

-J

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#22
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Re: Dark Energy Superfluous?

08/25/2010 7:11 PM

I believe that Jorrie's statement about Zero energy goes a long way to realising the truth.

" The totoal energy does not necessarily have to be Zero.

The zero point energy could be any thing, but the main thingis that it remains

constant."

The potential energy source, is as far as our preesent mehtods of detection are concerned, will indictae Zero.

We have no instruments that can detect potenrtial energies.

We see only that which we can measure.

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#23
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Re: Dark Energy Superfluous?

08/26/2010 2:57 AM

Thanks Jorrie. I didn't take into account the potential energy of the universe. That makes sense.

Although, what is the meaning of the negative cosmic potential energy??? (Or what is the meaning of the negative energy in general… This reminds me the "negative mass" issue and all the strange things related to this weird concept…) Is it probable that the total cosmic energy is positive??? It makes more sense than being zero.

After all, I think that the total cosmic energy is equal to the initial potential energy of the cosmic singularity (which was exploded, i.e. the Big Bang). As the universe is expanded this potential energy is converted to kinetic energy. (I ignore the vacuum energy.) This means that the cosmic potential energy is decreased (going to zero).

(Maybe my perception is wrong. After all, if the contribution of the vacuum energy is to increase the kinetic energy and decrease the potential energy (continuously), this means that the potential energy, sooner or later, will have negative values… Or this contribution will stop when the potential energy will become zero (???)… Hmm…)

Some help will be appreciated…

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#24
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Re: Dark Energy Superfluous?

08/26/2010 5:17 AM

Hi George, you asked: "Although, what is the meaning of the negative cosmic potential energy???"

The same as in Newton's mechanical orbital energy: Em = ½mv2 - GmM/h, where the negative gravitational potential energy is of mass m at height h (>0) above a reference point associated with mass M.

There are probably better ways, but for me the easiest way of understanding the negative nature of gravitational energy is thus: assume all matter-energy are created out of the energy of the vacuum during the BB. The vacuum must now have a huge 'energy deficit', something like negative energy,1 manifesting itself as gravity.

Unless there is a huge expansion velocity,2 all that matter will collapse back into the vacuum and the net energy will be zero again. Given a 'just large enough' expansion rate, where the positive kinetic energy exactly balances the negative gravitational energy, the net energy will also be zero, and it may stay that way forever.

"Maybe my perception is wrong. After all, if the contribution of the vacuum energy is to increase the kinetic energy and decrease the potential energy (continuously), this means that the potential energy, sooner or later, will have negative values…"

No, the common wisdom is that increasing expansion actually increases the gravitational potential energy, obviously by making it less negative. Only at infinite range potential energy approaches zero...

-J

1. We are not talking anti-matter here, because antimatter particles have positive mass-energy, just opposite electric charge. But it is accepted that the vacuum can give up energy for a short time and take it back again, like in virtual particles.

2. If it was not for quantum effects, the gravitational energy could have been at negative infinity at the BB singularity, requiring an infinite expansion rate. However, we simply do not know what happened exactly at the BB - waiting for quantum gravity to tell us...

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#25
In reply to #23

Re: Dark Energy Superfluous?

08/26/2010 8:50 AM

Hi GK, further to my prior reply, here is another easy-to-understand (I hope) consideration regarding the coupling between the energy content and accelerated expansion of the cosmos.

As the cosmos expands, vacuum energy increases proportionally to the volume of any sufficiently large area the we care to look at.1 At the same time, matter energy in that area remains constant and radiation energy decreases, but the latter is negligible today. Hence, the total negative gravitational potential energy of expansion (-E/r) decreases (goes more negative). Consequently, positive kinetic energy of expansion must increase in order to keep the universe 'flat'.2

Viola, we have accelerated expansion.

-J

1. Say we look at an area with constant comoving radius, but increasing proper radius due to the expansion.

2. If not perfectly 'flat', it should stay close enough for all practical purposes, or in any case change much slower than the potential energy.

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#26
In reply to #24

Re: Dark Energy Superfluous?

08/26/2010 9:14 AM

Erratum:

I wrote in post #24: "No, the common wisdom is that increasing expansion actually increases the gravitational potential energy, obviously by making it less negative. Only at infinite range potential energy approaches zero..."

This is only correct for a matter-only case, where total mass-energy remains constant, just less dense. It is incorrect in relation to vacuum energy, as described in my follow-on post #25.

So, your original statement on vacuum energy ("After all, if the contribution of the vacuum energy is to increase the kinetic energy and decrease the potential energy (continuously)") was actually correct!

-J

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#27
In reply to #24

Re: Dark Energy Superfluous?

08/27/2010 11:16 AM

Jorrie, thanks for your extended response and sorry for my late one. (I had problems with my pc. And that's why I have to login again.)

I had the sense that when we are talking about the cosmic potential energy, we actually mean the initial potential energy of the cosmic singularity just before the BB. (Actually it should be equal to the total cosmic energy, as -at this time- there were no kinetic energy.) In a way, it is equivalent to a compressed coil, ready to be released. After the BB (coil is released) the universe obtained kinetic energy which is increased, leading to a corresponding decrease of the potential energy (and so the total cosmic energy remains constant).

Probably, this point of view is wrong. Anyway, the whole "cosmic potential energy" issue was, always, a little "blurred" to me. You said: "... assume all matter-energy are created out of the energy of the vacuum during the BB. The vacuum must now have a huge 'energy deficit', something like negative energy,1 manifesting itself as gravity..." Is this a kind of "quantic pseudovacuum", totally different from the "conventional vacuum" of our 3D space??? It must be so, as there was no 3D space (hence vaccum) on t=0 (i.e. the BB). And this pseudovacuum must had (and still have) negative potential energy in order to counterbalance the positive kinetic energy of the new-born universe. You said: "... manifesting itself as gravity..." Is this a kind of "metaphor" (because of its negative potential energy) or this vacuum (or pseudovacuum) actually depicts gravitational behavior???

G.K. - George Kokotas -

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#28

Re: Dark Energy Superfluous?

08/27/2010 6:13 PM

Hi Jorrie,

Below is the picture of Shu that you referred to as "she". He looks like a man to me. Have you read the article in your footnote 2? Do you believe that he is a crank?

-S

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#29

Re: Dark Energy Superfluous?

08/27/2010 9:29 PM

Suppose, just for the sake of argument, that there is something outside of our visible universe.And suppose that something is a fluid similar to a liquid,and futhurmore,that our observable universe is a rising bubble in that liquid.Would we not find the same thing happening inside our bubble as it "rose" relatively upward toward the "surface" of this fluid.An increase in expansion as it rises, and if obstructed by outside forces, a decrease in rise rate.Imagine a bubble that encounters an inclined surface on the way up.It will slow down until clear of the obstruction,then resume expanding at an accellerating rate.

I know this is a very simplistic representation, but it seems to generally parallel our current understanding,without getting too technical.

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#30
In reply to #28

Re: Dark Energy Superfluous?

08/28/2010 1:24 AM

Hi S,

I thought I correctly referred to Shu as 'him'. Maybe you mis-associated the "she" that I later used for the pretty Sarah Kavassalis. Or, maybe I mixed them up somewhere...

No, I don't think Wun-Yi Shu is a crank - he found some mathematical expression that looks promising, although it does not seem to fit mainstream ideas of what is valid and what not. I think others may improve on his ideas and perhaps get closer to a good answer...

-J

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#31
In reply to #27

Re: Dark Energy Superfluous?

08/28/2010 4:36 AM

Hi George, you wrote: "... when we are talking about the cosmic potential energy, we actually mean the initial potential energy of the cosmic singularity just before the BB."

This can't quite work, as I pointed out in post 24, note 2: "If it was not for quantum effects, the gravitational energy could have been at negative infinity at the BB singularity, requiring an infinite expansion rate. However, we simply do not know what happened exactly at the BB - waiting for quantum gravity to tell us..." The best definition of the BB that I've read somewhere (I think on Physics-Forums), can be paraphrased as follows: 'The BB was not an event - it is simply the point where our theories break down...' So, we can only speculate about t=0.

You asked: "You said: "... manifesting itself as gravity..." Is this a kind of "metaphor" (because of its negative potential energy) or this vacuum (or pseudovacuum) actually depicts gravitational behavior???"

It is a known fact that the gravitational field itself gravitates, i.e. it has mass-energy of its own, a sort of extension of the primary mass. It is one of the key differences between Newton's and Einstein's gravity. I think whether we call potential energy "negative energy" is a matter of model. In the models that I know, the kinetic expansion energy must equal the gravitational potential energy for a 'flat universe', but whether that necessarily makes the gravitational potential energy negative1 is perhaps a matter of taste.

I'm not confident that I understand the concept of "pseudovacuum" or "false vacuum" correctly (it's quantum physics), but in standard ΛCDM cosmology, Λ is linked to vacuum energy density, not the "false vacuum", AFAIK.

-J

1. Einstein's gravity does not require negative energy. Look at the difference between Newton's mechanical (orbital) energy:

and Einstein's mechanical energy per unit orbiting mass:

where gtt = 1-2GM/r and grr = 1/gtt. Newton's goes positive and negative, while Einstein's remains positive (outside of black holes, at least). For orbital escape velocity, Newton's E = 0, while Einstein's E = mc2 (to get the same units as Newton's, one simply needs to multiply Eq. 6.7 through by orbiting mass m ).

The latter equation does not quite apply to cosmology, but it serves to illustrate the principle.

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#32
In reply to #29

Re: Dark Energy Superfluous?

08/28/2010 4:56 AM

Hi guest, I suppose we can "suppose" lots of things, but without it making technical sense, we will lose out.

I do not think you will be able to demonstrate the actual observable effects of the cosmos with your idea, but you are welcome to try... In this thread, we have used some 250+ entries to try and engineer a vaguely similar principle into a "perfect cosmic balloon", but we failed. In the end, we had to program a pump to "blow up the balloon" according to a pre-programmed set of equations.

BTW, the common wisdom is that the observable universe has a huge amount of material around it (possibly unlimited), but it is just more of the same that we see 'inside'. The minimum total radius compatible with observations is something like 10 times the radius of the observable universe.

-J

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#33
In reply to #31

Re: Dark Energy Superfluous?

08/30/2010 5:37 AM

Hi Jorrie. About the quantic pseudovacuum that I mentioned in my previous post, I meant sth like in the followng drawing that I made.

Once again, I took off one dimension, making a "reduced model" (my favourite way of thinking). My concept is that there is a kind of "prexisting space" with quantic fluctuations of space and energy (i.e. pseudovacuum). One of these (otherwise small) fluctuations happened to be very large creating our universe (the bubble in the drawing). (Quantum physics permits that, as in very small dt there is a chance a lot of energy to be created... you just have to wait enough for such a big fluctuation...)

Of course, the vacuum of our universe is, also, quantic. But, in my previous post, by saying "quantic pseudovacuum" I actually meant this "quantic preexisting space". Because of the positive energy of our space, this pseudovacuum must have a "lack of energy" or, in other words, a negative energy (in order to have total energy=0).

What is your opinion???

As far as I understood from your previous posts, you consider that the cosmic potential energy is, actually, the overall gravitational energy of our universe. Am I right???... (This is not what I thought about the cosmic potential energy... see my "metaphor" about the compressed coil...) If it is so, the potential (i.e. gravitational) energy of the universe must be negative, counterbalancing the cosmic positive kinetic energy. (And this is okay, as the gravitational energy is considered to be negative). Am I right???... And the increased vacuum energy of our space is adding in both the potential and kinetic energy of the universe: the gravitational energy of the vacuum is adding to the rest of the gravitational (potential) energy, making it more negative, while the "pressure" of the vacuum increases the kinetic energy, making it more positive. Am I right???...

I need your comments.

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#34
In reply to #33

Re: Dark Energy Superfluous?

08/30/2010 7:56 AM

Hi George, you said: "Because of the positive energy of our space, this pseudovacuum must have a "lack of energy" or, in other words, a negative energy (in order to have total energy=0)."

As I tried to point out before, the 'negative energy' and 'total energy=0' are not givens, but actually quite arbitrary - the 'zero point energy' can be any value, with zero just a convenient point to choose.1 It is correct as you said: "... the gravitational energy of the vacuum is adding to the rest of the gravitational (potential) energy, making it more negative, ...", but it may still mean that there is a net positive energy in the cosmos.

Also note that the present (presumed) non-zero vacuum energy is not necessarily the same as the energy that may have been doing cosmic inflation. I do not think Λ is today taken as a quantum fluctuation, but just a characteristic of empty space, doing the 'pushing' and adding to the cosmic gravitational energy. During inflation, things were not balanced, AFAIK. But then, without a working quantum gravity theory, who knows?

-J

1. The choice zero is particularly convenient in the case of a flat, infinite size cosmos, because otherwise the total energy may have been infinite. With a finely balanced zero, it could have zero total energy, despite being infinite. But, I'm sure that our understanding is still very incomplete...

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#35

Re: Dark Energy Superfluous?

08/30/2010 9:23 AM

Jorrie, this is a little OT, but isn't it kind of funny that physics moved away from the concept of "Aether" but has now resurrected the concept under a different name in "zero point energy" or "Dark Energy"? What is old is new again..... Maybe the ancient Greeks understood more about the universe than we give them credit for....=b (I'm joking BTW.)

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#36
In reply to #30

Re: Dark Energy Superfluous?

08/30/2010 10:11 AM

Maybe you mis-associated the "she" that I later used for the pretty Sarah Kavassalis.

Yes, I think I did, sorry. One thing I must say about the gravitational "potential" energy. In order for it to be potential, it must have a possibility of doing work in the future. With the universe expanding (with or without acceleration), the possibility doesn't seem to exist except in a few cases where the matter is close together. Things just keep getting farther and farther apart, reducing the probability that they will ever come together by gravitational force. So that seems contrary to the balance you described. Any comments about this?

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#37
In reply to #34

Re: Dark Energy Superfluous?

08/30/2010 10:21 AM

Jorrie, you said: "... I do not think Λ is today taken as a quantum fluctuation, but just a characteristic of empty space ..." Do you mean that Λ has to do with the "foamy structure" of the space (and it has nothing to do with the virtual particles produced in the space)???

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#38
In reply to #36

Re: Dark Energy Superfluous?

08/30/2010 11:39 AM

Hi S, you said: "One thing I must say about the gravitational "potential" energy. In order for it to be potential, it must have a possibility of doing work in the future."

I think it does. Without vacuum energy, a closed universe (gravitational energy more than kinetic energy) would collapse into a hot, high-density state again. For any open/flat cosmos, or a closed one with vacuum energy (accelerated expansion), it is a bit more difficult to see how 'work' would be done in the future.

But, isn't it the same as a probe shot away from Earth at (or exceeding) escape velocity? It changes kinetic energy into potential energy all the time, but barring an accident, it never falls back to Earth; hence, it never puts its potential energy 'to work' again. It just has it.

Makes any sense?

-J

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#39
In reply to #37

Re: Dark Energy Superfluous?

08/30/2010 11:54 AM

Hi George, you asked: "Do you mean that Λ has to do with the "foamy structure" of the space (and it has nothing to do with the virtual particles produced in the space)?"

I would not say it has nothing to do with virtual particles, but the problem is that they apparently yield 10120 times too high a vacuum energy! The "foamy structure" is one quantized possibility, but we simply do not have a theory of quantum gravity that predicts the correct strength for Λ.

This is one reason why other forms of dark energy, e.g. quintessence and phantom energy are receiving lots of attention. Must say I know even less about them I know about vacuum energy... :(

-J

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#40
In reply to #38

Re: Dark Energy Superfluous?

08/31/2010 11:59 AM

Hi Jorrie,

I think it does. Without vacuum energy, a closed universe (gravitational energy more than kinetic energy) would collapse into a hot, high-density state again.

But it does have vacuum energy, so it doesn't.

But, isn't it the same as a probe shot away from Earth at (or exceeding) escape velocity?

I suppose, but we're talking about the whole universe here. I guess what I am trying to say is that 'potential' is a bad name for it. A term like 'vacuum energy' is fine. Kinetic energy has a similar problem - it must be in reference to another object. What do you reference to for a whole universe? There is no center of it by present theory. Each object needs its own reference.

-S

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#41
In reply to #40

Re: Dark Energy Superfluous?

08/31/2010 3:11 PM

Hi S, you wrote: "But it does have vacuum energy, so it doesn't [collapse into a hot, high-density state again] ."

Roger Penrose (Oxford) does not necessarily agree with this,1 but I'll give you this one - Penrose's story is quite a complex one and I do not understand all of the details. :(

'Potential' may be a bad name, but 'gravitational potential energy' is very well defined and as such a very valid concept. You asked/stated: "What do you reference to for a whole universe? There is no center of it by present theory. Each object needs its own reference."

In such reference frames (for each object in the cosmological sense2), the total gravitational (potential) energy AND the total kinetic energy of everything that we observe add up to the same value for every object we care to choose as reference. Quite democratic.

We then make a 'leap-of-faith' that this holds for the entire universe, because the sample that we have is extremely large. In any case, to talk about the 'energy of the universe as a whole' is a bit presumptuous. All of us tend to use those words from time to time, but technically they have little meaning, IMO. We should rather try and stick to the observable universe...

-J

1. http://www.newton.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/frames.html

2. For the large scale, where isotropy and homogeneity are assumed and confirmed by observation as a very close approximation.

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#42
In reply to #41

Re: Dark Energy Superfluous?

08/31/2010 9:07 PM

Hi J,

Thought you might counter with the "Big Rip" or something. Thanks for the link. Its too bad we can't see the diagrams he's making.

Getting back to Shu. He says that singularities can't exist. I have also heard earlier that white holes can't exist. You pointed out that the total energy doesn't have to be zero. In the past, some descriptions of the BB had it starting from a 'seed'. Anyway, whether Shu is right or wrong, I think he has made a contribution to science with his theory.

-S

p.s. The term 'observable universe' is a bit muddy because of the fact that we see how it was in the past, but not how it is.

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#43
In reply to #42

Re: Dark Energy Superfluous?

09/01/2010 4:22 AM

Hi S.

The "Big Rip" is apparently only applicable to the 'phantom energy' variety of dark energy, not for vacuum energy.

"Anyway, whether Shu is right or wrong, I think he has made a contribution to science with his theory."

I agree.

"p.s. The term 'observable universe' is a bit muddy because of the fact that we see how it was in the past, but not how it is."

This is very fortunate! Otherwise we would have known nothing of the past of the larger cosmos. 'Observable universe' may be 'muddy', but nevertheless pretty well defined. :)

-J

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#44
In reply to #32

Re: Dark Energy Superfluous?

10/29/2010 8:15 AM

Jorrie a simple question for you

Can you, or would you give the explanation for <0> a thing that the pre and post Euclidean postulates never got to grips with. And in the real world not the metaphysical one, explain how two times a minus, equals a plus

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#45
In reply to #44

Re: Dark Energy Superfluous?

09/04/2012 8:15 AM

Zero is simply a definition in mathematics: if you subtract A from A you have zero left. In physics and engineering, it is almost an impossibility: if you subtract B from B you have some noise left, not zero. Actually, AFAIK, quantum physics forbids a zero state.

Essentially, we sidestep the issue by choosing a zero point, like zero voltage, and then assume it is precisely zero. We know that there are positive voltages and there are negative voltages and if we plot a curve of voltages, it may pass through zero volts.

BTW, "two times a minus" is still a negative number, as in 2 x (-4) = -8. I think you meant multiplying two negative numbers and getting a positive answer. If you accept the existence of negative numbers, then you have to accept that multiplying a positive number by a negative number gives a negative result. Then what else than a positive number could you get when you multiply a negative by a negative?

-J

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