The Start of Dark Energy and the limits of the Universe

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The Start of Dark Energy and the limits of the Universe

Postby lateralsuz on June 21st, 2019, 8:31 pm 

Although I can't find it again, I read somewhere that Dark Energy was believed to have 'begun' to form approx. 6 billion years ago - some 8 billion years after the Big Bang, and just before the origination of our solar system.

Does anyone know what evidence this notion is based on?

As Dark Energy was (at least in part) dreamt up to explain the accelerating expansion of the Universe, does this mean that the acceleration was also believed to begin 6 billion years ago?

Finally, this thinking suggests that Dark Energy must still be being generated today in vast and increasing quantities in order to maintain the acceleration.

Given the scale of that supposed production, isn't it strange that we can't detect the potential source?

The only speculation I have heard of is that Dark Energy is effectively leaking from other dimensions of existence - but there is no real evidence for other dimensions, either - it's just theory.

If we can't point to a specific source within the known Universe, then is the accelerating expansion of the physical Universe proof that there is something beyond it?
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Re: The Start of Dark Energy and the limits of the Universe

Postby PaulN on June 22nd, 2019, 8:57 am 

Forum rules are to provide citations in the science section. Could you use a search engine and possibly find the dark energy article you refer to? Would be most helpful.
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Re: The Start of Dark Energy and the limits of the Universe

Postby TheVat on June 22nd, 2019, 10:59 am 

Neither quintessence, vacuum energy, nor cosmological constant theories would have dark energy starting 6 gY ago. NASA has a good summary page on the topic....

https://science.nasa.gov/astrophysics/f ... ark-energy

Most theories have DE as intrinsic to spacetime, thus present from the beginning.
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Re: Eyeballing Darkness

Postby Faradave on June 22nd, 2019, 7:44 pm 

That's my impression as well, which is not to say that dark energy increases linearly. As space appears to expand isotropically (same in every direction) over time, and dark energy appears proportional to volume, then dark energy will tend to increase at something like the cube of cosmological age.
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Re: The Start of Dark Energy and the limits of the Universe

Postby lateralsuz on July 4th, 2019, 9:34 am 

I tracked-down one reference - below

NASA Website - http://science.nasa.gov/astrophysics/fo ... rk-energy/



Credit: Ann Feild (STScI)
Dark Energy, Dark Matter
In the early 1990s, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.
What Is Dark Energy?

Universe Dark Energy-1 Expanding Universe
This diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.
NASA/STSci/Ann Feild



There's lots of diagrams in this article too which I tried to copy here, but they didn't seem to work. They all show patterns pointing to 7.5 billion years ago - but this doesn't change the basic premise.
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Re: The Start of Dark Energy and the limits of the Universe

Postby BurtJordaan on July 10th, 2019, 1:21 am 

Sorry, haven't been here for some time. Occam's razor would put inherent negative spacetime curvature that existed (or pre-existed), after the BB (coming from Einstein's cosmological constant) as the simplest explanation for dark energies' exponential expanding tendency.

Because of the high energy density of matter and radiation at that time, they overwhelmed this and slowed the initial rapid expansion down considerable over time. As the said densities dropped, the exponential expanding tendency became dominant about 5 to 7 billion years ago, as its equivalent energy density stays constant.

Hence the present slowly accelerating expansion.
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Re: Nighty Knight

Postby Faradave on July 10th, 2019, 10:20 am 

Nice to have you back Jorrie.
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 10th, 2019, 2:53 pm 

lateralsuz » June 21st, 2019, 7:31 pm wrote:
Given the scale of that supposed production, isn't it strange that we can't detect the potential source?


You have some valid conclusions and I share your skepticism about Dark Energy. I see no reason to doubt the observations but it is the conclusions about accelerated expansion that I find to be on shaky ground because the conclusions are contrary to the laws of physics and it is the conclusions about the evidence for which there is no support.

This is an old debate going back to the days of Edwin Hubble. Even Hubble never considered the redshifting of distant galaxies to be sufficient evidence to support the conclusion of an expanding universe because redshifting can have so many other possible explanations.

Sir Arthur Eddington considered the contraction of the material world in a universe of constant size to be equivalent to the theory of an expanding universe. We have no external god’s eye view of the universe that can tell us which possibility or any combination of the two is more likely because they can both be explained by the same set of observations and one explanation is the simple inverse of the other. Contraction and expansion are indistinguishable without an external reference for size. The universe may be expanding relative to the length of a meter or the meter may be growing smaller relative to the radius of the universe but the observations are the same in either case.

If the universe is expanding, gravity should be slowing the rate of expansion because expansion works against gravity but, if the universe remains the same size while all the material within is contracting, then an acceleration should be expected since contraction works with gravity and characteristically accelerates with time. The observation of an “expanding” universe favors the contraction view over expansion because accelerated contraction does not require the invention of a Dark Energy.

Christof Wetterich has a recent theory along the lines of Eddington’s “Shrinking Atom” theory or, as I like to call it, an “Inverse Expansion” theory.

https://www.science20.com/hammock_physi ... ing-118673

Wetterich's approach is not different from the standard cosmological model. All Wetterich asks us is to view the standard cosmological model from a different perspective.

Wetterich's starting point is a somewhat trivial observation:

Only dimensionless ratios as the distance between galaxies divided by the atom radius are observable."

So although the hard truth remains that the size of the universe expressed in atomic sizes is increasing, one could maintain that atoms are shrinking while the universe itself remains constant in size.
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 10th, 2019, 2:57 pm 

Welcome back, Jorrie.
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 10th, 2019, 3:26 pm 

BurtJordaan » July 10th, 2019, 12:21 am wrote:

Because of the high energy density of matter and radiation at that time, they overwhelmed this and slowed the initial rapid expansion down considerable over time. As the said densities dropped, the exponential expanding tendency became dominant about 5 to 7 billion years ago, as its equivalent energy density stays constant.

Hence the present slowly accelerating expansion.


I don't see how this is possible unless gravity is a push as well as a pull which may be a possibility. Once the initial momentum of expansion has been slowed by a high energy density of matter and radiation, it would take another influx of energy to get things going again and what would be the source of that energy?
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Re: The Start of Dark Energy and the limits of the Universe

Postby BurtJordaan on July 12th, 2019, 4:19 pm 

bangstrom » 10 Jul 2019, 21:26 wrote:Once the initial momentum of expansion has been slowed by a high energy density of matter and radiation, it would take another influx of energy to get things going again and what would be the source of that energy?

In the LCDM model, the source is empty space itself - it is working like anti-gravity. Its overall energy density is constant over time, but presently seems to be about double that of all matter added together - hence the accelerated expansion.

Einstein's general relativity predicts the existence of vacuum energy, but it makes no prediction of its density. Quantum theory does make a prediction, but its prediction seems to be 120 orders of magnitude larger than what is cosmologically observed, so there is clearly something at work that we do not understand. Hence the term 'dark energy'.
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 12th, 2019, 8:59 pm 

BurtJordaan » July 12th, 2019, 3:19 pm wrote:
Einstein's general relativity predicts the existence of vacuum energy, but it makes no prediction of its density.


If you are referring to Einstein’s lambda-parameter, this was not predicted by relativity. It was added ad hoc to GR to achieve a static universe which was the general consensus for the state of the universe at the time.
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Re: The Start of Dark Energy and the limits of the Universe

Postby BurtJordaan on July 14th, 2019, 2:44 am 

bangstrom » 13 Jul 2019, 02:59 wrote:If you are referring to Einstein’s lambda-parameter, this was not predicted by relativity.

Not quite, because vacuum energy is a more general concept than the cosmological constant (Lambda), which is a special case. It was soon (1920's) realized by Einstein, Friedman, de Sitter etc. that a general theory of relativity (gtr) must include all forms of energy and pressure, which includes possible negative pressure. Lambda is a special case of negative pressure and acts in opposition to gravity, almost like anti-gravity.

One must also keep in mind that cosmological solutions to the full equations of gtr (including Einstein's first one) are all simplified special cases, because the full equations are generally unsolvable without that caveat.

Einstein's "biggest blunder" was not inserting Lambda in the first place, but later "dropping" it, because without it (or something similar), gtr could make no claim of being "general". In the 1930's, in his work with Willem de Sitter, it is clear that Einstein accepted the possibility of Lambda, but thought that due to lack of evidence, its value must be zero. As we know, that was the accepted view until some 40 years after Einstein's death, when it all changed...
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 14th, 2019, 5:10 am 

BurtJordaan » July 14th, 2019, 1:44 am wrote: Lambda is a special case of negative pressure and acts in opposition to gravity, almost like anti-gravity.


Any pressure involves a vector. Gravity clearly has a vector but where is the vector for Lambda? It is my understanding that the galaxies are not being accelerated in any direction but instead it is space-time itself that is expanding between the galaxies producing the appearance of an expanding universe. A quickening in the cosmic rate of time could produce the illusion of an expanding space.
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Re: The Start of Dark Energy and the limits of the Universe

Postby JohnD on July 15th, 2019, 6:51 am 

I am finding this debate fascinating. Certainly I don't have the IQ or knowledge that you have but this is interesting. I wonder if I may suggest an alternative. What if it isn't about expansion of the universe of the contraction of atoms but the interaction between the different forces causing some to change sides every-so-often? Dark becomes light, etc... Wouldn't it explain what is observable?
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Re: The Start of Dark Energy and the limits of the Universe

Postby BurtJordaan on July 16th, 2019, 2:46 pm 

bangstrom » 14 Jul 2019, 11:10 wrote:Any pressure involves a vector. Gravity clearly has a vector but where is the vector for Lambda?

On cosmic scales, neither gravity nor pressure is represented by a vector. In which direction would it be if it was?
A quickening in the cosmic rate of time could produce the illusion of an expanding space.

Do you have any mainstream references for this? It seems to fly in the face of accepted theory and observation.

Note that speculation is not discussed in the science section - we have the 'personal theories' section for that.
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Re: Getting Personal

Postby Faradave on July 16th, 2019, 10:19 pm 

JohnD wrote:What if it isn't about expansion of the universe of the contraction of atoms but the interaction between the different forces causing some to change sides every-so-often? Dark becomes light, etc... Wouldn't it explain what is observable?

There's not enough here yet to form an opinion. Follow your fascination. Keep reading until you can flesh out a more detailed hypothesis. Then, as Jorrie suggests, drop it on us in Personal Theories.
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 17th, 2019, 2:29 am 

JohnD » July 15th, 2019, 5:51 am wrote: What if it isn't about expansion of the universe of the contraction of atoms but the interaction between the different forces causing some to change sides every-so-often? Dark becomes light, etc... Wouldn't it explain what is observable?


Yes, the right changes among the physical constants could explain the observation of an expanding universe provided they conform to physical observations and mathematical theory.

Even if expansion theory is correct, the expansion of space should cause a rescaling of the other dimensional constants and their components. For example, if space expands, time must also quicken if the constants c, Newton’s G, and Planck’s h are to remain constant. Conversely, a universal quickening of time could appear as an expansion of space. We can observe long term changes in the cosmos by observing light from distant galaxies but we usually can’t identify the cause of change and no one parameter can change without inducing changes in our measurements of the others.
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Re: The Start of Dark Energy and the limits of the Universe

Postby bangstrom on July 17th, 2019, 3:51 am 

BurtJordaan » July 16th, 2019, 1:46 pm wrote:
On cosmic scales, neither gravity nor pressure is represented by a vector. In which direction would it be if it was?


Gravity has a vector towards the greatest concentration of mass even on cosmic scales where galactic clusters serve as gravitational attractors because of their mass.

What evidence do we have for a “pressure” forcing the expansion of the universe?


BurtJordaan » July 16th, 2019, 1:46 pm wrote:
A quickening in the cosmic rate of time could produce the illusion of an expanding space.

Do you have any mainstream references for this? It seems to fly in the face of accepted theory and observation.

Note that speculation is not discussed in the science section - we have the 'personal theories' section for that.


I don't see where you find a scenario of quickening time leading to the appearance of an expanding space contrary to theory or observation. A redshift can indicate a recessional velocity, an expansion of space, a slower rate of time at the point of emission relative to a faster rate of time at the point of reception, or any combination of these. The distant galaxies show no direct evidence of recessional velocities, or expanding space. All we can say with certainty is that they appear redshifted. This is what we observe and anything beyond that is speculation.

Consider an example from GR of what happens when an object enters and emerges from a gravitational well. The object entering the well shows evidence of time slowing and distances shortening from the perspective of an outside observer as it descends into the well and the opposite happens when an object emerges- time quickens and space expands- relative to an outside observer but space and time remain unchanged for an observer on the object itself. A local observer on the object should find the opposite changes in emissions coming from the remote observer. Light from the remote observer should appear blueshifting as the observer enters the well and redshifting as they leave.

In the Big Bang scenario, we are like an observer emerging from a gravity well with the “bottom” of the well being our highly dense big bang origin and the distant galaxies as surrounding observers. We can’t observe changes taking place locally but we can observe light from distant galaxies as being uniformly redshifted in all directions.

We don’t observe any particular direction of motion for our own galaxy and we can presume that the same observation applies to all galaxies if the universe is truly curved and four dimensional. The redshifting we observe in distant galaxies is not necessarily indicative of motion but it is the pattern of change we can expect from a decline in the strength of gravity.

This has prompted some physicists to propose cosmologies based on the premise of a gradual decline in the strength of the gravitational constant G rather than redshifting due to recessional velocities. A decline in the strength of gravity should appear as a quickening of time and the apparent expansion of space just as observed when an object emerges from a gravity well but no motion is necessary to complete this explanation.

The conventional view of an expanding universe imposes a 3D geometry on what is a 4D event.
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Re: The Start of Dark Energy and the limits of the Universe

Postby BurtJordaan on July 18th, 2019, 1:04 am 

Bang, you wrote:
The distant galaxies show no direct evidence of recessional velocities, or expanding space. All we can say with certainty is that they appear redshifted. This is what we observe and anything beyond that is speculation.

Not quite speculation. General relativity says that on cosmic scales there must have been either metric expansion or metric contraction in the past. All observations agree with metric expansion, meaning that clusters of galaxies are getting farther away from each other, as expressed in proper distance units (that of a tape measure or meter stick).

The gravitational redshift that you are referring to does not apply at cosmic scale, because all clusters are approximately at the same gravitational potential in an overall approximately flat space. Cluster form relatively small gravitational wells, with distant light going in and out of many such wells, but suffers negligible change in frequency due to that (blueshifted on the way in and redshifted on the way out). Overall, light is simply redshifted due to Hubble's law, directly proportional to the changing proper distance of the source. We observe light all the way to the time of the CMB radiation release, 13 billion years ago and observations support the above.

There is also no evidence that the gravitational constant (G) or the speed of light has changed over cosmic time. We observe distant objects moving/orbiting according to the same law and the same two values that we see today, within experimental errors of course. Don't be fooled by the problems of pinning down the precise value of G, which seems to fluctuate in the tens of parts per million - which is utterly negligible when considering cosmic time and distances.
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