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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
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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?
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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.
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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?
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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.
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bangstrom » 13 Jul 2019, 02:59 wrote:If you are referring to Einstein’s lambda-parameter, this was not predicted by relativity.
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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.
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bangstrom » 14 Jul 2019, 11:10 wrote:Any pressure involves a vector. Gravity clearly has a vector but where is the vector for Lambda?
A quickening in the cosmic rate of time could produce the illusion of an expanding space.
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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?
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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?
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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?
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.
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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.
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BurtJordaan » July 18th, 2019, 12:04 am wrote:
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).
BurtJordaan » July 18th, 2019, 12:04 am wrote: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).
BurtJordaan » July 18th, 2019, 12:04 am wrote: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.
BurtJordaan » July 18th, 2019, 12:04 am wrote:
There is also no evidence that the gravitational constant (G) or the speed of light has changed over cosmic time.
BurtJordaan » July 18th, 2019, 12:04 am wrote:
We observe distant objects moving/orbiting according to the same law and the same values that we see today, within experimental errors of course.
BurtJordaan » July 18th, 2019, 12:04 am wrote:
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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The universe is observed to be spatially flat, or so close to it that it makes no difference. Reading that together with all the other observational evidence, can only mean that it is spatially infinite, or so large that it does not matter if it isn't. Also observed is that it is homogenic and isotropic on the large scale. All of these means all observers, wherever they may be in this infinite space, will see themselves as the center of the expansion.Relative to, What, is the universe expanding?
A gravitational redshift, as I explained, blueshifts on the way in and redshifts on the way out.
Redshifting is not due to Hubble’s law since it was happening before Hubble's time and Newton’s law does not cause gravity.
Bangstrom wrote:Jorrie wrote: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.
The same can be said about an expanding universe where the assumed expansion can not be observed locally.
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BurtJordaan » July 20th, 2019, 2:25 am wrote:Bangstrom asked:The universe is observed to be spatially flat, or so close to it that it makes no difference. Reading that together with all the other observational evidence, can only mean that it is spatially infinite, or so large that it does not matter if it isn't. Also observed is that it is homogenic and isotropic on the large scale. All of these means all observers, wherever they may be in this infinite space, will see themselves as the center of the expansion.Relative to, What, is the universe expanding?
BurtJordaan » July 20th, 2019, 2:25 am wrote:Gravitational cosmological redhift that we observe is not the classical gravitational type. As I wrote some posts back, the large scale cosmos is essentially at the same gravitational potential, with the clusters small pockets of lower potential that the distant light may pass through, but with the blue/redshift influences largely canceling out.
BurtJordaan » July 20th, 2019, 2:25 am wrote:Bangstrom wrote:Jorrie wrote: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.
The same can be said about an expanding universe where the assumed expansion can not be observed locally.
This is totally different. We can measure expansion only on the large scale, because there is no cosmic expansion in gravitationally bound structures, like super-clusters of galaxies and smaller. "G" we measure locally, in the lab, in the solar system in our galaxy and even in our super-cluster. We cannot measure it on the large scale, but all collaborative evidence points to a constant G over cosmic timescales.
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bangstrom » 21 Jul 2019, 12:13 wrote:I am familiar with the theory but how does that answer the question, “Relative to, What, is the universe expanding?"
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BurtJordaan » July 25th, 2019, 12:30 am wrote:Bang, this is taking way more time that I have available, and I have no expectation that scientific mainstream answers will satisfy you, but I will give it one more try.
BurtJordaan » July 25th, 2019, 12:30 am wrote:bangstrom » 21 Jul 2019, 12:13 wrote:I am familiar with the theory but how does that answer the question, “Relative to, What, is the universe expanding?"
The short answer is: relative to any inertial frame, wherever you may choose one. This is essentially what "space is relative" mean. The long answer is, well, very long...
BurtJordaan » July 25th, 2019, 12:30 am wrote:'Occam's razor' then makes it very easy to pick gr as the most probable and simplest explanation.
BurtJordaan » July 25th, 2019, 12:30 am wrote:I can understand the frustration of readers that want a simple, concrete answer to such a simple question, but 'cooking up' some ordinary down-to-earth toy models/examples to help explain a deep underlying complexity, also leads to confusion. An example is my efforts with The Infinite Cosmic Lattice post some long time ago.
I still use the lattice pic as my avatar, so perhaps it is still a useful crutch... ;-)
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bangstrom » 14 Sep 2019, 08:44 wrote:My only objection to the SM is its use of distant galactic redshifts as straight forward recessional velocities.
Astronomical observations indicate a universe that is much larger and older than indicated by the interpretation of galactic redshifts as recessional velocities.
I see your lattice model is also infinite but is it infinite in the same way?
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BurtJordaan » September 18th, 2019, 9:40 am wrote:
How so? The standard interpretation of the cosmological model gives an age of 13.8 billion years and a size tending to infinite. I know of no observation that challenges this.
BurtJordaan » September 26th, 2012, 1:11 am wrote:
2. An observer somewhere near any red cube (say at the viewpoint in the diagram) will get the impression that she is at the center of a large expanding system of cubes, with all cubes moving away from her at speeds depending upon their relative distance. To her, the color of distant red cubes will appear redder than nearby cubes (light with longer or 'stretched' wavelengths), from which she will be able to deduce that more distant cubes recede faster than nearby ones. Whether she attributes the reddening to Doppler shift or to stretching of the wavelengths does not matter at this point, but it is equivalent to the distance/redshift relationship that astronomers use.
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bangstrom » 20 Sep 2019, 08:13 wrote:Our best alternative to a calculated age for the universe is direct observation and and this is where the 13.8 billion years appears to be too short.
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bangstrom » 20 Sep 2019, 08:13 wrote:Your observer has no external source of measurement so how is she to tell if the blue lines are getting longer or if the red squares are getting smaller?
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BurtJordaan » September 22nd, 2019, 4:25 am wrote:bangstrom » 20 Sep 2019, 08:13 wrote:Our best alternative to a calculated age for the universe is direct observation and and this is where the 13.8 billion years appears to be too short.
Which direct observation are you referring to?
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