Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 30/01/2015 00:48:10
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I have been wondering if the accumulation of dark energy over time could be causing the observation of linear redshift of galaxies. Can someone explain where I have gone wrong?
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I'm not sure how you consider 'dark energy' here? Seen two definitions of it, as a 'repulsive force' sundering (splitting) galaxies from each other, or as a result of some sort of reaction from matter and anti matter acting on each other. Dark energy is mostly used as a answer to the question why an expansion is accelerating. If you mean that there should be more redshift as the process accelerates I think you're right.
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going out from conservation laws and the idea of a commonly agreed on 'container universe' I would say such a behavior is the expected one as a expanding universe then should 'dilute' the energy. On the other hand, a light quanta does not change itself, unless 'interacting' with something else. Which in this case then presume it to 'interact' with a vacuum.
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You have the idea of two photon interactions naturally, to consider, but in this case the interaction is slightly weirder. The simplest definition of a light quanta is in the way you find a recoil, as it 'leaves' due to conservation of energy, to its annihilation by some detector. In between its not 'measurable'. By that I really mean that it doesn't exist until measured.
What a redshift states though, is that we have a accelerating expansion of the vacuum, which as it is 'neutral' in all manners to us, makes for an idea in where something without a measurable pressure still is able to 'add up' into something more. That one would be interesting to see cleared up. and you have to remember that we have four forces defined, acting on us. Those then keeping us and all rest mass together, making the older definition of a expansion 'only acting between galaxies' an untruth to me. It acts everywhere. And it does it without this measurable 'pressure'. Dark energy is a theory trying to explain it, as in a idea.
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The Hubble redshift relationship is linear over distance which doesn't seem right to my mind. If this was a non-linear relationship I would be happier to take it at face value. No other force shows such a linear relationship. We have an inverse square relationship for both electromagnetism and gravity. As we view the past light cone I would expect this linearity to diverge over time and not stay consistent.
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Not sure. v = H x d
where v is the galaxy's radial outward velocity, d is the galaxy's distance from Earth, and H is the constant of proportionality called the Hubble constant.
I think that if you have a (even) expansion in each point, then it gets 'quicker' the further away something is? And that becomes a linear expression sort of. How would you like to see it otherwise?
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Think of a light quanta, think of a 'point like' particle. Is it a 'point'? Can you move it by introducing more space in it? where does this new vacuum introduce itself? Around a point like particle? inside it?
Where it makes sense is when using an idea of something 'holding together' as a wave, then presuming that this wave can't be 'split', only elongated.
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all of this assuming lights 'propagation' naturally. Without a propagation a 'expanding accelerating vacuum' doesn't matter. Neither does how one want to define light, because you can keep the duality. Well, keep it, and use, observer dependencies and conservation laws.
Observer dependencies will then cover the way you set up your experiment and the way you detect. This makes lights duality a result of your choice of observation. You don't get two results, only one at a time.
And conservation laws will cover why it has to red shift in a expansion.
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Where it becomes really weird is when you consider measuring the 'photons' existing in this redshift. Because they must 'interact' with the vacuum somehow. As they lose energy, by a vacuum? How? Either that or you somehow find each 'photon' to keep its 'energy' although now becoming more 'spaced out' by propagating inside a expanding space. But that has nothing to do with any definition of a photon.
That's a pure wave picture, and a fail.
A fail because it pretend to consider quanta, but it doesn't, it consider a 'split' wave. Which it can't be, if you want it to be a wave?
Einstein's miraculous argument of 1905. (http://www.pitt.edu/~jdnorton/papers/miraculous.pdf)
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I do think conservation laws to exist, what I don't believe in is lights propagation, and a 'container universe' where we need to define the 'real one' as being outside my observations. The real one is the one I measure, as you do. The rest is rules. The symmetry is about 'time'.
sorry: the link didn't work properly, hope it does now :)
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Consider the attached graph. The line represents the path of photons in the light cone up to a particular point in time. This may be the present day or any other point we choose in the lifetime of the universe. The Y axis is time and the X axis is the distance that a galaxy is perceived to be and is being redshifted according to the Hubble relationship. If we consider a photon leaving point E and moving forward in time it will eventually arrive at point D. If we imagine that when this photon arrives at point D then another photon sets off from point D and carries on with the first photon and they both then arrive at point C. We can carry on in this manner until 4 photons arrive at point A. These are then detected and the redshifts measured. This then determines how the objects are moving away from each other and at what relative velocities. If we now wind this backwards until we reach point B then from point B's perspective an onserver sees the relationships between himself and point C in exactly the same way as an observer at point A saw his relationship to point B. This is because of the linear nature of the relationship. The observer at point B will also see his relationship to point D in excactly the same way as an observer at point A observes point C. That is at both points in time the expansion will appear to have the same velocities at the same relative distances. We can then do the same comparison between observers at points A and C. This is because we have to make a correction as we travel back in time for the velocity at point A and subtract this first.
This makes it possible to go even further back and find that the same increase occurs right back to the limit of the observabe universe. This indicates a uniform expansion over time. Because we have moved backwards with the photons an observer at point B is experiencing the photon that the observer at point A would have detected but at an earlier period in time so his view in the light cone is identical at the earlier time.
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Another point about the graph and points A and B. It will take the same amount of time for a photon to go from point A to point B as from Point B to point A. Determining where point A was in the past light cone of point B is the only way to resolve the issue. This is like looking out at a galaxy x light years from the Milky Way and then determining where the Milky Way was at that time in the past. The time a photon from the Milky way would take to get from the earth to that particular galaxy in that position can only then be calculated.
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You know. To get ones head around the way the Hubble constant is thought to work with a universe is tricky. In the simplest representation is can be seen as a linear function, sort of, depending on how we define the density of a universe, dark energy, GR, etc etc. but :) It still seems to be accelerating, although very slowly. Try this one.
"The Hubble parameter measures the relative speed of the scale factor a(t) of the universe, formally H(t) = [da(t)/dt]/a(t). It comes form the FLRW metric and is a function of universal cosmological time; its value is determined by the mass/energy content of universe by the first Friedman equation (obtained by the Einstein's field equations) H^2(t) = (8piG/3) x ϱ(t) - kc^2/a^2, where ϱ = ϱ_m + ϱ_d + ϱ_r is the total mass/energy density given by matter (luminous and dark), dark energy and radiation; k is the curvature parameter.
The value of the Hubble parameter *now* is called Hubble's constant.
Since it is possible to define a well precise relationship between time and redshift z, they measure the same thing and H can be expressed as a function of z. Now, the various densities decrease as the scale factor increases. The key point is that they have *different* dependencies on the scale factor: ϱ_m ~ a^-3, ϱ_r ~ a^-4 and ϱ_d ~ constant.
This implies that at early times (ie small a(t)) the radiation was the dominant component and the expansion speed, and therefore H, had a different value from now, then matter started to dominate since is decreasing more slowly than radiation and again speed changed. At some point in the past, the third component, the dark energy, started to dominate since matter was more and more diluted by the expansion (the most recent data seem to indicate around 5 billion ago). Since ϱ_d stays constant during the expansion (this *the feature* of dark energy, together with its negative pressure (!) ), the speed *increases* with time, that is the universe accelerates expansion. Apparently, we are living now in a dark energy-dominated universe. " By Emanuele Fiandrini ยท INFN - Istituto Nazionale di Fisica Nucleare
Then look at how cosmologists define a Comoving distance. (https://en.wikipedia.org/wiki/Comoving_distance) "Comoving distance and proper distance are defined to be equal at the present time; therefore, the ratio of proper distance to comoving distance now is 1."
Finally look at Value of the Hubble parameter over time (https://physics.stackexchange.com/questions/18301/value-of-the-hubble-parameter-over-time) from Stackexchange
Think Emanuele's presentation is the one easiest to follow myself, but I think we still can use it as a 'linear representation'?
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There is a way to define a comoving frame that helps intuitively. It's the 2 dimensional representation of a universe as existing on the plane (skin?) of a balloon. If you draw a grid on the surface, then as the balloon is blown up this grid will expand, but still keep to a same balance, and that could be seen as something 'comoving', as I get it.
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If we then place a observer on this surface of a balloon, presuming that he somehow is able to 'stay inertially' anchored at some (non comoving) position, he then should find the universe to redshift around him as the balloons surface expands, every sun 'moving away' from him. If we don't presume him to 'stay anchored', then I don't know how he would describe it :)
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There is a way to define a comoving frame that helps intuitively. It's the 2 dimensional representation of a universe as existing on the plane (skin?) of a balloon. If you draw a grid on the surface, then as the balloon is blown up this grid will expand, but still keep to a same balance, and that could be seen as something 'comoving', as I get it.
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If we then place a observer on this surface of a balloon, presuming that he somehow is able to 'stay inertially' anchored at some (non comoving) position, he then should find the universe to redshift around him as the balloons surface expands, every sun 'moving away' from him. If we don't presume him to 'stay anchored', then I don't know how he would describe it :)
But your light cone describes the balloon at past states of inflation. The observer is at the apex on the surface in the present time frame. You can think of your light cone originating at the centre of the balloon which in this analogy would coincide with a proposed big bang.
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I have been wondering if the accumulation of dark energy over time could be causing the observation of linear redshift of galaxies. Can someone explain where I have gone wrong?
Remember, we were able to explain cosmological redshift long before we even knew that dark energy existed. Perhaps it accounts for some of it but not all of it.
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Science often proceeds in two alternating phases: Integration and Differentiation.
Hubble & colleagues integrated the observed, apparently random redshifts of galaxies into a relatively simple linear relationship between distance and recession velocity, when you consider "nearby" galaxies (out to 5 billion light-years or so).
Schmidt & colleagues extended the Hubble relationship to redshift measurements of much more distant galaxies. And they were able to differentiate the expansion rate of these distant galaxies from the expansion rate of nearby galaxies as identified by Hubble. Gravitational attraction should cause the rate of expansion to slow down, over time. Surprisingly, they found that the redshift expansion rate is faster now than it was in those older galaxies. Their proposed explanation was Dark Energy.
So the linear, Hubble-style expansion accounts for part of the redshift, and the accelerating expansion due to dark energy accounts for another part of the observed redshift.
I have been wondering if the accumulation of dark energy over time could be causing the observation of linear redshift of galaxies.
I would conclude that Dark Energy is not required to explain the linear distance-redshift relationship of nearby galaxies (simple inertia can produce this).
But something like Dark Energy is necessary to explain the accelerating redshift seen from examination of more distant galaxies.
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How do you mean there Evan "And they were able to differentiate the expansion rate of these distant galaxies from the expansion rate of nearby galaxies as identified by Hubble. Gravitational attraction should cause the rate of expansion to slow down, over time." That they measured the gravitational attraction between galaxies far away, versus redshift? Then "Surprisingly, they found that the redshift expansion rate is faster now than it was in those older galaxies. Their proposed explanation was Dark Energy." Is faster where?
Or did you mean they used it as some 'time machine', finding galaxies far away (in distance and 'time) to have a slower accelerating expansion than those we can see closer to us in 'distance and time'? That one I would like to read :)
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How does that fit a inflation? "According to the simplest inflationary models, inflation ended at a temperature corresponding to roughly 10−32 second after the Big Bang. As explained above, this does not imply that the inflationary era lasted less than 10−32 second.
In fact, in order to explain the observed homogeneity of the Universe, the duration must be longer than 10−32 second. In inflationary cosmology, the earliest meaningful time "after the Big Bang" is the time of the end of inflation."
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I have been wondering if the accumulation of dark energy over time could be causing the observation of linear redshift of galaxies. Can someone explain where I have gone wrong?
Remember, we were able to explain cosmological redshift long before we even knew that dark energy existed. Perhaps it accounts for some of it but not all of it.
I think your point about a small effect is likely near the mark. I need to think about this a bit more.
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Science often proceeds in two alternating phases: Integration and Differentiation.
Hubble & colleagues integrated the observed, apparently random redshifts of galaxies into a relatively simple linear relationship between distance and recession velocity, when you consider "nearby" galaxies (out to 5 billion light-years or so).
Schmidt & colleagues extended the Hubble relationship to redshift measurements of much more distant galaxies. And they were able to differentiate the expansion rate of these distant galaxies from the expansion rate of nearby galaxies as identified by Hubble. Gravitational attraction should cause the rate of expansion to slow down, over time. Surprisingly, they found that the redshift expansion rate is faster now than it was in those older galaxies. Their proposed explanation was Dark Energy.
So the linear, Hubble-style expansion accounts for part of the redshift, and the accelerating expansion due to dark energy accounts for another part of the observed redshift.
I have been wondering if the accumulation of dark energy over time could be causing the observation of linear redshift of galaxies.
I would conclude that Dark Energy is not required to explain the linear distance-redshift relationship of nearby galaxies (simple inertia can produce this).
But something like Dark Energy is necessary to explain the accelerating redshift seen from examination of more distant galaxies.
Do you have a reference for data on the difference in expansion rates over time? I have been looking for a source of this information for a while.