Is the Universe Expanding or Are Galaxies Moving Apart?

[jonmtkisco]

Hi myseach,

Is it related to the implication that GR does not always conserve energy? I am raising this issue because there are derivations of the Friedmann and Fluid equations that seem to have the conservation of energy as a root assumption.

The Friedmann equations do conserve energy at least for a spatially flat model with zero Lambda.

This was a helpful clarification. However, is the value 6.7E-27kg/m^3 quoted associated with the critical density [\rho_c] normally inferred from Friedmann’s equation, i.e. \rho_c = 3H^2/8 \pi G?

Yes, you use the Friedmann equations to calculate it, using the estimated figures for matter and Lambda density, as a function of time. An easy way to calculate it is to run the Friedmann equations into the arbitrarily distant future when Lambda becomes the only significant component of the universe's mass-energy.

I have paraphrased the quote above from #82, but wanted to check whether matter included CDM?

Yes it is; as you know the great majority of matter mass is believed to be dark energy.

If I describe dark energy in terms of a negative pressure [P] does this effectively negate the need for the [\Lambda] term, as its units, i.e. m^{-2}, do not really seem indicative of pressure? The following form of the equation above simply removes all constant values and [\Lambda] to highlight the dependency on just [\rho] and [P]:

\frac {\ddot a}{a} = - \rho - (3P)

No, Lambda is in the equation for a reason, so you cannot simply exclude it.

However, what seems even stranger is the fact that the dark energy, which is the energy per unit volume that expands space, does not get ‘diluted’ in the process, i.e. the suggestion appear to be that it remains constant. If I have interpreted this correctly, it gives the impression that mass and energy are being `created` in the process or, at least, tapping into some other source, i.e. zero point energy/vacuum energy.

The simplest concept for dark energy is the cosmological constant, and every cubic meter of vacuum comes with its own cosmological constant, so by definition it can never be diluted. On the contrary it helps to dilute and eventually dominate everything else.

The ongoing creation of additional space filled with its own (additional) Lambda seems to be a characteristic of both the "dust ball model" and standard cosmology, which suggests that indeed new energy is being constantly created. Perhaps one could also posit a model where the empty vacuum "outside" the dust ball has Lambda but does not expand in accordance with the de Sitter model. In which case the existing Lambda was always there and is not being newly created. But any such theory is entirely speculative. In any event, little satisfaction is gained by saying that infinite energy was "always there" instead of incremental energy being "newly created" with the passage of time.

However, one point that I would like to raise in this thread is simply a level of surprise that nobody has challenged the assumption of a centre of gravity, as I thought this was not normally accepted as part of the standard model?

Standard cosmology readily admits that the lack of a center of gravity is a simplifying assumption, rather than a fact or theory which has been demonstrated to be highly likely. I believe it is a widely accepted assumption because of its mathematical simplicity (no "edge" effects) and philosophical elegance (cosmological principle), and because as a practical matter the observable characteristics of the universe so far have not depended on whether the assumption is correct.

Jon

 

[mysearch]

Response to #91:

Hi Jon,
Given the excessive length of my response in another thread, I will keep this one brief. Thanks for all the help, you given me a lot to think about, however, I now recognise the need do some more detailed reading based on what I hope is now a more ‘expansive’ perspective. Much appreciated.

P.S. Your last statement is very interesting, especially if nobody challenges it!

Standard cosmology readily admits that the lack of a center of gravity is a simplifying assumption, rather than a fact or theory which has been demonstrated to be highly likely. I believe it is a widely accepted assumption because of its mathematical simplicity (no "edge" effects) and philosophical elegance (cosmological principle), and because as a practical matter the observable characteristics of the universe so far have not depended on whether the assumption is correct.

 

[Cuetek]

Standard cosmology readily admits that the lack of a center of gravity is a simplifying assumption, rather than a fact or theory which has been demonstrated to be highly likely. I believe it is a widely accepted assumption because of its mathematical simplicity (no "edge" effects) and philosophical elegance (cosmological principle), and because as a practical matter the observable characteristics of the universe so far have not depended on whether the assumption is correct. Jon

We base a great deal of the Big Bang on the cosmological principle. And it is indeed an excellent model by which to hash out the local nature of the universe. But it is based on limited data in pursuit of an complete description of a universe that always seems to have more in store than our descriptions can convey. To attempt to contrive a complete and sufficient view of the universe, rather than presuming our models to be local characterizations, is ill-advised in my view.

Across 40 orders of spatial magnitude the material universe has proven to be uniformly hierarchical, yet in all our cosmologies we humans like to terminate that hierarchy at the limits of our data (flat earth, crystal sphere, island universe, steady state, etc). Currently we choose to presume the the mere 2 orders of magnitude of homogeneous galactic clustering we see at the limits of our view are sufficient to completely characterize a potentially infinite universe. We do this even though we can easily find in excess of 14 orders of magnitude of homogeneity (water molecule in 10 cubic km of ocean) nested within the 40 orders of hierarchy we have already fully examined. To invest all our efforts in a model based on such a selective application of the data is poor science in my estimate.

The Big Bang is an idealized model of how the universe could work if the material hierarchy did somehow stop at the limits of our ability to examine it. Like the 19th century "Island Universe" model of the Milky Way, the Big Bang is a beautiful, comprehensive, ostensibly accurate model of how the universe would behave if the hierarchy did stop. But like every cosmology we ever devised, the limits we habitually place on the hierarchy have always proven to be false and the hierarchy has always persisted beyond them.

The advantage to presuming an ongoing hierarchy is to better address the anomalous data in the current model. Influences from a greater context often show up as anomalous data in the current model (Al Sufi's nebula, Hubble's red shift). It is just as likely that the questions of dark matter, dark energy and curvature of the universe are related to the material and energetic effects of the surrounding hierarchical context rather than strictly local variable adjustments in the current model.

Sure the Big Bang will be BIG, but it won't be "everything." There will always be more to the hierarchy than we can see from here, just as has always been the case. That's what the material and historical data both consistently indicate. If the cosmological principle is good science, then the hierarchical principle is better science.

-Mike

 

[mysearch]

Response to #93:

There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.
Hamlet by William Shakespeare: 16th Century

Hi Mike,
In many respects, I think you have eloquently summarised a valid view of, not only, the Big Bang model, but possibly science as a whole. This is not an anti-science or anti-establishment view, far from it, simply a recogniition of the reality of the endeavour and we all need to be reminded of this reality occasionally.

Concepts that have proven useful in ordering things can easily attain an authority over us such that we forget their worldly origin and take them as immutably truths. They are then rubber-stamped as a "sine-qua-non of thinking" and an "a priori given". Such errors often make the road of scientific progress impassable for a long time.
Albert Einstein: 20th Century