Astrophysics/Thanks for answer to DOES RED SHIFTING ACTUALLY PROVE UNIVERSE EXPANSION
Dear Dr. Stahl:
Thanks for your answer to previous question DOES RED SHIFTING ACTUALLY PROVE UNIVERSE EXPANSION. Thanks for all corrections to the mixup I made and your excellent explanations, as well as your suggestions for further readings.
I have a deep feeling of awe towards Astronomy and Astrophysics, as these fields of Science are reshaping our view of the universe and even our philosophical and religious beliefs.
I have taught some 50 courses at 4 national universities in my country, for undergraduate (bachelor and M.Sc.) most of them related to Chemistry and Biological Sciences, none of them requiring advanced mathematical tools. Occasionally with my students we stumble upon topics such as the origin of life and the universe. I feel the ethical obligation to understand more about the Physics of the universe and the status of current theories before even discussing it students.
So, it is inevitable that tough questions arise. I honestly don't have the answers or even the mathematical tools to approach most of them:
1. A photon travelling forever with the same amount of energy (and wavelength) is a violation to 2nd law of thermodynamics?
2. An expanding universe, a continuously increasing speed in expansion and the Earth (or Milky Way) being the apparent geometrical center of this expansion are all parts of the same, current and accepted theory?
3. Does comparing evidence of the universe expansion that comes from distance sources of light (i.e. Hydrogen lines red shifting from distant galaxies, quasars, ...) observed magnitude vs redshift Z analysis(particularly 1a supernova in the accelerating universe region of the graph) and evidence independent from any doppler effect suggest contradicting explanations.
4. What are the limitations to Physics and Astrophysics for having a single point of observation of the kwnown universe? Whay if neighbors from a galaxy very far away (i.e. Vulcans for Trekkies, of Coruscants for SW fans)also perceive that their galaxies (not Earth) are the apparent center of the universe expansion?
I feel deeply uncomfortable with question 1, as light and other forms of radiation (doppler effect) is one of the main measuring tools in Astronomy. Having a ruler whose dimensionals might actually be confusing (or at least mixing up) the space-time fabrics expansion (extensively discussed and demonstrated) AND a gradual change in light wavelength over time* (more obscure and not fully understood/discussed) may be at least a metrological issue for Astronomy and related Sciences. *Thanks for the clarification that this is not a novel theory.
For question 2 and part of 4, I will resort to a mechanical analogy: We fully accept Techtonics to explain lots of geological, biological and other effects. But if pepople from Kansas told me that this State/City is the geometrical center of Earth, that all techtonic plates move away from Kansas AND the speed is increasing, I would raise both eyebrows, try to confirm this information with people OUTSIDE Kansas and be ready to compare with alternative explanations. I understand that in our current technological state we are not able to ´make a phone call´ outside Earth to confirm Vulcans or Coruscants point of view.
Again, Thanks for your time and patience.
And thanks for your use of All Experts. I will try to address your concerns here, though it might also be good to see if you can obtain some texts that might also be of help. One of the best I can think of, although a bit dated (1977) is Steven Weinberg's 'The First Three Minutes. He writes in an easily understandable form and also shows in his appendix how the Big Bang and expanding universe concepts make sense from a thermodynamics point of view. He specifically uses the established observation of a 2.7K microwave background temperature to extrapolate backwards to a superhot Big Bang. It is done in a logical succession which ought to appeal to you as an inquiring scientific mind.
Another useful text is Lawrence Krauss'e 'A Universe from Nothing', wherein he tied the observational evidence to the existence of dark energy, and its repercussions. The book, 'Infinity - New Rsearch Frontiers' is an anthology of scientific -mathematical essays wtih the one entitled 'Cosmological Intimations of Infinity' (Chapter 8) by anthony Aguirre, the most useful for your concerns.
His warning concerning the widespread assumption of the FLRW (Friedmann-Lemaitre- Robertson- Walker) metric is the one to note. That is, in faling to distinguish the Big Bang theory (which is well supported by observations) from the "idea that the FLRW metric applies to arbitrarily early times. That is, conceivably leading to some time t= 0 for which the radius R(t) -> 0.
In terms of the cosmological red shift actual "proving expansion" - let me say instead it validates the concept, if one follows the simplest hypothesis (by Ockham's razor) that the recession of distant objects is analogous to the Doppler effect for sound waves. The proviso or caution is that we still have not fully integrated the observation of divergent z -values associated with some quasar pairs, i.e. one may appear to have large z, another small. This is why "proof" is perhaps too strong a word.
But still there is a coherence and beauty in tying the redshift to expansion, including the ability to use the Hubble law to track back in time to obtain the age of the cosmos, e.g.
Now as to some of your other concerns:
1) A photon "traveling forever" and "not losing energy" is a statement that is impossible to prove unless one knows ab inito the reference frame for the photon. In cosmological arguments one has to take care to define an *inertial reference* frame, otherwise all observations and assumptions amount to comparing apples and oranges. What does this mean? It means that a photon has differing amounts of energy in different reference frames. The redshift, then, is associated with a change of inertial reference frames.
Such change could also conceivably occur owing to gravitational effects. In one experiment performed as early as 1960 (Pound and Rebka) for example, gravity was found to produce a fractional red shift in the frequency of gamma rays, on the order of 2.46 x 10^15 .
The question that must be asked then, is whether the photon detected is accelerating or not. If not, then it is not an inertial reference frame. (The photon's energy in the original inertial reference frame is constant, but in an accelerating reference frame, it is changing. )
2) On account of the phenomenon of cosmic inflation (please google to learn more) the expansion has not been constant. It was much more rapid in earlier epochs. We also know this because of differing values of the Hubble "constant" H. Currently, we estimate H ~ 70 km/ sec/Mpc, where Mpc denotes ‘megaparsec’ – e.g. 3.26 light years is one parsec. But not so long ago it was as high as 100 km/ sec/Mpc.
Also, it isn't corect - since the expansion is one of space as well as time- to see Earth as the "geometrical center". There is no preferred reference frame for the expansion, so Earth is only one vantage point buy which to arrive at the Hubble law. A fairly ok analogy to invoke is that of an inflating balloon with ink dots on its surface. As you inflate the balloon each of the dots moves away from its neighbors, analogous to how galactic clusters move away from each other in the real cosmos. However, as can be seen, NO single dot can be the center of said expansion.
3) Actually, apart from the QSO "bridge" effect intruding (differing z's for paired QSOs), and the over-extrapolations from the use of FLRW metrics, there aren't any "contradictory explanations", at least that can be said to comport with Ockham's Razor. If you can get hold of Steven Weinberg's excellent book, you will see what I mean.
4) In this regard, there's no need to feel uncomfortable about a "single point of observation". While that is certainly true in terms of the *location* perspective, we have what is known as the 'cosmological principle' i.e.
That the properties of the large scale universe (and the laws of physics) are the same for all observers. This is what makes our spectroscopic work on stars, to detect their chemical elements, meaningful. If it were not the case, we'd be surrounded by chaos and could make no deductions. We would not have a coherent theory of stellar evolution, which we do, nor a way to model stars of different types.
The expansion model 'balloon' analogy, described above, also discloses that the 'one location' perspective is not credible, again since all observers will experience the same observations and ought to make the same red shift measurements from their locations, for objects of a given distance, D.
Again, the key to eliminating most of your concerns is to eliminate the "geometrical center" viewpoint. There is no special or preferred viewpoint to any observer in the cosmos. For each observer- whether located in the Andromeda galaxy or one much more distant, the same type of redshift effects will be observed for the objects of a given D. If any sentient beings on these other galaxies are able to determine distances to objects, they will be able to verify what we observe here on Earth.
Hopefully these answers have helped, and again, please try to obtain the texts indicated earlier to provide even more information.