andrew s

Looking at the last few posts between you and @vlaiv it might be worth noting that in GR spacetime is about geometry and how stress energy shapes it. The standard model has the initial state (or at least as far as we can reliably go back) as hot and dense and it has been evolving from there. Spacetime evolves dynamically as the the stress energy has continued to dilute from it starting point as space metrical expanded with time. The geometry does not stop and start it just is. Regards Andrew

I would put it the gravitationally bound system don't expand as the force is order of magnitude greater that the tension due to metric expansion. Even more so for electromagnetically bound systems. However, metric expansion is still is going locally even if we can't see it. Regards Andrew

Sorry missed that. I think you may have missunderstood the paper. Maybe you could point out which diagrams are incorrect and mathematically why. It's very difficult to get a handle on your concern via "words" when discussing mathematics. The integral is well defined and I belive your concept of " retaining the scale factor from the past" is mistaken or at least I don't know what it means. For a given model, LCDM for example, the scale factor is well defined so the integration has no mysteries. In cosmology concepts such as distance are model dependent, and while I am sure LCDM is not the last word on the topic, there are several other predictions which are observationaly supported such as the time delay in super nove light curves which respect the distance scale it defines. Regards Andrew

Of course they exist and are among us, they are called teenagers. They appeared in the late 50s early 60s coinciding with Sputnik. Before then they were unheard of. We as a species went from child to adult but now this alien infection has modified its behavior much as the jewel wasp turns a cockroach into a zombie. Regards Andrew

You may find this paper worth studying. While it can take some effort to get into the various diagrams I think if will illuminate the issues you are discussing. Regards Andrew

Sadly fraud is not unkown in science especially now the competition for fund makes verifying results unattractive to competing groups. Micro biology has had it's fair share of issues this for example is an interesting read. Regards Andrew

As I mentioned above, the CMB does provide a reference that can be used as an almost "universal " reference for local, on a cosmological scale, motion. At very large distances and due to the dynamics of spacetime comparing relative positions, velocities etc. is very problematic and very model dependent. As a simple example of the effects of spatial curvature. Consider two cars on opposit sides of the equator heading north at a constant speed v. They are initially driving parallel to each other with zero relative velocity. However, by the time they reach the poles the are in for a head on collision at a relative speed of 2v. Physics is fun. Regards Andrew

This effect has been proposed as a solution to the Hubble tension between "local" and CMB derived value. Any very large scale significant non uniformity in the mass energy distribution will impact the local metrical expansion. Regards Andrew

Seems fair enough to me. You can tell if your moving with respect to the CMB by the red / blue shift dipole caused by the motion. It is removed from the CMB images normally shown. Regards Andrew

Yes many times but as @vlaiv said they are not comparable. This is true of both the proposed initial "inflation" and the early stages of the currently observed Hubble expansion. Even now if you go far enough away it is expanding faster than c. Remarkably we can see things that are receding at about 3c. 😊 Regards Andrew

Diffractive optics is already alive and well in commercial photographic lenes https://www.canon.co.uk/pro/infobank/lenses-multi-layer-diffractive-optical-element/ Regards Andrew

Thanks to all. To clarify what I ment by not being mainstream was that I was neither your classic visual observer nor a taker of classic astronomical images. More generally thanks for all the good wishes and I am sure you have not heard the last from me on the forum. Regards Andrew

Today my equipment at Castillejar will be dismantled and put into storage. After nearly 4 yrs of great service from PixelSkies I decided to give up serious observing. Unfortunately, the low resolution spectroscopy I did was fascinating but I could not extract the data I needed. Switching to Exoplanets was productive but just too routine and predictable for my taste. I still retain an interest in astronomy, cosmology and physics but I will also be developing my interest in wildlife and birds in particular. Lots of new spotting scopes, binoculars and photographic equipment to play with. Given I was never a main stream SGLer I will still stick around if that's ok. Regards Andrew

To exclude things from a search put a - sign in front if it e.g. -m31 -galaxy. Make sure there is a space before the - sign. Regards Andrew

Our Universe is not symmetrical! Due to it expansion the Universe is not time symmetric and so energy is not conserved. So no we don't need symmetry for the Universe to work. There are also symmetry breaking mechanisms at the microscopic level as well. However, in everyday life it is locally symmetric so maybe some comfort remains. 😊 Regards Andrew

Emmy Noether should be much more widely known and recognised. Up there with Newton and Einstein in explaining why the world is as we find it. What could be more fundamental than explaining why we see conservation laws and when they are violated. Modern physics rest fundamentally on her work. Regards Andrew

Not all symmetries of interest are in 3d space or 4d space time some are in more complex spaces like Hilbert spaces in QM. Regards Andrew

Both, the same thing can have more than one lable. You can be both a father and son. Regards Andrew

Yes your right just a 180 deg (pi) rotation brings it back to the same state . Regards Andrew

I don't think so as it's a spin 1 particle and 2pi rotation does it. Regards Andrew

It's an example of a modulo sequence. Another example is the 0 to 360 degrees in a circle ( or 0 to 2pi radians). 5 mod(ulo) 4 is 1 etc. It crops up a lot in symmetries in physics. There is even one particle property that needs a 4pi rotation to get to its starting point (twice round the circle) ! Sad to say I can't recall which. Regards Andrew It's spin 1/2 particles. They have a phase shift of -1 for 360 deg rotation and need another loop round to get back to 1

We are all different. One of my delights, as I grow old, is to see the development of my 4 grandchildren. You could not make up such 4 different characters - brilliant. My motivation is/was to understand how things work. Once I am satisfied I know I lose interest. Regards Andrew

Indeed, I think it is the motivation that differs. In fact physicist have led mathematician in some areas. The Dirac delta function was invented by physicists and mathematicians played catch up finally formalising it after years of turning their noses up at it. Regards Andrew

Absolutely, it, as the early posts pointed out, displaces the focal plane. Regards Andrew

Just consider where the principle plane is in a compound telescope. It is not always one of the lenes or mirrors! This illustrates how effective focal length of the whole system is defined using marginal rays. Regards Andrew