The Universe, and Space Time.
The universe is expanding in such a way that when Hubble measured the velocities of galaxies at varying distances from us he found that the further away they were, or the greater the distance, the greater was their speed of recession.
The ratio between the distances of the individual galaxies and their respective speeds of recession is known as Hubble's Constant. This is because when the velocities and distances were plotted on a graph, it showed the ratios to be clustered along a straight line. This straight line indicated a constant, rather than a variable.
In terms of relativistic distances at which recession velocities of regions of the universe are themselves relativistic, a problem arises when the Hubble value is regarded as constant.
This problem is relative to any observer who may be reasonably regarded to be at rest relative to his local region of the universe.
This problem manifests itself in various different ways and respects. Whilst it may be attractive to focus on these and try to resolve the issue full on, this is not a constructive approach as another that I would prefer to follow.
(To be extended & continued ..... )
It could be that the first reason for my interest in physics is that physics does not involve and is not subject to human opinion.
Essentially, nature does not care what we think.
Physics is physics, nature is nature, and opinions are not reality.
It is for this reason that the observation of completely unexpected or inexplicable phenomena is always exciting:
in such observations are the clues to what we do not yet understand.