Can we definitively know the global structure of spacetime? This is a good question. It is one that is actively discussed in the area of the philosophy of modern physics.1,2
However it is a question that highlights the fundamental weakness of cosmology and hence of cosmogony. (Cosmology is the study of the structure of the cosmos whereas cosmogony is the study of the origin of the universe.) That weakness is the inherent inability to accurately construct any global cosmological model, i.e. a model that accurately represents the structure of the universe at all times and locations. The reason for this is underdetermination.3
“There seems to be a robust sense in which the global structure of every cosmological model is underdetermined.”1
In the philosophy of science, underdetermination means that the available evidence is insufficient to be able to determine which belief one should hold about that evidence. That means that no matter what cosmological model one might conceive of, in an attempt to describe the structure of the universe, every model will be underdetermined. Or said another way, no matter what amount of observational data one might ever (even in principle) gather, the cosmological evidence does not force one particular model upon us. And this underdetermination has been rigorously proven.1 Continue reading
In 1929 Edwin Hubble published his observations of the redshift and distances of nearby galaxies. Hubble observed in the light from most of those galaxies that the spectral lines were shifted towards the red end of the spectrum as compared to a local laboratory source of the same atomic gas species. From this he interpreted that it was a Doppler effect (ie. due to the motion of the source), where the galaxies were receding from us, the observer. Thus the idea of the expanding universe was founded.
Expanding universe with us at the centre. The galaxies are moving away from us at the same rate in every direction.
But one other important idea came from those same observations. He observed roughly the same redshift in light from the galaxies as a function of distance in every direction he looked. This became known as the Hubble law, which is the basis for the standard cosmology today–the big bang model. But the fact that this was in every direction and that the proportionality between the redshift and distance was the same in every direction meant that it looked to him like we, that is, our galaxy, was at the centre of the Universe. This is because the galaxies were moving away in a spherically symmetric way, putting us at the centre. This view of the Universe then would look something like the image in the figure on the right. Continue reading