But physicists already made spacetime redundant by dividing it back to space and time. This was started by Dirac who restated general relativity with Hamilton formalism. The slicing of spacetime was completed in the sixties with ADM formulation. Also we know spacetime does not exist in practice because when we say "universe is expanding" we mean "space is expanding." It makes no sense to say spacetime is expanding.
Your comment is divorced from the coordinate freedom of general relativity.
In any spacetime you care to do an ADM split on, there are an infinite number of real-valued smooth scalar fields whose gradient is everywhere non-zero and timelike available to serve as the coordinate time.
In the standard cosmology the at-rest isotropic and homogeneous distribution of matter provides an obvious coordinate time function, but physics still has to work for other inertial and accelerated observers, so there is nothing preventing anyone from using a non-comoving observer's proper time in the ADM split.
Nits:
* Hamiltonian formulation of general relativity
* ADM formalism
The expansion is in the metric, and most visible (and most amenable to interpretation as an expansion of space) when using comoving coordinates. However, we are allowed to work in any system of coordinates, and when we do a general coordinate transform, we lose the interpretation of the metric expansion as an expansion in space.
("We see in both [Milne & de Sitter] cosmologies that the interpretation of redshift as an expansion of space is dependent upon the coordinates one chooses to calculate z." -- https://academic.oup.com/mnras/article/422/2/1418/1036317 final sentence in §3.4 de Sitter space).
The standard cosmology's FLRW spacetime (and also de Sitter space, where the vacuum constant positive scalar curvature makes for an easier-to-understand expansion history) is time-orientable. There are more points in spacetime in the future direction from an arbitrary point anywhere in the entire spacetime, than in that point's past.
However, the real distribution of matter is in our universe lumpy, as is easily demonstrated right here on Earth. Standing on the ground you can tell that locally there is neither isotropy nor homogenity (you can see quite far up into the sky, but not so far into the ground). Moreover, there are differences in isotropic pressure, convection, heat conduction, and shear stresses between the air and ground, none of which are features of the perfect fluids in the FLRW model. So locally the FLRW metric is not suitable, and since the Earth is very much not vacuum de Sitter is even less suitable. Consequently it should not offend anyone to read that Manhattan isn't undergoing cosmological expansion. Most things within ten kiloparsecs of here are gravitationally collapsing. It requires coordinate contortions to interpret interplanetary or interstellar space in the Milky way as expanding, and switching to different systems of coordinates will blow up that interpretation.
A "swiss cheese" universe with collapsing vacuoles in an expanding cosmology reflects that it's only when we go from ~kpc length scales to ~Gpc length scales that the distribution of matter behaves enough like perfect fluids to allow for an exact solution to the Friedmann equations. The collapsing vacuoles are usually modelled like Lemaître-Tolman-Bondi with a thin shell boundary; this results in a vacuole that time-orientable and has fewer spacetime points in the future direction. "Hole" time functions and "cheese" time functions must be different (hole clocks tick slower as they fall towards the centre).
This is just another proof that spacetime as a real entity does not exist. If there are infinitely many spacetimes, spacetime cannot be the fabric of the universe but it can only be fodder for academic careers. There is one universe but an infinite number of spacetimes. So which spacetime is the true one? None of them. You pick and choose one and write a paper on your chosen spacetime and collect your academic points. Another physicist chooses another popular spacetime and she writes a paper on that spacetime. The whole thing is a joke.
Of course one picks a simplified spacetime and investigate how that works, extracting ways to relate the simple model to actually-observed astrophysical phenomena. Just like how quantum physicists 70 years ago were working with simple models of the weak interaction, comparing their models to tracks in cloud chambers, ticks of geiger counters, or ratios of radioactive elements found in spectral lines.
It turns out that one can knit together simplified models and build up a good description of a real complex system, but this has been known since at least the dawn of thermodynamics, if not since the time of Newton.
Indeed, Nasa and its counterparts have been working for decades with an approximation of the solar system: new bodies inside Neptune's orbit keep being found practically every year. It's not remotely likely that we know all the bodies of the solar system even out to Neptune (much less beyond), let alone their orbital parameters and how those evolve over mere millions of years <https://en.wikipedia.org/wiki/Stability_of_the_Solar_System>. Does that mean publishing the results of studies of long-running models of our solar system "can only be fodder for academic careers"? Even if it spots anomalies that lead to the actual discovery of very dim bodies?
> collect your academic points
The academic points mostly come from being cited by an author poking holes in your paper. Go investigate google scholar.
This is called the academic dialogue. And yes, a variety of scores are kept (e.g. the loathsome h-index).
But I guess you don't care, because you are happy writing obviously ignorant nonsense on hacker news for engagement and upvotes, right?
> There is one universe but an infinite number of spacetimes
There is possibly one unique spacetime that fully describes the universe, but guess what, we simply do not have enough computer power on the planet to validate such a model.
Here's the recent state of the art in computational cosmological simulation, n-body with n in the hundreds of billions:
There are more than a hundred billion stars in this galaxy; there are more than a hundred billion galaxies in our sky. And there are more than a hundred billion particles in a star. And there are lots of motes of dust and blobs of gas between stars. So we're quite a few orders of magnitude too small in n in our n-body simulating to be able to pick out our own universe, exactly described, from a large set of simulations.
We also obviously don't have an infinite number of observations, since neutrinos and gravitational waves are hard to detect at all (and we only see a small part of the frequency space of both), we've only just started having really good views in the near infrared (JWST), our views in X-Rays and gamma rays are fuzzy because of technological limits, and so on and so forth. We are just at the start of https://en.wikipedia.org/wiki/Multi-messenger_astronomy four centuries after Galileo used a telescope to find the four biggest moons of Jupiter. (Incidentally, three moons of Jupiter were just discovered two years ago, because hey telescopes are not all-powerful and all-seeing. Guess how they figured out where to point the Victor M. Blanco Telescope?).
> The whole thing is a joke
Honestly, it's amazing that you aren't embarrassed by how obvious your ignorant contrarianism is.
Write back if you're actually interested in expanding your knowledge rather than mocking people you don't know whose work you know very very very very little about.
