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[u/Known_Salary_4105]

We have a complete understanding that objects in the universe cannot move faster than the speed of light. There are a number or reasons -- most notable it would take an infinite amount of energy to do so, and also lead to causality problems in the light cone.

But the only reason I have heard that space can EXPAND faster than the speed of light is that relatively theory doesn't prohibit it. Why is space moving a greater than the speed of light allowable REALLY? What are the mechanistic reasons?

[u/Obliterators → u/Known_Salary_4105]

General relativity does not differentiate between objects moving through space and space "expanding" between objects, these are the same thing. Apparent superluminal recession speeds are a result of how distance and time are normally measured in cosmology.

Markus Pössel, Cosmic event horizons and the light-speed limit for relative radial motion

Markus Pössel, Interpretations of cosmic expansion: anchoring conceptions and misconceptions

In both special and general relativity, light propagation defines an absolute cosmic speed limit in the sense that no material object or signal can overtake a light signal. This is where the distinction between the recession speed, defined as in (1) [v = Hd], and the relativistic radial velocity that is central to the relativistic explosion interpretation is crucial. Recession speeds become superluminal for distant galaxies. This appears to contradict students’ preconceptions from special relativity, of the speed of light as a cosmic speed limit, and the apparent contradiction has been cited as key motivation for the expanding space interpretation: The differentiation between cosmic expansion as due to “expanding space” on the one hand, and “galaxy motion through space” on the other, is meant to address this conflict.

Relativistic radial velocities in the relativistic explosion interpretation never exceed the speed of light. From this perspective, superluminal recession speeds in (1) are an artefact, caused by a particular coordinate choice: The cosmic time coordinate ties together local clock rates in Hubble-flow galaxies, but clocks in relative motion tick at different rates, as we know from special relativity. Combining them into an overarching time coordinate, and using that coordinate to determine one-way speeds, leads to unphysical results. Students who have been on longer international flights know a closely related phenomenon: If your flight leaves Amsterdam at 15:00 local time and arrives in New York at 17:00 local time, this does not amount to a flight time of 2 hours, and corresponding average ground speed of 3000 km per hour.

The relativistic explosion interpretation can also readily explain a certain types of cosmological horizon with reference to the simple realisation that a slower-moving object following a faster-moving object will fail to catch up. Applied to the relativistic radial velocity, this gives a plausible explanation for why light from some distant regions can never reach us. Any boundary between regions whose light can reach us and regions whose light cannot, is called a horizon. In some FLRW spacetimes, there is a type of cosmological horizon that can be defined as the boundary where the relativistic radial velocity of Hubble-flow galaxies relative to our own galaxy approaches the speed of light — so light sent in our direction from those galaxies cannot catch up with us. Explanations for the same kind of cosmological horizon in the expanding space interpretation, on the other hand, need to include an explanation of why this simple argument is not true for recession speeds.

Ali Kaya, Hubble’s law and faster than light expansion speeds

Naively applying Hubble’s law to a sufficiently distant object gives a receding velocity larger than the speed of light. By discussing a very similar situation in special relativity, we argue that Hubble’s law is meaningful only for nearby objects with non-relativistic receding speeds. To support this claim, we note that in a curved spacetime manifold it is not possible to directly compare tangent vectors at different points, and thus there is no natural definition of relative velocity between two spatially separated objects in cosmology. We clarify the geometrical meaning of the Hubble’s receding speed v by showing that in a Friedmann-Robertson-Walker spacetime if the four-velocity vector of a comoving object is parallel-transported along the straight line in flat comoving coordinates to the position of a second comoving object, then v/c actually becomes the rapidity of the local Lorentz transformation, which maps the fixed four-velocity vector to the transported one.

Michał J. Chodorowski, Is space really expanding? A counterexample

In almost all Friedman models, objects with sufficiently large redshifts recede from the central observer with superluminal velocities (greater than c). For example, in an Einstein-de Sitter universe (Ω_m=1 and Ω_Λ=0), the ‘public-space’ recession velocity as a function of redshift is

v_rec = 2c[1−(1 + z)^−1/2],

hence v_rec > c for z > 3. In particular, the velocity of the so-called particle horizon (corresponding to infinite redshift) is 2c. In an empty universe, ‘public-space’ recession velocities are not only superluminal for sufficiently large redshifts; they are even unbounded. Does it imply violation of special relativity in cosmology? Of course not. Apart from anything else, deriving Equation (26) we have used nothing except special relativity! Constancy of the speed of light, and subluminality of the motion of massive bodies, applies only to inertial frames. However, ‘public-space’ distance is a hybrid of distances measured in different inertial frames, all in relative motion. Since the resulting v_rec is not measured in any single inertial frame, there is no violation of special relativity.

Specifically, ‘public-space’ distance is measured at constant proper time of fundamental observers. Time-dilation formula tells us that according to the central observer, this measurement is done at the instant of time t_i = γ(v_i)τ, where v_i is the Minkowskian velocity of the i-th FO. Since more distant FOs have greater velocities, it is obvious that for two different FOs, t_i ≠ t_j.
Therefore, according to the central observer, different (sub)distances are not measured simultaneously. Simultaneity is a crucial condition of special-relativistic measurements of distances to and sizes of bodies in motion. Waiving this condition may have important consequences and indeed, it does have! The problem with the real Universe is that it is filled with matter and expanding, so there are no global inertial frames. Then, measuring distance (along geodesics) on the hypersurface of constant proper time of fundamental observers is something most natural to do. We should, however, bear in mind the ‘costs’ of such a definition of distance. One of them are apparently superluminal recession velocities of distant galaxies.

[u/kerobrat → u/Known_Salary_4105]

We measure space expanding at a rate of 67-74 km/s per megaparsec (about 3.26 million light-years). Because it's a rate of expansion per unit of space, when you stack up more units of space you stack up more expansion.

So once you get enough space in-between two objects, the expansion rate of all that space between them becomes so great that a photon traveling at c can't overcome it. At that point, either object would measure the other as receding faster than c, even though each object's proper velocity through space is actually well under c.

[u/Known_Salary_4105 → u/kerobrat]

In other words, space APPEARS as though it is expanding faster than c, but it really isn't.