Patrick Reany
9 May 2025
Patrick:
Recently, scientists are wondering if the way out of the Hubble Tension, as it is called, is to postulate that the entire universe is rotating. But, rotating with respect to what? Whatever it is, it would have to be a preferred rest frame. So, by it, we potentially solve one problem, only to introduce another: There goes the cherished Copernican Principle of cosmology.
Copilot:
You're diving into one of the most fascinating debates in modern cosmology! The Hubble Tension---the discrepancy between different measurements of the universe's expansion rate---has puzzled scientists for years. Now, some researchers are exploring the idea that the universe might be rotating once every 500 billion years.
This hypothesis could help reconcile the conflicting expansion rate measurements, but as you pointed out, it raises deeper questions. If the universe is rotating, what is it rotating relative to? That would imply a preferred frame of reference, which challenges the Copernican Principle---the idea that Earth (and by extension, any observer) does not occupy a special place in the cosmos.
Interestingly, this model suggests that such a slow rotation wouldn't create paradoxes like closed time-like curves (which could allow time travel). Some hints of rotational patterns in early galaxies lend support to the idea, but more evidence is needed before it can be considered a serious contender for solving the Hubble Tension.
What do you think---could this be a revolutionary shift in our understanding of the universe, or just another theory that will fade away?
Patrick:
But wait, there's more. If the universe is rotating, then it has an axis of rotation, and the center of the universe (or a 'center') would presumably lie on that axis. Yet another break in the Copernican Principle. The universe isn't supposed to have a center. Maybe the way out is to suppose that the universe does not rotate, but that there is some special universal frame that rotates with respect to the universe (sounds Machian), and effects it because of that rotation. After all, motion is relative, they tell me.
Copilot:
You're absolutely right---if the universe is truly rotating, that would imply an axis and possibly a center, which flies in the face of the standard cosmological model. The universe, as we understand it, is supposed to be homogeneous and isotropic on large scales, meaning no preferred directions or special locations. If rotation introduces an axis, that breaks the fundamental assumption of the Cosmological Principle and shakes the very foundations of modern cosmology.
Your alternative idea---a special universal frame rotating relative to the universe---sounds intriguingly Machian. Mach's principle suggests that inertia arises from the interaction between an object and the rest of the universe. If some large-scale cosmic frame exists and rotates, it could influence the dynamics of spacetime without requiring the entire universe itself to be in motion. That way, you preserve isotropy while still allowing rotational effects to seep into cosmological observations.
And yes, as relativity tells us---motion is always relative. So perhaps the question isn't whether the universe rotates, but rather whether there exists some deeper cosmic mechanism that creates effects similar to rotation without violating the principles we hold dear.
It's a wild time for cosmology, and ideas like this show just how little we truly understand about the deep structure of reality. What's your gut feeling---are we on the verge of a paradigm shift, or just chasing another fleeting mirage?
Patrick:
I'll paraphrase an insight I once read from a physicist: In all of physics, the most difficult concept to apprehend is that of motion, because it is much more subtle than human intuition can grasp. Back in the day, Einstein was very concerned about the concept of absolute spaces --- which he referred to as 'ghosts': Meaning that they are capable of acting upon ponderable matter, yet are not reciprocally affected by ponderable matter. What status do we grant to this Universal rotating frame that's supposed to fix the Hubble Tension? Is it absolute or not? If it's not, what are its dynamics?
Copilot:
That's a brilliant insight! Motion, especially on cosmic scales, is far more subtle than our intuition allows. Einstein's critique of absolute space---which he saw as a "ghost" exerting influence without being influenced---still echoes in debates today. If we introduce a universal rotating frame to resolve the Hubble Tension, we're essentially questioning whether such a frame is absolute or if its existence can be justified dynamically.
If this rotating frame were absolute, then we're back to the old idea of a privileged reference frame---the very thing that relativity sought to eliminate. It would mean the universe has an underlying structure that dictates motion, rather than motion being purely relative. That would be a monumental shift in physics, implying a cosmic ether-like entity guiding the behavior of spacetime.
But if it's not absolute, then the key question is: What governs its dynamics? Could this frame emerge from the collective motion of cosmic structures, in a Machian way, where the rotation is dictated by the distribution of mass-energy across the universe? Or could it arise as a large-scale effect of quantum spacetime, meaning it's not a fundamental structure but a consequence of deeper, hidden physics?
There's also the possibility that what we perceive as a rotation is not a literal spin, but a manifestation of some yet-unknown fundamental principle. Maybe the very fabric of spacetime has a torsion or anisotropic effect that mimics rotation without violating isotropy.
So, whether it's a true cosmic rotation or an emergent effect, the deeper issue is whether we must revise our understanding of relativity and motion altogether. Are we seeing cracks in our foundation, or just uncovering a more nuanced layer of reality?
Patrick:
Many physicists complain, rightly or wrongly, that there has been a long-standing stagnation of progress in the foundations of physics. Perhaps now is the right time for physicists to take another look at the philosophical ideas of absolute vs relative space as proferred by Mach and Einstein over a hundred years ago. Maybe that subject should be rebooted to provide a fresh outlook on the foundations of modern physics. After all, so much of the invention of new physical theories over the last 300 years has been in the name of the 'saving of appearances' of older established theories. Perhahps the Copernican Principle needs to be re-examined in light of recent physical data.
Copilot:
You've tapped into a profound concern that many physicists and philosophers have wrestled with in recent years. Some feel that fundamental physics has reached a plateau, where new theories are often extensions or reinterpretations of existing frameworks rather than truly groundbreaking shifts. Instead of discovering entirely new principles, physicists often tweak established models to "save the appearances"---a pattern that has played out across centuries.
Revisiting Mach and Einstein's ideas on absolute vs. relative space could be a vital step in refreshing foundational physics. The nature of space and motion remains elusive, despite general relativity's success. Mach's principle, for instance, still lacks a precise mathematical formulation within modern theories, and its implications for cosmic structure could hold the key to understanding the deeper nature of spacetime. If a universal rotating frame is needed to resolve the Hubble Tension, then physics may be forced to confront the question: Is there a preferred reference frame after all?
And the Copernican Principle, which states that we are not in a privileged position in the cosmos, has been a cornerstone of modern cosmology. But new observations---such as large-scale anisotropies in cosmic data---raise questions about whether this principle remains ironclad. If rotation or other large-scale structures suggest some form of cosmic preference, then cosmology may need a conceptual shift.
Perhaps rather than a stagnation, what we're experiencing is the prelude to a paradigm shift---a phase where foundational assumptions must be reconsidered before true progress can be made. The universe may be nudging us toward deeper insights, forcing a reassessment of what we thought was settled.
Patrick:
If the universe were rotating, there ought to be some centrifugal forces acting on matter, which might add to the expansion of the universe. But this effect would be more pronounced perpendicular to the rotation axis than along it, which should have been noticed by now if it were true. Still, with a slow rotation rate, maybe not.
Copilot:
That's a sharp observation! A rotating universe would introduce centrifugal effects, meaning matter should feel an outward force---especially in the plane perpendicular to the rotation axis. This anisotropic influence could potentially enhance cosmic expansion in certain directions more than others, creating detectable deviations in the shape of the universe.
Yet, as you pointed out, if the universe's rotation rate is exceedingly slow---on the order of once every 500 billion years---then any resulting distortions or differential expansion would likely be too subtle to have been clearly detected. The standard cosmological model, which assumes isotropy and homogeneity, has so far matched observations reasonably well. That means if rotation is happening, it must be incredibly faint, or its effects must be masked by other cosmic phenomena.
However, some studies looking at large-scale galaxy spins have hinted at potential alignments that could be linked to an underlying rotation. But without definitive evidence, the idea remains speculative. If physicists do find subtle variations in cosmic expansion rates along different directions, that could be a crucial clue pointing toward an overlooked rotational effect.
Wouldn't it be wild if we eventually found out that the universe has been slowly spinning all along, hiding its rotation in the subtle details of cosmic dynamics?
Patrick:
The light emitted from distant galaxies to us on earth would experience a transverse doppler shift if the galaxies are moving with a component besides a longitudinal one (due to the universe rotating). Maybe that has some hope of being detected, or being ruled out above a certain degree of accuracy.
Copilot:
Yes, that's a fascinating observational test! If the universe is indeed rotating, then distant galaxies wouldn't just be moving away from us due to cosmic expansion---they would also have transverse motion as a result of the rotational component. This would cause a transverse Doppler shift, where light from those galaxies would undergo a relativistic redshift due to their perpendicular motion relative to us.
In principle, this effect could be detectable, but the challenge lies in its tiny magnitude. Given the proposed slow rotation rate---on the order of one full rotation per 500 billion years---the transverse velocity would be exceptionally small compared to the dominant longitudinal redshift from cosmic expansion. That means current observational techniques might struggle to pick up such a faint signal.
However, if researchers could perform high-precision spectroscopic measurements across many galaxies and look for subtle deviations from expected redshifts, they might be able to constrain whether such an effect exists. If they find no transverse Doppler contribution beyond a certain accuracy, that could help rule out rotation above a given threshold.
It's an intriguing angle---do you think astronomers should start looking deeper into these subtle shifts to settle the question? Or will it remain one of those elusive cosmic curiosities for now?