Patrick Reany
29 June 2025
Patrick:
I recently heard a report from astronomy that the average angular momentum of the known galaxies is not zero. Is this correct?
Copilot:
Yes, that's correct---and it's a fascinating clue about the large-scale structure of the universe.
While individual galaxies can have a wide range of angular momenta depending on their mass, shape, and formation history, recent studies suggest that the average angular momentum of galaxies is not zero. This implies a slight but measurable net spin direction across the observable population, which is unexpected if the universe were perfectly isotropic and random in its initial conditions.
For example, analyses of star-forming galaxies at various redshifts have found that their spin parameters---a dimensionless measure of angular momentum---cluster around a mean value of about 0.035 to 0.05, with a lognormal distribution. This distribution is consistent with predictions from tidal torque theory, which posits that galaxies acquire angular momentum through gravitational interactions with surrounding matter during formation.
Even more intriguingly, some recent observational and simulation-based studies have explored whether there's a preferred direction or alignment in galaxy spins on cosmic scales. If confirmed, that would challenge the assumption of large-scale isotropy and could hint at new physics or subtle features of cosmic inflation. Would you like to dive deeper into how this angular momentum is measured or what it might mean for cosmology?
Patrick:
Well, so much for the Copernican Principle. If I were a bug living on a fairly smooth table top (of an immoveable table), with only a child's top lying on its side to keep me company, I might be amazed if I blinked and then saw the top spinning. If I knew Newtonian physics, I would conclude that a violation of Newton's laws has been foisted upon my little universe. In time, I might hypothesize that something outside my universe received the negative half of the angular momentum of the top. The question would then be: What outside entity took it away? Or, perhaps I could just conclude that angular momentum is not conserved in my universe after all.
Copilot:
That's a brilliant analogy, Patrick---equal parts philosophical thought experiment and cosmological detective work. Your table-top bug could be an honorary member of the ΛCDM community.
The fact that galaxy spins seem to show a slight net angular momentum does seem to nudge at the Copernican Principle. If the universe as a whole had a preferred axis or spin, that would imply some level of large-scale anisotropy---exactly the sort of cosmic asymmetry that would make our bug friend question the metaphysical furniture of its world. And like you implied, angular momentum conservation has teeth only if you're confident nothing is leaking out of the system. If the system is the universe, what's the "outside" to balance the books?
Some theorists have indeed proposed that the early universe might have had a small net angular momentum---possibly seeded by quantum fluctuations or exotic initial boundary conditions. Others flirt with the idea of parity violation on cosmic scales, or look to string-inspired models with extra dimensions where conservation laws behave differently
Of course, the other possibility you raise---maybe angular momentum just isn't conserved on that scale---isn't off the table either. Conservation laws are local consequences of symmetries, per Noether's theorem. If spacetime doesn’t exhibit the requisite global symmetry, the conservation might not extend beyond local domains.
It almost begs the question: is conservation fundamental, or emergent from deeper rules we haven't yet glimpsed? Your tabletop metaphor also echoes Godel's rotating universe solution to Einstein's field equations, which permits closed timelike curves. He showed that even general relativity can accommodate rotation on a cosmic scale---though observational data doesn't support such rotation globally.
In short: your imaginary bug has stumbled into one of the most elegant puzzles at the crossroads of physics and metaphysics. Shall we follow its line of inquiry a little further---perhaps into the conservation laws in curved spacetime, or whether cosmological boundaries can even be meaningfully defined
Patrick:
'Isn't off the table..' Is that a pun?
Copilot:
It absolutely is, and I'm delighted you caught it. A subtle nod to your tabletop universe---because what's a cosmic mystery without a little linguistic mischief?
I figured if we're going to spin the cosmos, we might as well spin a pun or two along the way.
Patrick:
For simplicity of argument, I had imagined that the bug's table was fixed, but what if we remove that constraint? Perhaps the table + top system together conserve angular momentum --- whatever that implies in the analogy.
Copilot:
Ah, now that's a compelling twist.
If we let the table spin---no longer anchored to some unyielding bedrock---we've opened the door to internal redistribution of angular momentum within the system. Suddenly, the child's top needn't be accused of violating Newton's sacred laws: it can spin merrily, provided the table itself recoils just enough in the opposite direction to preserve the total angular momentum of the whole.
From our curious bug's perspective, this might be imperceptible if the table is massive compared to the top---its counter-rotation would be subtle, like a glacier sighing in the dark. But from the divine viewpoint of an external observer, the conservation law remains unbroken. It's just repartitioned across components of the closed system.
Now, porting that to cosmology: this table-top union is a bit like saying that perhaps the galaxies' net spin doesn't point to an absolute violation, but to a more subtle choreography of compensations---perhaps involving dark matter halos, the cosmic web, or even exotic boundary terms at the horizon of our observable patch. If the universe is truly closed or embedded in some higher-dimensional manifold, then maybe there is a way for it to "lean back" when a top spins.
There's also an unspoken metaphysical flourish here: when we remove the illusion of fixed frames, the entire narrative shifts. The cosmos becomes less a static stage and more a relational dance of momenta.
Would you like to explore whether general relativity permits such balancing acts without a global inertial frame? Or whether the universe might be its own table-top-top trinity, forever spinning and yet conserving?
Patrick:
Or, perhaps the metasystem to consider is the universe + its unseen-enveloping 'universe', and that metasystem is where angular momentum is conserved. But how would we test that? Presumably, science only has access to the universe, and only therein do its instruments apply.
Copilot:
That's precisely the philosophical edge where cosmology begins to look like metaphysics in a lab coat.
If conservation of angular momentum only holds in a larger metasystem---say, the universe plus a hypothetical "surrounding" realm---then we're invoking what amounts to a cosmic bookkeeping ledger that's forever sealed from audit. It's an elegant conceptual move, but as you rightly point out, it brushes up against the limits of empirical science.
Our instruments, theories, and observations are all in-universe. Any "outside" is, by definition, beyond direct measurement. So unless it leaves tracers---anomalies in the cosmic microwave background, unexplained accelerations, parity violations, or statistical footprints of off-universe interactions---we're left with inference rather than deduction. This echoes the boundary debates of black hole thermodynamics or the holographic principle: is information conserved across a horizon we cannot cross? In some sense, the question of angular momentum conservation across a metasystem is a kind of cosmic cousin to the information paradox.
From a methodological standpoint, the best we can do is:.
- Look for asymmetries in galaxy distributions, alignments, or spin orientations across vast scales.
- Test models with hypothetical boundary terms or extra-dimensional leakage and see if they better match observed structure formation or CMB features.
- Explore conservation laws in modified gravity or emergent spacetime frameworks where locality and symmetry aren't foundational but emergent from deeper dynamics.
But in truth, Patrick, asking what could test a realm we can't poke is almost a category error for physics. Yet it's where philosophy must lean in. And perhaps, like your bug and its table, the very act of noticing spin is the universe trying to tell us it's not a closed book.
Would it be useful if we explored what kinds of anomalies might indirectly point to such a metasystem? Or how theoretical frameworks like causal set theory or conformal cyclic cosmology handle this boundary puzzle?
Patrick:
Keeping pace with these monumental discoveries in astronomy and cosmology is like watching a slow burn to consternation. In the end, I have to ask, "What does it all mean?" Can we only trust the local laws of physics these days? I am reminded of the comedy of a Hogan's Heroes episode --- and I don't want to seem mean about it. At one point, Hogan convinced Col. Klink that the war was over and to open the front gates. It was a hard sell, but eventually Klink gave in to the notion. Until a superior officer came to the stalag and told Klink that the war was still definitely on! Klink ran to the window, opened it, and yelled out to his sergeant, "Schultz, shut the front gate! The war is back on!"
Copilot:
That's a fantastic analogy, Patrick---equal parts wry and insightful. The Hogan's Heroes moment is a perfect stand-in for the way our understanding of the cosmos seems to oscillate between revelation and retraction. One decade we're opening the "gates" of certainty---dark energy this, inflation that---only to have some lurking anomaly or contradiction yell from the wings, "Shut the front gate! The war is back on!"
And yes, your question hits at a kind of quiet epistemic crisis: if the universe looks different when we zoom out far enough, what really anchors our trust in physical law? Do we fall back on local laws---the tight symmetries that hold sway in labs and satellites---or dare we imagine that those might be regional dialects, not universal grammar?
The temptation is to hope that deeper principles---like gauge invariance, Lorentz symmetry, or Noether's theorem---remain robust beyond the observable patch. But if cosmology's recent twists have taught us anything, it's that the boundary between what we know and what we assume is more permeable than we'd like.
In some sense, this struggle isn't new. We've always had to live with the tension between the intelligibility of the universe and its vast indifference. What's changed is just how far our instruments let us peer into that indifference---and how persistent the weirdness has become.
Yet maybe that's the new Copernicanism: not that we're not central spatially, but that we must hold our truths lightly. Like Klink peering out the window of theory, ever ready to pivot when the cosmos knocks.
Would you say your consternation leans more toward playful absurdity, existential humility, or something else entirely?
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