Fumfer Physics 24: Neutron Stars and the Non–Black-Hole Universe
Author(s): Scott Douglas Jacobsen
Publication (Outlet/Website): Vocal.Media
Publication Date (yyyy/mm/dd): 2025/10/22
Rick Rosner explains compact objects without hype: compressing matter triggers quantum degeneracy pressure (electrons in white dwarfs, neutrons in neutron stars). When gravity exceeds these pressures—around the Tolman–Oppenheimer–Volkoff limit (~2–3 solar masses)—collapse forms a black hole. Dimming is due to gravitational redshift, not ‘acceleration.’ Exterior fields encode only mass, spin, and charge (“no-hair”). The information paradox’s modern view favors unitarity; black holes preserve information, though mechanisms remain debated. Crucially, the universe is not a black hole: large-scale expansion fits FLRW cosmology, with horizons from cosmic expansion, not an event horizon. Scale matters—bigger systems have gentler curvature and tidal gravity overall.
Rick Rosner: Last night on Naked at Night, before the show, Lance said, “You have this high IQ, but you talk in everyday words. You need to say something that boggles people.” So I talked about what actually happens when matter is compressed by gravity. When matter is squeezed to extreme densities, quantum mechanics bites back: electrons (and, at higher densities, neutrons) resist being packed into the same states. That resistance is called degeneracy pressure (from the Pauli exclusion principle). In that regime—white dwarfs for electrons, neutron stars for neutrons—matter becomes “degenerate,” meaning its pressure comes mostly from quantum effects, not that it “loses information.” From far away, ultra-compact objects are hard to see not because they are “accelerated so much,” but because intense gravity redshifts and dims their light. Cross an event horizon, though, and classical general relativity says signals cannot get back out. That is why the interior of a black hole is not observable from the outside.
Scott Douglas Jacobsen: To confirm for readers: “degenerate” here refers to quantum-statistical effects?
Rosner: Inside degenerate matter, if gravity overcomes all known pressures, collapse continues. For neutron stars there is a maximum mass—the Tolman–Oppenheimer–Volkoff limit—beyond which no known equation of state can halt collapse, and a black hole forms. Stable neutron stars are not “making new sub-spaces and times” inside; they are held up by degeneracy pressure and nuclear forces. Whether a black hole interior “bounces” or spawns a new expanding region is speculative and model-dependent; it is not established physics. Appeals to Mach’s principle are not needed here. In standard general relativity, a neutron star’s stability is governed by its own mass, rotation, magnetic fields, and equation of state, not by an “informational umbilical” to the rest of the universe. The exterior only “knows” the star by global charges like mass, spin, and (if any) charge—the spirit of the no-hair theorems.
Jacobsen: Numerically, do you place that limit in the ~2–3 solar mass range, contingent on the equation of state?
Rosner: On information: the phrase “loses almost all of its information” is misleading. Hawking’s original calculation suggested information loss in black-hole evaporation, but the modern consensus in quantum gravity leans toward unitarity—information is preserved in principle (for example, via black-hole microstates)—even if we do not yet have the complete mechanism nailed down. As for “the universe is a black hole,” tempting numerology aside, it is not. Our universe is well-described on large scales by an expanding Friedmann–Lemaître–Robertson–Walker spacetime, not by a static, asymptotically flat Schwarzschild geometry. The fact that the Hubble radius and a naive Schwarzschild radius for the mass inside it can be of similar order is a coincidence of scales, not evidence that the cosmos is a black hole. Light does propagate within the observable universe; what limits us are cosmological horizons set by expansion, not an event horizon confining everything to a single “cosmic black hole.” Short version: gravity can crush matter into degenerate states; degeneracy and nuclear forces hold up white dwarfs and neutron stars until a mass threshold triggers black-hole formation; black-hole interiors and “baby universes” are speculative; Mach’s principle is not required; and the universe is not a black hole.
Jacobsen: For clarity: may we state that gravitational redshift lowers photon energy and apparent brightness, which is why distant compact objects appear dim, rather than attributing this to “acceleration”?
Rosner: We are not crushed by gravitational forces because the universe is so vast that it does not take much curvature to make space fold back on itself on a grand scale—tens of billions of light-years in circumference—to keep the universe self-contained. That is why it can appear, at a glance, somewhat analogous to a black hole. But to make a black hole of, say, three solar masses, the local gravitational pressure must be enormous to compress all that matter into such a tiny volume. The matter is squeezed beyond recognition. As you scale upward from a stellar-mass black hole to something like the entire universe—which, in rough terms, would correspond to a mass on the order of 10²² solar masses—the amount of local gravitational force experienced by the matter decreases. That is because as the radius and curvature scale up, the local curvature of spacetime at any given point is smaller. So the larger the system, the less local pressure is needed to curve space back on itself. The total mass of what is being compressed also affects whether it can internally differentiate—whether new structures, fields, or even separate regions of spacetime could form. One could imagine particles appearing to “explode outward,” but another way to look at it is that the internal scale of space changes, effectively creating more internal volume where new configurations of matter or energy can emerge. There is a lot happening in that idea—though much of it remains speculative.
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