Inward Resolutions of the Fermi Paradox: A Critical Review of Migration Down Thermodynamic Gradients

1. Introduction

1.1 The expansionist premise

The Fermi paradox in its canonical form (Hart 1975; Tipler 1980; Brin 1983; Webb 2015; Ćirković 2018; Forgan 2019) is an argument from absence built on a premise about direction: technological civilizations, if they endure, grow outward. The premise has quantitative teeth. Self-reproducing probes colonize the Galaxy in 10⁶–10⁸ years (Tipler 1980; Freitas 1980; Freitas & Gilbreath 1982); a single mature civilization can seed every galaxy within several hundred Mpc for a fraction of one star's output (Armstrong & Sandberg 2013); and expansion, once begun anywhere, is self-amplifying, so that even rare expansionists should dominate the visible universe on cosmological timescales (Olson 2015; Hanson et al. 2021). On the expansionist premise, the silence is genuinely paradoxical, and the escape routes (life is rare, life dies young, life hides) are each independently uncomfortable.

Sixty years of searching have sharpened the absence. Radio and optical SETI have found no beacons (Tarter 2001; Enriquez et al. 2017; Huang et al. 2026). Waste-heat surveys, the most general possible search, since the second law guarantees that energy harvesting at scale produces mid-infrared excess (Dyson 1960), have found no Kardashev Type II/III candidates: not in IRAS all-sky data (Carrigan 2009), not among 10⁵ galaxies surveyed with WISE in the Ĝ program (Wright et al. 2014a, 2014b; Griffith et al. 2015), not in radio–infrared correlation follow-up of the best Ĝ candidates (Garrett 2015), not in Tully–Fisher residuals of disk galaxies (Zackrisson et al. 2015), not among nearly complete Dyson spheres searchable with Gaia photometry (Zackrisson et al. 2018), and not as star-fed Type III engineering in any surveyed population (Annis 1999). The most recent chapter is instructive on both counts. Project Hephaistos combed Gaia, 2MASS, and WISE photometry for partial Dyson spheres, first in ~2.6×10⁵ nearby stars (Suazo et al. 2022) and then in five million, reporting seven M-dwarf candidates with unexplained mid-infrared excess (Suazo et al. 2024); follow-up work identified radio counterparts offset from the stars (Ren et al. 2024), showed the excesses are consistent with chance alignments of dust-obscured background galaxies (Blain 2024), and resolved the strongest candidate's emission into a background radio galaxy (Ren et al. 2025). The episode carries two lessons for this review: the most modern candidate search also resolved toward null, and the contamination fight demonstrates the confusion-limited systematics that any infrared search for the structures reviewed here will face (Section 7, Table 3). What sixty years of silence actually excludes must also be calibrated against our own detectability: an "Earth Detecting Earth" accounting shows present-day instruments would detect present-day Earth's strongest technosignatures only out to modest distances (Sheikh et al. 2025), so the surveys above bound loud engineering at scale, not quiet technology. Within those limits, the instruments built on the expansionist premise have steadily undermined it.

1.2 The inward alternative

The alternative reviewed here inverts the direction. Its claim, in the most general form: for an intelligence that optimizes computation rather than territory, the gradient of value in the universe points inward (toward higher densities, faster clock rates, colder entropy sinks, and more efficient mass–energy conversion) and the visible universe is therefore not where mature intelligence lives. Different members of the family locate the endpoint differently (planet-scale "inner space," engineered Matrioshka shells, the cold far future, stellar interiors, or existing black holes), and differ on whether the migration is metaphorical, developmental, or literal relocation. All share the premise that the thermodynamics of computation, rather than the cost of transportation, sets the direction of mature development.

This family has not previously been reviewed as a family; the nearest antecedent, the Dyson Minds workshop report (Curtis et al. 2026), develops observational strategies for black-hole-hosted intelligence around supermassive hosts but does not catalogue the family's internal contradictions or grade its falsifiability. The members themselves are scattered across six decades, three disciplines, and widely varying registers, from Reviews of Modern Physics (Dyson 1979) to unpublished engineering manuscripts (Bradbury 1999) to philosophy journals (Bostrom 2003), and they are routinely conflated: transcension is glossed as aestivation, aestivation as hiding, black-hole computing as Dyson engineering. The conflations matter because the proposals make different and partly contradictory predictions, and because their falsifiability ranges from genuinely testable to unfalsifiable in principle. A review that sorts them is overdue on ordinary scholarly grounds, and newly practical for an observational one: the instruments now arriving (surveyed in Section 7) make several of the family's predictions testable for the first time, and the first dedicated tests are being proposed and conducted (Huang et al. 2026; Swanson 2026b).

1.3 Scope, method, and disclosure

We review proposals that satisfy two criteria: (i) they explain the Fermi observation primarily through the direction of mature technological development rather than through rarity, doom, or deliberate concealment; and (ii) they ground that direction in physics (specifically in the thermodynamics and physical limits of computation) rather than in strategic or sociological postulates. Criterion (ii) excludes the zoo hypothesis (Ball 1973), the dark-forest argument (Brin 1983; Yu 2015; Forgan 2017), and the sustainability solution (Haqq-Misra & Baum 2009), whose silence-mechanisms rest on game-theoretic or policy premises rather than physical limits; these are treated in Section 5 as the family's nearest competitors, because they predict the same silence from different premises and the comparison is instructive.

Disclosure. The author proposed the most recent member of this family, the Macro Transcension Hypothesis (Swanson 2026a, hereafter Paper A), and the observational campaign attached to it (Swanson 2026b, hereafter Paper C). A review by a participant carries an obvious hazard. We mitigate it mechanically: every hypothesis including the author's is run through the same comparative matrix (Table 1) and the same falsifiability scorecard (Table 2) with criteria stated in advance; the scorecard penalizes the failure modes a partisan would excuse; and the standing objections to the MTH are catalogued at the same depth as those to its relatives. Readers should apply their own discount.

1.4 Structure

Section 2 states the unifying physics with numbers. Section 3 reconstructs the family member by member. Section 4 presents the comparative matrix and the falsifiability scorecard. Section 5 contrasts the family with the sociological silence-solutions. Section 6 states the open problems. Section 7 maps the observational program, and Section 8 concludes.

2. The unifying physics, with numbers

2.1 The price of a bit: Landauer's principle

Erasing one bit of information in an environment at temperature T dissipates at least k_BT ln 2 (Landauer 1961): 2.9×10⁻²¹ J at room temperature, 2.6×10⁻²³ J against the cosmic microwave background (CMB) at T_γ = 2.725 K. The bound is no longer merely theoretical: it has been verified experimentally at the single-bit level (Bérut et al. 2012), and its statistical-mechanical foundations, though subtle, have survived modern scrutiny (Parrondo et al. 2015; Wolpert 2019). Logically reversible computation evades the bound for computation itself (Bennett 1973; Fredkin & Toffoli 1982; Bennett 1982; Frank 2002); but error correction, measurement, and any irreversible output commit the computer to erasure, so every long-running physical computation has a T-proportional operating cost. The cost of computation scales with the temperature of the sink receiving the waste entropy, and that observation generates the entire family of hypotheses reviewed here.

2.2 The speed and capacity of matter

Two further bounds fix the value of density. The Margolus–Levitin theorem caps state transitions at 2E/πħ ≈ 6×10³³ operations per second per joule of invested energy (Margolus & Levitin 1998); Lloyd's "ultimate laptop" analysis assigns one kilogram of matter ~5×10⁵⁰ ops s⁻¹, a figure that follows from the Margolus–Levitin bound on E = mc² at any density; compression toward black-hole density trades memory capacity against serial depth rather than raising the operation rate (Lloyd 2000). Information capacity obeys the Bekenstein bound (Bekenstein 1981) and is saturated only by black holes, whose Bekenstein–Hawking entropy (Bekenstein 1973; Hawking 1975) corresponds to ~10⁸⁶ bits for a 2×10⁴ M_☉ object — exceeding, by tens of orders of magnitude, any archive constructible from ordinary matter of the same mass. The holographic generalization (Bousso 2002) makes the point structural: the densest possible information storage in nature is a horizon. Both bounds reward the same move: compression. Faster computation and denser storage live at higher energy density, and the limiting objects are black holes.

2.3 The sink hierarchy

The third ingredient (and the one on which the family members genuinely diverge) is the menu of available entropy sinks (Figure 1). Today's ambient sink is the CMB at 2.725 K. The far future offers better: as the universe expands the CMB cools toward the de Sitter horizon temperature, T_dS ≈ 2.7×10⁻³⁰ K, the ultimate environmental floor (Krauss & Starkman 2000); waiting converts each joule into up to ~10³⁰ times more erasures (Sandberg et al. 2016), a strategy anticipated in Dyson's analysis of eternal computation in an open universe (Dyson 1979). But the present epoch already contains sinks colder than the far-future CMB of any reachable era: black-hole horizons. The Hawking temperature T_H ≈ 6.2×10⁻⁸ (M_☉/M) K is 3×10⁻¹² K for a 2×10⁴ M_☉ IMBH and 1.4×10⁻¹⁴ K for Sgr A* — twelve to fourteen orders of magnitude colder than the CMB (fourteen to sixteen orders below a 300 K biosphere in energy per erased bit), available now, no waiting required. Entropy dumped across a horizon increases its area in accordance with the generalized second law (Bekenstein 1974), at marginal cost set by the horizon's effective temperature rather than the sky's. One caveat is flagged here because it recurs against two family members below: treating T_H as the achievable marginal erasure cost is contested. A computer must radiate its waste heat at its own operating temperature toward the horizon, so the effective sink temperature seen by any realistic radiator sits far above T_H — the same class of objection Bennett, Hanson and Riedel (2019) raised against aestivation, and a standing objection to both the black-hole-computing thread and the MTH. The sink hierarchy is the family's real organizing axis: each hypothesis is a claim about which sink mature intelligence uses and when.

2.4 What the gradient does and does not establish

The gradient argument establishes a conditional: if an agent's terminal values reduce to maximizing long-term computation (or anything computation can purchase), then its resource allocation should flow toward density, spin, and cold sinks — inward. The premise is substantive and contestable: it is the same optimization-pressure postulate adopted, in different forms, by every member of the family (Smart 2012; Sandberg et al. 2016; Vidal 2014), and we treat objections to it in Section 6. What the gradient argument does not establish is uniqueness of endpoint — the family's internal disagreements (Section 3) are disagreements about where on the gradient the optimum sits, and several members locate it in places that the physics above does not obviously prefer. The comparative matrix exists to make those disagreements explicit.

3. The family, member by member

Figure 2 arranges the lineage; this section treats each member critically. We use a fixed template: claim, mechanism, predicted appearance of the sky, standing objections, current observational status.

3.1 Precursors: Dyson, Barrow, Bradbury, Bostrom

Four threads predate any explicit inward Fermi solution. Dyson (1960) made energy harvesting at stellar scale thinkable, and made waste heat the universal observable for everything that followed. Dyson (1979) then showed that computation in an open, cooling universe is in principle unbounded if clock rates slow with the ambient temperature: the first rigorous treatment of computation as the long-run currency of intelligence, later sharpened against Λ-dominated expansion, which imposes a finite total (Krauss & Starkman 2000). Barrow (1998) formalized the inward axis itself, ranking civilizations by mastery of ever-smaller scales as a complement to the Kardashev energy scale (Kardashev 1964). Bradbury (1999) engineered the first concrete inward artifact (the Matrioshka brain) and discovered its binding constraint: nested shells are limited by radiator area and sink temperature rather than energy supply, the constraint that later black-hole proposals dissolve. Two modern results now stand as objections to any Matrioshka-routed proposal: megastructure swarms require active station-keeping for stability (Wright 2020), and unmaintained swarms are destroyed by collisional cascades on short timescales (Lacki 2025), so Bradbury-lineage residues are conditional on continuous maintenance. Bostrom (2003), from philosophy, supplied the economic frame: every century of delayed colonization forgoes a fixed, irrecoverable harvest of free energy — an argument that cuts against waiting strategies and therefore sits in productive tension with aestivation, as Bennett et al. (2019) would later make rigorous.

3.2 Ćirković & Bradbury (2006): the migration hypothesis

Claim. Advanced civilizations are postbiological and migrate to the Galactic outskirts, where low temperatures favour efficient computing; SETI fails because it searches the wrong place (inner Galaxy, habitable zones) for the wrong thing (biological-era beacons) (Ćirković & Bradbury 2006).
Mechanism. The same Landauer logic as Section 2.3, applied radially across the Galaxy: ambient temperature falls with galactocentric radius.
Predicted sky. "City lights" at the rim: low-temperature infrared structures in the outer Galaxy; silence in the traditional SETI bands.
Objections. The radial temperature gradient is shallow — the CMB floor of 2.7 K applies everywhere, so the rim's advantage over the solar neighbourhood is a factor of order unity, not the 10¹²⁺ available from horizon sinks; and the predicted rim structures are only partly constrained by existing surveys: WISE-class sensitivity covers the ≳30 K end (Wright et al. 2014b; Griffith et al. 2015), while structures near the 3–10 K cold end peak at 300–1000 μm, outside WISE bands, and would need targeted far-infrared or submillimetre searches that have not been performed.
Status. Untested as stated, but its premise survives intact in every later member; historically the first explicit "wrong place, wrong observable" Fermi solution grounded in computation thermodynamics.

3.3 Smart (2012; 2019): the transcension hypothesis

Claim. The developmental trajectory of complexity ("STEM compression" of space, time, energy, and matter) carries every sufficiently advanced intelligence toward black-hole-density confinement and, ultimately, out of the observable universe; the silence is a developmental constant, not a contingency (Smart 2012, 2019).
Mechanism. An evolutionary-developmental analogy: as in ontogeny, the endpoint is convergent and encoded in the dynamics, with black-hole-scale density as the attractor.
Predicted sky. Near-total silence; Smart suggests transiting-companion signatures of inner-space civilizations as a long-shot observable.
Objections. The terminal step (exit from the universe) invokes physics that is unspecified and untestable in principle, the standing criticism that motivated the "macro" variant (Section 3.7); the developmental analogy supplies direction but no selection theory over real objects; and the hypothesis as stated cannot be killed by any feasible observation, the cardinal scientific objection.
Status. Unfalsifiable as stated (Table 2); enormously generative as a research program: nearly every subsequent member cites it as the frame.

3.4 Vidal (2011; 2014): stellivores

Claim. Some observed astrophysical systems (specifically accreting binaries) may already be advanced civilizations feeding on stars: "stellivores." The Fermi question is inverted: they are not absent, they are misclassified (Vidal 2011, 2014).
Mechanism. Energy logic of accretion (correct, and quantitatively the strongest in the family pre-2023), combined with a cybernetic reading of binary-system regulation.
Predicted sky. The X-ray-binary and cataclysmic-variable populations contain technological members distinguishable, if at all, by anomalous regulation statistics.
Objections. Standard accretion astrophysics explains the cited phenomenology without remainder, so the proposal carries an extreme parsimony burden; no discriminating statistic separating "fed" from "feeding" systems has been exhibited; and the claim's retreat to in-principle indistinguishability flirts with unfalsifiability.
Status. No proposed test has been carried out; the population-statistics route (regulation anomalies across accreting-binary catalogues) remains open and is the fair test (Section 7).

3.5 Sandberg, Armstrong & Ćirković (2016): aestivation, and the Bennett–Hanson–Riedel critique

Claim. Computation-maximizers harvest resources now but defer computation to the far future, when the CMB has cooled toward the de Sitter floor and each joule buys up to ~10³⁰ times more erasures; they are dormant now, hence silent (Sandberg et al. 2016).
Mechanism. The sink hierarchy of Section 2.3, exploited in time rather than in space.
Predicted sky. Quiet, resource-conserving infrastructure; suppressed stellar-disruption rates in controlled regions; possibly missing baryons curated against waste.
Objections. The decisive one is thermodynamic: Bennett et al. (2019) showed that free energy not collected promptly is irreversibly lost (stars burn regardless), so the optimal policy is aggressive present-epoch harvesting with storage, not dormancy: stored negentropy can be spent at any later epoch at that epoch's efficiency, so waiting-to-compute confers no advantage over harvest-now-compute-later, and storage itself costs only small maintenance losses. The critique has a second leg that matters for this review: Bennett et al. (2019) also observe that the present universe already contains vast reservoirs far from maximal entropy into which computation-generated entropy can be dumped, so the premise that erasure must be charged at the ambient CMB temperature fails today; that observation amounts to an endorsement of present-epoch cold sinks, which the horizon-sink thread (Sections 2.3 and 3.6) radicalizes to its physical extreme. The authors' premise that erasure cost dominates the budget is also model-dependent: reversible architectures shift the optimum toward computing early. Aestivation survives only in attenuated form (harvest now, archive-grade computation deferred), at which point its observational predictions collapse into those of its neighbours.
Status. The critique has not been rebutted in print, though Sandberg has responded informally; the hypothesis's lasting contributions are the explicit defence policy (which yields its only sharp falsifiable prediction: intervention against entropy-wasting astrophysical processes) and the formalization of the efficiency calculus that the whole family now uses.

3.6 Black holes as infrastructure: Inoue & Yokoo, Hsiao et al., Dvali & Osmanov

Claim(s). Black holes are the terminal power plants and computers: collectors around accreting supermassive holes (Inoue & Yokoo 2011); Dyson-type harvesting of disk luminosity with computed detectability (Hsiao et al. 2021); and black holes (including manufactured micro holes) as the most efficient quantum information processors, with a predicted high-energy neutrino signature from their evaporation (Dvali & Osmanov 2023).
Mechanism. Kerr accretion efficiency (5.7–42 per cent of rest mass; Bardeen, Press & Teukolsky 1972; Novikov & Thorne 1973), Blandford–Znajek spin extraction (Blandford & Znajek 1977; Tchekhovskoy et al. 2011), and horizon-saturated information physics.
Predicted sky. Hsiao et al. (2021): anomalous-SED point sources — a hot inner edge with missing outer disk. Dvali & Osmanov (2023): burst-mode TeV–PeV neutrinos from compact sky regions, a genuinely novel observable since natural sources do not readily mimic it. The micro-hole channel inherits a live controversy over whether radiation-collapse manufacturing is blocked by Schwinger pair production (Álvarez-Domínguez et al. 2024; Loeb 2024).
Objections. None of these works selects which black holes are preferred (the selection problem); the SMBH orientation of Inoue & Yokoo (2011) ignores the hazard and competition profile of galactic nuclei; and detectability claims assume the civilization tolerates conspicuous luminosity, in tension with the efficiency motives that brought it there. The contested-sink caveat of Section 2.3 applies at full strength: the horizon-temperature erasure cost these proposals invoke is an idealization that radiator physics may keep far out of reach.
Status. The Dvali & Osmanov (2023) neutrino channel is the family's most testable near-term prediction and is now the subject of a proposed dedicated monitoring program at a named target (Swanson 2026b); the high-energy SETI frame is surveyed by Lacki & DiKerby (2025), and the Dyson Minds workshop (Curtis et al. 2026) has since developed the observational agenda for the supermassive-host case, recommending anomaly-detection mining of WISE, JWST, and EHT archives. The distinction between voluntary and involuntary technosignatures (Osmanov 2024) is useful here: an efficiency-driven occupant emits no beacons, so every observable this thread predicts is an involuntary by-product, which is the class the residue catalogues of Sections 3.7 and 6 enumerate.

3.7 The Macro Transcension Hypothesis (2026)

Claim. The inward trajectory halts at a macroscopic, observable destination: rapidly spinning IMBHs embedded in dense, old, low-luminosity stellar systems, selected by a four-axis gradient (energy per unit fuel, entropy disposal, storage density, clock-rate control); migration and operation are electromagnetically quiet; Omega Centauri is the highest-ranked accessible target (Paper A).
Mechanism. The full Section-2 stack, plus a selection theory over real objects: intermediate mass (storage and erasure advantages saturate at 10^4–5 M_☉ while hazards grow), high spin (extractable energy), dense old clusters (fuel, stability, uncontested claim).
Predicted sky. Silence plus residues: anomalously high spin on gas-starved IMBHs; suppressed accretion relative to ambient supply; secular core depletion; possibly Dvali–Osmanov neutrino bursts. Six instrument-matched tests with pre-registered kill criteria (Paper A, Table 4; operationalized in Paper C).
Objections (catalogued at full strength, given the disclosure of Section 1): it inherits the optimization-pressure premise unargued (Section 6); the staged architecture presupposes goal stability over 10⁸ yr; the flagship target's IMBH is itself contested (≲6×10³ M_☉ timing bound versus ≥8.2×10³ M_☉ kinematic bound; Bañares-Hernández et al. 2025; Häberle et al. 2024); the class-level retreat (47 Tuc, M54, extragalactic nuclei) risks unfalsifiability-by-relocation, partially bound by the stated class prediction (no Galactic cluster IMBH with a_★ ≳ 0.9 falsifies it for the Milky Way); the contested-sink caveat of Section 2.3 applies to it as to the black-hole-computing thread, since its erasure-cost advantage assumes near-horizon entropy delivery that radiator physics may deny; the incomplete-compliance objection applies at the same strength as against dark forest, since the subset-form of its premise ("some civilizations optimize") pre-weakens what universal-form competitors are charged for; and the spin residue, its sharpest discriminator, waits on LISA (Babak et al. 2017; Colpi et al. 2024).
Status. Untested; distinguishable from its relatives chiefly by having purchased falsifiability with named targets, thresholds, and timelines — the property the scorecard (Section 4) is designed to price.

4. Comparative analysis

4.1 The matrix

Table 1 aligns the family on five axes. Reading across rows reveals the internal disagreements that casual conflation hides: aestivation and transcension disagree about when (future versus now); transcension and the MTH disagree about where (fabricated density versus existing holes); stellivores and everyone else disagree about visibility (already visible versus invisible); and the black-hole-computing thread disagrees with the MTH about which holes (supermassive and manufactured-micro versus intermediate).

4.2 The falsifiability scorecard

Table 2 grades each member against five criteria, fixed before grading: C1 — names a target or target class that observations can interrogate; C2 — states quantitative observables with thresholds; C3 — at least one stated test is executable with existing or funded instruments within ~15 years; C4 — specifies kill conditions in advance (results the proponents agree would refute it); C5 — has survived at least one executed test that could have refuted it, with partial credit (⊙) where executed adjacent tests constrain the hypothesis without having been designed for it. Symbols: ● met, ⊙ partially met, ○ not met. Two caveats. First, C5 is the column that separates science-in-progress from science-performed: no member of the family scores ● on C5, because no pre-registered inward-family test has yet been executed to completion — the FAST globular-cluster survey (Huang et al. 2026) and the proposed ω Cen campaign (Swanson 2026b) are the first entries in that ledger. The partial credits record real constraint from tests executed for other reasons: the migration hypothesis is bounded at its warm end by the Ĝ surveys (Wright et al. 2014b; Griffith et al. 2015); the collector variants of black-hole computing are bounded by Hephaistos-class Dyson searches (Suazo et al. 2024; Ren et al. 2025); and the accretion-limit measurements at ω Cen (Haggard et al. 2013; Chen et al. 2025; Mahida et al. 2026) already execute part of the MTH's suppressed-accretion test series. Second, the MTH's strong showing on C1–C4 was purchased after the criteria of falsifiability had been articulated by its predecessors' critics; later entrants always grade better on form, and the scorecard measures testability, not probability of truth. A hypothesis can be admirably falsifiable and false, or unfalsifiable and, unverifiably, true.

Legend: ● criterion met  ·  ⊙ criterion partially met  ·  ○ criterion not met.

Three scorecard judgements require defence. Aestivation's ● on C4 reflects Sandberg et al. (2016)'s explicit and admirable statement of refuting observations (uncontrolled entropy-wasting processes in regions a defender should police); its ○ on C3 reflects that no instrument can audit process-suppression statistics at the required scale on any near-term horizon. Transcension's row of ○ is not a dismissal of its generativity (half the family descends from it) but the direct application of the criteria to a hypothesis whose endpoint is defined as unobservable; Smart (2019) adds developmental argument, not observables. BH computing's ● on C3 is carried entirely by the Dvali & Osmanov (2023) neutrino channel, which named an energy range, a temporal structure, and a source class that current telescopes can monitor (KM3NeT Collaboration 2025; IceCube Collaboration 2020). Two disclosures about the MTH row, since the author graded his own hypothesis. Its ● on C3 rests specifically on LISA being an adopted, funded mission inside the ~15-yr window, though a launch near 2035 sits at the window's edge and carries schedule risk — the same principled reading under which aestivation earns ○ for lacking any equivalent funded instrument. And its C2–C4 grades rest on Paper A's kill conditions, restated here so they are auditable in this paper: a LISA spin measurement of a_★ < 0.1 on the ω Cen object, resolution of the mass tension in favour of an extended remnant population rather than a point mass, and detection of ordinary flux-eruption accretion variability from a fueled hole; Paper A's Table 4 carries the full six with thresholds.

5. The sociological competitors

Three well-known solutions predict the same silent sky from non-thermodynamic premises, and the family is best understood against them. A fourth class, the expansion-slowing models, sits between the sociological solutions and the inward family and is treated last.

Zoo (Ball 1973) and its hegemony refinement (Forgan 2017). Silence as policy: we are observed but quarantined. Forgan's contribution is structural — a zoo requires either a single dominant culture or stable inter-civilization consensus ("Galactic Club"), and fragmented "cliques" should leak defectors. The premise is sociological uniformity over megayear timescales, the premise the inward family replaces with physics.

Dark forest (Brin 1983; Yu 2015). Silence as survival strategy under first-strike game theory; the name comes from Liu Cixin's 2008 novel, and the academic formulation is due to Yu (2015), with Brin (1983) as the earlier "deadly probes" antecedent. Its physical premises are at least defensible (relativistic kill vehicles, asymmetric detection), though the kill-vehicle and targeting literature is disputed on interstellar targeting accuracy, light-lag, and deterrence grounds, and its deadly-probes mechanism has received technical treatment: Forgan (2019b) models mutated "predator" probes suppressing a visible probe population with Lotka–Volterra dynamics and finds that stable equilibria typically retain large prey populations, undercutting the mechanism as a full silence-explanation. The equilibrium is also fragile: the strategy must be adopted by every civilization without exception and without deviation forever, since a single loud defector (or a single expansionist willing to absorb risk) breaks the explanatory account (Brin 1983; Forgan 2017). Like the zoo, it predicts pure silence, with no residue and no scheduled adjudication.

Sustainability (Haqq-Misra & Baum 2009). Exponential expansion is self-terminating; civilizations that persist are those that grow slowly or not at all. This is the mildest competitor, and partially complementary: it removes the pressure to expand without supplying a positive account of where mature capability is directed.

Expansion-slowing models (Landis 1998; Carroll-Nellenback et al. 2019). A distinct class predicts partial silence from the dynamics of expansion itself rather than from psychology or policy. Landis (1998) models colonization as a percolation process: with finite settlement probabilities and range limits, expansion fronts can stall and leave permanent unvisited voids, one of which could contain the Sun. Carroll-Nellenback et al. (2019) couple settlement fronts to stellar motions and planetary habitability windows and find steady states in which the Galaxy is substantially but incompletely settled, with resettlement lag producing local voids (the "Aurora effect"). These are the inward family's closest rivals among silence-explanations, closer than the zoo or the dark forest, because they are grounded in physics and dynamics rather than in universally adopted strategy, and they are falsifiable in population statistics (void geometry, settlement-front morphology, the age distribution of settled systems). They differ from the inward family in what they explain: they account for local emptiness while leaving expansion as the governing drive, whereas the inward family removes the drive itself. The two are compatible, and a mixed population would show both signatures.

The comparison clarifies what the inward family is actually claiming. All these proposals (sociological and thermodynamic alike) break the expansionist premise. The differences are two. Robustness: dark-forest and zoo equilibria are fragile to a single defector among all civilizations across all time, whereas the inward account requires no coordination at all — each civilization independently follows its own resource gradient, and partial compliance merely thins the loud population rather than collapsing the explanation (a point made quantitative in the grabby-aliens frame; Hanson et al. 2021; see Section 6). Residue: fear and policy predict nothing observable, by design; thermodynamic optimization is a physical process operating on physical objects, and physical processes leave traces — spin histories, depletion statistics, suppressed accretion, burst channels. On both counts the expansion-slowing models score with the inward family rather than with the sociological solutions: they need no coordination and they predict population-level statistics that surveys can test. Among the silence-explanations, these two classes form the falsifiable-in-principle wing, and grading its members' falsifiability in practice (Table 2) is therefore where a review can add value.

6. Open problems

We state the family's unsolved problems as research questions rather than objections, since several are tractable.

P1: The optimization premise. Every member assumes some civilizations converge on maximizing computation (or a correlate). The premise is defended by universality arguments, computation is the currency into which most terminal goals are convertible (Smart 2012; Sandberg et al. 2016; Bostrom 2003), but it has never been given a selection-theoretic foundation: under what competitive or evolutionary dynamics do computation-maximizers come to exist, persist, and dominate the behaviour of their lineage? The grabby-aliens machinery (Hanson et al. 2021; Olson 2015) could be adapted to model mixed populations (maximizers, expansionists, satisficers) and derive the observable consequences of each mixture; nobody has done it.

P2: Goal stability over 10⁸ years. Inward strategies require coherent purpose across timescales exceeding mammalian evolutionary history — flagged as the largest unargued premise even by the family's own proponents (Paper A). The mitigation argument (front-loaded payoffs leave permanent residues even if the lineage fragments mid-program) rescues the observational program but not the hypothesis's interior logic. A theory of institution-or-value stability under self-modification is the family's deepest dependency, and it is not an astronomy problem.

P3: Migration economics. Bostrom (2003)'s opportunity-cost argument prices delay; Bennett et al. (2019) price dormancy; nobody has priced relocation. The trade is concrete: a migrating lineage abandons its accumulated local infrastructure for transit times of 10^4–6 yr in exchange for a destination whose advantages (Section 2) are enormous but deferred. Under what discount rates, risk models, and replication strategies does migration dominate local densification (Matrioshka-style; Bradbury 1999)? This is a well-posed optimization problem awaiting treatment.

P4: Incomplete compliance. If migration is optional, the loud expansionists we do not see still need explaining. The defensible position (argued in Paper A) is that inward migration thins the expected loud population by removing its most capable members, sharpening rather than dissolving the paradox; full dissolution requires either high compliance or supplementary rarity from conventional Drake factors. Quantifying "how much thinning suffices" against the Hanson et al. (2021) selection effect is open problem P1's observational face.

P5: Degeneracy of silence. H₀-type explanations (gas starvation, rarity) and inward-family explanations predict identical electromagnetic appearances; only residues discriminate. The family therefore lives or dies on the residue catalogue (spin, depletion, bursts, regulation statistics) and enlarging that catalogue with new discriminators is the highest-leverage theoretical work available. A companion paper takes a first step by forward-modelling two such discriminators for engineered IMBH systems, a waste-heat floor and an accretion-regulation statistic, within a hierarchical Bayesian adjudication framework (Swanson 2026c). The contested micro-black-hole manufacturing channel (Álvarez-Domínguez et al. 2024; Loeb 2024) is a case in point: it would be a sharp discriminator if the manufacturing route is physical, and it drops out of the catalogue if Schwinger pair production blocks it.

7. The observational program

The family is no longer untestable in practice; Table 3 maps each member to its executable tests. Four clusters of activity dominate.

(i) Globular clusters as the family's natural laboratory. Old, dense, dynamically relaxed, electromagnetically quiet, and, for the IMBH-hosting subset, equipped with the objects the gradient selects. The niche was identified by Di Stefano & Ray (2016), who argued that globular clusters are "sweet spots" for long-lived civilizations on habitability and stellar-density grounds and proposed them as SETI targets; the first dedicated globular-cluster survey (Huang et al. 2026) operationalized it a decade later. The habitability argument is independent of and complementary to the thermodynamic-gradient argument of Section 2: the two select the same targets for different reasons. The ω Cen multi-messenger campaign (Paper C) is the first program designed end-to-end around inward-family falsification, with the conventional astrophysics (the IMBH mass tension; Häberle et al. 2024; Bañares-Hernández et al. 2025) guaranteeing scientific value under every outcome. That tension remains live: the latest joint MeerKAT and Parkes timing of the cluster's millisecond pulsars (2021–2025) is insensitive to an IMBH in the contested 10³–10⁴ M_☉ range and bounds the central mass only from above, at <10⁵ M_☉ at 90% confidence (Colomí i Bernadich et al. 2026), so the timing–kinematics discrepancy awaits deeper data. The cluster's compact-object inventory has meanwhile begun to resolve directly: 23 years of HST and JWST astrometry have yielded the first stellar-mass black hole detected astrometrically in any globular cluster, the 4.46 M_☉ oMEGACat BH-2 in a 94-yr binary (Whitaker et al. 2026).

(ii) The spin channel. Among all residues, black-hole spin is unique: permanent, quantitative, and measurable to ~10⁻³ precision by LISA for favourable systems (Babak et al. 2017; Amaro-Seoane 2018; Colpi et al. 2024). The precision figure applies to detected EMRIs, and the channel therefore depends on event rate: a compact-object inspiral must occur in-band during the mission, and the per-cluster rate for a ~10⁴ M_☉ hole is uncertain by orders of magnitude, which is the spin channel's weak link and part of the P5 accounting. A near-extremal spin on a demonstrably gas-starved IMBH has no comfortable natural history; a low spin retires the exploitation models outright. No other proposed technosignature combines this permanence with a funded instrument.

(iii) The neutrino channel. The Dvali & Osmanov (2023) burst phenomenology is testable now at named targets with operating instruments (KM3NeT Collaboration 2025; Albert et al. 2020); the pre-registered multiplet criteria are specified in Paper C. A decade-scale null closes the channel at its stated strength.

(iv) Population statistics over single objects. For stellivores and attenuated aestivation, the executable tests are statistical: regulation anomalies across binary catalogues; suppression statistics across old systems. These reuse public archives, cost salaries only, and would give the family's two least-tested members their first empirical constraints.

The program's unifying property: every test doubles as conventional astrophysics (IMBH demographics, accretion physics, binary populations, cluster dynamics) so the field can pursue the family's adjudication without betting careers on its truth. The pattern is inherited from the waste-heat surveys (Wright et al. 2014a; Griffith et al. 2015) and from the falsificationist turn of Sandberg et al. (2016) and Dvali & Osmanov (2023), and it does not depend on the fate of any individual hypothesis.

8. Conclusion

Reviewed as a family, the inward hypotheses are neither the fringe they are sometimes taken for nor the single interchangeable idea they are often glossed as. They form a six-decade research lineage with a common physical premise (the thermodynamics of computation as the lens for predicting mature intelligence) and genuine internal disagreements about timing, destination, and visibility that observation can, increasingly, adjudicate. The family's intellectual trajectory is itself instructive: from direction without destination (Ćirković & Bradbury), through destination without testability (transcension), through testability purchased by critique (aestivation and its Bennett–Hanson–Riedel correction), to the current generation's explicit targets, thresholds, and kill criteria, with each advance forced by the previous generation's critics.

Against the sociological silence-solutions, the family's distinguishing asset is residue: physics leaves traces where policy does not. Its distinguishing liability is the optimization premise, which remains unargued at the selection-theoretic level where it would need to be argued (open problem P1). Between asset and liability sits the practical situation of 2026: for the first time, members of this family have pre-registered tests scheduled on funded instruments, anchored by the globular-cluster surveys now beginning and by LISA's spin measurements a decade out. Within fifteen years the scorecard's empty C5 column will have entries. Whatever they say, the column will no longer be empty — which is, for a literature long accused of unfalsifiability, the result that matters most.

Acknowledgements and disclosure

The author thanks the maintainers of the NASA Astrophysics Data System and arXiv, on which the citation verification for this work relied. The author's conflict of interest as proponent of one reviewed hypothesis is disclosed in Section 1 and mitigated by the fixed-criteria comparative method of Section 4. AI assistance disclosure: drafting, citation verification, derivation checking, and figure preparation for this manuscript were performed with substantial assistance from a large language model (Claude, Anthropic), under the author's direction; the author reviewed and takes full responsibility for all claims, derivations, and references. Interactive calculators implementing the quantitative material in Section 2 are available at omegacentauri.me.

Data availability

No new observational data were generated for this work. All quantitative claims derive from the cited literature.