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Peer-reviewedCC BY 4.0Monthly Notices of the Royal Astronomical Society

Forward-modelling the Tolman and distance-duality tests with IllustrisTNG

  • Harry Desmond1
  • Tariq Yasin2
  • Richard Stiskalek2
  • Sebastian von Hausegger3
  1. 1Institute of Cosmology, and Gravitation, University of Portsmouth , Portsmouth PO1 3FX ,
  2. 2Astrophysics, University of Oxford, Denys Wilkinson Building , Keble Road, Oxford, OX1 3RH ,
  3. 3Rudolf Peierls Centre for Theoretical Physics , University of Oxford, Parks Road, Oxford OX1 3PU ,
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Abstract

The Tolman surface-brightness test and the angular-size distance-duality test are two complementary probes of the same underlying relation between luminosity and angular-diameter distance, DL = (1 + z)2DA, as holds in any metric theory of gravity where photon number is conserved. Both tests have recently delivered a priori surprising signals: JWST/ASTRODEEP measurements yield a surface brightness scaling with redshift much flatter than the expected value, and ultracompact radio sources also appear to follow a flatter DL/DA scaling with redshift. These results have been suggested to support non-expanding cosmologies, however they are also sensitive to astrophysical and instrumental effects. We test whether these results indicate genuine departures from standard cosmology by forward-modelling observed surface-brightness evolution in the IllustrisTNG cosmological hydrodynamical simulation, with an empirical mock-spectroscopic selection trained on ASTRODEEP. We show that the astrophysical evolution relevant for both tests may be effectively parametrised as a single power-law exponent for the luminosity density as a function of redshift, for which the simulation gives γ = 2.23 ± 0.20 across realistic aperture conventions. This value is approximately sufficient to explain both the Tolman and distance-duality signals within standard cosmology and galaxy formation physics, with a small discrepancy for the latter suggesting that radio AGN evolve slightly more strongly than bright galaxies.

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Cite

BibTeXRIS
BibTeX
@article{desmond2026forwardmodelling,
  title = {{Forward-modelling the Tolman and distance-duality tests with IllustrisTNG}},
  author = {Harry Desmond and Tariq Yasin and Richard Stiskalek and Sebastian von Hausegger},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = nov,
  doi = {10.1093/mnras/stag2124},
  url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2124/9013654},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Forward-modelling the Tolman and distance-duality tests with IllustrisTNG
AU  - Harry Desmond
AU  - Tariq Yasin
AU  - Richard Stiskalek
AU  - Sebastian von Hausegger
PY  - 2026
DA  - 2026/11/05
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag2124
UR  - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2124/9013654
AB  - The Tolman surface-brightness test and the angular-size distance-duality test are two complementary probes of the same underlying relation between luminosity and angular-diameter distance, DL = (1 + z)2DA, as holds in any metric theory of gravity where photon number is conserved. Both tests have recently delivered a priori surprising signals: JWST/ASTRODEEP measurements yield a surface brightness scaling with redshift much flatter than the expected value, and ultracompact radio sources also appear to follow a flatter DL/DA scaling with redshift. These results have been suggested to support non-expanding cosmologies, however they are also sensitive to astrophysical and instrumental effects. We test whether these results indicate genuine departures from standard cosmology by forward-modelling observed surface-brightness evolution in the IllustrisTNG cosmological hydrodynamical simulation, with an empirical mock-spectroscopic selection trained on ASTRODEEP. We show that the astrophysical evolution relevant for both tests may be effectively parametrised as a single power-law exponent for the luminosity density as a function of redshift, for which the simulation gives γ = 2.23 ± 0.20 across realistic aperture conventions. This value is approximately sufficient to explain both the Tolman and distance-duality signals within standard cosmology and galaxy formation physics, with a small discrepancy for the latter suggesting that radio AGN evolve slightly more strongly than bright galaxies.
SN  - 0035-8711
SN  - 1365-2966
ER  -