About

Dillon Brout is an Assistant Professor in the Department of Astronomy at Boston University. His research interests include cosmology, with a focus on cosmological distance indicators, Type Ia supernovae, dark energy, dark matter, and the Hubble Constant. He is engaged in advancing understanding of the universe's expansion and the fundamental components that influence cosmic evolution.

Research topics

• Astrophysics
• Physics
• Astronomy
• Mathematical physics
• Statistics

Selected publications

• SNDATA_ROOT for SNANA software

  Zenodo (CERN European Organization for Nuclear Research) · 2026-04-10

  datasetOpen access

  Environment for the Supernova Analysis software package (https://github.com/RickKessler/SNANA). This environment Includes public light curve data sets, filter transmissions, primary SEDs, calibration files, models for SNIa & CC, cadence and host-galaxy libraries for simulations, etc ...

  PublisherDOI

• Dark Energy Survey: Implications for cosmological expansion models from the final DES baryon acoustic oscillation and supernova data

  Physical review. D/Physical review. D. · 2026-01-30 · 6 citations

  article

  International audience

  PublisherDOI

• The Complete Sample of Available SNe Ia Luminosity Calibrations from the TRGB Observed with either HST or JWST

  ArXiv.org · 2025-04-11

  preprintOpen access

  Distance ladders which calibrate the luminosity of Type Ia supernovae (SNe Ia) currently provide the strongest constraints on the local value of H0. Recent studies from the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) show good consistency between measurements of SNe Ia host distances. These are calibrated to NGC 4258 using different primary distance indicators (Cepheids, Tip of the Red Giant Branch (TRGB), J-region Asymptotic Giant Branch, and Miras). However, some sub-samples of calibrated SNe Ia employed to measure H0 yield noteworthy differences due to small sample statistics but also due to differences in sample selection. This issue is particularly important for TRGB-derived calibrations owing to the smaller volume they reach compared to Cepheids, reducing sample size and enhancing the size of statistical fluctuations. To mitigate this issue, we compile the largest and complete (as currently available) sample of HST or JWST measurements of the TRGB in the hosts of normal SNe Ia for a total of N=35, 50% larger than the previous largest. Most are present in the literature, and we compile multiple measures when available. We also add 5 SNe Ia hosts from the HST archive not previously published. The full sample together with the Pantheon+ SN catalog gives H0=72.1-73.3 +/- 1.8 km/s/Mpc (depending on methodology), in good agreement with the value of 72.5 +/- 1.5 km/s/Mpc from HST Cepheids in hosts of 42 SNe Ia calibrated by the same anchor, NGC 4258. We trace the difference in the result of H0=70.4 +/- 1.9 km/s/Mpc from Freedman et al. 2025 to 11 hosts not selected for that CCHP compilation (of N=24) which alone yield H0=74.1 km/s/Mpc, 2$σ$ higher than the selected sample. A smaller increase of 0.6 km/s/Mpc comes from a commonly employed correction for peculiar velocities.

  PublisherOA PDFDOI

• The Dark Energy Bedrock All-Sky Supernova Program: Cross Calibration, Simulations, and Cosmology Forecasts

  ArXiv.org · 2025-08-14

  preprintOpen access

  International audience

  PublisherOA PDFDOI

• It's not $σ_8$ : constraining the non-linear matter power spectrum with the Dark Energy Survey Year-5 supernova sample

  ArXiv.org · 2025-01-31

  preprintOpen access

  The weak gravitational lensing magnification of Type Ia supernovae (SNe Ia) is sensitive to the matter power spectrum on scales $k>1 h$ Mpc$^{-1}$, making it unwise to interpret SNe Ia lensing in terms of power on linear scales. We compute the probability density function of SNe Ia magnification as a function of standard cosmological parameters, plus an empirical parameter $A_{\rm mod}$ which describes the suppression or enhancement of matter power on non-linear scales compared to a cold dark matter only model. While baryons are expected to enhance power on the scales relevant to SN Ia lensing, other physics such as neutrino masses or non-standard dark matter may suppress power. Using the Dark Energy Survey Year-5 sample, we find $A_{\rm mod} = 0.77^{+0.69}_{-0.40}$ (68\% credible interval around the median). Although the median is consistent with unity there are hints of power suppression, with $A_{\rm mod} < 1.09$ at 68\% credibility.

  PublisherOA PDFDOI

• Uniting the Observed Dynamical Dark Energy Preference with the Discrepancies in Ω_<i>m</i> and <i>H</i> ₀ across Cosmological Probes

  The Astrophysical Journal Letters · 2025-04-09 · 15 citations

  articleOpen access

  Abstract Recent results from Type Ia supernovae, baryon acoustic oscillations (BAOs), and the cosmic microwave background (CMB) indicate (1) potentially discrepant measurements of the matter density Ω m and Hubble constant H 0 in the ΛCDM model when analyzed individually and (2) hint of dynamical dark energy in a w 0 w a CDM model when data are combined in a joint analysis. We examine whether underlying dynamical dark energy cosmologies favored by data would result in biases in Ω m and H 0 for each probe when analyzed individually under ΛCDM. We generate mock data sets in w 0 w a CDM cosmologies, fit the individual probes under the ΛCDM model, and find that expected biases in Ω m are ∼0.03. Notably, the Ω m differences between probes are consistent with values observed in real data sets. We also observe that mock DESI-BAO data sets generated in the w 0 w a CDM cosmologies will lead to a biased measurement of H 0 higher by ∼1.2 km s −1 Mpc −1 when fitted under ΛCDM, appearing to mildly improve the Hubble tension, but as the true underlying H 0 is lower, the tension is in fact worsened. We find that the Ω m discrepancies, the high BAO H 0 relative to the CMB, and the joint dynamical dark energy signal are all related effects that could be explained simultaneously with either new physics or new systematics. While it is possible to unite many of the discrepancies seen in recent analyses along a single axis, our results underscore the importance of understanding systematic differences in data sets, as they have unique impacts in different cosmological parameter spaces.

  PublisherDOI

• Comparing the DES-SN5YR and Pantheon+ SN cosmology analyses: investigation based on ‘evolving dark energy or supernovae systematics’?

  Monthly Notices of the Royal Astronomical Society · 2025-07-07 · 26 citations

  articleOpen access

  ABSTRACT Recent cosmological analyses measuring distances of type Ia supernovae (SNe Ia) and baryon acoustic oscillations (BAO) have all given similar hints at time-evolving dark energy. To examine whether underestimated SN Ia systematics might be driving these results, Efstathiou (2025) compared overlapping SN events between Pantheon+ and DES-SN5YR (20 per cent SNe are in common), and reported evidence for an $\sim$0.04 mag offset between the low- and high-redshift distance measurements of this subsample of events. If this offset is arbitrarily subtracted from the entire DES-SN5YR sample, the preference for evolving dark energy is reduced. In this paper, we show that this offset is mostly due to different corrections for Malmquist bias between the two samples; therefore, an object-to-object comparison can be misleading. Malmquist bias corrections differ between the two analyses for several reasons. First, DES-SN5YR used an improved model of SN Ia luminosity scatter compared to Pantheon+ but the associated scatter-model uncertainties are included in the error budget. Secondly, improvements in host mass estimates in DES-SN5YR also affected SN standardized magnitudes and their bias corrections. Thirdly, and most importantly, the selection functions of the two compilations are significantly different, hence the inferred Malmquist bias corrections. Even if the original scatter model and host properties from Pantheon+ are used instead, the evidence for evolving dark energy from CMB, DESI BAO Year 1 and DES-SN5YR is only reduced from 3.9$\sigma$ to 3.3$\sigma$, consistent with the error budget. Finally, in this investigation, we identify an underestimated systematic uncertainty related to host galaxy property uncertainties, which could increase the final DES-SN5YR error budget by 3 per cent. In conclusion, we confirm the validity of the published DES-SN5YR results.

  PublisherOA PDFDOI

• The Hourglass Simulation: A Catalog for the Roman High-Latitude Time-Domain Core Community Survey

  ArXiv.org · 2025-06-05

  preprintOpen access

  We present a simulation of the time-domain catalog for the Nancy Grace Roman Space Telescope's High-Latitude Time-Domain Core Community Survey. This simulation, called the Hourglass simulation, uses the most up-to-date spectral energy distribution models and rate measurements for ten extra-galactic time-domain sources. We simulate these models through the design reference Roman Space Telescope survey: four filters per tier, a five day cadence, over two years, a wide tier of 19 deg$^2$ and a deep tier of 4.2 deg$^2$, with $\sim$20% of those areas also covered with prism observations. We find that a science-independent Roman time-domain catalog, assuming a S/N at max of &gt;5, would have approximately 21,000 Type Ia supernovae, 40,000 core-collapse supernovae, around 70 superluminous supernovae, $\sim$35 tidal disruption events, 3 kilonovae, and possibly pair-instability supernovae. In total, Hourglass has over 64,000 transient objects, 11 million photometric observations, and 500,000 spectra. Additionally, Hourglass is a useful data set to train machine learning classification algorithms. We show that SCONE is able to photometrically classify Type Ia supernovae with high precision ($\sim$95%) to a z &gt; 2. Finally, we present the first realistic simulations of non-Type Ia supernovae spectral-time series data from Roman's prism.

  PublisherOA PDFDOI

• The Hubble Tension in Our Own Backyard: DESI and the Nearness of the Coma Cluster

  The Astrophysical Journal Letters · 2025-01-15 · 40 citations

  articleOpen access

  Abstract The Dark Energy Spectroscopic Instrument (DESI) collaboration measured a tight relation between the Hubble constant ( H 0 ) and the distance to the Coma cluster using the fundamental plane (FP) relation of the deepest, most homogeneous sample of early-type galaxies. To determine H 0 , we measure the distance to Coma by several independent routes, each with its own geometric reference. We measure the most precise distance to Coma from 13 Type Ia supernovae (SNe Ia) in the cluster with a mean standardized brightness of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>m</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>15.710</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.040</mml:mn> </mml:math> mag. Calibrating the absolute magnitude of SNe Ia with the Hubble Space Telescope (HST) distance ladder yields D Coma = 98.5 ± 2.2 Mpc, consistent with its canonical value of 95–100 Mpc. This distance results in H 0 = 76.5 ± 2.2 km s −1 Mpc −1 from the DESI FP relation. Inverting the DESI relation by calibrating it instead to the Planck+ΛCDM value of H 0 = 67.4 km s −1 Mpc −1 implies a much greater distance to Coma, D Coma = 111.8 ± 1.8 Mpc, 4.6 σ beyond a joint, direct measure. Independent of SNe Ia, the HST Key Project FP relation as calibrated by Cepheids, the tip of the red giant branch from JWST, or HST near-infrared surface brightness fluctuations all yield D Coma &lt; 100 Mpc, in joint tension themselves with the Planck-calibrated route at &gt;3 σ . From a broad array of distance estimates compiled back to 1990, it is hard to see how Coma could be located as far as the Planck+ΛCDM expectation of &gt;110 Mpc. By extending the Hubble diagram to Coma, a well-studied location in our own backyard whose distance was in good accord well before the Hubble tension, DESI indicates a more pervasive conflict between our knowledge of local distances and cosmological expectations. We expect future programs to refine the distance to Coma and nearer clusters to help illuminate this new local window on the Hubble tension.

  PublisherDOI

• The Hourglass Simulation: A Catalog for the Roman High-latitude Time-domain Core Community Survey

  The Astrophysical Journal · 2025-07-15 · 4 citations

  articleOpen accessCorresponding

  Abstract We present a simulation of the time-domain catalog for the Nancy Grace Roman Space Telescope’s High-Latitude Time-Domain Core Community Survey. This simulation, called the Hourglass simulation, uses the most up-to-date spectral energy distribution models and rate measurements for 10 extragalactic time-domain sources. We simulate these models through the design reference Roman Space Telescope survey: four filters per tier, a five-day cadence, over 2 yr, a wide tier of 19 deg 2 , and a deep tier of 4.2 deg 2 , with ∼20% of those areas also covered with prism observations. We find that a science-independent Roman time-domain catalog, assuming a signal-to-noise ratio at a max of &gt;5, would have approximately 21,000 Type Ia supernovae, 40,000 core-collapse supernovae, around 70 superluminous supernovae, ∼35 tidal disruption events, three kilonovae, and possibly pair-instability supernovae. In total, Hourglass has over 64,000 transient objects, 11,000,000 photometric observations, and 500,000 spectra. Additionally, Hourglass is a useful data set to train machine learning classification algorithms. We show that SCONE is able to photometrically classify Type Ia supernovae with high precision (∼95%) to a z &gt; 2. Finally, we present the first realistic simulations of non-Type Ia supernovae spectral time series data from Roman’s prism.

  PublisherDOI

Frequent coauthors

• D. Scolnic

  173 shared

• E. Bertin

  Orange (France)

  163 shared

• D. Gruen

  159 shared

• L. N. da Costa

  Laboratório Interinstitucional de e-Astronomia

  158 shared

• A. Carnero Rosell

  155 shared

• M. Smith

  136 shared

• D. L. Burke

  134 shared

• K. Kuehn

  Netherlands Institute for Radio Astronomy

  133 shared