About

Nils Halverson develops millimeter-wavelength instrumentation to study the origins of the universe through observations of the Cosmic Microwave Background (CMB). His research focuses on using CMB observations to gather evidence for gravitational waves released by an early period of inflation, as well as constraining neutrino mass and understanding the growth of structure and dark energy. He is a co-investigator on two ground-based CMB experiments: the South Pole Telescope (SPT) and Polarbear/Simons Array. Additionally, he is a Co-Investigator on a NASA Technology Development Grant aimed at advancing focal plane technology for next-generation CMB space missions.

Research topics

• Physics
• Astronomy
• Astrophysics
• Optics
• Remote sensing
• Computer Science
• Particle physics
• Geology
• Geography
• Statistics
• Mathematics
• Algorithm
• Quantum mechanics

Selected publications

• Millimeter-wave observations of Euclid Deep Field South using the South Pole Telescope

  Astronomy and Astrophysics · 2026-01-29 · 2 citations

  preprintOpen access

  Context . The South Pole Telescope third-generation camera (SPT-3G) has observed over 10 000 square degrees of sky at 95, 150, and 220 GHz (3.3, 2.0, 1.4 mm, respectively) and will significantly overlap the ongoing 14 000 square-degree Euclid Wide Survey. The Euclid collaboration recently released Euclid Deep Field South (EDF-S) observations of 23 square degrees at wide field depths in the first quick data release (Q1). Aims . With the goal of releasing complementary millimeter-wave data and encouraging legacy science, we performed dedicated observations of a 57-square-degree field overlapping the EDF-S. Methods . The observing time totaled 20 days, and we reached noise depths of 4.3, 3.8, and 13.2 μK-arcmin at 95, 150, and 220 GHz, respectively. Results . In this work we present the temperature maps and two catalogs constructed from these data. The emissive source catalog contains 601 objects (334 inside EDF-S) with 54% synchrotron-dominated sources and 46% thermal dust emission-dominated sources. The 5σ detection thresholds are 1.7, 2.0, and 6.5 mJy in the three bands. The cluster catalog contains 217 cluster candidates (121 inside EDF-S) with median mass M 500c = 2.12 × 10 14 M ⊙ /h 70 and median redshift z = 0.70, corresponding to an order-of-magnitude improvement in cluster density over previous tSZ-selected catalogs in this region (3.81 clusters per square degree). Conclusions . The overlap between SPT and Euclid data will enable a range of multiwavelength studies of the aforementioned source populations. This work serves as the first step toward joint projects between SPT and Euclid and provides a rich dataset containing information on galaxies, clusters, and their environments.

  PublisherDOI

• High-Frequency Silicon Platelet Feedhorns for <i>LiteBIRD</i>

  IEEE Transactions on Applied Superconductivity · 2026-01-01

  article

  <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LiteBIRD</i> is a planned space mission to observe the polarized cosmic microwave background in the frequency range 34 GHz to 448 GHz, with the goal of measuring or constraining the tensor-to-scalar ratio <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i>. We have developed silicon platelet feedhorns for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LiteBIRD</i>, consisting of stacked metalized silicon wafers with deep-etched holes to create an array of feedhorns with profiled tapers and a coefficient of thermal contraction that is matched to the detector wafer. We present beam pattern measurements for silicon platelet feedhorn prototypes designed for 166-322 GHz, 200-388 GHz, and 356-448 GHz, corresponding to each of the planned <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LiteBIRD</i> high-frequency pixel types. The measurements were carried out on a room-temperature millimeter-wave vector network analyzer and are in excellent agreement with simulation.

  PublisherDOI

• SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G main field

  Physical review. D/Physical review. D. · 2026-01-05 · 10 citations

  preprintOpen access

  Plenary talk presented at the XXI International Workshop on Neutrino Telescopes - Padova 29 September - 3 October 2025 (https://agenda.infn.it/event/44606/)

  PublisherOA PDFDOI

• Unified and Consistent Structure Growth Measurements from Joint ACT, SPT, and Planck CMB Lensing

  ArXiv.org · 2025-04-28

  articleOpen access

  We present the tightest cosmic microwave background (CMB) lensing constraints to date on the growth of structure by combining CMB lensing measurements from the Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT) and \textit{Planck}. Each of these surveys individually provides lensing measurements with similarly high statistical power, achieving signal-to-noise ratios of approximately 40. The combined lensing bandpowers represent the most precise CMB lensing power spectrum measurement to date with a signal-to-noise ratio of 61 and an amplitude of $A_\mathrm{lens}^\mathrm{recon} = 1.025 \pm 0.017$ with respect to the theory prediction from the best-fit CMB \textit{Planck}-ACT cosmology. The bandpowers from all three lensing datasets, analyzed jointly, yield a $1.6\%$ measurement of the parameter combination $S_8^\mathrm{CMBL} \equiv σ_8\,(Ω_m/0.3)^{0.25} = 0.825^{+0.015}_{-0.013}$. Including Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) data improves the constraint on the amplitude of matter fluctuations to $σ_8 = 0.829 \pm 0.009$ (a $1.1\%$ determination). When combining with uncalibrated supernovae from \texttt{Pantheon+}, we present a $4\%$ sound-horizon-independent estimate of $H_0=66.4\pm2.5\,\mathrm{km\,s^{-1}\,Mpc^{-1}} $. The joint lensing constraints on structure growth and present-day Hubble rate are fully consistent with a $Λ$CDM model fit to the primary CMB data from \textit{Planck} and ACT. While the precise upper limit is sensitive to the choice of data and underlying model assumptions, when varying the neutrino mass sum within the $Λ\mathrm{CDM}$ cosmological model, the combination of primary CMB, BAO and CMB lensing drives the probable upper limit for the mass sum towards lower values, comparable to the minimum mass prior required by neutrino oscillation experiments.

  PublisherOA PDFDOI

• Constraints on Inflationary Gravitational Waves with Two Years of SPT-3G Data

  ArXiv.org · 2025-05-05

  preprintOpen access

  We present a measurement of the $B$-mode polarization power spectrum of the cosmic microwave background anisotropies at 32 $\le$ $\ell$ $&lt;$ 502 for three bands centered at 95, 150, and 220 GHz using data from the SPT-3G receiver on the South Pole Telescope. This work uses SPT-3G observations from the 2019 and 2020 winter observing seasons of a $\sim$1500 deg$^2$ patch of sky that directly overlaps with fields observed with the BICEP/Keck family of telescopes, and covers part of the proposed Simons Observatory and CMB-S4 deep fields. Employing new techniques for mitigating polarized atmospheric noise, the SPT-3G data demonstrates a white noise level of 9.3 (6.7) $μ$K-arcmin at $\ell \sim 500$ for the 95 GHz (150 GHz) data, with a $1/\ell$ noise knee at $\ell$=128 (182). We fit the observed six auto- and cross-frequency $B$-mode power spectra to a model including lensed $Λ$CDM $B$-modes and a combination of Galactic and extragalactic foregrounds. This work characterizes foregrounds in the vicinity of the BICEP/Keck survey area, finding foreground power consistent with that reported by the BICEP/Keck collaboration within the same region, and a factor of $\sim$ 3 higher power over the full SPT-3G survey area. Using SPT-3G data over the BICEP/Keck survey area, we place a 95% upper limit on the tensor-to-scalar ratio of $r &lt; 0.25$ and find the statistical uncertainty on $r$ to be $σ(r) = 0.067$.

  PublisherOA PDFDOI

• Pointing Accuracy Improvements for the South Pole Telescope with Machine Learning

  Journal of Astronomical Instrumentation · 2025-05-09 · 1 citations

  article

  In this paper, we present improvements to the pointing accuracy of the South Pole Telescope (SPT) using machine learning. The ability of the SPT to point accurately at the sky is limited by its structural imperfections, which are impacted by the extreme weather at the South Pole. Pointing accuracy is particularly important during SPT participation in observing campaigns with the Event Horizon Telescope (EHT), which requires stricter accuracy than typical observations with the SPT. We compile a training dataset of historical observations of astronomical sources made with the SPT-3G and EHT receivers on the SPT. We train two XGBoost models to learn a mapping from current weather conditions to two telescope drive control arguments — one which corrects for errors in azimuth and the other for errors in elevation. Our trained models achieve root mean squared errors on withheld test data of 2[Formula: see text]14 in cross-elevation and 3[Formula: see text]57 in elevation, well below our goal of 5[Formula: see text] along each axis. We deploy our models on the telescope control system and perform further in situ test observations during the EHT observing campaign in April 2024. Our models result in significantly improved pointing accuracy: for sources within the range of input variables where the models are best trained, average combined pointing error improved 33%, from 15[Formula: see text]9 to 10[Formula: see text]6. These improvements, while significant, fall shy of our ultimate goal, but they serve as a proof of concept for the development of future models. Planned upgrades to the EHT receiver on the SPT will necessitate even stricter pointing accuracy which will be achievable with our methods.

  PublisherDOI

• Measurements of the temperature and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>E</mml:mi></mml:math>-mode polarization of the cosmic microwave background from the full 500-square-degree SPTpol dataset

  Physical review. D/Physical review. D. · 2025-06-10 · 7 citations

  articleOpen access

  The authors build upon, and extend, previous work to now use the Full 500-square-degree South Pole Telescope Cosmic Microwave Background (CMB) polarization dataset. They confirm previous \ensuremath{\Lambda}CDM-consistent results and obtain the most sensitive measurements of the lensed-CMB damping tail to date.

  PublisherOA PDFDOI

• Measurement and Modeling of Polarized Atmosphere at the South Pole with SPT-3G

  The Astrophysical Journal · 2025-03-11 · 7 citations

  articleOpen access

  Abstract We present the detection and characterization of fluctuations in linearly polarized emission from the atmosphere above the South Pole. These measurements make use of data from the SPT-3G receiver on the South Pole Telescope in three frequency bands centered at 95, 150, and 220 GHz. We use the cross-correlation between detectors to produce an unbiased estimate of the power in Stokes I , Q , and U parameters on large angular scales. Our results are consistent with the polarized signal being produced by the combination of Rayleigh scattering of thermal radiation from the ground and thermal emission from a population of horizontally aligned ice crystals with an anisotropic distribution described by Kolmogorov turbulence. The measured spatial scaling, frequency scaling, and elevation dependence of the polarized emission are explained by this model. Polarized atmospheric emission has the potential to significantly impact observations on the large angular scales being targeted by searches for inflationary B-mode CMB polarization. We present the distribution of measured angular power spectrum amplitudes in Stokes Q and I for 4 yr of Austral winter observations, which can be used to simulate the impact of atmospheric polarization and intensity fluctuations at the South Pole on a specified experiment and observation strategy. We present a mitigation strategy that involves both downweighting significantly contaminated observations and subtracting a polarized atmospheric signal from the 150 GHz band maps. In observations with the SPT-3G instrument, the polarized atmospheric signal is a well-understood and subdominant contribution to the measured noise after implementing the mitigation strategies described here.

  PublisherDOI

• SPT-3G D1: Constraints on inflationary gravitational waves with two years of SPT-3G data

  Physical review. D/Physical review. D. · 2025-12-08 · 6 citations

  article

  We present a measurement of the $B$-mode polarization power spectrum of the cosmic microwave background anisotropies at $32\ensuremath{\le}\ensuremath{\ell}&lt;502$ for three bands centered at 95, 150, and 220 GHz using data from the SPT-3G receiver on the South Pole Telescope. This work uses SPT-3G observations from the 2019 and 2020 winter observing seasons of a $\ensuremath{\sim}1500\text{ }\text{ }{\mathrm{deg}}^{2}$ patch of sky that directly overlaps with fields observed with the BICEP/Keck family of telescopes and covers part of the proposed Simons Observatory and CMB-S4 deep fields. Employing new techniques for mitigating polarized atmospheric noise, the SPT-3G data demonstrates a white noise level of 9.3 $(6.7)\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{K}\text{\ensuremath{-}}\mathrm{arcmin}$ at $\ensuremath{\ell}\ensuremath{\sim}500$ for the 95 GHz (150 GHz) data, with a $1/\ensuremath{\ell}$ noise knee at $\ensuremath{\ell}=128$ (182). We fit the observed six auto- and cross-frequency $B$-mode power spectra to a model including lensed $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ $B$-modes and a combination of Galactic and extragalactic foregrounds. This work characterizes foregrounds in the vicinity of the BICEP/Keck survey area, finding foreground power consistent with that reported by the BICEP/Keck collaboration within the same region and a factor of $\ensuremath{\sim}3$ higher power over the full SPT-3G survey area. Using SPT-3G data over the BICEP/Keck survey area, we place a 95% upper limit on the tensor-to-scalar ratio of $r&lt;0.25$ and find the statistical uncertainty on $r$ to be $\ensuremath{\sigma}(r)=0.067$.

  PublisherDOI

• Cosmology from CMB lensing and delensed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>E</mml:mi><mml:mi>E</mml:mi></mml:math> power spectra using 2019–2020 SPT-3G polarization data

  Physical review. D/Physical review. D. · 2025-04-16 · 57 citations

  articleOpen access

  From CMB polarization data alone, we reconstruct the CMB lensing power spectrum, comparable in overall constraining power to previous temperature-based reconstructions, and an unlensed $E$-mode power spectrum, with clear detections of the third through the tenth acoustic peaks. The observations, taken in 2019 and 2020 with the South Pole Telescope (SPT) and the SPT-3G camera, cover $1500\text{ }\text{ }{\mathrm{deg}}^{2}$ at 95, 150, and 220 GHz with arcminute resolution and roughly $4.9\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{K}\text{\ensuremath{-}}\mathrm{arcmin}$ coadded noise in polarization. The power spectrum estimates, together with systematic parameter estimates and a joint covariance matrix, follow from a Bayesian analysis using the marginal unbiased score expansion (MUSE) method. The $E$-mode spectrum at $\ensuremath{\ell}&gt;2000$ and lensing spectrum at $L&gt;350$ are the most precise to date. Assuming the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model, and using only these SPT data and priors on $\ensuremath{\tau}$ and absolute calibration from Planck, we find ${H}_{0}=66.81\ifmmode\pm\else\textpm\fi{}0.81\text{ }\text{ }\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$, comparable in precision to the Planck determination and in $5.4\ensuremath{\sigma}$ tension with the most precise ${H}_{0}$ inference derived via the distance ladder. We also find ${S}_{8}\ensuremath{\equiv}{\ensuremath{\sigma}}_{8}({\mathrm{\ensuremath{\Omega}}}_{\mathrm{m}}/0.3{)}^{0.5}=0.850\ifmmode\pm\else\textpm\fi{}0.017$, providing further independent evidence of a slight tension with low-redshift structure probes. The $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model provides a good simultaneous fit to the combined Planck, ACT, and SPT data, and thus passes a powerful test. Combining these CMB datasets with BAO observations, we explore extensions to the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. We find that the effective number of neutrino species, spatial curvature, and primordial helium fraction are consistent with standard model values, and that the 95% confidence upper limit on the neutrino mass sum is 0.075 eV, close to the minimum sum expected from observations of solar and atmospheric neutrino oscillations. The SPT data are consistent with the somewhat weak ($&lt;3\ensuremath{\sigma}$) preference for excess lensing power seen in Planck and ACT data relative to predictions of the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model given the combined Planck, ACT, and BAO datasets. We also detect at greater than $3\ensuremath{\sigma}$ the influence of nonlinear evolution in the CMB lensing power spectrum and discuss it in the context of the ${S}_{8}$ tension. Forthcoming SPT-3G analyses will feature deeper and wider observations in temperature and polarization, providing even tighter constraints and more powerful tests of the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model.

  PublisherOA PDFDOI

Recent grants

• CAREER: Large focal plane array development for CMB polarization studies with the South Pole Telescope

  NSF · $875k · 2010–2016

Frequent coauthors

• M. Dobbs

  Canadian Institute for Advanced Research

  481 shared

• C. L. Chang

  Argonne National Laboratory

  467 shared

• B. A. Benson

  Netherlands Institute for Radio Astronomy

  462 shared

• J. E. Carlstrom

  405 shared

• S. S. Meyer

  388 shared

• L. E. Bleem

  University of Chicago

  368 shared

• W. L. Holzapfel

  359 shared

• K. K. Schaffer

  University of Chicago

  335 shared

Education

• PhD, Applied Physics

  California Institute of Technology

  2002

• MS, Applied Physics

  California Institute of Technology

  1993

• BS, Physics

  Stanford University

  1991