How Do Cosmologists Use the CMB to Measure Properties of the Early Universe?

Abstract

The Cosmic Microwave Background (CMB) is a relic of the early universe that is essentially a snapshot of the universe shortly after recombination. This paper presents a comprehensive review of CMB Cosmology and the tools used to probe it, synthesizing foundational concepts from textbooks and complementary information from peer-reviewed literature. Beginning with the theory of cosmic inflation, the paper discusses how CMB power spectra, temperature anisotropies, and polarization spectra reveal the geometry and constituents of the universe and could provide evidence supporting inflationary models. Together, these results show why the CMB has become the cornerstone of modern observational cosmology. This paper is intended to inform early researchers and students about how studying the CMB contributes to our understanding of the fundamental properties of the universe.

Keywords: Cosmic Microwave Background, cosmology, CMB power spectrum, CMB polarization, ΛCDM, inflation

Introduction

The CMB was first detected by Penzias and Wilson in 1965 as unexpected background noise from their radio telescope. Since then, we have been able to make more precise measurements of the CMB, improving our understanding of how the universe was formed and its relevance to modern astrophysics.

Despite these advances, CMB cosmology remains a technically demanding subject that can be difficult to approach without a strong physics background. Introductory treatments that connect the underlying physics to what we actually observe are harder to find. The present work addresses this gap by explaining where the CMB comes from, what we see in it, and what it tells us about the universe without requiring graduate-level knowledge.

This review is grounded in the standard ΛCDM cosmological model, which describes how inflation magnified quantum fluctuations, leading to density fluctuations that propagated through the photon-baryon plasma as acoustic oscillations and are observed as CMB temperature anisotropies. The scope is limited to the discussion of major pieces of evidence from the CMB with minimal mathematical derivations.

Section 1 covers the theory of inflation, describing how the universe expanded exponentially in less than a fraction of a second. Inflationary theory solves three discrepancies observed in the universe: the Monopole Problem, the Equal Horizons Problem, and the Flatness Problem. Sections 2-4 describe the CMB, as well as its formation and what current measurements have told us about fundamental properties of our universe. These sections also introduce key jargon and concepts essential to understanding how we can probe the early physical dynamics of the universe. Section 5 connects the CMB and inflation via CMB polarization signal properties, discussing how probing the CMB in search of a very specific signal would lead to a definitive measurement for inflation and determine whether inflation actually occurred.

Methods

This review was conducted with a structured search of primary CMB literature and citing established articles in CMB Cosmology. Sources were obtained using two databases: Google Scholar and NASA ADS, with search terms including “inflation”, “CMB polarization”, “ΛCDM cosmological parameters”, and “CMB power spectrum”. Sources were included if they were relevant to cosmological parameter estimation, or satellite mission results (COBE, Planck, WMAP). Sources were excluded if they focused on topics outside CMB cosmology, such as gravitational wave detection.

Quality was assessed based on the number of citations and journal reputation (e.g. The Astrophysical Journal, Astronomy & Astrophysics). From an initial pool of roughly 50 sources, 33 were selected for inclusion. After gathering these sources, research notes were organized thematically and synthesized into a narrative format.

Inflation

Immediately after the big bang, the universe underwent an accelerated expansion known as inflation, during which the scale factor grew by at least 60 e-folds (a factor of at least e⁶⁰ ~ 10²⁶), though the exact duration and timescales are model-dependent and not yet precisely constrained. Direct proof of inflation has not yet been found but it is a compelling theory, as it solves three critical problems.

The Horizon Problem

 The temperature of the universe is isotropic, contrary to what the standard model of cosmology predicts. Regions separated by 2° were not in causal contact at the time of the last scattering. The maximum distance two points could have exchanged information, the comoving particle horizon, was far smaller than the distance between regions on the last scattering surface in a standard non-inflationary FLRW universe. Despite having no causal contact, these separate regions have nearly identical temperatures^1,2. Inflationary theory resolves this conflict: during inflation, the local, observable universe is thought to have inflated rapidly from a very small region already in thermal equilibrium, such that the temperature distribution became ‘locked in’.

The Monopole Problem

The second issue is that there are no magnetic monopoles in the current universe³.  In the Planck Epoch, 4 fundamental forces, electromagnetic, gravitational, weak, and strong, were separated. This process, according to many Grand Unified Theories (GUTs) models, would have produced magnetic monopoles^4,5. Thus, there should be many magnetic monopoles today, yet none are observed³. Inflation diluted the density of monopoles by expanding space so they became spread across a volume exceeding the observable universe, making detection impossible⁶.

The Flatness Problem

The universe is surprisingly ‘finely-tuned’ with a critical density (Ω) of 1, indicating flatness. Inflation is thought to have flattened all inhomogeneities, increasing the scale factor a.¹ This is analogous to the surface of a balloon: as it expands, any local patch will appear flatter. These observational restraints come from CMB peaks positions, acoustic oscillations, and Type IA Supernovae. When combined with BAO data, Planck constrains the curvature parameter to Ω_k = 0.001 ± 0.002, consistent with a spatially flat universe⁷.

Cosmic Microwave Background

In the early universe, temperatures were so hot that free electrons were coupled to photons⁸. Before recombination, photons couldn’t travel far before scattering off free particles via Thomson Scattering, making the universe an opaque plasma. Free electrons were coupled to photons and baryons through electromagnetic coulomb interactions, thereby forming a photon-baryonic system that behaved like a single fluid⁸.

At 380,000 years, the universe cooled down sufficiently so that the equilibrium shifted in favor of hydrogen. Electrons combined with protons to form neutral hydrogen, a process called recombination. This reduced the number of free electrons available to scatter photons, causing photon decoupling. During the slow transition from an opaque to clear universe, different photons underwent the last scattering at slightly different times, which caused the last scattering surface to have finite thickness². Therefore, fluctuations on scales smaller than the thickness get washed out through photon diffusion⁹, and polarization is generated as photons scatter in an anisotropic radiation field. Today, these photons are detected through the Cosmic Microwave Background (CMB), redshifted from the stretching of the universe^10,11.

The radiation spectrum of these photons follows that of a blackbody, an object that absorbs incoming radiation and emits it in the form of thermal radiation¹². The nearly identical blackbody intensity distribution of the CMB¹², as measured by the COBE satellite (Cosmic Background Explorer), indicates that the CMB originated from an early universe in thermal equilibrium.

Instruments and Observational Challenges

CMB measurements are made using microwave detectors sensitive to frequencies of roughly 30–300 GHz, cooled to near absolute zero to prevent the instruments themselves from introducing thermal noise into the signal. The clearest CMB maps have come from satellites, which avoids atmospheric interference. COBE first detected temperature anisotropies in 1992¹³, WMAP followed with improved precision¹⁴, and Planck provided the most detailed fully sky CMB map to date with angular resolution down to ~5 arcminutes⁷. Ground-based telescopes such as the Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT) measure the CMB with even higher resolutions at small angular scales, though they observe only a fraction of the sky.

A major observational challenge is foreground contamination. Interstellar dust, for example, absorbs energy generated from star formation and re-emits it in the far-infrared, producing a background signal in the CMB¹⁵. This radiation must be separated from the CMB signal by observing at multiple frequencies, though this process creates its own uncertainties into the final maps.

Dark Matter

A key component of the ΛCDM model is dark matter, matter that does not interact electromagnetically with baryonic matter and does not reflect or emit any radiation, making it invisible in telescopes. Dark matter plays an important role in shaping the large scale structure of the universe as we observe it today.

CMB Temperature Anisotropies

Since recombination, the average temperature of the CMB has decreased from ~3000K to 2.725K ± 0.00057.¹² Although the CMB is nearly uniform, small variations appear as anisotropies, shown in Figure 1.

After inflation, quantum fluctuations stretched to macroscopic scales: low-density regions became potential hills and high-density regions became gravitational potential wells. Gravitational attraction led to compressions, and photons resisted this compression by producing radiation pressure, a restoring force that caused rarefactions¹⁷.

Large Angular Scales (≥1°): Sachs-Wolfe Effect

At large angular scales (≥1°), these density variations explain temperature variations via the Sachs-Wolfe Effect.

Under the normal Sachs-Wolfe Effect, photons decoupling from overdense or underdense regions will gain or lose energy, and leave hot or cold spots on the CMB¹⁸.

Under the Integrated Sachs-Wolfe Effect (ISW), potential gravitational wells start to get shallower and ‘decay’ as the universe expands. When photons enter a well in the process of decaying, they lose less energy when climbing out, resulting in a slight net energy gain, appearing hotter¹⁹.

Degree Scales (≤1°): Acoustic Oscillations

At small scales (≤1°), this spring-like effect formed acoustic oscillations, where higher density regions compressed photons and lower density regions rarefied them, appearing as hotter or colder spots²⁰.

Damping Tail

The tail of the CMB is an indication of diffusion damping. At high multipoles (low angular scales), acoustic peaks suddenly decrease in amplitude due to photon diffusion through plasma. Energy is moved into cooler regions, smoothing out fluctuations and damping acoustic oscillations.

Power Spectrum

The CMB anisotropies are explained through its power spectrum, shown in Figure 2. The power spectrum essentially plots the relationship between angular degrees converted into multipoles (l = 180/θ) and the temperature fluctuation power C_l. The y-axis physically measures the variance of temperature fluctuations at angular scale l, in units µK². In practice, most plots do not show Cl directly; instead, they show the rescaled quantity D_l =  l(l+1)C_l/2𝜋. This form generally makes the graph easier to read and makes the peaks more visually comparable.

How to read the power spectrum:

• Left side (low l): Large angular scales with relatively low power
• First acoustic peak (l ~ 200): The largest peak, corresponds to roughly 1° on the sky
• Subsequent peaks: Smaller peaks representing harmonics of acoustic oscillations in the photon-baryon plasma
• Damping tail (high l): Power decreases due to photon diffusion (silk damping)

Peaks of the CMB

Acoustic Horizon and Peak Conditions

The physical positions of the peaks in the CMB angular power spectrum are determined by the maximum distance a sound wave could travel through the plasma from the end of inflation to recombination. This distance is known as the sound horizon (r_s). As the photon-baryon fluid oscillated, it alternated between compression, where the photon-baryon fluid falls into gravitational potential wells, and rarefaction, where it bounces back out.

When photons decoupled from baryons, these sound waves ‘froze’. The standard condition for the wavenumber k_n of the nth acoustic peak is: k_nr_s = n𝜋. The factor of 𝜋 shows that each peak corresponds to a mode that has completed n half oscillations before recombination. This condition directly links the observed peak positions with the physical size of the sound horizon, allowing it to be used as a “ruler” in extracting information about the early universe.

First Peak

The first three peaks of the power spectrum tell us about the geometry of the universe and the distribution of baryonic and dark matter.

The angular scale of the first peak on the power spectrum reflects the critical density of the universe. Through the Angular Diameter Distance test, astronomers compare sound^* waves in the early universe to the physical distance CMB photons have travelled after recombination. Using this ‘Standardized Ruler’, we measure the angular length of the photons to determine the curvature of the universe²².

The first peak occurs at ~1°, which is consistent with flat geometry (Ω ≈ 1) within the ΛCDM framework and other cosmological parameters such as the power spectrum. WMAP constrained the total density parameter to Ω = 1.02 ± 0.02²³, and Planck approximated it even further to Ω = 1.0007 ± 0.0019⁷. If the universe were negatively or positively curved, the first peak would shift to smaller or larger angular scales, respectively.

Second Peak

When low-density baryons fall into potential wells, the temperature fluctuations of the CMB oscillate symmetrically about zero. However, baryon loading (odd-numbered) produces greater compression, leading to higher peaks, while maximal rarefactions (even-numbered) stay constant, resulting in asymmetric amplitudes (Figure 3)²⁰. The asymmetry between odd and even peaks is sensitive to the baryon density, but the relationship is not one-to-one. Besides baryon density, other cosmological parameters such as the matter density and the spectral index can produce similar effects on peak heights. In practice, the baryon density is constrained by fitting the entire power spectrum at once, adjusting all parameters together until the best match to the data is found. Planck Collaboration (2020) used this method to report a baryon density of Ω_bh²= 0.02237 ± 0.00015⁷.

The first peak being higher than the second indicates a significant baryon density. Since the second peak is unaffected, the amplitude difference demonstrates a large baryon content.

Third Peak

The higher the dark matter density, the deeper the wells, resulting in less acoustic oscillations generated and lower CMB peaks. The third peak represents a compression, mainly driven by dark matter. If the third peak is significantly higher than the second peak, there is a higher proportion of dark matter than normal matter. In this case, gravity is doing more work than baryons are resisting. Think of baryons as jumpers on a trampoline: the more baryons, the higher they jump — a result of their ‘self-gravity’⁹, while more dark matter acts like a heavy weight and deepens the well.

What we see is it that the third peak is only a little higher than the second peak. Since the third peak is only slightly higher than the second, we can conclude that dark matter is the dominant constituent in the universe, making the universe simply ‘less bouncy’.

CMB Quadrupole Anisotropy

Images of the CMB from WMAP show that the CMB has a quadrupole temperature anisotropy pattern. A distinctive feature of the quadrupole anisotropy is the four colored spots centered in Figure 4.

A Quadrupole Anisotropy is generated when the radiation field on an electron has a difference in intensity between the two perpendicular axes. Before the last scattering surface, different parts of the photon-baryon fluid move in different directions, and this variation in velocity from place to place is the “gradient”. The fluid’s motion doppler shifts the photons inside it.

Polarization and Polarization Power Spectra

By the time Thomson Scattering occurs, the radiation field already has a hot axis and cold axis. When an electron scatters this uneven light, the asymmetry of the intensity along the hot and cold axes produce a net linear polarization²⁶.

The CMB exhibits two polarization modes, E-mode polarization (E-modes) and B-mode polarization (B-modes), which are distinguished by their parity properties, or how their patterns behave under reflection. E-modes have even parity: their polarization pattern looks the same after a mirror reflection. B-modes have odd parity: their pattern picks up a sign change under reflection. The two resemble a gradient and a curl, respectively²⁷.

E-modes arise from scalar perturbations, the same density fluctuations that produce the temperature anisotropies seen in the CMB power spectrum. Because scalar perturbations dominated the early universe, E-modes are the dominant polarization signal and have been measured in detail by WMAP and Planck^7,28. In contrast, B-modes cannot be generated by scalar perturbations alone. They require either tensor perturbations (primordial gravitational waves produced during inflation) or the gravitational lensing of E-modes into B-modes by large-scale structure at later times.

This makes primordial B-modes a target for detecting inflationary gravitational waves. If inflation occurred, it would have produced a background of gravitational waves that imprinted a faint B-mode signature on the CMB polarization. The amplitude of this signal is constrained by the tensor-to-scalar ratio r, and detecting it would be direct evidence for inflation and constrain its energy scale.

In practice, outside contamination makes detecting B-modes much more difficult. For example, thermal emission from interstellar dust may mimic or interfere with the B-mode signal. Experiments address this problem by observing at multiple frequencies and by targeting the clean regions of the sky.

E- and B-mode power spectra map polarization across angular scales, but both are much weaker than the temperature spectrum (black curve in Figure 5).Cosmologists are currently developing more sensitive telescopes to detect the primordial B signal, which could provide evidence for both inflation and the presence of gravitational waves.

Limitations

One limitation in cosmology is cosmic variance – we observe only one universe. At large angular scales, there are limited areas of the sky to observe. This low sample size can cause statistical uncertainty.

A major area of ongoing debate is the Hubble tension. The Hubble Constant H₀ describes how fast the universe is expanding; specifically, how fast two galaxies are moving apart per unit distance between them. Two different measurements from different sources give conflicting answers: Planck extrapolates H₀ = 67.4 ± 0.5 km/s/Mpc³⁰, while direct measurements using the local distance ladder constantly give H₀ = 73.04 ± 1.04 km/s/Mpc³¹. This discrepancy is statistically significant and may indicate systematic errors or physics beyond the ΛCDM framework.

Finally, while inflation predicts the presence of gravitational waves, the primordial B-mode has not yet been detected. Therefore, the energy scale of inflation remains unknown and a key prediction of inflationary theory unconfirmed.

Conclusion

The primary objective of this review was to provide a clear yet rigorous overview of the Cosmic Microwave Background. This objective was met by applying fundamental physics to interpret data and using analogies to explain challenging topics. While the review does not provide original data analysis, the focus on conceptual understanding acts as a stepping stone for students seeking to dive deeper into technical CMB research.

The topics discussed in this review also reflect areas of ongoing research in modern cosmology. Despite the progress from decades of research, many questions regarding the universe remain unsolved. Current CMB-focused research projects are centered around developing higher technology to be more efficient, such as Simons-Observatory in Chile, with the aim of using multiple telescopes to observe different patches of the sky at once³².

At the same time, due to limited data from the CMB, modern cosmology has begun to shift towards using complementary probes rather than relying on a singular observational window. Further progress will depend on more research to grasp unknown elements such as the nature of dark energy and the driving force behind inflation.

Notes

* Though commonly referred to as sound waves by the scientific community, these acoustic oscillations are not real waves oscillating in space, but are fluctuations in density, as explained in Section 3.

References

1. Baumann, Daniel. “TASI Lectures on Inflation.” arXiv preprint arXiv:0907.5424 (2009). https://arxiv.org/abs/0907.5424. [↩] [↩]
2. Dodelson, Scott. Modern Cosmology. San Diego: Academic Press, 2003. [↩] [↩]
3. Milstead, David, and Erick J. Weinberg. “Magnetic Monopoles.” In Review of Particle Physics. Particle Data Group, 2017. https://pdg.lbl.gov/2017/reviews/rpp2017-rev-mag-monopole-searches.pdf. [↩] [↩]
4. Kolb, Edward W., and Michael S. Turner. The Early Universe. Redwood City, CA: Addison-Wesley, 1990. [↩]
5. Preskill, John P. “Cosmological Production of Superheavy Magnetic Monopoles.” Physical Review Letters 43, no. 19 (1979): 1365–1368. https://doi.org/10.1103/PhysRevLett.43.1365. [↩]
6. Guth, Alan H. “Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems.” Physical Review D 23, no. 2 (1981): 347–356. https://doi.org/10.1103/PhysRevD.23.347. [↩]
7. Planck Collaboration. “Planck 2018 Results. VI. Cosmological Parameters.” Astronomy & Astrophysics 641 (2020): A6. https://doi.org/10.1051/0004-6361/201833910. [↩] [↩] [↩] [↩] [↩] [↩]
8. Peacock, John A. Cosmological Physics. Cambridge: Cambridge University Press, 1999. [↩] [↩]
9. Hu, Wayne, and Naoshi Sugiyama. “Small-Scale Cosmological Perturbations: An Analytic Approach.” The Astrophysical Journal 471 (1996): 542–570. https://doi.org/10.1086/177989. [↩] [↩]
10. Penzias, Arno A., and Robert W. Wilson. “A Measurement of Excess Antenna Temperature at 4080 Mc/s.” The Astrophysical Journal 142 (1965): 419–421. https://doi.org/10.1086/148307. [↩]
11. Fixsen, D. J., E. S. Cheng, J. M. Gales, John C. Mather, R. A. Shafer, and E. L. Wright. “The Cosmic Microwave Background Spectrum from the Full COBE FIRAS Data Set.” The Astrophysical Journal 473 (1996): 576–587. https://doi.org/10.1086/178173. [↩]
12. Fixsen, D. J. “The Temperature of the Cosmic Microwave Background.” The Astrophysical Journal 707, no. 2 (2009): 916–920. https://doi.org/10.1088/0004-637X/707/2/916. [↩] [↩] [↩]
13. Smoot, G. F., C. L. Bennett, A. Kogut, E. L. Wright, et al. “Structure in the COBE Differential Microwave Radiometer First-Year Maps.” The Astrophysical Journal 396 (1992): L1–L5. https://doi.org/10.1086/186504. [↩]
14. Hinshaw, G., D. Larson, E. Komatsu, D. N. Spergel, et al. “Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results.” The Astrophysical Journal Supplement Series 208, no. 2 (2013): 19. https://doi.org/10.1088/0067-0049/208/2/19. [↩]
15. Gawiser, Eric, and Joseph Silk. “The Cosmic Microwave Background Radiation.” Physics Reports 333–334 (2000): 245–267. https://doi.org/10.1016/S0370-1573(00)00025-9. [↩]
16. Planck Collaboration. “Planck 2015 Results. I. Overview of Products and Scientific Results.” Astronomy & Astrophysics 594 (2016): A1. https://doi.org/10.1051/0004-6361/201527101. [↩]
17. Ma, Chung-Pei, and Edmund Bertschinger. “Cosmological Perturbation Theory in the Synchronous and Conformal Newtonian Gauges.” The Astrophysical Journal 455 (1995): 7–25. https://doi.org/10.1086/176550. [↩]
18. Sachs, R. K., and A. M. Wolfe. “Perturbations of a Cosmological Model and Angular Variations of the Microwave Background.” The Astrophysical Journal 147 (1967): 73–90. https://doi.org/10.1086/148982. [↩]
19. Manzotti, Alessandro, and Scott Dodelson. “Mapping the Integrated Sachs-Wolfe Effect.” Physical Review D 90, no. 12 (2014): 123009. https://doi.org/10.1103/PhysRevD.90.123009. [↩]
20. Hu, Wayne, and Scott Dodelson. “Cosmic Microwave Background Anisotropies.” Annual Review of Astronomy and Astrophysics 40 (2002): 171–216. https://doi.org/10.1146/annurev.astro.40.060401.093926. [↩] [↩] [↩]
21. Hinshaw, G., M. R. Nolta, C. L. Bennett, R. Bean, et al. “Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature Analysis.” The Astrophysical Journal Supplement Series 170, no. 2 (2007): 288–334. https://doi.org/10.1086/513698. [↩]
22. Eisenstein, Daniel J., Ice Zehavi, David W. Hogg, Roman Scoccimarro, Michael R. Blanton, Robert C. Nichol, Ryan Scranton, et al. “Detection of the Baryon Acoustic Peak in the Large‐Scale Correlation Function of SDSS Luminous Red Galaxies.” The Astrophysical Journal 633, no. 2 (2005): 560–574. https://doi.org/10.1086/466512. [↩]
23. Spergel, D. N., L. Verde, H. V. Peiris, E. Komatsu, et al. “First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters.” The Astrophysical Journal Supplement Series 148, no. 1 (2003): 175–194. https://doi.org/10.1086/377226. [↩]
24. Hu, Wayne. “Baryons in the Power Spectrum.” Ringing in the New Cosmology. University of Chicago. https://background.uchicago.edu/~whu/intermediate/baryons.html. [↩]
25. Copi, Craig J., Dragan Huterer, Dominik J. Schwarz, and Glenn D. Starkman. “Large-Angle Anomalies in the CMB.” Advances in Astronomy 2010 (2010): 847541. https://doi.org/10.1155/2010/847541. [↩]
26. Hu, Wayne, and Martin White. “A CMB Polarization Primer.” New Astronomy 2, no. 4 (1997): 323–344. https://doi.org/10.1016/S1384-1076(97)00022-5. [↩]
27. Kamionkowski, Marc, Arthur Kosowsky, and Albert Stebbins. “A Probe of Primordial Gravity Waves and Vorticity.” Physical Review Letters 78, no. 11 (1997): 2058–2061. https://doi.org/10.1103/PhysRevLett.78.2058. [↩]
28. Kogut, A., D. N. Spergel, C. Barnes, C. L. Bennett, et al. “First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature-Polarization Correlation.” The Astrophysical Journal Supplement Series 148, no. 1 (2003): 161–173. https://doi.org/10.1086/377219. [↩]
29. Achúcarro, Ana, Gonzalo A. Palma, Benjamin D. Wandelt, William Coulton, Giovanni Cabass, and Jacopo Fumagalli. “Inflation: Theory and Observations.” Preprint, 2022. https://cds.cern.ch/record/2805417. [↩]
30. Planck Collaboration. “Planck 2018 Results. V. CMB Power Spectra and Likelihoods.” Astronomy & Astrophysics 641 (2020): A5. https://doi.org/10.1051/0004-6361/201936386. [↩]
31. Riess, Adam G., Wenlong Yuan, Lucas M. Macri, Dan Scolnic, et al. “A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team.” The Astrophysical Journal Letters 934, no. 1 (2022): L7. https://doi.org/10.3847/2041-8213/ac5c5b. [↩]
32. Ade, Peter, et al. (Simons Observatory Collaboration). “The Simons Observatory: Science Goals and Forecasts.” Journal of Cosmology and Astroparticle Physics 2019, no. 02 (2019): 056. https://doi.org/10.1088/1475-7516/2019/02/056. [↩]