4 Illuminate
the
Invisible
Universe

Looking out into the night sky, we see pinpricks of light surrounded by darkness. Everything that we see in the universe, including stars, planets, and plumes of interstellar gas comprise only 5% of the universe. There is a vast hidden universe of dark matter and dark energy that is responsible for guiding the evolution of the cosmos itself, from forming the structures of matter to driving an accelerated expansion that will determine the ultimate fate of the universe.

The ɅCDM cosmological paradigm captures the physics that describes how the universe has changed over cosmic time. In the first tiny fraction of a second after the Big Bang, the universe underwent rapid, accelerated expansion called cosmic inflation during which the seeds of the structure that was to come were created. The universe continued to expand and cool, evolving from a universe filled with light in the earliest moments, to the epoch of galaxy formation driven by dark matter, to the current era of accelerated expansion driven by dark energy.

While we do not yet understand the underlying nature of dark matter and dark energy, or the specific physical processes driving cosmic inflation, we do know that they are not described by the subatomic structures in the Standard Model of particle physics. Answering some of the deepest questions about particle physics itself requires detailed studies of cosmic evolution, revealing the underlying nature of dark matter, dark energy, inflation, and other particles in the universe that might have played a role in driving cosmic evolution.

To illuminate the hidden universe, our efforts are focused on two main science drivers:

Determine the Nature of Dark Matter. The gravitational evidence for dark matter is overwhelming. We have many ideas for what dark matter could be, with a handful of particularly compelling candidates with viable cosmological histories. The number of strong candidates inspires a multifaceted campaign to determine the nature of dark matter, leveraging underground facilities, quantum sensors, telescopes, and accelerator-based probes.

Understand What Drives Cosmic Evolution. The evolution of the universe has been determined by physical processes not described by the Standard Model of particle physics, from the exponential expansion called inflation during the first moments of time, to intermediate periods dominated by radiation (potentially including unknown light species) and dark matter, to the cosmic acceleration of today. Measuring the growth of cosmic structure and the expansion history of the universe through multiple complementary methods offers unique explorations into inflation and dark energy, while also yielding insights into neutrino properties and the possible existence of cosmic relic particles from the earliest moments of the cosmos.

4.1.1  –Science Overview

Dark matter constitutes a vast majority of the universe’s mass, influencing its structure and galaxy formation through gravity. Despite this dominant influence, its particle composition and interactions beyond gravity remain unknown. This profound mystery drives research across all frontiers of our field.

Unraveling the dark matter enigma requires a diverse approach that falls into four main categories: cosmic surveys, accelerator-based experiments, indirect detection experiments, and direct detection experiments. Cosmic surveys probe the distribution of dark matter on a variety of length scales, yielding essential data about its properties and guiding the other approaches.The other three categories search for the particles themselves. Accelerator-based experiments attempt to produce the dark matter particles, while indirect detection experiments look for the cosmic messengers resulting from dark matter interactions. Finally, direct detection experiments focus on detecting dark matter’s interactions here on Earth. Each approach, guided by theoretical work, plays a vital role in increasing understanding of the nature of dark matter.

The weakly interacting massive particle (WIMP) has been the primary candidate for dark matter for three decades. With a mass between a few GeV and about a hundred TeV, the particle would have been created in great abundance at early cosmological times in thermal equilibrium with the universe. As the cosmos expanded and cooled, the WIMP would fall out of equilibrium, resulting in the formation of the cold dark matter structures we observe today.

The US has led significant efforts, using tools from all areas of particle physics, to seek evidence of the WIMP through its interactions with Standard Model particles. These efforts have shown that if WIMPs are the dark matter, they must couple to the Standard Model very feebly. Detecting these extremely rare interactions requires large detectors. Because of strong motivation from theory for WIMPs as a dark matter candidate, the pursuit of these feeble couplings remains an important benchmark.

Since the 2014 P5 report, progress in theory has expanded our understanding of other plausible dark matter candidates that have both compelling implications for particle physics and viable cosmological histories. These advances have been accompanied by significant advances in direct-detection technologies, particularly in quantum sensing, enabling discernment of the most minute signals. This convergence of theoretical developments and cutting-edge detection capabilities holds tremendous potential for discovery.

One theoretical approach leads to models of dark matter interacting with the Standard Model through hidden sector particles beyond the dark matter candidate. These new interactions allow for the dark matter to be produced through mechanisms distinct from those of the WIMP, as well as permitting new dynamics within the hidden sector, such as dark matter self-interactions. Hidden sector particles can be produced in accelerators, while innovative techniques can search for the cosmic dark matter particles themselves. This synergy of accelerator experiments and advanced detection methods could shed light on the nature of hidden sector models and their crucial role in the dark matter puzzle.

Another theoretical approach has led to wave-like dark matter candidates. These candidates possess masses less than 1 eV, making them so light that they behave more like waves than particles. As a result, detection techniques are inherently quantum in nature, pushing the boundaries of quantum sensing. Within this category are the highly compelling Quantum Chromodynamic (QCD) axion, which provides a solution to why interactions involving the strong force do not appear to show CP violation, and the related Axion-Like Particles (ALPs). These particles would have been abundantly produced during the early universe, but in contrast to the WIMP, they would not have been in thermal equilibrium due to their light mass and small couplings. Instead, their abundance would have been dictated by the initial conditions of the universe. They can be searched for directly, and cosmology and astrophysics measurements play a pivotal role in constraining the mass range of particles of interest.

Dark matter experiments currently taking data are venturing into unexplored territories and hold the potential for groundbreaking discoveries. Developments in detector instrumentation lay the foundation for future campaigns to identify dark matter in new scenarios (Recommendation 4d). Our recommendations provide a portfolio of projects, research, and tools, including theory and computational work (Recommendations 4b, 4.f), that can comprehensively target and characterize the dark matter model benchmarks.

4.1.2  –Ongoing Projects: Direct Detection, Indirect Detection, and Collider Searches

Ongoing projects probe dark matter using a complementary suite of techniques and technologies. The larger ones include: the LHC, which can produce electroweak-scale dark matter candidates in a controlled environment; IceCube, which has sensitivity to spin-dependent and ultra-heavy particle dark matter candidates; and the second-generation (G2) direct detection experiments, such as the DOE-funded LZ and ADMX-G2, the NSF-funded DarkSide-20k and XENONnT, and the jointly-funded SuperCDMS SNOLAB.

In the coming decade, the HL-LHC will be in a unique position to explore whether dark matter couples to the Higgs boson, and also to test weak scale supersymmetry and many other theories with dark matter candidates. The high energy and intensity of LHC collisions also enable auxiliary experiments searching for long-lived or feebly interacting hidden sector particles. Both LZ and XENONnT started data-taking in 2021, while SuperCDMS SNOLAB and DarkSide-20k are scheduled to begin data-taking in 2025 and 2026, respectively.

These experiments will improve the sensitivity to a wide range of models by more than an order of magnitude. The ADMX experiment has already excluded the QCD axion for masses between 2.66-4.2 µeV and is currently working to push sensitivity to higher masses. These experiments should be supported to achieve their maximum sensitivity and potential (Recommendation 1a, 1d). This support should include the necessary theory and simulation work, as well as background modeling (Recommendation 4b).

4.1.3  –New Initiative: A Portfolio of Agile Projects for Dark Matter

In pursuit of understanding dark matter, a diverse and agile portfolio of ASTAE experiments, as described in Section 6.2, offers the potential for significant discoveries and technological advancements. Small but sensitive detectors are ideal for studying low mass dark matter since the needed size of the detectors scales roughly with the dark matter mass. This strategic approach focuses on two promising areas: hidden sector models and QCD axions, both of which boast high-priority benchmark models that can best be addressed by this scale of experiment.

Accelerator-based searches for the production of hidden sector particles leverage beam dumps at existing beamlines and have sensitivity to thermal benchmark models in the MeV-GeV mass ranges. The direct searches for these hidden sector particles utilize innovative materials and ultra-low noise detectors with the ability to detect down to sub-eV energy depositions to reach the benchmarks. This synergistic combination of approaches is necessary to understand and unlock the secrets of hidden sector dark matter.

The search for axions and ALPs is also well-suited for this agile portfolio. Specific QCD axion models provide definitive benchmarks, and through a series of carefully designed experiments, the parameter space spanning masses from 40 neV to 1 eV can be thoroughly explored. Additionally, these endeavors lay a foundation for even more ambitious projects that target the lightest masses falling within the range of 1 peV to 40 neV.

This multi-faceted approach maximizes the potential for seminal discoveries and pushes the boundaries of what is measurable in the realm of dark matter. Notably, this portfolio has already been set in motion by the Dark Matter New Initiatives (DMNI) experiments, which have completed their design phases and now await construction funding. These initiatives are integral components of the broader portfolio of ASTAE experiments (Recommendation 3a; Section 6.2).

4.1.4  –Major Initiative: G3, the Ultimate WIMP Dark Matter Search

The next phase of the search for WIMP dark matter requires experiments capable of reaching roughly order-of-magnitude weaker interaction strengths than current experiments. A large Generation-3 (G3) WIMP dark matter search would build on the most successful designs of the current G2 experiments, providing sensitivity to dark matter-Standard Model interactions that are small enough that neutrinos become an irreducible background (the “neutrino fog”).

This improvement in reach would provide coverage of important benchmark WIMP models, such as most remaining potential dark matter parameter space under the constrained minimal supersymmetric extension to the Standard Model. Such a G3 experiment would also perform important measurements of solar and possibly supernova neutrinos. A G3 direct detection experiment would be the ultimate WIMP search within the current approach; moving past the reach of the G3 experiment and deeper into the neutrino fog would require significant changes in method and technology.

Although supporting more than one G3 experiment would be beneficial, expected costs are high enough, especially compared to the costs of the portfolio of smaller dark matter projects, that funding two does not appear feasible. Our recommendation supports one G3 experiment, preferably sited on US soil to help maintain US leadership (Recommendation 2d). Investment in the expansion of SURF, taking advantage of the DUNE excavation infrastructure and potential private funding, would enable such siting. Continued support by both DOE and NSF is needed to maximize the science and US leadership. A second, complementary G3 experiment would maximize the discovery potential and would teach us more about dark matter if one of the G2 experiments has promising results.

4.1.5  –New Initiatives: IceCube-Gen2 & CTA

In the next decade, NSF-funded astrophysical gamma-ray and neutrino observatories will provide unprecedented views of the high-energy universe. These observatories will look for cosmic messenger particles that are made in dark matter interactions (Recommendations 2e and 3c). In addition, observations of photons, cosmic rays, neutrinos, and gravitational waves can give a more complete picture of the physics that drives the most energetic sources in the universe.

The IceCube-Gen2 Observatory will provide a ten-fold improvement in sensitivity to astrophysical neutrinos over the IceCube Observatory, and will be the most sensitive probe of heavy decaying dark matter. IceCube-Gen2’s wide-ranging particle-physics portfolio also includes searching for signatures of neutrino physics beyond the Standard Model. In addition, IceCube-Gen2 has a wide-ranging multi-messenger astrophysics portfolio, which gives us a more complete picture of the physics that drives the most energetic sources in the universe that produce the highest energy neutrinos and cosmic rays.

The Cherenkov Telescope Array (CTA) offers a parallel improvement in sensitivity to very high energy gamma rays, along with refined energy resolution over an expanded energy range and a more sharply-resolved picture of the gamma-ray sky. CTA and the Southern Wide-field Gamma-ray Observatory (SWGO) provide sensitivity to WIMP thermal targets that lie beyond the reach of the G3 direct detection experiments. CTA’s excellent energy and angular resolution play a key role in disentangling a dark matter signal from conventional astrophysical backgrounds. Beyond dark matter, CTA’s broad astrophysics portfolio will provide insights into the most extreme environments in the universe and the origin and role of relativistic cosmic particles.

4.1.6  –Complementarity: Astrophysical & Cosmological Probes

Astrophysical and cosmological probes, such as observations of the Milky Way satellite galaxies, stellar streams, strong lensing systems, and the cosmic microwave background, can map the distribution of dark matter on small length scales where the standard cold and collisionless nature of dark matter may break down. On these scales new interactions among dark matter particles, which are predicted in many hidden sector models, can lead to structure-formation phenomena such as halo core formation or gravo-thermal collapse that would be absent in WIMP or QCD axion models. The insights gained could guide terrestrial experiments.

Similarly, a dearth of dark matter structure on small scales could be indicative of dark matter with a significant thermal velocity, the quantum pressure from an ultra-light axion, or dark matter particles that had significant interactions with relativistic species in the recent past. CMB-S4 will have exquisite sensitivity to such light species that reside in hidden sector models. Over the next decade the Rubin Observatory LSST is expected to discover a large number of new Milky Way satellites, stellar streams, and strong lensing systems. Follow-up observations of these objects with existing ancillary telescope resources or future observatories, such as DESI-II and Spec-S5, combined with state-of-the-art cosmological simulations and analyses, will access parameter space inaccessible to laboratory experiments.

Thus, developing a comprehensive understanding of the nature of dark matter requires the complementary support of both astrophysical and terrestrial probes. In practice this can be done within the DOE HEP Cosmic Frontier model by supporting individuals contributing to the primary HEP science goals of ongoing projects (DESI, Rubin Observatory LSST and DESC) to also carry out complementary work on dark matter as a secondary science goal. In addition, computational and modeling work relevant to astrophysical probes can be performed with theory support.

4.1.7  –20-Year Vision

The program outlined above encompasses a series of experiments planned for this decade that hold immense potential for discovery. Simultaneously, it nurtures the growth of next-generation experiments. Over a 20-year timeframe, this carefully curated portfolio of experiments will conduct targeted searches encompassing WIMP, hidden sector, and QCD axion dark matter models. The investments in the construction of a Higgs factory and in the R&D and technology tests and demonstrators for a 10 TeV pCM collider will be essential steps toward achieving unprecedented sensitivity to feebly coupled particles (see Section 5.1). Reaching the 10 TeV scale is needed to achieve definitive covering of the thermal targets for minimal WIMP candidates.

Meanwhile, astrophysical probes will provide complementary insights into the nature of dark matter. Discoveries would be followed up with studies by multiple means with improved sensitivity, informed by this proposed portfolio of experiments. These endeavors not only foster the development of novel technologies but also challenge the limits of what can be effectively measured. Acting as a vital bridge, theory interconnects diverse measurements and guides us toward uncharted avenues of exploration. This multi-pronged strategy, harmonizing experiments of varying scales, theoretical frameworks, and technological advancements, provides a coherent roadmap that optimizes the potential for seminal discoveries.

4.2.1  –Science Overview

The dynamical evolution of the universe is deeply connected to its energy content. At its earliest moments, the universe was sensitive to particle physics processes at energies far beyond what can be probed even in the LHC, today’s most powerful particle accelerator. As the universe expanded and cooled, clues about those early high-energy phenomena were imprinted on the distribution of cosmic matter and light, the latter known as the Cosmic Microwave Background (CMB). Those clues allow us to probe new physics that is inaccessible by other means, playing an essential role in advancing our knowledge of particle physics.

As we peer into the night sky, we see the cumulative effect of the multiple eras that the universe has undergone to reach its present rich structure. The earliest epoch was an era of apparent accelerated expansion, referred to as inflation, during which the initial seeds of structure were created. Once inflation ended, the universe transitioned to a hot radiation era in which ultra-relativistic particles dominated its energy density. During this era, new light species predicted by many promising theories beyond the Standard Model could leave subtle signatures on the evolution of the universe and give insight to the nature of dark matter. The matter era that followed allowed the universe to mature under the dominant influence of dark matter and form the stars, galaxies, and clusters now populating the cosmos. Finally, and recently, the universe entered another era of accelerated expansion, requiring dark energy to form the majority of the energy budget of the universe today.

The key particle physics questions about the universe’s evolution that cosmic surveys seek to answer are the following: What physics is responsible for the rapid, accelerated expansion during the early inflationary era? Were there extra light species beyond photons and neutrinos present in the universe during the radiation-dominated era? What is driving the current accelerated expansion of the universe?

Answering these questions requires scientists to develop a detailed understanding of (i) the nature, properties, and type of primordial fluctuations created during the inflationary era, (ii) the evolution and growth of these initial fluctuations into the visible objects we observe today, and (iii) the cosmic expansion history of the universe. The current paradigm for addressing these questions attributes the recent cosmic acceleration to a cosmological constant, a uniform repulsive energy throughout the universe. However, there are tensions that arise when attempting to consistently interpret both early and recent expansion data within this framework. These tensions may hint at physics that requires a significant paradigm shift.

The early universe’s primordial fluctuations, resulting from inflation, reveal critical insights into the physics of this era. These primordial fluctuations comprise energy density variations shaping the universe’s structure and gravitational waves indicating space-time’s response to high inflation energies. Analyzing the statistical properties of density fluctuations through the distribution of galaxies and CMB anisotropies uncovers the inflationary dynamics. Primordial gravitational waves leave a unique signature in CMB polarization, offering clues about fundamental physics at high energies. Post-inflation, the universe transitioned into a hot radiation phase, allowing investigation of new light particle species beyond the Standard Model. CMB measurements provide essential constraints on these species. Future measurements may probe relics present during the quark-hadron transition and, eventually, light species present at temperatures above the electroweak scale.

The matter-dominated epoch that followed the radiation-dominated era was recently interrupted by a burst of accelerated expansion driven by what is termed dark energy. The simplest explanation for dark energy, a cosmological constant (Ʌ), is a pillar of the ɅCDM cosmological paradigm—but confirming the nature of dark energy as either Ʌ or something else is a key science driver for our field. Investigating dark energy’s impact on cosmic structure growth and expansion history is crucial, with both observational and theoretical advances needed to distinguish between the cosmological constant of ɅCDM, more complex dark energy models, and alternative cosmological models with modified gravity.

Since the last P5 report, advances in our understanding of the early universe have been made through precise observations of the CMB’s temperature and polarization fluctuations. The US-led ground-based observing program, including the BICEP program, South Pole Telescope (SPT), the Atacama Cosmology Telescope (ACT), and POLARBEAR, combined with the European Planck Satellite, have provided us with crucial bounds on the energy scale of inflation, the abundance of light relics in the early universe, and the sum of neutrino masses, in addition to shedding light on the fundamental nature of dark matter and dark energy.

Significant progress has also been made in building statistically consistent portfolios of observations from galaxy surveys with well-understood systematics. On the growth of structure, a combination of (i) gravitational lensing; (ii) the distribution of galaxies across time and distances; (iii) the abundance of galaxy clusters; and (iv) the velocities of galaxies near over-dense regions, can precisely probe the parameters characterizing dark energy, determining whether it is consistent with a cosmological constant or has a dynamic of its own. For the cosmic expansion history, the transition from the past decelerating phase to the current era of accelerated expansion can be probed primarily through four types of measurements: the brightness of distant supernovae, strong lensing cosmography, the evolution in distance of a distinctive pattern in the distribution of galaxies known as the baryon acoustic oscillation signature, and gravitational wave standard sirens, which indicate cosmic distance. The major advances made by the Dark Energy Survey (DES) and the extended Baryon Oscillation Spectroscopic Survey (eBOSS) in development and application of these measurement methods, and in theoretical modeling, have directly influenced the ongoing and new projects recommended in this P5.

Our recommended portfolio will enable seminal discoveries connected with these crucial scientific questions along with other secondary science cases noted below. It is important to note that support for computational work and theory efforts (Recommendations 4f and 4b), as well as a healthy workforce (Recommendation 5), are essential for realizing the scientific potential of the experiments listed below.

4.2.2  –Major initiative: CMB-S4

CMB-S4 is the transformative next-generation CMB experiment, with the ambitious primary science goals of constraining the energy scale of inflation and determining the abundance of light relic particles in the early universe. CMB-S4 will also measure the sum of neutrino masses and probe the physics of dark matter and dark energy, in addition to a rich and exciting astrophysics program. This project will provide a significant advancement in sensitivity to gravitational waves produced by inflation and to light relics. Importantly, this sensitivity to light relics will extend beyond the quark-hadron transition, which is an important benchmark in searches for new physics. CMB-S4 construction is planned to begin in Chile and at the South Pole late in this decade (Recommendation 2a).

The current generation of ground-based CMB experiments includes the South Pole Observatory (BICEP Array and the South Pole Telescope, both currently operating), and Simons Observatory. CMB-S4 builds on decades of experience from US-led ground-based CMB experiments, but with increased sensitivity achieved by scaling up to nearly 500,000 detectors.

CMB-S4 presents an important opportunity for the field of particle physics: the discovery of gravitational waves produced by inflation in the extremely early universe would provide a direct window to this previously inaccessible epoch in cosmic history, and to the highest energy scales in the universe. CMB-S4 is designed to cross critical thresholds in the search for gravitational waves from inflation. Even a non-detection from CMB-S4 would rule out large classes of inflation models, placing interesting constraints on the theoretical landscape.

Achieving CMB-S4’s ambitious science goals requires installing telescopes at and observing from both the South Pole and Chile, which are proven sites with good observing conditions and infrastructure. For ground-based CMB experiments, the unprecedented sensitivity to the physics of inflation that probes the highest energy scales in the universe is possible only at the South Pole. The site at the South Pole is unique, providing a dry, stable atmosphere with continuous observation of the same patch of sky. The site in Chile is complementary because Earth’s rotation leads to the ability to observe large portions of the sky, which is important for constraining the abundance of light relic particles. Coordination between DOE-HEP, NSF-AST, and NSF-OPP is critical for the success of CMB-S4. NSF-OPP and CMB-S4 should continue to work closely together to ensure that the logistics footprint of the project at the South Pole is consistent with site capabilities (see section 6.6.4).

Given the planned landscape of ground- and space-based CMB experiments, CMB-S4 plays a unique role in using demonstrated technology with a two-site survey design that is crucial for addressing the key science goals. The two-site design ensures that the whole is far greater than the sum of its parts, enabling important crosschecks on systematics that would otherwise be impossible. CMB-S4 also provides important synergies with the space-based LiteBIRD instrument, which aims to launch in the next decade and constrain the energy scale of inflation through a complementary technique.

4.2.3  –Ongoing Projects: Rubin Observatory LSST and DESC, DESI

This decade will see tremendous advances in our understanding through the galaxy survey program established by the last P5 report: DESI (a spectroscopic survey), and Rubin Observatory LSST. Rubin Observatory will carry out the Legacy Survey of Space and Time (LSST), a 10-year imaging survey, and the LSST Dark Energy Science Collaboration (DESC) will carry out the fundamental physics analyses of LSST. DESI and LSST will enable analyses with multiple complementary methods of both structure growth and expansion history of the universe, with extensive programs to control systematics (Recommendation 1e). In particular, these experiments will provide unprecedented constraints on cosmic acceleration using several probes of structure growth and expansion rate during the entire time period of cosmic acceleration. The experiments will also reach back into the weakly matter-dominated era, when the expansion was still decelerating. Strong support for operations and data analysis will ensure a return on the investment in these experiments. This program will stress-test the standard cosmological paradigm, and is particularly powerful when combined with spaced-based datasets (e.g. from Euclid or the Nancy Grace Roman Space Telescope), with current CMB surveys, and even more so with CMB-S4. These combinations would benefit from dedicated efforts towards joint analysis including N-body simulations and simulated survey products.

As these surveys yield discoveries about cosmic evolution and improve our understanding of how to robustly constrain the cosmological model despite astrophysical and observational systematics, the particle physics community should use that new understanding to formulate future galaxy surveys. This includes, during the first 5 years of LSST, engagement with discussions of the post-LSST future of Rubin Observatory; and during DESI/DESI-II, refinement of Spec-S5 survey concepts.

These cosmic surveys also have important secondary science goals, such as constraining the sum of the neutrino masses (providing complementary information with the measurements of the mass ordering by DUNE) and dark matter (where astrophysical probes play an important role among other types of measurements).

4.2.4  –New Initiative: DESI-II upgrade

The DESI-II program (Recommendation 3c) is a first step at going beyond the galaxy surveys constructed as a result of the previous P5 report. Besides providing an opportunity for testing technology for next-generation spectroscopic surveys, its scientific goals include constraining cosmic acceleration by extending DESI dark energy constraints deeper into the matter-dominated regime, and complementing/enhancing dark energy and dark matter science with Rubin Observatory LSST by leveraging the power of overlapping spectroscopic and imaging surveys.

For example, the program could provide ~5%-level constraints on the dark energy density at a time when the standard cosmological paradigm predicts it is only a few percent of the energy density of the universe; it also provides opportunities for spectroscopic observations that would reduce key systematic uncertainties in Rubin Observatory LSST measurements of structure growth and cosmic expansion. Given the low construction cost for this extension to the DESI project, executing DESI-II provides a high scientific return on the existing investments in both DESI and Rubin Observatory LSST, especially as DESI-II will serve as a pathfinder for the next proposed major initiative, Spec-S5.

4.2.5  –Future Opportunity: Spec-S5

The proposed next-generation spectroscopic survey, Spec-S5, holds great promise to advance our understanding and reach key theoretical benchmarks in several areas: inflationary physics via the statistical properties of primordial fluctuations, late-time cosmic acceleration, light relics, neutrino masses, and dark matter. The balance between these scientific goals, which affects survey design, should be refined in light of early DESI and Rubin Observatory LSST results.

The coming years will see important preparations for Spec-S5, with support for necessary instrumentation R&D along with refining the survey concept—site selection and other crucial decision points. Going beyond the capabilities of DESI-II and Rubin Observatory LSST, Spec-S5 will permit us to map cosmic expansion deep into the matter-dominated regime, while also enabling order of magnitude improvements in our understanding of the early era of cosmic inflation. Carrying out these preparations during the 2020s, including the computational and theory work necessary to interpret the data, is essential to continuing the robust program of spectroscopic surveys.

Spec-S5 could be ready for construction at the end of this decade if key decisions regarding issues such as siting are resolved. With limited funds under the baseline budget scenario, the difficult choice was made to support Spec-S5 R&D (Recommendation 4e) but not construction, resulting in a significantly more mature survey concept for consideration for immediate construction by the next P5 (see Figure 1 timeline). However, in the event of exciting discoveries in DESI and/or Rubin Observatory LSST, and in better funding scenarios, a more mature Spec-S5 concept should be considered for construction at the end of this decade (Section 2.5.2).

4.2.6  –Future Opportunities: Line Intensity Mapping & Gravitational Waves

Line intensity mapping (LIM) techniques are potentially a valuable future method to address key particle physics science cases during the next twenty years by probing the expansion history and the growth of structure deep in the matter-dominated era when the first galaxies were forming. LIM observations of this era could enable tests of the theory of inflation by providing a precise map of the primordial hydrogen gas which is theoretically clean for interpretation. This technique has the potential to access an earlier epoch in the universe than Spec-S5. Work to prove the viability of this method (encompassing both analysis and instrumentation) should continue with multi-agency support (Recommendation 4e), including low-cost instrumentation development competed through the DOE R&D program. DOE has already partnered with NASA to construct one pathfinder LIM experiment, LuSEE-Night, and there are exciting opportunities for investment in ground-based activities in the coming decade.

Gravitational waves are a powerful new tool for exploring a range of astronomical and particle physics topics, including probing the expansion history of the universe using standard sirens. NSF has been an excellent steward of this program and should support the development of new capabilities and a next-generation project. The particle physics case for studying gravitational waves at all frequencies should be explored by expanded theory support.

4.2.7  –20-Year Vision

We are entering an exciting era in our study of cosmic evolution. The projects recommended by the last P5 report that are beginning operations, the project portfolio recommended by this P5 report, and the future projects for which R&D and project definition will occur in this decade, will allow for great progress in our knowledge of the entirety of our cosmic history, from the inflationary era, through the radiation and then matter dominated eras, to the dark energy era. Together with strong theory and computational support, that progress lays the foundation for the next generation of projects.

To support the success of this portfolio of cosmic surveys at a range of wavelengths, continued work and advocacy will be important to prevent or mitigate the effects of human-produced nuisances, including light pollution, satellite constellations in low-earth orbit, and radio-frequency interference.

The knowledge gained from CMB-S4 and eventually from Spec-S5 will enlighten us about the nature of inflation at the earliest cosmic times, both in terms of the energy scale and the inflationary dynamics. We recommend pathfinding works in the next decade, specifically LIM R&D and research, that will allow us to follow up any detected primordial signal from the inflationary era. Moving forward in cosmic time to the radiation and matter eras, we will have a window to new relics during the quark-hadron transition, and lay the groundwork for future projects that can push down to the electroweak scale.

In the event of a discovery beyond the standard cosmological paradigm, LIM and high-resolution CMB experiments could be formulated to confirm and characterize the discovery. Future gravitational wave experiments could provide complementary means to probe the expansion history deeper in the matter era. And finally at late times, our recommended portfolio sets us up with multiple complementary means to rigorously test the cosmological constant hypothesis and discover the time evolution of dark energy.

The flexibility of Spec-S5 to address multiple scientific goals (inflation, late-time cosmic acceleration, dark matter) depending on the priorities that emerge from DESI, early DESI-II, and Rubin Observatory LSST results makes it a crucial part of this 20-year vision. Similarly, future survey concepts for Rubin Observatory, to be developed later this decade after early LSST science results are available, could address key questions that come to the forefront of particle physics studies of cosmic evolution in five to ten years.