Observed supernovae, galaxy clusters, and gravitational lensing to uncover the nature of dark energy
The Mysterious Cosmic Accelerator
In 1998, two independent teams made a surprising discovery: distant Type I supernovae appeared dimmer than expected under a decelerating or nearly constant expansion of the Universe. This indicated that the Universe's expansion is accelerating. This shift in results gave rise to the idea of "dark energy" – an unknown "repulsive" effect driving the Universe to speed up. The simplest explanation is the cosmological constant (Λ) with an equation of state w = -1, but it remains unknown whether dark energy is truly constant or can dynamically change. Fundamentally, determining the nature of dark energy could initiate a new era in fundamental physics, linking cosmic-scale observations with quantum field theory or new definitions of gravity.
Dark energy surveys – specialized observational programs utilizing various methods to assess the imprint of dark energy on cosmic expansion and structure growth. The key methods are:
- Type I supernovae (standard candles) – to study the distance-redshift relation.
- Galaxy clusters – to track the evolution of matter overdensities over time.
- Gravitational lensing (strong and weak) – to study mass distribution and the geometry of the Universe.
By comparing observational data with theoretical models (e.g., ΛCDM), these surveys attempt to constrain the dark energy equation of state (w), possible time evolution w(z), and other cosmic dynamics parameters.
2. Type I Supernovae: Standard Candles for Expansion Studies
2.1 Discovery of Acceleration
Type I supernovae – thermonuclear explosions of white dwarfs, characterized by a fairly uniform peak brightness that can be "standardized" based on the light curve shape and color corrections. In the late 1990s, the High-Z Supernova Search Team and the Supernova Cosmology Project observed supernovae up to z ∼ 0.8 that appeared dimmer (thus farther) than expected in a Universe without accelerated expansion. This finding indicated cosmic acceleration, for which the 2011 Nobel Prize in Physics was awarded to the leading members of these projects [1,2].
2.2 Modern Supernova Surveys
- SNLS (Supernova Legacy Survey) – Canada–France–Hawaii telescope, which collected hundreds of supernovae up to z ∼ 1.
- ESSENCE – focused on the intermediate redshift range.
- Pan-STARRS, DES supernova programs – wide-field surveys detecting thousands of Type I supernovae.
By combining supernova distance moduli with redshift data, the “Hubble Diagram” is constructed, directly tracing the Universe's expansion rate over cosmic time. Results indicate dark energy likely has w ≈ -1, but small variations are not excluded. Current local supernova–Cepheid calibrations also contribute to the “Hubble tension” debate, showing a higher H0 value than predicted by CMB data.
2.3 Future Prospects
In the future, deep variable object surveys – the Rubin Observatory (LSST) and the Roman Space Telescope – will capture tens of thousands of Type I supernovae even up to z > 1, enabling tighter constraints on w and its possible variations w(z). The main challenge is systematic calibration – ensuring that unaccounted luminosity evolution, dust, or population changes do not mimic dark energy variations.
3. Galaxy Clusters: Massive Halos as Cosmic Probes
3.1 Cluster Abundance and Growth
Galaxy clusters are the largest gravitationally bound structures dominated by dark matter, hot intracluster gas, and galaxies. Their number over cosmic time is very sensitive to matter density (Ωm) and dark energy's effect on structure growth. If dark energy slows structure formation, fewer less massive clusters will form at high redshift. Therefore, counting clusters at various redshifts and measuring their masses can constrain Ωm, σ8, and w.
3.2 Detection Methods and Mass Calibration
Clusters can be identified by:
- X-ray radiation from hot gas (e.g., ROSAT, Chandra).
- Sunyaev–Zeldovich (SZ) effect: CMB photon distortions caused by collisions with hot electron gas in clusters (SPT, ACT, Planck).
- Optical or IR radiation: higher density of red galaxy regions (e.g., SDSS, DES).
To calculate the total cluster mass from observed indicators, mass-observable scaling relations are needed. Weak lensing helps calibrate these relations and thus reduce systematics. Surveys like SPT, ACT, or DES have already used clusters for dark energy studies, although mass uncertainties remain important.
3.3 Key Surveys and Results
DES cluster catalog, eROSITA X-ray survey, and Planck SZ cluster catalog together cover thousands of clusters up to z ~ 1. They confirm the ΛCDM model Universe, although some studies have shown minor discrepancies among results regarding the amplitude of structure growth. By expanding cluster mass calibration and detection functions, cluster data can further constrain dark energy.
4. Gravitational Lensing: Study of Mass and Geometry
4.1 Weak Lensing (Cosmic Shear)
Shapes of distant galaxies are slightly distorted (shear) by foreground mass distribution. By analyzing millions of galaxy images, one can reconstruct matter density fluctuations and their growth, sensitive to Ωm, σ8, and dark energy effects. Projects like CFHTLenS, KiDS, DES, and future Euclid or Roman will achieve cosmic shear measurements at percent-level precision, potentially revealing deviations or confirming ΛCDM [3,4].
4.2 Strong Lensing
Massive clusters or galaxies can create multiple images of background sources or light arcs, magnifying them. Although this is more local information, strong lensing allows precise measurement of mass distribution and, using quasar time delays (e.g., H0LiCOW), independent estimation of the Hubble constant. Some studies show H0 ≈ 72–74 km/s/Mpc, close to local supernova measurements, thus contributing to the "Hubble tension."
4.3 Combination with Supernovae and Clusters
Lensing data complement cluster constraints well (e.g., cluster mass calibrated by lensing) and supernova distance measurements, all combining into a joint cosmological parameter fit. The synergy of lensing, clusters, and supernovae is crucial to reduce degeneracies and systematics for reliable dark energy constraints.
5. Major Current and Future Dark Energy Surveys
5.1 Dark Energy Survey (DES)
Conducted 2013–2019 with the 4m Blanco telescope (Cerro Tololo), DES observed ~5000 sq. degrees of sky in five filters (grizY), and also ran a supernova survey in dedicated fields. It includes:
- Supernova sample (~thousands of Type I SNe) to build the Hubble diagram.
- Weak lensing (cosmic shear) to study matter distribution.
- Cluster observations and BAO in galaxy distributions.
Its third-year and final analysis yielded results similar to ΛCDM, showing w ≈ -1 ± 0.04. Combining Planck + DES data reduces errors even further, finding no clear sign of evolving dark energy.
5.2 Euclid and Nancy Grace Roman Space Telescope
Euclid (ESA) is expected to launch around 2023, performing near-IR imaging and spectroscopy over ~15,000 sq. degrees. It will measure both weak lensing (shapes of billions of galaxies) and BAO (spectral shift measurements). A ~1% distance accuracy up to z ≈ 2 is expected – enabling very sensitive tests of possible w(z) ≠ constant.
Roman Telescope (NASA), planned for the 2030s, will have a wide-field IR camera and conduct the "High Latitude Survey," including lensing measurements and supernova detection. These projects aim for sub-percent level constraints on w and its possible variations, or will confirm that it is indeed a constant cosmological constant.
5.3 Other Projects: DESI, LSST, 21 cm
Although DESI is primarily a spectroscopic BAO survey, it complements dark energy studies by measuring distances at various redshifts with 35 million galaxies/quasars. LSST (Rubin Observatory) will observe ~10 million supernovae over 10 years and record billions of galaxy shapes for weak lensing. 21 cm intensity maps (SKA, CHIME, HIRAX) also promise to measure large-scale structure and BAO at high redshift, further constraining dark energy evolution.
6. Scientific Goals and Significance
6.1 Precise Determination of w and Its Evolution
The goal of many dark energy surveys is to measure the equation of state parameter w, searching for possible deviations from -1. If w ≠ -1 or changes over time, it would indicate a dynamic field (e.g., quintessence) or modifications of gravity. Current data show w = -1 ± 0.03. Upcoming surveys could narrow this to ±0.01 or better, either confirming an almost constant vacuum energy or opening the way to new physics.
6.2 Testing Gravity on Large Scales
The growth rate of structures, measured through redshift-space distortions or weak lensing, can reveal whether gravity matches GR (general relativity). If structures grow faster or slower than predicted by ΛCDM given a certain expansion history, there may be hints of modified gravity or dark energy interactions. So far only minor discrepancies have been observed, but more data will be needed for decisive results.
6.3 Hubble Tension Resolution?
Dark energy surveys can help by reconstructing the expansion history at intermediate redshifts (z ∼ 0.3–2), thus connecting local ladder and early Universe (CMB) expansion estimates. If the “tension” arises from new physics in the early Universe, such intermediate measurements can confirm or refute it. Or they may show that local measurements systematically differ from the cosmic average, helping to understand (or sharpen) the tension.
7. Challenges and Next Steps
7.1 Systematic Errors
Each method has its own challenges: supernova calibration (dust absorption, standardization), cluster mass and observed quantity relations, lensing shape measurement errors, photometric redshift errors. Surveys pay great attention to ensuring systematic accuracy. Combining independent methods is crucial for cross-validation.
7.2 Large Data Volumes
Upcoming surveys will provide enormous data: billions of galaxies, millions of spectra, thousands of supernovae. Automated data processing systems, machine learning classifiers, and advanced statistical analyses are essential. Large research teams (DES, LSST, Euclid, Roman) collaborate to ensure the most robust results, sharing data and intersections between different methods.
7.3 Possible Surprises
Historically, each major set of cosmic observations either confirms the standard model or reveals new anomalies. If we detect even a slight deviation of w(z) from -1, or persistent discrepancies in structure growth, the theory may need revision. Some propose early dark energy, additional relativistic species, or exotic fields. So far, ΛCDM dominates, but persistent long-term discrepancies could spur breakthroughs beyond the conventional model.
8. Conclusion
Dark energy surveys, utilizing supernovae, galaxy clusters, and gravitational lensing, are the core of modern cosmological progress aimed at understanding the accelerating expansion of the Universe. Each method surveys a different spectrum and properties of cosmic epochs:
- Type I supernovae allow extremely precise distance measurements based on redshift, reflecting the nature of late-time expansion.
- The abundance of clusters shows how structures form under the influence of dark energy "pushes," revealing matter density and growth rate.
- Weak lensing shows the overall mass fluctuation, linking the Universe's geometry with structure growth; strong lensing, by measuring time delays, can even determine the Hubble constant.
Major projects – DES, Euclid, Roman, DESI and others – are approaching a percent or even more precisely measured cosmic expansion parameter, allowing to refine whether ΛCDM with a cosmological constant remains unbroken or signs of evolving dark energy appear. These reviews can also contribute to solving the Hubble tension, test possible gravity modifications, or even discover new cosmic phenomena. Indeed, as data volumes grow over the coming decade, we are increasingly approaching a conclusion on whether dark energy is simply vacuum energy or if new physics lies behind it. This perfectly illustrates how cosmic observations and advanced instruments lead to fundamental astrophysical discoveries.
Literature and Further Reading
- Riess, A. G., et al. (1998). "Observational evidence from supernovae for an accelerating universe and a cosmological constant." The Astronomical Journal, 116, 1009–1038.
- Perlmutter, S., et al. (1999). "Measurements of Ω and Λ from 42 high-redshift supernovae." The Astrophysical Journal, 517, 565–586.
- Bartelmann, M., & Schneider, P. (2001). "Weak gravitational lensing." Physics Reports, 340, 291–472.
- Abbott, T. M. C., et al. (DES Collaboration) (2019). "Dark Energy Survey Year 1 results: Cosmological constraints from galaxy clustering and weak lensing." Physical Review D, 99, 123505.
- Laureijs, R., et al. (2011). "Euclid Definition Study Report." arXiv:1110.3193.