Our understanding of the Universe's origin, evolution, and large-scale organization has undergone revolutionary changes over the past century, driven by increasingly precise observations and theoretical breakthroughs. Cosmology, once merely a speculative field, has expanded into a data-rich discipline thanks to measurements of the cosmic microwave background radiation, galaxy surveys, and advanced detectors. This wealth of data not only illuminates the early Universe—when quantum fluctuations stretched to astronomical scales—but also reveals how filaments, clusters, and voids formed, creating the vast "cosmic web" we observe today.
In Topic 10: Cosmology and the Large-Scale Structure of the Universe, we examine the main pillars of modern cosmological research:
-
Cosmic inflation: theory and evidence
Early Universe inflation posits that during the first tiny fraction of a second, there was an extremely rapid exponential expansion that solved the horizon and flatness problems. It left imprints in density fluctuations later captured in the cosmic microwave background (CMB) and large-scale structure. Current data on CMB anisotropies and polarization strongly support this scenario, although the detailed physics of inflation (and the exact mechanism) is still actively studied. -
Detailed structure of the cosmic microwave background radiation
The CMB is the hot early Universe's radiation echo, encoding small temperature and polarization variations reflecting density perturbations about 380,000 years after the Big Bang. Such maps (e.g., Planck, WMAP) reveal the seeds of galaxies and clusters and precise cosmological parameters like matter density, the Hubble constant, and Universe curvature constraints. -
Cosmic web: filaments, voids, and superclusters
Gravity acting on dark matter and baryons from early fluctuations created the "cosmic web," where galaxies cluster along enormous filaments surrounding voids, forming superclusters. Dark matter and gas N-body simulations compared with redshift surveys show how structure hierarchically formed over billions of years—smaller halos merging into larger complexes. -
Baryon acoustic oscillations
In the hot primordial plasma before recombination, sound waves (acoustic oscillations) propagated through the photon-baryon fluid, leaving a characteristic scale in matter distributions. These BAO now serve as a "standard ruler" in galaxy correlation functions, allowing precise measurements of cosmic expansion and geometry, complementing supernova methods. -
Redshift surveys and Universe mapping
From the first CfA redshift surveys to modern initiatives like SDSS, DESI, or 2dF, astronomers have recorded millions of galaxies, constructing a three-dimensional reconstruction of the cosmic web. Such studies provide knowledge about large-scale flows, expansion rate, clustering amplitude, and dark energy's influence on the Universe over time. -
Gravitational lensing: a natural cosmic telescope
Massive galaxy clusters or cosmic structures distort background light propagation, creating multiple images or magnifying brightness—a natural telescope of nature. Beyond impressive astrophysical images, lensing allows precise measurement of total mass (including dark matter), assessment of cluster mass distribution, distance calibration, and study of dark energy through cosmic shear (weak lensing). -
Hubble constant measurement: tension
One of the latest cosmology questions is the discrepancy between "local" Hubble constant measurements (using distance ladders, e.g., Cepheid stars and supernovae) and "global" methods (ΛCDM analyses fitted to CMB data). This so-called Hubble tension has sparked discussions about possible new physics, systematic errors, or unknown phenomena in the early or late Universe. -
Dark energy surveys
Specialized projects like the Dark Energy Survey (DES), Euclid, and the Roman Space Telescope observe supernovae, galaxy clusters, and lensing signals to better understand the dark energy equation of state and evolution. These observations test whether dark energy is a simple cosmological constant (w = -1) or a dynamic field with variable w. -
Anisotropies and inhomogeneities
From temperature anisotropies in the CMB to local inhomogeneities in galaxy distributions—these phenomena are extremely important. They not only confirm cosmic inflation but also show how dark matter and baryons, under gravity, accumulate to form the large-scale environment of the Universe we see today. -
Current debates and unanswered questions
Although the ΛCDM model works well in many respects, open questions remain: details of inflation, the nature of dark matter particles, possible alternative gravity theories to explain cosmic acceleration, resolving the Hubble tension, and deeper Universe topology. These questions drive further theoretical development and new observational projects.
Reviewing these key topics—inflation, CMB structure, cosmic web, BAO, redshift surveys, gravitational lensing, dark energy observations, and unanswered questions—this topic reveals a grand portrait of the Universe's large-scale structure: how it formed from the early inflationary epoch, evolved under dark matter and dark energy, and still poses unsolved mysteries awaiting answers.