The activities described below can be summarized in a single sentence: “Using the universe as a general-purpose laboratory for probing fundamental physics.”
Just like particle physics, cosmology also has its standard model – the ΛCDM model – which has been phenomenologically very successful. However, only about 5% of its content, being covered by the standard model of particle physics, is well understood. The remaining 95% is composed of two enigmatic components: dark matter and dark energy (DE), accounting for approximately 25% and 70% of the universe’s total energy density, respectively.
Accordingly, two central questions are repeatedly addressed by members of our theory group:
To account for the observed large-scale structure and its evolution, dark matter must be sufficiently cold, meaning its initial random velocities must be small. To remain “dark”, dark matter’s interactions with the visible sector must be strongly suppressed. Surprisingly, however, dark matter self-interactions (beyond gravity) can be sizable.
In its simplest form, dark matter is modeled as a cold, collisionless, self-gravitating gas, known as CDM (Cold Dark Matter), which has been the standard dark matter paradigm for about four decades. CDM performs exceptionally well at the largest scales. However, at the smaller scales of dwarf galaxies or galactic cores, deviations have been observed. These could arise from our incomplete understanding of feedback effects in the visible sector (e.g., star formation, supernova feedback, or interactions with supermassive black holes). Alternatively, they might indicate the need for modifications to the CDM paradigm.
Several modifications can be considered:
Warm Dark Matter (WDM): Relaxing the “coldness” assumption by allowing for more random motion.
Self-Interacting Dark Matter (SIDM): Abandoning the collisionless assumption.
Fuzzy Dark Matter (FDM): Assigning the DM particle an extremely small mass, so its de Broglie wavelength becomes comparable to the size of galactic cores.
Primordial Black Holes (PBHs): DM could consist of macroscopic objects, such as primordial black holes, rather than elementary particles.
Compared to dark matter, DE is even more mysterious.
In the ΛCDM model, dark energy is attributed to the cosmological constant Λ or vacuum energy, which suffers from severe fine-tuning issues: why is the resulting vacuum energy so tiny despite contributions that should naturally be enormous? One potential resolution involves anthropic reasoning. However, more elegant solutions might involve a symmetry or mechanism which sets Λ=0, with dark energy arising from a dynamical scalar field, as in quintessence models.
In such scenarios, dark energy varies over time, a signature that could potentially be observed. Recent DESI data has provided intriguing hints toward these models. Another possibility, also explored by our group, is that dark energy reflects modifications to general relativity, rather than a new energy component.
Cosmic inflation describes the universe’s rapid expansion during its earliest moments, solving key cosmological puzzles such as the uniformity of the cosmic microwave background (CMB) and the universe’s flatness. Inflation also explains the origin of large-scale structure.
The simplest models attribute inflation to a particle known as the inflaton, whose energy density drove the exponential expansion. Researchers at NICPB explore leading inflaton models, comparing them with experimental data and connecting them to other phenomena, such as dark matter, Higgs physics, and more.
In recent decades, black holes have emerged as one of the fastest-growing research fields in astrophysics and cosmology. They intersect with topics such as gravitational waves, cosmic inflation, dark matter, cosmic structure formation, and quantum gravity.
At NICPB, we study primordial black holes (PBHs) as potential windows into exotic early-universe physics. PBHs are also investigated as DM candidates or as seeds for supermassive black holes. Gravitational wave data from LIGO/Virgo/KAGRA has been used to constrain stellar-mass PBHs, while gravitational wave measurements from pulsar timing arrays (PTAs) and electromagnetic signals from the James Webb Space Telescope (JWST) have helped probe the formation and evolution scenarios of supermassive black holes at galactic centers.
The cosmic medium between the CMB’s last-scattering surface and the observer acts as a natural ionization chamber for probing energy inputs from exotic phenomena, such as:
Decaying or annihilating dark matter.
Evaporating or accreting black holes.
Observations of 21cm hydrogen absorption/emission features in the radio band can serve as powerful calorimeter for these processes. Similarly, gamma-ray or X-ray observations may reveal direct signatures of dark matter annihilation or decay. Cosmic-ray spectra can also be exploited to study energy inputs from the dark sector.
All of the above cosmology research directions overlap significantly with other theory topics at NICPB and are developed in close synergy with them.
