Dark Matter’s Role in Universe Expansion and Temperature Evolution: New Insights

Dark matter, accounting for approximately 15% of the universe’s total matter, plays a significant role in cosmology. Its mass and interactions influence the universe’s expansion rate, temperature evolution, and the cosmic microwave background (CMB) anisotropies. Research by scientists from the University of Southern California, the University of Texas at Austin, and the California Institute of Technology provides new cosmological bounds for dark matter models. By considering the effects of dark matter mass and interactions simultaneously, the sensitivity of CMB measurements can be increased, potentially leading to a more accurate understanding of dark matter’s nature and role in the universe.

What is the Role of Dark Matter in Cosmology?

Dark matter (DM) is a significant universe component, accounting for approximately 15% of its total matter. Despite its prevalence, the physical nature of dark matter remains largely unknown. Standard cosmology posits that dark matter is cold and collisionless, interacting only gravitationally with the Standard Model of particle physics. However, a range of theories beyond this standard paradigm are being explored, which could lead to unique observational consequences.

In the standard thermal freeze-out scenario, dark matter is in chemical equilibrium with the thermal bath at early times. As the temperature drops below the dark matter mass, dark matter becomes non-relativistic, and its equilibrium number density drops exponentially. This process, known as decoupling, is primarily dictated by dark matter mass, while the freeze-out abundance of dark matter is governed by its annihilation rate to Standard Model particles.

During this decoupling process, the contribution of dark matter to the entropy density of the universe is transferred to the thermal bath, slowing down the cooling of the bath. Additionally, dark matter behavior as a radiation-like or matter-like fluid affects the expansion rate of the universe.

How Does Dark Matter Influence the Universe’s Expansion and Temperature Evolution?

For high dark matter masses (greater than 20 MeV), the decoupling process occurs sufficiently early in cosmic history that there are no observable effects on the temperature evolution and expansion rate. However, for dark matter masses between 10 keV and 20 MeV, the decoupling process occurs around the time of Big Bang Nucleosynthesis (BBN) and may affect standard BBN predictions through changes to the expansion rate, photon-to-baryon density ratio, and weak interaction rates.

As a result, dark matter mass can affect the production of light chemical elements and the effective number of relativistic particle species. For even lower dark matter masses (less than 10 keV), the decoupling process does not impact BBN, but the presence of relativistic dark matter still alters the expansion rate during BBN. These effects that originate during the BBN era can be captured in the cosmic microwave background (CMB) anisotropies and provide some of the most stringent bounds on the mass of light thermal-relic dark matter.

How Do Dark Matter Interactions Affect the Cosmic Microwave Background?

Apart from its mass, the non-gravitational interactions of dark matter with baryons can also affect CMB anisotropies in a more direct manner, leading to changes in the CMB power spectra. For example, dark matter-baryon elastic scattering suppresses the clustering of matter in the universe through dark matter-baryon momentum transfer, which is absent in the standard ΛCDM model.

Additionally, late-time post freeze-out residual annihilation of dark matter into Standard Model particles injects energy into the plasma, potentially altering the recombination history and increasing the optical depth of CMB photons. In summary, the fundamental properties of dark matter affect cosmological observables in three distinct ways: the mass of dark matter controls the onset of its decoupling from the thermal bath in the early universe, the dark matter-baryon scattering cross-section quantifies the rate of momentum transfer and affects clustering of matter, and the dark matter annihilation cross-section determines the energy injection into the plasma at late times as well as the dark matter relic abundance at the time of freeze-out.

How Do Dark Matter Mass and Interactions Influence Cosmological Data?

The effects of dark matter mass and interactions are typically considered separately in cosmological data analyses. However, under the assumption of a thermal-relic scenario, these effects can be simultaneously relevant to the same observables, and their joint consideration may alter cosmological bounds on individual parameters.

For example, dark matter-baryon scattering at the level of current cosmological bounds typically implies that dark matter is in equilibrium with the thermal bath during BBN, thus the effects of dark matter mass and scattering with baryons should be considered and analyzed together. When considering late-time residual dark matter annihilation, the effects of dark matter mass and annihilation cross-section should also be considered together.

What are the Implications for Future Dark Matter Research?

The research conducted by Rui An, Kimberly K Boddy, and Vera Gluscevic from the Department of Physics and Astronomy at the University of Southern California, the Texas Center for Cosmology and Astroparticle Physics at the University of Texas at Austin, and the California Institute of Technology, provides new self-consistent cosmological bounds for dark matter models.

These findings have significant implications for future dark matter searches. By considering the effects of dark matter mass and interactions simultaneously, researchers can increase the sensitivity of CMB measurements to both fundamental properties of dark matter. This approach could lead to a more accurate understanding of the nature of dark matter and its role in the universe.