12.5 Dark Matter and Dark Energy

Dark Matter and Dark Energy

Dark matter and dark energy together make up roughly 95% of the total energy content of the universe. Understanding them is essential for explaining how galaxies form, why the universe is structured the way it is, and what will happen to the cosmos in the far future.

Definitions and Cosmic Composition

Dark matter is a hypothetical form of matter that exerts gravitational effects on visible matter but does not interact with electromagnetic radiation. You can't see it, and it doesn't emit, absorb, or reflect light. It's detected only through its gravitational influence.

Dark energy is a hypothetical form of energy that permeates all of space and drives the accelerating expansion of the universe. While dark matter pulls things together, dark energy pushes the universe apart on cosmic scales.

The observable universe breaks down roughly as follows:

• ~68% dark energy
• ~27% dark matter
• ~5% ordinary (baryonic) matter: protons, neutrons, electrons

The Lambda-CDM model (Lambda–Cold Dark Matter) is the current standard model of cosmology. It incorporates both dark matter and dark energy as fundamental components and successfully accounts for a wide range of observations, from the cosmic microwave background to large-scale structure.

Dark matter provides the gravitational scaffolding that allows visible matter to clump together into galaxies and clusters. Dark energy counteracts gravity's attractive force at cosmic scales, causing the expansion of space to accelerate over time.

Roles in Cosmic Structure

Dark matter organizes the universe on the largest scales. It forms the cosmic web, a vast network of filaments and voids that guides the distribution of visible matter. Galaxies and galaxy clusters tend to sit along these filaments, while enormous voids stretch between them.

• Dark matter's enhanced gravitational attraction drives the formation of galaxy clusters and superclusters.
• The hierarchical model of galaxy formation depends on dark matter providing initial density fluctuations in the early universe. Small structures merge over time to build larger ones.
• Dark matter halos surround galaxies and stabilize their rotation, preventing spiral arms from flying apart.

Dark energy works in the opposite direction. Its repulsive effect causes the expansion of space to accelerate, which over time counteracts the gravitational pull of matter. The balance between dark matter's attraction and dark energy's repulsion determines the universe's ultimate fate, whether it ends in a Big Freeze (continued expansion and cooling) or a Big Rip (accelerating expansion that tears apart all bound structures).

The relative densities of dark matter and dark energy also influence the overall geometry and curvature of space-time.

Evidence for Dark Matter and Dark Energy

Galactic and Cluster Observations

Several independent lines of evidence point to the existence of dark matter and dark energy:

• Galactic rotation curves: Stars in the outer regions of galaxies orbit faster than expected based on visible mass alone. If only luminous matter were present, orbital velocities should drop off with distance from the galactic center (following Keplerian dynamics). Instead, they remain roughly flat, implying a large halo of unseen mass.
• Gravitational lensing: Galaxy clusters bend the light of background objects more than their visible mass can explain. The extra bending requires additional, invisible mass.
• The Bullet Cluster: When two galaxy clusters collided, the hot gas (visible via X-rays) was separated from the bulk of the mass (mapped via gravitational lensing). This provides direct empirical evidence that most of the mass is non-luminous and non-baryonic.
• Cosmic microwave background (CMB): Temperature fluctuations in the CMB encode information about the density of matter in the early universe. The pattern of these fluctuations matches models that include dark matter.
• Type Ia supernovae: Observations of these "standard candle" explosions in distant galaxies showed that the expansion of the universe is accelerating, not slowing down. This was the key discovery that established the existence of dark energy (1998).
• Large-scale structure: The observed distribution, clustering, and velocities of galaxies require dark matter to explain the patterns we see.

Cosmological Implications

Multiple independent measurements converge on the same picture:

• The CMB power spectrum (the pattern of hot and cold spots at different angular scales) matches predictions from models that include both dark matter and dark energy.
• Baryon acoustic oscillations (BAOs) are regular patterns in the distribution of galaxies, imprinted by sound waves in the early universe. Their measured scale is consistent with the Lambda-CDM model.
• Weak gravitational lensing surveys map the distribution of dark matter across large swaths of sky, confirming its role in shaping the cosmic web.
• The integrated Sachs-Wolfe effect (a subtle correlation between CMB temperature and large-scale structure) provides additional evidence for dark energy's influence on cosmic expansion.
• Big Bang nucleosynthesis predicts the abundances of light elements (hydrogen, helium, lithium) produced in the first few minutes after the Big Bang. These predictions constrain the total amount of baryonic matter and confirm that most matter in the universe must be non-baryonic, supporting the need for dark matter.

Effects on the Universe

Structural Impact

Dark matter creates gravitational wells, regions of concentrated mass that trap ordinary matter and seed the formation of galaxies and stars. Without these wells, baryonic matter would not have clumped together fast enough to form the structures we observe.

• The filamentary structure of the cosmic web is shaped by dark matter, guiding the flow of baryonic matter along its strands.
• Dark matter halos stabilize galaxy rotation and prevent the rapid disintegration of spiral arms.
• Concentrations of dark matter enhance gravitational lensing, which astronomers use to observe extremely distant galaxies and quasars that would otherwise be too faint to detect.
• Dark matter bridges between galaxies in clusters facilitate mergers and interactions, shaping galactic evolution.
• The abundance and distribution of dwarf galaxies around larger galaxies is explained by dark matter substructure within halos.

Evolutionary Consequences

The expansion history of the universe reflects the changing balance between dark matter's gravitational attraction and dark energy's repulsive push.

• In the early universe, matter density was high and dark matter dominated, allowing structures to grow through gravitational collapse.
• As the universe expanded, matter density dropped while dark energy density remained roughly constant. Dark energy eventually became dominant (around 5 billion years ago), and the expansion began to accelerate.
• The rate of structure growth slows as dark energy takes over, because accelerating expansion works against gravitational collapse.
• Cosmic voids grow larger over time as dark energy pushes matter away from underdense regions.
• Galaxy cluster formation and evolution are modulated by the interplay between dark matter concentration and dark energy expansion.
• The long-term fate of cosmic structures depends on whether dark energy remains constant or changes over time. If dark energy strengthens, a Big Rip scenario becomes possible, in which accelerating expansion eventually tears apart galaxies, stars, and even atoms.

Theories of Dark Matter and Dark Energy

Dark Matter Candidates

No dark matter particle has been directly detected yet. Several candidates have been proposed:

• WIMPs (Weakly Interacting Massive Particles): The leading candidate for decades, predicted by supersymmetry theories in particle physics. WIMPs would interact via gravity and the weak nuclear force, making them extremely difficult to detect. Large underground experiments (like LUX-ZEPLIN and XENON) search for WIMP interactions.
• Axions: Hypothetical ultra-light particles originally proposed to solve the strong CP problem in quantum chromodynamics. If they exist in sufficient numbers, they could account for dark matter.
• Sterile neutrinos: Hypothetical particles related to standard neutrinos but with no standard model interactions other than gravity. They could contribute to dark matter if they have the right mass.
• Primordial black holes: Black holes formed in the very early universe (not from stellar collapse) have been proposed as an alternative to particle dark matter, though observational constraints limit how much of the dark matter they could represent.
• Self-interacting dark matter: Models where dark matter particles interact with each other (not just gravitationally) attempt to explain observed galaxy core densities that standard cold dark matter models struggle with.
• Fuzzy dark matter: Ultra-light bosonic particles with very long de Broglie wavelengths. Their quantum behavior on galactic scales could explain some small-scale structure observations.

Dark Energy Models

The nature of dark energy is even less understood than dark matter. Several theoretical frameworks exist:

• Cosmological constant ($\Lambda$): The simplest model. It represents a constant energy density of the vacuum itself. Einstein originally introduced $\Lambda$ into his field equations, then abandoned it, but observations of accelerating expansion brought it back. The cosmological constant fits current data well, but its observed value is vastly smaller than quantum field theory predicts, a discrepancy known as the cosmological constant problem.
• Quintessence: A dynamic scalar field that changes over time, unlike the fixed cosmological constant. Quintessence models allow dark energy density to evolve, which could be tested with future precision measurements.
• Phantom energy: A hypothetical form of dark energy with an equation of state parameter $w < -1$. If real, phantom energy would grow stronger over time and could lead to a Big Rip.
• Modified gravity theories (e.g., MOND): These attempt to explain galactic dynamics by modifying the laws of gravity at large scales rather than invoking dark matter. MOND (Modified Newtonian Dynamics) works well for individual galaxy rotation curves but struggles to explain cluster-scale observations and the CMB.
• Chameleon fields: Scalar fields whose properties vary depending on local matter density. In dense environments, the field is suppressed; in voids, it behaves like dark energy. This could explain why dark energy effects are only apparent on cosmological scales.
• Quantum gravity approaches: The holographic principle and quantum field theory in curved spacetime are being explored to reconcile dark energy with quantum mechanics, but no complete theory exists yet.