12.2 The nature of dark matter and dark energy

Dark Matter and Dark Energy

Dark matter and dark energy together account for roughly 95% of the universe's total energy content, yet neither can be observed directly with any telescope. Dark matter provides the gravitational scaffolding that holds galaxies and large-scale structures together, while dark energy drives the accelerating expansion of the universe. Understanding both is central to cosmology because they determine the universe's structure, evolution, and ultimate fate.

Evidence for Dark Matter and Dark Energy

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

• Galactic rotation curves. Stars in the outer regions of spiral galaxies orbit at speeds far higher than predicted by the visible mass alone. If only luminous matter were present, orbital velocities should drop off with distance from the galactic center (following Keplerian decline). Instead, rotation curves stay flat, implying a massive halo of unseen matter surrounding each galaxy.
• Gravitational lensing. Light from distant background galaxies is bent as it passes through foreground galaxy clusters. The degree of bending is much greater than the visible mass of those clusters can account for, revealing large concentrations of dark matter acting as a gravitational lens.
• CMB anisotropies. Tiny temperature fluctuations in the cosmic microwave background encode information about the composition of the early universe. The pattern of acoustic peaks in the CMB power spectrum is consistent with a universe containing about five times more dark matter than ordinary (baryonic) matter. This CMB-derived estimate agrees with independent measurements from galaxy surveys and Big Bang nucleosynthesis.
• Large-scale structure formation. The observed distribution of galaxies and galaxy clusters across cosmic time requires dark matter. Without it, the small density fluctuations in the early universe would not have grown fast enough to produce the cosmic web of filaments and voids we observe today.

The primary evidence for dark energy comes from a different observation:

• Type Ia supernovae. In the late 1990s, two independent teams measured the distances and redshifts of distant Type Ia supernovae (used as "standard candles"). They found that these supernovae were dimmer than expected, meaning they were farther away than a decelerating or coasting universe would predict. The conclusion: the expansion of the universe is accelerating, driven by a component with negative pressure now called dark energy.

Properties of Dark Matter vs. Dark Energy

Dark matter and dark energy are both "dark" because they do not interact electromagnetically, meaning they neither emit, absorb, nor scatter light. Beyond that shared trait, they behave very differently.

Dark matter (~27% of the universe's energy density):

• Interacts gravitationally but not electromagnetically
• Clumps together, forming halos around galaxies and clusters
• Plays a direct role in structure formation by providing gravitational wells into which baryonic matter falls
• Likely consists of one or more undiscovered particle species beyond the Standard Model

Dark energy (~68% of the universe's energy density):

• Acts as a negative pressure that counteracts gravitational attraction on cosmological scales
• Does not clump; its density appears uniform throughout space
• Becomes dominant over matter as the universe expands (matter density dilutes, but dark energy density stays roughly constant)
• Its exact nature remains unknown; the simplest model is Einstein's cosmological constant $\Lambda$

Dark Matter Particle Candidates

No dark matter particle has been detected in a laboratory yet, but several well-motivated candidates exist. Each comes with distinct mass ranges and detection strategies.

Weakly Interacting Massive Particles (WIMPs)

WIMPs are hypothetical particles with masses in the range of a few GeV to several TeV. They interact through gravity and the weak nuclear force but not electromagnetism. WIMPs gained popularity because a particle with weak-scale interactions naturally produces the observed dark matter abundance (sometimes called the "WIMP miracle").

Detection approaches:

1. Direct detection — Highly sensitive underground detectors (e.g., XENON, LZ) look for the tiny recoil of atomic nuclei when a WIMP scatters off them. These experiments must be shielded deep underground to reduce background noise from cosmic rays.
2. Indirect detection — Telescopes search for products of WIMP annihilation or decay in regions of high dark matter density, such as the galactic center. Signals could appear as excess gamma rays, neutrinos, or cosmic-ray positrons.

Axions

Axions are ultralight particles with predicted masses in the range of $10^{-6}$ to $10^{-3}$ eV. They were originally proposed to solve the strong CP problem in quantum chromodynamics (QCD), which asks why the strong force does not violate CP symmetry as much as the theory allows. It turns out that if axions exist, they could also be produced abundantly in the early universe, making them a natural dark matter candidate.

Detection approaches:

1. Axion haloscopes (e.g., ADMX) — A strong magnetic field inside a resonant cavity converts axions from the local dark matter halo into detectable microwave photons.
2. Axion helioscopes (e.g., IAXO) — These instruments point at the Sun, searching for axions produced in the solar core that convert to X-ray photons in a laboratory magnetic field.

Sterile Neutrinos

Sterile neutrinos are hypothetical neutrinos that interact only through gravity, not through the weak force like ordinary (active) neutrinos. They could have masses in the keV range, placing them in the "warm dark matter" category.

Detection approaches:

1. X-ray observations — A sterile neutrino can slowly decay into an active neutrino and an X-ray photon. Astronomers search for a faint, narrow X-ray emission line from galaxy clusters or the Milky Way halo at the energy corresponding to half the sterile neutrino's mass.
2. Structure formation signatures — Because warm dark matter particles move faster than cold dark matter, they smooth out small-scale structure. Comparing observed distributions of dwarf galaxies and the Lyman-alpha forest with simulations can constrain sterile neutrino properties.

Dark Energy and the Universe's Fate

As the universe expands, the density of matter (both ordinary and dark) drops because the same amount of mass occupies an ever-larger volume. Dark energy, however, maintains a roughly constant density. Over time, dark energy therefore becomes the dominant component, and the expansion accelerates.

The ultimate fate of the universe depends on how dark energy behaves, which is characterized by the equation of state parameter $w$, defined as the ratio of dark energy's pressure to its energy density.

Three possible scenarios:

1. Big Freeze (Heat Death) — If $w = -1$ (a cosmological constant), the universe expands forever at an accelerating rate. Galaxies beyond our local group recede beyond the observable horizon. Stars exhaust their fuel, black holes slowly evaporate via Hawking radiation, and the universe asymptotically approaches a cold, dilute, near-empty state. This is the most likely outcome based on current data.

2. Big Rip — If $w < -1$ (so-called "phantom" dark energy), the dark energy density increases over time. The accelerating expansion eventually becomes so violent that it overwhelms all binding forces: first galaxy clusters are torn apart, then galaxies, then solar systems, then atoms themselves. The universe ends in a singularity after a finite time.

3. Big Crunch — If $w > -1$ and dark energy weakens sufficiently over time, gravity could eventually dominate. The expansion would slow, halt, and reverse, causing the universe to collapse back into an extremely hot, dense state. Some speculative models suggest this could trigger a new Big Bang (an oscillating universe).