8.2 The nature and properties of dark energy

Dark Energy

Dark energy and universe expansion

Dark energy is a hypothetical form of energy that permeates all of space and drives the accelerating expansion of the universe. It's the leading explanation for a startling discovery made in the late 1990s: observations of distant Type Ia supernovae showed that the universe's expansion is speeding up, not slowing down as most physicists had expected.

• Two independent teams (the Supernova Cosmology Project and the High-z Supernova Search Team) found that distant supernovae were dimmer than predicted, meaning they were farther away than a decelerating universe would allow.
• The simplest explanation was that some unknown energy is pushing space apart at an increasing rate.

Dark energy is thought to make up roughly 68% of the total energy density of the universe, making it the dominant component by a wide margin. Dark matter accounts for about 27%, and ordinary (baryonic) matter makes up only about 5%.

Properties of dark energy

What makes dark energy so unusual is its negative pressure. In general relativity, pressure contributes to gravity. Normal matter and radiation have positive pressure, which adds to gravitational attraction and works to slow expansion. Dark energy flips this: its negative pressure produces a repulsive gravitational effect that accelerates expansion.

Two other key properties set dark energy apart:

• Uniform distribution. Dark energy doesn't clump the way matter does. It's spread evenly throughout space, with the same density everywhere.
• Constant density over time. As the universe expands, matter and radiation get diluted because the same amount of stuff occupies a larger volume. Dark energy does not dilute. Its energy density stays constant even as space stretches, which means its total contribution to the universe's energy budget grows over time relative to matter and radiation.

This combination of traits is why dark energy increasingly dominates the universe's behavior as expansion continues.

Dark energy vs other cosmic components

Dark energy is fundamentally different from both ordinary matter and dark matter:

• Matter has positive pressure; dark energy has negative pressure.
• Matter density decreases as the universe expands (spread over more volume); dark energy density remains constant.
• Dark matter interacts gravitationally and clumps into halos around galaxies; dark energy is smooth and uniform everywhere.
• Neither dark matter nor dark energy interacts with electromagnetic radiation, but dark matter's gravitational effects on visible structures (galaxy rotation curves, gravitational lensing) are more localized and easier to trace than dark energy's influence.

The simplest theoretical candidate for dark energy is the cosmological constant $\Lambda$, a term Einstein originally added to his field equations to allow for a static universe. He later dropped it, but the 1998 supernova results brought it back. If dark energy is indeed a cosmological constant, it represents a fixed energy density intrinsic to space itself, unchanging across all of cosmic history.

Challenges in dark energy detection

Dark energy doesn't emit, absorb, or scatter light. You can't point a telescope at it and see it. Its existence is entirely inferred from its effect on the large-scale expansion history of the universe.

Studying dark energy therefore requires measuring cosmic expansion with extreme precision over billions of years of lookback time. This demands:

• Massive sky surveys covering large fractions of the observable universe
• Sophisticated statistical techniques to extract tiny signals from noisy data
• Multiple independent methods (supernovae, baryon acoustic oscillations, weak gravitational lensing, galaxy cluster counts) to cross-check results

Several major experiments are designed to tighten constraints on dark energy's properties:

1. Dark Energy Survey (DES) — mapped hundreds of millions of galaxies to trace expansion and structure growth
2. Vera C. Rubin Observatory (formerly LSST) — will conduct a decade-long survey of the southern sky, tracking billions of objects
3. Nancy Grace Roman Space Telescope (formerly WFIRST) — a NASA space mission designed to measure dark energy through supernovae and weak lensing from above Earth's atmosphere
4. Euclid (ESA) — a space telescope launched in 2023, mapping the geometry of the universe over the last 10 billion years

These experiments aim to pin down the equation of state parameter $w$, which relates dark energy's pressure to its energy density ($w = P / \rho$). For a cosmological constant, $w = -1$ exactly. If observations find $w$ deviating from $-1$, or changing over time, that would point toward a more complex form of dark energy and reshape our understanding of fundamental physics.