Dark Matter and Dark Energy

Learning Objectives

Define dark matter and dark energy.

Describe the primary evidence for the existence of dark matter, including galaxy rotation curves.

Explain the role of dark energy in the accelerating expansion of the universe.

State the approximate composition of the universe in terms of dark energy, dark matter, and ordinary matter.

The Invisible Universe

All the stars, planets, and galaxies we can see make up only a tiny fraction of the total mass and energy in the universe. The rest is composed of two mysterious, invisible components: dark matter and dark energy.

Dark Matter

Dark matter is a hypothetical form of matter that does not emit, reflect, or absorb light, making it completely invisible to all electromagnetic observation. Its existence is inferred from its gravitational effects on visible matter.

Evidence for Dark Matter:

Galaxy Rotation Curves: The primary evidence comes from observing the speed of stars orbiting the center of spiral galaxies. According to Newton's laws, stars farther from the dense galactic center should orbit more slowly. However, observations show that stars in the outer regions orbit just as fast as stars closer in. This implies that there is a huge, invisible 'halo' of extra mass surrounding the galaxy, providing the extra gravitational pull to keep these fast-moving stars in orbit. This unseen mass is what we call dark matter.

Gravitational Lensing: The bending of light from distant galaxies around massive clusters is far greater than can be accounted for by the visible matter in the cluster, indicating a large amount of unseen dark matter.

Dark Energy

While dark matter provides an extra gravitational pull, dark energy is an even more mysterious force that acts as a sort of anti-gravity. It is a form of energy that is inherent to the fabric of spacetime itself and is causing the expansion of the universe to accelerate.

Discovery: In the late 1990s, astronomers studying distant Type Ia supernovae discovered that these galaxies were moving away from us faster than predicted. This meant the expansion of the universe was not slowing down due to gravity, as expected, but was speeding up.

Dark energy is the name given to the unknown repulsive force driving this acceleration. It is the dominant component of the universe.

The Composition of the Universe

According to our best current models (like the Lambda-CDM model), the total mass-energy content of the universe is approximately:

68% Dark Energy

27% Dark Matter

5% Ordinary (Baryonic) Matter (everything we can see)

Key Terms

Dark Matter: A non-luminous component of the universe's mass that is inferred to exist from its gravitational effects on visible matter.
Dark Energy: A mysterious form of energy that is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.
Galaxy Rotation Curve: A plot of the orbital speeds of visible stars or gas in a galaxy versus their radial distance from that galaxy's center. The discrepancy between observed and theoretical curves is key evidence for dark matter.
Cosmological Constant (Lambda): A term introduced by Einstein into his field equations of general relativity to represent a static universe. It is now associated with dark energy and the vacuum energy of space.