The universe is mostly made of stuff we can't see. Only 5% is ordinary matter like stars and planets. The rest? Dark matter and dark energy, mysterious substances that shape cosmic structure and expansion.

Our understanding of the universe has evolved dramatically. Observations of galaxies and supernovae led to the discovery of dark matter and dark energy. These findings revolutionized our view of cosmic composition and evolution.

The Universe's Composition and Evolution

Density contributions in universe composition

• Stars contribute a tiny fraction of the universe's total density, less than 1%
• Galaxies, including their stars, planets, and interstellar gas, make up around 5% of the universe's total density (Milky Way, Andromeda)
• Ordinary matter, which includes everything made of protons, neutrons, and electrons, accounts for only about 5% of the universe's total density
  • This familiar matter composes stars, planets, galaxies, and interstellar gas (hydrogen, helium)
  • Also known as baryonic matter, it is the type of matter described by the standard model of particle physics
  • The remaining 95% of the universe consists of non-ordinary matter, such as dark matter and dark energy, which do not interact with electromagnetic radiation

Evolution of cosmological understanding

• In the 1970s, observations of galaxy rotation curves revealed galaxies contain more mass than visible matter alone can explain (Vera Rubin's work)
  • This discrepancy led to the concept of dark matter, an invisible form of matter that interacts gravitationally but not electromagnetically
• In the 1990s, observations of distant supernovae showed that the universe's expansion is accelerating (Type Ia supernovae)
  • This discovery led to the concept of dark energy, a mysterious form of energy that permeates space and drives the accelerating expansion of the universe
• Current estimates of the universe's composition:
  • 5% ordinary matter
  • 27% dark matter
  • 68% dark energy (cosmological constant, quintessence)
• These proportions form the basis of the lambda-CDM model, the current standard model of Big Bang cosmology

Challenges of dark matter identification

• Dark matter does not interact with electromagnetic radiation, making it invisible to telescopes and instruments that detect light (optical, radio, X-ray telescopes)
• Potential dark matter candidates include:
  1. Weakly Interacting Massive Particles (WIMPs), hypothetical particles with mass that rarely interact with ordinary matter (neutralinos, axions)
  2. Massive Compact Halo Objects (MACHOs), astronomical bodies that emit little or no radiation (black holes, neutron stars, brown dwarfs)
• Direct detection efforts to identify dark matter particles have been ongoing but have not found conclusive evidence yet (LUX, XENON, SuperCDMS experiments)
• Indirect detection methods, such as gravitational lensing, provide evidence for the presence of dark matter in galaxies and galaxy clusters

Dark matter's role in galaxy formation

• Dark matter played a crucial role in the formation and evolution of galaxies in the early universe
• Shortly after the Big Bang, dark matter began to clump together due to its gravitational influence, forming dense regions that attracted ordinary matter
• These dense dark matter regions served as the seeds for galaxy formation, allowing ordinary matter to accumulate and form stars and galaxies (cosmic web, dark matter halos)
• Without dark matter, galaxy formation would have taken much longer, as ordinary matter alone would not have been able to create the necessary gravitational wells

Universe development since cosmic microwave background

• The cosmic microwave background (CMB) is the oldest light in the universe, originating about 380,000 years after the Big Bang
  • At this time, the universe had cooled enough for atoms to form, allowing photons to travel freely through space (recombination, decoupling)
• After the CMB emission, the universe entered the Dark Ages, a period lasting several hundred million years before the formation of stars and galaxies
• As dark matter clumped together, it attracted ordinary matter, leading to the formation of the first stars and galaxies during the Cosmic Dawn (Population III stars, quasars)
• Over billions of years, galaxies continued to form, evolve, and merge, forming larger structures such as galaxy clusters and superclusters (Virgo Cluster, Laniakea Supercluster)
• About 5 billion years ago, the universe's expansion began to accelerate due to the influence of dark energy, overcoming the gravitational attraction of matter
• Today, the universe continues to expand at an accelerating rate, with dark energy and dark matter dominating its composition, while ordinary matter makes up only a small fraction

Early Universe and Cosmic Inflation

• The theory of cosmic inflation proposes that the early universe underwent a period of rapid exponential expansion
• This inflationary period occurred just a fraction of a second after the Big Bang
• Cosmic inflation helps explain several observed features of the universe, including its flatness and the uniformity of the cosmic microwave background
• The end of the inflationary period is thought to have led to the production of matter and energy that formed the basis of our observable universe