The visible universe-including Earth, the sun, other stars, and galaxies-is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up less than 5 percent of the mass of the universe. The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent). Scientists have a few ideas for what dark matter might be. One leading hypothesis is that dark matter consists of exotic particles that don't interact with normal matter or light but that still exert a gravitational pull. Dark energy is even more mysterious, and its discovery in the 1990s was a complete shock to scientists. Previously, physicists had assumed that the attractive force of gravity would slow down the expansion of the universe over time. But when two independent teams tried to measure the rate of deceleration, they found that the expansion was actually speeding up. One scientist likened the finding to throwing a set of keys up in the air expecting them to fall back down-only to see them fly straight up toward the ceiling.

Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties"and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.Few fields, such as astrometry, are purely astronomy rather than also astrophysics.

  • Big bang theory
  • Forensic astronomy
  • Timeline of the big bang
  • Hadron epoch, lepton epoch and photon epoch
  • Cosmic calendar
  • Oscillating model
  • Timeline of the far future
  • Ultimate fate of the universe
  • Ancient astronomy
  • Archaeoastronomy
  • Astrostatistics
  • Astrochemistry
  • Astrodynamics
  • Solar astronomy
  • Planetary science
  • Stellar astronomy
  • Galactic astronomy
  • Extragalactic astronomy
  • Advanced software in astronomy
  • The Electromagnetic zero-point field

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