The universe refers to the vast, all-encompassing expanse that includes everything that exists, such as matter, energy, planets, stars, galaxies, and even the cosmic laws and forces governing them. It is often described in terms of the following key points: 1. **Scope**: The universe contains all celestial bodies and cosmic phenomena, including the Milky Way galaxy, other galaxies, nebulae, black holes, and dark matter.
Cosmology is the scientific study of the origin, evolution, structure, and eventual fate of the universe. It seeks to understand the large-scale properties and behavior of the cosmos as a whole. Cosmology covers a range of topics, including: 1. **The Big Bang Theory**: This is the leading explanation for the origin of the universe, proposing that it began as a singular, extremely hot and dense point around 13.8 billion years ago and has been expanding ever since.
Matter is anything that has mass and occupies space. It is composed of particles, which can be atoms or molecules, and it exists in various states, primarily solid, liquid, and gas. In some contexts, it can also include plasma, which is a high-energy state of matter where electrons are separated from atoms. Matter has properties such as volume, density, and viscosity, and it can undergo physical and chemical changes.
The term "World" can refer to several concepts depending on the context: 1. **Planet Earth**: At its most basic, "world" often refers to the Earth, the third planet from the Sun, which is home to all known life. 2. **Human Experience**: The "world" can also refer to the collective experience of humanity, encompassing cultures, societies, and human interactions.
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The universe is the total physical system described by cosmology, including spacetime, matter and radiation. A cosmological model specifies their large-scale geometry and evolution, for example through a Friedmann-Lemaître-Robertson-Walker metric and the matter content entering the Friedmann equations.
The observable region is limited by the history of light propagation. A finite particle horizon bounds what could have sent signals to an observer since the model's early evolution; it does not imply a physical wall or establish that the entire universe is finite. Measurements of expansion, the cosmic microwave background and large-scale structure constrain the model through different physical processes.
