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A picture of the Universe From the Hubble Space Telescope
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The Universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, momentum, and energy, and the physical laws and constants which govern them. The term "universe" may also be used in slightly different contextual senses, denoting such concepts as the world or Nature, or the cosmos.
Astronomical observations indicate that the universe is around 13.75 billion years old. Its diameter is at least 93 billion light years. According to the prevailing scientific theory, the universe has expanded from a gravitational singularity known as the Big Bang (a point in space and time at which all the matter and energy of the observable universe were concentrated). Since the Big Bang, the universe has expanded to its present form. Several experimental measurements support this theoretical expansion and the Big Bang theory in general. Recent observations show this expansion is accelerating and that most of the matter and energy in the universe in fundamentally different from that observed on Earth and not directly observable.
Experiments suggest that the universe has been governed by the same constants and physical laws throughout its history. The dominant force as cosmological distances is gravity, and currently, general relativity is the most accurate theory of gravitation. The Standard Model includes the remaining three fundamental forces and all the known particles on which they act. The universe has at least three dimensions of space and one of time. Space-time appears to be simply and smoothly connected.
The word "universe" is normally defined as encompassing everything. Using an alternate definition, some have theorized that this "universe" is one of many disconnected "universes" which are collectively denoted as the multiverse
Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative models were developed by the ancient Greeks, proposing that the universe possesses infinite space and has existed eternally, containing a single of concentric spheres of finite size which correspond to the fixed stars, the Sun, and various planets, rotating about a spherical but unmoving Earth. Over the centuries, precise observations and improved theories of gravity led to Copernicus' heliocentric model and the Newtonian model of the solar system. More improvements in astronomy led to the characterization of the Milky Way, the discovery of other galaxies, and the discovery of microwave background radiation. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.
Definitions
The broadest definition of the universe is found in De divisione naturae by the medieval philosopher Johannes Scotus Eriugena. He defined it as simply everything: everything that exists and everything that does not exist. Time is not considered in this definition.
More often, the universe is defined as everything that exists, has existed, and will exist. According to this definition, the universe consists of three elements:
- Space and time: collectively known as "space-time" or the vacuum
- Matter and various forms of energy and momentum occupying space-time
- Physical laws which govern the first two
A related definition of "universe" is everything that exists at a single moment of time.
Another restricted definition of "universe" is everything within our connected space-time that could ever interact with us and that we could interact with.
Characteristics
- Universe is large and possibly infinite in volume, with observable matter spread over a space of at least 93 billion light years across
- Observable matter is spread uniformly throughout the universe when averaged over distances longer than 300 million light years
- Observable matter of the universe is also spread isotropically, meaning that no direction of observation seems different from any other.
- Overall density of the universe is very low.
- 73% dark energy
- 23% cold dark matter
- 4% ordinary matter
- Dark matter gravitates as ordinary matter, working to slow the expansion of the universe
- Dark energy accelerates the expansion.
- Universe is old and evolving
- Precise estimate of the age of the universe is around 13.75 billion years old
- Universe has not been the same at all times in its history
- Space itself seems to have expanded
- Expansion consistent
- Expansion is accelerating
- Relative functions of different chemical elements seem to be identical throughout the universe and its history
- Universe seems to have much more matter than antimatter
- Universe seems to have no set electric charge
- Gravity appears to be the dominant interaction on cosmological length scales
- Universe appears to have no net momentum and angular momentum
- Universe appears to have a smooth space-time continuum
- Three spatial dimensions and one temporal (time) dimension
- Space observed to be very nearly flat
- Space-time appears to have a simply connected topology
- Universe appears to be governed throughout by the same physical laws and physical constants
- All matter is composed of three generations of leptons and quarks
- Those elementary particles interact via at most three fundamental interactions
- Electroweak Interaction: includes electromagnetism and the weak nuclear force
- Quantum Chromodynamics: the strong nuclear force
- General Relativity: gravity
Historical Models
Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed. They were based on the then-available data and conceptions of the universe. Cosmologies and cosmogonies were historically based on narratives of gods acting in various ways. Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the universe.
- Creation Myths
- Present in many cultures and can be roughly grouped into common types
- The world is born from a world egg
- The creation is caused by a single god emanating or producing something by themselves
- The world is created from the union of male and female deities
- The universe is created by crafting it from pre-existing materials
- The world is created by the command of a divinity
- The universe emanates from fundamental principles
- Philosophical Models
- The universe goes through repeated cycles of creation, destruction, and rebirth
- Matter can change and all matter is different forms of a single material, hence a single form of material makes up the universe
- Multiple fundamental materials are needed to explain the diversity of the universe and the Classical elements (Earth, Air, Fire, and Water) exist in different forms and combinations, hence making up the universe
- Universe composed of invisible atoms moving through empty space, a vacuum
- Some Greek philosophers believed that the universe had an infinite past with no beginning
- Some medieval philosophers and theologians believed that the universe had a finite past with a beginning (view inspired by the creation myth shared by the three Abrahamic religions)
- Astronomical Models
- Universe is a flat disk floating in the ocean (Babylonian astronomers)
- Space and time are infinite and eternal, the Earth is spherical and stationary, and all other matter is confined to rotating concentric spheres (Eudoxus of Cnidos, Greek philosopher)
- Heliocentric Theory (Aristarchus of Samos, Greek astronomer)
- Fixed stars and Sun remain unmoved
- Earth revolves around the sun on the circumference of a circle with the sun lying in the middle of the orbit
- Stars very far away
- Aristotelian Model
- Earth rotates on an axis
- Sun is the center of the universe
- Space and time were infinite and the stars in the universe has been burning forever
- Matter is arranged hierarchically in a fractal way such that the universe has a negligibly small overall density
- Stars are not distributed uniformly throughout space; they are grouped into galaxies
Theoretical Models
- General Theory of Relativity
- Provides a set of ten nonlinear partial differential equations for the space-time metric that must be solved from the distribution of mass-energy and momentum through the universe
- The universe is homogeneous and isotropic
- Gravitational effects of the various galaxies making up the universe are equivalent to those of a fine dust distributed uniformly throughout the universe with the same average density
- Special Relativity and Space-Time
- Universe has at least three spatial and one temporal (time) dimension
- Spatial and temporal separations are interconvertible, within limits, by changing one's motion
- Big Bang Model
- Universe created in one moment via the interaction of certain chemicals
- Universe constantly expands over time
- Slight imbalance of matter over antimatter was present in the universe's creation or developed very shortly thereafter
- Small amount of matter survived, fiving the present matter-dominated universe
- Dark matter and dark energy
- Dark matter slows the expansion of the universe
- Dark energy accelerates the expansion of the universe
Multiverse
Some speculative theories have proposed that this universe is but one of a set of disconnected universes, collectively denoted as the multiverse. By definition, there is no way for anything in one universe to affect another. The disconnected universes are conceived as being physical:
- Each has its own space and time
- Each has its own matter and energy
- Each has its own physical laws
The concept of a multiverse of disconnected universes is very old.
There are two scientific senses in which multiple universes can occur.
- Disconnected space-time continua may exist
- All forms of matter and energy are confined to one universe and cannot "tunnel" between them
- A parallel universe is born with every quantum measurement
- The universe "forks" into parallel copies, each one corresponding to a different outcome of the quantum measurement.
Both senses of the term "multiverse" are speculative and may be considered unscientific due to the fact that universes cannot interact and therefore other universes cannot be "found" by experiments in this universe.
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