Glossary

29.2 A Model of the Universe

5 min read•june 12, 2024

The universe's expansion rate determines its ultimate fate. Depending on its density, the universe could expand forever, stop expanding and collapse, or expand at an ever-decreasing rate. The interplay between matter, radiation, and shapes the universe's evolution.

Our cosmic future holds fascinating possibilities. An eternally expanding universe might lead to isolated galaxies in a cold, dark cosmos. Alternatively, the universe could collapse in a "" or undergo cycles of expansion and contraction. Current evidence points to .

The Universe's Expansion and Evolution

Rate of universe expansion

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  • Universe's expansion rate determines its ultimate fate
    • Above critical rate, universe expands forever ()
    • Matching critical rate, universe expands forever at ever-decreasing rate ()
    • Below critical rate, universe stops expanding and collapses ()
  • Expansion rate determined by universe's density
    • Higher density slows expansion due to increased gravitational attraction
    • Lower density accelerates expansion due to reduced gravitational attraction
  • Universe's density changes as it expands
    • Matter density decreases as universe expands, same amount of matter occupies larger volume
    • Radiation density decreases more rapidly than matter density
    • density remains constant, becoming more dominant over time

Possibilities for universe's future

  • (eternal expansion)
    • Universe continues expanding forever
    • Galaxies become increasingly isolated as space between them grows (, )
    • Universe becomes colder and darker as stars exhaust fuel and die
  • Flat universe (critical expansion)
    • Universe expands forever at ever-decreasing rate
    • Expansion rate approaches zero but never reaches it
    • Ultimate fate similar to open universe, with increasing isolation and cooling
  • (eventual collapse)
    • Universe stops expanding and begins contracting
    • Galaxies and stars move closer together as universe collapses
    • Universe becomes hotter and denser, potentially leading to ""
  • (cycles of expansion and contraction)
    • Hypothetical scenario where closed universe collapses and "bounces" into new expansion phase
    • Universe undergoes repeated cycles of expansion and contraction (, Big Crunch)
    • Feasibility depends on nature of Big Crunch and physics of extremely high densities

Understanding the Expansion of the Universe

Meaning of universe expansion

  • Expansion of universe refers to increasing distance between galaxies over time
    • Space itself expands, causing galaxies to move away from each other
    • Galaxies not moving through space, but carried along with expansion of space
  • Expansion not due to galaxies moving outward from central point
    • No "center" of expansion; all points in universe expanding away from each other
  • Expansion uniform on large scales
    • On average, galaxies moving away from us (and each other) at rate proportional to distance
    • Relationship described by : v=H0×dv = H_0 \times d, where vv is recession velocity, H0H_0 is , and dd is distance
  • Expansion a fundamental property of spacetime itself
    • Described by solutions to of
    • Influenced by presence of matter, radiation, and dark energy in universe
  • Accelerating expansion observed, attributed to dark energy

Critical density in cosmology

  • (Ωc)(\Omega_c) is density required for universe to have flat geometry
    • Represents boundary between open universe (Ω<1)(\Omega < 1) and closed universe (Ω>1)(\Omega > 1)
    • Depends on value of : Ωc=3H028πG\Omega_c = \frac{3H_0^2}{8\pi G}, where H0H_0 is Hubble constant and GG is gravitational constant
  • Actual density of universe (Ω)(\Omega) expressed as fraction of critical density
    • Ω=1\Omega = 1 indicates flat universe
    • Ω<1\Omega < 1 indicates open universe
    • Ω>1\Omega > 1 indicates closed universe
  • Critical density includes contributions from matter (Ωm)(\Omega_m), radiation (Ωr)(\Omega_r), and dark energy (ΩΛ)(\Omega_\Lambda)
    • In current universe, matter and dark energy are dominant components
    • Relative contributions change over time as universe expands
  • Determining actual density of universe a key goal of observational cosmology
    • Measuring , large-scale structure, and helps constrain universe's density and geometry

Early Universe and Cosmic Evolution

Big Bang Theory and Cosmic Inflation

  • describes the origin and evolution of the universe
    • Universe began in an extremely hot, dense state and has been expanding ever since
  • proposes a period of rapid expansion in the very early universe
    • Explains observed uniformity and flatness of the universe
    • Provides mechanism for generating primordial density fluctuations

Dark Matter and Observable Universe

  • is a form of matter that does not interact with electromagnetic radiation
    • Inferred to exist through its gravitational effects on visible matter
    • Plays crucial role in formation of large-scale structure in the universe
  • refers to the portion of the universe that can be seen from Earth
    • Limited by the speed of light and the age of the universe
    • Larger than the visible universe due to cosmic expansion

Cosmological Constant

  • is a term in Einstein's field equations of general relativity
    • Originally introduced to allow for a static universe
    • Now associated with dark energy and the accelerating expansion of the universe
  • Represents the energy density of empty space
    • Constant throughout space and time, unlike matter and radiation densities

Observational Evidence and the Universe's Fate

Evidence for universe's fate

  • Cosmic microwave background (CMB) measurements
    • CMB remarkably uniform in temperature across sky
    • Small temperature fluctuations in CMB consistent with flat universe
    • (WMAP) and provided precise CMB measurements
  • as standard candles
    • Type Ia supernovae have consistent peak luminosity, useful for measuring cosmic distances
    • Observations of distant supernovae indicate universe's expansion is accelerating
    • Acceleration attributed to presence of dark energy, which counters gravitational attraction of matter
  • Large-scale structure of universe
    • Distribution of galaxies and galaxy clusters on large scales sensitive to universe's density and geometry
    • Surveys like (SDSS) and (DES) map large-scale structure
    • Observed large-scale structure consistent with universe dominated by dark matter and dark energy
  • Combining multiple observational techniques
    • Combination of CMB, supernova, and large-scale structure data provides strong evidence for flat universe
    • Current best estimates indicate universe composed of ~5% ordinary matter, 27% dark matter, and 68% dark energy
    • Presence of dark energy suggests universe will continue expanding indefinitely, leading to open or flat fate, depending on nature of dark energy

Key Terms to Review (52)

Accelerating Expansion: Accelerating expansion refers to the observed phenomenon in cosmology where the rate of expansion of the universe is increasing over time, rather than slowing down as one might expect due to the gravitational attraction between galaxies and other matter. This accelerated expansion is a key feature of the current cosmological model known as the Lambda-CDM model.
Ancient Greek Astronomers: Ancient Greek astronomers were the pioneering scientists who made significant contributions to the field of astronomy in the ancient Greco-Roman world. They laid the foundations for our modern understanding of the universe and its celestial bodies.
Andromeda: Andromeda is the nearest major galaxy to the Milky Way, located approximately 2.5 million light-years away. It is a spiral galaxy similar in structure and size to our own Milky Way galaxy, and it is a prominent feature in the northern night sky, visible to the naked eye under clear conditions.
Andromeda galaxy: The Andromeda Galaxy, also known as M31, is the closest spiral galaxy to the Milky Way and is on a collision course with it. It is approximately 2.537 million light-years from Earth and is the largest galaxy in the Local Group.
Big Bang: The Big Bang is the prevailing cosmological model for the origin and evolution of the universe. It posits that the universe began as an extremely hot, dense state approximately 13.8 billion years ago, and has been expanding and cooling ever since. This theory provides a comprehensive explanation for the observed large-scale structure of the cosmos, the abundance of light elements, and the cosmic microwave background radiation.
Big Bang Theory: The Big Bang Theory is the prevailing cosmological model for the universe, describing its development from an extremely hot and dense initial state to its current, vast and complex state. It is the foundational model that explains the age and evolution of the universe.
Big crunch: The Big Crunch is a theoretical scenario in which the expansion of the universe eventually reverses, causing it to collapse back into a singularity. This hypothesis contrasts with the idea of an ever-expanding universe and suggests a cyclic model of cosmic evolution.
Big Crunch: The Big Crunch is a hypothetical scenario in which the current expansion of the universe reverses, leading to an eventual collapse of all matter back into an extremely hot and dense state, similar to the initial conditions of the Big Bang. This concept is closely tied to the overall model and evolution of the universe.
Closed universe: A closed universe is a cosmological model where the universe is finite and will eventually stop expanding, leading to a contraction or 'Big Crunch.' It implies that the geometry of the universe is positively curved, like the surface of a sphere.
Closed Universe: A closed universe is a model of the universe where the overall curvature of spacetime is positive, meaning the universe has a finite volume and will eventually re-collapse on itself in a 'Big Crunch'. This is one of the possible scenarios for the ultimate fate of the universe as described by the general theory of relativity.
Cold dark matter: Cold dark matter (CDM) consists of slow-moving particles that do not emit, absorb, or reflect light, making them invisible and detectable only through gravitational effects. It plays a crucial role in the formation and clustering of galaxies in the universe.
Copernican Revolution: The Copernican Revolution was a paradigm shift in the understanding of the universe, where the Earth was no longer considered the center of the cosmos, but rather the Sun became the central body around which the planets, including Earth, orbited. This revolutionary idea challenged the prevailing geocentric model of the universe and laid the foundation for modern astronomy.
Cosmic Inflation: Cosmic inflation is a theory that describes an extremely rapid exponential expansion of the universe in the first fraction of a second after the Big Bang. This rapid expansion is thought to have smoothed out irregularities and set the stage for the universe we observe today.
Cosmic Microwave Background: The cosmic microwave background (CMB) is the oldest light in the universe, a faint glow that permeates all of space and is a remnant of the early stages of the universe's formation. It provides crucial information about the origins and evolution of the universe, as well as its large-scale structure and composition.
Cosmological constant: The cosmological constant is a term introduced by Albert Einstein in his field equations of General Relativity. It represents a uniform energy density that fills space homogeneously and is associated with dark energy.
Cosmological Constant: The cosmological constant is a term in the field of cosmology that represents a constant energy density inherent in the fabric of space-time itself. It was originally introduced by Albert Einstein as a way to achieve a static, non-expanding universe, but has since been incorporated into our modern understanding of the accelerating expansion of the universe.
Cosmological principle: The cosmological principle states that the universe is homogeneous and isotropic when viewed on a large enough scale. This means that the distribution of matter and energy is uniform, and the universe looks the same in all directions.
Cosmological Principle: The cosmological principle is a fundamental assumption in cosmology that states the universe is homogeneous and isotropic on large scales. This means that the properties of the universe are the same everywhere (homogeneous) and look the same in all directions (isotropic), regardless of one's location or orientation.
Critical Density: The critical density is the average density of matter in the universe that would result in a flat, or Euclidean, geometry of space. It represents the density threshold that determines the overall curvature and fate of the universe.
Dark energy: Dark energy is a mysterious form of energy that makes up about 68% of the universe and is responsible for its accelerated expansion. Its exact nature remains unknown, but it is a crucial component in cosmological models.
Dark Energy: Dark energy is a mysterious and pervasive form of energy that appears to be driving the accelerated expansion of the universe. It is a fundamental component of the universe that makes up approximately 68% of the total energy content of the cosmos. The discovery of dark energy has revolutionized our understanding of the universe, as it challenges the traditional models of cosmology and the evolution of the universe. Dark energy is a crucial concept that helps explain the large-scale structure and dynamics of the universe, as well as its past, present, and future.
Dark Energy Survey: The Dark Energy Survey (DES) is a collaborative scientific project aimed at studying the mysterious dark energy that is believed to be driving the accelerated expansion of the universe. It involves the use of a powerful wide-field camera mounted on a telescope in Chile to map the distribution of galaxies and measure the properties of dark energy over a large portion of the sky.
Dark Matter: Dark matter is a hypothetical form of matter that cannot be seen directly but accounts for the majority of the matter in the universe. It is believed to interact gravitationally with itself and with ordinary matter, but does not emit, reflect, or absorb light, making it invisible to traditional astronomical observations.
Edwin Hubble: Edwin Hubble was an American astronomer who made groundbreaking contributions to our understanding of the universe. He is best known for his observations and discoveries that led to the realization that the universe is expanding, and that galaxies beyond our own Milky Way exist.
Einstein's Field Equations: Einstein's field equations are a set of 10 coupled, nonlinear partial differential equations that describe the fundamental relationship between the curvature of spacetime and the distribution of matter and energy within it. They form the foundation of Einstein's general theory of relativity, which revolutionized our understanding of gravity and the structure of the universe.
Flat Universe: A flat universe is a cosmological model in which the geometry of the universe is described as being flat, or Euclidean, rather than curved. This means that the large-scale shape of the universe is such that parallel lines never converge, and the sum of the angles in a triangle is exactly 180 degrees.
General Relativity: General relativity is a theory of gravity developed by Albert Einstein that describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the presence of mass and energy. This theory fundamentally changed our understanding of the universe and has far-reaching implications across various fields of astronomy and physics.
Geocentrism: Geocentrism is the belief that the Earth is the center of the universe, with all other celestial bodies, such as the Sun, Moon, and planets, orbiting around it. This model of the universe was the dominant view in many ancient and medieval civilizations.
Georges Lemaître: Georges Lemaître was a Belgian Catholic priest and professor of physics who is best known for proposing the theory of the expansion of the universe, which later became known as the Big Bang theory. He was a pioneering figure in the field of cosmology and made significant contributions to our understanding of the origin and evolution of the universe.
Heliocentrism: Heliocentrism is the astronomical model that places the Sun, rather than the Earth, at the center of the universe. This model contrasts with the earlier geocentric model, which placed the Earth at the center of the solar system and the universe.
Hubble constant: The Hubble constant is the rate of expansion of the universe, measured in kilometers per second per megaparsec (km/s/Mpc). It provides a relationship between the distance of galaxies and their recessional velocity due to cosmic expansion.
Hubble Constant: The Hubble constant is a fundamental parameter in cosmology that describes the rate of expansion of the universe. It represents the relationship between the distance to a galaxy and its recessional velocity, providing a measure of the expansion rate of the observable universe.
Hubble’s law: Hubble's law states that the velocity at which a galaxy moves away from us is directly proportional to its distance from us. This relationship supports the concept of an expanding universe.
Hubble's Law: Hubble's Law is a fundamental principle in cosmology that describes the relationship between the distance of a galaxy from the Milky Way and its recessional velocity. It states that the farther a galaxy is from our own, the faster it is moving away from us, indicating an expanding universe.
Milky Way: The Milky Way is the galaxy in which our solar system is located, comprising hundreds of billions of stars and vast amounts of gas and dust. It is a spiral galaxy, with a central bulge and a rotating disk of stars, gas, and dust. The Milky Way is an essential component in understanding the structure, formation, and evolution of the universe, as it provides a window into the larger cosmic landscape.
Milky Way Galaxy: The Milky Way Galaxy is the spiral galaxy that includes our Solar System, characterized by its barred structure and multiple spiral arms. It is one of billions of galaxies in the universe and contains over 200 billion stars.
Observable Universe: The observable universe is the region of the universe that can be observed from Earth or any other location, based on the maximum distance that light has traveled since the beginning of the universe. It encompasses all the matter and radiation that can, in principle, interact with or be detected by observers on Earth and other locations.
Open universe: An open universe is a cosmological model where the universe will expand forever without recontracting. It implies that the curvature of space is negatively curved, resembling a saddle shape.
Open Universe: An open universe is a model of the universe in which the overall geometry of the universe is open or hyperbolic, meaning the universe will continue to expand forever. This is in contrast to a closed universe, which would eventually re-collapse, or a flat universe, which would expand forever but at a decreasing rate.
Oscillating theory of the universe: The oscillating theory of the universe posits that the universe undergoes a series of expansions and contractions. This cyclical process includes a Big Bang, followed by expansion, eventual contraction, and then another Big Bang.
Oscillating Universe: The oscillating universe model is a cosmological model that proposes the universe undergoes a cyclic pattern of expansion and contraction, with each cycle beginning with a big bang and ending with a big crunch. This cyclical process is believed to repeat indefinitely, creating an oscillating or 'bouncing' universe.
Parallax Measurement: Parallax measurement is a technique used in astronomy to determine the distance to nearby stars by observing their apparent shift in position against more distant celestial objects as the Earth orbits the Sun. This method provides a fundamental way to measure the scale of the universe by establishing a reliable distance scale for the nearest stars.
Planck Satellite: The Planck satellite was a space observatory operated by the European Space Agency (ESA) from 2009 to 2013. It was designed to study the cosmic microwave background (CMB), the oldest light in the universe, in order to provide a better understanding of the early universe and the formation of structures within it.
Redshift: Redshift is the phenomenon where the wavelength of light from an object increases, making it appear more red. It is often observed in light from galaxies moving away from us, indicating the expansion of the universe.
Redshift: Redshift is the phenomenon where the wavelength of light emitted from a distant object is shifted towards longer, or redder, wavelengths compared to the original wavelength. This shift in the observed wavelength is caused by the relative motion between the object and the observer, as well as the expansion of the universe.
Sloan Digital Sky Survey: The Sloan Digital Sky Survey (SDSS) is a major multi-year project that has created the most detailed three-dimensional maps of the universe. It uses a specialized telescope and camera system to observe and analyze the properties of celestial objects, providing valuable data for studying the colors of stars, the evolution of quasars, the formation and evolution of galaxies, and the overall structure and model of the universe.
Spectroscopic Analysis: Spectroscopic analysis is a technique that uses the interaction of electromagnetic radiation with matter to determine the chemical composition and physical properties of a substance. It involves the study and interpretation of the spectrum produced when light or other radiation interacts with a sample, providing valuable insights into the nature and characteristics of the observed object or phenomenon.
Theory of general relativity: Albert Einstein's theory of general relativity describes gravity as the warping of spacetime by mass and energy. It revolutionized our understanding of gravity, replacing Newton's law of universal gravitation.
Type Ia supernovae: A Type Ia supernova is a powerful and luminous stellar explosion resulting from the thermonuclear disruption of a white dwarf in a binary system. It occurs when the white dwarf accretes matter from its companion star, reaching the Chandrasekhar limit and igniting carbon fusion uncontrollably.
Type Ia Supernovae: Type Ia supernovae are a specific class of supernovae that occur when a white dwarf star in a binary system accretes enough material from its companion to exceed the Chandrasekhar limit, causing the white dwarf to undergo a thermonuclear explosion. These events are remarkably consistent in their intrinsic brightness, making them valuable standard candles for measuring extragalactic distances and studying the expansion of the universe.
Wheeler: John Archibald Wheeler was an American theoretical physicist who made significant contributions to the field of general relativity and quantum mechanics. He is best known for popularizing the term 'black hole' and his work on the theory of nuclear fission and wormholes.
Wilkinson Microwave Anisotropy Probe: The Wilkinson Microwave Anisotropy Probe (WMAP) was a NASA space observatory launched in 2001 to measure the cosmic microwave background (CMB) radiation, the oldest light in the universe. It provided high-precision measurements that helped establish the standard model of cosmology and our understanding of the early universe's structure and evolution.
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