8.1 Hubble's law
6 min read•august 20, 2024
describes the universe's expansion, showing that move away faster. This fundamental relationship in cosmology provides evidence for the and our expanding universe model.
The law relates a galaxy's to its distance from Earth. It's expressed as v = H0 × d, where H0 is the , currently estimated at about 70 km/s/Mpc.
Hubble's law overview
- Fundamental relationship in cosmology that describes the expansion of the universe
- Named after , who first observed and described the relationship in 1929
- Provides evidence for the Big Bang theory and the expanding universe model
Velocity-distance relationship
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- States that the recessional velocity of a galaxy is directly proportional to its distance from Earth
- Mathematically expressed as , where is the recessional velocity, is the Hubble constant, and is the distance
- Implies that more distant galaxies are receding from Earth faster than nearby galaxies
- Applies to galaxies outside our local group (Andromeda, Triangulum)
Hubble constant
- Proportionality constant in Hubble's law, denoted as
- Represents the current expansion rate of the universe, expressed in units of km/s/Mpc
- Current best estimate is approximately 70 km/s/Mpc, meaning that for every megaparsec of distance, the recessional velocity increases by 70 km/s
- Precise value is still a topic of ongoing research and debate
Expansion of universe
- Hubble's law provides evidence that the universe is expanding uniformly in all directions
- Expansion is not due to galaxies moving through space, but rather the stretching of space itself
- Expansion is a fundamental property of the universe, not an explosion from a central point
Redshift of galaxies
- Galaxies appear to be moving away from Earth due to the expansion of the universe
- Light from receding galaxies is stretched, causing a shift towards longer (redder) wavelengths
- is proportional to the galaxy's distance and recessional velocity
- Measured using spectroscopy, by comparing the observed wavelengths of spectral lines to their rest wavelengths
Recessional velocity
- Apparent velocity at which a galaxy appears to be moving away from Earth due to cosmic expansion
- Not a true physical velocity, but a result of the expansion of space
- Increases linearly with distance, as described by Hubble's law
- Can exceed the speed of light for distant galaxies, as it is not limited by special relativity
Cosmological implications
- Hubble's law has significant implications for our understanding of the universe's origin, evolution, and ultimate fate
- Provides evidence for the Big Bang theory and against the
- Allows for the estimation of the universe's age and the
Age of universe
- Hubble's law can be used to estimate the age of the universe, known as the Hubble time
- Hubble time is the inverse of the Hubble constant, approximately 14 billion years
- Represents an upper limit for the universe's age, as the expansion rate was likely faster in the past
- More precise age estimates (13.8 billion years) come from combining Hubble's law with other observations ()
Big Bang theory
- Cosmological model that describes the universe as originating from a singularity and expanding over time
- Supported by Hubble's law, which shows that galaxies are receding from each other
- Predicts the existence of the cosmic microwave background radiation
- Currently the most widely accepted model for the origin and evolution of the universe
Steady State theory
- Alternative cosmological model proposed by , in which the universe has no beginning or end
- Suggests that new matter is continuously created to maintain a constant average density
- Contradicted by Hubble's law and the observed expansion of the universe
- Largely abandoned in favor of the Big Bang theory after the discovery of the cosmic microwave background
Observational evidence
- Hubble's law is supported by numerous observational studies using various telescopes and techniques
- Key evidence comes from the and the cosmic microwave background radiation
Hubble Space Telescope
- Orbiting space telescope launched in 1990, named after Edwin Hubble
- Provides high-resolution images of distant galaxies, enabling precise measurements of their distances and redshifts
- Observations have confirmed Hubble's law and provided more accurate estimates of the Hubble constant
- Played a crucial role in establishing the accelerating expansion of the universe ()
Cosmic microwave background
- Faint background of microwave radiation that fills the entire sky
- Predicted by the Big Bang theory as a remnant of the early universe, discovered in 1965 by Penzias and Wilson
- Provides a snapshot of the universe approximately 380,000 years after the Big Bang
- Exhibits a nearly perfect black body spectrum with a temperature of 2.7 K
- Supports the idea of a hot, dense early universe that has been expanding and cooling over time
Challenges and refinements
- Despite its success, Hubble's law faces several challenges and has undergone various refinements over time
- Key issues include the uncertainty in the Hubble constant, the role of dark energy and matter density, and local deviations from the global Hubble flow
Uncertainty in Hubble constant
- The precise value of the Hubble constant remains a topic of ongoing research and debate
- Different measurement techniques (Cepheid variables, , gravitational lensing) yield slightly different values
- Discrepancy between local and early universe measurements, known as the "Hubble tension"
- Resolving this tension may require modifications to the standard cosmological model or new physics
Dark energy vs matter density
- The expansion rate of the universe is influenced by the relative contributions of dark energy and matter density
- Dark energy causes the expansion to accelerate, while matter density (including dark matter) slows it down
- The balance between these components determines the universe's ultimate fate (eternal expansion, recollapse, or a flat, coasting universe)
- Current observations suggest that dark energy dominates, leading to an accelerating expansion
Local vs global Hubble flow
- Hubble's law describes the global expansion of the universe, but local deviations can occur due to gravitational interactions
- Galaxies in clusters and superclusters can have peculiar velocities that deviate from the Hubble flow
- These deviations must be accounted for when measuring distances and recessional velocities
- Techniques like the Tully-Fisher relation and Fundamental Plane are used to correct for peculiar velocities
Related concepts
- Hubble's law is closely connected to several other key concepts in cosmology, including the cosmological principle, Olbers' paradox, and the Tolman surface brightness test
Cosmological principle
- Assumption that the universe is homogeneous and isotropic on large scales
- Implies that there is no preferred location or direction in the universe
- Forms the basis for the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, which describes the geometry of the universe
- Supported by observations of the cosmic microwave background and the large-scale distribution of galaxies
Olbers' paradox
- The question of why the night sky is dark, despite the presence of countless stars in an infinite, static universe
- Resolved by the expanding universe model, as the light from distant stars is redshifted and diluted by the expansion of space
- Also explained by the finite age of the universe, which limits the distance from which light can reach us
Tolman surface brightness test
- Observational test that compares the surface brightness of galaxies at different redshifts
- In an expanding universe, the surface brightness of galaxies should decrease with increasing redshift, as predicted by Tolman's relation
- Provides independent evidence for the expansion of the universe, consistent with Hubble's law
- Helps to distinguish between cosmological models (Big Bang vs Steady State)
Key Terms to Review (22)
Big bang theory: The big bang theory is the leading explanation for the origin of the universe, suggesting it began as an infinitely small, hot, and dense point approximately 13.8 billion years ago and expanded rapidly. This expansion laid the groundwork for the formation of galaxies, stars, and planets, connecting to various fundamental concepts such as the uniformity of the cosmos, the relationship between distance and velocity in an expanding universe, observable redshift, and critical cosmological parameters that define the universe's structure and fate.
Cosmic Microwave Background: The cosmic microwave background (CMB) is the afterglow radiation from the Big Bang, permeating the universe and providing a snapshot of the early universe when it was just about 380,000 years old. This faint glow, detected in the microwave part of the electromagnetic spectrum, is crucial for understanding the formation and evolution of structures in the universe, linking various aspects of cosmology and astrophysics.
Cosmological expansion: Cosmological expansion refers to the phenomenon where the universe is continuously expanding, causing galaxies to move away from each other over time. This expansion is described by Hubble's law, which states that the recessional velocity of a galaxy is proportional to its distance from an observer, implying that the farther away a galaxy is, the faster it is receding. Understanding this concept is crucial for studying the large-scale structure of the universe and its evolution since the Big Bang.
Dark energy: Dark energy is a mysterious form of energy that makes up about 68% of the universe and is responsible for the accelerated expansion of the cosmos. It plays a crucial role in shaping the universe's large-scale structure, influencing phenomena like voids, the cosmological principle, and Hubble's law.
Distance Ladder: The distance ladder is a series of methods used by astronomers to measure astronomical distances, relying on a hierarchy of techniques that range from nearby objects to the farthest reaches of the universe. This concept connects various distance measurement methods, including parallax for nearby stars, standard candles like Cepheid variables and supernovae for intermediate ranges, and redshift for distant galaxies.
Distant galaxies: Distant galaxies are galaxies that are located far away from Earth, often billions of light-years away, and are seen as they existed in the past due to the finite speed of light. These galaxies provide crucial insights into the early universe, its formation, and evolution. Studying distant galaxies helps astronomers understand the structure and behavior of the cosmos over time, as well as the expansion of the universe.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in establishing the field of extragalactic astronomy and is best known for Hubble's law, which describes the expansion of the universe. His work not only led to the classification of galaxies but also revolutionized our understanding of the cosmos, connecting various concepts like the cosmic web and the cosmological principle.
Fred Hoyle: Fred Hoyle was a British astrophysicist who made significant contributions to the understanding of the universe, particularly in the areas of stellar nucleosynthesis and the Big Bang theory. He is well-known for coining the term 'Big Bang' in a 1949 radio broadcast, although he was a proponent of the steady state theory, which posited that the universe is eternal and unchanging on a large scale.
Galaxy surveys: Galaxy surveys are systematic astronomical studies aimed at mapping and analyzing the distribution, composition, and properties of galaxies in the universe. These surveys are crucial for understanding cosmic structure, the evolution of galaxies, and their relationship to cosmic expansion, particularly in the context of observations that support Hubble's law.
Hubble constant: The Hubble constant is a value that describes the rate at which the universe is expanding, typically expressed in kilometers per second per megaparsec (km/s/Mpc). This constant is crucial for understanding the relationship between distance and velocity of galaxies, providing insights into the universe's expansion history and its overall structure.
Hubble Parameter: The Hubble parameter is a measure of the rate of expansion of the Universe, represented by the symbol 'H'. It quantifies how fast galaxies are moving away from us as a function of their distance, which is a key aspect of understanding the dynamics of cosmic expansion and is central to both the observation of redshift in distant galaxies and the formulation of models describing the Universe's evolution.
Hubble Space Telescope: The Hubble Space Telescope is a large, space-based observatory launched in 1990 that has provided astronomers with unparalleled views of the universe. Operating outside the distortion of Earth’s atmosphere, it captures high-resolution images across various wavelengths, significantly enhancing our understanding of celestial phenomena and supporting critical discoveries, including insights into distant quasars, the expansion of the universe, and gravitational lensing effects.
Hubble time: Hubble time is a measure of the age of the Universe based on the rate of its expansion, specifically derived from Hubble's law, which describes how galaxies are moving away from each other at speeds proportional to their distances. This concept provides a way to estimate the time since the Big Bang by taking the reciprocal of the Hubble constant, giving an approximation of how long the Universe has been expanding. It connects deeply with the understanding of cosmic evolution and the large-scale structure of the Universe.
Hubble's Law: Hubble's Law states that the velocity at which a galaxy is receding from us is directly proportional to its distance from us. This fundamental observation supports the idea that the universe is expanding, linking it to various phenomena like galaxy formation and the structure of the cosmos.
Light-year: A light-year is a unit of distance that measures how far light travels in one year in a vacuum, approximately 5.88 trillion miles or about 9.46 trillion kilometers. This concept is crucial for understanding vast cosmic distances, as it provides a practical way to express distances between celestial objects and phenomena in the universe.
Metric expansion: Metric expansion is the phenomenon in cosmology where the distances between points in the universe increase over time due to the expansion of space itself. This concept indicates that galaxies are moving away from each other, which is a fundamental observation that leads to the understanding of an expanding universe and is critical for explaining Hubble's law.
Parsec: A parsec is a unit of distance used in astronomy, defined as the distance at which one astronomical unit subtends an angle of one arcsecond. It is equivalent to about 3.26 light-years and serves as a crucial measurement for understanding the vastness of the universe. This term plays a significant role in connecting distances between celestial objects, particularly when discussing the structure and expansion of the universe.
Recessional Velocity: Recessional velocity refers to the speed at which an astronomical object, such as a galaxy, is moving away from an observer due to the expansion of the Universe. This concept is critical in understanding the relationship between distance and velocity in the cosmos, particularly as it connects to the redshift phenomenon observed in light from distant galaxies.
Redshift: Redshift is the phenomenon where light from an object is shifted towards longer wavelengths, typically observed as a shift toward the red end of the spectrum. This effect occurs when an object moves away from the observer, providing key insights into the expansion of the universe and the nature of celestial bodies.
Steady state theory: Steady state theory is a cosmological model that posits the universe is eternal and unchanging on a large scale, with new matter continuously created to maintain a constant density as the universe expands. This theory contrasts with the Big Bang model, suggesting that while the universe evolves, it does so without a beginning or end, leading to an unchanging average appearance over time.
Supernovae: Supernovae are powerful and luminous explosions that occur at the end of a star's life cycle, marking the transition from a stable phase to a spectacular event. These explosions can significantly influence their surrounding environment, enriching the interstellar medium with heavy elements and impacting star formation in nearby regions. They also serve as critical distance indicators in the universe and play a role in understanding cosmic expansion.
V = h0 * d: The equation v = h0 * d describes the relationship between the recessional velocity (v) of a galaxy and its distance (d) from an observer, with h0 representing the Hubble constant. This formula indicates that the farther away a galaxy is, the faster it is moving away from us, a fundamental principle in understanding the expansion of the Universe. The connection between velocity and distance is central to Hubble's law, which supports the idea that the Universe is expanding uniformly.