14.1 Evidence for Cosmic Acceleration
4 min read•august 9, 2024
is driving the universe's accelerating expansion, a discovery that revolutionized cosmology. Evidence from , cosmic microwave background, and supports this surprising phenomenon.
This acceleration challenges our understanding of physics and the universe's fate. It suggests a cosmic timeline where dark energy now dominates, potentially leading to eternal expansion or more dramatic scenarios in the far future.
Observational Evidence for Cosmic Acceleration
Type Ia Supernovae as Standard Candles
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- Type Ia supernovae serve as standard candles in astronomy due to their consistent peak luminosity
- Astronomers use these supernovae to measure cosmic distances with high precision
- Peak luminosity of Type Ia supernovae reaches about 5 billion times that of the Sun
- Formation occurs when a white dwarf in a binary system accretes matter from its companion star
- Accretion process continues until the white dwarf reaches the Chandrasekhar limit (~1.44 solar masses)
- Exceeding the Chandrasekhar limit triggers a thermonuclear explosion, resulting in a Type Ia supernova
- Consistent explosion mechanism leads to remarkably uniform peak luminosities across different Type Ia supernovae
Hubble Diagram and Redshift Analysis
- plots the relationship between a galaxy's distance and its recession velocity
- Modern Hubble diagrams use Type Ia supernovae to extend measurements to high redshifts
- measures the stretching of light waves due to cosmic expansion
- Higher redshifts indicate greater distances and earlier cosmic epochs
- Astronomers calculate redshift by comparing observed spectral lines to their known rest wavelengths
- relates redshift to distance, expressed as , where v is recession velocity, H_0 is the Hubble constant, and d is distance
- Deviations from the expected linear Hubble relationship at high redshifts provide evidence for cosmic acceleration
High-z Supernova Search Team and Supernova Cosmology Project
- Two independent research teams formed in the 1990s to study cosmic expansion using Type Ia supernovae
- led by Brian Schmidt and Adam Riess
- headed by
- Both teams employed large telescopes and advanced CCD cameras to detect distant supernovae
- Researchers developed sophisticated techniques to correct for dust extinction and other observational biases
- Teams independently analyzed data from dozens of high-redshift Type Ia supernovae
- Results showed that distant supernovae appeared fainter than expected in a decelerating universe
- Findings published in 1998 and 1999 provided strong evidence for cosmic acceleration
- Schmidt, Riess, and Perlmutter awarded the 2011 Nobel Prize in Physics for their groundbreaking work
Complementary Evidence
Cosmic Microwave Background Observations
- Cosmic Microwave Background (CMB) radiation provides a snapshot of the early universe
- CMB observations reveal tiny temperature fluctuations across the sky
- Angular size of CMB temperature fluctuations depends on the universe's geometry and expansion history
- Precise measurements of CMB anisotropies conducted by satellites (WMAP, Planck)
- CMB data supports a with a non-zero cosmological constant
- Acoustic peaks in the CMB power spectrum constrain cosmological parameters
- Combining CMB data with other observations strengthens the case for cosmic acceleration
Baryon Acoustic Oscillations as Cosmic Rulers
- Baryon Acoustic Oscillations (BAO) result from sound waves in the early universe's plasma
- BAO create a characteristic scale in the distribution of matter, serving as a "standard ruler"
- This scale corresponds to the sound horizon at the time of recombination (~150 Mpc today)
- Large-scale galaxy surveys (SDSS, BOSS) measure the BAO signal in the cosmic web
- BAO measurements at different redshifts track the universe's expansion history
- Comparing observed BAO scales to theoretical predictions supports the accelerating expansion model
- BAO results complement and corroborate evidence from Type Ia supernovae and CMB observations
Implications
Accelerating Expansion and Dark Energy
- Observational evidence points to an accelerating expansion of the universe
- Acceleration requires a form of energy with negative pressure, dubbed "dark energy"
- Dark energy counteracts the attractive force of gravity on cosmic scales
- Current observations suggest dark energy comprises about 68% of the universe's energy density
- Leading candidate for dark energy is the cosmological constant, represented by Λ in Einstein's field equations
- Cosmological constant interpreted as the energy of the vacuum with an equation of state w = -1
- Alternative dark energy models propose dynamical fields (quintessence) or modifications to general relativity
- Accelerating expansion challenges our understanding of fundamental physics and the fate of the universe
Revised Cosmic Timeline and Future of the Universe
- Discovery of cosmic acceleration dramatically altered our view of the universe's history and future
- Early universe dominated by radiation, followed by matter domination
- Transition to dark energy domination occurred roughly 5 billion years ago
- Current acceleration expected to continue indefinitely in the ΛCDM model
- Future scenarios include eternal expansion, Big Rip, or Big Freeze, depending on the nature of dark energy
- Cosmic acceleration raises questions about the long-term fate of structures in the universe
- Implications for the far future include the potential isolation of galaxies and the erosion of bound systems
Key Terms to Review (19)
Adam G. Riess: Adam G. Riess is an astrophysicist renowned for his pivotal contributions to our understanding of cosmic acceleration and dark energy. He is best known for his work in measuring the rate of expansion of the universe using Type Ia supernovae, which provided compelling evidence that the expansion is not only ongoing but accelerating.
Alexandre V. Filippenko: Alexandre V. Filippenko is a prominent astrophysicist known for his groundbreaking research on supernovae and cosmic acceleration, particularly in relation to Type Ia supernovae as standard candles for measuring astronomical distances. His work has significantly contributed to the discovery of the universe's accelerated expansion, shedding light on the role of dark energy in the cosmos.
Baryon Acoustic Oscillations: Baryon acoustic oscillations are periodic fluctuations in the density of visible baryonic matter (normal matter) of the universe, which result from sound waves propagating through the hot plasma of the early universe. These oscillations are critical for understanding the large-scale structure of the cosmos, influencing the formation of galaxies and clusters, and providing insights into cosmic evolution and the expansion of the universe.
Cosmic microwave background radiation: Cosmic microwave background radiation (CMB) is the faint glow of radiation that fills the universe, a remnant from the Big Bang, and is a critical piece of evidence for understanding cosmic evolution. This radiation is isotropic and uniform, with slight temperature fluctuations that provide insights into the density and composition of the early universe. The study of CMB has profound implications for models of cosmic expansion and the acceleration of the universe.
Dark energy: Dark energy is a mysterious form of energy that permeates all of space and is responsible for the observed accelerated expansion of the universe. It makes up about 68% of the total energy content of the universe and plays a crucial role in shaping its large-scale structure and future dynamics.
Equation of State Parameter: The equation of state parameter, often denoted as $w$, is a dimensionless quantity that describes the relationship between pressure and density in a given cosmic fluid. Specifically, it is defined as the ratio of pressure to energy density, $w = \frac{P}{\rho c^2}$, where $P$ is the pressure and $\rho$ is the energy density. This parameter helps characterize different components of the universe, such as dark energy, matter, and radiation, and plays a crucial role in understanding cosmic acceleration.
Flat Universe: A flat universe is a model of the cosmos where the overall geometry of space is flat, meaning it follows Euclidean geometry on large scales. In this model, parallel lines never meet and the angles of a triangle sum up to 180 degrees. This characteristic is critical in understanding cosmic expansion and supports the idea that the universe's density is exactly at the critical density required to halt its expansion in the long term.
Friedmann Equation: The Friedmann Equation is a set of equations derived from Einstein's General Relativity that describe how the universe expands over time. It connects the expansion rate of the universe, represented by the Hubble parameter, to its matter and energy content, including dark energy. This equation is crucial for understanding the dynamics of cosmic expansion and provides insights into the evidence supporting cosmic acceleration.
High-z supernova search team: The high-z supernova search team is a collaborative group of astronomers focused on discovering and studying distant supernovae, particularly those at high redshift (high-z). This team played a crucial role in providing evidence for the accelerated expansion of the universe by observing Type Ia supernovae, which serve as standard candles for measuring cosmic distances. Their findings have significantly contributed to our understanding of dark energy and the fate of the universe.
Hubble Diagram: The Hubble Diagram is a graphical representation that illustrates the relationship between the distance of galaxies from Earth and their recessional velocity, which is determined by the redshift of their light. This diagram is a key tool in cosmology as it visually represents Hubble's Law, showing that more distant galaxies move away from us faster, indicating the expansion of the universe. The Hubble Diagram is also essential for understanding cosmic acceleration and measuring distances to faraway galaxies.
Hubble's Law: Hubble's Law states that the recessional velocity of a galaxy is directly proportional to its distance from Earth, implying that the universe is expanding. This relationship is foundational in understanding the dynamics of galaxies and the overall structure of the cosmos, linking distance measurements, cosmic acceleration, and redshift surveys in a comprehensive framework of modern astrophysics.
Inflationary Theory: Inflationary theory is a cosmological model that suggests a period of rapid exponential expansion of the universe occurred shortly after the Big Bang, specifically during the first 10^-36 to 10^-32 seconds. This theory helps to explain several observed phenomena in the universe, including the uniformity of the cosmic microwave background radiation and the large-scale structure of the cosmos. By proposing that regions of space expanded faster than the speed of light, inflation addresses why our universe appears so flat and homogeneous on large scales.
Lambda-cdm model: The lambda-cdm model is a widely accepted cosmological model that describes the universe as being composed of dark energy (represented by the Greek letter lambda, \(\Lambda\)), cold dark matter (cdm), and normal matter. This model explains the large-scale structure of the universe and its evolution, indicating that the universe is currently experiencing accelerated expansion due to dark energy, which is key in understanding cosmic acceleration and various dark energy models.
Large-scale structure: Large-scale structure refers to the organization and distribution of matter in the universe on scales larger than individual galaxies, including galaxy clusters, superclusters, and vast cosmic filaments. Understanding this structure helps us comprehend the formation and evolution of the universe, as well as the influence of dark energy and cosmic acceleration on the expansion of space.
Redshift: Redshift refers to the phenomenon where light from an object in space is shifted towards longer wavelengths, making it appear more red. This effect is primarily observed in astronomical objects moving away from us, allowing scientists to measure the velocity and distance of these objects, and providing crucial insights into the expansion of the universe and the nature of cosmic phenomena.
Saul Perlmutter: Saul Perlmutter is an American astrophysicist renowned for his groundbreaking work in cosmology, particularly in the discovery of the accelerating expansion of the universe. His research utilized Type Ia supernovae as standard candles to measure cosmic distances, providing crucial evidence that the universe's expansion is not slowing down as previously thought, but rather speeding up due to a mysterious force known as dark energy.
Supernova Cosmology Project: The Supernova Cosmology Project was a scientific initiative aimed at using Type Ia supernovae as standard candles to measure cosmic distances and investigate the expansion of the universe. This project significantly contributed to the discovery of cosmic acceleration, revealing that the universe's expansion is not just continuing but actually speeding up due to an unknown form of energy, often referred to as dark energy.
Type Ia Supernovae: Type Ia supernovae are a class of stellar explosions that occur in binary star systems, where one star is a white dwarf that accretes matter from its companion star until it reaches a critical mass, leading to a thermonuclear explosion. These explosions are crucial for understanding cosmic distances and the expansion of the universe due to their consistent peak brightness, allowing astronomers to use them as standard candles for measuring astronomical distances.
Voids in the Universe: Voids in the universe are vast, largely empty spaces between galaxy filaments that contain very few galaxies and matter. These regions can span millions of light-years and are significant for understanding the large-scale structure of the cosmos, particularly in relation to cosmic acceleration and dark energy.