12.2 The nature of dark matter and dark energy
4 min read•july 22, 2024
and are cosmic mysteries that shape our universe. They make up 95% of everything, yet we can't see them directly. Scientists have found evidence for their existence through various observations and measurements.
Dark matter holds galaxies together and helps form cosmic structures. Dark energy, on the other hand, pushes the universe apart, causing its expansion to speed up. Understanding these invisible forces is crucial for unraveling the universe's past, present, and future.
Dark Matter and Dark Energy
Evidence for dark matter and energy
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Top images from around the web for Evidence for dark matter and energy
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- show observed rotation speeds of galaxies are higher than expected based on visible matter suggesting the presence of additional unseen matter (dark matter) providing gravitational influence
- indicates light from distant galaxies is bent more than expected due to the presence of unseen matter (dark matter) in galaxy clusters and along the line of sight
- (CMB) anisotropies show fluctuations in the CMB temperature are consistent with the presence of dark matter in the early universe and the amount of dark matter inferred from CMB observations agrees with other independent measurements
- Large-scale requires the presence of dark matter to explain the observed distribution of galaxies and galaxy clusters and the growth of structure over cosmic time
- of the universe indicated by observations cannot be explained by ordinary matter and requires the presence of dark energy
Properties of dark matter vs energy
- Dark matter composes ~27% of the universe's energy density interacts gravitationally but not electromagnetically (does not emit, absorb, or scatter light) plays a crucial role in the formation and evolution of cosmic structures and likely consists of undiscovered particles beyond the Standard Model of particle physics
- Dark energy composes ~68% of the universe's energy density is responsible for the accelerating expansion of the universe acts as a negative pressure counteracting the attractive force of gravity on large scales and its exact nature remains unknown with the being the simplest explanation
- Both dark matter and dark energy are "dark" in the sense that they do not interact electromagnetically have significant effects on the evolution and fate of the universe and their nature remains one of the greatest mysteries in modern cosmology
Dark matter particle candidates
- () are hypothetical particles with masses ranging from a few GeV to several TeV that interact through the weak force and gravity but not electromagnetically and potential detection methods include:
- Direct detection: Observing the recoil of atomic nuclei when WIMPs scatter off them in highly sensitive detectors
- Indirect detection: Searching for the products of WIMP annihilation or decay such as gamma rays, neutrinos, or cosmic rays
- are ultralight particles with masses ranging from to eV introduced to solve the strong CP problem in quantum chromodynamics (QCD) and potential detection methods include:
- Axion haloscopes: Detecting the conversion of axions to photons in a strong magnetic field
- Axion helioscopes: Searching for axions produced in the Sun's core
- are hypothetical neutrinos that do not interact through the weak force only gravitationally could have masses in the keV range and potential detection methods include:
- X-ray observations: Searching for the decay of sterile neutrinos into active neutrinos and X-ray photons
- Kinematic effects: Investigating the influence of sterile neutrinos on the formation and evolution of structures in the universe
Dark energy and universe's fate
- Dark energy causes the expansion of the universe to accelerate over time and as the universe expands the density of matter decreases while the density of dark energy remains constant or increases
- Possible scenarios for the fate of the universe include:
- (Cold Death): The most likely scenario based on current observations where the universe continues to expand and cool indefinitely and galaxies, stars, and planets will eventually die out leaving a cold, dark, and empty universe
- : Occurs if the dark energy density increases with time (phantom dark energy) and the accelerating expansion becomes so strong that it tears apart galaxies, stars, and eventually atoms with the universe ending in a singularity after a finite time
- : Requires dark energy to weaken over time allowing gravity to dominate with the expansion of the universe slowing down, stopping, and then reversing causing the universe to collapse back into a singularity potentially leading to a new Big Bang (oscillating universe model)
- Current measurements of the parameter are consistent with a cosmological constant () and if the Big Rip scenario becomes more likely while if the Big Freeze remains the most probable outcome with further observations needed to better constrain the nature of dark energy and refine predictions for the ultimate fate of the universe
Key Terms to Review (21)
Accelerating expansion: Accelerating expansion refers to the phenomenon where the universe is expanding at an increasing rate over time. This behavior is driven by a mysterious force known as dark energy, which counteracts the attractive force of gravity and affects the dynamics of cosmic expansion.
Axions: Axions are hypothetical elementary particles proposed as a solution to the strong CP problem in quantum chromodynamics and are also considered a leading candidate for dark matter. These extremely light and weakly interacting particles are expected to have a significant role in explaining the nature of dark matter and dark energy, providing a possible mechanism to account for the missing mass in the universe and contribute to its overall energy density.
Big Crunch: The Big Crunch is a theoretical scenario for the ultimate fate of the universe, where the expansion of the universe eventually reverses, leading to a catastrophic collapse back into a singular state. This concept is closely linked to ideas about the density of matter and energy in the universe, suggesting that if the density exceeds a certain threshold, gravitational forces will dominate, pulling everything back together.
Big freeze: The big freeze is a scenario for the ultimate fate of the universe, characterized by a gradual decline in temperature as the universe continues to expand indefinitely. In this scenario, galaxies move away from each other, stars exhaust their nuclear fuel, and ultimately, only cold remnants like black holes and stellar remnants remain. The big freeze connects deeply with concepts of dark energy, the behavior of dark matter, and the ongoing debate about the cosmological constant.
Big Rip: The Big Rip is a hypothetical cosmological scenario in which the expansion of the universe accelerates to the point where it eventually tears apart all structures, from galaxies to atoms. This concept is closely tied to the effects of dark energy, suggesting that its density may increase over time, leading to an ultimate catastrophic end where everything is ripped apart.
Cosmic microwave background: The cosmic microwave background (CMB) is the remnant radiation from the Big Bang, filling the universe and providing a snapshot of the early cosmos when it was just 380,000 years old. This faint glow, almost uniform across the sky, carries crucial information about the universe's formation, composition, and expansion, connecting various areas of cosmological research and theories.
Cosmic Web: The cosmic web is the large-scale structure of the universe, composed of galaxies, galaxy clusters, and vast voids interconnected by filaments of dark matter and gas. This intricate network reveals how matter is distributed in the universe and plays a crucial role in understanding the formation and evolution of cosmic structures over time.
Cosmological Constant: The cosmological constant, denoted as $$\Lambda$$, is a term introduced by Albert Einstein in his equations of general relativity to represent a constant energy density filling space homogeneously. This concept is closely linked to the accelerated expansion of the universe and is a key component in explaining dark energy, which plays a vital role in our understanding of the universe's fate and structure.
Dark energy: Dark energy is a mysterious form of energy that makes up about 68% of the universe and is responsible for the observed accelerated expansion of the cosmos. This phenomenon challenges our understanding of gravity and cosmological models, as it seems to have a repulsive effect, counteracting the gravitational pull of matter.
Dark Energy Equation of State: The dark energy equation of state describes the relationship between the pressure and density of dark energy in the universe. It is often expressed using the parameter $w$, defined as the ratio of pressure ($p$) to energy density ($\rho$), represented mathematically as $w = \frac{p}{\rho}$. This relationship is crucial for understanding the dynamics of cosmic expansion and how dark energy influences the universe's fate.
Dark Matter: Dark matter is an unseen form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects. It plays a crucial role in the structure and evolution of the universe, influencing galaxy formation, cosmic expansion, and the distribution of galaxies within the cosmic web.
Galactic rotation curves: Galactic rotation curves are graphs that plot the rotational velocity of stars and gas in a galaxy against their distance from the galactic center. These curves provide crucial insights into the distribution of mass within galaxies, revealing unexpected flatness at large distances that suggest the presence of dark matter, a component that plays a significant role in understanding the nature of dark matter and dark energy.
Gravitational Lensing: Gravitational lensing is the phenomenon where the light from a distant object, such as a galaxy or quasar, is bent around a massive object, like a galaxy cluster, due to the effects of gravity. This bending of light can create multiple images, magnify the brightness of the source, and provide valuable insights into the distribution of mass in the universe, especially dark matter and its role in cosmic structure.
Lambda-cdm model: The lambda-cdm model, or Lambda Cold Dark Matter model, is the standard cosmological model that describes the evolution of the universe, incorporating dark energy (represented by lambda) and cold dark matter. This model explains how structures like galaxies form and evolve over time, while also accounting for the observed accelerated expansion of the universe.
Saul Perlmutter: Saul Perlmutter is an American astrophysicist best known for his groundbreaking work on the accelerating expansion of the universe, which has profound implications for understanding dark energy. His research, particularly with the Supernova Cosmology Project, provided critical observational evidence that the universe is not only expanding but doing so at an increasing rate, leading to the concept of dark energy as a driving force. This discovery has shaped modern cosmology and contributed significantly to the standard model of cosmology.
Sterile Neutrinos: Sterile neutrinos are a proposed type of neutrino that does not interact via the standard weak interactions that normal neutrinos do. They are theorized to be a component of dark matter and are distinct from the three known flavors of active neutrinos, making them an intriguing candidate for explaining the nature of dark matter and its role in the universe's structure and expansion.
Structure Formation: Structure formation refers to the process by which matter in the universe organizes into structures such as galaxies, clusters of galaxies, and the large-scale cosmic web. This concept is crucial for understanding how the universe evolved from a homogeneous state after the Big Bang into the rich, complex structures we observe today, influenced by dark matter, dark energy, gravity, and various physical laws.
Type Ia Supernovae: Type Ia supernovae are a specific class of stellar explosions that occur in binary star systems where one of the stars is a white dwarf. These supernovae are important for cosmology because they serve as standard candles for measuring astronomical distances and have been key in discovering the accelerated expansion of the universe.
Vera Rubin: Vera Rubin was an influential American astronomer known for her pioneering work on the rotation curves of galaxies, which provided strong evidence for the existence of dark matter. Her groundbreaking observations revealed that galaxies rotate at such speeds that they would fly apart if only visible matter were present, highlighting the need for unseen mass to account for this phenomenon and shaping our understanding of dark matter's role in the universe.
Weakly interacting massive particles: Weakly interacting massive particles (WIMPs) are hypothetical particles that are thought to make up dark matter and interact only through the weak nuclear force and gravity. This makes them difficult to detect, as they do not emit or absorb electromagnetic radiation, like regular matter does. WIMPs are appealing candidates for dark matter because they can potentially explain various astronomical observations related to the universe's structure and evolution.
WIMPs: WIMPs, or Weakly Interacting Massive Particles, are a class of hypothetical particles that are proposed as a primary candidate for dark matter. They are predicted to have mass and interact via the weak nuclear force, making them difficult to detect with conventional means. Their existence helps explain various cosmic phenomena, such as the formation of galaxies and the large-scale structure of the universe.