29.5 What Is the Universe Really Made Of?
4 min read•june 12, 2024
The universe is mostly made of stuff we can't see. Only 5% is ordinary matter like stars and planets. The rest? and , mysterious substances that shape cosmic structure and expansion.
Our understanding of the universe has evolved dramatically. Observations of and supernovae led to the discovery of and . These findings revolutionized our view of cosmic composition and evolution.
The Universe's Composition and Evolution
Density contributions in universe composition
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- Stars contribute a tiny fraction of the universe's total density, less than 1%
- Galaxies, including their stars, planets, and interstellar gas, make up around 5% of the universe's total density (, )
- Ordinary matter, which includes everything made of , , and , accounts for only about 5% of the universe's total density
- This familiar matter composes stars, planets, galaxies, and interstellar gas (, )
- Also known as , it is the type of matter described by the
- The remaining 95% of the universe consists of non-ordinary matter, such as dark matter and dark energy, which do not interact with electromagnetic radiation
Evolution of cosmological understanding
- In the 1970s, observations of galaxy rotation curves revealed galaxies contain more mass than visible matter alone can explain ('s work)
- This discrepancy led to the concept of dark matter, an invisible form of matter that interacts gravitationally but not electromagnetically
- In the 1990s, observations of distant supernovae showed that the universe's expansion is accelerating ()
- This discovery led to the concept of dark energy, a mysterious form of energy that permeates space and drives the accelerating expansion of the universe
- Current estimates of the universe's composition:
- 5% ordinary matter
- 27% dark matter
- 68% dark energy (, )
- These proportions form the basis of the , the current standard model of cosmology
Challenges of dark matter identification
- Dark matter does not interact with electromagnetic radiation, making it invisible to telescopes and instruments that detect light (optical, radio, X-ray telescopes)
- Potential dark matter candidates include:
- (), hypothetical particles with mass that rarely interact with ordinary matter (, )
- Massive Compact Halo Objects (), astronomical bodies that emit little or no radiation (, , )
- Direct detection efforts to identify dark matter particles have been ongoing but have not found conclusive evidence yet (, , experiments)
- Indirect detection methods, such as , provide evidence for the presence of dark matter in galaxies and galaxy clusters
Dark matter's role in galaxy formation
- Dark matter played a crucial role in the formation and evolution of galaxies in the early universe
- Shortly after the Big Bang, dark matter began to clump together due to its gravitational influence, forming dense regions that attracted ordinary matter
- These dense dark matter regions served as the seeds for galaxy formation, allowing ordinary matter to accumulate and form stars and galaxies (, )
- Without dark matter, galaxy formation would have taken much longer, as ordinary matter alone would not have been able to create the necessary gravitational wells
Universe development since cosmic microwave background
- The (CMB) is the oldest light in the universe, originating about 380,000 years after the Big Bang
- At this time, the universe had cooled enough for atoms to form, allowing photons to travel freely through space (, )
- After the CMB emission, the universe entered the , a period lasting several hundred million years before the formation of stars and galaxies
- As dark matter clumped together, it attracted ordinary matter, leading to the formation of the first stars and galaxies during the Cosmic Dawn (, )
- Over billions of years, galaxies continued to form, evolve, and merge, forming larger structures such as galaxy clusters and superclusters (, )
- About 5 billion years ago, the universe's expansion began to accelerate due to the influence of dark energy, overcoming the gravitational attraction of matter
- Today, the universe continues to expand at an accelerating rate, with dark energy and dark matter dominating its composition, while ordinary matter makes up only a small fraction
Early Universe and Cosmic Inflation
- The theory of proposes that the early universe underwent a period of rapid exponential expansion
- This inflationary period occurred just a fraction of a second after the Big Bang
- Cosmic inflation helps explain several observed features of the universe, including its flatness and the uniformity of the cosmic microwave background
- The end of the inflationary period is thought to have led to the production of matter and energy that formed the basis of our observable universe
Key Terms to Review (53)
Active galaxies: Active galaxies are galaxies that emit an exceptionally high amount of energy from their cores, often due to the presence of a supermassive black hole. They are characterized by their strong and variable emissions across the electromagnetic spectrum.
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.
Axions: Axions are hypothetical, extremely lightweight particles that were first proposed to resolve an apparent conflict in the theory of quantum chromodynamics (QCD), the fundamental theory of strong interactions. These particles are of great interest in the context of cosmology and astrophysics, as they may provide insights into the nature of dark matter and the overall composition of the universe.
Baryonic Matter: Baryonic matter refers to the ordinary, visible matter that makes up the majority of the observable universe. It consists of subatomic particles known as baryons, such as protons and neutrons, which are the building blocks of atoms and molecules that form the familiar structures we see around us, including planets, stars, galaxies, and the human body.
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.
Black Holes: A black hole is an extremely dense region of spacetime with a gravitational pull so strong that nothing, not even light, can escape from it. Black holes are formed when a massive star collapses in on itself at the end of its life cycle, creating a singularity surrounded by an event horizon.
Brown dwarfs: Brown dwarfs are celestial objects that are too large to be planets but not massive enough to sustain hydrogen fusion in their cores like true stars. They occupy the mass range between the heaviest gas giant planets and the lightest stars.
Brown Dwarfs: Brown dwarfs are substellar objects that are too large to be considered planets, yet not massive enough to sustain the nuclear fusion reactions that power stars. They occupy the mass range between the heaviest gas giant planets and the lightest stars, bridging the gap between these two celestial bodies.
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.
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.
Cosmic Web: The cosmic web is a large-scale structure of the universe, composed of galaxies, galaxy clusters, and filaments of matter that are separated by vast empty spaces called voids. It is a complex network that describes the distribution and organization of matter on the largest scales in the universe.
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.
Dark Ages: The Dark Ages refer to a period in European history following the fall of the Western Roman Empire, characterized by a decline in social, political, economic, and cultural conditions. This term is particularly relevant in the context of understanding the beginning of the universe and the composition 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 matter: Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible to current instruments. It exerts gravitational forces and is thought to constitute approximately 27% of the universe's mass-energy content.
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.
Dark Matter Halos: Dark matter halos are vast, spherical regions of invisible, gravitationally dominant matter that surround and envelop galaxies. They are a crucial component in the formation and evolution of galaxies, as well as the overall structure of the universe.
Decoupling: Decoupling refers to the process by which certain physical quantities or interactions become independent of each other in the early stages of the universe's evolution. This concept is particularly relevant in the context of the beginning of the universe and the fundamental nature of the cosmos.
Electrons: Electrons are negatively charged subatomic particles that are found in all atoms, orbiting the nucleus and playing a crucial role in various physical and chemical processes. These fundamental particles are central to understanding topics such as mass, energy, cosmic rays, and the composition of the universe.
Galaxies: Galaxies are vast, gravitationally bound systems of stars, stellar remnants, interstellar gas, dust, and dark matter. They range in size from dwarfs with just a few hundred million stars to giants with one trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way is the galaxy that contains our Solar System, and is just one of the hundreds of billions of galaxies in the observable universe.
Gravitational Lensing: Gravitational lensing is the bending of light by the gravitational field of a massive object, such as a galaxy or a black hole. This phenomenon occurs because the presence of matter distorts the fabric of spacetime, causing light to follow a curved path as it travels through this warped spacetime.
Helium: Helium is a colorless, odorless, and inert gas that is the second most abundant element in the universe, after hydrogen. It is a crucial component in various scientific and technological applications, as well as in the understanding of the universe and the evolution of stars and planets.
Hydrogen: Hydrogen is the simplest and most abundant element in the universe, consisting of a single proton and electron. It is a key component in the formation and composition of many astronomical objects and phenomena, playing a crucial role in the study of the very small, the formation of spectral lines, the atmospheres of the giant planets, the spectra of stars, the interstellar medium, and the fundamental makeup of the universe.
Lambda-CDM model: The lambda-CDM model, also known as the standard cosmological model, is the predominant theoretical framework used to describe the evolution and composition of the universe. It combines the cosmological constant (lambda) and cold dark matter (CDM) to provide a comprehensive model for understanding the large-scale structure and dynamics of the universe.
Laniakea Supercluster: The Laniakea Supercluster is a vast collection of galaxies, including our own Milky Way, bound together by gravity. It is one of the largest known structures in the observable universe, spanning hundreds of millions of light-years across and containing the equivalent of tens of thousands of Milky Way-sized galaxies.
Lux: Lux is a unit of illuminance, which measures the amount of light that falls on a surface. It is a fundamental unit used to quantify the intensity of light in various applications, including astronomy, lighting design, and photography.
MACHOs: MACHOs (Massive Compact Halo Objects) are hypothetical astronomical objects that were proposed as a potential explanation for the missing mass or dark matter in the universe. They are dense, massive objects that could be detected through their gravitational effects on the light from distant stars.
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.
Neutralinos: Neutralinos are hypothetical, electrically neutral particles that are predicted to exist in certain extensions of the Standard Model of particle physics, such as supersymmetry (SUSY) theories. They are considered to be a potential candidate for the mysterious dark matter that makes up a significant portion of the universe's mass.
Neutron Stars: Neutron stars are the collapsed cores of massive stars that have undergone supernova explosions. They are incredibly dense, with a mass comparable to that of the Sun compressed into a sphere only tens of kilometers in diameter, making them some of the most extreme objects in the universe. Neutron stars play a crucial role in various astronomical phenomena, including the life cycle of cosmic material, tests of general relativity, gravitational wave astronomy, and our understanding of the fundamental building blocks of the universe.
Neutrons: Neutrons are subatomic particles that, along with protons, make up the nucleus of an atom. They have no electric charge and a mass slightly greater than that of a proton, playing a crucial role in the stability and composition of atomic nuclei across the universe.
Photon decoupling time: Photon decoupling time is the epoch in the early universe when photons stopped interacting frequently with matter and began to travel freely through space. This event led to the formation of the Cosmic Microwave Background radiation.
Population III Stars: Population III stars are the earliest generation of stars that formed in the universe, composed primarily of hydrogen and helium with little to no heavier elements. These stars played a crucial role in the evolution of the universe, providing the initial sources of light and energy that shaped the formation of the first galaxies and quasars.
Protons: Protons are subatomic particles that are found in the nucleus of an atom. They are positively charged and, along with neutrons, make up the core of an atom. Protons are fundamental to the structure and behavior of matter, and they play a crucial role in various topics in astronomy, including mass, energy, cosmic rays, and the composition of the universe.
Quasars: Quasars are extremely luminous active galactic nuclei powered by supermassive black holes at their centers. They emit massive amounts of energy, often outshining entire galaxies.
Quasars: Quasars are extremely luminous, compact objects at the centers of some distant galaxies. They are powered by supermassive black holes that are actively accreting matter, releasing enormous amounts of energy across the electromagnetic spectrum. Quasars are important for understanding the large-scale structure of the universe, the formation of spectral lines, the Doppler effect, evidence for black holes, observations of distant galaxies, and the composition of the universe.
Quintessence: Quintessence refers to the hypothetical fifth element or fundamental substance believed to compose celestial bodies in ancient and medieval philosophy. In the context of modern cosmology, quintessence is a hypothetical form of dark energy that may be driving the accelerated expansion of the universe.
Recombination: Recombination is the process by which free electrons and protons in a plasma (ionized gas) combine to form neutral atoms, releasing energy in the form of photons. This process is a crucial aspect of the evolution of the early universe and the formation of spectral lines in various astrophysical contexts.
Standard Model of Particle Physics: The standard model of particle physics is the most comprehensive and well-tested theory that describes the fundamental particles and the forces that govern their interactions in the universe. It is a highly successful framework that has been extensively validated through experimental observations and has become the foundation of modern particle physics.
SuperCDMS: SuperCDMS (Super Cryogenic Dark Matter Search) is an experiment designed to directly detect dark matter particles by using ultra-sensitive detectors cooled to extremely low temperatures. It aims to identify the nature of dark matter, which is believed to make up the majority of the matter in the universe.
Supermassive black holes: Supermassive black holes are extremely large black holes, typically found at the centers of galaxies, including our Milky Way. They have masses ranging from millions to billions of times that of our Sun and significantly influence their galactic environments.
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.
Vera Rubin: Vera Rubin was an American astronomer who made significant contributions to the understanding of the universe's composition. She is best known for her pioneering work in the field of dark matter, which has revolutionized our understanding of the cosmos.
Virgo Cluster: The Virgo Cluster is a large, nearby galaxy cluster located in the northern constellation of Virgo. It is one of the most massive and densest concentrations of galaxies in the local universe, containing thousands of individual galaxies gravitationally bound together.
Weakly interacting massive particles: Weakly Interacting Massive Particles (WIMPs) are a hypothetical type of dark matter particle that interact through gravity and possibly the weak nuclear force. They are massive compared to other particles, yet their interactions with ordinary matter are extremely rare.
WIMPs: WIMPs, or Weakly Interacting Massive Particles, are hypothetical subatomic particles that are believed to make up a significant portion of the universe's dark matter. These particles are called 'weakly interacting' because they only interact with ordinary matter through the weak nuclear force and gravity, making them extremely difficult to detect directly.
XENON: Xenon is a dense, colorless, odorless noble gas that is the heaviest of the naturally occurring stable elements. It is found in the Earth's atmosphere in trace amounts and has a wide range of applications, particularly in the field of physics and technology.