Cosmology and the Big Bang Theory

• Cosmology is the study of the universe's origin, evolution, and fate.

Religious vs. Physical Cosmology

• Religious/mythological cosmology explains the origin of the universe based on religious beliefs.
• Creatio ex nihilo refers to God creating the universe, as in Genesis.
• Physical cosmology explains the origin of the universe based on scientific insights and experiments.
• Nicolaus Copernicus developed the heliocentric model.
• Albert Einstein theorized the expanding universe through his theory of relativity.
• The Big Bang Theory explains the universe.

The Big Bang Theory

• The Big Bang Theory is a cosmological model describing the universe's expansion from about 13.8 billion years ago.
• The universe began as a singularity containing all space, time, matter, and energy.
• Rapid expansion occurred through a process called Inflation
• Cooling occurred as the universe expanded.
• Subatomic particles formed, and light atom nuclei were created via nucleosynthesis/nuclear fusion between protons and neutrons.
• Electrons interacted with nuclei to form primordial atoms through recombination.

Evidences for the Big Bang Theory

• Vesto Slipher and Carl Wilhelm Wirtz (1910) measured redshift, observing most spiral galaxies moving away from Earth.
• Georges Lemaître (1927) proposed the alternative idea that the universe is expanding.
• Edwin Hubble (1929) calculated distances between Earth and galaxies using redshift and observed distant galaxies moving away from each other.
• Robert Wilson and Arno Penzias (1965) discovered cosmic microwave background radiation (CMBR), a low, steady humming noise believed to be energy remains.
• Modern astronomy (2014) estimates the universe to be 13.8 billion years old.

Definition of Terms

• Atoms form of matter and are made of subatomic particles: protons, neutrons, and electrons.
• Elements are pure substances representing a specific atom type.
• Isotopes are atoms of the same element with different atomic masses due to varying neutron numbers.

Big Bang Nucleosynthesis (BBN)

• BBN, or primordial nucleosynthesis, produces light elements during the Big Bang expansion.
• BBN yields two stable hydrogen isotopes, two helium isotopes, some lithium atoms, and beryllium isotopes.
• The BBN did not give rise to elements heavier than beryllium.
• Temperature drop resulted in insufficient energy for fusion reactions to proceed.
• Nucleosynthesis continued with expanding universe.

Stellar Nucleosynthesis

• Elements associated with living and nonliving things originated from stars.
• Processes inside stars formed these elements.
• Elements heavier than beryllium formed through stellar nucleosynthesis.
• Hydrogen and helium produced by BBN combined in nuclear fusion reactions.

Stellar Evolution

• Stellar evolution is the process in which a star changes through its lifetime.
• Element abundances inside stars change over time and their mass determines its path
• Stars are formed from nebulae, referred to as stellar nurseries.
• A nebula breaks into smaller fragments as it collapses, contracting into a protostar.
• Protostars evolve into main sequence stars upon reaching gravitational equilibrium.
• Nuclear reactions form subatomic particles called neutrinos and positrons.
• Red dwarf stars remain on the main sequence phase for at least 100 billion years due to the slow rate of Hydrogen Fusion
• The sun is thought to be in the middle of its main sequence phase for another five billion years.
• Brown dwarf stars are only able to fuel deuterium fusion reactions.
• Brown dwarf stars cool gradually and have an average lifespan of less than a billion years.

Stellar Evolution After the Main Sequence

• Main sequence stars become red giants when hydrogen atoms in their cores deplete.
• Low-mass stars become white dwarf stars when most helium in their cores is consumed.
• An inert carbon core eventually becomes the white dwarf.
• A white dwarf's composition depends on its mass.
• Massive stars evolve into multiple-shell red giant stars.
• Multiple elements formed in a carbon → oxygen → neon → silicon → iron sequence.
• Stellar nucleosynthesis of elements heavier than iron requires a supernova.
• Supernovae occur when a core can't maintain nuclear fusion to resist gravity. – During a supernova, the exploding multiple-shell red giant releases massive quantities of high-energy neutrinos.
• Neutrinos break nucleons and release neutrons, which are picked up by nearby stars.
• This neutron capture is a key step in forming elements heavier than iron.

Hydrogen Burning

• Hydrogen burning refers to stellar reactions that produces He-4 from H.
• Hydrogen burning produces energy in stars, accomplished by two dominant processes.
• The Proton-Proton chain reaction transforms H into He and form helium cores.
• The Carbon-Nitrogen-Oxygen (CNO) cycle is for stars 1.3 times more massive than the Sun.
• Beta-plus decay in the proton-proton chain reaction: two ps fuse to form a deuteron (deuterium nucleus), a positron, and a neutrino.
• Deuterium burning: D fuses with p to yield He-3 and gamma.
• Fusion of two He-3 to form He-4
• The Carbon-nitrogen-oxygen cycle: the main source of He for massive stars.

Carbon-Nitrogen-Oxygen (CNO) cycle

• Proton capture: 12C fuses with p to form 13N and gamma.
• Beta-plus decay: 13N producing 13C, a positron and a neutrino.
• Fusion of 13C with p to yield 14N and gamma.
• Proton capture: 14N fuses with p to form 15O and gamma.
• Beta-plus decay: 15O producing 15N, a positron and a neutrino.
• Fusion of 15N with p to yield 12C and 4He

Helium Burning

• Helium burning is the set of stellar nuclear reactions that use helium to produce energy and heavier elements such as Be, O, Ne, and Fe.
• Helium burning generates two dominant processes: the Triple-alpha process and the Alpha process.

Triple-Alpha Process

• Set of two-stage nuclear fusion reactions where three alpha particles (He-4 nuclei) convert to 12C.
• Two alpha particles fuse to yield 8Be and gamma
• 8Be fuses with another alpha particle to form 12C and gamma
• Creates an inert carbon core found in white dwarfs and larger stars.

Alpha Processes

• A set of nuclear reactions where He converts into heavier elements, consuming He and ultimately ending at Fe
• 56Fe is the most stable element with the lowest mass-to-nucleon ratio.
• The process increases the core size and density by forming heavier elements.
• Vital in converting main sequence stars to supergiants
• These reactions capture an alpha particle and release a gamma.
• 12C captures an alpha particle/4He to make 16O, then 16O captures an alpha particle to produce 20Ne.
• Processes continues where each product captures an extra alpha particle until producing the last atom in the series (52Fe).
• All atoms produced are from even-numbered elements.

Limitations of Big Bang/Stellar Nucleosynthesis

• Fusion reactions above Fe are unfavorable because Nuclear binding energy per nucleon holds the nucleus intact.
• Smaller nuclear binding energy per nucleon and further fusion reactions with Fe require more energy.

• Elements beyond Fe require different pathways through nonspontaneous nucleosynthesis.
• The neutrinos that supernovae release help to form neutrons and protons, which are then captured by the nuclei residing on nearby stars
• This neutron or proton-capture helps achieve higher-level nucleosynthesis.

S-Process Neutron Capture:

• Processes can be slow or rapid
• S-process or slow neutron capture happens when there are only a few available neutrons
• Neutron capture is slow compared to beta decay.
• The beta decay almost always occurs before another neutron capture.
• Usually found in red giant/supergiant stars, with each neutron capture taking a decade and cascade processes taking millennia.

R-Process Neutron Capture:

• R-process or rapid neutron capture happens when large numbers of neutrons are available.
• Neutron capture is fast enough that an unstable nucleus will still combine with another neutron before beta decay occurs.
• This process is usually in supernovae with incredibly high temperatures that causes neutrons to move fast
• Neutrons can combine with already heavy isotopes right away.

Explore cosmology, the Big Bang Theory, and the universe's origins. Understand the expansion of the universe and the rapid neutron capture process. Learn about the contributions of scientists like Edwin Hubble and the composition of the universe, including ordinary matter.