Cosmology Combined Flash Cards

Hot Big Bang Model

• Current standard model for universe

• States that universe has expanded from an initially hot and dense state, to the current relatively cool and tenuous state

• Expansion still ongoing

Olbers’ Paradox

• The fact that the night sky is dark at visible wavelengths (as opposed to being uniformly bright)

• Should be bright in a universe with infinite stars

• Resolution: the universe has a finite age, stars from too far away don’t have time for their light to have reached us

• Also the sky isnt dark in microwaves!

Universe is Isotropic and Homogeneous

• Isotropic: no preferred directions

  • Looks the same in all directions

• Homogeneous: no preferred locations, looks the same wherever you look

• On large scales, our universe is both (over 100 Mpc)

  • Nothing special about our location in the universe “cosmological principle”

Steady State Model

• Another cosmological model stating there are also no priviledged moments in time

  • Mean density and hubble constant remain constant with time

  • Infinitely old universe

  • Universe expands but new matter constantly created

• Eventually disproved when they realized the properties of the universe are changing with time

  • Discovery of cosmic microwave background

    • Doesnt explain uniform background raditation

  • We can see more younger galaxies at higher redshifts (used to be more common = change)

  • Large-scale formation?

Universe Makeup

The composition of the universe, including dark matter, dark energy, ordinary matter, and radiation that influences its structure and evolution.

• protons, neautrons, electrons

• universe is electrically nuetral on large scales

Dark Matter

Any massive component of the universe that is too dim to be detected readily

• WIMPS

Cosmic Microwave Background

• isotropic background of microwave radiation, fitted well by blackbody spectrum

  • Big Bang Model universe has this naturally arise

    • baryonic matter would have been completely ionized at much hotter temps, free electrons would make the universe opaque - which produces a blackbody

  • Relic of when universe was sufficiently hot and dense

  • Drop in CMB temp due to expansion of universe

Peculiar Motion

The irregular motion of an object in the universe, differing from the average motion of nearby galaxies, often caused by gravitational interactions.

Dark Energy

w< -1/3

Cosmological Constant

• Can a universe filled with nonrelativistic matter be static? No! it must be expanding or contracting.

• Solution: cosmological constant, lambda

• What is it physically? Vacuum energy?

Big Chill vs Big Crunch

Two competing theories about the ultimate fate of the universe; the Big Chill describes a cold, expanding universe, while the Big Crunch suggests a possible collapse back to a singular state.

Cosmic Microwave Background

The remnant radiation from the Big Bang that fills the universe, providing evidence for the early conditions of the cosmos and its expansion.

* Assumes universe was hot, dense, opaque and nearly homogeneous early on.

Epoch of Recombination

The time when baryonic component of the universe goes from ionized to neutral. Numerically, when number density of ions is equal to number density of neutral atoms.

Epoch of Photon Decoupling

rate at which photons scatter from electrons becomes smaller than the Hubble parameter (rate of universe expansion). When they decouple, they cease to interact with electrons → universe becomes transparent

Epoch of Last Scattering

The time when a typical CMB photon underwent its last scattering from an electron. Every observer is surrounded by a last scattering surface, from which CMB photons have been streaming freely. (very close to epoch of photon decoupling)

Thomson Scattering

The process by which photons interact with free electrons, causing the photons to scatter. This phenomenon is significant in the early universe, influencing the decoupling of photons and baryons.

Epoch of Big Bang Nucleosynthesis

The period in the early universe when temperatures were sufficiently high for nuclear reactions to occur, allowing the formation of light elements such as hydrogen, helium, and lithium.

Proton-Neutron Freezeout

The stage in the early universe when protons and neutrons became stable as the temperature fell, leading to the formation of deuterium and, subsequently, light nuclei.

Nucleosynthetic equivalent of SAHA equation

Deuterium favored in kT → 0 limit, protons/neutrons in kT→ infinity limit.

Asymmetry between baryons and antibaryons

products of a tiny asymmetry between quarks and antiquarks (most annihilated each other, leaving just a few quarks, leaving small baryon to photon ration)

The flatness problem

Refers to just how close Omega_0 is to 1 at all times

The Horizon Problem

The universe is nearly homogeneous and isotropic on very large scales. Why? Unexpected in Hot Big Bang scenario. 2 points not in causal contact cant have sent signal to eachother, so why is everything the same temperature?

The Monopole Problem

The apparent lack of magnetic monopoles in the universe.

Grand Unified Theory: attempts to unify EM force, weak nuclear force, and strong nuclear force.

• One prediction: universe underwent phase transition as temp dropped below GUT temp. Associated with loss of symmetry. They ask where have all the magnetic monopoles gone?

The Inflation Solution

A proposed explanation for the flatness, horizon, and monopole problems, suggesting a rapid expansion of the universe in its earliest moments. This expansion would smooth out irregularities and provide a mechanism for the uniformity observed in the cosmic microwave background. Associated with exponential expansion of universe for a brief period of time.

- explains flatness: 1-omega plumets exponentially with time here

-explains horizon: epoch of exponential inflation causes horizon size to grow exponentially, giving universe time to have been in causal contact

-explains monopole:the rapid expansion would dilute the density of magnetic monopoles, making them exceedingly rare in the observable universe.

Physics of Inflation

Take a scaler inflation field. Then energy density, with inflation field varying slowly, energy density approx - Pphi = V(phi). Potential large enough to dominate energy density of universe

Metastable false vacuum state: not truly stable, will fall into true vacuum state if nudges.

- makes universe too isotropic and homogeneous

Temperature Fluctuations in CMB

Arise from grav. effect of density fluctuations in distribution of nonbaryonic dark matter.

baryon-acoustic oscillations: inward/outward oscillations of photon-baryon fluid, potential well approx. sound horizon.

Large Scale Structure of the Universe

Clusters, superclusters, voids: all structures bigger than individual galaxies

Superclusters and Voids

Superclusters: Objects that are just now collapsing under their own self-gravity. Typically flattened or elongated structures.

• Contain one or more cluster (a fully collapsed object that has come to equilibrium, obeying virial theorem)

Voids: roughly spherical in shape, regions of space with very few or no galaxies, resulting in large-scale underdensities in the universe.

Basic Mechanism for growing large structures

Gravitational instability

• slight inhomogeneities in universes density

• Sufficient density: collapse and become gravitationally bound (clusters) → superclusters

• Draw matter to themselves from surrounding underdense regions

Jeans mass

• Abrupt decrease in baryonic jeans mass at time of decoupling → first structures formed

Jeans length

pressure,

• if becomes negligibly small, gravitational collapse can occur, leading to the formation of structures.

• density perturbations smaller than hubble distance grow in amplitude only when no longer pressure supported

• Collapse if R>R/Cs

Hot Dark Matter Structure Formation

• e.g neutrinos

• free-streaming: (fast DM, smooths out perturbations (r<ct).)

• Top-Down structure formation: galaxy superclusters, then fragmentation into galaxies later

• Expect holdest structures to be superclusters, galaxies as relatively young - but the opposite is true

  • Superclusters only just forming today

Cold Dark Matter Structure Formation

ex. WIMPS

• No free-streaming, smaller regions collapse first

• Small regions collapse first

• “bootom-up” structure formation

• galaxies, then superclusters

• more accurate, but baryon acoustic oscillations pose an issue

Neutron-Protons + Freezeout

A process in which neutrons and protons combine to form nuclei after the universe cools sufficiently, leading to the formation of light elements. After freezeout, nuclear reactions slow down significantly, affecting the composition of the universe.

Heavier Element Formation

• Heavier elements (for example 12C) require bringing together highly charged nuclei

• • Electrostatic repulsion requires high temperatures fuse heavier elements

• • But Universe is cooling down as time progresses! •

• Once Helium-4 is produced it is too cool to make anything heavier than Berylium!

Lighter Element Formation

• Once T < 3 x 108 K, no more nucleosynthesis • Abundances are “frozen” in

• Abundances after nucleosynthesis stops depend on the value of η = baryon-to-photon ratio

• Larger η means that reactants are closer, fusion is easier, and can take place at higher T.

• Higher Helium-4 production, less 2H, 3He

Inflation (Using slides!)

Spontaneous symmetry breaking

• inflation as rolls to true minimum (occurs after temp cools past certain temperature)

• shallow curve: slow-roll

Reheating:

• Rapid expansion cools universe

• inflation couples to radiation (oscillations from KE V differences), reheating universe

• power spectrum