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The Big Bang. Koji Mukai. Elements 2002 Workshop. What is the Big Bang? . ‘Big Bang’ Cosmology is The theory that the expansion of the universe began at a finite time in the past, in a state of enormous density and pressure (Weinberg) Tightly constrained by observations

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the big bang
The Big Bang

Koji Mukai

Elements 2002 Workshop

what is the big bang
What is the Big Bang?
  • ‘Big Bang’ Cosmology is
  • The theory that the expansion of the universe began at a finite time in the past, in a state of enormous density and pressure (Weinberg)
  • Tightly constrained by observations
  • Highly successful family of theories with no obvious competitor

Elements 2002: Big Bang

unanswerable questions
Unanswerable Questions
  • This subject can generate countless questions: some are based on misconceptions, some are about the unobservable.
  • Where did the Big Bang occur?
  • What is the Universe expanding into?
  • What happened before the Big Bang?
  • Are there many universes?

Elements 2002: Big Bang

key concepts
Key Concepts
  • The Universe as a whole is expanding.
  • Hubble’s Law: recession velocity is proportional to distance
  • Expansion implies early Universe was dense and hot, sufficient for fusion
  • Further back in time you go, the more uncertain the theory becomes
  • Predicts a Microwave Background

Elements 2002: Big Bang

a cosmic census
A Cosmic Census
  • What objects are there in the Universe?
  • A galaxy is made up of billions of stars
  • Many galaxies are found in groups and clusters

Elements 2002: Big Bang

cosmological principle
Cosmological Principle
  • The axiom that the universe is isotropic and homogeneous.
  • Does not apply to stars within Milky Way Galaxy, or to bright, nearby galaxies
  • Applies to the average distribution of galaxies on the largest scales
  • Implies that we are not a privileged observer.

Elements 2002: Big Bang

isotropic distribution an example
Isotropic Distribution: an Example

Elements 2002: Big Bang

hubble s discovery
Hubble’s Discovery

Elements 2002: Big Bang

hubble s law
Hubble’s Law
  • Hubble’s Law
  • Recession velocity is proportional to distance
  • Hubble constant: ~65 km/s per Mpc
  • I.e., the Universe is expanding!

Elements 2002: Big Bang

how do we know their distances
How Do We Know Their Distances?
  • Astronomers use a series of steps, or “the cosmic distance ladder”
  • Parallax for nearby stars
  • Cepheid and other “standard candles”

Elements 2002: Big Bang

how do we know their velocities
How Do We Know Their Velocities?
  • We use Doppler Shifts.
  • Redshifts when object is receding

Elements 2002: Big Bang

fingerprints of atoms
Fingerprints of Atoms
  • Atoms emit and absorb light at specific wavelengths; these can be used to identify composition and velocity.
  • Examples (hydrogen, helium, carbon)

Elements 2002: Big Bang

back to hubble s law
Back to Hubble’s Law
  • Space itself is expanding, not just a few galaxies!
  • Same expansion in every direction
  • Same seen from another galaxy

Elements 2002: Big Bang

looking back in time
Looking Back in Time
  • Hubble’s Law describes the current expansion of the Universe.
  • General Relativity is our current best theory of gravity and motion.
  • Inputting the current expansion speed and density to GR equations, we can “run the film backward”
  • Universe began as a “singularity” of infinite density (there is no “before”)

Elements 2002: Big Bang

smaller means hotter
Smaller Means Hotter
  • The early Universe was not only dense, but was also hot.
  • You can heat gas by compressing (bicycle pump experiment) or cool it by letting it expand (fridge)
  • Temperature is the measure of average energy per particle
  • This applies to photons as well as to protons, electrons, etc.

Elements 2002: Big Bang

the history of the universe
The History of the Universe

Elements 2002: Big Bang

the first second
The First Second
  • We can reconstruct the earliest history of the Universe, based on GR and quantum physics.
  • More uncertain earlier we go - a family of theories for this stage of Big Bang
  • T=10,000,000,000 K soup of particles at t=1 s (antiparticles mostly gone)
  • Electrons and positrons decreasing but numerous, photons, protons, ...

Elements 2002: Big Bang

electrons and positrons
Electrons and Positrons
  • Electron and its anti-particle, positron, are among the least massive particles.
  • At high temperatures, they can be created and destroyed ~equally
  • At T<~10,000,000,000K, annihilation starts to win (not enough energy)

Elements 2002: Big Bang

protons and neutrons
Protons and Neutrons
  • Isolated neutrons decay into protons with a life time of ~10 min.
  • When the universe is hot, reactions go both ways so there are equal numbers
  • Imbalance develops as Universe cools

Elements 2002: Big Bang

the first three minutes
The First Three Minutes
  • The first three minutes is the era of Big Bang nucleosynthesis.
  • Too hot (T~10,000,000,000K), and protons and neutrons won’t stick (left)
  • Still needs high T and high density for fusion to occur (right)

Elements 2002: Big Bang

deuterium bottleneck
Deuterium Bottleneck
  • Deuterium (heavy hydrogen) nuclei are the first step in creating heavier elements, but they don’t stick easily.
  • Almost all deuterium nuclei will immediately end up as helium
  • Deuterium formation requires T=1,000,000,000K or less, depending on density of protons+neutrons
  • Universe cools sufficiently at t=3 min.

Elements 2002: Big Bang

elements created
Elements Created
  • The ratio of neutrons to protons when deuterium begins to form determines the ratio of helium to hydrogen.
  • ~10% He4
  • Also He3 etc.
  • Neutrons disappear

Elements 2002: Big Bang

origin of hydrogen and helium
Origin of Hydrogen and Helium
  • The Big Bang created most of the hydrogen and helium in the Universe.
  • Protons are stable, and more numerous than neutrons; they survive to become hydrogen nuclei
  • Most neutrons end up in the very stable alpha particles (2 protons, 2 neutrons), or He4 nuclei
  • Observed ratios agree with theory

Elements 2002: Big Bang

recombination
Recombination
  • There are a lot of photons in the Universe.
  • Spectrum is determined by the temperature (blackbody)
  • When atoms form (recombination of nuclei and electrons) at ~3,000K, Universe becomes transparent
  • The 3,000K blackbody has been redshifting ever since

Elements 2002: Big Bang

cosmic microwave background
Cosmic Microwave Background

Elements 2002: Big Bang

cmb as evidence of big bang
CMB as Evidence of Big Bang
  • Cosmic Microwave Background and its approximate temperature was predicted before discovery in 1964: a triumph of Big Bang theory!
  • Current temperature is 2.725K, and uniform to better than 1 part in 1,000.
  • No alternative theory can explain the uniformity.
  • Small “wiggles” are seeds of galaxies.

Elements 2002: Big Bang

elements from the big bang
Elements from the Big Bang

Elements 2002: Big Bang