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Cosmology: The Origin and Evolution of the Universe. The universe shows structure at many scales subatomic particles atoms stars and planets star clusters and galaxies galactic cluster and superclusters voids and sheets. Does this structure ever end? .

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The universe shows structure at many scales
    • subatomic particles
    • atoms
    • stars and planets
    • star clusters and galaxies
    • galactic cluster and superclusters
    • voids and sheets
does this structure ever end
Does this structure ever end?
  • While the data is not totally conclusive, it appears that on the scale of greater than 200 Mpc the structure does ends in other words becomes homogenous and isotropic.
cosmological assumptions
Cosmological Assumptions
  • Homogeneous
    • –   Every region is like every other region
    • –   This is true over large regions (300 Mpc and more)
  • Isotropic
    • –   The Universe has the same properties, no matter what direction you look
  • •   Together these requirements make up the cosmological principle
  • •   This basic assumption is needed to get started
Something is isotropic at a particular point if it looks the same in all directions when you stand at that point. On the largest scales, the universe is thought to be isotropic at every point
the expanding universe
The Expanding Universe
  • Olbers's Paradox
    • The observation that the sky is dark at night contrasted with a simple argument that shows that the sky should be uniformly bright.
  • This haunted Kepler as long ago as 1610.
  • 1823 - Heinrich Olbers proposed paradox
  • Assume universe is infinite and stars are randomly scattered.
  • Isaac Newton argued that no other assumption made sense
  • Then - in every direction you will eventually come to a star and the sky will be glowing
resolution of the paradox
Resolution of the paradox
  • Stars are moving away so light is red-shift and not as bright.
  • The universe is not infinitely old - so some light hasn't had time to reach us.
The redshifts that we see from distant galaxies are caused by this expansion, not by the motions of galaxies through space
The redshift of a distant galaxy is a measure of the scale of the universe at the time the galaxy emitted its light
the darkness of the night sky tells us about the nature of the universe
The darkness of the night sky tells us aboutthe nature of the universe
  • The Cosmological Principle:Cosmological theories are based on the idea that on large scales, the universe looks the same at all locations and in every direction
  • It is meaningless to speak of an edge or center to the universe or of what lies beyond the universe
the big bang
The Big Bang

Explains the features of the universe

Present data favors this model of cosmology

Follows Einstein's General Theory of Relativity

Early on the universe was compressed to infinite density and it has been expanding ever since.
  • Was it compressed to a point?
  • could it be infinite in size still?
  • Hubble's Law tells us this happened about 15 billion years ago.
supporting data for the big bang theory
Supporting Data for the Big Bang Theory
  • Hubble's Law
    • As the universe expands, galaxies move apart
  • Quasars (i.e. the universe appears to have an early history)
  • Cosmic microwave background
    • We can reinterpret the cosmological redshift
    • As the universe expands, the wavelength of radiation is stretched
  • Helium abundance (cosmic microwave background)
exactly what is expanding
Exactly what is expanding?

The Universe is not expanding into empty space

This idea comes from watching explosions

Instead, space-time itself is expanding and carrying galaxies along with it

Another question: are we really at the center of the expansion?

    • critical density is about 5 hydrogen atoms per cubic meter, averaged over the entire universe,
  • or 10-26 kg/m3not much!
the observable universe extends about 14 billion light years in every direction from the earth
The observable universe extends about 14 billion light-years in every direction from the Earth

We cannot see objects beyond this distance because light from these objects has not had enough time to reach us

discovery of cosmic microwave background
Discovery of Cosmic Microwave Background
  • 1965 - Penzias and Wilson
  • Bell Labs, New Jersey
  • radio telescope and receiver (7 cm wavelength receiver)
tried to fix static problem
Tried to fix static problem
  • Tried to eliminate all static
  • fixed loose joints
  • repaired faulty connections
  • removed nesting pigeons and "sticky white deposits"
Still there remained an annoying low level static
  • coming from all directions
  • all times of day
  • through all the seasons
what was it
  • Blackbody Radiation
  • All objects emit radiation with the wavelengths characteristic of their temperature
what was it24
  • The Cosmic Microwave Background
  • Penzias and Wilson soon learned that this was predicted by Big-Bang.
The 100,000 year old universe was a 3,000 K blackbody.
    • peak wavelength = 1 micrometer = 10-6m
  • As we look across space we see it glowing
    • we see 1 millimeter = 10-3 meters
  • The shift of the spectrum of the cosmic microwave background from 3,000K to 3K is an enormous red-shift (cosmological red shift)
  • This "light", which we detect on earth now comes from the very early days of the universe
    • (Age~100,000 years or about 15 billion years ago)
the background radiation was hotter and more intense in the past
The background radiation was hotter and more intense in the past
  • The cosmic microwave background radiation, corresponding to radiation from a blackbody at a temperature of nearly 3 K, is the greatly redshifted remnant of the hot universe as it existed about 380,000 years after the Big Bang
  • During the first 380,000 years of the universe, radiation and matter formed an opaque plasma called the primordial fireball
When the temperature of the radiation fell below 3000 K, protons and electrons could combine to form hydrogen atoms and the universe became transparent
steady state theory
Steady-State Theory
  • The steady state theory pre-dates the Big Bang
  • Consistent with Hubble's Law
  • Assumption -
    • universe is homogeneous, isotropic, and unchanging in time
    • New matter is continuously created as universe expands to keep universe unchanged
bb sst
  • Both BB and ST assume a homogeneous, isotropic Universe:
  • (i) no edge
  • (ii) no centre
  • (iii) every part looks the same (on average, i.e. over a large enough volume)
big bang assumes also
Big Bang assumes also:
  • (i) initial 'singularity' (beginning)
  • (ii) expansion
  • (iii) evolution:
  • (iv) cooling, transformation of energy into matter, formation of structure, all through normal physical processes.
steady state also
Steady State also:
  • (i) expansion (in current theory; original theory did not!)
  • (ii) no evolution
  • (iii) no beginning
  • (iv) matter (Hydrogen) spontaneously created "in between" Galaxies
  • (v) all radiation is from stars
how much matter must be created
How much matter must be created?
  • 1 hydrogen atom/cubic cm every 1015 years
  • (1,000,000,000,000,000 years)
  • or 1,000 atoms per year in the Astrodome (cannot be detected)
  • not much
However, this theory is not in favor since Big Bang explains so much and Steady State Theory cannot explain the cosmic microwave background nor how matter or energy is created.
Einstein found that he couldn't allow universe to be static in his theory
  • it either expands or contracts
  • Einstein was unhappy with these two and invented a fudge factor (the cosmological constant) to force a static universe
    • Basically the force of expansion is equaled by the gravitational force of the mass
hubble later tells us it s expanding
Hubble later tells us it's expanding

3 Possibilities in Einstein's General Theory of Relativity:

  • infinite and unbounded - open
  • infinite and unbounded - flat
  • finite - closed (need not be bounded)

If ρ0 is greater than ρc, the density parameter Ω0 has a value greater than 1, the universe is closed, and space is spherical (with positive curvature)

if 0 is equal to c the density parameter 0 is equal to 1 and space is flat with zero curvature
If ρ0 is equal to ρc, the density parameter Ω0 is equal to 1 and space is flat (with zero curvature)

If ρ0 is less than ρc, the density parameter Ω0 has a value less than 1, the universe is open, and space is hyperbolic (with negative curvature)

future of the universe
Future of the Universe
  • Major Question - Will it continue to expand or will it collapse to big crunch?

Open, closed, or flat?

same question
Same Question -
  • Is there enough gravity (or matter) to bend universe back in on itself
    • to "close" the universe
  • the amount of mass needed to close the universe is call the critical density
Need to Determine Average Mass in a Given Volume (Density)
  • Finding all mass is tricky because of the Dark Matter
  • Neutrino Mass - very many neutrinos in universe so very small mass for each would add up to a lot open or closed??
the expanding universe emerged from a cataclysmic event called the big bang
The expanding universe emerged from a cataclysmic event called the Big Bang
  • The universe began as an infinitely dense cosmic singularity which began its expansion in the event called the Big Bang, which can be described as the beginning of time
  • During the first 10–43 second after the Big Bang, the universe was too dense to be described by the known laws of physics
the shape of the universe indicates its matter and energy content
The shape of the universe indicates its matterand energy content
  • The curvature of the universe as a whole depends on how the combined average mass density ρ0 compares to a critical density ρc

Observations of temperature variations in the cosmic microwave background indicate that the universe is flat or nearly so, with a combined average mass density equal to the critical density

observations of distant supernovae reveal that we live in an accelerating universe
Observations of distant supernovae reveal that welive in an accelerating universe
  • Observations of galaxy clusters suggest that the average density of matter in the universe is about 0.27 of the critical density
  • The remaining contribution to the average density is called dark energy
  • Measurements of Type Ia supernovae in distant galaxies show that the expansion of the universe is speeding up
  • This may be due to the presence of dark energy in the form of a cosmological constant, which provides a pressure that pushes the universe outward
key words
average density of matter

Big Bang

closed universe

combined average mass density


cosmic background radiation

cosmic microwave background

cosmic light horizon

cosmic singularity

cosmological constant

cosmological principle

cosmological redshift


critical density

dark energy

dark energy density parameter

dark-energy-dominated universe

density parameter

era of recombination

flat space


hyperbolic space


lookback time

mass density of radiation

matter density parameter

matter-dominated universe

negative curvature

observable universe

Olbers’s paradox

open universe

Planck time


positive curvature

primordial fireball

radiation-dominated universe


relativistic cosmology

spherical space

zero curvature

Key Words
If we accept 14 billion years as the age of the Universe, then we can only see 14 billion light years into space
  • Light from anything more distant hasn’t had time to reach us yet
  • There is a sphere around us with a 14 billion light year radius beyond which we can’t see
  • This sphere is called the cosmic particle horizon
  • The observable Universe is located inside this sphere