Lecture 2

Big Bang Time Line

The Birth of the Quantum• Max Planck

– The energy contained in radiation is related to the frequency of the radiation by the relationship

• n is a positive integer called the quantum number• f is the frequency of the oscillation

– A discreet packet of energy, later to become known as “a photon”

nhfE

Implications of Planck’s Law

• The energy levels of the molecules must be discreet

• Only transitions by an amount E=hf are allowed

• The implication is that light is discreet or quantised

These quantum levels are known as number states

43210

4hf3hf2hf1hf0

energy n

Spectroscope

Three Types of Spectra

Spectral Analysis of the Elements

Continuous Spectrum: a collection all possible wavelengths/ frequencies of light

Studying the light emitted by an object in order to know something about that object!

Emission Spectra

Pattern of bright spectral lines produced by an element.

Absorption Spectra

Pattern of dark spectral lines where light within a number of narrow frequency ranges has been removed.

HeliumArgon

NeonKrypton

Bright Line Emission Spectra

Hydrogen

Wavelength

Kirchoff’s Laws• 1st law: A luminous solid or

liquid, or a sufficiently dense gas, emits light of all wavelengths and produces a continuous spectrum of radiation.

• 2nd law: A low-density hot gas emits light whose spectrum consists of a series of bright emission lines which are characteristic of the chemical composition of the gas.

• 3rd law: A cool thin gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place superimposed on the continuous spectrum.

Spectra and BackgroundType of spectrum seen depends on the temperature of the

thin gas relative to the background temperature.

TOP: thin gas cooler than background, absorption lines seen.

BOTTOM: thin gas hotter than background, emission lines seen.

Studying the Stars:

Analyzing the light from a star can tell us:

1. The composition of the star.2. The relative motion & rotation of the star.3. The star’s temperature.

Shows limited Range of Light Energies Reaching Earth’s Surface

Hubble’s Discovery of the Expanding Universe (1929)

• Spiral nebulae known to have redshifted spectra

• Hubble and Humason carry out quantitative study

• Hubble shows velocity of recession is proportional to distance

Instrument of Discovery:Hooker 100” Telescope

Mount Wilson Observatory

The Hubble Law

• Hubble’s original data showing the galaxy velocities to be propor-tional to their distance

v=HoR

The Hubble Law

• Improved data showing that the Hubble law holds to much larger distances

= 75km/s/Mpc0H

v=HoR

6 61 10 3 10Mpc pc x light year

Cosmic Distance Ladder

Radar

METHOD

Supernovae

Britest Galx. In Cluster

Rotation Velocity Period-Lum. Relat.

Color-Mag Rel.

Stat. Parallax

Moving Cluster

Parallax

Objects

Remote Galax.

Remote Clusters Spiral Galaxies

Cepheid Var. Stars

Star Clusters

Hyades Star Cluster

Planets & Stars

Nearby Planets

Useful Distance

1010 Light years

1010 Light years 108 Light years

5x107 Light years 106 Light years 1000 Light years 120 Light years 100 Light years light minutes

Stellar Parallax

Parallax is the annual shift in a star’s apparent position in the sky due to the Earth’s orbital motion.

The parallax angle is half the annual shift.

The parallax angle of the nearest star, Proxima Centauri, is 0.77 arcseconds.

𝑑=𝐴 .𝑈 .𝑝

Parsec

An object with a parallax of 1 arcsecond is located at the distance of 1 parsec.

1 pc = 3.26 light-years = 3.09 1013 km

1d (in parsecs) = -------------------------- p (in arcseconds)

Parallax

A.U.

Earth

Sun

2x Parallax (p) in arcsecs

BackgroundStars

A.U. = Astronomical Unit = Earth-Sun Distance = 1.5x1011mParsec = pc = distance when parallax is 1 arcsec:

2 radians in circle = 360 deg ==>

161 . . 1 . .206000 . . 3.1 10

11 sec206000

AU AUd AU x m

arc rad

360deg1 57deg 57 60 min

257 60 60 sec 206000 sec

rad x arc

x x arc arc

The Hubble Law

·

dR

RdHV == 0

= 75km/s/Mpc0H

The Expansion of the Universe: One should consider the galaxies located on the surface of the sphere which expands with time. As the sphere expands all lengths, including that of light increase. That means all the photons redshift. The redshift increase with the distance.

The Expansion of the Universe

Raisin Cake Model

12

3

1

2

3

Like raisins in rising raisin cake, galaxies move awayaway from each other in our expanding universe.

Cosmology

• Hubble Time• The age of the universe if the expansion has been

constant.• t = 1/Ho = ?

• The expanding universe probably originated in an explosion called the Big Bang between 12 and 18 billion years ago.

91 10billion

Big Bang Timeline

We are here

Big Bang Timeline

• GUT period -age of quarks and gluons:

Dense concentration of matter and antimatter; gravity a separate force, more quarks than antiquarks

• Inflationary period: rapid expansion, strong force separate from electroweak force

• Electroweak era; age of leptons: Leptons distinct from

quarks; bosons mediate weak force ;

• Particle era:• Age of nucleons and antinucleons: quarks bind together to

form nucleons and antinucleons; energy too low for nucleon- antinucleon pair production at .

• Age of nucleosynthesis: stable deuterons; matter 74% H, 25% He, 1% heavier nuclei

• Age of ions: expanding, cooling gas of ionized H and He.

0W and Z 12(10 )s

210 s

Big Bang Timeline

Big Bang Timeline• Recombination era: age of atoms; neutral

atoms form, pulled together by gravity; universe becomes transparent to most light.

• Age of stars and galaxies

Thermonuclear fusion begins in stars, forming heavier nuclei

• Present era 915 10x years