Introduction to Astronomy and Astrophysics o Lectures 1. Astronomy – and Observational Science 2. The Sun 3. Planets of the Solar System 4. Extra-solar Planets 5. Observing the Universe 6. Properties of Stars 7. Life and Death of Stars 8. Galaxies and Large Scale Structure of the Universe 9. Cosmology – Origin and Evolution of the Universe Planets Cluster of Stars Cluster of Galaxies Lecture 9: Cosmology – Origin and Evolution of the Universe o Overview: o Expansion of the Universe o Big Bang or Steady State? o Cosmic Background o Chapter 9 of Introduction to Astronomy and Cosmology Einstein’s General Theory of Relativity o Einstein published General Theory of Relativity in 1915. o The Einstein field equation: where Λ is the cosmological constant which was introduced to hold back gravity – Einstein’s “greatest blunder”. o Einstein believed Universe was static but his equations predicted expanding Universe. Hubble and Expansion of the Universe o In 1920s, Edwin Hubble showed that the universe was expanding. o Measured Dopper shifts to get velocities of galaxies. o From Dopper shifts, can calculate galaxy velocity: where c is the speed of light. Δλ λ = v c => v = c Δλ λ Sun Galaxy
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Introduction to Astronomy and Astrophysics o Lectures
1. Astronomy – and Observational Science
2. The Sun
3. Planets of the Solar System 4. Extra-solar Planets
5. Observing the Universe
6. Properties of Stars
7. Life and Death of Stars
8. Galaxies and Large Scale Structure of the Universe
9. Cosmology – Origin and Evolution of the Universe
Planets
Cluster of Stars
Cluster of Galaxies
Lecture 9: Cosmology – Origin and Evolution of the Universe
o Overview: o Expansion of the Universe
o Big Bang or Steady State?
o Cosmic Background o Chapter 9 of Introduction to Astronomy and Cosmology
Einstein’s General Theory of Relativity
o Einstein published General Theory of Relativity in 1915.
o The Einstein field equation:
where Λ is the cosmological constant which was introduced to hold back gravity – Einstein’s “greatest blunder”.
o Einstein believed Universe was static but his equations predicted expanding Universe.
Hubble and Expansion of the Universe
o In 1920s, Edwin Hubble showed that the universe was expanding.
o Measured Dopper shifts to get velocities of
galaxies.
o From Dopper shifts, can calculate galaxy velocity:
where c is the speed of light.
Δλλ
= vc
=> v = c Δλλ
Sun Galaxy
Galaxy Distances: Period-Luminosity Relationship
o Henrietta Leavitt (1868-1921) discovered Cepheid variable stars. From period of pulsation, can determine luminosity – called “standard candle”.
o Below shows a Cepheid variable lightcurve (left) and the period-luminosity relationship (right). Page 288 of Morison.
o Can then use inverse square law to get distances.
Mag
nitu
de
Hubble and Expansion of the Universe
o Hubble plotted galaxy velocities (from Doppler shifts) against galaxy distances (from Cepheids).
o Showed than distant galaxies were receding at a speed proportional to their
distance.
o Georges Lemaître theorised an expanding universe such that
v = H0 × R where H0 is Hubble’s constant and R is distance. o Implies we live in expanding Universe.
Hubble age of the Universe
o Assume Universe has expanded at uniform rate thouout its existence.
o Can estimate time when it had not size – called the Hubble age.
o Hubble found that H0 = 500 km s-1 Mpc-1
o Age of Universe is therefore:
o Hubble found that t0 ~ 2 billion years.
o Modern value for H0 = 70.8 ± 1.6 km s-1 Mpc-1 => ~14 billion years.
t0 =1H0
Freidman “Big Bang” Models
o A. A. Friedman solved Einstein’s equations to produce models of expanding universe. Known as Big Bang models, a derogatory by Fred Hoyle.
o Universes begin in point singularity. Initial high rate of expansion then falls with time due to the mutual gravitation of all the matter in the universe.
o Critical or Flat universe is boundary between the open and closed universes. The universe then has a “critical density”
o If density is greater than this we have closed universes which will eventually collapse to a “Big Crunch”.
Freidman “Big Bang” Models
o The ratio of the actual density to the critical density is Ω = ρ / ρcritical
“Siz
e of
Uni
vers
e”
Freidman “Big Bang” Models
o If Ω > 1 => closed universe. o Space is positively curved.
o If Ω < 1 => open universe. o Space is negatively curved.
o If Ω = 1 => critical universe. o Space is “flat”
Cosmic Microwave Background
o George Gamow: If origin of Universe was very hot, should still be radiation from that time now. Called the Cosmic Microwave Background.
o For ~300,000 years, Universe too hot for atoms to form. Matter and radiation were interacting. So Universe was opaque – like a fog.
o Finally, it was cool enough for atoms (H and He) to form. Universe then became transparent – so this is as far back in time as we can see.
o Radiation then had the same temperature as matter (~3,000 K). It would have had a Black Body Spectrum with peak in the visible.
o Since then, Universe has expanded by 1000 times => temperature of this radiation has fallen by the same ratio – so now ~3K.
Cosmic Microwave Background
o Radio astronomers Arno Penzias and Robert Wilson serendipitously discovered this background radiation in 1963.
o Using “horn” radio antenna (left), they found a blackbody spectrum (right).
o Using Wien’s Law: T = 2.897 x 10-3 / λmax => TUniverse = 2.7 K
λmax
Cosmic Background Explorer (COBE)
o COBE was launched by NASA in 1989 and revolutionized our understanding of early cosmos.
o It precisely measured and mapped the oldest light in the universe – the cosmic microwave background.
o The results confirmed the Big Bang theory of the origin of the universe. .
COBE Blackbody Spectrum of the Universe
Read more at http://wmap.gsfc.nasa.gov/universe/bb_tests_cmb.html
COBE Blackbody Spectrum of the Universe
o CMB is remnant of Big Bang. Minute temperature variations (~150 micro-K) are linked to slight density variations in early universe.
o These variations are believed to have given rise to the structures that populate the universe today: clusters of galaxies, as well as vast, empty regions.
COBE Nobel Prize
o John Mather (below) from NASA Goddard Space Flight Center and George Smoot from UC Berkeley win Nobel Prize in Physics for COBE.
o Planck studied the CMB more sharply and sensitively than ever before (temperature resolution ΔT/T ~ 10-6).
o Determined the the spectrum of primordial fluctuation in the Big Bang and H0 = 67.11 ± 1.2 km s-1 Mpc-1 => t0 = 13.819 billion years.
o Found more dark matter and a little less dark energy than previously supposed.
ESA Planck Spacecraft Practical Task 9 – Follow ESA Launch to ISS Today
o Launch from Baikonur Cosmodrome in Kasakhstan at 11:03 GMT this morning.