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.

o  Watch live at www.esa.int

o  Follow at www.twitter.com/esa