Post on 11-Mar-2020
08 CosmologyAST1002 2017 London LN01Prof. Eric Hsiao
1
Cosmology: the science of the origin and development of the universe.
2
Where do we come from?What are we?Where are we going?
-Paul Gauguin
1908: PL relation for Cepheids• Henrietta Swan Leavitt
discovered the period-luminosity (PL) relation for cepheids.
• Longer period, brighter.
4
Henrietta Swan Leavitt
1920: The Great Debate
• Sun is not at the center of the Milky Way.
• Spiral nebulae are galaxies like the Milky Way.
5
ShapleyCurtis
1924: Distance to Andromeda
• Using Leavitt’s PL relation, Edwin Hubble found the distance to Andromeda Galaxy to be 100x the size of the Milky Way.
6
1915: General Relativity
• Gik + Λgik = 8πGTik
• Einstein laid out the mathematical foundation for cosmology.
• Yes, this 1 equation describes the whole universe.
7
Geometry ofthe Universe
Stuff inthe Universe
1915: General Relativity
• Gik + Λgik = 8πGTik
• Einstein prefers a static universe. If we don’t occupy a special place, why should we occupy a special time?
• The equation naturally produces a dynamic universe. Einstein added a “cosmological constant” to keep it static.
8
cosmologicalconstant
9
100-inch Hooker TelescopeMt Wilson Observatory
1929: Hubble’s law
10
Hubble diagram:Distance vs. velocity
How did he measure distance?
How did he measure velocity?A positive velocity means…?
1929: Hubble’s law
11
Edwin Hubble observed:
1. Practically all galaxies exhibit redshifts, thus moving away from us.
2. The more distant a galaxy, faster it is moving away from us.
What do these observations tell us?
1929: Hubble’s law
12
“It is thought premature to discuss in detail the obvious consequences of the present result.”
Edwin HubbleCarnegie, 1929
• Distance between MW and galaxy 2 increases much faster than thedistance between MW and galaxy 1.
• Velocity = distance/time; thus,galaxy 2 is moving away from us with faster speed than galaxy 1.
• Hubble showed that the universe is expanding!
13
time
MW
1
2
1929: Hubble’s law
• Hubble shows that the Universe is not static and is expanding.
• What happens when we rewind the tape?
• The expanding universe implies abig bang beginning.
14
time
MW
1
2
1929: Hubble’s law
1931: Einstein’s visit to Mt Wilson
15
1931: Einstein’s visit to Mt Wilson
16
1931: Einstein’s visit to Mt Wilson
17
Λ
Einstein reportedly remarked that the inclusion of the cosmological constant Λ is the “biggest blunder of his life.”
Tutorial: Hubble’s Law (p.155)
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one another.
• Come to a consensus you both agree on.
• If you get really stuck, ask me for help.
18
When looking at a Hubble diagram, what is the correct relationship between the slope, the age of the universe, and the universe’s expansion rate?
A steeper slope indicates a younger universe with a slower expansion rate.
A steeper slope indicates an older universe with a slower expansion rate.
A flatter slope indicates a younger universe with a faster expansion rate.
A flatter slope indicates an older universe with a slower expansion rate.
19
Which Hubble diagram represents a universe with expansion rate that is increasing with time?
20
Which Hubble diagram represents a universe with a constant expansion rate?
21
1965: Cosmic microwave background
Arno Penzias and Robert Wilson in a satellite experiment, detected an “excess light.”
After ruling out the Earth, Sun, Milky Way, and bird poop, they concluded the source is outside of our galaxy.
22
1965: Cosmic microwave background
This signal is the “cosmic microwave background,”a relic of the big bang emitting a blackbody spectrum of 3K.
Why does such a powerful event emit light at such low energy/long wavelength?(Recall the Hubble Diagram)
23
1985: Type Ia supernova
24
1985: Type Ia supernova
25
1993: Phillips relation
26
Fainter supernova fades faster.
Mark Phillips
1993: Phillips relation
27
Mark Phillips Henrietta Leavitt
28
Type Ia supernovae are bright.
Just like the cepheids, the absolute magnitude of a Type Ia supernova can be determined using by observing how its brightness changes.
31
Climbing the distance ladder
parallax recall the pencil experimentspectroscopic parallax HR diagramcepheids PL relationType Ia supernova Phillips relation
32
Armed with this new tool, astronomers set out to use Type Ia supernovae to measure the deceleration of the universal expansion.
Why do astronomer expect that the expansion should slow down?
33
Sandage 1972
Hubble 1929
Suzuki+ 2012Type Ia supernova
34
Sandage 1972
Hubble 1929
Suzuki et al. 2012Type Ia supernova
1998: Distant supernova race
35
Two teams of astronomers independently found the same answer.
The expansion rate is currently accelerating!
What is driving the acceleration?
36
Einstein’s original motivation for introducing the cosmological constant was to keep the Universe static.
Hubble showed that the Universe is expanding.
However, the cosmological constant currently provides the best fit to Type Ia supernova Hubble diagrams (and the best description of the acceleration).
Dark energy: the name astronomers have given to the unknown cause of the acceleration.
“On a good day, I can think of 3 or 4 plausible candidates for dark matter. The same cannot be said for dark energy.”
Rocky Kolb, 2004
37
today
Universe started acceleratingat ~2/3 of present size velocity
big bang
Type Ia supernova observations show past deceleration and current acceleration.
• Dark matter was discovered using rotation curves of galaxies.
• Dark energy was discovered using Type Ia supernovae.
• But we are only sure of their existence by getting the same answer with many different techniques.
39
1998: Distant supernova race
dark
ene
rgy
dark matter
1998: Distant supernova race
40
Nobel Prize in Physics 2011
“for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".