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Cosmology³The study of the origins, structure, and
evolution of the universe³Key moments:
²Einstein General Theory of Relativity²Hubble’s observation of expanding universe
17-2
© Dr. Joseph E. Pesce, Ph.D.
Olber’s Paradox³A simple question: Why is the sky at night
dark?
³Everything is the same for a long way in all directions
17-3
You are here
© Dr. Joseph E. Pesce, Ph.D.
Assumptions³Assumption: the universe is infinitely big³Assumption: the universe is infinitely old³Assumption: the universe is uniform
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© Dr. Joseph E. Pesce, Ph.D.
Olber’s Paradox³If the universe is infinitely large and uniform,
why is the night sky not bright?
³Every sight line will eventually hit a star
17-5
You are here
© Dr. Joseph E. Pesce, Ph.D.
DefinitionsCosmological Principle:³Universe is Isotropic & Homogenous³We are not in a special placeIsotropic: the universe is the same in all
directions³There’s no preferred directionHomogenous: At the largest scales, the
universe is uniform; pretty much the same everywhere; same physical concepts apply
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© Dr. Joseph E. Pesce, Ph.D.
Expansion & Big BangHubble’s Law: vr = Ho * D
³Universe expanding so must have been smaller, more dense in past
³George Gamow proposes an initial explosion:The Big Bang
³ 1/Ho gives age of universe = 13.74 Billion years old (confirmed independently)
³ Redshift: space stretches as photon flies through it = stretching redshift
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© Dr. Joseph E. Pesce, Ph.D.
Alternate Theories
³Steady State Model of Fred Hoyle, Herman Bondi, and Tom Gold
³Universe infinitely big and old, expands forever, matter constantly being created
²Coming from jets in AGN (in some views)
³Big Bang ironically name by Hoyle
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© Dr. Joseph E. Pesce, Ph.D.
Evidence for Big Bang
³Roll time back: universe begins as an extremely hot (1012 K) singularity
³High-energy radiation field³As universe expands, radiation cools³Radiation should permeate entire universe,
even today³Cosmic Microwave Background (CMB)
radiation
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© Dr. Joseph E. Pesce, Ph.D.
Detection of CMB³Arno Penzias & Robert
Wilson, Bell Labs (Nobel Prize)
³CMB at 2.74K³Isotropic and smooth
Consistent with Big Bang but NOT Steady State
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© Dr. Joseph E. Pesce, Ph.D.
Isotropy Problem
³To be as smooth as the CMB is, the parts of the universe had to �communicate� with each other
³But universe is too big for this to be possible
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13 Billion ly
26 Billion ly© Dr. Joseph E. Pesce, Ph.D.
Inflation
³Solution to Isotropy problem is inflation
³10-35 seconds after Big Bang, universe expanded 1050 orders of magnitude (atom to grapefruit)²The inflationary epoch²Somewhat strange, but related to dark energy
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© Dr. Joseph E. Pesce, Ph.D.
Unification of Forces
³Four fundamental forces today²Gravitation (dominant today, but VERY weak.
Operates over larger scales than others)²Electro-magnetic²Strong nuclear (holds protons & neutrons together)²Weak nuclear (important in radioactive decay)
³Were combined into one at beginning²During �Planck time� (0 – 10-43 seconds after BB)
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© Dr. Joseph E. Pesce, Ph.D.
Post Big Bang Timeline
³�Planck time� (0 – 10-43 seconds after BB)³10-43 s (T=1032 K): Gravity �freezes� out³10-35 s (T=1027 K): Strong force �freezes� out³10-35 -10-24 s: Inflation of universe by 1050
³10-12 s (T=1015 K): EM/Weak forces separate³10-6 s (T=1013 K): Protons & neutrons can exist
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© Dr. Joseph E. Pesce, Ph.D.
Matter/Antimatter
Pair Production Pair Annihilation³For T < 1s: matter/antimatter co-exist³There should be equal matter and antimatter,
annihilating each other. But not the case³1 EXTRA particle of matter for every billion
particles of antimatter³Good for us!
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© Dr. Joseph E. Pesce, Ph.D.
Timeline Continues³T = 1 – 3 minutes: nuclei form, destroyed by
gamma-rays³After T = 3 minutes, Temp too cool for fuse
nuclei²H, He, Li, Be formed
³T < 300,000 years: atoms ionized by photons; only nuclei survive: RADIATION DOMINATED
³T > 300,000 years: atoms can form: MATTER DOMINATED UNIVERSE (still more radiation than matter)
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© Dr. Joseph E. Pesce, Ph.D.
Decoupling³T=300,000 years Transition called: Decoupling
(Temp = 3,000)³T < 300,000 yrs: plasma only³T > 300,000 yrs: H atoms form, universe
becomes transparent²�Era of Recombination�²CMB is �Fossil� of this period²We can’t �see� further back in time (prior universe is
opaque)
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© Dr. Joseph E. Pesce, Ph.D.
Structures³Then what?
²Stars form²Galaxies form
q Where do blackholescome in?
³All happens very soon after decoupling
³Galaxies seem to be made up of small pieces
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© Dr. Joseph E. Pesce, Ph.D.
Age of the Universe
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³Related to Hubble’s Constant (1/H ~ age)
² Must be at least 10,000 yrs old (tree rings)² Must be at least 200 million yrs old (Cepheid light
travel time)² Must be at least 4.6 billion yrs old (meteor dates)² Must be at least 10 billion yrs old (redshifts of
quasars; globular cluster ages)
³Measured – accurately and with multiple methods – as: 13.74 billion years old
© Dr. Joseph E. Pesce, Ph.D.