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Why is the Night Sky Dark?

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Why is the Night Sky Dark?. ‘Cosmology’ Studies of the universe as a whole… Today… Brief history of ideas (Early Greeks  Big Bang ) The e x pa n d in g universe (Hubble, Relativity, density & destiny) An alternative theory: The ‘Steady State ’ model. - PowerPoint PPT Presentation

Text of Why is the Night Sky Dark?

Introduction: The Night Sky

Our Evolving Universe1Why is the Night Sky Dark?CosmologyStudies of the universe as a whole

TodayBrief history of ideas (Early Greeks Big Bang)The expanding universe (Hubble, Relativity, density & destiny)

An alternative theory: The Steady State model

1Our Evolving Universe2

A brief history of IdeasAre we in a special place?Early Greeks (BC)Geocentric - Earth centredCopernicus (1543 AD)Heliocentric - Sun centredNewton (1600s AD)Governed by law of gravity

GeocentricHeliocentricThe Cosmological Principle

From Newton on (1600s now)Were NOT in a special place.The universe looks the same from everywhere and in all directions (Cosmological Principle).2Our Evolving Universe3A brief history of IdeasWhy is the night sky dark?Pre-1930s the universe was widely thought to be infinite, eternal and staticBut if so Why is the night sky dark? (Heinrich Olbers ~ 1800s AD)

Something is WRONG!

Expect: Sky as bright as surface of average star!Overall temperature of sky > 3000K!

Olbers Paradox3Our Evolving Universe4A brief history of IdeasEinsteins UniverseSpecial & General Relativity (Early 1900s)Revolutionary new descriptions of space, time, gravity and the universe.

Einsteins model of the Universe assumed:The Cosmological PrincipleThe universe is STATIC

He invented the cosmological constant () to allow static universe in General RelativityNote that he could have predicted expansion!

4Our Evolving Universe5A brief history of IdeasHubbles Expanding Universe!1929 -Edwin Hubbles analysis of light spectra of distant galaxies revealed the universe is expandingDoppler effect shows: Redshift VelocityHubbles Law: Velocity DistanceHubbles 1929 data

300 Mpc430 MpcH + K15,000 km/sec22,000 km/secVirgoUrsa Major

Distance (Mpc)Velocity (km/sec)Einstein concedes: The universe IS expanding.The introduction of was his greatest blunder!5Our Evolving Universe6A brief history of IdeasCurrent Thinking: The Big Bang The universe is expanding & cooling after an initial explosion ~14 billion years ago.

T = 0NowVery hot~ 3KNext lecture - The 1st 400,000 yearsGalaxy formation

InflationParticle/photon SoupNucleosynthesisRecombination6Our Evolving Universe7How does the Big Bang theory solve Olbers paradox?Olbers paradox assumes the Universe is Infinite, Eternal and Static.

The Observable Universe

The Observable Universe1. The Big Bang universe is NOT eternal.So the observable universe is not infinite.

2. The Big Bang universe is NOT static.Light from distant objects cools as the universe expands

7Our Evolving Universe8The Expanding UniverseDoes Hubbles law go against the cosmological principle?No - the appearance of everything receding from US would be the same anywhere..

1996 type 1 SNeHubbles findings revisited.Modern measurements of distant galaxies confirm Hubbles findingsHubbles Law: V = H0DH0 = Hubbles ConstantRecent estimates: H0 ~ 70 km.s-1.Mpc-18Our Evolving Universe9ageThe expanding universeThe age of the universeHubbles law can be used to calculate the age of the universe:If expansion (V) is constant: Age = D0/V = 1/H0 If H0 ~70 km.s-1.Mpc-1 Age ~ 14 billion years

Separation of galaxies (D)Time (t)nowV=H0D0D0Age is less if expansion is slowed by gravityWith =0, would expect age ~ 9 billion years(With >0, age goes back to~ 14 billion years see later)

Expansionslowed by gravityage9Our Evolving Universe10Not like a normal explosion Not IN a particular place AT a particular time.Big bang thought to be the beginning of time itself.Expansion OF space not WITHIN space.

The expanding universeWhat does the expansion look like?

Raisins in breadanalogySurface of balloon analogyHard (very hard) to visualise.

10Our Evolving Universe11The expanding universe Einsteins RelativitySpecial Relativity (1905)

The speed of light (c) is always constant (3108 m s-1) Nothing can go faster than c.Space & time are not independent.Mass & energy are related: E = mc2

Concerns motion, space, time & the speed of light.

General Relativity (1915)

Concerns motion, space-time, gravity and mass. Mass WARPS Space-time.Warping perceived as gravity SPACE-TIME11Our Evolving Universe12The expanding universe Evidence for General RelativityGeneral Relativity predicts light is bent by massive objects

Gravitational Lensing12Our Evolving Universe13The expanding universe Curvature of the UniverseGeneral relativity says:Space-time in the universe is CURVED!Curvature depends on mass density (). If

= c > c < c Critical Densityc = Flat Closed OpenInfinite Finite InfiniteSize of the Universe13Our Evolving Universe14The expanding universe What about the future?Fate of the universe is dependent on density () & critical density (c)Which is it? 3rd lecture - Vital Statistics of the Universe Separation of galaxies (D)Time (t) < c=0 = c In simple models (=0): If < c: (Open Universe) Universe will expand forever If = c: (Flat universe) Expansion decelerates towards zero If > c: (Closed Universe) Universe will eventually collapse (the Big Crunch!)


(See later!)14Our Evolving Universe15Summary so farBrief history of ideas..The Cosmological Principle: The universe looks the same in all directions and in any location.

Olbers Paradox: If the universe is infinite, eternal and static - Why is the night sky dark?

Now believe the universe is in a state of expansion and cooling after an initial Big Bang ~14 billion years ago. 15Our Evolving Universe16Summary so farThe expanding universe:

Distant objects are receding according to Hubbles law (V=H0D).

The Big Bang is not like a normal explosion.Expansion OF (not WITHIN) spaceBig bang = beginning of time

Einsteins Relativity describes gravity, space, time and the structure of the universe.

The Age and Fate of the universe dependant on density ()

16Our Evolving Universe17An alternative model? The Steady State ModelSo far we knowDistant objects receding according to Hubbles LawStructure of universe described by General Relativity.Is the Big Bang the only Model that fits?

Fred Hoyle (1915-2001)The Steady State ModelThere was no big bangThe universe has always looked same as it does now. It is infinite but expandingMass is created as space expands

new mass17Our Evolving Universe18Does the Steady State model also solve Olbers Paradox?An alternative model? The Steady State Model

Yes Light is cooled by expansion Big Bang or Steady State?Any evidence for change over time?Any evidence for hot beginnings?3rd lecture - Vital statistics of the Universe 18