The Dawn of Time - II. A Cosmos is Born

! Why does Olbers’ paradox show the Universe began? ! How does Hubble’s Law tell us the age of the Universe? If

Hubble’s Constant is large, is the Universe old or young? ! What is the Big Bang? Where in the Universe did it occur?! What is the CMB? Why are the expansion of the Universe

(Hubble’s Law), and the CMB, convincing evidence that, at its birth, the Universe was very hot and very small?

! What is the temperature of the CMB? If the Big Bang was very hot, why is the CMB very cold? It may help to think about both cosmological redshift and Wien’s Law

! Is the CMB perfectly uniform in temperature? How do we see the seeds of large clusters of galaxies in the CMB?

Learning Objectives

Olbers’ Paradox! If the Universe extends

infinitely in space, as it might, then the accumulated light from an infinite number of distant stars and galaxies should seemingly cause the sky to be bright at all times, night and day

If space is infinite, every line of sight from Earth out into space should eventually terminate on the surface of a star or galaxy

The necessity of a beginning!The more distant galaxies are so far away that

light from them has not yet reached the Earth ! If you look out far enough, you look back to a

time that is greater than the age of the Universe!The night sky is dark because the Universe had

a beginning

Looking Back…! The Universe is expanding! So, in the past, the Universe

must have been much smaller and much denser

! Looking backwards in time, the entire Universe was crammed into a singularity (similar, mathematically, to the center of a black hole)

! There must have been a point at which the expansion of the Universe began

Hubble’s Law and The Age of the Universe

! Imagine watching a movie of the expansion of the Universe!Now, reverse the movie !Expansion becomes

contraction! If the Universe expanded

at a constant rate…! time = distance/velocity

! Recall, v=Hod (Hubble’s Law)…!Time = 1/Ho~1/70 km/s/Mpc ~ 14 billion years

The early Universe must have been hot and dense

!Earlier in time, the Universe must have been more compact

!As light from the Universe was stretched to become more red (so, had less energy) the Universe cooled

!The expanding Universe must have begun from conditions of extremely high density and temperature

!Astronomers call those conditions the Big Bang

Not An Explosion!The Big Bang occurred

everywhere at once!The early Universe was

filled with energy–washot & dense–everywhere

!The beginning of space-time, matter, and energy

!As space expanded, the Universe redshifted to became less dense and cooler (redder = cooler)

!Eventually the cooling gas formed the galaxies we see today

Major Evidence for the Big Bang!1) Expansion of the Universe

!There must have been a moment in time at which this expansion began

!2) Cosmic Microwave Background (CMB)!Predicted remnant radiation from when the

Universe was hot and dense long, long ago!3) Structure Formation

!Galaxies are distributed as expected based on structure in the CMB + 13.8 billion years of galaxies being collected together by gravity

! + others I won’t discuss, e.g., light element formation

Evidence for the Big Bang! Stars are hot and

dense, so If the early Universe was hot and dense, shouldn’t it, too, have emitted blackbody radiation?

! Could we detect the spectrum of that radiation?

! Yes! It’s redshifted to microwaves

! Called the Cosmic Microwave Background

! Detected by Robert Wilson and Arno Penzias

Removed “white dielectric

material”“Other Guy

shot pigeons”

CMB: A Very Uniform Blackbody! All over the sky, we see

blackbody radiation! Its temperature is 2.73 K

(about -270oC)! Compelling evidence for the

Big Bang Theory! Almost perfectly isotropic

! i.e. it’s almost exactly the same in every direction

! This indicates that, over large distances, the Universe is uniformly spread out

How to Understand Sky Maps

Left: Sphere of the sky wrapped around

in projection (showing our view towards the center of our Galaxy)

Right: Sphere of the Earth wrapped

around in projection

The Milky Way

! Cosmological variations are less than 1 part in 100,000 around the 2.73 K background temperature of the Universe

Small Scale Variations - Cosmological

! Originally “quantum fluctuations”! instabilities on scales

comparable to the size of single atoms

! These small differences in temperature represent tiny differences in density of stuff in the early Universe

! Over billions of years, slightly more dense regions collect more mass through gravity to become superclusters, clusters and galaxies

Small Scale Variations - Cosmological

Structure Formation! Regions of higher density became the seeds

of galaxies, clusters, and superclusters! Collapsed

under their own gravity

! Well-fed supermassive black holes at galaxy centers became quasars, etc.

Structure of the Universe! Clusters of

galaxies are grouped together in superclusters

! Superclusters occur in filaments and sheets

! Large “voids” of nothing between them

Review: Main 3 Reasons the Big Bang Must Have Happened

! The Hubble Law: v=H0d+ Einstein’s General Relativity= Expanding Universe with an

age of ~14 billion years! Cosmic Microwave Background

Remnant of the Big Bang when the Universe is ~500,000 years old. Tiny fluctuations become galaxies

! Structure FormationThe tiny density fluctuations,over ~14 billion years of time,grow to become large clustersand empty spaces

Next Time

The Fate of the Universe