big bang, black holes, no math astr/phys 109 dr. david toback lectures 14 & 15

Evidence for the Big BangTopic 1: The Exploding Universe

Big Bang, Black Holes, No Math

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ASTR/PHYS 109Dr. David Toback

Lectures 15



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Was due Today – L15• Reading:

– (Unit 3)• Pre-Lecture Reading Questions:

– Unit 3, Stage 2• End-of-Chapter Quizzes:

– Chapter 9• Papers:

– Paper 1 Revision, Stage 1• Due Wed after Spring Break, but Stage 2 will be

Mon March 30– Paper 2, Stage 1:

• Due before class on Wednesday• (Text needs to be in turnitin on eCampus)

• Honors Paper:– Stage 1 due before Midnight tonight



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Starting Unit 3Finished Unit 2: Physics We Need

1.Light and Doppler Shifts 2.Gravity, General Relativity and

Dark Matter3.Atomic Physics and Quantum

Mechanics4.Nuclear Physics and Chemistry 5.Temperature and Thermal

EquilibriumStarting Unit 3: Evidence for Big

Bang1.The Exploding Universe2.Expanding Space-Time3.Photons and Hydrogen in the

Universe



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Where we are going…

•First give three pieces of evidence for the Big Bang–Don’t worry… you don’t need to memorize them now

–Will keep coming back to them over and over again

–Will be in the lecture notes•Then tell the story about how

they all fit together•Will be the topic of Paper 3



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Overview: Evidence for the Big Bang

1. We observe all distant galaxies to be moving away from us–The further away the galaxy is the faster it is moving away from us

–True no matter which direction we look

2. We observe low energy photons (microwaves) uniformly distributed in all directions which are consistent with a temperature of about 2.7 degrees above absolute zero (Kelvin)

3. The atoms in the Universe are basically Hydrogen and Helium and not much else



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3 Lecture Outline

1.The Exploding Universe•Today

2.Expanding Space-Time•Next Lecture

3.Photons and Hydrogen in the Universe•The Lecture After That



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Today’s Outline

•Looking at Galaxies•Light from distant galaxies is Red Shifted and appears the same in all directions

•The Exploding Universe•A problem…



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Galaxies before 1920’s

•Before the 20’s, when the telescopes became powerful enough, there was no convincing evidence that there was anything outside the Milky Way

•When Einstein first wrote about the Universe, he meant “The Milky Way”



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Edwin Hubble• In the 1920’s Hubble

established that there are distant galaxies made of individual stars– VERY far away

• Now know that in the same way that Sun is just one star of many, the Milky Way is just one of about a 100 billion galaxies



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What can we Learn from Galaxies?

Measure some important things:

•How far away are they?

•How fast are they moving?

•What directions are they moving?



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How do you measure the distance to a galaxy?

Use a special Star called a “Cepheid Variable”

•We know how much light they emit–Like a 100 Watt Light bulb

•If we measure how much light we see, we can figure out how far away it is

•If we can find a Cepheid in a galaxy, then we can figure out how far away that galaxy is



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What is Their Velocity?

•How fast are they moving?

•What direction are they moving?

Use the Doppler shift and measure their spectral lines!



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Observing a Galaxy

Galaxy Emitting LightNot MovingMoving away from us



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Put it all together

Measure distance from how much light reaches us

Measure speed from Doppler shift of Spectral lines



Distances

•Andromeda (the closest galaxy to our own) is ~2 million light years away! –More than 10 times farther away than the furthest thing in our own galaxy

•We see galaxies over 10 billion light years away

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Speeds

A fast star can travel a few hundred kilometers per second!

Put this in perspective Big: Can travel 10 billion kilometers in a year

Small: That’s less than a 1000th of the distance to the star nearest us



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Is there a trend?•Most of the galaxies are red shifted –Moving away from us

•More importantly: The further away they are the faster they are rushing away from us!

•Perhaps even more important: Looks the same in all directions!

IndividualGalaxies

Call this simple

relationship “Hubble’s

Law”



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Put this in Perspective•They are traveling away from us really quickly–From New York to California in ~1 second

•Can’t see this effect in our galaxy–Furthest stars in our galaxy are ~105

light years away–Thousand times smaller size



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So What?

•We live in a Universe where things are clearly moving!

•Galaxies are moving away from us very quickly

•Is something pushing them?



Why are Things Flying Apart?

•The answer turns out to be complicated

•We’ll start with a hypothesis that turns out to be wrong but helps us understand

•Then we’ll go back and explain it more correctly20



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An Explosion?

•Georges Lemaitre (1927): The galaxies are not flying apart because something is “forcing” them apart, they are moving apart because they were part of an “explosion” a long time ago



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Is the data consistent with

the simplest picture of an

exploding Universe?



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A Simple Explosion



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A Snapshot in TimeFast Stuff is far from

the explosion

Slow Stuff is close to

the explosi

on



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A (wrong) Hypothesis…

Fast Stuff is far from

the location of

the explosion

Slow Stuff is close to

the location of the explosi

on

Consistent with this

data



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A Beginning?

If galaxies are rushing apart from us then at some point in the past they must have all have been closer together



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Running Backward in Time



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Bottom Line

The data about the velocities of the galaxies

provides evidence that we live in a Universe that

exploded into existence a long time ago

We now know it was about 14 billion years ago



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The Story is Always more Complicated

Let’s think about our data a little more carefully

Say it again: Galaxies are moving away from us AND moving the same way in EVERY direction

Simple explanation? We’re located at the center of the explosion and we’re not moving!



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Consistent with what we observe!

We’re at the CENTER and NOT MOVING: Red Shifted Light



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If We’re at the Center

Equal amounts of stuff on each side

Consistent with what we observe!

Fast Stuff

Slow Stuff

Medium

Speed



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Not what we observe!

Another possibility: We’re at the center and

MOVING



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If We’re Not At The Center

Lots of stuff on this sideNot much stuff on this side

Not what we observe!

Fast Stuff

Slow Stuff

Medium

Speed



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???

•Do we REALLY believe we’re the center of the Universe AND that we’re not moving?

• Is there a better explanation?



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For Next Time– L15• Reading:

– (BBBHNM Unit 3)• Pre-Lecture Reading Questions:

– Let us know if you were misgraded• End-of-Chapter Quizzes:

– If we finished Chapter 10 then end-of-chapter quiz 10 (else just up to Chapter 9)

• Papers: – Paper 1 Revision, Stage 1

• Due Wed after Spring Break, but Stage 2 will due Monday, March 30

– Paper 2, Stage 2:• Due Wed March 25, before class

• Honors Paper:– Stage 1 due tonight before midnight



Full set of Readings So Far•Required:

–BBBHNM: Chaps. 1-12•Recommended:

–TFTM: Chaps. 1-3–BHOT: Chaps. 1-7, 9 and 11 (117-122)

–SHU: Chaps. 1-3, 4(77-86), 5(95-104), 6, 7 (up-to-page 153)

–TOE: Chaps. 1 & 236



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Clicker Question

Which best describes the two galaxies below? a)Both are moving towards us, but the left one is moving faster than the one on the rightb)Both are moving towards us, but the right one is moving faster than the one on the leftc)Both are moving away from us, but the left one is moving faster than the one on the rightd)Both are moving away from us, but the right one is moving faster than the one on the left



Clicker Question

Two stars in the Milky Way have the same temperature. Are they in thermal equilibrium with each other?

a)Yesb)No

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Clicker Question

True or False: Once a bunch of atoms are in thermal equilibrium, the energy of an individual atom doesn’t change

a)Trueb)False

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Clicker Question

True or False: If I have a bunch of electrons, positrons and photons in thermal equilibrium, we will never have another positron annihilate with an electron and raise the number of photons

a)Trueb)False

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Clicker Question

Why can electrons, positrons, and photons be in thermal equilibrium with each other, even if they are creating and annihilating one another?

a)They can be the same temperature

b)They are in thermal contactc)Can only become in thermal

equilibrium when interactions cease

d)A and B

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Clicker Question

On a cold winter morning, which is a lower temperature, a metal sign pole or a wooden bench?

a)The metal poleb)The wooden benchc)They are the same temperatured)They are at absolute zero

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Clicker Question

Let’s say you were the person observing the galaxies, but each other person in the room were a galaxy. Which person in the room is moving away from you most quickly?

a)Back leftb)Back rightc)Front leftd)Front right 44



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Prep For Next Time – L7• Reading:

–BBBHNM: All reading through Chapter 12• Reading Questions:

–All reading questions through 12, due before class

• eLearning Quizzes:–Through Chapter 9, due before class

• Papers:–Paper 2 text feedback (if desired) due Monday March 11th for guaranteed feedback.

–Due Tuesday March 19th in CPR and turnitin.



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For Next Time– L14• Reading:

– (BBBHNM Unit 3)• Pre-Lecture Reading Questions:

– Let us know if you were misgraded– Unit 3, Stage 2

• End-of-Chapter Quizzes:– If we finished Chapter 10 then end-of-chapter quiz 10

(else just up to Chapter 9)• Papers:

– Paper 1 Revision (if desired), Stage 1 due Monday before class• (Text needs to be in turnitin on eCampus)

– Paper 2, Stage 1 due before class Monday• (Text needs to be in turnitin on eCampus)

• Honors Paper:– (Stage 0)– Stage 1 due Mon Oct 20th



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Was due Today– L16• Reading:

– Chapter 10 • Pre-Lecture Reading Questions:

– Chapter 10• End of Chapter Quizzes:

– Chapter 9 • Papers:

– Paper 0 Revision (if you didn’t pass the first time – See eCampus score, not CPR Score): Calibrations/Reviews due Monday before class

– Paper 1: Scores are on CPR (won’t go on eCampus). If you want to do a revision, that will be due next Wednesday before class.

– Paper 2: Text due today before class, Calibrations/Reviews due next Wednesday before class

– Honors Paper: Stage 0 due Today



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Prep For Next Time – L16• Note: May change depending on how far we get in

lecture• Reading:

– BBBHNM: Chapters 11 and 12• Pre-Lecture Reading Questions:

– Chapter 11 and 12• End of Chapter Quizzes:

– Chapter 10 if we finished 10 today, if not just 9 (already assigned)

• Papers:– Paper 0 Revision (if you didn’t pass the first time –

See eCampus score, not CPR Score): Calibrations/Reviews due Monday before class

– Paper 1: Scores are on CPR (won’t go on eCampus). If you want to do a revision, that will be due next Wednesday before class.

– Paper 2: Text due today before class, Calibrations/Reviews due next Wednesday before class

– Honors Paper: Stage 0 due Today



49

Prep For Next Time – L7• Note: May change depending on how far we

get in lecture• Reading:

–BBBHNM: All reading through Chapter 12• Reading Questions:

–All reading questions through 12• eLearning Quizzes:

–If we finished Chapter 10 then end-of-chapter quiz 10 (else just through Chapter 9)

• Papers:–Paper 2 text feedback (if desired) due Monday March 11th for guaranteed feedback.

–Due Tuesday March 19th in CPR and turnitin.



50

Was due Today – L8•Reading:

–BBBHNM: All reading through Chapter 12

•Reading Questions:–All reading questions through 12

•eLearning Quizzes:–Chapter 9

•Paper 2:–Due Tuesday March 19th (tomorrow) in CPR and turnitin



Tentative Due Dates – L8• Monday March 18th

– Paper 3 Assigned– Reading for Chap 11 and 12, EOC quizzes for 9 due

• Tuesday March 19th – Paper 2 Text due in CPR and eLearning (1-day

extension)• Friday March 22nd

• Want feedback on a Paper 2 draft? Due today in eLearning

• Monday March 25th

– Paper 2 Reviews due in CPR• Wednesday March 27th

– Paper 3 text due in CPR and eLearning (2-day extension)

• Monday April 1st

– Paper 3 Reviews due in CPR51



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Prep for Next Time – P13•Reading:

– BBBHNM 11•Reading questions:

– Two questions on BBBHNM 11•End-of-Chapter Quizzes:

– If we finished Chapter 10, end-of-chapter 10 (else just 9)

•Paper 2:– Text due Tuesday at 5:45PM– Calibrations/Reviews due Monday

March 19



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Prep for Today (Is due now) – L14

•Reading: – BBBHNM 11

•Reading questions: – Chapter 11 or the recommended

reading•End-of-Chapter Quizzes:

– Chapter 9•Paper 2:

– Text was due Yesterday at 5:45PM– Calibrations/Reviews due Monday

March 19



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Prep for Next Time – P14•Reading:

– BBBHNM 11•Reading questions:

– Two questions on BBBHNM 11•End-of-Chapter Quizzes:

– If we finished Chapter 10, end-of-chapter 10 (else just 9)

•Paper 2:– Text was due Yesterday at

5:45PM– Calibrations/Reviews due Monday

March 19



Prep for Next Time – P14

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• Reading– BBBHNM 11

• Reading questions– Chapter 11

• End-of-Chapter Quizzes– If we finished Chapter 10 then

end-of-chapter quiz 10 (else just 9)

• Paper Stuff:– Paper 1 Revision: CPR due

Friday 6PM– Paper 2 Submission: Wed 4PM

in CPR



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Galaxies Not Moving?• What if the galaxies were not

moving in any way? What would happen?

• But, everything attracted by gravity!– All objects will eventually be

drawn to each other– A stationary rock will just fall to

the surface of the Earth• This would require something to

hold them in place



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Thermal Equilibrium?

•What would we expect if our universe were in thermal equilibrium? –If we (as observers) are stationary: Half the stars will be moving away from us, half will be moving towards us, all roughly with the same speed

–If we’re moving: Faster (higher temperature) on one side than the other



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Rotating?

Maybe the galaxies are rotating around some center of the Universe?

•Like the planets around the sun?

•Like the stars around the center of our galaxy?

•Is there something at the center that’s drawing them in?



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Other options?

Einstein’s Gravity (General Relativity) says that space-time can be getting smaller, but it can also be expanding–Things can be moving further apart, like a rock after it’s been throw up in the air

–Observe either?



Enough guessing…

what does the data say?

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So What?• Looks like Thermal Equilibrium in

that it appears to be the same in all directions• Not faster on one side than the

other• Doesn’t look like thermal

equilibrium in that we don’t observe half the stars moving away from us, half moving towards us!

• We live in a Universe where things are clearly moving!

• Is something pushing them apart?



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What Else Can I Learn?

• If the expansion rate is 15 kilometers per second per million light years of separation, then dividing these we get that everything was at the same place about 20 billion years ago– The age of the Universe!

• Not quite right… things must have been slowing down due to gravity (the galaxies attract each other and slow each other down). Take into account how long the light has been traveling.– Better estimate: The age of the

universe is ~13.5 billion years



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What Else Can I Learn?

•Take into account the expansion rate and how much its being slowed down by gravity get that everything was at the same place about 14 billion years ago–The age of the Universe!–Current best estimate is that the age of the universe is ~13.7 billion years



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Corroborating Evidence?

•Carbon dating and other studies suggest that the Earth is about 5 billion years old

•Similar studies show that the Milky Way is about 13 billion years old–The story hangs together



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The Opposition• In 1915, when Einstein came up with

General Relativity, we only knew about our own galaxy and that ISN’T big enough for these velocity effects to be noticeable– People thought the Universe was

essentially not moving– Discovery of the rotation of the Milky

Way was discovered in the 20s also• SOMETHING must be keeping it that

way Cosmological constant (anti-gravity force) was built into the fabric of space-time

• More on this when we come to Dark Energy



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For Next Time• Finished: The Exploding Universe• Next Time: Expanding Space-Time• After That: Cosmic Background RadiationReading for next time– BBBHNM: Chapter 11Full reading so far:• BBBHNM: Chaps. 1-11• TFTM: Chaps. 1-3• BHOT: Chaps. 1-7, 9 and 11 (117-122)• SHU: Chaps. 1-3, 4(77-86), 5(95-104), 6, 7 (up-

to-page 153) • TOE: Chaps. 1 & 2Lecture prep: Turn in two questions on Chapter

11 on eLearning before class



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Another Random Fact

In 1951 the Catholic church officially pronounced the idea of the Big Bang model to be in accordance with the Bible

Liked the idea of an experimental verification of a moment of creation



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History

•Belgian Jesuit priest (1927) working at the Vatican Observatory (Georges Lemaitre)

•History: The Vatican was interested in finding cosmological models consistent with the notion of creation in the bible: A beginning!



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The Reading For Unit 3Reading for the start of Unit 3• BBBHNM: Chapter 10• TFTM: 1-3• BHOT: 7• SHU: Rest of chapter 3, 4(77-86), 5(95-104)• TOE: 2 Full reading so far:• BBBHNM: Chaps. 1-10• TFTM: Chaps. 1-3• Prelude• BHOT: Chaps. 1-7, 9 and 11 (117-122)• SHU: Chaps. 1-3, 4(77-86), 5(95-104), 6, 7 (up-

to-page 153) • TOE: Chaps. 1 & 2



Paper 1 Stuff• Done with grades

– Graded unfairly? Let us know!• Passing score:

– Text score: more than 21 points (70%)– Calibrations: more than 15 points– Reviews: more than 15 points– Overall score: more than 70 points

• Revision due date was today at noon– Updated assignment requirements and

rubric– Do full CPR grading– Grading right after that Due one week

later 70



Paper 2 and Other Stuff

• Now open for submissions• Extended due date:

– Friday at noon• CPR grading right after that

due one week later

• Unit 2 eLearning was due today, before class

• Need to start Unit 371



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Get Velocity from Spectral Lines

• Expect Spectral lines at certain Wavelengths

• How much do they “shift” for each Galaxy?



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The Doppler Effect and Light

More Blue

More Red



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Writing Assignments1. Short Assignment 1 is due before the

beginning of Class Thurs Oct 15th • Any revisions will be due 1 week

after it is returned2. Short Assignment 2 is posted

• Currently Due Thursday Oct 27th which is one week after we finish Chapter 12

3. Research Paper is posted• Stage 0 due Thurs Oct 29th• Stage 1 due Thurs Nov 5th • Final paper due the last day of class,

Tuesday Dec 8th Unit 2 eLearning stuff now dueNeed to start Unit 3 eLearning stuff



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History, Part 1

•Late 1700’s: Lots of visible stars out there, mostly in specific set of directions–The Milky Way

•1750 Thomas Wright: Found more stars were seen clustered together when looking in one direction than in others

•First ideas of clustering of stars…



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The Milky Way

•Could be explained if most of the visible stars were arranged in a single disk-like configuration–Now: Can see about 5000 stars with the naked eye (~0.0001% of the total number)



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The 1920’s

•Started looking with more powerful telescopes

•“Hazy stuff” in the heavens–Dust clouds or “nebulae”

•Are these more clusters of stars?



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Problem

•The visible stars are close to us (in a cosmic sense)!–Inferring what the universe is like from them would be asking what are people around the United States is like by just looking at the people in College Station



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More on the Milky Way

• The Milky Way is a spiral galaxy about one hundred thousand light years across and slowly rotating

• The stars in the spiral arms orbit the center about every several hundred million years



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The Necessity of a Big Bang

If galaxies are moving away from each other with a speed proportional to distance, there must have been a beginning, when everything was concentrated in one single point:

The Big Bang!

?



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Olbers’s Paradox

•Why is the sky dark at night?

•If the universe is infinite, then every line of sight should end on the surface of a star at some point

•The night sky should be as bright as the surface of stars!



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Solution to Olbers’s Paradox

•If the universe had a beginning, then we can only see light from galaxies that have had time to travel to us since the beginning of the universe

•The visible universe is finite!



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The Reading For Unit 3



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•Some text



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Expanding Universe

•Goes here on in next section on GR?

•Clearly there is an expansion–Lecture 1: Big Bang–GR: Space is expanding rather than we are expanding into an already expanding space



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•But everything expands away from everything else as well. Hmmm… Experimental evidence for this? How do we know the difference between us being at the center of the explosion and being in an expanding space?



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•TFTM: 1-3•BHOT: 7•SHU: 3 (69-77)-XX•TOE: Does TOE 2 go here?

•Entered in: TFTM 1 - 3, BHOT 7, part of SHU 3.



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If we not at the Center

•Different amounts of stuff on either side of us…

Stuff that’s close Stuff that’s far away



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•Keep them here for now. Cosmic Background Radiation, Red Shift, Isotope Abundance as Evidence of age, (nucleosythesis, WMAP experiment, particle antiparticle asymmetry)



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TFTM 1 cont…

• Basic particles…• Electrons• Positrons• Photons• Neutrinos• Proton• Neutrons• Hydrogen• Helium



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This chapter marks a watershed in our study of astronomy. Since Chapter 1, our discussion has focused on learning to understand the universe. Our outward journey has discussed the appearance of the night sky, the birth and death of stars, and the interactions of the galaxies. Now we reach the limit of our journey in space and time, the origin and evolution of the universe as a whole.

The ideas in this chapter are the biggest and the most difficult in all of science. Our imaginations can hardly grasp such ideas as the edge of the universe and the first instant of time. Perhaps it is fitting that the biggest questions are the most challenging.

But this chapter is not an end to our story. Once we complete it, we will have a grasp of the nature of the universe, and we will be ready to focus on our place in that universe—the subject of the rest of this book.

Guidepost



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Hubble’s Law

Distant galaxies are receding from us with a speed proportional to distance



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• I. Introduction to the Universe• A. The Edge-Center Problem• B. The Necessity of a

Beginning• C. Cosmic Expansion• D. The Necessity of a Big Bang• E. The Cosmic Background

Radiation• F. The Story of the Big Bang



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• II. The Shape of Space and Time• A. Looking at the Universe• B. The Shape of the Expanding

Universe• C. Model Universes• D. Dark Matter in Cosmology•Does this all go here? Some in

GR section…



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TFTM 2 Cont…

•The history of why we believe in galaxies… First observed nebulae ()… lots of them. With better telescopes we could begin to pick out individual stars within them. Kant in 1755 said “gee, maybe these are galaxies like our own Milky Way?” (paraphrase…)



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•Cephids BHOT page 51



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The Age of the Universe

Knowing the current rate of expansion of the universe, we can estimate the time it took for

galaxies to move as far apart as they are today:

T ≈ d/v = 1/H ~ 14 billion years

Time = distance / velocity

velocity = (Hubble constant) * distance

The most distant galaxies known are seen as they were only about 1 billion years after the Big Bang.



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•Yet more text…

big bang, black holes, no math astr/phys 109 dr. david toback lectures 14 & 15

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