Chapter 17 Cosmology

Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 17Cosmology

Copyright © 2010 Pearson Education, Inc.

Chapter 17Cosmology

Ultra Deep Space Field HST


Units of Chapter 17The Universe on the Largest ScalesThe Expanding UniverseCosmic Dynamics and the Geometry of SpaceThe Fate of the CosmosThe Early UniverseThe Formation of Nuclei and AtomsCosmic InflationThe Formation of Large-Scale Structure in the Universe


a) the Milky Way must be located at the edge of the universe.

b) the Milky Way is at the center of the universe.

c) the universe is expanding.d) the sky is dark at night.e) the universe has not changed

significantly.

Question 1

Because distant galaxies in every direction are moving away from us,


This galaxy map shows the largest structure known in the universe, the Sloan Great Wall. No structure larger than 300 Mpc is seen.

The Universe on the Largest Scales


Therefore, the Universe is homogenous (any 300-Mpc-square block appears much like any other) on scales greater than about 300 Mpc.The universe also appears to be isotropic – the same in all directions.The cosmological principle includes the assumptions of isotropy and homogeneity.

The Universe on the Largest Scales


Olbers’s Paradox:If the universe is homogeneous, isotropic, infinite, and unchanging, the entire sky should be as bright as the surface of the Sun.

The Expanding Universe


So, why is it dark at night?The universe is homogeneous and isotropic – It must not be infinite and/or unchanging.

We have already found that galaxies are moving away from us faster the farther away they are:

recessional velocity = H0 distance



So, how long did it take the galaxies to get there?



Using H0 = 70 km/s/Mpc, we find that time is about 14 billion years.

Note that Hubble’s law is the same no matter who is making the measurements.


1 Mpc = 3.3 x 1019 km1/h0 = 3.1 x 1019 km / 70 km/s = 4.4 x 1017 s4.4 x 1017 s = 14 billion years


a) the size of the universe.b) the age of the universe.c) the shape of the universe.d) the temperature of the universe.e) the distance the universe has

expanded.

Question 2

Hubble’s constant, H0, can be related to


a) the size of the universe.b) the age of the universe.c) the shape of the universe.d) the temperature of the universe.e) the distance the universe has

expanded.

Question 2

Hubble’s constant, H0, can be related to

H0 is currently estimated to be about

70 km/sec/Mpc.This translates to an

age for the universe of about 14 billion years.


If this expansion is extrapolated backward in time, all galaxies are seen to originate from a single point in an event called the Big Bang.So, where was the Big Bang?It was everywhere!No matter where in the universe we are, we will measure the same relation between recessional velocity and distance, with the same Hubble constant.



This can be demonstrated in two dimensions. Imagine a balloon with coins stuck to it. As we blow up the balloon, the coins all move farther and farther apart. There is, on the surface of the balloon, no “center” of expansion.



The same analogy can be used to explain the cosmological redshift.



a) a Doppler shift of the random motions of galaxies.

b) an aging of light as the universe ages.c) space itself expanding with time,

stretching light.d) the result of the Milky Way’s position at

the center.e) due to the temperature differences in the

early and late universe.

Question 3

The redshift of galaxies is explained best as


a) a Doppler shift of the random motions of galaxies.

b) an aging of light as the universe ages.c) space itself expanding with time,

stretching light.d) the result of the Milky Way’s position at

the center.e) due to the temperature differences in the

early and late universe.

Question 3

The redshift of galaxies is explained best as

As the universe expands, photons

of radiation are stretched in

wavelength as they move

through space.


These concepts are hard to comprehend, and not at all intuitive. A full description requires the very high-level mathematics of general relativity.However, there are aspects that can be understood using relatively simple Newtonian physics – we just need the full theory to tell us which ones!



a) is infinitely old and getting larger.b) began expanding long ago, and has a

finite age.c) will slow down because of dark matter.d) has repeatedly expanded and

contracted.e) will eventually stop and recollapse.

Question 4

Hubble’s law implies that the universe


a) is infinitely old and getting larger.b) began expanding long ago, and has a

finite age.c) will slow down because of dark matter.d) has repeatedly expanded and

contracted.e) will eventually stop and recollapse.

Question 4

Hubble’s law implies that the universe

Using the Hubble constant H0,

astronomers can estimate that the

universe was born about 14 billion

years ago.


There are two possibilities for the universe in the far future:1. It could keep expanding forever.2. It could collapse.Assuming that the only relevant force is gravity, which way the universe goes depends on its density.

Cosmic Dynamics and the Geometry of Space


If the density is low, the universe will expand forever.

If it is high, the universe will ultimately collapse.



If space is homogenous, there are three possibilities for its overall structure:1. Closed – this is the geometry that leads to

ultimate collapse2. Flat – this corresponds to the critical density3. Open – expands forever



These three possibilities are illustrated here. The closed geometry is like the surface of a sphere; the flat one is flat; and the open geometry is like a saddle.



In a closed universe, you can travel in a straight line and end up back where you started.



a) the universe will eventually stop expanding and recollapse.

b) dark matter will dominate over dark energy.c) the universe will stop expanding and remain

stationary.d) dark energy will dominate over dark matter.e) the universe will not stop expanding.

Question 7

In a closed universe


a) the universe will eventually stop expanding and recollapse.

b) dark matter will dominate over dark energy.c) the universe will stop expanding and remain

stationary.d) dark energy will dominate over dark matter.e) the universe will not stop expanding.

Question 7

In a closed universe

Greater density means more matter in a smaller

volume, and gravity will be strong enough to stop the

expansion and cause a “Big Crunch.”


The answer to this question lies in the actual density of the universe.Measurements of luminous matter suggest that the actual density is only a few percent of the critical density.But – we know there must be large amounts of dark matter.

The Fate of the Cosmos


However, the best estimates for the amount of dark matter needed to bind galaxies in clusters, and to explain gravitational lensing, still only bring the observed density up to about 0.3 times the critical density, and it seems very unlikely that there could be enough dark matter to make the density critical.



Type I supernovae can be used to measure the behavior of distant galaxies. If the expansion of the universe is decelerating, as it would if gravity were the only force acting, the farthest galaxies had a more rapid recessional speed in the past, and will appear as though they were receding faster than Hubble’s law would predict.



However, when we look at the data, we see that it corresponds not to a decelerating universe, but to an accelerating one.



Possible explanation for the acceleration: Vacuum pressure (cosmological constant), also called dark energy.



The cosmic microwave background was discovered fortuitously in 1964, as two researchers tried to get rid of the last bit of “noise” in their radio antenna.

Instead they found that the “noise” came from all directions and at all times, and was always the same. They were detecting photons left over from the Big Bang.

The Early Universe

Robert Wilson & Arno Penzias


When these photons were created, it was only one second after the Big Bang, and they were very highly energetic. The expansion of the universe

has redshifted their wavelengths so that now they are in the radio spectrum, with a blackbody curve corresponding to about 3 K.

The Early Universe


Since then, the cosmic background spectrum has been measured with great accuracy.

The Early Universe


Cosmic Microwave Background

• COBE 1990s WMAP 2000s

Video


Question 5

The cosmic microwave background radiation is

a) evidence supporting the Big Bang.b) proof that the universe is getting warmer.c) a result of the hot intergalactic gas between

clusters.d) the observable form of dark energy.e) released from the first generation of stars in

the universe.


a) evidence supporting the Big Bang.b) proof that the universe is getting warmer.c) a result of the hot intergalactic gas between

clusters.d) the observable form of dark energy.e) released from the first generation of stars in

the universe.

Question 5

The cosmic microwave background radiation is

The radiation observed is the “fossil remnant” of the

primeval fireball that existed at the very

beginning of the universe.


The total energy of the universe consists of both radiation and matter.

As the universe cooled, it went from being radiation-dominated to being matter-dominated.Dark energy becomes more important as the universe expands.

The Early Universe


Question 9

In the first few minutes after the Big Bang

a) the universe cooled and formed neutral matter.

b) the cosmic microwave background radiation was released.

c) electrons recombined with protons.d) hydrogen fused into deuterium and

then helium.e) the universe was governed by one

unified super-force.


a) the universe cooled and formed neutral matter.

b) the cosmic microwave background radiation was released.

c) electrons recombined with protons.d) hydrogen fused into deuterium and

then helium.e) the universe was governed by one

unified super-force.

Question 9

In the first few minutes after the Big Bang

The production of elements heavier than hydrogen by nuclear fusion is “primordial nucleosynthesis.”

The amount of deuterium we see today is an important clue to the density of this early universe.


Hydrogen will be the first atomic nucleus to be formed, as it is just a proton and an electron.Beyond that, helium can form through fusion.

The Formation of Nuclei and Atoms


Most deuterium fused into helium as soon as it was formed, but some did not.Deuterium is not formed in stars, so any deuterium we see today must be primordial.



The time during which nuclei and electrons combined to form atoms is referred to as the decoupling epoch. This is when the cosmic background radiation originated.



The horizon problem: Cosmic background radiation appears the same in diametrically opposite directions from Earth, even though there hasn’t been enough time since the Big Bang for these regions to be in thermal contact.

Cosmic Inflation


The flatness problem: In order for the universe to have survived this long, its density in the early stages must have differed from the critical density by no more than 1 part in 1015.

Cosmic Inflation


Geometry

• What shape does the universe have?

Geometry


Between 10-35 s and 10-32 s after the Big Bang, some parts of the universe may have found

themselves in an extreme period of inflation, as shown on the graph. Between 10-35 s and 10-32 s, the size of this part of the universe expanded by a factor of 1050!

Cosmic Inflation


Inflation would solve both the horizon and the flatness problems. This diagram shows how the flatness problem is solved – after the inflation the need to be very close to the critical density is much more easily met.

Cosmic Inflation


a) there is more matter than energy.b) the universe is closed, and will recollapse.c) the universe is open, and will keep

expanding.d) dark matter will dominate, and galaxies will

stop expanding.e) there was more helium than hydrogen

created in the Big Bang.

Question 8

If the density of the universe is greater than “critical”,


Cosmologists realized that galaxies could not have formed just from instabilities in normal matter:• Before decoupling, background radiation

kept clumps from forming.• Variations in the density of matter before

decoupling would have led to variations in the cosmic microwave background.

The Formation of Large-Scale Structure in the Universe

Ripples


• Because of the overall expansion of the universe, any clumps formed by normal matter could only have had 50–100 times the density of their surroundings.

Dark matter, being unaffected by radiation, would have started clumping long before decoupling.



Galaxies could then form around the dark-matter clumps, resulting in the universe we see.



This figure is the result of simulations of a cold dark matter universe with critical density.



Although dark matter does not interact directly with radiation, it will interact through the gravitational force, leading to tiny “ripples” in the cosmic background radiation.

These ripples have now been observed.



This is a much higher-precision map of the cosmic background radiation.



a) the Steady State Theory.b) the Grand Unified Theories.c) the Inflationary epoch.d) dark matter.e) decoupling of matter from radiation.

Question 10

The universe appears flat; this is explained by


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Final Exam Study Guide

Density is defined as…

A. Weight per square inchB. Mass times weightC. Size divided by weightD. Weight divided by a planet’s radiusE. Mass per unit volume


E. Mass per unit volume

kg / m3


Comets

The tail of a comet always points…

A. toward EarthB. toward the SunC. In the direction of the comet’s motionD. Away from the sunE. Toward the galactic center


D. Away from the sun


Techniques

When we detect Doppler shifting of a star we can assume that…

A. The star is about to go supernovaB. The star is a class OBC. There is intelligent life near byD. There might be an orbiting planet


D. There might be an orbiting planet


Water

Water in the form of permafrost has been found on which planet?A. EuropaB. VenusC. JupiterD. MercuryE. Mars


E. Mars


Saturn

Saturn is the only known planet that has rings.

A. TrueB. False


B. False

Rings of Uranus


Jupiter

The light and dark bands of Jupiter are caused by…A. Zones of rising and sinking gasB. Dirty sooty ringsC. Huge magnetic fieldsD. Differential rotationE. Ice crystals


A. Zones of rising and sinking gas


Venus

What greenhouse gas does Earth and Venus have in common?A. OxygenB. ArgonC. HydrogenD. Carbon dioxideE. Nitrogen


D. Carbon dioxide


Bright!

What is the third brightest object in the sky?A. JupiterB. MoonC. SiriusD. VenusE. Mars


D. Venus


Moon

It takes a full year for us to see the entire surface of the moon.A. No, only one monthB. No, only one dayC. No, we see only one half of its surfaceD. Yes, but it is a sidereal year E. Yes, but it is a tropical year


C. No, we see only one half of its surface


Moon

The reason we see only one side of the moon is because of something called…A. Tidal lockB. Lunar gravityC. Solar equalizationD. Earth syncE. Kepler’s Law


A. Tidal lock


Kepler’ Laws

Kepler’s Laws allow us to determine what if we know the period and distance of one object orbiting another?A. Length of yearB. Distance to that objectC. Composition of the objectD. Total mass of both objects


D. Total mass of both objects

Chapter 17 Cosmology

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