Galaxies and Cosmology

Attendance Quiz

Are you here today?

(a) yes

(b) no

(c) Cosmetology? Like hair and nails and makeup?

Here!

Next Tuesday, 5/30:Dr. Jorge Moreno is unavailable,

so class will be cancelled

Final Exam

• The final exam is Thursday, 6/8, from 11:30am to 1:30pm (2 hours), in this room; please arrive early!

• The final exam will be comprehensive, i.e., it will cover all the material you have studied this quarter

• It will be multiple choice, so make sure to bring a 100-question (2-sided) scantron to class!

Today’s Topics

• Large Scale Structure in the Universe• The Distance Ladder and Galactic Distances• Hubble’s Law

• The Distances to the Galaxies• Redshifts and Hubble’s Law• Expansion of the Universe• The Age of the Universe

Local Group

2.5  Million  ly

The Andromeda Galaxy and the Milky Way are approaching each other at 119 km/s, and will collide and merge into a giant elliptical galaxy in approximately 5 billion years

Local Supercluster

52  Million  ly

Abell  1060

100s-­1000s  of  galaxies

Abell  1689

Galaxy Clusters

Virgo  cluster

Giant  elliptical  galaxies10s  of  Millions  of  ly

Large Scale Structure in the Universe

1  billion  light  years

The Distance Ladder• Radar ranging (few AU)• Parallax (AU → few 1000 light years)• Main sequence fitting (100s → 10s of thousands of light years)• Cepheid variable stars (100s of thousands → 100s of millions of light years)• Distant standards (Tully-Fisher, White Dwarf supernovae; 10s of millions to 10+

billion light years)

Standard Candles

• With the exception of radar and parallax, all these methods rely on the concept of a standard candle

• Recall, the apparent brightness of an object is related to its intrinsic brightness (luminosity) and distance by

• Thus, if we know an object’s luminosity, and measure its apparentbrightness, we can find its distance

Interactive Figure

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Apparent brightness = Luminosity4π × (distance)2

Standard Candles - Examples • Main sequence stars (MS stars as SCs)

• Spectral type ⇒ Luminosity• Calibrated for stars near enough to

measure parallax• Many stars at same distance is better

(clusters) ⇒ Main Sequence Fitting• Calibrated by cluster near enough to

show parallax - Hyades• Cepheid variable stars (Cepheids as SCs)

• Very bright stars which vary in luminosity in a regular way

• A relation exists between variation period and the star’s luminosity

• Calibrated using nearby Cepheids of known distance

• White Dwarf (Type I) supernovae (SCs)• All WD Sne cross the Chandrasekhar

limit and explode in the same way; hence all have similar luminosities

• Calibrated in nearby galaxies of known distance

Galactic Distances QuizCepheid variable stars are located in two different galaxies, A and B. Both stars have the same average apparent brightness. The star in galaxy A has a bright-dim-bright period of 10 days, while the one in galaxy B has a bright-dim-bright period of 30 days. Which of the two galaxies is at a greater distance from us?

a) Galaxy Ab) Galaxy Bc) They are located at the

same distance.d) There is insufficient

information to tell.

Edwin Hubble and Galactic Distances• Before Hubble, the nature of so-called

“spiral nebulae” was unknown• Other galaxies (“island universes”)• Gas clouds in our galaxy

• The key missing piece of evidence was the distances to these objects

• Hubble, using the Mt. Wilson 100” telescope in Pasadena (then the largest in the world) observed Cepheid variable stars in other galaxies

• Using the period-luminosity relationship, he found that these objects were muchfurther away than the size of the Milky Way, meaning they are independent galaxies

Redshifts and Hubble’s Law• It was known since the 1910s that the spectra of most spiral galaxies were

redshifted, meaning they were moving away from us• Once Hubble found the distances to these galaxies he found something startling!• Rather than the motions of the galaxies being random, there is a relationship

between redshift and the distance to the galaxy• The more distant the galaxy, the faster it recedes (Hubble’s Law)

Expansion of the Universe• If all the galaxies are moving apart, then

earlier, they must have been closer together, i.e., the universe is expanding!

• Since all the galaxies are moving away fromthe Milky Way, does this mean that we are at the center of the universe?

• Our knowledge of the history of astronomy (Copernicus, Shapley) should make us skeptical of this idea

• How else can we explain these facts?• If the entire universe is expanding then every

galaxy sees all others receding, and if that expansion is uniform, then the rate of motion apart is proportional to distance (Hubble’s Law)

• Note: because gravity holds them together, individual galaxies do not expand as the space they occupy expands

Space Expansion + Int. Fig. 20.22

Cosmology Quiz I

Consider three widely separated galaxies in an expanding universe. Imagine that you are located in galaxy 1 and observe that both galaxies 2 and 3 are moving away from you with a speed proportional to their distance from you. If you asked an observer in galaxy 3 to describe how galaxy 2 appears to move, what would he or she say?

a) “Galaxy 2 is not moving”b) “Galaxy 2 is moving toward galaxy 3”c) “Galaxy 2 is moving away from galaxy 3”

Lecture Tutorial: Expansion of the Universe, pp. 161-162

• Work with one or more partners - not alone!• Get right to work - you have 15 minutes• Read the instructions and questions carefully.• Discuss the concepts and your answers with one another.

Take time to understand it now!!!!• Come to a consensus answer you all agree on.• Write clear explanations for your answers.• If you get stuck or are not sure of your answer, ask another

group.• If you get really stuck or don’t understand what the Lecture

Tutorial is asking, ask me for help.

Cosmology Quiz I

Consider three widely separated galaxies in an expanding universe. Imagine that you are located in galaxy 1 and observe that both galaxies 2 and 3 are moving away from you with a speed proportional to their distance from you. If you asked an observer in galaxy 3 to describe how galaxy 2 appears to move, what would he or she say?

a) “Galaxy 2 is not moving”b) “Galaxy 2 is moving toward galaxy 3”c) “Galaxy 2 is moving away from galaxy 3”

Cosmology Quiz II

According to modern ideas and observations, what can be said about the location of the center of our expanding universe?

a) The Earth is at the centerb) The Sun is at the centerc) The Milky Way Galaxy is at the centerd) The universe does not have a center

Hubble’s Law and the Age of the Universe• Hubble’s Law tells us that all galaxies

are moving apart at a rate proportional to their separation distance

• The slope of this relationship is called Hubble’s Constant, or H0

• If that rate stayed the same throughout the history of the universe, we could run a “movie” of the universe backwards to find out when all the galaxies were on top of each other

• The faster the galaxies are receding now (the larger H0), the shorter the time until the “movie” reaches its beginning

• Thus, the age of the universe is proportional to 1/H0

• This simple calculation gives an age of the universe of 13.6 billion years

Interactive Figure 1: Estimating Age of the Universe

Interactive Figure 2: Dependence of Age on H0

Cosmology Quiz III

Astronomers currently believe that the Hubble constant has a value of about 22 km/s/Mly. If some new measurement revealed that instead the Hubble constant is closer to 60 km/s/Mly, what would this imply about the age of the universe?

a) It is much younger than current estimatesb) It is much older than current estimatesc) This change would have no effect on estimates of the age of the

universe