Hubble’s Law

• Hubble was an American astronomer, born in Missouri.

• In the 1920’s Hubble discovered a relationship between recessional velocities of galaxies and the distances of those galaxies from Earth.

• (See next slide)

Edwin Hubble

Edwin Hubble at Mt. Wilson

Hubble guiding the Hooker100 inch telescope in 1923.

The Hooker 100 inch telescope atop Mt. Wilsonnear Pasadena, CA. It was the largest telescope in the world from 1917-1947.

Photos courtesy Mt. Wilson: http://www.mtwilson.edu/History

Hubble’s observations at the 100 inch during the1920’s led him to the conclusion that the universe is expanding, and that an object’s recession velocityis proportional to its distance from the observer.

Hubble’s Law• The following pictures all

magnified equally.• Virgo is closest and Hydra

is furthest from Earth.• Notice that the redshift is

greatest for Hydra, and therefore the recessional velocity is also greatest.

• Recessional velocity is proportional to distance (Hubble’s Law)

v d

Definition of Various Distance measuring tools

• Astronomical Unit (AU) = distance from Earth to the Sun (93,000,000 miles)

• Light year (ly) = distance light travels in 1 year. (3 x 108 m/s)(1year) = 9.46 x 1012km = 63,240 AU (only 6 trillion miles!)

• Parsec = 3.26 ly (19,560,000,000,000 miles!)

Hubble’s Law – Consider the universe to be expanding like a balloon.Let Galaxy A = Milky WayThen we are 100Mpc from both galaxies B and E at time t=0We are 200Mpc from galaxy C, etc.

Hubble’s Law – Consider the universe to be expanding like a balloon.A time t=1 later, what is our distance from galaxies B and E? From C? From D?

• The so what is that …– The separation of

galaxies is proportional to the original separation of the galaxies.

– Since for each equal time interval the separation will increase, then we can say…

Hubble’s Law – Consider the universe to be expanding like a balloon.

vd

tv d

bu t t is constant

therefore

Hubble’s Data (1929)

1000

500

0

0 1 2

Distance (Mpc)

Re

cess

ion

Ve

loci

ty

(km

/se

c)

Hubble & Humason (1931)

10 20 30

5000

10,000

15,000

20,000

Re

cess

ion

Ve

loci

ty

(km

/se

c)

Distance (Mpc)

1929 Data

Hubble’s Law – A closer look

This graph plots the recessional velocities of 20 galaxies

(PSRT)

v H d

th e s lo p e is th e H u b b le C o n stan t

v d

Hubble’s Law – A closer look

Hubble’s Constant is… Slope = 75Km/sec/Mpc

(PSRT)

v H d

th e s lo p e is th e H u b b le C o n stan t

v d

Limitations to Hubble’s Law

• A common misconception is that all galaxies have positive recessional velocities. (i.e. they are moving away from the Milky Way)

• Consider galaxy M-31 (Andromeda)• This galaxy actually has a blueshift

Limitations to Hubble’s Law

• The problem is that the relative velocities of neighboring galaxies is relatively small.

• Conversely, at larger distances, a number of methods can be used to determine the distance. – Parallax, absolute and apparent magnitude, Cepheid

variables. (stay tuned)

• The problem is that there is a vast disagreement what the Hubble Constant actually is.

• The range is 40 – 90 km/s/Mpc

Calculating the Age of the Universe• Consider a galaxy

– measure its distance d– measure recession velocity v

• Combining

• With

• We see that

v

dtage

Hdv

HHd

d

v

dtage

1

This is called the Hubble time.Its important to recognize it is the same for all of the galaxies

Calculating the Age of the Universe

km/s/Mpc 75H

Hv

dtage

1

• This time = the origin of the universe (galaxies must have been collected at the same location of space and time.

• The slope of this graph gives a Hubble Constant of…

Calculating the Age of the Universe

years!billion 13

aka

Gyr131

0

H

tage

km/s/Mpc 75H

Htage

1constant

• Substituting for H and converting to correct units, the age of the universe is…

(Note: 1Mpc = 3.09 x 1010 years1 year = 3.156 x 107 seconds) Gyr 20 and 10between is

universe that theaccepted isIt

universe the toagesdifferent

give H of valuesdifferent

aware beagain However,