hubble’s law
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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,