The Expanding Universe
Discovery of Expansion
1929: Edwin Hubble measured the
distances to 25 galaxies:
• Compared distances and recession velocities
• Calculated recession velocity by assuming the
redshift of spectral lines is due to the Doppler
Effect
Discovered:
• Recession velocity gets larger with distance.
Systematic expansion of the Universe.
Redshifted Spectral Lines
Increasing Distance
Recession Velocity (km/sec)
Hubble’s Data (1929)
1000
500
0
0
1
Distance (Mpc)
2
Added more data :Hubble & Humason
(1931)
Recession Velocity (km/sec)
20,000
15,000
10,000
5000
1929 Data
10
20
Distance (Mpc)
30
Hubble’s Law
v = H0 x d
v = recession velocity in km/sec
d = distance in Mpc
H0 = expansion rate today (Hubble Parameter)
Measure Hubble Parameter by calculating slope of the
linear relationship
Best value: H0 = 22 ± 2 km/sec/Mly
where Mly = Mega lightyear=1 million ly
Interpretation
Hubble’s Law demonstrates that the Universe
is expanding in a systematic way:
• The more distant a galaxy is, the faster it
appears to be moving away from us.
• Hubble Parameter: Rate of expansion today.
Comments:
• Empirical result - based only on data
• Actual value of H0 is important. Allows us to get
a rough idea of the Age of the Universe (time
elapsed since the Big Bang)
Age of the Universe (Analogy)
You leave Columbus by car for Florida, but
leave your watch behind.
How long have you been on the road?
• Your speed = 100 km/h
• Your trip meter reads: distance = 300 km
Time since you left: T = distance  speed
• T = 300 km  100 km/h = 3.00 hours
The Hubble Time: T0
Hubble’s Law says
• A galaxy at distance d away has a recession
speed, v = H0d
So as in the analogy:
• T0 = d / v
• but since, v = H0d, T0 = d / H0d = 1 / H0
Hubble Time: T0 = 1 / H0
Estimate of the Age of the Universe
Best Estimate of the Age:
14.0  1.4 Gyr
This age is consistent with the ages of the
oldest stars seen in globular clusters.
• 1 Gyr = 1 Gigayear = 1 billion years
Common Misconception of Universe Expansion
Milky
Way
Common Misconception
Description:
• Galaxies are all moving away from each other
through space
• Explosion of the Big Bang sent them flying
• Big Bang sent all galaxies flying away from MW
because that is what we observe
Problems:
• Why is the Milky Way the Center of the Universe?
• Why is Hubble’s Law obeyed?
Should speed vs distance be linear?
Does the galaxy movement have to be uniform?
Space Itself is Expanding: Hubble Flow
Correct Explanation
Description:
• Galaxies typically have small (compared to Hubble flow),
gravitationally influenced motions in any direction in space.
(More on this later)
• SPACE ITSELF IS EXPANDING
Distance between galaxies is growing, they only appear to be
moving away
Solutions:
• Nothing special about the Milky Way. Every galaxy would
see the others receding from them (in the same manner)
• Hubble’s Law follows naturally.
Galaxy A is 1 Mly from MW : dA=1 Mly. Galaxy B has
dB=3 Mly
Expansion of universe doubles the scale of the
coordinate system
Now: A distance is 2 Mly B distance is 6 Mly
VA~ (2-1)=1 Mly = dA VB ~ (6-3)=3 Mly = dB V ~ d
Two Dimensional Analogy
Cosmological Redshift
Expansion of space stretches
light:
• Wavelengths get stretched into
redder (longer) wavelengths
• The greater the distance,
the greater the stretching
Result:
• The redshift of an object
gets larger with distance.
• Just what Hubble actually
measured
Two Dimensional Analogy
Time to be more precise
Most galaxies are found in groups & clusters
Galaxies are held in them by gravity
It is the distance between clusters of galaxies that
is getting bigger due to the expansion of the
universe
Within a cluster, galaxies can have other motions
due to the gravity produced by the total matter in the
cluster. Gravitational Force is stronger on these
“small” scales than the expansion.
For example, the Andromeda Galaxy and the Milky
Way are on a collision course!
Groups & Clusters of Galaxies
Basic Properties:
• Groups: 3 to 30 bright galaxies
• Clusters: 30 to 300+ bright galaxies
• Sizes: 1 - 10 Mpc across
• Extremely large objects in the universe
separated by extremely large distances
The Local Group
Group of 39 galaxies including the Milky Way
and Andromeda:
• Size: ~1 Mpc
• 5 bright galaxies (M31, MW, M33, LMC, IC10)
• 3 Spirals (MW, M31, & M33)
• 22 Ellipticals (4 small Es & 18 dEs)
• 14 Irregulars of various sizes (LMC, SMC
nearest neighbors)
Total Mass ~5x1012 Msun
The Local Group
1 Megaparsec (Mpc)
Virgo Cluster
Nearest sizable cluster to the Local Group
Relatively loose cluster, centered on two
bright Ellipticals: M87 & M84
Properties:
• Distance: ~18 Mpc
• Size: ~ 2 Mpc
• 2500 galaxies (mostly dwarfs)
• Mass: ~1014 Msun
Rich Clusters
Contain 1000’s of bright galaxies:
• Extend for 5-10 Mpc
• Masses up to ~1015 Msun
• One or more giant Elliptical Galaxies at center
• Ellipticals found near the center.
• Spirals found at the outskirts.
10-20% of their mass is in the form of a very
hot (107-8K) intracluster gas seen only at
X-ray wavelengths.
Rich Cluster
Abell 1689
(Hubble Space Telescope)