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The Expanding Universe
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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.
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Redshifted Spectral Lines
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Increasing D
istance
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Hubble’s Data (1929)
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0 1 2Distance (Mpc)
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Added more data :Hubble & Humason (1931)
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1929 Data
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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
Hubble’s Law
v = H0 x d
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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)
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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
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The Hubble Time: T0
Hubble’s Law says• A galaxy at distance d away has a recession
speed, v = H0d
So as in the analogy:• T0 = d / v
• but since, v = H0d, T0 = d / H0d = 1 / H0
Hubble Time: T0 = 1 / H0
Estimate of the Age of the Universe
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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
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Common Misconception of Universe Expansion
Milky Way
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Common MisconceptionDescription:
• 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?
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Space Itself is Expanding: Hubble Flow
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Correct ExplanationDescription:
• 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 MlyVA~ (2-1)=1 Mly = dA VB ~ (6-3)=3 Mly = dB V
~ d
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Two Dimensional Analogy
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Cosmological Redshift
Expansion of space stretches light:
• Wavelengths get stretched intoredder (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
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Two Dimensional Analogy
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Time to be more preciseMost galaxies are found in groups & clustersGalaxies 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!
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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
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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
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The Local Group
1 Megaparsec (Mpc)
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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
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Rich Clusters
Contain 1000’s of bright galaxies:• Extend for 510 Mpc
• Masses up to ~1015 Msun
• One or more giant Elliptical Galaxies at center• Ellipticals found near the center.• Spirals found at the outskirts.
1020% of their mass is in the form of a very hot (1078K) intracluster gas seen only atX-ray wavelengths.
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Rich ClusterAbell 1689(Hubble Space Telescope)