Transcript Hubble Law Talk
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
Hubble’s Data (1929)
1000 500 0 0 1 Distance (Mpc) 2
Added more data :Hubble & Humason (1931)
20,000 15,000 10,000 5000 1929 Data 10 20 Distance (Mpc) 30
Hubble’s Law
v = H
0
x d
v
= recession velocity in km/sec
d
= distance in Mpc
H 0
= expansion rate today (
Hubble Parameter
) Measure Hubble Parameter by calculating slope of the linear relationship Best value:
H 0 = 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
appears to be moving away from us.
it • Hubble Parameter : Rate of expansion
today
.
Comments: •
Empirical
result - based only on data • Actual value of H 0 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 • T = 300 km 100 km/h = 3.00 hours speed
The Hubble Time: T
0 Hubble’s Law says • A galaxy at distance
d
speed,
v = H 0
d
away has a recession So as in the analogy: • T 0 = d / v • but since, v = H 0 d, T 0 = d / H 0 d = 1 / H 0
Hubble Time
:
T 0 = 1 / H 0
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
moving away to be 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 : d A =1 Mly. Galaxy B has d B =3 Mly Expansion of universe doubles the scale of the coordinate system Now: A distance is 2 Mly B distance is 6 Mly V A ~ (2-1)=1 Mly = d A V B ~ (6-3)=3 Mly = d B 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 universe
clusters
of galaxies that is getting bigger due to the expansion of the 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 ~5x10 12 M sun
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: ~10 14 M sun
Rich Clusters
Contain 1000’s of bright galaxies: • Extend for 5 10 Mpc • Masses up to ~10 15 M sun • 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 (10 7 8 K) intracluster gas seen only at X-ray wavelengths.
Rich Cluster Abell 1689 (Hubble Space Telescope)