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)