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

Hubble’s Data (1929) Distance (Mpc) Recession Velocity (km/sec)

Added more data :Hubble & Humason (1931) ,000 15,000 20,000 Recession Velocity (km/sec) Distance (Mpc) 1929 Data

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 Hubble’s Law v = H 0 x d

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 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  speed T = 300 km  100 km/h = 3.00 hours

The Hubble Time: T 0 Hubble’s Law says A galaxy at distance d away has a recession speed, v = H 0  d 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 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 : 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 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 ~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)