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.
Hubble’s Data (1929) Recession Velocity (km/sec) 1000 500 0 0 2 1 Distance (Mpc)
Added more data :Hubble & Humason (1931) 20,000 Recession Velocity (km/sec) 15,000 10,000 5000 1929 Data 20 30 10 Distance (Mpc)
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 km100 km/h = 3.00 hours
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
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 • 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?
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
Expansion of space stretches light: Wavelengths get stretched intoredder (longer) wavelengths The greater the distance,the greater the stretching Result: The redshift of an objectgets larger with distance. Just what Hubble actually measured Cosmological Redshift
Time to be more precise • Most galaxies are found in groups & clusters • Galaxies are held in them by gravity • It is the distance between clustersof 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 atX-ray wavelengths.
Rich Cluster Abell 1689 (Hubble Space Telescope)