Hubble’s Law

1

Dr. Bill Pezzaglia, Santa Clara University

A1. Static CosmologyNewton proposed that the universe must be

infinite to be balanced; a finite universe would collapse due to gravity

2

•1920 Eddington shows that Newton’s infinite universe is unstable and would collapse

1917 Einstein proposes universe:•Is finite, curved like a ball•But gravity would still collapse it•Proposes negative pressure(cosmological constant) prevents collapse•Later calls this his “biggest blunder”

A2. The Expanding Universe 3

•1922 Friedmann shows that another solution would be that the universe is expanding from kinetic energy leftover from a “big bang” creation. No need for negative pressure.

•1929 Hubble verifies that the universe is indeed expanding.

B1. Hubble’s LawTo visualize the expansion of our three-dimensional universe, imagine a two-dimensional universe crisscrossed by a grid of parallel lines (like on a piece of graph paper). The animation shows five galaxies that happen to lie where gridlines cross. As the universe expands in all directions, the gridlines and the attached galaxies spread apart. From the viewpoint of any one of the galaxies, all the other galaxies appear to be moving away. The more distant a galaxy is, the more rapidly it appears to be receding.

4

B2. Hubble’s LawHubble’s Law is then

(recessional speed) =(const)x(Distance)

Or: v = H d

H=Hubble’s Constant, which will be the “slope” of the line. The approximate value is 75 km/(sec-MPC)

Note: MPC=megaparsec (a million parsecs).

5

B3. Hubble’s Law MeasurementThe velocity is measured using the Doppler redshift:

v/c = Z (approximately)

c=speed of lightExample: Z=0.2 means the speed is 20% the speed of light.

Redshift: Z = /

Example: if wavelength is 520 nm, but you measure it to be 580 nm, then =50 nm, and Z=50/520=0.096

6

C1. Decelerating Universe 7

CLOSED UNIVERSE: Just like a ball thrown upward will fall back to earth due to gravity, we might expect the universe will slow down, and collapse.

OPEN UNIVERSE: If you throw a ball upward fast enough, it won’t fall back, but it certainly will slow down due to gravity

Everyone assumed that the universe must be decelerating due to gravity, it was only a question of how fast it was slowing down.

C2. The Universe is Accelerating? 8

1998 Measurements of distant supernova (i.e. in the distant past) were 20% fainter than expected.

Interpretation: universe WAS slowing down for first half of lifetime

BUT, since then, it has been ACCELERATING

This is a big surprise

C3. Dark Energy

9

                                                            

                                            

How do we explain this?

•Propose Universe is•4% normal stuff•23% weird dark matter•73% “DARK ENERGY”

The Dark Energy provides the negative pressure.

But is the universe really 96% weird stuff?

•Alternative: Revive Einstein’s Cosmological Constant?