Measuring the Hubble Constant

Scales of the Universe in

Perspective

AU

~0.5 LY ~4 LY

~200,000 LY across

Andromeda Galaxy is

~2 million LY away

100 million LY

500 million LY

15 Billion Light Years4.6 Billion Parsecs

Virgo Supercluster

Large-

Scale

Structure

of the

Universe

The Cosmic Distance Ladder

Each successive “rung”

must be calibrated against

the preceding “rungs”

RR Lyraes

&

Edwin Hubble &

the Great Debate

1920s—astronomers were

divided in 2 camps: those

who thought “spiral

nebulae” were in the Milky

Way, & those who thought

they were separate galaxies

Hubble resolved the debate

in 1924, using Cepheid

variables to prove M31 was

a separate galaxy

Hubble's Redshift Observations

Nearby galaxies with resolved stars: Hubble derived

distances using Cepheid variables

Distant galaxies, where stars are unresolved: Hubble used H & K spectral lines

Hubble's Observations cont.

Hubble's Law

Most galaxies are receding from us

More distant galaxies are rushing away faster!

What

does it

mean?

Cosmological Principle: our place

in the universe is not special.

If this is true, then…

1. The universe is expanding.

2. Therefore the universe used to

be smaller.

3. At some time t = 0, the

universe may have had no

physically meaningful size

(singularity)

Implications

Continued

The universe’s age is

finite (no word on its size)

The beginning of its

expansion is called “The

Big Bang” (a derisive

term that stuck)

The observable universe,

extrapolated forward

from the observed rates

of expansion, would

appear to be ~93 billion

LY across if light traveled

infinitely fast

Observable universe ≠

entire universe

Brief History of the Universe

IN 1998, WE FOUND THAT SINCE THE UNIVERSE WAS ~9 BILLION

YEARS OLD, ITS EXPANSION RATE HAS BEEN INCREASING.

DARK ENERGY WAS PUT FORTH AS AN EXPLANATION

Procedures, Tools, & Tips (demo)

K H

Ca K (left) = 3934 Å Ca H (right) = 3969 Å

Redshift (z) Formula:

Velocity in terms of Redshift:

Use c = 3 x 105 km/s (*Conversion factors don't

affect sig-figs)

Distance

m = Apparent magnitude of the galaxy (measured)

M = Absolute magnitude (assume -22 [exact])

d is in units of parsecs

Reminder (see also

Appendix A):

Hubble's Law (written 2 ways):

H0 has weird units: km/s/Mpc –

must convert d to Mpc!

Average all your H0 values &

compute the % discrepancy using

the following theoretical value:

You will also get an H0 value from the slope of the best-fit

line on your graph.

Compute the % discrepancy between the H0 from your

graph & the theoretical value above.

67.8 ± 0.8

Next Week:

Worksheets for Hubble’s Law lab due

Next week’s lab TBA (we’re debating between

2 labs – watch your emails)