E6 continued- HUBBLE'S LAW

E6.4: State Hubble's law.

E6.6: Explain how the Hubble constant may be determined.

In the 1920s, Edwin Hubble plotted the distance between the Earth and distant galaxies, and their recession speed against each other- he realized that the recession speed was directly related to the distance to the galaxy. This supports the theory that the universe is EXPANDING.

Source: Study Guide, Tom Kirk

Where v= recession velocity- this is calculated by finding the RED SHIFT of the galaxies,
and d= distance between the Earth and the galaxy- calculated using either the STELLAR PARALLAX, SPECTROSCOPIC PARALLAX or CEPHEID VARIABLE methods.

Source: Heinemann HL Physics, Chris Hamper

NOTE: REMEMBER THE UNITS.
Distance: Mpc
Recession Velocity: Km s^-1

E6.5: Discuss the limitations of Hubble's law.

The data points are scattered about the best fit line on the graph, indicating that there are big RANDOM ERRORS in the measurements.

Also, the gravitational attraction slows down the recession speed of the galaxies, but this is not accounted for, and we assume that the recession speeds are constant.

There are fewer data points near the origin of the line because the galaxies closest to the Earth are red-shifted only SLIGHTLY- the shift is too small, so the errors would be significant.

E6.7: Explain how the Hubble constant may be used to estimate the age of the universe.

At the time of the BIG BANG- all parts of the universe were in the same place. If we know how FAST any 2 parts are moving apart, and how FAR apart they are now, we can calculate the age of the universe :)

Source: Heinemann HL Physics Textbook, Chris Hamper

But, this calculation assumes that the recession velocity is constant. We know that the gravitational attraction slows down the galaxies, so the recession velocity today is much smaller than what it was in the past. Hence, our value is too large, and thus the age of the universe cannot be MORE than 1.35 x 10^10 years.