Hubble's law (A-level only) Flashcards – AQA A-level Physics (7408)

What does it mean for light from a distant galaxy to be redshifted?

Its observed wavelengths are longer than its emitted wavelengths.

What does the redshift of distant galaxies suggest about their motion and the universe?

Galaxies are receding as the universe expands.

What is Hubble’s law for a galaxy’s recession speed $v$ and distance $d$?

$v = Hd$, where $H$ is the present Hubble constant.

According to Hubble’s law, how does a galaxy’s recession speed generally relate to its distance?

More distant galaxies generally recede faster.

What is the approximate Hubble time in terms of distance $d$, recession speed $v$ and the Hubble constant $H$?

$t \approx \frac{d}{v} = \frac{1}{H}$

What assumption underlies the Hubble time estimate?

A galaxy’s present recession speed is extrapolated backwards as if it had remained constant. The estimate is approximate because the expansion history need not follow this extrapolation.

What does the Big Bang theory say about the universe’s early state and subsequent development?

The universe has expanded and cooled from an earlier hot, dense state.

What two observations support the Big Bang theory?

The cosmic microwave background radiation and the relative abundance of hydrogen and helium.

When could radiation from the early universe first travel freely?

When the universe cooled enough for neutral atoms to form.

Why is radiation from the early universe now observed at microwave wavelengths?

The universe’s expansion stretched it to microwave wavelengths.

What does the presence of the cosmic microwave background throughout space suggest about the early universe?

It is consistent with an early hot, dense universe.

How did some helium nuclei form in the early hot universe?

Some hydrogen nuclei combined to form helium nuclei.

How do the observed abundances of hydrogen and helium compare with predictions for the early universe?

Hydrogen predominates, with substantial helium; both are broadly consistent with the predicted amounts.