The Expanding Universe & Hubble's Observations

What is meant by the expansion of the Universe?

The distances between widely separated galaxies generally increase over time.

What evidence suggests that the Universe is expanding?

Light from most distant galaxies is red-shifted.

What did Edwin Hubble's observations help establish?

That more distant galaxies generally have greater recession speeds.

What is Hubble's law?

The recession speed of a galaxy is proportional to its distance, on sufficiently large scales.

What does Hubble's law suggest about the Universe?

The Universe is expanding.

What happens to the observed red-shift as galaxy distance generally increases?

The red-shift increases.

Why does the expansion of the Universe stretch light?

As light travels through expanding space, its wavelength increases.

Does the expansion of the Universe mean Earth is at the centre?

No, observers in other galaxies would also see distant galaxies generally moving away.

Why do distant galaxies appear to recede from one another?

The space between them expands.

Does the expansion of the Universe mean galaxies themselves must be expanding?

No, gravitationally bound systems generally remain held together.

Why does the Milky Way not simply expand along with the Universe?

Gravity holds its stars and other matter together.

What does a greater recession speed generally indicate about a galaxy's distance?

It is generally further away.

Why is red-shift evidence rather than direct visual observation of expansion?

Astronomers measure changes in light wavelengths and use them to infer expansion.

How can the expansion of the Universe be represented using a balloon model?

As the balloon expands, points on its surface move further apart.

What is a limitation of the balloon model?

It can incorrectly suggest the Universe has a centre or expands into an external space.

Why does the expanding Universe support the idea of a denser past?

If distances are increasing now, matter was generally closer together in the past.

What does the expansion of the Universe suggest about its history?

The observable Universe evolved from an earlier, hotter and denser state.