What is the approximate age of the Universe, in terms of the Hubble constant?

1 ÷ H₀. A faster expansion means less time has passed.

The Hubble constant turns out to be larger than was thought. What does that do to the estimated age of the Universe?

It makes it smaller. Age ≈ 1 ÷ H₀.

matter that gives out no light and is detected only by its gravity. We know it is there from how galaxies move.

What is the evidence for dark matter?

The outer parts of galaxies rotate faster than visible matter can explain. More gravity is needed than the visible matter provides.

What decides whether the Universe will expand for ever?

its average density. More mass means more gravity pulling back.

The age and fate of the Universe · Edexcel IAL Physics

In plain words

If the galaxies are all flying apart, two questions follow. How long ago did they set off? And will they keep going for ever? The answers depend on how fast the expansion is, and on how much stuff there is out there pulling back, most of which we can't even see.

4 things to know

If the galaxies have always moved apart at the speeds they have now, the age of the Universe is about 1 ÷ H₀. So the value of the Hubble constant sets the age.
H₀ is hard to measure well, because the distances to far-off galaxies are uncertain. That makes the age uncertain too.
The fate of the Universe depends on its average density. Gravity slows the expansion: with enough mass the expansion could stop and go into reverse, and with too little the Universe expands for ever.
Dark matter: the outer parts of galaxies rotate too fast to be held in by the gravity of the matter we can see. There must be extra matter that gives out and absorbs no light. Because we can't see it, the total density, and so the fate of the Universe, is uncertain.

Worked example

Estimate the age of the Universe, taking H₀ as 2.2 × 10⁻¹⁸ s⁻¹.

Age ≈ 1 ÷ H₀ = 1 ÷ 2.2 × 10⁻¹⁸ = 4.5 × 10¹⁷ s.
One year is 3.15 × 10⁷ s.
Age ≈ 4.5 × 10¹⁷ ÷ 3.15 × 10⁷ = 1.4 × 10¹⁰ years: about 14 billion years.

Tips and tricks

A larger value of H₀ means a younger Universe.
Dark matter has never been seen directly. It is detected only by the pull of its gravity.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

The age and fate of the Universe: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is the approximate age of the Universe, in terms of the Hubble constant?
- 1 ÷ H₀ (the answer)
- H₀
- H₀²
- c ÷ H₀
A faster expansion means less time has passed.
The Hubble constant turns out to be larger than was thought. What does that do to the estimated age of the Universe?
- It makes it smaller. (the answer)
- It makes it bigger.
- It makes no difference.
- It doubles it.
Age ≈ 1 ÷ H₀.
What is dark matter?
- matter that gives out no light and is detected only by its gravity (the answer)
- matter that has cooled down
- matter inside black holes only
- the space between galaxies
We know it is there from how galaxies move.
What is the evidence for dark matter?
- The outer parts of galaxies rotate faster than visible matter can explain. (the answer)
- Galaxies are redshifted.
- Stars are black bodies.
- The night sky is dark.
More gravity is needed than the visible matter provides.
What decides whether the Universe will expand for ever?
- its average density (the answer)
- its temperature
- the brightness of its stars
- the number of galaxies we can see
More mass means more gravity pulling back.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 9 marks. Write your answers on paper, then check them.

The age and fate of the Universe

Edexcel International A Level Physics WPH · 9 marks · papermunch.org

Name ______________________________ Date ______________

Estimate the age of the Universe if H₀ is 2.5 × 10⁻¹⁸ s⁻¹.[2]
Show answerHide answer
4.0 × 10¹⁷ s, which is about 1.3 × 10¹⁰ years.
I got this one
Explain why the value of the Hubble constant is uncertain.[2]
Show answerHide answer
Finding H₀ needs the distances to very distant galaxies as well as their speeds. The speeds come accurately from redshift, but the distances are hard to measure and have large uncertainties.
I got this one
Describe the evidence for dark matter.[3]
Show answerHide answer
Stars in the outer parts of galaxies orbit faster than can be explained by the gravitational pull of the visible matter. There must be additional mass that we cannot see, providing the extra gravitational force.
I got this one
Explain how the average density of the Universe affects its fate.[2]
Show answerHide answer
Gravity slows the expansion. If the density is high enough, gravity could eventually stop the expansion and reverse it. If it is too low, the Universe will go on expanding for ever.
I got this one

Answers: The age and fate of the Universe

1. 4.0 × 10¹⁷ s, which is about 1.3 × 10¹⁰ years.
2. Finding H₀ needs the distances to very distant galaxies as well as their speeds. The speeds come accurately from redshift, but the distances are hard to measure and have large uncertainties.
3. Stars in the outer parts of galaxies orbit faster than can be explained by the gravitational pull of the visible matter. There must be additional mass that we cannot see, providing the extra gravitational force.
4. Gravity slows the expansion. If the density is high enough, gravity could eventually stop the expansion and reverse it. If it is too low, the Universe will go on expanding for ever.