Exam practice

The universe is expanding: the space between galaxies is stretching everywhere, at the same rate. Two galaxies far apart have more stretching space between them than two galaxies close together, so the distance between them grows faster, just as dots far apart on an inflating balloon separate faster than dots close together. This is Hubble’s law, v = Hd.

Every celestial body is pulled by the gravity of the bodies around it, especially the large one it orbits (a planet by the Sun, a moon by its planet). But each also moves sideways along its orbit at high speed. Gravity bends its path into a curve around the bigger body instead of pulling it in, so it keeps going round in its orbit. The balance between gravity and its motion keeps it in its place.

The universe is the whole of space and everything in it: all the matter and energy, including the stars, planets, satellites, comets, asteroids, meteors, galaxies, gas and dust, and the light and radiation between them.

The Big Bang theory says that the universe began about 13.8 billion years ago from an extremely hot, dense and compressed state, smaller than an atom, in which the four fundamental forces (gravitational, electromagnetic, strong nuclear and weak nuclear) were united as one. A sudden huge expansion (the “explosion”) began, and the universe has been expanding and cooling ever since; the matter formed in it became the stars and galaxies, which are still moving apart.

1. All distant galaxies are moving away from us and from one another.
2. The farther a galaxy is, the faster it is moving away: its speed is proportional to its distance, v = Hd (Hubble’s law), with H, Hubble’s constant, about 70 km/s/Mpc (measurements give 67 to 73).
3. So the universe is expanding, and its galaxies must have been together long ago: this supports the Big Bang theory.

Whether there is a limit depends on the average density of the universe compared with the critical density, because gravity, which depends on the density, slows the expansion:

• If the density is more than critical, gravity will stop the expansion and the universe will collapse (closed universe): there is a limit.
• If it is equal, the expansion slows and stops only after infinite time (flat universe).
• If it is less, there is no limit; the universe expands forever (open universe).

Today’s measurements show that the universe is flat, but that dark energy is making its expansion speed up, not slow down. So, as far as we know now, there is no limit: the universe will most likely keep expanding forever. (This could change if dark energy turns out to change over time, which some 2025 results hint at.)

1. It keeps the planets revolving around the Sun and the moons around their planets, each in its own orbit.
2. It holds the stars of a galaxy together, and galaxies together in groups and clusters.
3. It pulled clouds of gas and dust together to form the stars, the Sun and the planets in the first place.
4. It keeps the planets round, and holds their atmospheres.
5. It slows the expansion of the universe, and its strength (the density of the universe) decides the universe’s future.

Each body is moving sideways along its orbit at high speed: the Earth at about 30 km/s around the Sun, the Moon at about 1 km/s around the Earth. Gravity pulls each towards the body it orbits, but instead of falling straight in, the moving body’s path is bent into a near-circle around it: it is always falling and always missing, as in Newton’s cannon. The pull of gravity provides just the force needed to keep it on its curved path. Also, the orbits are at different distances and don’t cross, so the bodies never meet.

In a flat universe, the average density of the universe is exactly equal to the critical density. The galaxies keep moving apart, but gravity slows them down, and both their speed and gravity’s effect reach zero at the same time, after an infinitely long time, when the galaxies are infinitely far apart. The expansion then stops, and the universe stays as it is, neither growing nor collapsing. It is like a rocket launched at exactly the escape velocity. (“Flat” also means that space is not curved. Measurements show our universe is indeed flat, though dark energy is making it expand faster.)

This asks for your opinion, so any answer with good reasons can earn marks. Here is one based on today’s evidence:
The universe is most likely to expand forever.

1. The afterglow of the Big Bang shows that space is flat: the total density is equal to the critical density.
2. But most of that density is dark energy, which pushes space apart. Since 1998, measurements of distant supernovae have shown that the expansion is speeding up, not slowing down.
3. So gravity will not stop the expansion, and there will be no Big Crunch. The galaxies will keep moving apart, the stars will slowly burn out, and the universe will grow ever colder and darker.
4. Scientists are still not sure what dark energy is, and new results (2025) hint it may be changing. If it weakened and reversed, the universe could still slow down, or even collapse. So the question isn’t completely settled.