Universe

The universe is everything there is: all of space, and everything in it. You met many of the objects in it in earlier grades. Objects in space are called celestial bodies (or heavenly bodies):

Celestial body	What it is	Example
Star	a huge ball of hot gas that makes its own light and heat by nuclear fusion	the Sun, Sirius, Polaris
Planet	a large, round body that goes round a star and shines only by reflected light	the Earth, Mars, Jupiter
Satellite (moon)	a body that goes round a planet	the Moon; Jupiter’s moon Ganymede
Baby planet	a small, planet-like body going round the Sun (scientists say minor planet: the dwarf planets, such as Pluto, and the rocky asteroids)	Pluto, Ceres, the asteroid Vesta
Comet	a lump of ice and dust that grows a glowing tail when it comes near the Sun	Halley’s Comet
Meteor	a streak of light, a “shooting star”: a grain of space rock burning up in our air	the Perseid meteors every August
Meteorite	a piece of space rock that survives the fall and lands on the ground
Galaxy	a huge group of billions of stars, gas and dust, held together by gravity	the Milky Way, the Andromeda Galaxy

Some celestial bodies are enormous and very massive; others are tiny. Some, like stars, are made of gas; others, like the Earth and the asteroids, are solid. Our Solar System (the Sun with its planets, moons, asteroids and comets) is a very small part of the Milky Way galaxy, and the Milky Way is just one of hundreds of billions of galaxies in the part of the universe we can see.

Gravity holds every one of these bodies in its place, and keeps them moving in their paths. Scientists have proposed many ideas about how the universe began; the one with the strongest evidence by far is the Big Bang theory. According to it, the universe is expanding: the galaxies are moving apart. Edwin Hubble’s study of their motion was the key. And the tug-of-war between this motion and gravity gives three possible futures for the universe: open, flat and closed. This unit takes these ideas one at a time.

The distances in space are so huge that kilometers become useless. The nearest star after the Sun, Proxima Centauri, is about 40,000,000,000,000 km away. So astronomers measure distances by how long light takes to cover them.

Light is the fastest thing there is: 300,000 km every second. It could go round the Earth seven and a half times in one second.

• Light from the Moon reaches us in about 1.3 seconds.
• Light from the Sun takes about 8 minutes 20 seconds. You always see the Sun as it was 8 minutes ago.

A light-year is the distance light travels in one year:

$$1\ \text{light-year} = 300\,000\ \text{km/s} \times 365.25 \times 24 \times 60 \times 60\ \text{s} \approx 9.46 \times 10^{12}\ \text{km}$$

That is nearly ten trillion kilometers. A light-year is a distance, not a time.

Use the slider to travel outwards and feel how fast the distances grow.

Look at Figure 12.2 and think about these questions.

The Earth has mass, so it has gravity. So do the Sun and the Moon. As Figure 12.2 shows, they all pull one another:

• the Sun pulls the Earth towards its center, and the Earth pulls the Sun towards its center, with an equal force;
• the Earth pulls the Moon, and the Moon pulls the Earth;
• the Sun pulls the Moon, and the Moon pulls the Sun.

Every celestial body in space attracts every other one in this way. This mutual force of attraction is gravitation (Unit 7). Because of it, the Earth revolves in its orbit around the Sun, and the Moon revolves around the Earth. Every body in the Solar System revolves around the Sun, and gravity keeps the planets in their orbits.

Gravity does even more. It built these objects in the first place. The Sun and the planets formed 4.6 billion years ago from a huge, slowly spinning cloud of gas and dust: gravity pulled the cloud together, most of it into the Sun at the center, and the rest into a flat disc that clumped into the planets. That is why the planets all go round the Sun in the same direction and in nearly the same plane. On a larger scale, gravity holds the hundreds of billions of stars of a galaxy together, and holds galaxies together in groups.

How did the universe begin? What shape is it? How big is it? Scientists have studied these questions for centuries. The branch of science that studies the universe as a whole, its history and its future, is called cosmology. A closely related branch, astrophysics, uses physics to study the stars, galaxies and the universe: how they are made, how they work, and how they change.

Our galaxy, the Milky Way

Our Solar System is in a spiral galaxy called the Milky Way. It is a flat, spinning disc about 100,000 light-years across, with a bulge of old stars in the middle and long spiral arms full of young stars, gas and dust. The Sun is in one of the arms, about 26,000 light-years from the center.

The Milky Way has somewhere between 100 billion and 400 billion stars. The Sun, carrying the Earth with it, goes round the center of the galaxy at about 230 km/s, and even so, one trip round takes about 230 million years. The last time the Sun was where it is now in its orbit, the first dinosaurs were appearing.

The Big Bang theory

The Big Bang theory grew out of one astonishing observation: all the distant galaxies are moving away from us, and from one another, at tremendous speed. If they are all moving apart now, then in the past they were closer together. Run the film backwards far enough, and everything was packed together in one tiny, unimaginably hot and dense state.

This is what the theory says:

1. About 13.8 billion years ago, the universe was in an extremely hot, dense and compressed state, full of energy: everything we can see today was packed into a space smaller than an atom.
2. At that time, the four fundamental forces of nature, gravitational force, electromagnetic force, the strong nuclear force and the weak nuclear force, are thought to have been joined as a single force.
3. Then the universe began to expand, very fast, and as it expanded it cooled. The forces separated. The energy turned into particles, the particles into atoms (mainly hydrogen and helium), and over hundreds of millions of years gravity pulled the gas together into stars and galaxies. All the celestial bodies came from it.
4. The galaxies are still moving apart, so the universe is still growing. For a long time, astronomers expected gravity to be gradually slowing them down; you’ll see below what they found when they checked.

Many books describe the beginning as the explosion of a single “atom”, flinging matter outwards like the pieces of a bursting firecracker. That picture is easy to remember, but it is misleading in two ways. (In an exam, write that the universe began with the sudden expansion, the “explosion”, of an extremely hot, dense state smaller than an atom: that is right whichever book your teacher uses.)

So the Big Bang theory rests on three main pieces of evidence:

1. The galaxies are moving apart (Hubble’s law, below).
2. The cosmic microwave background, the afterglow, fills the whole sky.
3. The universe’s ordinary matter is about three-quarters hydrogen and one-quarter helium, just what the theory predicts the early hot universe would have made.

Time after the Big Bang	What happened
a tiny fraction of a second	the forces separate; space expands extremely fast
about 3 minutes	the first nuclei of hydrogen and helium form
about 380,000 years	the first atoms form; light can travel freely (the afterglow we see today)
about 200 million years	the first stars light up
a few hundred million years	the first galaxies; the oldest stars of the Milky Way form soon after
9.2 billion years (4.6 billion years ago)	the Sun and the Earth form
13.8 billion years	today

Activity 12.3: an expanding balloon

Hubble’s study

In the 1920s, the American astronomer Edwin Hubble used the 100-inch Hooker telescope on Mount Wilson in California, then the biggest telescope in the world, to measure how far away various galaxies are. Other astronomers, especially Vesto Slipher, had already measured how fast they were moving, from the color of their light: light from a galaxy moving away is stretched to longer, redder wavelengths, an effect called redshift, like the falling pitch of a bus’s horn as it drives away.

In 1929, Hubble plotted each galaxy’s speed against its distance and found a simple pattern: galaxies are moving away from us, and the farther away a galaxy is, the faster it is moving away. (Farther galaxies also look smaller and fainter, which helped him judge their distances.)

This relation is Hubble’s law:

$$v = H d$$

where v is the speed at which a galaxy is moving away (its recession velocity), d is its distance, and H is Hubble’s constant. On a graph of speed against distance, the galaxies lie close to a straight line through zero (Figure 12.6): twice as far, twice as fast. H is the slope of that line.

Distances between galaxies are measured in megaparsecs (Mpc). A parsec is an astronomer’s unit of distance, about 3.26 light-years, and a megaparsec is a million parsecs, about 3.26 million light-years. Measuring nearby galaxies gives H = 73 km/s per Mpc. That means two galaxies one megaparsec apart are moving apart at 73 km/s; two galaxies 10 Mpc apart, at 730 km/s; and so on.

Hubble’s law says that the farther the galaxies are, the faster they separate, exactly like the dots on the balloon. The whole universe is getting bigger. So the galaxies must have been closer together in the past, and long ago, all at the same place. In this way, Hubble’s study supports the Big Bang theory.

The universe has a history, and so does every star: stars, too, are born, live and die. Here is the life cycle of a star.

A star is born in a cold, dark cloud of gas and dust called a nebula. Gravity pulls a clump of the cloud together; as it shrinks, it heats up, until its center reaches about 10 million °C. There, hydrogen nuclei start fusing into helium, releasing enormous energy: a star has switched on. It shines steadily for most of its life, its gravity squeezing in and the heat of fusion pushing out, perfectly balanced.

What happens when the hydrogen in the core runs out depends on the star’s mass:

• A star like the Sun swells into a red giant, then gently puffs off its outer layers as a glowing shell called a planetary nebula. The core left behind is a white dwarf: about as big as the Earth but as heavy as the Sun, a teaspoon of it weighing tonnes. It slowly cools and fades.
• A star much more massive than the Sun (over about 8 times) swells into a red supergiant, then its core collapses in a fraction of a second and the star blows apart in a supernova, which can outshine a whole galaxy for weeks. What remains is a neutron star (a city-sized ball so dense that a teaspoon would weigh a billion tonnes) or, for the very biggest stars, a black hole, whose gravity is so strong that not even light escapes.

The Sun is about halfway through its life: 4.6 billion years old, with about 5 billion years to go before it becomes a red giant.

The galaxies are moving apart, but gravity is pulling them all back towards one another. Which will win? To think about it, start with something smaller: a rocket.

The universe is in the same situation. All the galaxies are moving apart, and the universe is getting bigger, but the gravitational pull of all the matter in the universe slows them down. As the galaxies get farther apart, that pull gets weaker. Does their speed run out first, or does the pull fade first? That decides the future of the universe.

Critical density

By Newton’s law, every mass pulls every other mass, with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. So how strongly the universe pulls itself back depends on how much mass there is and how spread out it is: in other words, on its average density (mass ÷ volume).

The average density the universe would need for gravity to just stop its expansion (after an infinitely long time) is called the critical density. It is incredibly small: about the mass of five or six hydrogen atoms in every cubic meter of space. (The air in your room has about 10²⁵ molecules in each cubic meter.)

Comparing the real density with the critical density gives three possible futures:

a. Open universe

If the average density is less than the critical density, gravity is too weak to stop the expansion. When gravity’s effect has faded away, the galaxies still have some speed left, so they keep moving apart forever, and the universe keeps growing. This is an open universe, like a rocket launched faster than the escape velocity.

b. Flat universe

If the average density is exactly equal to the critical density, the expansion keeps slowing down, and the galaxies’ speed and gravity’s effect both reach zero together, only after an infinitely long time, when the galaxies are infinitely far apart. The universe then stops growing and stays as it is. This is a flat universe, like a rocket launched at exactly the escape velocity. (Flat doesn’t mean like a pancake: it means space isn’t curved, so that the angles of a triangle drawn between three faraway galaxies would add up to exactly 180°.)

c. Closed universe

If the average density is more than the critical density, gravity wins. The expansion slows, stops, and turns into a collapse: the galaxies fall back together, and the universe shrinks back to a single point. This end is called the Big Crunch. Some scientists imagined that the crunch could be followed by a new Big Bang, and a new universe, again and again, so that the universe would have a life cycle of its own, like a star. This is a closed universe, like a rocket launched too slowly, which falls back.

	Open	Flat	Closed
Average density	less than critical	equal to critical	more than critical
Expansion	forever	slows, stops only after infinite time	stops, then reverses
End	galaxies spread apart forever	galaxies infinitely far apart, at rest	the Big Crunch
Like a rocket launched	faster than escape velocity	at exactly escape velocity	slower than escape velocity

The main thing that decides the future of the universe is gravity, and the strength of gravity depends on the mass and size of the universe, that is, on its density. Astronomers estimate the density by weighing everything they can see in space, and many things they can’t.

What do we know now (as of 2026)? Measurements of the Big Bang’s afterglow show that space is flat, to within less than half a percent: the total density is equal to the critical density. But the ordinary matter that stars, planets and people are made of makes up only about 5% of it. About 27% is dark matter, which we know from its gravity but can’t see, and about 68% is dark energy. If dark energy stays as it is, the universe will expand faster and faster forever: no Big Crunch, but a slow, cold fading, as the stars burn out one by one, sometimes called the Big Freeze. In 2025, a large survey of galaxies (DESI) found hints that dark energy may be weakening over time; if that is confirmed, the story could change again. That is how science works: the question isn’t closed.

1. Choose the correct option

2. Differentiate between

3. Give reasons

4. Answer these questions