A few things we know about the universe. Science in Society 7

The scale, origin and future of the universe

The Earth is one of the 9 known planets which orbit the Sun. It takes one year to make a complete orbit. The planets are very small compared with the distances between them. Other smaller objects also orbit the Sun, some on orbits which are very elliptical. These can come close to Earth or even hit it (meteors, meteorites). Others can form comets. Several of the outer planets are quite different in composition from the Earth, much larger and largely composed of gases. Only the Earth appears to be able to support life as we know it.

The solar system was formed about 5 billion years ago. Surface features of several other planets and moons show similarities to features on the Earth, suggesting a common origin. The Sun is a star. Stars form from clouds of gases being drawn together by the force of gravity. In stars hydrogen atoms join together (fuse) to form helium atoms. This fusion releases energy. This energy is emitted at a fairly constant rate until most of the hydrogen is used up, probably in about 5 billion years for our Sun.

The Sun is one of billions of stars clustered in a group called the Milky Way galaxy. The size of this galaxy is huge, even compared to the distances between our Sun and the nearest stars. Light travelling at 300 million metres per second takes about 100 000 years to get from one side of the galaxy to the other. Most of the galaxy is empty space. There are millions of other galaxies in the Universe. The distances between galaxies are very large compared to the size of the galaxies themselves.

The distances between the galaxies are not fixed, but increasing. The Universe is expanding. Furthermore, the further apart galaxies are from each other, the faster they are moving apart. This suggests that all the Universe, all of space, matter, energy and time started from a tiny size with a huge explosion, the Big Bang, which occurred 13.7 billion years ago (or 1.37×1010 years ago).

“The discovery of the Big Bang and the recession of the galaxies came from a commonplace of nature called the Doppler effect. We are used to it in the physics of sound. An automobile driver speeding by us blows his horn. Inside the car, the driver hears a steady blare at a fixed pitch. But outside the car we hear a characteristic change in pitch. To us, the sound of the horn elides from high frequencies to low…if the car is racing away from us, it stretches out the sound waves, moving them, from our point of view, to a lower pitch and producing the characteristic sound with which we are familiar….

Light is also a wave…The Doppler effect works here as well…An object receding from us at very high velocities has its spectral lines red-shifted. This red shift, observed in the spectral lines of distant galaxies and interpreted as a Doppler effect, is the key to cosmology.”

From ‘The Edge of Forever’ chapter 10 of Cosmos by Carl Sagan.

Sagan goes on to describe the partnership of Milton Humason, who worked the mule teams during the construction of Mount Wilson observatory in California and then became an skilled telescope operator, and Edwin Hubble, the astronomer who worked at Mount Wilson from 1919. Humason and Hubble analysed the spectra of different galaxies and found that the further away they were the more they were red shifted, suggesting that the universe is expanding. The theory of an expanding universe was proposed by the Belgian priest, poet and astronomer Georges Lemaitre in 1927, two years before Hubble published his findings.

Time, size and gravity

Time since other events: 5×109 years ago: birth of the Sun, 4×109 years ago: formation of the Earth, 2×109 years ago: first sign of life on Earth, 105 years ago: early humans.

Size of the observable universe: 1028m, a typical galaxy: 1020m, distance from Earth to Sun: 1011m, diameter of Earth: 1.3×107m, a human: 1.7×100m, an atom: 10-10m, an atomic nucleus: 10-15m.

Gravity is the force of attraction between any two masses, including planets, stars and other objects in the universe. The strength of the force is proportional to the masses of the two attracting objects and inversely proportional to the square of the distance between them. Gravity is a very weak force and is only significant for very large masses.

Newton’s law of universal gravitation                F = G (m1m2) / r2

Where F is the force of gravity (in N), G is the gravitational constant (6.67×10-11 Nm2kg-2), m1 and m2 are the two masses (in kg) and r is the distance between them (in m).

Worth watching:

The history of the universe in 10 minutes: https://www.youtube.com/watch?v=-Ip5BAEfZuA

Carl Sagan’s Cosmos, Episode 10 The Edge of Forever covers Hubble, the redshift, the expanding the universe and the Big Bang: https://www.youtube.com/watch?v=o8ENNgO4z5c&index=10&list=PLBA8DC67D52968201The series dates from 1980 but is still a brilliant introduction. The whole series is available on YouTube.

Neil Degrasse Tyson’s Origins documentary is also excellent: https://www.youtube.com/watch?v=D-EXw5CdPtM

A useful interactive guide to scale in the universe: http://htwins.net/scale2/

In 2005, the story of Milton Humason and Edwin Hubble was made into a musical The Expanding Sky by Stan Peal.


  1. Research the size, ‘day’ length, ‘year’ length and composition of the planets in our Solar system.
  2. What do you think would count as evidence for extra-terrestrial life?
  3. How might our ideas about ourselves change if we found evidence of extra-terrestrial life?
  4. Define the following words: cosmology, elliptical, galaxy, light year.


About Eddie Playfair

I am a Senior Policy Manager at the Association of Colleges (AoC) having previously been a college principal for 16 years and a teacher before that. I live in East London and I blog in a personal capacity about education and culture. I also tweet at @eddieplayfair
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