The Universe, the Big Bang Theory and Galaxies
Astronomical geography begins with the largest question of all - how did the universe begin, and what is it made of? The accepted answer is the Big Bang Theory, which explains the origin and expansion of the universe. From that starting point we study galaxies, the groups and clusters they form, and two invisible ingredients that control their fate, dark matter and dark energy. Examiners ask this topic through facts about the age of the universe, the evidence for the Big Bang, the types of galaxies and the names of famous objects.
The Big Bang Theory
The Big Bang Theory states that the universe began from a single, extremely hot and dense point about 13.8 billion years ago and has been expanding ever since. The term "Big Bang" was actually coined by Fred Hoyle, while the theory itself was developed by Georges Lemaitre, who proposed that the universe began from a primeval atom. Before the Big Bang the universe existed as a singularity - an infinitely hot and dense point with no space and no time. The first elements to form were hydrogen and helium, with hydrogen being the most abundant element in the universe.
Evidence for the Big Bang
Three main pieces of evidence support the theory. The first is the cosmic microwave background radiation (CMB), the faint thermal radiation left over from the Big Bang that fills the whole universe; it was discovered by Penzias and Wilson in 1965. The second is the redshift of distant galaxies, summarised in Hubble's Law, which shows that the universe is expanding. The third is the observed abundance of light elements such as hydrogen and helium, which matches what the theory predicts. Redshift means the increase in the wavelength of light from an object that is moving away from us, which tells us that the object is receding.
Hubble's Law and the expanding universe
Hubble's Law states that the velocity at which a galaxy recedes from us is directly proportional to its distance. This was discovered by Edwin Hubble in 1929 from the redshift of galaxies. The law means that, on average, the distances between galaxies are increasing over time, so distant galaxies are moving farther apart. It is important not to confuse the expansion of the universe with galaxies moving through space - it is space itself that is stretching.
Galaxies
A galaxy is a huge system of stars, gas, dust and dark matter bound together by gravity. Our galaxy is the Milky Way, which is a barred spiral galaxy with a central bar-shaped structure and spiral arms. It is estimated that there are more than 100 billion to 200 billion galaxies in the observable universe. The three main types of galaxies are spiral, elliptical and irregular. At the centre of the Milky Way lies a supermassive black hole called Sagittarius A* (Sgr A*). The Large and Small Magellanic Clouds are satellite galaxies of the Milky Way visible from the southern hemisphere, and the nearest large spiral galaxy to us is the Andromeda Galaxy (M31), about 2.5 million light-years away.
Dark matter, dark energy and special objects
The universe is not made only of the matter we can see. Dark matter is an invisible form of matter that does not emit or absorb light but exerts gravitational effects, and it makes up about 27 per cent of the universe. Dark energy is a mysterious force causing the accelerated expansion of the universe, and it makes up about 68 per cent. Ordinary matter is only a small remaining share. A quasar is an extremely bright, distant galactic nucleus powered by a supermassive black hole at the centre of a galaxy. The Local Group is a cluster of about 80 galaxies that includes the Milky Way, Andromeda and the Magellanic Clouds, and the Great Attractor is a massive gravitational anomaly towards which our galaxy and many others are moving.
Quick revision
For quick revision, remember the universe is about 13.8 billion years old and began from a singularity in the Big Bang, a term coined by Fred Hoyle from the theory of Georges Lemaitre. The evidence is the CMB (Penzias and Wilson, 1965), the redshift of galaxies and Hubble's Law (Edwin Hubble, 1929), and the abundance of hydrogen and helium. A galaxy is a gravitationally bound system of stars, gas and dust; the Milky Way is a barred spiral with Sagittarius A* at its centre, and galaxies are of three main types - spiral, elliptical and irregular. Finally, note the proportions: dark energy about 68 per cent, dark matter about 27 per cent and ordinary matter the rest.
How to prepare this topic
- Learn the Big Bang basics: age 13.8 billion years, singularity, Georges Lemaitre the developer and Fred Hoyle the coiner of the name.
- Fix the three evidences in order - CMB, redshift and Hubble's Law, and the light-element abundance.
- Remember the galaxy facts: Milky Way a barred spiral, Sagittarius A* its centre, Andromeda the nearest large galaxy and the Magellanic Clouds as satellites.
- Keep the three galaxy types and the three special objects (quasar, Local Group, Great Attractor) ready.
- Memorise the percentages of dark energy and dark matter, as they are asked as direct questions.
Common exam traps
- The Big Bang did not happen at a point in space; it was the beginning of space and time itself.
- Fred Hoyle coined the term "Big Bang", but Georges Lemaitre developed the theory.
- The Milky Way is a barred spiral galaxy, not an elliptical or irregular one.
- Redshift means a galaxy is moving away, while blueshift would mean it is approaching.
- Dark energy (about 68 per cent) is different from dark matter (about 27 per cent).
Frequently Asked Questions
What is the Big Bang Theory and who developed it?
The Big Bang Theory states that the universe began from a single, extremely hot and dense point called a singularity about 13.8 billion years ago and has been expanding ever since. The theory was developed by Georges Lemaitre, who proposed that the universe began from a primeval atom, while the term "Big Bang" was coined by Fred Hoyle. Before the Big Bang there was no space and no time, and the first elements to form were hydrogen and helium.
What evidence supports the Big Bang Theory?
The main evidence includes the cosmic microwave background radiation, the redshift of distant galaxies described by Hubble's Law, and the abundance of light elements such as hydrogen and helium. The cosmic microwave background radiation was discovered by Penzias and Wilson in 1965, and Edwin Hubble showed in 1929 that galaxies are moving away from us, proving that the universe is expanding.
What is a galaxy and what type is the Milky Way?
A galaxy is a huge system of stars, gas, dust and dark matter bound together by gravity. Our galaxy is the Milky Way, which is a barred spiral galaxy with a central bar-shaped structure and spiral arms. The three main types of galaxies are spiral, elliptical and irregular, and there are an estimated 100 to 200 billion galaxies in the observable universe. Sagittarius A* is the supermassive black hole at the centre of the Milky Way.
What are dark matter and dark energy?
Dark matter is an invisible form of matter that does not emit or absorb light but exerts gravitational effects, and it makes up about 27 per cent of the universe. Dark energy is a mysterious force causing the accelerated expansion of the universe, and it makes up about 68 per cent. Ordinary matter forms only a small remaining part of the universe.
What is Hubble's Law and what is redshift?
Hubble's Law states that the velocity at which a galaxy recedes from us is directly proportional to its distance, which shows that the universe is expanding. It was discovered by Edwin Hubble in 1929. Redshift is the increase in wavelength of light from an object moving away from us, indicating that the object is receding; the farther a galaxy is, the greater its redshift.