Red-shift and the Big Bang

This whole topic (4.8 Space physics) is Separate Physics only. It is not assessed in Combined Science: Trilogy.

When astronomers study the light coming from most distant galaxies, they notice something striking. The light has been shifted towards the red end of the visible spectrum. This is called red-shift.

Red-shift means the wavelength of the light has increased. Red light has a longer wavelength than blue light, so light shifted towards red has been stretched to a longer wavelength than it started with.

Red-shift is the observed increase in the wavelength of light from a distant galaxy.

Red-shift happens because these galaxies are moving away from us, a motion described as receding. As a light source moves away, the waves reaching us are stretched out to longer wavelengths, producing the shift towards red.

Crucially, this is observed for most distant galaxies, in almost every direction. Nearly everything in the distant universe is moving away from us.

The single most important pattern in this topic is a relationship between a galaxy's distance and how it moves.

The further away a galaxy is, the faster it is moving away (receding), and the greater its red-shift.

So red-shift is not the same for every galaxy. A nearby galaxy shows a small red-shift; a very distant galaxy shows a large red-shift. Measuring the size of the red-shift lets astronomers work out how fast a galaxy is receding.

Galaxy	Distance from us	Speed of recession	Red-shift
Nearby galaxy	Small	Slow	Small
Distant galaxy	Large	Fast	Large

This link between distance and speed is not a coincidence. It is exactly the pattern you would expect if space itself is expanding, carrying the galaxies apart, which is the idea explored next.

The fact that distant galaxies are receding, and that more distant galaxies recede faster, is strong evidence that the whole universe is expanding.

It is important to understand what is expanding. It is not that galaxies are flying outwards through fixed space from one central point. Instead, space itself is stretching, and this carries the galaxies apart from one another.

A helpful model is dots drawn on a balloon. As the balloon inflates, every dot moves away from every other dot, and dots that start further apart separate faster. No dot is the centre; the whole surface is stretching. Galaxies behave like the dots, and expanding space is like the stretching rubber.

The observation that the speed of galaxies increases with distance is evidence that space (the universe) is expanding.

This is why we see red-shift in almost every direction. From any galaxy, including our own, everything else appears to be moving away.

If the universe is expanding now, then in the past everything must have been closer together. Running that idea backwards leads to the Big Bang theory.

The Big Bang theory states that the universe began from a very small, hot, dense region and has been expanding ever since.

Red-shift is the key evidence for this model. Because red-shift shows the universe expanding, it supports the idea that all matter and space started off packed together and has been spreading out since.

This is also a good example of how science works. Scientists make observations (the red-shift of galaxy light), look for a pattern (speed increases with distance), and build a theory (the Big Bang) that best explains those observations. A theory is accepted because it is supported by evidence, and it can be revised or replaced if new evidence appears, as the next slide shows.

For most of the twentieth century, scientists knew the universe was expanding but were unsure how the rate of expansion was changing over time. New observations changed the picture.

Since 1998, observations of supernovae in very distant galaxies suggest that distant galaxies are receding ever faster. The expansion of the universe appears to be accelerating.

Certain supernovae are useful because their true brightness is known, so astronomers can use them to measure the distances to faraway galaxies very accurately. Comparing these distances with the galaxies' red-shifts showed that the most distant galaxies are moving away faster than expected.

This was a surprise. An accelerating expansion is difficult to explain, and it introduced problems that scientists are still working on today. It shows that even a well-supported theory keeps being tested against new evidence.

The Big Bang theory explains a great deal, but there is still much about the universe that is not understood. Two big unknowns appear in the specification.

Dark mass (dark matter). Galaxies behave as though they contain far more mass than we can see. This unseen matter is called dark mass or dark matter. We can detect its gravitational effects, but we do not know what it is made of.

Dark energy. To explain the accelerating expansion found since 1998, scientists use the idea of dark energy. Like dark matter, its true nature is unknown.

Scientists cannot yet fully explain dark mass (dark matter) or dark energy. Much about the universe remains not understood.

For the exam, the key point is honest and simple: our current theories work well, but they are incomplete, and dark matter and dark energy are the standard examples of what is still unexplained.

1. Saying red-shift means light "turns red"

Red-shift means the wavelength increases (the light is stretched towards the red end of the spectrum). Describe it as an increase in wavelength, not simply as the light changing colour.

2. Getting the distance relationship backwards

More distant galaxies recede faster and show a greater red-shift. Do not write that distant galaxies move more slowly.

3. Saying galaxies fly outwards through space from a central point

It is space itself that is expanding, carrying galaxies apart. There is no single centre that everything is moving away from.

4. Treating the Big Bang as proven fact rather than the best-supported theory

The Big Bang is a theory strongly supported by evidence, chiefly red-shift. Explaining that it is built from observations shows the examiner you understand how science works.

5. Confusing dark matter and dark energy

Dark mass (dark matter) is unseen matter detected by its gravity. Dark energy is used to explain the accelerating expansion. Both are still not understood, but they are not the same thing.