GCSE Physics: Redshift

This is our third blog for CGSE Physics students.

It’s the first of two blogs on cosmology.

It explores the evidence for our current understanding of the universe.

We are looking at redshift.


In our second blog, next month, we’ll see how redshift led to the idea of the Big Bang.

The Big Bang was the creation of space and time, the beginning of our universe.



By 1920, astronomers had measured the distance to many stars.

You can see how they did it in a previous blog.


Many of these astronomers thought that our galaxy, the Milky Way, was the whole Universe.

The universe was around 100,000 light years across.

That’s around 950 000 000 000 000 000 kilometres across, pretty big!

File:Milky Way 2010.jpg

Milky Way.    Credit: NASA


Other astronomers had a different view.

Among the stars were hazy, whirlpool patches of light.

They called them ‘spiral nebulae’, spiral-shaped clouds.

Andromeda spiral.    Credit: Stellarium


These astronomers said that spiral nebulae were other Milky Ways far beyond our star system.

They were what we now call galaxies.

Our Milky Way was just one of these galaxies in an enormous universe.


Edwin Hubble

The debate was settled in 1923 by American astronomer Edwin Hubble.

He measured the distance of the brightest spiral, the Andromeda nebula.

He used the huge telescope at Mount Wilson to find a particular kind of star in Andromeda.

Called a Cepheid Variable, it told Hubble that the distance was immense.

NASA Andromeda Galaxy Wallpaper - WallpaperSafari

Andromeda Galaxy, M31.   Photo: NASA


The Andromeda nebula was 2.5 million light years away!

This put it way outside the Milky Way.

It and the other spirals were galaxies, star cities like the Milky Way.

Between the galaxies was an immense void of empty space.


Big – and getting bigger

Hubble had shown that the universe is enormous.

Space goes on for millions, billions of light years.


His next discovery, in 1927, was even more amazing.

He found that the universe is expanding, getting bigger every day.

To understand his discovery, we must explore the science of spectroscopy.



A spectroscope splits starlight into its component colours:
red, orange, yellow, green, blue, indigo, violet.

Blue = shorter wavelength —————————————-red = longer wavelength

This is the continuous spectrum.


In the spectra of some gas clouds, nebulae, bright vertical lines can be seen in the colours.

These are emission lines.

They come from individual elements that are giving out energy at specific wavelengths.

Infographic showcasing different types of spectra. A continuous spectrum contains all wavelengths emitted by a hot, dense light source and creates a continuous curve graph. An emission spectrum shows colored lines of light emitted by glowing gas, creating a broken graph with short peaks representing the elements or molecules present. An absorption spectrum shows dark lines or gaps in light after the light passes through the gas, creating a curve graph with dips corresponding to specific elements or molecules.


In a star’s spectrum, dark vertical lines are seen in the coloured spectrum.

These are absorption lines, where each element in the star absorbs light of those same wavelengths.

Data from ESA's Gaia mission reveals our Sun's future and past ...

We can identify elements in stars from their lines in spectra, like a barcode.

Galaxy Spectra

Distant galaxies are too far away to resolve individual stars.

But the light from a galaxy still shows absorption lines from stars inside it.

Field of Galaxies.   Webb Space Telescope


The lines from the galaxy are not at the usual wavelength.

They are all at a longer wavelength, towards the red part of the spectrum.

This is redshift.


Doppler redshift

Redshift is caused by movement of an object away from us.


As the object moves away, the light waves are ‘stretched’ and become longer.

An approaching galaxy shows blueshift, where waves are ’squashed’.

Only the nearby Andromeda Galaxy shows a blueshift.

How Is A Stars Emission Spectrum Used To Study Stars - Study Poster

All other galaxies show redshift – they are moving away from us.
The greater the redshift, the faster the galaxy is moving away – and the further away it is.

In fact, the galaxies are not moving away.

Instead space is getting bigger and taking the galaxies with it.

We live in an expanding universe.


It follows that in the past, the universe was smaller.

Calculations show that 13.8 billion years ago, the universe was small, hot and dense.

This tiny universe suddenly became bigger in the ‘Big Bang’, the creation of the universe.

The universe has been expanding ever since.


Coming Next

The evidence for an expanding universe is conclusive.

Redshift shows that space is getting bigger all the time, and taking galaxies with it.


In our next blog, we’ll see where this discovery took astronomy.

It took us to the beginning, the ‘Big Bang’.


Dennis Ashton, blog author

The author: Dennis Ashton is a Fellow of the Royal Astronomical Society and a Wonderdome presenter.

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