How light is stretched by the expansion of the Universe

What is the scientific meaning of the term 'redshift', and how does it relate to the expansion of the Universe?

A NASA image showing how wavelengths of light are stretched - or redshifted - as the Universe expands. Credit: NASA/JPL-Caltech//R. Hurt (Caltech-IPAC)
Published: February 17, 2022 at 9:19 am
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You may have heard astronomers and cosmologists talk about the term 'redshift' in relation to the expansion of the Universe and the 'stretching' of light that occurs, but what does the term mean?

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To understand redshift, we must first go all the way back to the birth of the Universe.

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The Universe has continued expanding since the Big Bang. Credit: BSIP SA / Alamy Stock Photo
The Universe has continued expanding since the Big Bang. Credit: BSIP SA / Alamy Stock Photo

The Universe is expanding in the aftermath of the Big Bang explosion 13.7 billion years ago.

During the time that light from an astronomical object has been travelling across space to the Earth, the Universe has grown in size.

The effect of this is to stretch the ‘wavelength’ of the light, much as a wave drawn on the fabric of a balloon would be stretched if the balloon were inflated.

The wavelength of red light is about twice that of blue light, so the stretching of visible light shifts it towards the red end of the spectrum: thus the term ‘redshift’.

The further away a galaxy is, the more its light is shifted to longer wavelengths – an effect known as redshift
The further away a galaxy is, the more its light is shifted to longer wavelengths – an effect known as redshift

What is the redshift z value?

The letter ‘z’ is used by astronomers to denote the ‘redshift’ of a celestial object such as a galaxy.

The redshift is defined as the fractional change in the wavelength of the light: that is, the observed wavelength minus the emitted wavelength divided by the emitted wavelength.

An object with a redshift of z emitted its light when the Universe was 1 / (1 + z) of its present size.

With this definition, objects on our doorstep have a redshift of zero.

A snapshot of the Cosmic Microwave Background - heat left over from the Big Bang - when the Universe was just 380,000 years old, as seen by the Planck Telescope. It shows tiny temperature fluctuations that correspond to regions of different densities: the seeds that would grow into the stars and galaxies of today. Credit: ESA and the Planck Collaboration
A snapshot of the Cosmic Microwave Background - heat left over from the Big Bang - when the Universe was just 380,000 years old, as seen by the Planck Telescope. Credit: ESA and the Planck Collaboration

Conversely, a quasar with a redshift of 6 existed when the Universe was 1/7th its current size and the cosmic microwave background radiation – the ‘afterglow’ of the Big Bang fireball – with a redshift of about 1,000, comes from an epoch when the Universe was about 1/1,000th its present size.

The redshift of an object is not related to its distance in a simple way. This is because the Universe has not expanded at a constant rate.

It expanded faster than average in the beginning and is expanding faster than average again today.

https://svs.gsfc.nasa.gov/vis/a010000/a012800/a012856/Redshift_Expansion_v3_1.mp4

A NASA animation showing how wavelengths of light are stretched - or redshifted - as the Universe expands. Credit: NASA/JPL-Caltech//R. Hurt (Caltech-IPAC)

Could galaxy redshifts locate the centre of the Universe?

The answer is yes… and the centre is here!

The redshift in wavelength of light emitted by a galaxy is related to its velocity – and other galaxies are rushing from us.

But before you think that the Milky Way is in a super-privileged location at the centre of creation, if we were in the Andromeda Galaxy we would also think we were at the centre.

How come? Well, the Big Bang didn’t happen at any one location. Instead, space and matter came into being and started expanding everywhere.

Think of the galaxies in the expanding Universe as raisins in a giant rising cake.

It wouldn’t matter which raisin you sat on – all the other raisins would be moving away and it would seem as if you were at the centre.

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This article originally appeared in the October 2006 and March 2010 issues of BBC Sky at Night Magazine.

Authors

Marcus Chown is an award-winning writer and broadcaster and a former radio astronomer at the California Institute of Technology in Pasadena. He is the author of Breakthrough: Spectacular stories of scientific discovery from the Higgs particle to black holes (Faber & Faber, 2021).

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