Super fact 79 : Distant galaxies appear redder (redshifted) because of the universe’s expansion. The farther away the galaxy, the redder it is. This cosmological redshift is also increasing because the Universe’s expansion is accelerating. Our Universe is getting more and more red every day.
Esther’s writing prompt: January 14 : Red
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Many people are aware that the Universe is expanding but may not know that this results in a measurable redshift. The faster a galaxy is moving away from us the redder it appears. This is called the cosmological redshift. Measuring these redshifts was how we discovered that our Universe is expanding.
At the end of the 1990’s it was also discovered that the Universe’s expansion was accelerating. This discovery came from measuring the redshifts (and apparent dimness) of distant Type Ia supernovae. I should add that measuring the increase in the cosmological redshift directly is difficult. However, it is expected to be possible when the Extremely Large Telescope (ELT) starts gathering data in 2027. I can add that there are also other types of evidence showing that the Universe’s expansion is accelerating.
Another surprising aspect is, if the Big Bang was like an explosion, gravity would pull back the galaxies making the expansion slow down and perhaps eventually even reverse. However, the opposite is happening. A common explanation is that a repulsive force, a sort of anti-gravity, referred to as dark energy, is responsible for this acceleration.
I call this a super fact because this is an observed phenomenon, it is important knowledge for how we view the world, and it is surprising. Many people have heard bits and pieces of this but do not have the fuller picture.
The Doppler Effect
The cosmological redshift is an example of the so called doppler effect. The doppler effect is the change in the frequency or wavelength of a wave in relation to an observer if the origin of the wave and the observer are moving compared to each other. If an object is moving towards you, let say an ambulance, the frequency of its sound will be higher (wavelength shorter).
After it passes you and moves away from the frequency will be lower (wavelength longer). In other words, the sound changes when the ambulance passes you. The same is true for other kinds of waves, including waves on the water, and light. If a light is travelling towards you at a high speed, it will look bluer. If a light is travelling away from you at a high speed it will look redder. The latter is what is called a redshift, cosmological redshift in our case.
Below is a three minute video explaining the doppler effect and redshift.
I can add that the constancy of the speed of light in vacuum changes the exact size of the doppler effect. The doppler effect remains as is for the most part but the formulas for the classical doppler effect, which you would use for sound, and the relativistic doppler effect, which you would use for light in space, are different. It is just to pick the right formula.
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- We are Star Dust
- The vastness of space and the beginning of infinity
- GPS uses relativity for accuracy
- There is strong evidence for the Big Bang
- Time Dilation Goes Both Ways
- The Edge of the Observable Universe is 46.5 billion Light Years Away
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A very interesting post as usual, Thomas. Thank you.
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Thank you so much Lynette
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I didn’t know this, Thomas, and I’ve never heard of the doppler effect. Thank you once again for your sharing your knowledge and explaining it clearly for us.
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Thank you so much Esther and thank you for doing the fun prompts
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But we aren’t surprised the Universe is expanding, are we? That’s what it does. Well, until it doesn’t. Now that would be interesting.
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No you are right Jacqui. Most people have heard of the expansion of the Universe. However, that it is red shifted as a result might be surprising and that the expansion is accelerating and therefore the redshift increasing might be surprising too. In fact if the dark energy explanation is correct and dark energy keeps increasing then it would keep expanding faster and faster and eventually ripping the Universe apart.
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WOW Thomas…. your wealth of knowledge is unimaginable my friend. The pieces are slowly being put together.. Will we ever know the bigger picture of the Universe? I doubt it.
But like the universe…. We are expanding our consciousness 🙂 xx
Another mega fact finding post my friend x
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Thank you so much Sue and Mega Fact is a good name for super fact. Maybe I should have called it that.
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