THE HUBBLE LAW
If you take a spectrum of anything out in space, you will usually see spectral lines, caused by electrons jumping up and down energy levels and either emitting or absorbing light. Each element has characteristic wavelengths at which it produces lines: for example, the very famous H-alpha line of hydrogen occurs at 656.3nm wavelength. Here is a plot of a star spectrum showing some of the strongest and most common spectral lines.
Spectral Lines. The strongest line ks a hydrogen one at 656.3nm
If you look at distant galaxies, however, all these spectral lines appear shifted to longer wavelengths. This effect is called redshift, and the redshift of a given object is defined as:
z=Δλλ0
where z is the redshift and Δλ is the shift in wavelength (i.e. observed wavelength minus the wavelength λ0 you would get for this spectral line in a laboratory on Earth.
This shift is caused by the doppler effect: all these distant galaxies are moving away from us at a speed v, which can be found from the equation v=cz where c is the speed of light.
Edwin Hubble showed that the recession velocity is proportional to distance: i.e. that v⃗ =H0r⃗ , where H0 is Hubble's constant and has a value of around 70km s−1Mpc−1 (i.e. a galaxy that is one mega-parsec from us is moving away at a speed of around 70 km/s).
Does this mean we are in a special place and everything is moving away from us? No - using vectors you can show that you would see exactly the same thing wherever you are.
So - we live in a uniform and seemingly endless universe, and wherever you are in this universe, everything seems to be moving away from you with a velocity proportional to its distance.
The Comprendia was founded in 2019 owned and managed by Neelabh Jaiswal is an Indian technology news and science website which covers the intersection of science, astronomy, technology and engineering. It's mission is to provide in-depth relevant information and knowledge.
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