Uploaded June 2020 | Updated September 2026, 2 hours ago
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Cosmology is the science of the origin and development of the universe.
In this series, we’ll dive into the known facts that lead to the most accepted theories regarding the origin of our universe. But it all starts out with a few basic concepts. The first one we’ll tackle is called Redshifting
Astronomers often use the term redshift when describing how distant objects behave far away from us. To understand what a redshift is, think of how the sound of a siren changes as it moves toward you and then away from you. As the sound waves from the siren move toward you, they are compressed into higher frequency sound waves. As the siren moves away from you, its sound waves are stretched into lower frequencies. This shifting of frequencies is called the Doppler effect.
A similar thing happens to light waves. When an object in space moves toward u, its light waves are compressed into higher frequencies or shorter wavelengths, and we say that the light is blueshifted. When an object moves away from us, its light waves are stretched into lower frequencies or longer wavelengths, and we say that the light is redshifted.
We believe our universe is expanding because the light from most objects in the Universe is redshifted, other-wise known as the cosmological redshift. The redshift of an object can be measured by examining the absorption or emission lines in its spectrum. These sets of lines are unique for each atomic element and always have the same exact pattern and spacing. When an object in space moves toward or away from us, the absorption or emission lines will be found at different wavelengths than where they would be if the object was not moving (relative to us).
The change in wavelength of these lines is used to calculate the objects redshift.
You may now be thinking: if all the stars are moving and motion changes the wavelength of each spectral line, how can astronomers possibly tell what elements are present in the stars? I mean, isn’t it the precise wavelength (or color) that tells astronomers which lines belong to which element?
I thought the same thing. The truth is that Astronomers rarely judge the presence of an element in an astronomical object by a single line. It is the pattern of lines unique to hydrogen or calcium that enables us to determine that those elements are part of the star or galaxy we are observing. The Doppler effect does not change the pattern of lines from a given element—it only shifts the entire pattern slightly toward redder or bluer wavelengths. Then, the shifted pattern is still quite easy to recognize. Best of all, when we do recognize a familiar element’s pattern, we get a bonus: the amount the pattern is shifted can enable us to determine the speed of the objects in our line of sight.
You see, the training of an astronomer includes hours learning to decode light measuring these wavelengths in containers of gas in laboratories, which are not moving. An experienced “decoder” can learn the temperature of a star, what elements are in it, and even its speed in a direction toward us or away from us just by looking at it.
Some interesting questions arise from this for me though regarding why the universe is increasing in its expansion. Please ask YOUR questions in the comments section and we’ll tackle them in part 2. Thanks for watching and please consider sharing this video and supporting this channel in the links below. Thanks!
Many of the facts mentioned here were quoted from http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html and NASA.gov.
Support me for $1/month: patreon.com/spacebutmessier
Buy this shirt and others!: spacebutmessier.com/shop
Cosmology is the science of the origin and development of the universe.
In this series, we’ll dive into the known facts that lead to the most accepted theories regarding the origin of our universe. But it all starts out with a few basic concepts. The first one we’ll tackle is called Redshifting
Astronomers often use the term redshift when describing how distant objects behave far away from us. To understand what a redshift is, think of how the sound of a siren changes as it moves toward you and then away from you. As the sound waves from the siren move toward you, they are compressed into higher frequency sound waves. As the siren moves away from you, its sound waves are stretched into lower frequencies. This shifting of frequencies is called the Doppler effect.
A similar thing happens to light waves. When an object in space moves toward u, its light waves are compressed into higher frequencies or shorter wavelengths, and we say that the light is blueshifted. When an object moves away from us, its light waves are stretched into lower frequencies or longer wavelengths, and we say that the light is redshifted.
We believe our universe is expanding because the light from most objects in the Universe is redshifted, other-wise known as the cosmological redshift. The redshift of an object can be measured by examining the absorption or emission lines in its spectrum. These sets of lines are unique for each atomic element and always have the same exact pattern and spacing. When an object in space moves toward or away from us, the absorption or emission lines will be found at different wavelengths than where they would be if the object was not moving (relative to us).
The change in wavelength of these lines is used to calculate the objects redshift.
You may now be thinking: if all the stars are moving and motion changes the wavelength of each spectral line, how can astronomers possibly tell what elements are present in the stars? I mean, isn’t it the precise wavelength (or color) that tells astronomers which lines belong to which element?
I thought the same thing. The truth is that Astronomers rarely judge the presence of an element in an astronomical object by a single line. It is the pattern of lines unique to hydrogen or calcium that enables us to determine that those elements are part of the star or galaxy we are observing. The Doppler effect does not change the pattern of lines from a given element—it only shifts the entire pattern slightly toward redder or bluer wavelengths. Then, the shifted pattern is still quite easy to recognize. Best of all, when we do recognize a familiar element’s pattern, we get a bonus: the amount the pattern is shifted can enable us to determine the speed of the objects in our line of sight.
You see, the training of an astronomer includes hours learning to decode light measuring these wavelengths in containers of gas in laboratories, which are not moving. An experienced “decoder” can learn the temperature of a star, what elements are in it, and even its speed in a direction toward us or away from us just by looking at it.
Some interesting questions arise from this for me though regarding why the universe is increasing in its expansion. Please ask YOUR questions in the comments section and we’ll tackle them in part 2. Thanks for watching and please consider sharing this video and supporting this channel in the links below. Thanks!
Many of the facts mentioned here were quoted from http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/redshift.html and NASA.gov.
