how can you infer that the universe is under constant expansion
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Short answer:
According to Albert Einstein's theory of General Relativity the universe should be expanding or contracting. After that astronomer Edwin Hubble discovered all the galaxies are moving away from us, which fits Einstein's predections really well. Scientist today measure the distance between the galxies using the Doppler shift.
Detailed answer:
After Albert Einstein developed his famous theory of General Relativity, he applied it to the whole universe and found something remarkable. He found that either the entire universe should be expanding or contracting.
Few years after that astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He discovered that almost all galaxies are moving away from us!
Since the time of Hubble we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us. And in fact, the farther away a galaxy is the faster it is moving away from us. This fits in very well with Einstein's predictions.
Astronomers measure the movement of objects relative to us using Doppler shift. When you hear a train coming, its whistle is heard at a different frequency compared to when it is receding(moving away), right? In the same way, light also has a Doppler shift, whereby its frequency is shifted depending on the motion of the emitting object.
Astronomers observed that light from distant objects in the universe is redshifted (shift in the frequency of light towards red color), which tells us that the objects are all receding away from us. This is true in whatever direction you look at: all the distant galaxies are going away from us. This can only be due to the fact that the Universe is expanding.
A lot of people also say this is caused by Dark Matter. You can also watch this video-
https://m.youtube.com/watch? v=QAa2O_8wBUQ or search in YouTube 'What is dark matter' there will be a video by Kurzgesagt.
Keep on learning.
According to Albert Einstein's theory of General Relativity the universe should be expanding or contracting. After that astronomer Edwin Hubble discovered all the galaxies are moving away from us, which fits Einstein's predections really well. Scientist today measure the distance between the galxies using the Doppler shift.
Detailed answer:
After Albert Einstein developed his famous theory of General Relativity, he applied it to the whole universe and found something remarkable. He found that either the entire universe should be expanding or contracting.
Few years after that astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He discovered that almost all galaxies are moving away from us!
Since the time of Hubble we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us. And in fact, the farther away a galaxy is the faster it is moving away from us. This fits in very well with Einstein's predictions.
Astronomers measure the movement of objects relative to us using Doppler shift. When you hear a train coming, its whistle is heard at a different frequency compared to when it is receding(moving away), right? In the same way, light also has a Doppler shift, whereby its frequency is shifted depending on the motion of the emitting object.
Astronomers observed that light from distant objects in the universe is redshifted (shift in the frequency of light towards red color), which tells us that the objects are all receding away from us. This is true in whatever direction you look at: all the distant galaxies are going away from us. This can only be due to the fact that the Universe is expanding.
A lot of people also say this is caused by Dark Matter. You can also watch this video-
https://m.youtube.com/watch? v=QAa2O_8wBUQ or search in YouTube 'What is dark matter' there will be a video by Kurzgesagt.
Keep on learning.
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