Two things that come to my mind when I think of the Universe… It has to be either finite or Infinite, and both set sail thoughts in my mind. Over the past 80 years scientists have started believing that our universe is constantly expanding. How did they come to this conclusion?
The light coming from distant objects would be redshifted (Visible light emitted or reflected by an object is shifted towards the red end of the electromagnetic spectrum) as it traveled through the expanding universe. The redshift would increase with increasing distance to the object.
Edwin Hubble, measured the redshifts of a number of distant galaxies. He also measured the relative distances of those galaxies by measuring the apparent brightness of a class of variable stars in each galaxy. When he plotted redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance (ie; more the distance between galaxies, more will be the redshift). The only explanation for this observation is that the universe was expanding! Edwin Hubble also realized that galaxies were rushing away from each other at a rate proportional to their distance, i.e. the farther away the galaxies are, the faster they are moving away from each other.
Once scientists understood that the universe was expanding, they immediately realized that it would have been smaller in the past. At some point in the past, the entire universe would have been a single point. This point, later called the big bang, was the beginning of the universe as we understand it today.
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