How old is the Universe? (3)

The prevailing view through to the early 1920s was that the Universe consisted entirely of our own Milky Way galaxy, which contained stars clustered in nebulae. When Edwin Hubble, building on the work of Henrietta Leavitt, assessed the distances to the cepheids located in several nebulae he realised they were way too far away to be part of Milky Way. In fact the nebulae appeared to be separate galaxies. Hubble’s discovery was so revolutionary that he was advised not to publish it at the risk of being ridiculed. In the end the evidence he presented prevailed and the Universe we knew grew in size by a factor of millions.

For those of you wondering why my last two posts, supposed to be dealing with the age of the Universe, have instead focused on the distances to the stars the answer is near. You may remember how, evidenced by the redshift of their visual spectrum, many of the distant stars appeared to be receding (i.e. moving away from the Earth). Armed with the new measurements of the distance to these stars Edwin Hubble plotted a simple graph which is reproduced below for your convenience. It shows that the recessional speed of stars (here measured in km/s) appears roughly proportional to their distance from the Earth (here measured in megaparsecs).


I will briefly pause to consider the physical reality this implies. If objects far away from us recede faster than those close by there appears to be a general stretching of the distances across the Universe. In other words – the Universe, as a whole, is expanding. But if bits of cosmic matter are all flying off in the outward direction their paths can be traced back to a common point of origin from which they emerged some time ago. This gave the Big Bang model first proposed in 1927 by Georges Lemaitre a quantitative dimension. Knowing the speed of recession and spacial distribution of the stars we have worked out when they had started their journey. In other words we have found out the age of the Universe.

I am finding the way humanity arrived at this insight fascinating. It is as if we once were in a dark room with distant faint lights all around. Then, without ever being able to directly measure anything, we managed to work out the size of the room, location of the lights and even how and when the room came together. Truly impressive.



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