Hubble’s law says that galaxies are receding from us, faster the farther away they are. But the correct reading of this fact is subtle and surprising. The interpretation is not that galaxies move through a static space, like projectiles fired from a central explosion. It is that space itself is expanding, and the galaxies float in their own corners while the distance between them grows.
The classic model is that of paper dots glued to the surface of an inflating balloon. As the balloon expands, every dot moves away from every other dot, and the recession velocity between any two of them grows with their distance — just as in Hubble’s law. Above all: on the balloon there is no “centre” of the expansion. Every dot sees all the others moving away from it, and none occupies a privileged position. In the same way, in the expanding universe there is no centre from which everything is fleeing.
Velocity of a distant galaxy
A galaxy shows (a tenth of a wavelength shift). What is its distance according to Hubble?
From the relation : Then, from the law : The light we receive from that galaxy set off when the universe was about billion years younger.
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Topics: Relatività ristretta Concepts: Effetto Doppler
Related exercises: Problema — Un semaforo rosso visto verde · Problema — Composizione per via Doppler · Problema — Redshift di una galassia