Hubble's Law

Some Universe Wallpapers in FullHD.

The expansion of the Universe is such that the speed v at which a galaxy seems to move away from us is on average proportional to the distance r of that galaxy, according to the formula

(Eq. 1) v = H r

This formula is know as Hubble's Law, and H is Hubble's Constant. The best measurements we have yield a value for H that is about 71 km/s/Mpc.

Our Universe is expanding, but you only notice that at scales much bigger than the scale of a galaxy. Within a galaxy, gravity is strong enough to keep the stars together against the expansion of the Universe.

According to Hubble's Law (see above), the expansion speed over the diameter of a galaxy such as ours (about 100,000 lightyears or 30,000 pc or 0.03 Mpc) is about 2 kilometers per second, which is much smaller than the roughly 200 km/s at which stars orbit around the center of the Galaxy.

In our Galaxy, about as many stars move towards us as move away from us, so about as many stars in our Galaxy have a redshift as have a blueshift.


More Universe Wallpapers in FullHD.