Astronomy 100 -- Hubble's Law and Large Scale Structure


For measuring distances to galaxies greater than 5 Megaparsecs (15 Million LY) away, astronomers use the HUBBLE LAW.


"The More Distant an Object is,
the Greater is its Recessional Speed."

Hubble Law out to 150 Megaparsecs.

The Hubble Law is a linear relation between how fast an object is receding from us and the distance to that object (such as a distant galaxy). For ALL distant objects,

Definition of the Hubble Constant.

or this can be written as

Speed = Hubble Constant x Distance.

The Hubble Constant, H, is a speed divided by a distance and astronomers use units of kilometers per second per Megaparsec, written as km/s/Mpc or km s-1 Mpc-1. The VALUE of the Hubble Constant ranges between 50 and 90 km s-1 Mpc-1, with the most recent "best" determination being

H = 75 km s-1 Mpc-1.


Outside of clusters of galaxies, the overall galaxy distribution is not smooth. Below is a model of a LARGE VOLUME OF SPACE. This cube is approximately 500 Mpcs on a side!

Galaxy Distribution in a 500 Mpc Cube.

The Universe on a large scale looks like a sponge!

Rotation Curves and Dark Matter


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