Can someone explain Hubble's Law to me in plain English?

Question

My College Astronomy homework this week involves Hubble's Law, but I'm just not grasping the whole concept, more specifically, Hubble's Constant. It's completely going right over my head. I know it involves velocity and distance but I'm just not putting 2 and 2 together. HELP!!!

Answer 1

the universe is expanding.

just like a balloon expands when you put air in it.

the speed at which the universe is expanding is hubble's constant.

which at this point in time is i think 71kms/mpc. which means for every megaparsec [3.2 million light years], the universe is expanding at 71 km a second.

oh ****, ive totally forgotten how to explain this myself.

Answer 2

The Hubble Constant simply means for every million parsecs, or 3.26 million light years away from us, a galaxy would be moving away from us at a particular speed because of the expansion of the Universe alone. Think of the Universe as a balloon on which dots were inked on. As you blow up the balloon, the dots move apart as the balloon expands. If you were on one of them, all the other dots would appear to speed away from you as more air is blown into the balloon. The more distant dots would appear to be moving away from you faster than the ones closer to you. It's no different with galaxies, galaxy clusters and the expansion of the Universe. A galaxy that is 300 million light years away on average would be moving away from us ten times faster than one that is 30 million light years away. Recent observations indicate the value for the Hubble Constant is 70 kilometers or 45 miles per second for every mega-parsec of distance. Under this value of the Hubble Constant, a galaxy 300 million light years away would be on average speeding away from the Earth at  about 4,150 miles per second, whereas a galaxy 30 million light years away would be heading away from us at about 415 miles per second. This ignores the galaxies individual motions through space, and because of this there are some nearby galaxies that show a blue-shift instead of a red-shift. That is why galaxies at the same distance have somewhat different red-shifts, or for a few galaxies as far away as the Virgo Cluster, no red-shift at all. Therefore the Hubble Constant wouldn't be of much help for members of the Local Group, for which we have other reliable methods of finding their distances, but it is of great value for distant galaxies. All you would need to find the distance to a remote galaxy is to take a spectrum of it and determine the red-shift, which directly tells you how quickly it is receding from the Earth. From there, its a simple task for astronomers to derive the distance to that galaxy.

Answer 3

Hubble's law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. The law was first formulated by Edwin Hubble and Milton Humason in 1929[1] after nearly a decade of observations. It is considered the first observational basis for the expanding space paradigm and today serves as one of the pieces of evidence most often cited in support of the Big Bang. The most recent calculation of the proportionality constant used 2003 data from the satellite WMAP combined with other astronomical data, and yielded a value of 70.1 ± 1.3 (km/s)/Mpc. This value agrees well with that of 72 ± 8 km/s/Mpc obtained in 2001 by using NASA's Hubble Space Telescope[2]. In August, 2006, a less precise figure was obtained independently using data from NASA's orbital Chandra X-ray Observatory: 77 (km/s)/Mpc or about 2.5×10−18 s−1 with an uncertainty of ± 15%.[3]