What is meant by the expansion of 70km per second per megaparsec?

Question

Hey there,
Reading a book today, I stumbled upon a section where the expansion of the universe was discussed. The numbers were not constant, and although I did not get to finish, there is something that puzzled me. At a certain point, the book talked about an expansion rate of 70 kilometers per second per megaparsec. As far as I figured, a parsec is 3.26 light years, and a megaparsec is one million times that numbers. However, what exactly does it mean to expand per second per megaparsec? Any explanations pertaining to the topic will be greatly appreciated. Thanks!

Answer 1

If it just said 70 km per sec, you'd have no trouble. It means that all galaxies are moving away from us at that speed on the average. But the farther away something is, the faster it is receding from us. So it means for things a megaparsec away from us, they are receding about 70 km/sec. At two megaparsecs, that speed has increased to 140 km/sec. At 3 MP, it's 210 km/sec. Etc.

Answer 2

The further away from us something already is, the faster it's moving away. The amount of the increase in the recessional speed has been measured to be about 70 kilometers per second per megaparsec. That is, if you add one more megaparsec to the distance, you should find that the galaxies at the increased distance are moving 70 km/sec faster, away from us, than the nearer galaxies were.

Answer 3

If you had a steel rod (on earth) and heated it uniformly, it would expand (get longer). If you waited for a while, it would stablize at the final temperature and length. The ratio of the expansion to length is the coefficient of expansion, usually given per degree of temperature. If you start with a cool bar and raise the heating temperature evenly, so the change in length takes place over time, you can report that per second (or minute or hour) of time the bar gets a certain amount longer. Since different length bars would have different total length change, you would divide by the length of the bar to report that the change in length was 1.2 mm per second per meter of length.
The universe is like a rod 3,260,000 light years long which is expanding at 228.2 kilometers per second, which when divided give the number you gave of 70 kilometers per second per megaparsec.
If the universe was a balloon one megaparsec across, one second later it would be 70 kilometers bigger in diameter.

Answer 4

This is the Hubble constant, which shows the rate of expansion of the universe. The rate is, as you can see, proportional to the distance from here to the point of interest. Work this backwards, and you can see that in the past things were closer together. The Hubble constant is equivalent to a time, specifically 13.5 billion years, so that long ago everything was in one place. BANG!

Answer 5

Megaparsec