Iqra and the two scallywags!

1. Hubble's constant is the gradient of the graph and it tells us how quickly the universe is expanding. Does that mean that Hubble's law does not take into account that the universe could possible result in a Big Crunch? It seems to me that Hubble's law assumes that the universe will expand forever...

Hubble’s law does in fact assume expansion for ever but it is based only on the start of the graph. We don’t yet know how the graph continues! I am not sure why they call it Hubble’s LAW and not Hubble’s THEORY! Three of the possibilities for the future are shown on the graphs on page 196. I think they are pretty self explanatory.

  • The universe may continue to expand for ever.
  • The universe may stop expanding and remain constant
  • The universe may stop expanding and start to collapse.

2. Is majority of the universe made of dark matter? If it is then is there no way of finding out what it is (because it doesn't emit any light)?

Just because we don’t know what dark matter IS and cant see it doesn’t mean that it doesn’t exist. The rotational speed of galaxies suggests that there MUST be much more mass in a galaxy than we can see. It is a challenge to either find it or come up with another explanation. Maybe there’s a mistake in the theories we have at present. Maybe YOU can find it.

3. Also I don't quite understand the maths on page 194 and am getting completely confused with all the units as well

This is not nearly as complex as it looks. Trust me!

You know that y = mx + c is the equation of a straight line. Yes? So far so good

You also know (or jolly well should know) that the gradient of a distance time graph represents the speed.

Why is it that this is so?

A distance time graph of constant speed is a straight line

Distance on the y axis time on the x axis

The thing on the y axis = the gradient x the thing on the x axis + some start value.

It follows therefore since we know that distance = speed times time then the gradient represents the speed. Easy? I knew you would agree.

OK lets change the things on the axes. Lets put speed on the y axis and distance on the x axis and see what the formula suggests the gradient of our line represents now.

OK here goes

Speed = gradient x distance

Now we know that

so the gradient must represent 1 over time.

Now relating this to the Hubble constant problem, the gradient of the hubble graph is the Hubble constant so the Hubble constant H0 = 1 / T

A word about the units kms-1Mpc-1 is kilometres per second per megaparsec

Now a Mpc is a unit of distance (let me know if you need further clarification of what a parsec is) so Hubble’s constant has units of

km (Length) s-1(time-1)Mpc-1(length-1)

the length cancels the length-1 and you are left with time-1! How lucky is that because that is exactly what we just said above Hubble’s constant = 1 / time.