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A2 OCR G485: June2011 11a(i) Hubble constant

The question was to find the hubble constant in km/s/Mpc using the age of the universe (13.7E9), it also gave the value of 1pc=3.09E16m.

I did the inverse of the age in seconds and got 2.31E-18(1/s) then since they asked for the value of H in km/s/Mpc I multiplied 2.31E-18 by 10^3 for km and divided it by (3.09E16)*10^6 for Mpc. Got the wrong answer - apparently you need to multiply the 2.31E-18 by 1Mpc and divide the value by 1km. I dont understand this, could someone explain.

june13 11a(i).png16.0KB
Original post by tomusaA
The question was to find the hubble constant in km/s/Mpc using the age of the universe (13.7E9), it also gave the value of 1pc=3.09E16m.

I did the inverse of the age in seconds and got 2.31E-18(1/s) then since they asked for the value of H in km/s/Mpc I multiplied 2.31E-18 by 10^3 for km and divided it by (3.09E16)*10^6 for Mpc. Got the wrong answer - apparently you need to multiply the 2.31E-18 by 1Mpc and divide the value by 1km. I dont understand this, could someone explain.



2.31x10-18 is H0 in SI units... meters per second per meter

you convert meters per second to km per second by dividing by 1000

2.31x10-21 km/s /m
i.e. for every meter further away something is, it's recession speed is 2.31X10-21 km/s greater

so for every million meters more distant the recession speed in km/s would be 1 million times 2.31X10-21

and for every parsec more distant the recession speed in km/s would be 3.09X1016 times 2.31X10-21

... and for every million parsecs it's 3.09X1022 times 2.31X10-21

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