View Full Version : Big Rip calculation

2013-Mar-27, 10:00 PM
I'm not sure if this is in the right section... I've become a bit interested in the Big Rip possibility of the end of the universe and I'm trying to calculate possible times with multiple values of w (phantom energy) with the formula presented here http://en.wikipedia.org/wiki/Big_Rip . However, I am getting shorter times until Big Rip if I increase the w (say -1.001 instead of the -1.5 = Big Rip in just 20 billion years), while it should be the opposite. What am I doing wrong?

By the way, looking at this little wikipedia article really says a lot about much "popular science" media who quoted the 20 billion time until Big Rip figure even through that was an example used in the formula, not a prediction, not even a guess (in fact such a short time is pretty much impossible if current observations of galaxy clusters are right). I also "love" all those media stating that "universe is gonna rip itself apart after all" even through it just one among many hypotheses about the possible far, far future of the universe.

2013-Mar-28, 05:55 AM
Not sure what you are doing wrong but I don't get that it decreases. For your w = -1.001 I get a huge value for t - t0 (500 times the value I get for w = -1.5)

2013-Mar-28, 09:49 AM
Thanks, what is the value you are getting for w = -1.5 ? Anyways, should I interpret those straight "brackets" same as regular brackets? I'm not too good at math.

2013-Mar-28, 01:04 PM
The straight brackets represent a modulus or absolute (i.e. the value of the number without the sign). So you work out the value of what is in them, then discard the sign. You can do this in most spreadsheets with an ABS() function.

I just worked it through - they are messing up the units in the wikipedia article. Most of the parameters are dimensionless except H in that equation. H is in units of km per second per megaparsec. So you need to put in a scale factor to get H0 in units of km per sec per km (or the equivalent Mpc per sec per Mpc). One megaparsec is about 31e18 kilometres. So in fact what you need to work out is:

trip - t = (2/3) / (abs(1+w) * (H0 / 30.9e18) * sqrt(1 - Omega))

This comes out in seconds. To get to years you have to divide by 365.22*24*60*60

So combining the constants to get time in years you just have to calculate:

trip - t = (6.5e11) / ( abs(1+w) * H0 * sqrt( 1 - Omega) )

Where H0 is in km per sec per Mpc and the output is in years

Doing this I get 22,293,685,049 or about 22 Gyr as required.