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cosmos0
2011-Jul-07, 05:25 PM
In the following at would consider the followings are true:
- the Universe is flat (i.e. Omega matter + Omega vacuum = 1)
- the critical density is a constant

My question is that considering the expansion of the Universe, it is common sense that the density of the Universe was higher in the past, therefore wouldn'it make sense that the ratio of Omega matter to Omega vacuum was higher in the past.

In the lambda-CDM model is was hypothetised that the ratio of Omega matter to Omega vacuum is a constant. The Omega matter was estimated at 0.27, and Omega vacuum at 0.73.

How was it hypothetised that Omega matter is a constant? How was Omega matter estimated? is there any evidence that Omega matter is a constant?

If the Omega matter to Omega vacuum ratio was higher in the past that would imply that we are overestimating light travel distances for high redshifts. See Ned Wright cosmological calculator (http://www.astro.ucla.edu/~wright/CosmoCalc.html)

caveman1917
2011-Jul-07, 08:55 PM
In the lambda-CDM model is was hypothetised that the ratio of Omega matter to Omega vacuum is a constant.

When you use the equations that model our universe in a cosmological sense, you have to calibrate them at some time which is free for you to choose (technically you'd have to set some cosmological time as being where the scale factor equals 1). The standard way this is done is calibrating it at the current time, so right now the scale factor is 1 (this is done because it implies that comoving distance and proper distance are numerically equal at the current time). So when we plug in the values for \Omega_m and \Omega_\lambda we also have to use the ones at the current time. The equations will by themselves take care of the time-evolution of those quantities once you have calibrated them and plugged in the correct numerical values for your time of calibration. There is nothing preventing you from calibrating at say 10 billion years after the big bang (scale factor equals 1 then) and plug in the omega values as they were then, it will give you the same result in the end.

cosmos0
2011-Jul-07, 09:14 PM
Thanks, it is more clear now. Now I can trust the model.

Cougar
2011-Jul-07, 10:09 PM
...it is common sense that the density of the Universe was higher in the past, therefore wouldn'it make sense that the ratio of Omega matter to Omega vacuum was higher in the past.

Yes.

In the lambda-CDM model is was hypothetised that the ratio of Omega matter to Omega vacuum is a constant.

I would disagree with that. Omegam is fixed, and Omegalambda is increasing, AFAIK. So the ratio isn't constant.

How was it hypothetised that Omega matter is a constant? How was Omega matter estimated? is there any evidence that Omega matter is a constant?

Well, additional "new" matter is not spontaneously appearing anywhere in the visible universe. And baryon number is conserved (http://en.wikipedia.org/wiki/Baryon_number#Conservation) (for all practical purposes) in all quantum interactions.

If the Omega matter to Omega vacuum ratio was higher in the past that would imply that we are overestimating light travel distances for high redshifts.

Please provide your chain of logic that allows you to reach this conclusion.

cosmos0
2011-Jul-08, 06:05 PM
caveman1917 answered my question. I am using Ned Wright calculator in my study titled "Frawework for the conversion of light travel distances to Euclidean distances". See the other thread: http://www.bautforum.com/showthread.php/117283-age-of-the-Universe-around-20-billion-years