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## Main Question or Discussion Point

How can we tell the values of dark matter and mass in the universe when it is say five times its original size?

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How can we tell the values of dark matter and mass in the universe when it is say five times its original size?

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Nabeshin

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What do you mean by "original size"?five times its original size?

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marcus

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How can we tell the values of dark matter and mass in the universe when it is say five times its original size?

That's the logical question to ask, for sure. Nabeshin you seem really knowledgeable and reliable----would you be willing to check me on some numbers?What do you mean by "original size"?

Some years back I took the usual numbers we were hearing, on critical (energy) density and dark matter fraction and dark energy fraction and I imagined everything put into energy terms (instead of grams per unit volume, say joules).

And I recall getting something like this for the CURRENT energy densities:

Critical density today was around 0.85 nanojoule per cubic meter, and the U looks flat or nearly flat so let's say the real overall average density is 0.85 nJ/m

And then dark energy density (which in the standard cosmo model stays constant) comes out to about 0.6 nJ/m

I suppose the OP ("jc09") could be asking what the densities would be like when distances have expanded by a factor of cuberoot 5 and when volumes are 5 times what they are NOW.

(he could mean "present" instead of "original")

So then because the dark matter fraction is around 23 or 24% of the mix now, which translates to

about 0.20 nJ per cubic meter, it would then be 0.04 nJ per cubic meter.

And ordinary matter is now about 0.03 nJ per cubic meter, so when volume is 5 times what it is now the ordinary would be down to the level of roundoff error.

What is 0.03 divided by 5? 0.006. Doing rough estimates we can forget that.

So the present densities are about 0.6 nJ per cubic meter DE

and about 0.2 nJ per cubic meter DM

and in future when volume has increased by factor of 5 we will have

0.6 nJ per cubic meter DE (because in standard LCDM cosmology it does not change) and

0.04 nJ per cubic meter DM

My aim is to say something concrete at an effective pedagogical level. I'd welcom any comment correction or improvement

It may also be irrelevant to the OP question since he or she may not have meant 5 times bigger than present, when they said 5 times bigger than original.

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Nabeshin

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Yep, checks out!would you be willing to check me on some numbers?

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marcus

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Thanks! This is an example of how well PF astro/cosmo forum can work sometimes. I probably trust you on numbers and technical detail more than I trust myself, so it's reassuring to hear from you something checks out.Yep, checks out!

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Nabeshin

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Haha! This is a scary thought though!I probably trust you on numbers and technical detail more than I trust myself, so it's reassuring to hear from you something checks out.

It definitely is nice to have someone check your work, especially when you're doing numerical answers where order of magnitude errors sometimes creep in unnoticed :)

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