# I Understanding dark energy problem

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1. Apr 15, 2016

### windy miller

It is often said that dark energy is fine tuned to 120 orders of magnitude.
What I understand from this is that if you use quantum field theory to predict the energy of the vacuum and compare it to observations then you are off by 120 orders of magnitude
I have a few questions regarding this.
1) is the above correct?
2) if 1 is correct then why not just assume that dark energy is not vacuum energy but something else?
3) this is not the same degree as saying that dark energy is fine tuned for life in the universe or is it? i.e if dark energy were smaller than 120 order of magnitude could life still exist.?It seems to me there are two different senses of saying something is fine tuned 1)for agreement with observations and 2) for conditions for life and these should be different.

2. Apr 15, 2016

### bapowell

Yes.
People have considered alternative approaches, like modifying the "geometry side" of the Einstein equations.
Weinberg argued that if the CC was larger, the universe would have expanded too rapidly for bound structures to form. There's no problem though with the CC being smaller, even zero.

3. Apr 15, 2016

### windy miller

Thanks for the quick reply Bapowell. When Weinberg says dark energy can't be larger; is it by the same degree as in (1) i.e 120 orders of magnitude or is it a different amount? I don't see any reason, a prior,i to think these different senses of the phrase"fine tuning" should have the same value. Also if it were zero I agree there should be no problem there, but what if it were negative? how negative could it be without the universe collapsing before stars could form?

4. Apr 15, 2016

### bapowell

Here's Weinberg's paper: http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.59.2607 [Broken]. His upper "anthropic" bound on the CC is only a few orders of magnitude above the then currently known density. I do not know what the observational constraints on a negative cosmological constant are.

Last edited by a moderator: May 7, 2017
5. Apr 15, 2016

### Chalnoth

Not quite.

There is no solid way to predict the level of vacuum energy from current quantum theory. There have been a large number of attempts, but no single result has stood above as "the" prediction.

Rather, the 120 orders of magnitude number stems from dimensionless units. It's very common in theoretical physics to write down units so that constants like c and G are equal to one. In these units, the value of the cosmological constant is approximately $10^{-122}$. That seems extremely odd to physicists (who would expect something more like 0.01 - 100 or so), and many have been trying hard to find an explanation for the discrepancy.

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