# Understanding dark energy problem

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• windy miller
In summary: 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. Dimensionless units are used to keep constants like c and G equal to one, and the level of vacuum energy is estimated to be around 10^{-122}. This number is extremely odd to physicists, as they would expect something closer to 0.01 - 100. Many researchers are still trying to find an explanation for the discrepancy.
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.

windy miller said:
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?
Yes.
2) if 1 is correct then why not just assume that dark energy is not vacuum energy but something else?
People have considered alternative approaches, like modifying the "geometry side" of the Einstein equations.
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.
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.

QuantumQuest and windy miller
bapowell said:
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.
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?

windy miller said:
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?
Here's Weinberg's paper: http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.59.2607 . 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.

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windy miller
windy miller said:
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
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.

## What is dark energy?

Dark energy is a theoretical form of energy that is thought to make up about 70% of the universe. It is believed to be responsible for the observed accelerating expansion of the universe.

## What is the dark energy problem?

The dark energy problem refers to the mystery of what exactly dark energy is and why it exists. It is currently one of the biggest unsolved problems in physics and cosmology.

## How do scientists study dark energy?

Scientists study dark energy by using various observational techniques, such as measuring the acceleration of the expansion of the universe and observing the cosmic microwave background radiation. They also use theoretical models and simulations to better understand its properties and behavior.

## What are the current theories about dark energy?

There are several theories about dark energy, but the most widely accepted one is the cosmological constant theory, which suggests that dark energy is a constant, uniform energy throughout space. Other theories propose that dark energy is a dynamic field or that it is an indication of flaws in our understanding of gravity.

## What are the implications of understanding dark energy?

Understanding dark energy is crucial for our understanding of the universe and its future. It could also have important implications for other areas of physics, such as the study of gravity and the search for a unified theory of the fundamental forces of nature.

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