# I Where did the Dark Energy (or similar mechanism) come from?

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1. Jan 15, 2017

### Bolhuso

Hi all!

I'm new to this forum and, as a matter of fact I only recently rediscovered my interest for physics. I am starting to catch up, yet I find that Wikipedia sometimes falls sort for some basic questions, and I was hopping that this community could gently help me understand some concepts and suggest books and sources to continue my learning.

Please forgive my boldness posting a question given my lack of academic training and my thin knowledge of physics and cosmology. Yet I am very excited that this is my very first post.

The question at hand is:

I was reading about one of the not very well explained mysteries of the current state of science, which is why the expansion of the universe is accelerating when there was a phase in which it was decelerating. I have read that it has been postulated the existence of dark energy which would accelerate the expansion of the Universe. Lets assume that it exists (in the postulated way or an equivalent mechanism). For the sake of this discussion the nature of the dark matter (or equivalent mechanism) is not relevant. My question is very simple. Where did this dark energy (or equivalent mechanism) come from?

We know that there was a phase in which the expansion was decelerating. Which means that the balance of all effects controlling the expansion (whatever dark energy was present then in the Universe and whatever other effects) was negative. Yet now we observe that this balance is positive, causing the acceleration. In a rough way, this would lead to following four possibilities:
(1) The amount of dark energy has increased (where from?)
(2) The strength of dark energy has increased (how, given that this would break temporal isomorphism and maybe other symmetries?)
(3) The amount of sources of negative acceleration has decreased (where did they go to? and, wouldn't be this easily observable?)
(4) The strength of sources of negative acceleration has decreased (how, given that this would break temporal isomorphism and maybe other symmetries? and, wouldn't this be easily observable?))

I was wondering if someone can tell me if this is already into consideration in the existing models, and how do they address this. Or maybe I am missing out something essential. In any case, if someone can bring some light, preferably referring to sources, I will be very grateful.

2. Jan 15, 2017

### Bandersnatch

Hi, welcome to PF!

4 is the closest to what is happening.

There are basically three types of energy in the universe, whose densities affect expansion: matter (dark + baryonic), radiation (including hot neutrinos), and dark energy. You may have seen the equation for total energy denisty in the universe: $\Omega=\Omega_{m}+\Omega_{rel}+\Omega_{\Lambda}$

Each of those three evolves differently with the expansion of space:
- $\Omega_m$ matter density decreases with the third power of the scale factor (scale factor $a$ being roughly equivalent to average distances between galaxies), since as space expands you get the same amount of matter confined to an increasingly larger volume (volume being 3D, hence third power)
- $\Omega_{rel}$ radiation density decreases with the fourth power of the scale factor, since not only there's the same amount of photons in an increasing volume, but these photons additionally experience cosmological redshifting proportional to the growth of the scale factor
- $\Omega_{\Lambda}$ dark energy density does not change as the scale factor grows, i.e. is constant per unit volume. As you get more volume, you get more dark enegy

Since all three parameters evolve with different powers of $a$, it necessarily implies that there must have been or will be a time, when each of these was dominating.
The epoch of radiation domination ended very early in the history of the universe and by now is negligible, while dark energy started to dominate over matter later on.

3. Jan 15, 2017

### Bolhuso

Thanks very much! That was a really fast reply.

So, as I supposed, it is postulated that overall dark energy increases over time (since overall volume increases). I am sorry but I have further questions.

(1) Which specific model are you referring to (the one which postulates that equation)?
(2) Does the model elaborate on the underlying physical basis for the constant ΩΛ darkenergy? Or is it just a necessary (unexplained) component for the equation to match observation?
(3) Can you refer me to further sources?

Cheers!

4. Jan 15, 2017

### Bandersnatch

The model is LCDM, i.e. Lambda-Cold Dark Matter. It's the standard model of cosmology, which itself is a parametrisation of the FRLW metric, which in turn is a solution of Einstein's field equations.

The model itself gives no direct reason for DE to be there. Its constancy is the simplest parametrisation that fits the observations.

In any case, I'm told that DE can be understood as residual curvature left over after inflation, but you'll need somebody with deeper insight to elaborate on that.

As for resources to follow up on, I'm not sure what exactly are looking for and at what level. Having said that,
Wikipedia has good articles on LCDM, FLRW, Friedmann Equations, and Cosmological Constant.
If you can follow the maths, this excerpt may shed some light on the density parameters:
https://ned.ipac.caltech.edu/level5/Peacock/Peacock3_2.html
Anything detailing derivation of Friedmann equations should cover those as well.

If you're looking for resources on more general cosmology, Ned Wright's tutorial is a good start:
http://www.astro.ucla.edu/~wright/cosmolog.htm
A good introductory textbook is A.Liddle's 'Introduction to modern cosmology', which one can mostly follow without advanced maths knowledge.
Leonard Susskind's cosmology lectures are available on youtube: