**Ask a monkey a physics question thread**

[jwilliams]

What is the dark energy? I know this is not the easy question, but I really want to get the answer.

[Renton]

What is the dark energy? I know this is not the easy question, but I really want to get the answer.

The farthest that my layman's brain could get on it is that it's the default tendency of empty space to expand when not acted upon by a dominant gravity field. It's more a property of space itself than it is actual stuff i.e. dark matter. Dark matter to me is far more mysterious.

[MadMojoMonkey]

The farthest that my layman's brain could get on it is that it's the default tendency of empty space to expand when not acted upon by a dominant gravity field. It's more a property of space itself than it is actual stuff i.e. dark matter. Dark matter to me is far more mysterious.

I'd love to know if you have a source for the underlined bit. As I understand it, this has not been shown.

The rest is hypothetical, but I hesitate to even argue with it, because it's at least plausible that you're on he right track.


***
This kinda ties in with BID's question. If dark energy is a property of spacetime, then it seems like 'empty space' is ultimately a perpetual motion machine... While this is fine with me personally, the physicist in me is appalled. Violating the conservation of energy is kinda unthinkable to me at this point in my life. It's so evident to me that this conservation holds in our universe that I cannot truly believe that there is some mystical breaking of that law going on all around me.

It is a purely emotional response, though.

[Renton]

I'd love to know if you have a source for the underlined bit. As I understand it, this has not been shown.

Sorry, probably me not being a scientist and saying shit wrong. What I meant to say is that it seems to only expand intergalactic space. As I understand it, dark energy isn't causing the volume of the earth to expand, nor the space between the earth and the sun, nor the space within the galaxy. I guess the force is acting weakly in opposition to gravity. Am I correct?

[MadMojoMonkey]

Sorry, probably me not being a scientist and saying shit wrong. What I meant to say is that it seems to only expand intergalactic space. As I understand it, dark energy isn't causing the volume of the earth to expand, nor the space between the earth and the sun, nor the space within the galaxy. I guess the force is acting weakly in opposition to gravity. Am I correct?

This is correct. The energy density of the dark energy is overwhelmed by gravitational attraction over these kinds of distances.

Perhaps over even larger distances. The distance between the Milky Way and Andromeda is not affected by dark energy. In fact, the entire Virgo Supercluster of galaxies may be unaffected by dark energy.

It is a very weak effect. It's just that the universe is quite big. Have I mentioned that? It deserves mentioning again.

[MadMojoMonkey]

What is the dark energy? I know this is not the easy question, but I really want to get the answer.

Well, it's easy in the sense that it's not too hard to say, "I don't know."

It's not just me, no one really knows what it is. So why are all these supposedly "smart" physics type people going on about it if they don't know what it is?

First of all, this is our normal state of being. Every answer leads to more questions (at least so far). It is common for people, even scientists, to lose sight of this when discussing the discoveries of the scientific process.

What we know that by our observations, the universe is not only expanding, but accelerating. Not only is it accelerating, which we would expect, due to all the gravitational acceleration sources about the place, but it's accelerating in the wrong direction... away from the attraction of gravity.

Expanding is easy enough to explain. There was a Big Bang... stuff is still 'sploding out from that bang. Did I mention it was Big? It was truly the Biggest of Bangs.

Now the universe is full of all this mass, and mass causes gravitational attraction. So we expect the rate of the expansion to be slowing down. However, we observe the rate of the expansion speeding up. That requires energy.

What is the source of this energy? We don't know. We have applied the name "dark energy" as a placeholder name in the mean time.

We don't know what it is, or even if it's a single thing or many things. We only know we observe a property of our universe which is beyond the ability of our current model to understand.