Dark energy question

twofish-quant

I don't think this is right. In the Einstein equations, dark energy is just a constant pressure term, but in gravitationally bound objects it's balanced out by the stress-energy tensor term.

So the "gravity that is due to matter being there" does counteract the "pressure that is everywhere."

Also all of this assumes that dark energy is the cosmological constant which isn't absolute certain.

Personally, I prefer to call this "dark pressure" than "dark energy" since global energy isn't a well defined quantity in GR.

e.bar.goum

True enough.

I was deliberately conflating dark energy and the cosmological constant term since they seemed to be being used interchangeably (and seems to be used interchangeably in lay discussions everywhere at the moment) and I didn't want to introduce more confusion. My cosmology lecturer wouldn't approve.

twofish-quant

I worry that the explanation that dark energy is "energy that appears as space expands" is going to be one of those "almost but not quite correct" explanations that people will regret in a few years (like the metaphor that things "gain mass" as they get close to the speed of light).

e.bar.goum

Probably, but as with many things, it's very hard to explain properly without going into the formalism of it all. Which I'm happy to do, but isn't helpful when you're talking to people without high level maths/physics.

Chronos

Under the 'big rip' scenario dark energy will eventually overcome even atomic bonds. But, it appears we still have trillions of years before that becomes as worrisome as global economic trends.

e.bar.goum

However, this scenario has been shown to be rather unlikely, Chandra studies have shown that the strength of dark energy isn't changing with time -ie, it behaves very similarly to the cosmological constant term. See http://www.universetoday.com/22382/n...o-dark-energy/

And http://arxiv.org/abs/0812.2720

To quote the Nobel committee - the universe will end in ice.

Chronos

Agreed the big rip appears unlikely, but, always a conversation point. By most accounts it would not occur before most stars have already expired.

binbots

So I am still confused. Does the expansion of space have any effect on matter or gravity? On any scale?

nikkkom

Expansion of space seems to happen everywhere uniformly both in space and time, and VERY slowly. Basically, within Earth, or even entire Solar System, it is so slow that it's hardly measurable. It may create enough "new" space between Sun and Earth to move Earth a few microns farther (a simplified picture, the reality is a bit more complex).

But when objects are millions of parsecs away and continue to fly away from each other, then those microns do add up to measurable values, which eventually make these objects seem to fly away from each other faster with time, not slower as we would expect from gravitation alone.

bm0p700f

Dark energy exisits everywhere and casue space to expand everywhere at least that what I understand.

If you have a steel metre stick and heat by 1 K you get a 11 microns extra length not really that noticeable unless you have the right kit. Have a million sticks and you get 11 metres expansion. You could detect that just by counting although it would take a long time. That how I visulise why dark energy expansion is only visible at larger scales. However if the rate of expansion get too high even atoms can become unbound hence the big rip idea.

binbots

Does this mean that the earth itself is actually expanding aswell?

bm0p700f

I think that what post #26 and mine #27 are getting at but the expansion is too small to measure. Also matter can move in the new space so the volume the earth occupies may not change for that reason at all.

Drakkith

No, the Earth is not expanding.

Tea Jay

If every point, including inside the earth, were expanding, would the earth be expanding as well? I'm assuming that due to the presence of gravity, etc, that we would not be able to measure this, but if the "fabric with the grid we're all drawn on" is expanding, that should include everything.

If the graph paper we are using for the analogy is drawn onto spacetime fabric, that has grids that are governed by local gravitational forces for example, as opposed to a logarithmic or linear rule, etc - Then the grid is influenced by all of the involved factors...but perhaps approaching zero impact for the earth itself.

So, the impression I have is that the effect is universal, but the scale of the impact is highly variable.

Drakkith

No, as gravity is pulling everything together. Think of it as a massive force pulling everything in to itself, and a much smaller force pushing outwards. The massive pulling force wins out. It might be 0.000000001% weaker, but it still overwhelmingly dominates.

Edit: Also, lets be clear. Are you asking about the Earth EXPANDING, or simply being slightly larger than it is without expansion but a static size?

korben dallas

Everything except matter expands!