# Dark Energy

1. Jul 1, 2011

### Cbray

Quick question - Is it that the bigger the space in between two bodies makes bigger room for more dark energy to allocate more force, making the two bodies distant of each other faster than the bodies such as me and my friend standing a meter away from each other?

2. Jul 1, 2011

### bcrowell

Staff Emeritus
Material objects are irrelevant, and there is no force involved. It's just a dynamical effect of spacetime itself, sort of like gravitational waves. Material objects are only relevant because you can use them as test particles to measure the effect (again, like gravitational waves).

3. Jul 1, 2011

### sankalpmittal

Yes , absolutely .

The reason for this may be the allocation of dark energy but your argument can easily be solved by the simple universal law of gravitation .

F=GMm/r2

So if mass of two bodies remain constant then if the square of the distance between the two bodies is inversely proportional to the force of attraction between then .

Hence the force between Moon and Earth is same .

Still I may be wrong . I am also 14 .

4. Jul 1, 2011

### Cbray

Haha, thanks - How is India and school? Are you thinking of coming to Australia for University?

5. Jul 2, 2011

### sankalpmittal

See your visitor message . :)

6. Jul 2, 2011

### Drakkith

Staff Emeritus
I think bcrowell covered it nicely. There is no force against the matter and energy of the universe, but merely an effect of spacetime itself. The larger the distance the more spacetime there is between objects, which increases the effect between points.

7. Jul 2, 2011

### author40

What is the prrof that there is dark energy. Is it the fact that greater red shift occurs at greater distances from us?

8. Jul 2, 2011

### Drakkith

Staff Emeritus
The evidence that there is greater redshift with increasing distance implies that the expansion of the universe is increasing. This dark energy is one reason theorized to cause this acceleration.
Edit: Actually, I don't know the exact way of determining the increase in acceleration, as I just confused myself trying to type out a reason...but in the end that is the conclusion we arrived to.

9. Jul 3, 2011

### author40

V = dx/dt and a = dV/dt
This is basic stuff. Where is the dt in the evidence. I only see dv/dx. dv/dx is not evidence of acceleration but rather evidence that higher velocity exists at higher x.
What do you say?

10. Jul 3, 2011

### bcrowell

Staff Emeritus
11. Jul 3, 2011

### marcus

There is no proof that there is an actual energy. There is proof of slightly accelerating expansion, or small extra curvature. Dark energy is simply one way of imagining what could cause this acceleration. Another reasonable explanation is a small positive cosmological constant in the classical 1915 law of gravity. "Dark energy" appeals to the popular imagination and is more common in journalism and mass market books. If you read NASA reports and professional cosmo lit you tend to see the term "cosmological constant" used as well. This puts less spin on it and sticks closer to the facts.

The evidence for acceleration came out first in 1998:
http://arxiv.org/abs/astro-ph/9805201

Drakkith, perhaps the simplest thing is just to quote the abstract of the original paper published by the High-Z Supernova Search Team
==quote http://arxiv.org/abs/astro-ph/9805201 ==

The distances of the high-redshift SNe Ia are, on average, 10% to 15% farther than expected in a low mass density (Omega_M=0.2) Universe without a cosmological constant. ...

==endquote==
Those are the most resounding words I can think of spoken in observational cosmology ever since 1965 when Penzias Wilson saw the Ancient Light and Robert Dicke told his team "Boys, we've been scooped!"

That is, with a zero cosmo constant the model didn't fit. To make the model fit you needed to put in a small cosmo constant. (No jazzy talk about "dark energy" just adjust the constant Einstein originally put in the statement of the law.)

The evidence was that they found that at the new farther distances they were looking the redshift at a given distance was LESS than expected. That was because they were looking farther back in time, and less accel had happened, so the recession speed was less.

Hubble's original measurements were of rather close by stuff. He found that redshift increased approximately proportional to distance. The precise Hubble law relates presentday distance to presentday rate of expansion of that distance. But for comparatively small distances and small lookback times, the redshift is approximately proportional to the expansion rate. So you get an approximately linear relation between distance and redshift.
This is the roughly linear relation that the High-Z Supernova Team expected to see continue as they looked farther out, measured greater distances using Supernovae, and logged correspondingly greater redshifts.

They and everybody else were assuming a zero cosmo constant in their model, in those days. But as you see from the 1998 paper they found the redshifts 10% or 15% smaller than they expected at each given distance in the range they were sampling.

To put it simply, they found some acceleration which hadn't happened yet. Because they were checking deeper into the past (with an independent yardstick) than people previously had.

Last edited: Jul 3, 2011
12. Jul 3, 2011

### author40

I thought I had read within the past year that Type Ia supernovae (SNe Ia) have some variability due to variations in the amount of heavy metals. Is that true?