Dark Energy

[Agent Smith]

From what I know, dark energy is the posited energy driving cosmic expansion. This seems to imply cosmic expansion is work (requiring energy), but then I was told cosmic expansion is the space stretching and space has no mass. How is work being done in cosmic expansion when no mass in involved, a work that requires dark energy?

[Janus]

Dark energy is not required for the universe to expand, it is needed to explain why the rate of expansion has been increasing over time.

The original assumption was that, starting from some initial impetus, the universe began to expand, and that over time, the mutual gravitational attraction of its matter would slow the expansion rate. 

From this there were two possibilities:

1. Gravity would eventually win, the universe would stop expanding and then collapse back in on itself.

2. The universe didn't have quite enough mass to stop the expansion completely, and it would continue to expand forever.

The study that opened the whole dark energy can of worms was trying to determine which of these was true.

What they did was measure the recession velocity of various galaxies at various distances.  Since the further a galaxy is from us the longer it took its light to reach us, you were looking further and further into the past as you looked at more and more distant galaxies.

You then plot a distance/recession graph.  If the rate of expansion had been constant over time, you would get a straight line. Of course, this was not what they expected to see, they expected to get a curve, the degree of which would indicate how fast the expansion was slowing. 

They got a curve, but one that curved the opposite direction, indicating that the expansion rate had increased over time.  Something was causing it to speed up.

They settled on calling it "dark energy" just for the simple fact that the term "dark matter" had already been in usage (And this is the only thing the two have in common).

As to the exact nature of dark energy, it is still an unsolved mystery.

[Agent Smith]

43 minutes ago, Janus said:

Dark energy is not required for the universe to expand, it is needed to explain why the rate of expansion has been increasing over time.

The original assumption was that, starting from some initial impetus, the universe began to expand, and that over time, the mutual gravitational attraction of its matter would slow the expansion rate. 

From this there were two possibilities:

1. Gravity would eventually win, the universe would stop expanding and then collapse back in on itself.

2. The universe didn't have quite enough mass to stop the expansion completely, and it would continue to expand forever.

The study that opened the whole dark energy can of worms was trying to determine which of these was true.

What they did was measure the recession velocity of various galaxies at various distances.  Since the further a galaxy is from us the longer it took its light to reach us, you were looking further and further into the past as you looked at more and more distant galaxies.

You then plot a distance/recession graph.  If the rate of expansion had been constant over time, you would get a straight line. Of course, this was not what they expected to see, they expected to get a curve, the degree of which would indicate how fast the expansion was slowing. 

They got a curve, but one that curved the opposite direction, indicating that the expansion rate had increased over time.  Something was causing it to speed up.

They settled on calling it "dark energy" just for the simple fact that the term "dark matter" had already been in usage (And this is the only thing the two have in common).

As to the exact nature of dark energy, it is still an unsolved mystery.

Okay Work = Fd 

F = ma

 

Work = mad 

 

The expansion is acclerating, but there's no mass i.e. Work = 0 × a × d = 0 joules.

[Genady]

AFAIK, expansion of the universe is a GR effect, and it cannot be consistently described in term of Newtonian mechanics. Specifically, there is no Newtonian 'F' in GR formulation.

Einstein field equations, EFE, relate geometry of spacetime (LHS of the equations) with distribution of energy-momentum in the spacetime (RHS). To get an accelerated expansion in these equations, you need either an extra term on the LHS or an extra term on the RHS. The former leads to inclusion of a "cosmological constant" in the EFE. The latter leads to inclusion of a peculiar energy source in the universe, the "dark energy."

[Agent Smith]

On 1/19/2023 at 1:04 AM, Genady said:

AFAIK, expansion of the universe is a GR effect, and it cannot be consistently described in term of Newtonian mechanics. Specifically, there is no Newtonian 'F' in GR formulation.

Einstein field equations, EFE, relate geometry of spacetime (LHS of the equations) with distribution of energy-momentum in the spacetime (RHS). To get an accelerated expansion in these equations, you need either an extra term on the LHS or an extra term on the RHS. The former leads to inclusion of a "cosmological constant" in the EFE. The latter leads to inclusion of a peculiar energy source in the universe, the "dark energy."

Could we be on a wild goose chase?

[Genady]

23 minutes ago, Agent Smith said:

Could we be on a wild goose chase?

Why? Is there a problem?

[Agent Smith]

46 minutes ago, Genady said:

Why? Is there a problem?

From a Newtonian perspective yes. Kindly go through the thread, it's short.

[Mordred]

The cosmological contribution is a kinetic energy term  if you take the critical density term. When you solve the critical density formula you will find it will give an answer that will be approximately 10^{-10} joules/cubic metre.

 much like photons can power a solar sail even though photons have no inavariant mass the photon has momentum.

Edited January 20, 2023 by Mordred

[Genady]

2 minutes ago, Agent Smith said:

From a Newtonian perspective yes. Kindly go through the thread, it's short.

But we already have a correction to Newtonian perspective. It is GR. Newtonian perspective doesn't work in this domain.

[Agent Smith]

30 minutes ago, Genady said:

But we already have a correction to Newtonian perspective. It is GR. Newtonian perspective doesn't work in this domain.

So, from a Newtonian perspective, dark energy is a wild goose.

[Genady]

33 minutes ago, Agent Smith said:

So, from a Newtonian perspective, dark energy is a wild goose.

No, it is not.

A Newtonian perspective would need to find an energy source causing the observed acceleration of galaxies. You said above that there is no mass, but this is not so. Galaxies have mass.

You've also said above that the universe expansion is "space stretching", but it is not so from a Newtonian perspective. It is so from the GR perspective. 

[Agent Smith]

38 minutes ago, Genady said:

No, it is not.

A Newtonian perspective would need to find an energy source causing the observed acceleration of galaxies. You said above that there is no mass, but this is not so. Galaxies have mass.

You've also said above that the universe expansion is "space stretching", but it is not so from a Newtonian perspective. It is so from the GR perspective. 

Some of these galaxies are moving away from us at speeds exceeding the speed of light I believe. Does that mean anything? Hence the space stretching patch I suppose. From Newton's perspective then the kinetic energy of some of these galaxies = [math]\frac{1}{2}mc^2[/math] which in Einstein's theory should be impossible. 

[Genady]

2 minutes ago, Agent Smith said:

Some of these galaxies are moving away from us at speeds exceeding the speed of light I believe. Does that mean anything? Hence the space stretching patch I suppose. From Newton's perspective then the kinetic energy of some of these galaxies = 12mc2 which in Einstein's theory should be impossible. 

One can get many inconsistencies if one mixes freely these two different models. To be consistent, one should not generally do it, although it works sometimes if applied very carefully and in a restricted domain.

In particular, if you use Newtonian perspective, then there is no speed limit. And if use GR, then the galaxies do not move faster than light. Each picture is consistent within itself. 

[Bufofrog]

11 minutes ago, Agent Smith said:

Some of these galaxies are moving away from us at speeds exceeding the speed of light I believe.

No, the galaxies are not moving away from us through space at that speed. 

12 minutes ago, Agent Smith said:

From Newton's perspective then the kinetic energy of some of these galaxies = 1/2mc^2 which in Einstein's theory should be impossible. 

 The galaxies are not moving through space at that speed relative to us so the KE equation you presented does not apply.

[Agent Smith]

On 1/20/2023 at 9:00 PM, Genady said:

One can get many inconsistencies if one mixes freely these two different models. To be consistent, one should not generally do it, although it works sometimes if applied very carefully and in a restricted domain.

In particular, if you use Newtonian perspective, then there is no speed limit. And if use GR, then the galaxies do not move faster than light. Each picture is consistent within itself. 

So Newton is inconsistent with Einstein. So, depending on the theory we use, either no work is being done or some work is being done. 🧒

On 1/20/2023 at 9:00 PM, Bufofrog said:

No, the galaxies are not moving away from us through space at that speed. 

 The galaxies are not moving through space at that speed relative to us so the KE equation you presented does not apply.

Thank you for the reply

[Genady]

4 minutes ago, Agent Smith said:

So Newton is inconsistent with Einstein.

Yes.

[studiot]

17 minutes ago, Agent Smith said:

So Newton is inconsistent with Einstein. So, depending on the theory we use, either no work is being done or some work is being done.

No that is not true.

 

Einstein's theories are more detailed than Newton's and cover more situations, but reduce (simplify) to Newton's, when the circumstances (situations) are the same.

In other words they extend Newton.

 

This fact has always played a major part in the acceptance and preference for Einstein's General Relativity over other competing explanations of the observed facts.

 

And yes I know that we have now discovered situations where Newton's analysis is inadequate.

[Genady]

36 minutes ago, Agent Smith said:

depending on the theory we use, either no work is being done or some work is being done

Not exactly. Both require a source of energy to account for the accelerated expansion, albeit for different reasons.