Dark Energy

[curium96]

Can the expansion of the universe be explained in terms of time dilation?

If we accept observational evidence at face value, it appears that all the matter and energy in the universe was originally confined to an extremely small space.

Would the extremely high density of the early observable universe not lead logically to a universe that, given the initiation of expansion, expands at an accelerating (but also uneven) rate in accordance with the distribution of matter and energy in space?

Where there is less gravity, time moves faster, right? So wouldn't it make sense that the cosmic voids would expand at a faster and faster rate relative to the galactic centers of mass?

[Mordred]

Ordinarily if the mass/energy density distribution was non uniform you would get different regions with different time rates as per GR. We see that today with large scale structure formation, galaxies stars etc. 

 However the Cosmological principle of a homogeneous and isotropic uniform mass distribution still occurs at the large size scales. It's like looking at the waves of a lake  as you move further from the surface the more uniform the lakes look.

The uniform mass distribution becomes apparent on a scale of 100 Mpc (megaparsec) at the time of the CMB that scale is greatly reduced. However we cannot forget that dark matter fills up most of the universe and baryonic matter is only 3 percent of the mass.

Gravity itself However only attracts it doesn't expand. 

We also already take into account the result from matter forming into stars galaxies etc in the matter only solutions of the FLRW metric.  This is rather complex to understand but essentially a matter only universe can expand due to structure formation because the global mass distribution reduces due to matter forming those structures on the local scale.

So yes the mass distribution does contribute to expansion However that isn't the same as being due to time dilation ( I'm going to assume the time dilation statement is more a poor choice of descriptive for non uniform mass distribution).

Now dark energy is not the only contributor to expansion it's the current most dominant contributor..

Expansion also can be caused by matter, radiation (relativistic particles photons etc) spacetime curvature as well as the Cosmological constant. At one point (except the curvature term as k=0 for the near flat universe we see) each of these contributors was dominant.

The three eras are radiation dominant, matter dominant and Lambda dominant (Cosmological constant aka Dark energy).

 

In essence yes non uniform mass can cause expansion so you are correct however we already factor that detail in the FLRW metric via the equations of state 

See here for the equations of state

https://en.m.wikipedia.org/wiki/Equation_of_state_(cosmology)

Edited June 1 by Mordred

[curium96]

4 hours ago, Mordred said:

Ordinarily if the mass/energy density distribution was non uniform you would get different regions with different time rates as per GR. We see that today with large scale structure formation, galaxies stars etc. 

 However the Cosmological principle of a homogeneous and isotropic uniform mass distribution still occurs at the large size scales. It's like looking at the waves of a lake  as you move further from the surface the more uniform the lakes look.

The uniform mass distribution becomes apparent on a scale of 100 Mpc (megaparsec) at the time of the CMB that scale is greatly reduced. However we cannot forget that dark matter fills up most of the universe and baryonic matter is only 3 percent of the mass.

Gravity itself However only attracts it doesn't expand. 

We also already take into account the result from matter forming into stars galaxies etc in the matter only solutions of the FLRW metric.  This is rather complex to understand but essentially a matter only universe can expand due to structure formation because the global mass distribution reduces due to matter forming those structures on the local scale.

So yes the mass distribution does contribute to expansion However that isn't the same as being due to time dilation ( I'm going to assume the time dilation statement is more a poor choice of descriptive for non uniform mass distribution).

Now dark energy is not the only contributor to expansion it's the current most dominant contributor..

Expansion also can be caused by matter, radiation (relativistic particles photons etc) spacetime curvature as well as the Cosmological constant. At one point (except the curvature term as k=0 for the near flat universe we see) each of these contributors was dominant.

The three eras are radiation dominant, matter dominant and Lambda dominant (Cosmological constant aka Dark energy).

 

In essence yes non uniform mass can cause expansion so you are correct however we already factor that detail in the FLRW metric via the equations of state 

I appreciate all the info!

As you can tell, I'm not a scientist, just a curious bystander. I respect your offering your time to me.

It was not my intention to explain or even raise the question of what causes universal expansion (which seems to me a natural consequence of observation in an infinite universe, but I don't want to get sidetracked on that point). I specifically used the phrase "given the initiation of expansion..." in my initial comment. In other words, taking the causal mechanism of expansion for granted, can expansion then be explained in terms of time dilation?

It is my understanding that expansion is something that occurs in the spaces between matter and energy as opposed to within them, or am I misunderstanding?

My question, to clarify, is whether the absence of matter and energy in the Cosmic voids could cause a time dilation effect which would naturally lead to an accelerating expansion of the universe, as opposed to a steady expansion.

It seems like you are telling me that time dilation is already factored into the models, but on that point you have me confused.

Isn't the whole mystery of Dark Energy the fact that the Universe appears to be expanding at an accelerating rate, which was not predicted by theory but rather discovered through observation? Couldn't that reasonably be explained by differences in the relative passage of time in empty space as opposed to spaces occupied by mass?

[MigL]

5 hours ago, curium96 said:

Where there is less gravity, time moves faster, right?

The gravitational wells of galaxies are extremely 'shallow'; the galactic voids where expansion occurs, just slightly more so.
The dark energy term ( Cosmological Constant ? ) only slightly exceeds gravitational potential in the voids between clusters, but is not enough to overcome it at galactic scales.
The gravitational potential difference is not enough to account for sufficient time dilation which would explain expansion.

[Mordred]

18 hours ago, curium96 said:

I appreciate all the info!

As you can tell, I'm not a scientist, just a curious bystander. I respect your offering your time to me.

It was not my intention to explain or even raise the question of what causes universal expansion (which seems to me a natural consequence of observation in an infinite universe, but I don't want to get sidetracked on that point). I specifically used the phrase "given the initiation of expansion..." in my initial comment. In other words, taking the causal mechanism of expansion for granted, can expansion then be explained in terms of time dilation?

 

My question, to clarify, is whether the absence of matter and energy in the Cosmic voids could cause a time dilation effect which would naturally lead to an accelerating expansion of the universe, as opposed to a steady expansion.

 

No time dilation would not explain dark energy but don't feel bad this conjecture has already been considered and tested for. Back when we first discovered the accelerating expansion physicists looked at the possibility of being observer effect due to time dilation.

The problem is the cosmological constant is too constant to be explained by the non linear curve of time dilation formulas.

This turned out not to work but at least the good news is that your idea had merit. 

To add detail it was low redshift values for the late time accelerating portion of expansion ( Lambda dominant era) compared to the deceleration portion(matter dominant era) that showed that time dilation could not explain the cosmological constant. ( the transition between matter dominant and Lambda dominant occurs roughly when the universe is between 6 and 7 billion years old.) It is the redshift variations (which incorporates time dilation effects) that showed that time dilation could not explain the cosmological term w=-1.

In point of detail it was the redshift data between this transition which became further evidence supporting the existence of the cosmological term when initially it was felt it would support an observer effect due to time dilation.

( you get similar results from luminosity distance and angular size via distance relations ).

We have used all three methods above to try to eliminate or validate the cosmological term at various redshift distances and at different matter/radiation densities 

Edited June 2 by Mordred

[curium96]

On 6/2/2024 at 9:47 AM, Mordred said:

No time dilation would not explain dark energy but don't feel bad this conjecture has already been considered and tested for. Back when we first discovered the accelerating expansion physicists looked at the possibility of being observer effect due to time dilation.

The problem is the cosmological constant is too constant to be explained by the non linear curve of time dilation formulas.

This turned out not to work but at least the good news is that your idea had merit. 

To add detail it was low redshift values for the late time accelerating portion of expansion ( Lambda dominant era) compared to the deceleration portion(matter dominant era) that showed that time dilation could not explain the cosmological constant. ( the transition between matter dominant and Lambda dominant occurs roughly when the universe is between 6 and 7 billion years old.) It is the redshift variations (which incorporates time dilation effects) that showed that time dilation could not explain the cosmological term w=-1.

In point of detail it was the redshift data between this transition which became further evidence supporting the existence of the cosmological term when initially it was felt it would support an observer effect due to time dilation.

( you get similar results from luminosity distance and angular size via distance relations ).

We have used all three methods above to try to eliminate or validate the cosmological term at various redshift distances and at different matter/radiation densities 

We as human beings seem to share a connection in the form of our observed reality.

This is evidenced by the indisputable fact that our spacetime heritage can be traced back, irrespective of the individual perspective, to a common origin.

If the nature of the common origin is infinite, there would be no upwards or downwards limit to the frequency of light, and the observable universe would be defined in terms of the visible spectrum of light shared by the group (the parameters of shared experience).

Time dilation as a consequence of perspective in relation to mass and motion would lead automatically to an accelerating expansion of spacetime, in other words, this visible effect should have been predicted given a sufficient understanding of observation, and this is evident in the fact that the effect is observed.

The sum of human efforts in observation is directly tethered to the mass and motion of the Earth in relation to the Solar System, the Solar System in relation to the Milky Way, the Milky Way in relation to the Local Cluster, and so on and so forth.

You are telling me that you know for a fact that the effects of time dilation on an infinite variety of scales is insufficient to explain or predict the phenomenon in question?

I know there is much I do not know. I hope that you will understand when I state plainly that I remain skeptical and unconvinced.

On 6/1/2024 at 4:46 PM, MigL said:

The gravitational wells of galaxies are extremely 'shallow'; the galactic voids where expansion occurs, just slightly more so.
The dark energy term ( Cosmological Constant ? ) only slightly exceeds gravitational potential in the voids between clusters, but is not enough to overcome it at galactic scales.
The gravitational potential difference is not enough to account for sufficient time dilation which would explain expansion.

Again, and I thought I said this, but I know words are trickier than they seem:

I am not trying to explain expansion. I explicitly said, as a basic premise, that this discussion will be based in the idea of observation as a fact (e.g. empirical science), therefore we will take expansion for granted as a prerequisite of the conversation as such.

The question I raised, to clarify, is whether or not it is a matter of basic logic that the universe should expand at a static or accelerating rate given the nature of observed space and time.

Obviously, given massive areas in spacetime which are relatively void of matter and energy relative to other areas which are relatively dense in terms of matter and energy, we would observe expansion which can be predicted in terms of this discrepancy.

In other words, try assuming space that appears void of matter and energy is still massive, and think about what that means.

"Problem solved.

Or, am I missing something?"

LVW

[MigL]

There are two types of time dilation; one due to motion relative to the observer, and the other due to depth in a gravitational potential well relative to the observer.

In the first case we would notice time dilation of distant galaxies if they were moving away from us, but this effect would be non-linear ( as Mordred has explained ) since dilation increases asymptotically as c is approached. The Hubble expansion constant is, however, just that, constant, and mostly linear ( except for the slight up-tic due to accelerated expansion ).

The second case would involve everything we observe, in all directions, being deeper in a gravity well than we are, in order to see a time dilated red shift; and that makes no sense due to the shell theorem.

If you know of another type of time dilation, please educate us ...

[Mordred]

20 hours ago, curium96 said:

 

You are telling me that you know for a fact that the effects of time dilation on an infinite variety of scales is insufficient to explain or predict the phenomenon in question?

Yes I know for a fact that time dilation cannot account for the cosmological constant.  When you do the math you will find that you get the wrong ratios of change. You will also discover as Migl mentioned Newtons Shell theorem that time dilation isn't involved when you have a homogeneous and isotropic mass distribution.

Let's do a simple example if you use recessive velocity from Hubbles Law

\[v_{r}=H_0d\] and apply that velocity term to the time dilation formula  you will think it will work out. However once you get to the Hubble horizon it approach infinity.

The Observable universe itself is far larger than the Hubble horizon. The recessive velocity formula will have a value for recessive velocity of 2.3 c. Thankfully recessive velocity is not a true velocity but an apparent velocity due to separation distance. As our time dilation formula would break down when velocity is greater than c. Leading to causality issues. Our light cone would be restricted to the Hubble horizon. The recessive velocity equals c at the hubble horizon. We observe beyond that. Does that help ?

Edited June 7 by Mordred