# No need for dark energy , gravity will suffice.

1. Feb 2, 2009

### Peter Watkins

No need for "dark energy", gravity will suffice.

In 1929 it was observed that, with rare exception, galaxies in all directions are exhibiting degrees of red-shift that increase with distance. This information alone is sufficient to, (a), describe the structure of the universe, (b), state that collapse is inevitable, and, (c), predict that the rate at which galaxies separate will increase. This being the case, why was the "dark energy" theory ever put forward? It is totally unnecessary.

What the "faster with distance" view shows is that the rate of expansion is slowing. As this continues, galaxies will move apart at an increasing rate. This is what I mean by unnecessary. It is the restraint of gravity that is causing the rate of galaxy separation to increase. This is why it can be stated that collapse is inevitable

Last edited: Feb 3, 2009
2. Feb 2, 2009

### ThalorB

Re: No need for "dark energy", gravity will suffice.

I recall reading something not too long ago stating that Einstein was never satisfied with his general theory of relativity because it included a "fudge" factor: I believe it was a cosmological constant. Some have found that calculations considering dark energy can produce that value almost exactly. So, it's not "necessary," unless you are looking for complete understanding of the universe. For example, is it always constant?

3. Feb 2, 2009

### Nabeshin

Re: No need for "dark energy", gravity will suffice.

It is not unnecessary. Hubble's expanding universe model predicts that the recession velocity of galaxies is strictly tied to its distance. However, we observe the expansion to actually be increasing in rate (not predicted in the older models).

(Correct me if I'm wrong but I believe that we see deviations in the recession velocity for very far away galaxies, indicating that in the past they were moving slower than they are today, indicating an accelerated motion?)

4. Feb 2, 2009

### Janus

Staff Emeritus
Re: No need for "dark energy", gravity will suffice.

Simply this: The receding galaxies all gravitationally attract with each other. This acts against the expansion. IOW, the mutual gravitational attraction of the Universe tends to want to pullit together. Of course, as the galaxies get further apart, this gravitational attraction gets weaker.

So, the question became, Is the universe expanding fat enough that the distance between the galaxies grows at a great enough rate that the gravitational attraction weakens faster than it can slow down the expansion, or will gravity eventually stop the expansion and cause the universe to collapse?

In both cases you would expect the expansion to slow with time, but with the first, it never slows to zero.

In the 1990's observations were made to test which of these possible futures the universe had. As one looks out at the universe, the further away you look, the further back in time you look. If you plot a number of galaxies distances against how fast they are receding, you can get a graph that shows how the universe has expanded over time. This graph would show the rate at which the expansion was slowing and tell us if it could be expected to continue doing so.

The surprise came when it was found that the graph didn't show the universe's expansion slowing at all, but speeding up instead. It was the same as throwing a ball in the air and expecting to to slow down as it climbed higher, but instead seeing the ball climb at ever increasing speeds. And just like there would have to be something actively pushing the ball upwards for it to behave this way, the universe would need something actively pushing it apart for it to behave as the observations showed. This "something" was labeled Dark Energy.

5. Feb 2, 2009

### Chronos

Re: No need for "dark energy", gravity will suffice.

As a footnote to Janus, supernova studies by Perlmutter pushed the dark energy hypothesis into the mainstream - e.g., http://arxiv.org/abs/astro-ph/0303428

6. Feb 3, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

The point is that gravity, when combined with a model for the contents of the universe, provides a very specific prediction for the relationship between redshift and distance (among a great many other things which I won't go into here...). If we assume that normal matter plus dark matter are the only contents in the universe, we get the wrong answer. Clearly this means one of two things:
1. Our model of gravity is incorrect a very large distances (potential solution: modified gravity).
2. Our understanding of the contents of the universe is incorrect (potential solution: dark energy).

Both are being investigated.

Last edited: Feb 3, 2009
7. Feb 10, 2009

### Peter Watkins

Re: No need for "dark energy", gravity will suffice.

My whole point is that the increase in the rate of recession, or separation from us, is not due to an increase in the rate of universal expansion but rather, a decrease in the rate of expansion due to gravitational drag. That is why "dark energy" is not necessary.

8. Feb 10, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

Nobody's going to take you seriously without a well-motivated mathematical model that describes this expansion and also fits with current experiment. And even then, the fact remains that the experiments to date simply aren't good enough to distinguish between different potential explanations for the accelerated expansion, so unless your idea here has significantly fewer hypothetical entities than the cosmological constant (which only has one additional degree of freedom), there's no way anybody is going to think it likely to be true.

9. Feb 11, 2009

### mysearch

Re: No need for "dark energy", gravity will suffice.

Purely for my own education, I am trying to understand which accepted model is said to explain the expansion of the universe. As I understand it, so far, the following equation is used by many cosmology calculators to derive the expansion of the universe with time as a function of the energy density make-up:

$$\Delta t = \frac {\Delta a}{aH_0 \sqrt{ (\Omega_M /a^3) + (\Omega_R /a^4) +(\Omega_\Lambda) } }$$

1. This energy-density model appears to only include of baryons matter, cold dark matter, radiation and dark energy, but does not seem to explain how any expansion took place during the 1st 7 billion years.

$$H^2 = \left( \frac {\dot a}{a} \right)^2 = \frac{8}{3} \pi G( \rho_m + \rho_{cdm} + \rho_\lambda + \rho_\Lambda )$$

2. Based on the equation above, why is Friedmann’s equation thought to define an expansive velocity as implicit within the Hubble constant (H), when the energy density of each component would seem to be describing gravitation collapse?

$$\left( \frac {\ddot a}{a} \right)^2 = -\frac{4}{3} \pi G( \rho_m + \rho_{cdm} + 2 \rho_\lambda - 2 \rho_\Lambda )$$

3. In addition, is the acceleration equation above, derived from the Friedmann equation, not implying that the accelerated expansion of the universe would be negative, i.e. collapsing, until the negative pressure of dark energy exceeded the normal gravitational effects, i.e. post + 7 billion years?

4. If the universe is homogeneous, at the very large scale, and for most of its existence has had a very low energy density per unit volume of space, wouldn’t the universe, as a whole, approximate to a weak gravitational field model without some overall gravitational centre?

5. Can this large-scale homogeneous model be said to have any other centre of gravity, which would explain the overall gravitational slow-down of expansion as implied by Janus’ statement?

6. What is meant by the expansion of space that results in distance between galaxies increasing, i.e. if gravity is no longer described as a force, what geodesic curvature is causing the galaxies to roll away from each other?​
I am not trying to suggest an alternative model, simply trying to understand the details of the current accepted model. Thanks

10. Feb 11, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

How does it not explain the expansion? The expansion is explicitly given by the density of these components through Friedmann's equations.

As the matter dilutes, the Hubble parameter H approaches a constant value. H is defined as:
$$H \equiv \frac{1}{a}\frac{da}{dt}$$

So if H = constant, we have:
$$\frac{da}{dt} = H_0 a$$

...which is the equation for exponential growth, meaning:

$$a(t) = a(0) e^{H_0 t}$$

Right, after inflation ended, the expansion of the universe decelerated until relatively recently.

There is no gravitational center in the big bang theory.

I don't get what you're asking.

Well, if you write down the FRW metric, there exist timelike geodesics where objects are stationary with respect to the coordinate system. It's really that simple.

11. Feb 11, 2009

### mysearch

Re: No need for "dark energy", gravity will suffice.

Hi Chalnoth,
Thanks for the response. Let me try to clarify some of the issues raised:
$$\Delta t = \frac {\Delta a}{aH_0 \sqrt{ (\Omega_M /a^3) + (\Omega_R /a^4) +(\Omega_\Lambda) } }$$

When I look at this equation, the Omega components appear to relate to energy density, which in the case of matter and radiation have no expansive properties, at least, within the general assumptions of the model after matter-radiation decoupling at +380,000 years. Dark energy is virtually a negligible issue until +5 billion years. As such, the equation seems more applicable to describing the rate of expansion in reverse, i.e. gravitational collapse, rather than expansion, especially as all components are added with the same sign.
I don’t disagree that there is an exponential rate involved, but what puzzles me is what physical process describes this as exponential rate as expansive, when all the active components, in the 1st 7 billion years, seem to be associated with gravitational attraction, i.e. collapse.
My issue here is slightly tangential. As I have understood the expansion model, matter in the form of galaxies do not expand apart by travelling through space, but rather by the initial space that separates them continuing to uniformly expand per unit volume. This seem to suggest that ‘something’ had to actively continue to expand each unit volume throughout the 1st 7 billion years, as I don’t see that expansive momentum attributed to initial inflation is applicable to this model.
I accept that this is the conclusion of the standard model. However, I would like to better understand how gravitational slowdown of the universe, as a whole, works without any notion of a centre of gravity.
If I assume a spatially flat model, i.e. k=0, the only geodesic seems to be associated with spacetime. For example, 2 photons initially travelling in parallel to each will eventually diverge in an expanding universe as a function of time. This seems to be the main conclusion of the FRW metric, especially if k=0. However, this appears to be very different to the curvature of spacetime due to an extreme gravitational centre of mass like a black hole. Again, this is why I was raising issues that suggested that a large-scale homogeneous model would appear to approximate a weak gravitational field model without any overall gravitational centre.

Again, I would like to stress that I have no pet theory of my own and the only reason for raising these issues is to get a better handle on the accepted model. Thanks

12. Feb 11, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

I really don't see why you find this disturbing. Those component dilute away, and when the cosmological constant remains, it takes over and drives an accelerated expansion.

I think what you're looking for are the initial conditions of the system. Those were set by cosmic inflation, which is still poorly-understood, but appears to have been a period of very rapidly accelerated expansion (with a vastly greater Hubble factor than we have now) that occurred right at the limit of the earliest portion of our universe we can see.

I guess I don't really know what to say, except that when you construct a stress-energy tensor for a homogeneous, isotropic universe (which becomes diagonal with only two different components: energy density and pressure), and plug in the properties of the various types of matter, for normal matter, dark matter, and radiation domination, you get a decelerated expansion. The math just works. I don't really see a way to explain this other than mentioning that fact, though perhaps you might gain some satisfaction from working through it yourself.

Well, while the spatial curvature of a k=0 FRW universe is indeed zero, it has space-time curvature, which is just another way of describing the expansion.

13. Feb 13, 2009

### PeopleSmoks

Re: No need for "dark energy", gravity will suffice.

Greetings,

I'm new to this forum, and don't claim to be a physicist or cosmologist, however I am fascinated by these subjects and hope my questions and comments aren't seen as silly.

A friend and I have often discussed the theory of chaos. Chaos simply being 'that without structure' and nothing more. When you speak of dark energy, I assume you refer to the energy of dark matter which was recently introduced as a means to account for a lack of mass in the comos. The origins of dark matter are, to my knowledge, yet unknown. My thoughts are that dark matter is a form of matter that is within chaos, but is neutral to both chaos and the cosmos which is what allows our universe to move through it.

I also buy into the theory that expansion will eventually slow, stop and the universe will collapse in on itself. I see three factors as contributing to this eventual collapse.

First, I believe that there is a black hole at the center of the cosmos which is the remnants of the singularity of the big bang acting in a manner similar to any star gone super nova. Having released it's energy to form the cosmos, it itself collapsed to form a gravity well. As it increases in mass by 'swallowing' matter, it's mass will increase, and with an increase in mass, an increase in gravity.

Second, I believe that there is little if any new energy created beyond that of the original big bang and that everything we see today is occurring as a result of that initial release of energy.

Third, I see the expansion into chaos as being resistive. Anything within chaos that is not dark matter is being pushed against and the rule of equal but opposite reaction fits my thinking. Eventually, enough pressure will build to were there is a push inward, with equal force to that which is currently pushing out.

In conclusion, I also believe that as chaos pushes, the central black hole's pull will increase exponentially to reach the speed of light to reform the singularity, and that the pressure of chaos on this sigularity will start the whole process again.

I strongly hope for comments and constructive criticism of my thoughts.

14. Feb 13, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

The difficulty with this is that it's just not well-defined enough to actually determine what you mean by it. Any answer for what dark matter actually is must necessarily be mathematical. Mathematics is the only language that is specific enough to accurately describe the nature of dark matter.

The evidence does not support this view.

There is no center of the universe. Far away from a black hole, its gravitational field is nothing special. If, for example, our Sun was suddenly replaced by an equal-mass black hole, the Earth would continue on its orbit precisely as before.

Finally, the big bang didn't act anything like a star going supernova. They aren't even comparable.

Based upon what?

Except if the cause of the accelerated expansion is actually dark energy, then the nature of the pressure is what's driving the expansion in the first place.

15. Feb 13, 2009

### PeopleSmoks

Re: No need for "dark energy", gravity will suffice.

Chalnoth, I honestly appreciate your response. What I mean by chaos in my example is what the universe is expanding into. Now, from what I have read on the subject of dark matter, this is nothing more than another assumption needed to explain a lack of mass in the cosmos and it's true nature can't be measured aside from gravitational effects, the same way dark energy is used to fill gaps in so many theories. Really, I am only trying to propose a possibility as to it's origin and another alternative to how it fits in. You're more than right that mathematics is necessary to find the nature of dark matter. It seems no one has done it yet?

Einstein's adjustment to general relatiivity attempted to eliminate the possibility of acceleration and collapse, by establishing what was called the cosmological constant. This still is dependent on the density parameter, Omega and the effects of dark energy being assumed to be a positive constant. Basically, as I have read, dark energy has been theorized to be positive in order to explain constant and or accelerated expansion. I am currently trying to find something related to it possibly being neutral. String theory appears to be the only thing that seems to completely exclude the possibilty of cosmic collapse or the cyclic model.

As is the case with everything related to black holes, there is some debate as to a difference between the gravitational effects of a black hole compared to a collapsed star when matter enters the event horizon. I've read that matter entering a dwarf star, polsar, super nova etc is converted to energy and expelled where as a black hole retains matter, light, etc and thus it's mass, adding it that of the black hole. This done, the mass of the black hole increases and so does it's gravitational effect. Yes, if the sun were to be replaced by a black hole of equal mass, all would remain constant according to Newton's laws. Reality is that planetary orbital decay has been measured. To use our solar syatem as a model, once Mercury enters the black hole, it's mass is added to the black hole making it's pull on Venus that much greater and so on. This is the basis for my thinking.

The whole debate about the center of the universe, whether there is a center and what's there, is exactly that, a debate. But, of what we can see within our universe most things seems to have a center. Many see no reason the universe should be any different.

Some debate also exists regarding accelerated expansion, since much of the evidence is based on red shift. Some problems with the affect of gravity on the infra red, the curvature of time/space and an unknown rate of decay of information have made red shift less accurate the further out we look. Using dark energy to fuel the rate of expansion is useless since we simply don't know it's nature. Where as if the energy produced by the big bang, which can't be measured either, is what is still fueling expansion, we at least have an event that pretty much everyone agrees happened.

The need for more energy from dark energy is only because of limitless expasion theories.

16. Feb 13, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

You would be wrong. This is how it started, of course. But the evidence has mounted since then. The Bullet Cluster is particularly striking evidence, though even before that our observations of the cosmic microwave background dramatically solidified the case for dark matter.

Well, the thing is, the cosmological constant is a feature of General Relativity. It would be in the theory whether Einstein wrote it in or not. Theorists have supposed for a long time that there was some symmetry that set it to zero, but nobody has yet found a symmetry that could do that. The real problem, then, isn't that the cosmological constant exists, but instead why its value is so absurdly small. Granted, that is, that it's a cosmological constant at all, but then nobody has yet come up with a well-motivated alternative.

This is incorrect. What goes on inside the event horizon is irrelevant to what occurs outside it. Mercury's orbit would, for instance, be every bit as stable with a black hole as it currently is with the Sun. And if Mercury were to fall into the Sun, then Venus would see exactly as much mass inside its orbit as before, and so would have almost the exact same orbit (there would be a slight difference due to the minuscule difference in the distribution of that mass, but it wouldn't be significant).

Naturally, the orbits of the planets would decay on extremely long time scales. I'd have to look the exact numbers up, but we're talking a number of orders of magnitude beyond billions of years here.

There is no debate. There simply is no center.

Well, there was no singularity in the finite past of our universe. As we look back in time, we see no starting point. What we do see is that our universe has changed with time in such a way as to hide the nature of what happened before. A number of experiments are currently underway to tease out the precise properties of the earliest epoch of our universe that we can observe.

And one thing that we do know, by the way, that the energy that currently exists in the universe was generated during inflation, which was a period of accelerated expansion early-on, the earliest period we can detect. It isn't clear what the precise properties of inflation were, but its behavior was much like a cosmological constant (with a very large value).

17. Feb 14, 2009

### PeopleSmoks

Re: No need for "dark energy", gravity will suffice.

Greetings Chalnoth. Thanks again for taking your time to reply to my post. When I say that dark matter and dark energy are only used to fill in gaps, I don't dispute their existense, rather the values given to them. Aside from a way to "prove" accelerated expasion, what reason is their for these to be given positive charge values? I read an article which inferred that dark matter would contain an equal number of positive and negative particles, thus making it neutral.

Once again, regarding the cosmological constant, my understanding is that Einstein made it part of his theory of general relativity for the purpose of avoiding the possibilty of cosmic collapse, by giving it a value of zero? Yet, it seems that modern physicists have inferred a positive value to Omega, disregarding Einstein's original model? Also, what I meant by it being 'added' is that while working on his theory, he himself saw a variable that he didn't like and formulated the cosmological constant to correct this variable prior to general relativity being published.

Here I again ask, if all energy was created during this period, why has a positive value been give to Omega and dark energy when Einstein's model gives Omega a value of zero? Also, I ask, if dark matter already exists within the cosmos, and is not being 'added' from an external source, wouldn't that mean that there is too much mass in the universe today, since we will need more tomorrow and it MUST be there tomorrow?

I'm currently searching articles and papers pertaining to things you've said in your comments.

18. Feb 14, 2009

### Janus

Staff Emeritus
Re: No need for "dark energy", gravity will suffice.

You are confusing Omega, which is the critical density of the universe, how it will , with Lambda which is Einstein's cosmological constant. Lambda was added by Einstein to provide an influence that would counter a static universe's (one that isn't expanding or contracting) tendency to collapse under its own mutual gravitational attraction. In order to work as needed, Lambda had to given a precise value to exactly counter gravity on the cosmological scale. If it varied by the least amount, the universe would undergo either runaway expansion or collapse.

When Hubble determined that the universe was expanding, the need for the Lambda term was dropped, as it was felt that the inertia of the galaxies' recession explained why the universe hadn't collapsed on itself.

When it was discovered that this expansion was speeding up with time, Lambda was given a new look as this was like the runaway expansion predicted by the "slightly off" Lambda mentioned above.

19. Feb 14, 2009

### ZapperZ

Staff Emeritus
Re: No need for "dark energy", gravity will suffice.

Try this on a cluster collision:

http://www.journals.uchicago.edu/doi/pdf/10.1086/508162 [Broken]

There's also an unbelievable amount of material already out there on DM/DE in case you are still looking.

http://arxiv.org/abs/0706.2986
http://arxiv.org/abs/0901.4090
http://arxiv.org/abs/0901.0632

Zz.

Last edited by a moderator: May 4, 2017
20. Feb 14, 2009

### Chalnoth

Re: No need for "dark energy", gravity will suffice.

Well, not quite. Theorists never forgot about the cosmological constant. Very simple arguments demonstrated that the cosmological constant, if it were nonzero, had to be absurdly small, around 10^-120 in natural units. So most just figured that there must be some unknown symmetry that sets it to zero. No such symmetry has been found, but there are many theorists that still think it's more likely.