I believe from what I have read, the observations (1998) that indicate the universe’s expansion, started to accelerate 8.5 billion years ago, were a surprise from the standpoint of cosmological theory and fundamental physics.

I have been looking for review papers that discuss the data that is used to justify the accelerating universe hypothesis and explain the hypothesis. Attached below are a couple of papers I found readable.

It looks like scientists are continuing to challenge the observations which indicate that 8.5 billion years ago the universe expansion started to accelerated, to confirm or try to disprove that hypothesis. I guess a portion of the scientific community are rechecking the data, to reduce the risk that the scientific community will spend significant time and effort, trying to explain something, which did not happen. From what I have read I think there are still questions, but nothing that disproves the hypothesis.

To me cosmology is interesting because all of the questions have not been answered. I enjoy trying to understand the standard cosmological model, as well as the data that both supports and challenges it. Some very basic questions, such as why did the accelerated expansion start 8.5 billion years ago, need to be answered. From what I have read the term “dark energy” is just a holding spot for a future theory which will explain what is causing the universe to expand.

What are other people thoughts? What astrophysics/cosmology problems are you interested in? Any thoughts concerning the accelerating universe or dark energy?

Cosmic Acceleration, Dark Energy and Fundamental Physics, by Michael Turner and Dragan Huterer

http://arxiv.org/pdf/0706.2186


Accelerating Universe: Observational Status and Theoretical Implications by L. Perivolaropoulos

This paper explains some of the science associated the observational analysis such as Supernovae Ia data, which is fundamental to the accelerating universe hypothesis.

http://arxiv.org/pdf/astro-ph/0601014

This paper discusses how to tune the cosmological model to match observations. (Without a mechanism the issue is how to tune the model to match observations.)

When Did Cosmic Acceleration Start ? By Alessandro Melchiorri and Luca Pagano, Stefania Pandolfi

http://arxiv.org/pdf/0706.1314

I think that the theory is that after the beginning of the expansion, the gravitational attraction between the massive bodies tended to pull the pieces back together (slow down the expansion), but that dark energy, right from the beginning, tried to accelerate that expansion. At first, gravity (which diminishes as the volume of space grows) was comparatively large, and dark energy (which increases as the volume of space grows) was comparatively small, so, at first, the expansion decelerated. At a certain point, however, the strengths of the two opposing tendencies reached equiibium, and after that, dark energy increasingly began to dominate, and the expansion began to accelerate. So it isn't as if the dark energy suddenly "kicked in" at some point - there was simply a point at which it began to surpass gravity.

Many of those who firmly believed in inflation theory were expecting a nonzero energy density because inflation predicted that the universe should be flat. Flatness required a certain amount of mass and energy density, and during the 1990s, it became increasingly obvious that there just was not enough mass to produce flatness.[/quote]

Skepticism lies at the heart of science.

It is an extremely intellectually stimulating time.

That is my understanding also. We seem to be in the stage of verifying the details of the expansion as a function of time, with no really compelling "explanation."[/quote]

Hi Fortunate,

I understand what you are saying and agree that a fixed expansion mechanism change works theoretically. Is that hypothesis, however, only a model change. Does the fixed vacuum energy suggestion make physical sense?

The authors of the attached paper in figure 8 have drawn a line on the graph which is called vacuum energy. Why is the vacuum energy flat? Wouldn't the vacuum energy content change with space density, does vacuum energy follow the conservation of energy law?

Cosmic Acceleration, Dark Energy and Fundamental Physics, by Michael Turner and Dragan Huterer

http://arxiv.org/pdf/0706.2186

Hi William,

It seems we have some common interests.

I don't think that we have any convincing explanatory models yet, so while the theorists grind down their pencils, all we can do is trace out the details of the expansion as a function of time. There are a number of projects and groups working on that as we type.

They are trying to observe more type 1A supernovae, but they also want to employ other, independent methods as a crosscheck.

HETDEX http://www.as.utexas.edu/hetdex/ will attempt to directly observe the large-scale distributions of galaxies back to a very early age.

NASA's Joint Dark Energy Mission will be chosen from among several proposals, each relying on observation of type 1A supernovae plus one other method (different for different proposals).

There are several other projects, some in progress.

One obvious thing to look for is whether the "energy" scales directly with the amount of space (cosmological constant) or not (quintescence). Note that these terms do not apply to different explanatory models, but only to the whether the corresponding terms in the relativity equation are constant or not. So far, i'm told, observations seem to favor the constant.

Well, it's certainly an attempt to make sense of the situation. And oddly enough, Einstein's cosmological constant, or as you say "fixed vacuum energy", does seem to explain the observations. Significantly, Riess found what Goldsmith describes as follows:

"When astronomers succeed in observing supernovae with redshifts and distances much larger than those of the supernovae with redshifts between 0.4 and 0.7, the Hubble diagram for the universe actually reverts toward the original line describing a cosmos with no acceleration produced by a cosmological constant. This reversion occurs because as we look farther out in space, we look further back in time, to eras when the cosmological constant had produced a cumulative effect much smaller than at the present time or at times 'only' 4 to 7 billion years ago. We can effectively recapture the Hubble diagram for a universe without a cosmological constant by looking so far back in time that we observe epochs when the cosmological constant had produced negligible results."

Or as Fortunate says, "so, at first, the expansion decelerated [due to the gravity of all the mass, which was much closer together]. At a certain point, however, the strengths of the two opposing tendencies reached equiibium, and after that, dark energy increasingly began to dominate [because the masses got further and further apart]."

It goes against common sense, but somehow I don't find it too hard to imagine that as space expands, the "new" space has the same vacuum energy as the "old." As I understand it, that's how Einstein's lambda works in his equations.

Apparently not -- if the cosmological constant accurately describes it. When that law was established, dark energy had not set foot on the stage. It wasn't even in the wings.

__________________
Everyone is entitled to his own opinion, but not his own facts.

Vacuum energy is flat because a given spatial volume of fixed (non-co-moving) lengths will always have the same amount of vacuum in it, and it is this sort of volume that is used to define density parameters.

Vacuum energy does follow the conservation of energy law. However, the effect of the law comes up differently in the mathematics. It is this conservation law that indicates that the pressure term of the vacuum energy is proportional to the negative of the density term. (The requirement that the vacuum energy equation of state also be Lorentz-invariant also plays a role in the derivation, I believe, but my mind is a little fuzzy on the details this morning.)

There is more discussion of dark energy on the following thread:
What is the evidence for dark enegry?

Hi Kwalish Kid,

There are three different levels to discuss dark energy (Moving beyond questioning whether the effect is real and accepting the hypothesis that the effect is caused in some manner by space.)

A) Physically (fundamental)
Physically what in space is causing this negative pressure, which is independent of space density? All particle based forces change based on distance from particle to particle. As space expanses wouldn’t the negative pressure become less? Integrate the vacuum pressure within a fixed volume. As space expanses there is less space density within the fixed volume. Why does the negative pressure remain constant?

(See the quote below from Wikepedia concerning quintessence and the requirement for a tracker function. The tracker function adjusts “the quintessence field density to closely track (but is less than) the radiation density until matter-radiation equality, which triggers quintessence to start having characteristics similar to dark energy, eventually dominating the universe. This naturally sets the low scale of the dark energy.”)


B) Standard Model (consistency)
The properties of space I believe are defined in the standard model. I thought there is no theoretical means to create a fixed negative pressure with the standard model. Is a new or modified standard model necessary to create a fixed negative pressure in space?

Cosmologists can freely adjust parameters to get a model that matches observations. In physics I believe the standard model is constrained. To change the properties of space it is necessary to reconstruct, the entire model.

From Wikipedia.



C) Cosmological Models (Adjust parameters in model to match observations)
As you say if w=-1 is entered into the cosmological model, the model matches observations.


Cosmological parameters from SDSS and WMAP, by 60 authors.

http://arxiv.org/pdf/astro-ph/0310723

From what I've read, the pressure term involved in the Einstein Field Equation is really only be analogous to pressure as we see it in standard gas theory (even for standard matter and radiation). The so-called vacuum energy really has no state in itself (though it has an effective equation of state) if it is merely a constant of the theory. This seem inelegant to some, but gravity already includes one constant.