Necessity of Dark Energy Questioned

ttnuagmada points us to an article about scientist David Wiltshire's suggestion that theorized dark energy is not needed to describe the expansion of the universe. His work challenges assumptions made about the distribution of matter in the universe. Early solutions to general relativity were based on a "smooth distribution" of matter. Wiltshire's approach focuses on a "lumpy" dispersal, which more accurately fits data from modern studies. We have discussed other theories about dark energy in the past. Quoting: "Through observational projects like the Sloan Digital Sky Survey and the 2 Degree Field survey, we now have a much better picture of the large-scale structure of the universe and we know that galaxies are not uniformly distributed. 'Rather, they are in clusters sprinkled thinly in filaments and "bubble walls" surrounding huge voids hundreds of millions of light-years across,' Wiltshire says.

Comments

Relatively readable survey of solution approaches

[gyepi]

... can be accessed here: http://arxiv.org/abs/astro-ph/0510059 . A bit less recent (but even more readable) account is http://xxx.lanl.gov/abs/astro-ph/0310342 . The first linked article also mentions the approaches featured in the slashdot post (this is an ongoing business for a while). For starters the flow diagrams in the front pages describing the options might be particularly useful.

Can someone please explain?

[martyb]

I did some googling and found David Wiltshire's home page which had links to his recent publications. That brought me to this full article which I am guessing is the one that corresponds to what was discussed in the original /. article here.

I had a couple courses in astronomy and cosmology way back in my college days. That said, I can't begin to understand the details. I'm hoping someone with more knowledge and experience could elaborate. Is he really onto something that can dispense with the need for dark energy? And, if he is, am I correct in thinking this would be Nobel-Prize-Candidate-Worthy?

Re:Can someone please explain?

[SoberVoiceOfReason]

I am, for the record, a physicist.

Here's the slightly more condensed version of this story. Einstein's theory of General Relativity (GR), which incidentally should the Law of GR by today's standards, gives a large set of differential equations to be solved. When this was first being applied to Cosmology in the 1920's, some basic assumptions about the universe had to made in order to solve the GR equations: it is isotropic (same in all directions), and homogeneous (uniform everywhere). They were primarily made for two reasons: mathematical expediency (this is the simplest sort of non-trivial universe you can have), and this didn't conflict with any observations at the time. Solving the GR equations with these assumptions gives fairly simple equations for the time evolution of the universe, leading to the standard model of Cosmology (called the Lambda-CDM model).

As you would imagine, we have vastly more astronomical data now then we did in the 20's. To explain what we observe now, particularly the cosmic microwave background data, with these evolution equations we need to include a constant expansion term. This expansion would have to be from something uniformly distributed throughout the universe with negative pressure (very reminiscent of phlogiston, isn't it?) which we call "Dark Energy". So, based on current data and using the standard model to explain certain properties of the universe, it must consist of around 73% dark energy. Considering that this is the bulk of the universe and that, other than negative pressure, we have no idea what dark energy is or what it's properties are, this leads to a scientifically troubling state of affairs.

However, modern sky surveys show that the universe is neither isotropic nor homogeneous. Instead there is a tendency towards a bubble-like structure with large empty spaces surrounded by thin "filaments" of galaxies. Even still, the standard model which requires dark energy ignores these differences. So, Wiltshire's contribution is to replace the standard assumptions with this "bubble" model, re-solve the GR equations, and get new equations for the evolution of universe based on it's *observed structure*, not some simplified model. In his new equations, dark energy is completely unnecessary. Since the structure of these "bubbles" is so large, fits to the data with Wiltshire's model are statistically just as good (actually indistinguishable) as the standard model, though as a caveat not all of the calculations have been done. Not only is Wiltshire's model much better from an Occam's Razor standpoint, it may actually solve some mysteries which the standard model cannot explain.

I really can't go any further and still call this a "condensed" version with a straight face. In /. articles in other fields, I enjoy reading the commentary from experts, so here's an attempt to reciprocate. Hope this helped.

Re:Dark matter balloney

[vertinox]

If you cannot detect something at all with light or gravity effects, then it very likely isn't there.

Are you so sure there aren't other spectrum's yet to be discovered? We just might not have the technological know how to detect certain things. Doesn't mean they aren't there.

Take radiation for example. You can't see it, can't taste it, can't feel it and without the proper tools you'd never know you're sitting in it.

Same thing for "dark matter". Yes it could be a bunch of baloney, but its the only thing that somehow makes the model of everything else work on a astronomical scale. Eventually, we might find some other explation but we can't discount anything until we can prove it false.

Currently we don't have the means to prove it false so its just a big assumption. Hopefully the LHC will shed more light this spring on how matter works so we can stack the evidence for and against dark matter in general.

Re:KISS

[ceoyoyo]

Actually, physicists hate complex answers. The overwhelming guiding principle of physics is to describe the universe with as few axioms and rules as possible. Leon Lederman (former director of FermiLab) has a neat little passage in his book about the goal of physics being to produce the ultimate t-shirt: everything that's needed to describe everything written on a shirt. And not one of those XXXL shirts couch potatoes wear.

If this guy is correct then it's a nice advancement of cosmology. From what's described in the article it appears that at least the sign of the effect in his argument is correct. You hear a LOT of these claims though, that explain one or two observations and conveniently omit a hundred or so others.

And the article is terrible. It sets this up somehow as a battle between this guy and Einstein. Einstein postulated a cosmological constant (the equivalent of dark energy) because he wanted a STATIC universe and then retracted it when Hubble came up with experimental evidence that the universe isn't static at all. Einstein's theories have nothing to do with whether matter is smoothly distributed or not.

Re:Dark matter balloney

[ceoyoyo]

You do know that the temperature of the corona has been explained by observation of the sun's magnetic field lines right? Your confusion stems from modeling the sun as a light bulb, rather than the physics.

The shortage of detected solar neutrinos was explained by hypothesizing that neutrinos actually have a very small mass. That would imply that they oscillate between types, only one of which we were detecting. That implies that the shortage is made up by neutrinos of the two other types that we couldn't detect. Now, with better equipment, we've discovered that neutrinos do have some very small mass and everything adds up nicely. Sorry, but that's a TRIUMPH of quantum mechanics, nuclear physics and the standard model, not a failing.

But if you don't like all that math, that's no problem. Please sell or recycle your computer, which is only possible due to all that math.

Re:Relatively readable survey of solution approach

[Chemicalscum]

You picked on a particularly apposite article in your ref: http://arxiv.org/abs/astro-ph/0510059

from the article

A central question for this approach is whether the feedback of non-linearities into the evolution equations can significantly modify the background, volume-averaged FRW universe and explain the accelerated expansion without the introduction of new matter, or a cosmological constant [13]. Key to this issue is that interpreting observations made on a particular scale tacitly also requires the smoothing of theoretical predictions on that scale, and the smoothing operation does not commute with time evolution [14]. The Einstein equations are non-linear, and this non-commutivity means that the FRW equations, for which quantities have been averaged prior to inclusion, will not be the same as the equations and then averaging the equations.

This defines exactly the questions Wiltshire seems to be addressing. His most recent paper on arXiv posted on 24 Dec (from a yet unpublished confrerence contribution) is here:

http://arxiv.org/abs/0712.3984

It seems like a good review. He may be right, but then again he might not. Only careful testing against the observational data will tell. He proposes to outline the differences in observational predictions between his "Fractal Bubble" model and the current Lamba CDM model in a forthcoming paper.

Re:Dark matter balloney

[John Hasler]

You confound dark energy with dark matter. They are very, very different concepts. This paper deals with dark energy.

Re:Seems to Make Sense

[boot_img]

IAAAP: The explosion model was ruled out in the late 1980's. Why? velocities of galaxies are not compatible with observations.