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

Meanwhile, on the Neutral Planet

[davidwr]

As Zapp Brannigan is in lukewarm discussions with the Neutral Planet president, the planet's scientists are holding a lukewarm debate over the possible existence of Grey Matter.

I need my dark energy

[russlar]

'Dark energy', which researchers have spent years trying to fathom, isn't necessary to explain our universe after all I don't know about those guys, but I usually do my best work after a cup of dark energy.

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:Mini-Inflation events in Voids

[LionKimbro]

I don't think so; My understanding is that it's the force of gravity.

Here is the picture I have heard:

The universe basically, from any point, stretches out in all directions. Gravity pulls a given lump in all directions at a given time. But local things are more powerful by the law of gravity, than far things. So things start lumping with their neighbors.

Some lumpings occur earlier than other lumpings, which cause then to exert a stronger pull. These become the super-clusters (joining points between filament; such as the Virgo Cluster.)

So masses are basically pulled towards the closest super-cluster. But, ah-hah, some are pulled strongly by *two* super-clusters. These become the filament ("bubble walls.")

If you download Mitaka, you can see a lot of these things first hand, with data directly from the Sloan Digital Sky Survey.

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:Dark matter balloney

[John Hasler]

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

That makes sense, but

[shadowofwind]

Why have they been wasting our time with this dark energy stuff for the last decade then? Why posit the dark energy if its only needed to fix a model that was derived with what has for a while now known to be a false assumption? It seems stupid. Instead of endless science articles on dark energy, instead there should have been articles on scientists working to solve pde's with really hard constraints that match modern astronomical observations. I don't get it. Is there more to the story?