Universe's expansion may be understood without dark energy

http://www.physorg.com/news/2011-10-supernovae-universe-expansion-understood-dark.html

by joeyespo • 15 years ago

28 20 15 years ago

J3L24043• 15 years ago

"Annila shows that, when gravitational lensing is analyzed with this concept, it does not require dark matter to explain the results."

Both dark energy and dark matter seem contrived and virtually anything would be less hand-wavy. Just for aesthetic reasons I hope these are found to be the modern equivalents of "ether" as a medium for EM waves.

jonnathanson• 15 years ago

Something's always struck me as very "ethereal" about the concepts of dark matter and dark energy, and I'd be inclined to agree with you. Sooner or later, a robust theoretical model will come along that suffices without either of these constructs, which will be the kiss of death for them.

bluedanieru• 15 years ago

Dark energy especially so, I think. I do agree with you somewhat, but dark matter at least is not so preposterous, simply being matter that only interacts with other matter via the gravitational force.

J3L24043• 15 years ago

Yes, I think you have a point. Galaxy rotation is going to be hard explain without some missing mass.

splat5• 15 years ago

I have to disagree with you about dark matter and dark energy seeming contrived.

First it's hard to call dark energy "contrived" when it hasn't really been contrived, conceived, or created at all. That is, no one has really made any strong, convincing claims as to what it actually is. The term is really just a placeholder for the fact that something is providing a long-scale repulsive force (and so probably has negative energy density). Although it can be "explained" by a cosmological constant, that just punts the question to what the cosmological constant physically is.

As for dark matter, it is the simplest, most elegant way to explain the variety of observations that point to its existence. These observations include small-scale dynamics of dwarf spheroidal galaxies, the rotation curves of galaxies, gravitational lensing due to galaxy clusters, and more. All of them can be explained if there exists some unknown particle which interacts only weakly with other particles. While it's true that the standard model predicts no such particle, it's also true that it's known that the standard model is an incomplete theory.

You might argue that it could just as well be that our theory of gravity is wrong. (Certainly at the quantum level we know for sure that that is the case.) But inaccuracies in our theory of gravity at large scales have difficulty explaining the dark matter problem in an elegant matter. Consider dwarf spheroidals. These are the smallest of galaxies; in terms of luminous mass, they are only slightly more massive than globular clusters. Yet globular clusters are 1000s of times smaller than dwarf spheroidals. How can they possibly remain gravitationally bound on the scale of 1000s of light-years when globular clusters are only bound on scales of a few light-years? Dark matter can explain this by simply saying that dwarf spheroidals have relatively massive dark matter halos, whereas globular clusters have almost no dark matter. (Which one would expect in standard galaxy formation scenarios.) But modified gravity theories have difficulty explaining why two systems with similar masses interact so differently.

We can similarly see that galaxies of all sizes exhibit a range of dark matter fractions. If the problem was merely with our theory of gravity one would expect all galaxies of a given luminous mass to have the same dark matter fraction. Any modified theory of gravity which explained these observations would not be aesthetic. The aesthetic explanation would be some hitherto unknown weakly interacting massive particle.

J3L24043• 15 years ago

>The term is really just a placeholder for the fact that something is providing a long-scale repulsive force

How is that not contrived?

>You might argue that it could just as well be that our theory of gravity is wrong.

As an armchair physicist, I was rather hoping the Pioneer anomaly would lead us down that road, oh well.

>Any modified theory of gravity which explained these observations would not be aesthetic.

Well aesthetics do invoke subjective qualities. Relativity, modifying Newtonian physics, could have be seen as ugly by some. E = mc^2 is so elegant as to seem improbable.

redwood• 15 years ago

I'd suggest that the terms are contrived, rather than the concept. "Dark" is used to imply something unknown in this context, but many laymen believe "Dark Matter" and "Dark Energy" are understood quantities, since they sound as such. A more appropriate term would be "Unknown Matter" etc... but this would be far less cool sounding.

lutorm3• 15 years ago

"Dark energy" isn't that contrived. When Einstein formulated GR, he showed that when you set up the theory, there is an additional free parameter. People assumed that it was zero, but there was really no empirical reason for this. But it's just as much part of the equations of GR as matter-energy is.

If a dark energy had been not been allowed by GR and still been observed, then the hand-waving would have started.

bh42222• 15 years ago

I viscerally distrust anything from physorg.com. Anyone have a better option?

gjm11• 15 years ago

Actual paper at author's web site: http://www.helsinki.fi/~aannila/arto/light.pdf. I have to say that a brief perusal sets off my crank alarms, although the author does appear to be a real physicist. ... Oh. On googling a bit more, I find that the author has also claimed a proof that P != NP [EDITED to add: available at http://arxiv.org/abs/0906.1084]. The signs aren't looking good.

splat5• 15 years ago

Another good rule of thumb is not to put too much stock in any papers typeset in Microsoft Word rather than LaTeX (as this one was). Perhaps I'll give it a closer look if it ever gets any citations, but I wouldn't put much money on that happening.

tzs• 15 years ago

Considering that some top journals (such as Nature and Science) require or recommend Word format, your rule of thumb is ridiculous.

Jabbles• 15 years ago

Well he starts off by considering the kinetic energy of light...

lutorm3• 15 years ago

a brief perusal sets off my crank alarms

That's what I was going to say, too. And I got more convinced the further down I got. The weird, convoluted wording was a big tipoff. And then this sealed it:

When a star explodes and its mass is combusted into radiation, conservation requires that the number of quanta stays the same, whether in the form of matter or radiation.

There is no conservation law limiting the number of photons. This is obviously observed any time you turn on a light bulb.

splat5• 15 years ago

If I were being charitable I might think that he was referring to conservation of baryon number or lepton number...but all other indicators point to him not knowing what he's talking about. I'm actually pretty surprised that this paper was accepted. MNRAS is a fairly prestigious journal.

Plimsoll• 15 years ago

yes the author is real physicist but it should be noted that he is a professor of biophysics. So cosmology is not exactly in his field of specialty.

Universe's expansion may be understood without dark energy

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