New Clues About the Nature of Dark Energy

Jim Mansfield writes "With the Hubble space telescope no longer being serviced by NASA, it's good to see one of their hardest working and most famous satellites in the news again. According to their press release on the nature of dark energy, Einstein may have been right after all - and even if he turns out to have been wrong, it seems that dark energy is not going 'to cause an end to the universe any time soon' ... whew, that's a relief." See also a space.com story.

Correct me if I'm wrong

[TheRealMindChild]

But the "Cosmological Constant" Einstein was credited for theorizing on was Ether, and eventually disproved the existance of Ether himself by somehow using the earths revolution around the sun.

While this may be a completely seperate idea, it definitely appears that the author is mixing these two (Dark Energy and Ether) Einstein theories.

Re:Correct me if I'm wrong

[GammaRay Rob]

You're wrong. Aether was thought to be a physical fluid whose ripples were the basis of the wave-like nature of light. This was proven not to be so by Michelson and Morely, who showed that the speed of light was the same no matter if it were going with or against the aether (which was presumably flowing past the moving Earth). Dark energy is a field, like light or gravity, which presumably has no preferred frame of reference (like light or gravity).

Re:The restaurant at the end of the universe

[mog007]

I assume you're using "far" to discuss lengths of time, and not distance. Everyone who's anyone knows that the Restaurant at the End of the Universe is located on Magrathera, just 30 billion years in the future.

Re:Relief?

[Psiren]

Stephen Baxter (I think?) wrote a very good book (Time) based around the idea of heat death. Some of the ideas that civilzations come up with to make the most out the last remaining energy in the universe is very neat. Well worth a read.

Re:Dark Matter and Ether

[snake_dad]

The article is about dark energy, not dark matter. Those are two distinctly different things. Dark matter is simply matter that has not been found, but that astronomers assume must exist to explain certain gravitational behaviour as observed in galaxies. AFAIK there is not much controversy over wether dark matter is real or not. Dark energy however is theorized to be a force that acts opposite to gravity, and that could explain why the rate of expansion of the universe seems to be increasing.

IANA astronomer, but that's what I've understood from the stuff that I've read about it. Pop science ofcourse because the math is way over my head.

http://www.ebtx.com/ntx/ntx16.htm

[Anonymous Coward]

Whats more likely? This mysterous dark energy exists and compromises 70% of the mass/enery of the universe even though we can't see it anywhere locally, or our theories are wrong?

I suggest reading www.ebtx.com on the nature of dark energy. This guy is right, or at least close.

Matter attracts matter; this we know. The rest of the theory explains that space attracts space, and matter repels space. Matter and space are polar opposites (as well as logical opposites).

Einstein wasn't relative enough in his theories. He declares C as constant and bases all other observations off it, when in fact you can change all the physical constants continuously and arrive at the same results. If C changed, as long as h, G, and about 18 other 'constants' also changed, we couldn't tell, from our point of view.

Is the universe expanding, or are we all shrinking? From a relative point of view there is no difference.

Re:Einstein was wrong anyway

[khallow]

Actually, the cosmological constant can result in an expanding universe.

Big Rip a Big improbability

[Pi_0's don't shower]

If you fear things involving physics, skip the rest of this post. Alright, for those who are interested, it seems like 70% of the current energy density of the universe is in some form of "dark energy", as was previously stated. The Universe is currently 13.7 billion years old. We say that every component in the universe has an energy density and a pressure. Dark energy is different from things like normal matter and light, because these have positive pressures. (Normal matter has a very small pressure). But dark energy has a negative pressure, which means it works opposite to gravity. Everything that has a pressure that we can physically think of (well, that I can physically think of) has a pressure between (-1)*energy density and (+1)*energy density. A big rip will only occur (and it will only occur in the very distant future) if the dark energy has a pressure that is outside this range, such that pressure is less than (-1)*energy density. This is, of course, possible, but unlikely in my view.

Re:Dark Matter and Ether

[barawn]

It can also explain phenomena which the dark matter hypothesis can't, such as the Tully-Fisher relationship observed in the surface brightness of galaxies.

The Tully-Fisher relation has been explained by dark matter for some time. You can find a brief derivation in Carroll & Ostlie p. 1002, for instance. There's no need to invoke MOND at all - it just comes from the fact that the luminosity is proportional to the maximum velocity to the 4th power, which you can get by using the expression for total mass contained within the galaxy derived from rotational velocity curves.

Re:non-physical physics

[EnVisiCrypt]

Actually, Galileo was excommunicated, which at the time was almost as bad as being driven out entirely. In addition, he was placed on house arrest and was confined there until he died, refusing to recant his observations.

Great man.

Comment removed

[account_deleted]

Comment removed based on user account deletion

Big Rip != Acceleration

[jpflip]

The fact that the universe is accelerating is not the same as the "big rip". The accelerating universe, as we understand it now, sort of means that the space between everything and everything else is getting bigger all the time. However, in order to discover this (and the expansion of the universe in general), we have to look at very distant galaxies - we don't see our own galaxy flying apart, and some other galaxies bound together in our local galaxy cluster are orbiting or moving toward ours. In general, objects that are in bound states - whether gravitational bound states (like solar systems and galaxies) or other bound states (atoms, etc.) will remain held together even as the distant galaxies which are not tightly bound to us zoom away. Our own situation on earth would be completely unaffected - you'd need a big telescope to even tell the difference. The idea of the "Big Rip" is that this condition that "bound things stay bound" (the dominant energy condition) might be violated, that dark energy might be so extreme that not even bound objects could keep from eventually dissipating. That idea is HIGHLY theoretical - there's no particular evidence for it, and until recently most theorists thought it was ridiculous. But, of course, this is science - we have to think about even the weird possibilities.

There's more to dark matter...

[jpflip]

You're right, the natural step when we learn that the universe doesn't obey Newton's laws should be to try to modify Newton's laws, not to imagine that there is a magic 95% of the universe with funny unobserved properties. The thing is that this isn't the only evidence for dark matter. There are a number of different lines of evidence which lead to the same conclusion - the orbital behaviors of galaxies and their clusters, the adundances of various light elements in the universe, the behavior of the cosmic microwave background, x-ray emission from clusters, etc. It turns out that no matter how hard we try, we can't modify Newton's laws to get the right answer to all of these. Gravitational lensing (the bending of light by the mass of distant galaxies and clusters) is really impressive in this regard - modifying Newton's laws (and general relativity) in the desired ways should have essentially no effect on it, and it definitely looks like there's dark matter (and even allows us to map its distribution). Dark matter really seems like the SIMPLEST answer, from the point of view of someone who knows the data! Dark energy was the subject of the article, however, and that's quite a bit different. As of right now, I'd say that we DON'T have very convincing evidence that this isn't just a modification of general relativity. All of our particle physics-related ideas seem far too complicated. Oh, and chaotic systems still obey the laws of classical physics - the systems are just so complicated that knowing how the individual atoms are behaving is not very helpful for predicting the behavior of the macroscopic system.

Re:...End of time?

[V_M_Smith]

I always wonder whether the "It's accelerating so it'll drift apart in the end" folks understand basic calculus. The rate of expansion accelerating doesn't mean it will continue accelerating

Well, if you've done any General Relativity you'll know that for a standard cosmology (FLRW cosmology), the final state is one of recollapse, asymptotic expansion, or accelerating expansion. This end state depends on the total mass-energy content of the universe and the nature of the dark energy (cosmological constant). It really isn't a lack of understanding of "basic calculus", but rather a deeper understanding of the physics involved. So, basically, we don't need to know all the derivatives -- we just need to have an understanding of the potential in which our universe evolves.

Re:non-physical physics

[beanyk]

The thing is, he did recant. That doesn't mean he changed his mind, but he did change his official line. There's a writeup here [firenze.it], for instance.

Re:...End of time?

[Bootsy Collins]

Of course they understand basic calculus. They just also understand the currently prevailing model for the constitution of the universe and its evolution. To have the accelerating expansion stop accelerating, decelerate, or turn over would require some additional, extremely bizarre physics that's not indicated by any observation or experiment we presently have. This may seem like an odd constraint for me to place when we're talking about something as bizarre as "dark energy", but it isn't. There were a lot of theoretical reasons from both cosmology and elementary particle physics (and even a few vague extragalactic observational reasons) to at least consider that the cosmological constant may be nonzero; that's why the two high-z supernova teams did their work. And now there's still harder data suggesting same. In contrast, there's just no reason whatsoever to presume unbelievably bizarre physics of the form necessary to produce the behavior to which you appeal. The scale-factor dependence of the currently-known components of the Universe don't have the higher-order derivative behavior you appeal to; while coming up with a hypothetical field that does is pretty damned hard. That doesn't mean you're wrong, of course; it just means the odds are very highly against you. The claims they're making are almost certainly true.

Re:Einstein was wrong anyway

[tgibbs]

This whole "Eistein was right after all" angle is misinformed. He wanted a static universe because that was the historic conception of the universe. His own science didn't allow for it, but he wrangled an equation for one out of it anyway. Turns out he was wrong, is wrong, and will always have been wrong. Einstein's motivation for putting in the cosmological constant was ideological, not observational -- and that's a recipe for Dumb Science.

Not exactly. Einstein didn't "put in" the cosmological constant; it emerged naturally from the derivation of the equations of General Relativity. But the theory did not provide its value; it was a free variable. It needed to be given some value, and there was at the time no firm observational data to do that. The mathematically simplest course would have been to arbitrarily assume that it had a value of zero, effectively "getting rid of it." That would have implied an expanding universe, and Einstein would have scored quite a coup by predicting the expansion well before the data came in to confirm it. Instead, Einstein chose to assume a value that brought his theory into line with the then-current astrophysical view of the universe--i.e. that it was static. So Einstein didn't "put it in;" he merely chose not to arbitrarily take it out. Yet another possible value of the cosmological constant yields an accelerating expansion. But that is different from the value that Einstein assumed. So the only sense in which the "Einstein was right after all" statement applies is that the correct value may not be zero, after all.

The main problem with Dark Energy...

[little1973]

...is that no mainstream theory predicts its existance. It is based solely on observations. Scientists try to bend/modify current theories in order to include Dark Energy.

Many formulas and theories are based on observations, however, a good theory not only describes current observations, but predicts things which are not observed, yet. Like Einstein's theory predicted time-dilation, the curvature of space-time, etc. and gave a solution to the orbit of Mercur (which Newton's theory was unable to explain).

A new theory may be needed to include the Dark Enegy from its foundations or to explain these phenomenas without Dark Energy.

Re:No info...

[Silburn_Luke]

If you want more detail you should check out the February issue [sciam.com] of Scientific American, which has four or five feature articles discussing the cosmological theories these experiments are addressing.

Regards Luke

Re:Dark Matter and Ether

[barawn]

I was under the impression that dark matter needs fine tuning to explain Tully-Fisher

Yes and no: The typical Tully-Fisher coefficients for Sa, Sb, and Sc type galaxies are 9.95, 10.2, and 11.0 or so. These are all within 10%, and for Sa and Sb types, within 5%, of 10. Simple assumptions get you a coefficient of 10, if you assume that the mass-to-light ratio is the same for all spirals, and that the surface brightness is the same for all spirals.

The first assumption (mass-to-light ratio) is a clearly idiotic assumption. It assumes that galaxies form with same proportions of light and dark matter, which we *know* is not true for other types of galaxies (dwarf ellipticals, in particular). Aside: This is also the "nail in MOND's coffin", more or less - MOND was hoping to replace the dark matter hypothesis by saying physics works differently at large distances. The problem is that galaxies which contain the same amount of light-emitting matter and have the same spatial extent should therefore have the same rotation curves. This isn't true. You then have to add a new parameter with MOND to fit it, which is OK, sure, but now you've started to lose the elegance originally intended, and now MOND becomes a more complicated theory than the dark matter hypothesis, which just says "well, that galaxy formed around less dark matter."

Anyway, back to the subject: the point is that those two assumptions clearly are not completely true, and therefore there's plenty of room for a 10% correction due to forming biases in spiral galaxy types. If the mass-to-light ratio is a very weak function of mass (which is believable - perhaps smaller galaxies formed when the dark matter density was slightly lower, due to their late formation times), you can easily get those corrections.

MOND allows you to get that 10% correction due to the parametric fit of the rotation curve, which is essentially identical to the way that it's done in the dark matter case - the corrections are due to the variation in the rotation curve, which MOND says is due to a modified Newtonian field, and dark matter says is due to a dark matter density. It's the same reasoning - one isn't more natural than the other.

(It should also be noted that the Tully-Fisher data has a crapload of spread to it, just like all astronomical data. Each galaxy varies a fair amount.)