Posted on 06/30/2004 4:52:28 PM PDT by PatrickHenry
In the last few decades, scientists have discovered that there is a lot more to the universe than meets the eye: the cosmos appears to be filled with not just one, but two invisible constituents –dark matter and dark energy – whose existence has been proposed based solely on their gravitational effects on ordinary matter and energy.
Now, theoretical physicist Robert J. Scherrer has come up with a model that could cut the mystery in half by explaining dark matter and dark energy as two aspects of a single unknown force. His model is described in a paper titled “Purely Kinetic k Essence as Unified Dark Matter” published online by Physical Review Letters on June 30 and available online at Astrophysics, abstract: Purely kinetic k-essence as unified dark matter.
“One way to think of this is that the universe is filled with an invisible fluid that exerts pressure on ordinary matter and changes the way that the universe expands,” says the professor of physics at Vanderbilt University.
According to Scherrer, his model is extremely simple and avoids the major problems that have characterized previous efforts to unify dark matter and dark energy.
In the 1970’s, astrophysicists postulated the existence of invisible particles called dark matter in order to explain the motion of galaxies. Based on these observations, they estimate that there must be about 10 times as much dark matter in the universe as ordinary matter. One possible explanation for dark matter is that it is made up of a new type of particle – dubbed Weakly Interacting Massive Particles, or WIMPs – that don’t emit light and barely interact with ordinary matter. A number of experiments are searching for evidence of these particles.
As if that weren’t enough, in the 1990s along came dark energy, which produces a repulsive force that appears to be ripping the universe apart. Scientists invoked dark energy to explain the surprise discovery that the rate at which the universe is expanding is not slowing, as most cosmologists had thought, but is accelerating instead. According to the latest estimates, dark energy makes up 75 percent of the universe and dark matter accounts for another 23 percent, leaving ordinary matter and energy with a distinctly minority role of only 2 percent.
Scherrer’s unifying idea is an exotic form of energy with well-defined but complicated properties called a scalar field. In this context, a field is a physical quantity possessing energy and pressure that is spread throughout space. Cosmologists first invoked scalar fields to explain cosmic inflation, a period shortly after the Big Bang when the universe appears to have undergone an episode of hyper-expansion, inflating billions upon billions of times in less than a second.
Specifically, Scherrer uses a second-generation scalar field, known as k-essence, in his model. K-essence fields have been advanced by Paul Steinhardt at Princeton University and others as an explanation for dark energy, but Scherrer is the first to point out that one simple type of k-essence field can also produce the effects attributed to dark matter.
Scientists differentiate between dark matter and dark energy because they seem to behave differently. Dark matter appears to have mass and to form giant clumps. In fact, cosmologists calculate that the gravitational attraction of these clumps played a key role in causing ordinary matter to form galaxies. Dark energy, by contrast, appears to be massless and spread uniformly throughout space where it acts as a kind of anti-gravity, a repulsive force that is pushing the universe apart.
K-essence fields can change their behavior over time. When investigating a very simple type of k-essence field – one in which potential energy is a constant – Scherrer discovered that as the field evolves it passes through a phase where it can clump and mimic the effect of invisible particles, followed by a phase when it spreads uniformly throughout space and takes on the characteristics of dark energy.
“The model naturally evolves into a state where it looks like dark matter for a while and then it looks like dark energy,” Scherrer says. “When I realized this, I thought, ‘This is compelling, let’s see what we can do with it.’”
When he examined the model in more detail, Scherrer found that it avoids many of the problems that have plagued previous theories that attempt to unify dark matter and dark energy.
The earliest model for dark energy was made by modifying the general theory of relativity to include a term called the cosmological constant. This was a term that Einstein originally included to balance the force of gravity in order to form a static universe. But he dropped the constant cheerfully when astronomical observations of the day found it was not needed. Recent models reintroducing the cosmological constant do a good job of reproducing the effects of dark energy, but do not explain dark matter.
One attempt to unify dark matter and dark energy, called the Chaplygin gas model, is based on work by a Russian physicist in the 1930’s. It produces an initial dark-matter-like stage followed by a dark-energy-like evolution, but it has trouble explaining the process of galaxy formation.
Scherrer’s formulation has some similarities to a unified theory proposed earlier this year by Nima Arkani-Hamed at Harvard University and his colleagues, who attempt to explain dark matter and dark energy as arising from the behavior of an invisible and omnipresent fluid that they call a “ghost condensate.”
Although Scherrer’s model has a number of positive features, it also has some drawbacks. For one thing, it requires some extreme “fine-tuning” to work. The physicist also cautions that more study will be required to determine if the model’s behavior is consistent with other observations. In addition, it cannot answer the coincidence problem: Why we live at the only time in the history of the universe when the densities calculated for dark matter and dark energy are comparable. Scientists are suspicious of this because it suggests that there is something special about the present era.
OF course they are the same. Ultimately, everything is all one thing -- different manifestations of one God.
bump for later.
Scientists are suspicious of this because it suggests that there is something special about the present era.
Of course, WE are here to observe it!
Cool.
Is it how they described the ether hundred years ago?
"Invisible fluid," yes. As for the rest, no. The aether was assumed to have no effects at all. Or so I recall.
I continue to suspect that the most likely substance for dark matter/energy is 'fudge'.
Numbers made up to make theories match data.
Many of the observed/inferred numbers underlying the observation that matter is missing (such as the size and age of the universe) can be changed by new observations. This will require recalculation of the % dark matter and energy in the universe.
I'd like to know the error values for these impressive sounding numbers. If the average radius of Galaxies turns out to be 10% off the currently estimate what does that do to the amount of missing matter?
Maybe the old time philosophers weren’t far off when they spoke of the “ether”
I continue to suspect that the most likely substance for dark matter/energy is 'fudge'.
A sign noted in town: "So what is the speed of dark?"
Wow. Now that you've explained it, the nature of everything, so succinctly - there is no need for any further scientific inquiry.
It always seemed to make more sense to me that Dark Matter and Dark Energy were everything 'observable' by the observer that is not in the 'present' of the observer.
Dark Matter being matter independent of time and the dark energy being the 'potentialities' of all the possible quantum states of the space not in the observers 'present'.
Recall that this was the same complaint raised with regard to the original BB theory, when it was discovered that the matter density is exquisitely close to the critical value. But under the Inflationary BB model, the fine tuning needed to produce a Universe of critical matter density was eliminated; regardless of the intital conditions, the Inflation process always drives the matter density to the critical value.
Perhaps a similar mechanism will rescue k-essence from the fine-tuning problem.......
Rather I think they're up to their necks in dark matter and without a paddle.
I belive most dark matter is centered around Bill, Hillary and Jabba the Moore.
Whether the "fine tuning" problem is truly a problem, or just a spasm of one's worldview isn't clear to me. But as long as it's regarded as an anomoly worthy of research, and it may lead somewhere, that's fine with me.
Yes. It has a good ring to it. When stuff falls together like that, it's a good indication that you may really be on to something.
The æther was actually considered to be an infinitely-rigid solid.
Great, does this mean we need Yoda to explain this?
It surrounds us, and binds us.
"Dark" has no speed as "Dark isn't a 'thing'. "Dark" is the absence of light (which is a 'thing').
at least that's how I remember it :-)
BTW, That sounds like a Steven Wright line. That guy cracked me up. Here's a few;
One of the British physicists from two centuries ago (maybe Bill Thompson, about 1895) modeled the æther as a set of tightly coupled gyroscopes; I think the arrangement was dense. It was similar to a modern field theory.
I don't think that's right -- the Michelson-Morley Experiment was based on the assumption that the ether would affect the measured speed of light.
From the linked site:
Prevailing theories held that ether formed an absolute reference frame with respect to which the rest of the universe was stationary. It would therefore follow that it should appear to be moving from the perspective of an observer on the sun-orbiting Earth. As a result, light would sometimes travel in the same direction of the ether, and others times in the opposite direction. Thus, the idea was to measure the speed of light in different directions in order to measure speed of the ether relative to Earth, thus establishing its existence.
Note that this is not the same effect as what the folks in this article are talking about, but it is an effect....
Cripes, man ... you post as if with a very sharp axe.
Thanks for expressing in succinct terms the types of niggling questions I couldn't quite translate from reptilian brain to fingertips.
That's what I was trying to say, but you said it better.
Still, I think if they'd carefully thought through the and applied the implications of the idea that the ether had an effect on the measured speed of light -- especially in light of later discoveries -- the theory of the ether might well have had to include other "observable effects" as well.
I'm just wondering if the Dark Matter/Energy stuff is serving the same purpose as the earlier version of the ether -- a stopgap measure that will go away once a better explanation comes along.
So far we have some clever theories that purport to explain certain observations. But until we get some testable predictions out of these theories, we just won't know. The aether washed out, eventually. Maybe this newly proposed concept will too. These things take time.
Or maybe not.... ;-)
I think that was Maxwell. Regardless, those mechanical analogies were not to be taken literally, they were simple visualization tools to show storage of energy and stresses, etc.
No one was able to come up with a serious mechanical explanation for electromagnetic waves. Heaviside tried repeatedly and failed -- and he was probably the only person who could come up with a satisfactory model. His final electromagnetic theory was one that included gravity called "Moving Compressible Ether" in Electromagnetic Theory Vol III. In it, em waves were reduced to little more than density changes in the ether (or space density since the changes were proportional in permittivity and permeability, ie. only the speed of light changes). He published the results for plane waves in 1902 and the general results in 1909.
If someone with a far larger brain than me were to examine his general theory in detail they would discover some very interesting results.
What happened to the "measurements"?
According to the latest estimates, dark energy makes up 75 percent of the universe and dark matter accounts for another 23 percent, leaving ordinary matter and energy with a distinctly minority role of only 2 percent.
Estimates??? You said it was "known". And his percents are slightly different too.
but Scherrer is the first to point out that one simple type of k-essence field can also produce the effects attributed to dark matter.
So then dark matter might not even exist. Obviously Scherrer didn't consult you. You would have told him how it really is.
Yep. Strange how we're back to an "essence" explanation.
I just need something to get a handle on just what it is they're talking about.
Your fault?
I'm far from an expert on dark matter, but my understanding is that at this point there's no answer to your question. The stuff is presumed to exist, in order to account for a host of gravitational activity, but by its nature it's not observable. It may be particles that are everywhere, but that don't interact much with ordinary matter, so you can't raise a dust cloud of the stuff. Some of our resident experts may have more to say on this.
bttt
Well that helps narrow it down a little. If we know it by its gravity, then it can't be evenly spread out everywhere, since that would cause its gravity to cancel out to nothing.
Right. I'm guessing here, but I assume the stuff would be concentrated where normal matter is, and for the same reason -- gravitation. Thus, where we see galaxies of normal matter, there is also present, where that galaxy is, a great deal of dark matter. As to its distribution within the galaxy, and possibly between galaxies, I'm not able to even speculate.
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