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- Gungnir
- Junior Member
- Registered: 2007-02-20
- Posts: 21
Dark Energy: What's the deal?
I'm having some difficulty with dark energy and would appreciate it if someone would be kind enough to do some clarification for me.
I know that dark energy is the result of the discovery that the universe isn't just expanding, as previously thought, but that it's expansion is accelerating. Physicists propose that dark energy is sort of the inverse of gravity, an energy that is pushing on astral bodies with greater force relative to distance.
My problem is, I don't know what the problem is. The universe is expanding; not from a central point, like an explosion, but every point of space is expanding relative to every other. The common analogy I've seen is that it's like dots drawn on the surface of a balloon. As the balloon inflates the dots recede from one another. But this analogy seems wrong, since the surface of the balloon is stretching. Space isn't stretching; stretching implies that the same amount of material is spread out over time. While we may not "feel" the expansion of space around us, I would think a diminishing of the fabric of existence as it is stretched thinner and thinner wouldn't go unnoticed!
Therefore it seems to me that space isn't really stretching, the expansion is more like reproduction. Imagine a batch of bacteria whose natural color is dark, but we paint them yellow, and each one reproduces into two bacteria every second. As seconds go by, our initial yellow bacteria are driven apart from one another as more of their dark progeny fill space. This seems similar to how galaxies are driven apart by expansion.
But these yellow bacteria would be pushed apart exponentially. The more dark bacteria there are between the yellows, the more new bacteria are created. If two yellows are separated by four darks, in the next second they would be separated by eight, then sixteen, and so on. Switching bacteria back to individual points in space time, the more units of space exist between two astral bodies, the more expansion is taking place as more units are created, and these new units create new units.
In conclusion, it only seems logical that galaxies at rest relative to an expanding space would experience accelerated motion relative to one another.
So what am I missing? Where does dark energy fit in?
- physicsgal2627
- New Member
- Registered: 2007-03-11
- Posts: 2
Re: Dark Energy: What's the deal?
I'm confused as to why you are confused. The dark energy or the cosmological constant is believed to cause the expansion. Without this energy there would be no expansion.
Also, your analogy of bacteria in a dish is a good one, but the universe isnt expanding exponentially. It would have flown apart and we wouldnt be here. This would be a model of a universe that were dominated by say, vacuum energy. This cosmological constant isnt as powerful as vacuum energy but it has similar properties.
- Gungnir
- Junior Member
- Registered: 2007-02-20
- Posts: 21
Re: Dark Energy: What's the deal?
Thanks for your response, physicsgal! For clarification, you said "The dark energy or the cosmological constant is believed to cause the expansion. Without this energy there would be no expansion." I was under the impression that the theory of dark energy is intended to explain ACCELERATED expansion. So it's actually intended to be a theory describing the force behind the expansion itself, accelerated or otherwise?
Also, is it possible that the universe is expanding exponentially, and that the boundaries of space extend far, far beyond the edges of the visible universe? As you said, it makes sense that exponential expansion would rip matter apart, but maybe gravity is strong enough to hold us together. Perhaps gravity itself is a far more potent force, but much of it's power is consumed in countering the effects of accelerated expansion, and the everyday gravity we experience is the remainder. Without accelerated expansion, maybe the curving geometries of spacetime would be far more pronounced.
- physicsgal2627
- New Member
- Registered: 2007-03-11
- Posts: 2
Re: Dark Energy: What's the deal?
In the first part of the 20th century physicists believed that the universe was expanding due to some residual momentum from the big bang. Sometime before that at the turn of the century however, Albert Einstein basically created the cosmological constant because he realized that eventually, without a counter force, the universe as he knew it, which was just the milky way, would collapse due to gravity. In the late 20th century however, physicists saw that the universe was accelerating and that there was not enough matter, which would be baryonic matter, or matter that interacts with light so that we can see it, to explain the acceleration. The only way to explain this EXTREMELY (its really infinitesimal compare to the size of the universe and incidentally its related to the scale R of the universe) small acceleration would be to create a random energy called dark matter, or Einstein's cosmological constant, sort of revamped.
There might actually be some residual momentum left from the Big Bang, I dont know but the expansion is mostly cause by dark energy. The solving of Einstein's field equations explained the properties of various universes. For instance, a universe that contains only the cosmological constant will be saddle shaped and open so it will fly apart, a universe containing only radiation energy will be spherical and closed. Our universe is thought to be flat with and infinite, it is a mixture of the three energies, radiation, matter and dark. However, in the past it is believed it was dominated by radiation energy, today some believe that it is dominated by dark energy. That is significant because the concentrations of radiation and matter energy decrease, the concentration of dark energy stays constant because it is built into the fabric of space.
So to answer the second question only a universe dominated by the cosmological constant would expand close to exponentially. There isnt enough visible matter to counter an exponential expansion if we assume that gravity is correct. It is entirely possible that gravity is a lot stronger than we observe, its a property of M-theory, but the expansion only occurs on cosmological scales. The density of matter energy (baryonic) in the universe as a whole is quite small and gravity goes like one over r-square, so that on larger scales it gets weaker and weaker. If gravity gets weaker on larger scales and the density of matter is small compared to say the density of the earth, then the earth would be completely crushed by gravity. It would take a ridiculously strong gravitational force to counter exponential expansion on a universal scale. But I suppose you could say that gravity doesnt get weaker with distance and that the cosmological constant simply gets stronger with distance. In which case your theory might be plausible. The only problem I would have with it is that it assumes that gravity is rationing itself. Unless it is conscience I don’t think it would care how it is distributed. But again anything is possible really.
As for space extending beyond what we can detect, it’s a mystery. The universe is though to be flat and infinite according to the standard model but its impossible to envision such a space. In any event your model would require that the solution to Einstein’s field equations are incorrect, but that is what predicts and infinite universe.
Well I got a little wordy there. Hope that helps. And anyone reading this please feel free to correct me Im not expert. I only have a BS in physics. I think someone with a higher degree would be able to answer your questions more thoroughly.
- safertr
- New Member
- Registered: 2007-07-04
- Posts: 9
Re: Dark Energy: What's the deal?
About redshift and how astronomers (or astrophysicists or comsomologists) came to the conclusion that the expansion is accelerating.
Redshift (and blue shift) are characterized by the relative difference between the observed and emitted wavelengths of the light from an object. The key here is the emitted wavelength as we can only ever assume this figure, based on our observations and assumptions. In astronomy, redshift is defined by the quality known as z, where z = (observed wavelength - emitted wavelength) / emitted wavelength.
If z < 0 then the object is moving towards us, and if z > 0 then the object is moving away from us. At redshift figures below z = 0.1 (i.e the stars in our galaxy) observed redshifts are almost always related to the line of sight velocities associated with the objects being observed. We cannot calculate any expansion from this as the amount of expansion would stretch the light by such a small amount (even if the expansion was accelerated and thus at its highest factor) that it is almost negligable.
In contrast, the luminous point-like cores of quasars were the first "high-redshift" (z > 0.1) objects discovered before the improvement of telescopes allowed for the discovery of other high-redshift galaxies. Currently, the highest measured quasar redshift is z = 6.4, with the highest confirmed galaxy redshift being z = 7.0 while as-yet unconfirmed reports from a gravitational lens observed in a distant galaxy cluster may indicate a galaxy with a redshift of z = 10. The cosmic background radiation on the other hand has a redshift of z = 1089!
Hubble suggested (and other methods using other assumptions have seemed to confirm) that there seems to be a linear relationship between redshift and distance. The bigger the redshift, the further away it is. This seems to hold true in most cases. There aren't many things closer to us moving away faster than things further away (which might have been the case if the universe was static).
But then the study of quasars threw up new possibilities. Quasars are a lot 'brighter' than galaxies and thus their redshift is easier to measure. But they seem to have a higher redshift than they should have, according to their percieved distance. And the closer the quasar the more the difference between expected redshift and measured redshift. This is why we think the expansion may be accelerating. But that's assuming we aren't missing something about the nature of quasars and how we are observing them, which is why the jury is still out on accelerated expansion.
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