Banned
The universe
70% Dark Energy
25% Dark matter
5% Baryonic Matter
If Dark Energy is responsible for the accelerated expansion, what "stuff" makes up the original Hubble expansion???
I think you say that it is "in" the 70% Dark Energy, but I want to be sure.
Established Member
Established Member
Who knows?
The answer is entirely dependent on the model or theory on chooses to explain the bang. In generic big bang cosmology, the cause of the original expansion is the bang itself. So, whatever made the bang go bang, made the universe expand. After the bang, it's just the universe coasting along, with gravity pulling back. Depending on the initial energy of the bang, as compared to the collective braking of gravity, the universe might slow to a stop, and then contract under the pull of gravity. Or, it might coast to a full stop only at infinity, which means it slows asymptotically. Or, it might continue to expand forever, even at infinity, always slowing because of gravity, but always expanding because the bang was too strong for gravity to stop it.Originally Posted by RussT
The accelerating expansion idea has changed all that. Now the idea is that the universe is expanding too fast, the bang alone can't do it. But if there is even a small, constant repulsive pressure in the universe, then the expansion must accelerate, because the gravitational attraction of the galaxy clusters gets smaller as they get farther apart, while the repulsive pressure remains constant. That's the model where the accelerated expansion is caused by something like the cosmological constant. But it could be more complicated, something like quintessence, a time variable outward pressure. Then we are even more beholden to the functional form of the expansion field.
Order of Kilopi
And I would add one tiny correction to this nice summary, which is that the dark energy "pressure" is actually negative, and in any event does not vary with position, so it's not the pressure per se. It is the antigravity that comes from negative pressure. I'm sure Tim Thompson glossed over this to avoid being bogged down, but it is an interesting point-- it is in fact still gravity that is causing the acceleration, a growing component that is all part of the gravity that was causing the reduced expansion in past times.
Order of Kilopi
I would have thought that "gravity" had little to do with the negative pressure, other than the fact that the negative pressure works in opposition to gravity. Isn't it a very different phenomenon?
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
Actually, in order for energy density to transform properly under relativity, you need pressure to appear in a way very similar as energy in the formula for gravity. In other words, if you say that energy causes gravity, you are stuck with pressure also contributing. It is just normally negligible, because most of the gravity we know comes from rest mass and not kinetic energy or pressure. But this is not so for dark energy, which if it is a cosmological constant, then it has no rest mass associated with it but does have a significant negative pressure, giving rise to an antigravity that causes the acceleration. The negative pressure comes about because if vacuum requires energy, then more vacuum involves more energy, so expansion of space is an energy sink. Positive pressure works the opposite way-- when you expand positive pressure, you end up with lots of energy you can do things with (like drive cars). But in the most bizarre twist of all, the energy sink required to make space expand does not cost the energy of the universe anything-- because it is not necessary for energy to be conserved when space expands. I suppose at some level everything I'm saying just comes out of the equations, and I may be oversimplifying because that's really all I know, or at least, have heard from those who know.
Established Member
Further reading
It's all much to complicated to go through here. But the concept of negative pressure & graivty is handled in detail in two recent papers. I suggest they should be pursued as an exercise in further reading.
- The Meaning of Einstein's Equation; John Baez & Emory Bunn, 5 Jan 2006, an expanded version of their paper recently published in the American Journal of Physics (the journal of the American Association of Physics Teachers).
- Accelerating Universe: Observational Status and Theoretical Implications; L. Perivolaropoulos, 1 Jan 2006, an invited review for the Third Aegean Summer School: `The Invisible Universe Dark Matter and Dark Energy'.
Order of Kilopi
Nice links!
Banned
Thanks Gentlemen!
Good information.
Let me restate my question though...If the Big Bang made the whole universe, then it must have made the Dark "Stuff" that was here the first 8 billion years the universe was expanding. What category would you put that first Dark Stuff in...the 70% Dark Energy or the 25% Dark Matter???
Order of Kilopi
It still sounds like you are imagining some kind of conservation principle that is not in effect. The dark matter should have appeared early in the Big Bang, the dark energy (if a cosmological constant) is appearing all the time, more and more. There is nothing particularly special about 8 billion years in this regard. And dark energy would not be considered "stuff", although dark matter certainly would be. Of course, until more is known about dark matter and dark energy, it is all highly speculative.
Established Member
An unaswerable question
The question is not answerable because it is too vague. Dark "stuff" won't do, dark energy & dark matter are distinctly different things, distinctly different kinds of things, and cannot be sensibly lumped together like that.
Dark matter & dark energy are indeed both assumed to exist, for reasons very much the same as the neutrino was asumed to exist. Wolfgang Pauli is credited with first predicting (or "assuming" if you prefer) the existence of the neutrino, in order to explain the otherwise disturbing observation that energy & momentum were not always conserved in some radioactive decay processes. Of course, he might have been wrong. Maybe energy & momentum are just not conserved, no matter how much we wish it were otherwise. But in this case, Pauli was right, they are conserved, and the particle he predicted was discoverd 28 years later, in 1959 (for which discovery Frederick Reines won a share of the 1995 Nobel prize in physics). The point of this paragraph is to illustrate that it is by no means unreasonable to assume the existence of something, as a consequence of observation, and neither is success unprecedented in such affairs.
Dark matter is the easier to understand. Like ordinary matter, it likely came into existence at some point when the expanding universe had cooled enough for it to exist. Matter in all forms can be thought of as "condensed energy"; just as amorphous water vapor can become solid if it is cooled, so can amorphous energy become solid if it is cooled. That's the basic matter making process in a nutshell. But, unlike ordinary, baryonic matter, non-baryonic dark matter does not interact with photons, which is why it is "dark". It does not scatter photons, nor itself scatter off of them, nor does it emit or absorb them. Indeed, one form of non-baryonic dark matter is known from laboratory experience: neutrinos are non-baryonic dark matter. I cannot comprehend why so many people are so outraged by the thoroughly ordinary idea that extra gravity might be caused by extra matter, yet the idea is treated with such constant disdain in forums like this.
Dark energy is perhaps harder to get a grip on, perhaps because of the press release language; "dark energy" is a bullet designed to go along with "dark matter". The idea is simple enough. We already know from general relativity that spacetime is a dynamic thing with pseudomechanical properties, akin to those of the 19th century ether. In generic big bang cosmology, the universe bangs into existence, and then coasts through the rest of its history, always expanding only because of the initial push. So maybe someday it will stop & fall back "into itself" in the legendary big crunch.
About 7 years ago, it began to become evident, based on the observation of distant type-Ia supernovae, that the expansion rate of the universe was not constant, but was rather increasing with time. This assessment may be controversial, but nowhere near as controversial in the science community, as one might think from the skewed view of things that appear in forums like this one. The seminal paper was Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, Adam G. Reiss, et al., Astronomical Journal 116(3): 1009-1038, September 1998, which has already gathered unto itself an astounding 2070 citations & counting!
So how does that happen? How can the expansion rate be time variable? The easiest way to pull that off is to allow for a non-zero cosmological constant in Einstein's equations of general relativity. That way the universe is not just coasting after the bang, but is being pushed along. If the push is constant, then the expansion will obviously accelerate, as the gravitating elements of the universe (i.e., galaxy clusters) get farther apart, and therefore pull against the expanding push with constantly diminishing force. But you could make things more complicated by eschewing the cosmological constant, in favor of a more complex field to be included in the momentum-energy (or "stress-energy") tensor. That allows the push to be variable in both time & space, and not everywhere & everywhen constant. Or you could allow both, and really get lost fast.
Here the bottom line is simply this: if the expansion of the universe is accelerating, then something is pushing the expansion. That's a bare bones logic that is inescapable. That the expansion of the universe is accelerating is widely accepted in the community of astronomers, astrophysicists, cosmologists & etc., it is not a very controversial idea. Most alternative explanations for the SNIa data have been laid to rest, or at least seriously damaged. The words "dark energy" refer in general to whatever it is that pushes the expansion along, and at the moment, that could be just about anything in a very wide assortment of possibilities. It is far too early in the history of the idea to try to pin it down more precisely than that.
With luck, this will help to make what astronomers are thinking about a bit easier to understand. And it might make it easier to craft answerable questions.
Banned
Tim, I fully agree that the universe was expanding (Hubble Expansion), then began decelerating, maybe even stopped, and possibly even began contracting, and is now "Accelerating", and that Dark Energy and Dark Matter are the best current ways to account for the extra gravity.
The problem is...the only way we know how to define the extra gravity is to give it some kind of Matter or Energy like quality!
Since I started this thread, I'm going to take the liberty of inviting you to see how I have incorperated "M" Theory into what I consider a plausible answer for all of this.
Pleae go to ATM > The Universe A, C, B
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