Somewhat less even than that, given that light itself does not travel at the speed of light.
That is curious: 5 billion years ago was right around the time that this here star was starting to form. That suggests to me that there is more going on with our system than we have been able to account for (i.e., our observations are more distorted than we realize).
Well thank you very much, the perfect and concise debunking to my half-baked notion. Well done, bravo!
It’s still 13.7 billion years for any rounding level. Since neutrinos, even with mass, have a speed less than light with an upper bound of 2 parts in a billion, any difference for light itself would be negligible.
That these two numbers are similar may be the effect of rounding error. Or one could be not accurately known. Or it’s just pure coincidence.
The explanation behind the mechanism of the expansion of the fabric of space-time has to have a common-sense answer you find plausible? Do you understand the implications of this? It’s exactly why Creationism maintains its clammy hold on such a large percentage of the population. Please rethink this and then reject it in its entirety.
The best fit we have to the data does suggest that the phenomenon causing the acceleration only started very recently… but this is not a very robust result. Given the noise levels in the data we have, a constant-magnitude dark energy is also within the error bars.
Note that even a constant-magnitude dark energy will result in a period of deceleration early in the Universe followed by a period of acceleration. This is because the force due to the dark energy isn’t the only one at work: You do still have the attractive force from ordinary matter, which is more significant when the matter is closer together.
As I understand it:
Essentially, space is expanding everywhere at all times. More space is being created uniformly across the universe. The rate is slow enough that it isn’t going to significantly affect things that are bound together (by gravity, molecular bonds, etc), but essentially, between everything not bound, about 72km of space is being created over every megaparsec every second.
As an analogy, imagine the perspective of any particular raisin in raisin bread during when it rises. Every other raisin gets further away from it, there’s no center, just more bread expanding between everything.
So as two particular things in space are being made further apart by this expansion, there’s more space between them, which means there’s more space to expand, which means they go apart even faster. This alone can account for distant galaxies accelerating away from us - well, not actually accelerating, but the distance between us gets further faster. It’s my understanding that this alone could account for an expanding universe.
But evidently we’re flying apart even faster than that, and that’s the unexplained force - dark energy.
Nag, nag. FWIW, I don’t consider Creationism to be either common sense nor plausible. A lot of modern physics can be described that way too, but at least it is demonstrable. The d
expansion of spacetime is mind boggling and fascinating, and it won’t hurt to break some lousy ideas against it.
There has to be some explanation. It is hard just to get used to the idea of space as a thing, let alone a malleable one, let alone a spontaneously, constantly changing one.
Beef: So, dark energy and space inflation are potentially unrelated??
In the big bang model the Universe started off with a sort of natural momentum (I use the term ‘momentum’ loosely and not in it’s scientific sense) that caused the matter within the Universe to fly apart much like an explosion. However the difference between the big bang and common or garden explosion is that the latter is the same in all places and all directions, so that in many ways every observer everywhere in the Universe sees themselves as being at the centre of a giant cosmic explosion.
The natural dynamics of matter (and again use a term loosely, so that ‘matter’ includes radiation) are such that they tend to slow the rate of expansion, and as these are the same dynamics that cause clouds of gas to collapse into stars, we might as well call this gravity. Gravity can either slow the expasnion so that it approaches a constant rate of expansion, slow it so it approaches a zero rate of expansion or slows down so much it becomes negative and the Universe collapses (the Big Crunch).
Now the natural question may be where did this initial ‘momentum’ come from? The answer is we simply don’t know and the best we can say is that it comes from the initial/early conditions of the Universe. Unless we understand those conditions better we can’t say much more. We know though that it must exist as Universe containing matter can only collapse without this initial momentum. The observation of an expanding Universe is proof positive that it must exist.
But this is not the full story of the expansion of the Universe (and we’ll completely skip over the part that is covered by theories of cosmic inflation) as at the end of the last century it was noticed that in the Universe’s relatively recent past the rate of expansion has actually been increasing. However, as I said at the end of the 2nd paragraph, matter can only decrease the rate of expansion so there must be something else going on.
The catch-all term for all the proposed mechanisms/models behind what is causing the expansion to increase is ‘dark energy’, though it needn’t be viewed as energy as perhaps we would normally think of it. We know for the reasons stated above it cannot be caused by the matter and radiation (including dark matter) in the Universe though.
The most popular model for accelerated expansion is a positive cosmological constant because it is arguably the simplest model and it fits best with the evidence. Models with cosmological constants are actually pretty much as old as big bang theory and have been used to propose solutions to various problems in cosmology before the discovery the acceleration in the Universe’s rate of expansion. Most usually though as soon as models with cosmological constants were wheeled out to solve a problem in cosmology a better solution had been found (until the discovery of the accelerating expansion of the Universe that is). In fact Einstein was the first to propose a positive cosmological constant it what he later called his “greatest mistake”.
One way of viewing a cosmological constant is that empty spacetime, rather than being flat, as in special relativity, has a natural and constant curvature. This what I mean when I talked earlier not having to see it as energy as we would perhaps normally think of it.
In our current best models the Universe expanded due to its initial momentum with gravity putting on the brakes to slow the expansion, but as the Universe became more spread out the slowing effect of gravity became less until some point in the relatively recent past (i.e. a few billion years ago) dark energy (whose effect on the rate of expansion is constant, if we take it as the cosmological constant) became the dominant effect on the rate of expansion of the Universe.
Getting back to this… the ‘shell theory’ is about the effect of gravity on matter. What about the effect of gravity on space? If two opposing gravitational fields pull on a given, vast amount of space, is the result the stretching of that portion of space?
As for the observed isotropy- I suspect that the effect in a multiverse would be the same as in the universe, that is, it always appears to the observer as a perfect, expanding sphere.
“Asympotically fat” mentioned, “Cosmic Inflation”. I don’t mean to hijack this thread but what is the current sense about cosmic inflation based upon current findings? I thought I heard something about faulty findings/bad data. OR should I create a new thread about this if the “current sense” explanation does not figure into answering the OP’s question??
Spacetime appears to be a strange medium with non-material properties. Another poster was ranting on about æther in a rather long thread, but the basic model he was using was more-or-less valid, if you leave out the material aspect of it.
Picture a huge tank of fluid that behaves rather oddly. You can move through it effortlessly, at least in a straight line, by itself it seems to have no effect. Now put a really massive body into the fluid: the region around that body will become less dense the nearer you get to the body, in a more-or-less spherical gradient. So, as you approach the body, the lower density toward it will cause your path to curve in its direction, as if the fluid itself (spacetime) were exerting a force (non-flat inertia) on the side of you away from the body. Kind of like the way an airplane loses altitude when it flies into a region of air that is abruptly and significantly less dense than it had been flying through.
So, one way to look at gravity is that it pulls. But another way to look at it is that it sort of pushes. Either way, the math comes out the same.
Now, if spacetime were of a material nature, which it does not seem to be as far as we know, the effect of a body would cause an offset balance: if a body rarifies the inertial fluid near it, one would expect the fluid to, like, go somewhere (increase in density far from the body, as it pushes it away). But as the effect would be far away and hence extremely small (per the inverse square law), observing whether or not that happens is probably well beyond the limits of what we could reliably detect and would probably not make much or any difference.
As to your question, note that gravity fields, howsoever they actually work, are spherical. Hence, they cross minimally: if two tremendous gravity fields were opposing each other, their effect would be strongest in a very localized area, falling off perpendicularly to the common axis. Things pulling on the universe from outside where we can see would have to be incredibly massive, far away, and arrayed evenly abroad a spherical surface. The first two things are not inconceivable, but the third thing kind of really is. Massive bodies with immense gravity simply do not behave that way: they would start jostling each other around until they found some sort of harmonic equilibrium, which, in nature, is not an isotropic arrangement.
Start a new thread if you want. I don’t mind if you ask these questions in this one. Are you saying that the expansion of the universe is actually not accelerating, that this conclusion is the result of bad data? Show us a cite; that could blow the thread out of the water.
Maybe someone just left the spacetime tap running. It started out a drizzle, but now the washer’s well worn out and it’s full on gushing…
Yes Try2B Comprehensive. Looky here. Does this put a “crimp” in things? I know < 0 but I do have an inquiring mind. Do tell your interpretation of the link.
As a side note, Frederick I. Ordway III, the one who put “Odyssey” into “2001: A Space Odyssey”, has died,
“Could you please work out a brief, concise explanation of the propulsion system and general operating features of Discovery?” Mr. Kubrick asked of Mr. Ordway in a letter in September 1965."
Too Cool.
All right. Deep down I knew the difference between dark matter and dark energy, but that did not come out in the OP, or was in fact con-fused into a single idea that shouldn’t take that form.
Our understanding of the Big Bang is ultimately speculative- the phenomenon is infinite in scope, yet our understanding of it can only proceed step-wise.
There is a frission between these two viewpoints, which can create an illusionary, third phantom viewpoint, actually a set of them, which aren’t really real. You can observe this confusing our debates from time to time.
Determining the reality of the various competing claims becomes the real nub of the game.
I’m not actually qualified to parse microwave readings from immediately after the Big Bang, sorry about that, it is a failure of our system to be sufficiently liberal that is to blame. Looks like the claimants backtracked on detecting primordial gravitational waves, so there is a sort of tarot indication of conservative influence in the cards. Don’t trust anyone too much is advised.
Hard to argue with more data with a handle like mine…
Right now, we have something going on (which we label “dark energy” out of ignorance) which is causing the expansion of the Universe to accelerate. It’s very difficult to understand what this phenomenon is. Our best models of particle physics predict the existence of a phenomenon, vacuum energy, which would behave qualitatively in the same way as dark energy… but quantitatively, they’re way off. Our models of particle physics aren’t good enough to exactly calculate the strength of the vacuum energy, but we can still make estimates. When we do so, the estimates we get are extremely ridiculously ludicrously higher than the amount of dark energy we actually observe… and in saying that, I just made the biggest understatement on this message board. The estimate is so far off that one just can’t put it into plain language just how far off it is. Now, with an estimate of this sort, nobody would be particularly surprised if the estimate were off and the real value turned out to be zero… but it beggars belief that it’d be this far off while still not being exactly zero. So nobody really knows whether the particle physics estimate is relevant or not: It might be that it really is exactly zero, and the dark energy is due to some completely unrelated phenomenon.
OK, so that’s the state of our ignorance concerning dark energy. But there’s other evidence that, in the extremely early Universe, there was another time when there was a phenomenon causing expansion. This phenomenon, called “inflation”, was much stronger than the current dark energy, and in fact could very well have been about as strong as we expect the particle-physics vacuum energy to be. According to our models, this phenomenon lasted for a very short time, set the Universe to expanding, and then stopped, leaving us coasting ever since. It’s not a proven fact, of course (nothing in science ever is), but the evidence in favor is strong enough that almost all cosmologists treat the existence of inflation as the default assumption.
There might have been some new recent result which calls inflation into question. No, let me rephrase that: There certainly is some new recent result which calls it into question. There always is, at any given moment, for any given widely-established model in science. But historically, such new results questioning the status quo almost never pan out (that’s why it’s the status quo).
To my limited understanding, whether these results pan out or not they are irrelevant to the thread. Dark energy is an entirely separate thing and the expansion of the universe is happening regardless of the cause.
No, it’s about the gravitational field within a shell-like distribution of matter, which is exactly zero; it wouldn’t ‘pull’ on spacetime anymore than it pulls on matter inside.
Besides, the idea of gravity ‘pulling’ on space in this way is misleading in two ways: first, gravity couples to—jargon for: is caused by and influences in turn—mass, or more accurately, stress-energy, a somewhat abstract quantity that contains the local mass/energy-density, momentum flows, pressure and shear stresses. In empty space, this quantity is zero, leaving nothing for gravity to coupld to.
But things aren’t quite that simple, because in an important sense, the gravitational field is spacetime. Basically, the gravitational field tells us about the proper notion of distances between events—that is, their spatial as well as temporal separation. It’s essentially the set of rulers and clocks you cover space with in order to determine how far, in both space and time, it is from one event to another. The fact that this set of rulers and clocks is not everywhere uniform is what causes gravitational effects—the geometry of spacetime changes, and with it, the notion of a straight line, so in the vicinity of mass (or rather, stress-energy), motions will deviate from straight in a way described by the gravitational force law.
But then it turns out that even in empty space, the gravitational field need not be intrinsically zero: there may still be an overall positive or negative curvature. This intrinsic curvature may be either negative or positive, and can be captured in a quantity called the ‘cosmological constant’, as mentioned by Asympotically Fat. Our universe, seeing as how it is currently expanding in an accelerated fashion, will ultimately approach just such a state: an empty universe (all the matter having been infinitely diluted) with positive curvature, known as the de Sitter-universe.
Anyway, all this just to say that gravity shouldn’t be thought of as a force ‘pulling on’ spacetime, as if you took a sheet of rubber in both hands and gave it a good tug; rather, it’s a property of spacetime itself, which is ultimately determined by the configuration of matter (read: stress-energy) within it. Of course, even this picture is further complicated: the gravitational field itself has nonvanishing stress-energy, so it is a source of gravity—this ‘self-coupling’ makes the theory nonlinear, since you can’t just add the gravitational fields of two matter distributions together, but have to take into account the gravitational fields sourced by those gravitational fields, and so on. But this just as a final parentheses.
In general, they’re not. The shape of the gravitational field depends on the matter distribution—for instance, there are also gravitational fields corresponding to plane wave solutions.
This concerns not the accelerated expansion our universe is currently undergoing, but the expansion it underwent in the first few instants after the big bang, generally called ‘inflation’. This inflation is predicted to have left a characteristic signature in the cosmic microwave background due to the gravitational waves it produced; the BICEP-2 team has claimed to have detected the corresponding fluctuations, but now some have suggested that the claim was erroneous due to an incorrect modelling of interstellar dust effects.
However, inflation touches on another of the OP’s questions which I didn’t get to earlier, namely whether it’s possible to have different ‘big bangs’ in the same spacetime. It’s here a bit of a question of how you define ‘big bang’: if you take it to be the point of origin of our universe, as in ‘classical’ hot big bang theory, then no, this isn’t possible—our spacetime is like the surface of a balloon, all of which originates from the big bang.
But inflationary theory has led to a paradigm known as ‘eternal inflation’. Basically, inflation is caused by a hypothetical field, the inflaton, which, well, inflates; but there is always a slight probability for it to undergo spontaneous decay, in which case we get a region in which inflation stops. This region then undergoes reheating, which means, basically, particle creation—so in this post-inflationary region, sometimes called ‘bubble’, we have what looks like a big bang—a hot fireball creating a shower of particles that, as the whole thing cools, coalesce into complex structures, possibly including some like those we see in the universe.
In this scenario, due to the fact that there is always some probability for bubbles to nucleate, there will typically be many big bangs—and since the whole thing goes on forever, infinitely many of those. The space between most of these bubbles will typically be expanding exponentially, so it’s impossible for one to influence the other. But it’s in principle possible for a bubble to nucleate such that it’s within the lightcone of another bubble, in which case both universes will collide—what happens then is somewhat difficult, and depends on things like the vacuum energy of each bubble (a lower vacuum energy in one bubble will cause the other bubble to decay to this lower level, as well, for instance). Such bubble collisions can, in theory, leave observable evidence, again in the form of fluctuations in the cosmological microwave background, but none has as yet been observed.