I keep reading the same rhetoric about “dark energy” and the accelerating universe, but the real question is WHY are scientists so sure the universe should be slowing down already? Consider:
a) Like a ball and paddle (connected by rubber band), maybe we’re observing that the rubber band is still stretching outward from the last whack (i.e., the last Big Bang). Maybe the universe has YET to slow down let alone collapse back in upon itself, right?
b) Wasn’t it once postulated that the universe might actually expand forever and never collapse back on itself (i.e., an open universe) as opposed to collapsing back on itself (i.e., a closed universe)? Whatever happened to the open universe proponents? Did they sell out?
c) Are scientists assuming the acceleration due to the Big Bang is constant acceleration? Isn’t possible they are observing “jerk”, simply defined as accelerated acceleration? Why should the acceleration from the last Big Bang be constant, or say a linear function? Like so many researchers do, are they making the most basic of assumptions (like biology’s one gene, one trait theory) on how the universe behaves?
d) Is it possible they are witnessing the push from anti-gravity? If this exists, wouldn’t celestial objects appear to be moving faster and faster away from each other? [Wouldn’t it be wild if it behaved as the physical opposite to mGm/R^2? The further two masses are apart, the more they repel each other!?!]
e) Last, have scientists found that all celestial objects at a locus equidistant from the projected point of the Big Bang (as best we can determine by tracing their paths backwards)…are said objects ALL running away at the same rate? Or, is it a mixed bag of results? And, whatever happened to those rogue objects exhibiting blue shift, i.e., moving towards us? How do they fit any model? (Or, is it assumed something changed their initial “red shift” course?)
If gravity exerted by normal matter were the only force acting on the Universe, it would always slow down. The difference between the open and closed models is just that in the open models, it doesn’t slow down fast enough: In the hyperbolic models, it asymptotically approaches some nonzero speed, and in the flat ones, it asymptotically approaches zero speed.
The observations can be explained either in terms of gravity working differently than we expected (the so-called cosmological constant, which could be vaguely described as anti-gravity), or in terms of perfectly ordinary gravity acting on a peculiar sort of substance (the dark energy). The latter explanation is currently preferred, though mostly on aesthetic grounds: Either is still perfectly viable.
No matter what point you pick, everything at a given distance from that point is moving away from that point at the same speed. All points have equal claim to being the “projected point of the Big Bang”.
But the rate of expansion is accelerating. If there was one big push 15 billion years ago, what is causing the continued acceleration? That requires a continued force of some kind. One big push would slow down until the expansion reached zero; the universe’s expansion is not doing that.
It is currently believed by the majority of scientists that the universe will, indeed, expand forever and never collapse back on itself. I’m not sure where you’re getting the idea that the proponents of this idea have gone away.
I’m not sure I can parse your question, or what you mean by “jerk”. Acceleration requires a force, and acceleration is happening. Therefore there’s a force of some kind, which has been termed - for want of a better word - “dark energy”. Dark energy is a placeholder name for something we don’t know much about yet.
I guess there’s an infinitesimally small possibility that it is something we’ve never seen like anti-gravity or magic or unicorns or God or something. Doesn’t seem likely, though. But yes, one can never completely discount anything.
There isn’t a central point where the Big Bang took place. The entire universe was that central point, and that central point expanded to create the universe. Every point in the universe is that central point.
Galaxies aren’t all moving away from a single point. They are all moving away from each other.
This is true on a massive scale, but locally (as in within our galactic cluster, for example), things can still be moving towards us because their velocity towards us as induced by the gravitational pull between them and us currently exceeds the rate of the universe’s expansion, so they have a net movement towards us. On a much larger scale, things are moving apart.
With a ball and paddle, after the initial whack the ball will always be slowing down. All the energy is added by the whack and after that the ball slows down until the turning point and then speeds up in the opposite direction. This is of course because the force involved in rubber band physics increase as the band is stretched, we don’t expect the universe to act like a rubber band like that. But if it isn’t slowing down, then all our physics tell us something is adding energy to the system. And rather than slowing down at some rate the expansion apparently is accelerating. Thus “dark energy”.
