Please excuse and correct any errors in the following, but I’ve come up with this question after cobbling together info from various sources.
Galaxies are accelerating their separation from each other, possibly due to a ‘dark energy’.
Can we calculate that acceleration backwards in time and figure how close together everything was in distant times? And does that projection correspond to everything being very close shortly after the Big Bang?
I understand that the Big Bang was not a pinpoint of energy and/or mass hanging in a vacuum and then suddenly expanding. There was no ‘space’, or ‘distance’ before the big bang. But I am also led to believe that shortly after the bang commenced, perhaps within minutes, most of ‘space‘ had been created.
So, does the acceleration of the galaxies, projected backward, line up with the 13.5 or so billion years ago that is believed to be the ‘start of time’ ?
And, if doesn’t go back that far, what were galaxies doing prior to that?
The best evidence for, and measurement of, the expansion of space comes from the red-shift of light from distant galaxies. This is most of what leads us to believe space is expanding, and that it has expanded since the Big Bang.
The “cosmic inflation” refinement changes what we thought may have happened very, very early on.
The current “expansion is accelerating” refinement changes what we think will happen in the future.
But, really, neither of these refinements changes the basic idea: space is expanding, now, at a fairly steady rate, and this allows us to calculate the age of the cosmos. The estimates come in pretty close to each other these days: the variability in the estimates from different models is narrowing.
As the mayor of Halloweentown said, “We’re all waiting for the next surprise.”
Yes, they line up. If they didn’t there’d be something wrong with our model of how matter has been moving, or with whatever combination of methods used to estimate the age. (The wikipedia article on the age of the universe mentions “rate of cooling”, and though I wouldn’t be surprised to learn the expansion of space being a factor in that calculation I’m not an astrophysicist.)
You seem to be trying to check two different things against each other. And that’s a good thing to try to do, in science. But you’re actually just checking the same thing against itself. The Big Bang Theory originated precisely because people extrapolated back the motions of galaxies to a time when they were all very close together. So what you’re really asking isn’t something like “Is the sky blue?”, it’s “Is the sky the same color as the sky?”.
That said, however, there have since arisen a number of other ways of studying the evolution of the cosmos, aside from measuring the redshifts and distances of distant galaxies. Some of these methods are completely independent from each other (in particular, the measurements made from the cosmic microwave background are unrelated to the measurements from supernovae), and they do indeed agree with each other to a beautifully remarkable degree.
But doesn’t the increasing rate of expansion mean the Hubble Constant isn’t actually constant? Have the numbers changed by much given that it seems the younger universe must have been expanding at a slower rate?
Hubble’s Constant is changing, but if the dark energy has its way, it won’t keep on changing.
In the simplest models of an expanding universe, where everything just expands apart at a constant rate, the Hubble Parameter (note that I’m not calling it a constant here) would just be the reciprocal of the age of the universe. In a universe that’s expanding exponentially, it really is a constant. A universe with nothing but dark energy would expand exponentially. A universe which is almost entirely dark energy would expand nearly exponentially, and thus have a Hubble parameter which was nearly constant. In our current Universe, the dark energy appears to be dominant, but not yet overwhelmingly so. Assuming that nothing fundamental about the dark energy changes (a very large assumption, since we know almost nothing about how it works), the dark energy in our Universe will become increasingly dominant until everything else is negligible.
I would refine this to include “and has been for the last 13 billion years”, since we get these measures of expansions by observing the light that left galaxies billions of years ago.
Why are the galaxies sticking together-why don’t the stars in galaxies recede from eachother? it seems that galaxies have some “glue” that keeps them together.
Second: what accounts for the enormous kinetic energy of the receding star? Does it add up to the energy released in the Big Bang?
<snark> Dark energy is a mythological substance created by physicists so their equations will balance.</snark>
Seriously, though, there’s data that seems to imply that electron orbital distances are smaller today than 10 billion years ago. That’s not supposed to happen AFAIK. Noteworthy is that we’re trying to measure a few nanometers from 10 billion light-years away, there may be some instrumentation issues.
It comes down to the speed of light being constant, how do we know this?
First of all, the result you’re referring to is extremely small and only barely significant, and even the group that discovered it thinks it’s probably just random noise in the measurements. That was years ago, and nothing has changed meanwhile to make the result look any more solid.
Second, despite the popular reporting, even if true it does not imply that c is changing. It’s completely meaningless to speak of c changing, since it’s exactly equal to 1. It’s actually about something called the Fine Structure Constant (alpha), and while c is one of the fundamental constants which is used to derive the Fine Structure Constant, it’s not the only one. It would make far more sense to describe a change in alpha as a change in the strength of the electromagnetic force.
That’s not right, a red shift simply means they are receding from us. The original spacetime expansion is what “threw” everything out and it was thought that the expansion would either settle out, stop or collapse back down.
Observations show that the receding is accelerating and this is currently linked to the inherent energy in spacetime. It’s poorly understood, less so than dark matter, and is called dark energy. Think of it as spacetime “wanting” to expand and relax out.
My impression was the authors were unequivocal in the bogosity of any conclusions the data could bring, especially anything crazy like c changing.
The Wikipedia article leaves me believing this isn’t exact set in stone yet. Meh, I guess if we define them as constants, then they have to be constant … just seems a bit arbitrary to me.
Basically, what it comes down to is that whenever you speak of any quantity with units changing, you have to specify changing relative to what. alpha itself is dimensionless, so it’s perfectly fine to speak of it changing without needing to specify any further context. That doesn’t mean it does change, and any physicist would wager good odds that it doesn’t, but it’s meaningful to speak of the possibility of it changing. Pinning down which of the ingredients in it is responsible for the change, however, is not so clear-cut. It almost always makes sense, whenever we speak of an action or angular momentum changing, to mean changing relative to hbar, and similarly, when we speak of a speed changing, we mean changing relative to c. To say, based on a hypothetical change in alpha, that c is changing, means that it’s changing relative to the speed e^2/(4piepsilon_0*hbar), and why on Earth would anyone ever measure any speed relative to that?
Oh, and,
While I wouldn’t put it in quite those terms, neither can I really argue with that statement. Though I will note that very similar statements could be made about, say, the neutrino, which we’re now pretty sure we have a pretty good handle on.
Ah, so you’re saying the question is a cheapskate and I’m wasting my time begging it. That makes sense in a way that doesn’t make sense that’s just the right way to not make sense … time is relative after all. I’m looking for dc/dt ≠ 0, which would make some pretty cool conclusions, like a steady-state universe. I don’t know about you, but that would help me sleep better at night.
Here’s my guess: The galaxies would fly apart if space/time was a perfect plain, but space/time is full of cute little dimples such that even though stars are moving along a perfectly straight line in space, just that space itself is curved into a circle so it appears that some force is holding the stars in orbit …
My guess on the second question is dark energy … [giggle] …
Oh, about that energy: Conservation of Energy is, so far as we can tell, absolutely true, but it doesn’t mean what most people think it means. It’s a local law, not a global law. If you take a box, and measure the net amount of energy flowing through all of the walls per time, that’s the same as the change of the amount of energy in the box over that time. But the funny thing is, even if this is true of all boxes in the Universe, it doesn’t necessarily mean that it’s true of the Universe as a whole: The total energy content of the Universe can change, and this doesn’t violate local conservation.