Cosmoboffins think grav waves hold the key to sorting out the disputed Hubble Constant Scientists agree that the universe is expanding at an increasing rate, but by exactly how much is still an enigma. To measure the pace of expansion, slightly confusingly known as Hubble’s Constant because it's not actually constant, boffins need two bits of information. One is the speed at which an object is receding from …
The ESA gravitational-wave detector LISA will be a great help, sometime in 2034
https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna
The X-ray observatory Athena may pick up photons from the same observed events, for red shift analysis
https://en.wikipedia.org/wiki/Advanced_Telescope_for_High_Energy_Astrophysics
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If we were shrinking then the gravitational constant, the electromagnetic constants and possibly even the strong nuclear forces would all have to change and rebalance themselves in order to preserve orbital mechanics, nuclear physics and chemistry. Evidence suggests extremely strongly that these phenomena are not changing.
No matter what it is expanding into, in billions of years, all the known and undiscovered galaxies will have moved away from each other.
All the black holes (assumed to be) at the centre of galaxies will have evaporated due to Hawking radiation and all the stars will then start to recede from each other such that, they too will become ever more distant from each other and will become less visible.
And eventually as stars go super-nova or just fade to being brown dwarves, all the known energy will just dissipate to nothing, leaving a cold, dark universe with all known matter spread far and wide.
And unless they have collided with something (or been drawn into a gravity well), Voyager 1 and 2 will still be going.
Timbo - There is a theory that dark energy will eventually overcome gravity completely. So after the stars start receding from each other, the atoms of Voyager 1 and 2 will also do the same. Personally I find that sad, so I'm rooting for a gravity come-back, then a big crunch as everything collapses back ready for the next big bang. I am a romantic though.
A good question.
If we regard the Universe as a closed system, and the space-time within it is expanding, then where is the additional space-time coming from?
If only one of space or time were expanding, and the other was contracting, then we might have a way to keep the total amount of space-time within the Universe constant.
Time appears to 'expand' from its boundary, or 'end', whereas space seems to expand throughout its volume. In view of this, it seems to me that solutions that have space contracting or shrinking are going to be easier to deal with than solutions that have time contracting/shrinking.
There's no need to invoke something external for the universe to expand into. Our universe is probably finite but unbounded. Imagine 2D creatures exploring the surface of a sphere. Keeping on the same bearing, they will eventually return to their starting point; go where they may, there's no boundary.
We are 3D creatures, perhaps on the surface of a hypersphere, and would experience the same if we tried to cross the universe: there's no boundary for us to find, and a constant bearing would bring us home, but arriving from the opposite direction we started with.
There is no centre, and it's not expanding into anything. A reasonable 2D analogy is the skin of an inflating balloon, in which points on its surface are all getting further apart but there is no centre of expansion, and no 'empty skin' that the points are expanding into.
In the case of the universe it's as if the skin is somehow powering its own expansion - the question is what that expansion rate is, and how has it varied over time?
There is no mathematical requirement for a 3D spherical surface (a manifold) to be embedded in a 4D space. It's just that it's easier for most of us to imagine it as being embedded, just as we find it easier to imagine a 2D surface as embedded in a 3D space (rather than 3D embedded in 4D).
Some manifolds are weird enough so that an embedding in a 1-higher dimensional space is not possible (e.g. the Klein bottle is a 2D surface that is not embeddable in 3D)
Possibly into an 11-dimensional "multiverse", which is heavily populated by an endlessly-growing number of expanding universes or "branes" roughly similar to our own, a respectable cosmological theory known as eternal inflation.
Although ordinary inflation cannot currently account for the observed fluctuations in the rate of Universal expansion, it is the commonest explanation offered for the observed microwave background and Hubble Constant. It has no need for a multiverse or anything to expand space "into", any more than it needs a kind of "pre-existing" fixed future for time to expand forwards "into".
Small correction, the current measurements put it at 67 and 72 kilometers per second per megaparsec.
If they get a value that's within a 3% distance of either of the current ones (that'd be +/- 2.01 and +/- 2.16, respectively, which have no interlapse), it would reinforce that method of calculation.
Or they could get a totally new range. "That's odd", they would say....
One (or perhaps both) groups are wrong.
Science is proudly self-correcting. Obviously it follows that at least some of it is wrong*. Too many people keep forgetting this point.
- signed, Capt Obvious
(*And no, I'm not anti-science. I'm anti 'too much blind faith in the falible humans that are scientists'. It drives me up the wall when the science fan boyz display unlimited 'faith' in all things science. Faith, in science. They don't even see the hilarious contradiction of applying religious zeal to science...)
All of this gives me to think: According to Guth's initial expansion theory there was a large expansion of the universe coincident with the Big Bang which ceased a very short time later (no my terminology isn't precise but you can figure out what I mean). Now current measurements show that the universe is expanding. Is there a relationship between the initial expansion and the much slower current expansion? (Does Lisa Reynolds know?)
For those without a deep education in mathematics or physics, these concept can be hard to explain. It does not help that some of the core terms can have multiple meanings.
If you don't mind the very "of the times" language and imagery, the book Flatland, by A. Square is an extraordinary exposition of the subject of higher dimensions in the form of an engaging story. My explanation is going to take a very different tack, so if why I write does not help, you might look there.
As hinted, part of the problem is the need to be very clear about the terms in use. Let's start with "dimension". Consider a Cartesian plane. Such a thing is not real. Specifically, there is no corresponding physical entity to a Cartesian plane. Nevertheless, we are comfortable working with it to solve mathematical problems. As the problems get more interesting, we create more interesting mathematical objects to address them. Cartesian 3-dimentional space. Cartesian 4-dimentional space. Spherical spaces. Hyperbolic spaces. Whatever.
Most of the time, we find it useful to examine entities "from the outside". If we are considering the set of points x, y such that x * x + y * y = 1, we don't think of our selves as inside the curve--indeed it would be really difficult to do so. For the points x, y, z such that x * x + y * y + z * z = 1, however, we often talk as if we can. Because there is a (very rough) approximation that forms our lives, namely the surface of the earth.
Suppose you met with someone from an isolated tribe. In their experience, the world is flat. You inform them that the earth is round, and perhaps there is a fortuitous eclipse that even allows you so show the Earth's shadow. They hear your explanation, see your demonstration, and laugh at you. Why? "Because surely everything on the other side falls off!" In their minds, "down" is not "whatever direction gravity pulls", but is a coordinate in their Cartesian understanding of nature of the world.
This is key. Their mathematical model of world is a Cartesian 3-space. In such a space, a finite world has to have an edge. And a center that one might reach by walking or sailing. Of course, their model is wrong.
This kind of thinking is hard to overcome. I read a philosophy paper written in 2010 which began, "We know with probability one that the universe is infinite." That's a whole lot of wrong for so few words. Nevertheless, the author, and whomever he consulted for the paper, all educated people, thought such a sentiment true.
Every direct observation we can make tells us that the Space is at least roughly a Cartesian 3-space. Perhaps as a result of this, our brains are not wired to think about other possibilities. But our physicists tell us that it is almost certainly not the case. Recall that a circle (S1) is one-dimension. But to look _at_ this one-dimensional thing, you think of it as sitting inside a two-dimentional Cartesian system--specifically you think of it as the surface of a two-dimensional ball. Likewise, the sphere (S2) is a two-dimensional thing, but we think of it as sitting in a three-dimensional Cartesian system--again as the surface of a three-dimensional ball. Physicists tell us that space is probably shaped like the surface of a four-dimensional ball (S3).
But our brains don't really go there. At all.
So for your question, "where is the center"? There is in fact an answer. Just as the center of S1 is not in S1, and the center of S2 is not in S2, the center of S3 is not in S3. If we model Space as sitting in some four-dimensional space, however, we can specify it. But never go there. So the obvious follow up question, "Is the center 'real'?" becomes a matter of philosophy.
Unless further observations force us to consider forces operating on the Universe from the "outside" ...
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