Does time expand?

[Aki]

We all know that space in our universe is expanding, but like Einstein said, space does not exist on its own, only spacetime exists. So would time in our universe be expanding as well?

[Qfwfq]

We all know that space in our universe is expanding

This is so according to a model based on the so-called co-moving coordinate atlas. In this model the coordinates in a spacelike submanifold for a given cosmological time have an increasing scale.

 

Semantically, the gerund of a verb wouldn't make sense for the whole space-time manifold. It comprises all times, so it couldn't very well be anything-ing, could it? :)

[IDMclean]

Time == Measurement of anykind of motion.

Time dialation has a upper and lower limit to it.

All frames are realitive. Just cause the overall universe's scale increases or decrease the individual parts still operate on their own scale...

 

Am I making anysense?

[Bo]

standard cosmological models are based on the so called cosmological principle:

-Every point in space is equivalent

-Every direction is equivalent.

 

in the context of general relativity this means that:

- The spacial part of the universe can have a time depenedent scale factor (measuring the expansion of the universe)

- in principle the time part can also have such a scale factor,

 

However:

We can rescale the structure of spacetime in such a way that we only have the scale factor of the spacial part left, so an expansion of a bit of space and a bit of time, is equivalent to an expansion of space alone. (or for that matter: time alone, but space is intuitively somewhat better to understand)(still next to impossible though..)

 

So space and time are still intimitely linked :)

 

Bo

[Qfwfq]

Am I making anysense?

Well, I guess so... in your own little way! :)

 

Choose your atlas!

[UncleAl]

Is ds^2 a conservative quantity (re the metric signature)? If so, how does expansion of space affect the time quantity? Note the signs!

[paultrr]

A theory of time should describe the relationship between instants and events. Generally we look at time as a continum. In an absolute vs. relational theory of time, the question that is normally addressed can time exist if no event is happening. However, given that in this universe, especially at a quantum level, there actually is no such thing as a lack of an event. Another thing that a theory of time should address is the question of time's apparent direction. In one sence since under Einstein's relativity space and time are united and the direction of time is generally forward, time does expand. It expanded from the first instant in time onward as the universe itself expanded.

 

However, there is a qualification of this. Given that C seems to have been a constant at least since the end of the inflation period when we speak in physics about events generally our measuring rod we use is C or some fraction or multiple there of. If C is held to be constant then our measuring rod defining the duration of each event has not expanded. However, if the traditional VSL, proposed by guys like Jose is correct, then our measuring rod might be shrinking with time which would imply time shrinks as the universe expands. On this issue there is also varients to the VSL idea where while C globally has stayed rather constant it might not be constant in all local situations. In that sence it is possible to have at least the measuring rod for time to have expanded and contracted at the same time. So a lot of this really depends upon not only the perspective one looks at this as well as one's own side and theory as far as physics and cosmology goes.

 

On the metrics question, the assumption is usually made that this expansion results from spatial metrics that change with time. The idea of an expanding space naturally leads to the conclusion that the Universe originated in a spacetime singularity(ie the Big Bang or in certain models ways are found around this singular moment in time). Outside of certain odd metrics, the basic idea of these expanding models is that the Universe or any space discribed by these metrics evolves by changing the spatial metric relative to the temporal metric. Which results in expanding space while keeping the pace of time the same. So the proper answer is that time does not actually expand, its the spatial aspect of spacetime that expands unless you do have some VSL case in which there are different choices that depend upon the model. My own stance is that outside of possible local changes C has not varied since the end of inflation. So I'd tend to agree with the majority view that only space expands, not time.

[Qfwfq]

Is ds^2 a conservative quantity (re the metric signature)? If so, how does expansion of space affect the time quantity? Note the signs!

I'm not sure what you mean by "conservative quantity" UA, I've heard of conservative fields but not of conservative quantities, perhaps you meant conserved quantity. Well, that would depend on the Lagrangian or the Hamiltonian of the dynamical system you are describing. I'm not sure which one you are talking of. Further, ds^2 isn't really a quantity, it's a differential form. Unless you are talking about the interval between two specific points of space-time. Which two? In this sense it's a scalar, called the square of proper time. As for the metric itself, it's a tensor. Question unclear I'd say, UA.

 

Of course, if you mean that expanding space coordinates implies expanding time coordinates for a given proper time, alright, but does that mean that time is expanding?

 

In any case, co-moving coordinates are just a choice of atlas, I never understand why the notion that "space itself is expanding" is taken so literally. Just like the geometrical interpretation, you can't verify it without seeing the global topology of the universe.

[Bo]

ds^2 is not so much as a conserved quantity. it is a quantity that is invariant under lorentz transformations (which are basicly the relativistic equivalence of translation and rotation)

 

Bo