I am indebted to John at Johns Justice for his valuable comments prior to my posting.
The concept of space and time are usually considered separately. We say space is the empty gaps and time is a measurement. We measure our life or events in our life as an expression of the time between them. Ordinary time is a horizontal line left to right with the oldest events on the left hand side and later times on the right But when we speak of space time we are speaking in the context of the universe as whole, on a macro level, as if we are outside looking in. The same principles apply to the micro level on earth, but the differences are so infinitesimally small, we take no notice of them. In this context of the macro view of the universe time and space can be considered the same thing. Why is this so?
We start out with a hypothesis which becomes a verified theory through calculation and observation. Firstly space arises from the expression of matter. That is, the interaction of objects and energy through gravity. There can be no time or space before matter, only that which arose from the Big Bang and afterwards. For time has a definite beginning and an end. Space is a product of the expressions of matter and its matters interaction brought about by gravity, that's what we measure, the space from that interaction. That space, from the expression, also represents its time. For we live in a time dependent universe, as events take place that is time to its active participants. The matter in the universe is continually expanding or contracting as is time, and space, hence space time. Consider the aspect of a black hole, where time comes to an end, space then also does not exist. The corollary is true for the beginning of time and space from the big bang and henceforth there is nothing new, only a changed form in its space time. We measure the gap (space), which itself is a representation of the time of that gap that occurred as a mirror reflection of its expression. When I say a mirror reflection I mean it reflects what the expression is, which, as consequence of the effect of gravity, includes a bending of time as one heavier object, pulls another towards it, or by energy but its still the same space, just warped or bent time in that space. Like a taut rubber mat with heavy objects sagging and distorting its shape. A warped mirror image reflection. These theories that state time as having a definite beginning and end have stood up to untold scrutiny, by observation and backed up by mathematics and numerous scientific analyses. It represents the big leap forward from Newtonian thinking to Einstein and beyond Einstein to Stephen Hawking who proved time had definite beginning and end.
In fact Einstein has already discovered this, but fearing his calculations could not be correct invented a cosmological constant to cancel the effect of his equations that correctly indicated time indeed come to an end in singularities.
Space Time in the Universe
We can view the galaxies as they appeared almost at the beginning of time (at the time of the big bang singularity) as light has a finite speed. Looking back in time when matter was at a higher density over a period of roughly 15 billion years the universe can be imagined as an inverted ice-cream cone, as light is warped by gravity and energy from its big bang singularity tip beginning. So we can see it, mathematically prove it and measure it. I think our ancient philosophers and prophets would be in awe at what we are now are able to understand of creation, marveling at its majesty, in its light filled splendor. An amazing evolutionary story, of its untold mysterious energy from the ONE. It can only be partly understood by the use of our own energy and imagination, in imaginary time partnered by rational thought. The elusive imperfect partnership in our human quest for truth.
For when we take off in our future space ship we had better count on the existence of matter. All kinds. Seen and unseen or what we call dark matter, black holes. In between the vastness of space, expanding as we view it, for the stars may no longer fill the night sky as they expand out into expanding space in infinity
It's likely we will have trouble avoiding a collision with black holes, thought to account for say 90% of all matter in the universe. Singularities where space and time cease to exist.
We may also have to abandon the theory that there is a unique universe that we can observe in this manner. It's more likely there are any numbers of possible universes, mulltiuniverses, with a probability distribution determinable only by that which is trapped within it. Perhaps we live in the most probable of all possible universes. How could we possibly measure this in terms of time? Stephen Hawking suggests we introduce the idea of imaginary time, horizontal to our concept of ordinary time, down the page, at right angles to the horizontal line. This means it behaves in a similar way to the 3 directions that correspond to the moving space dimensions e.g. let's say there are 3 dimensions in space.
Our common experience tells us we live in a 3 dimensional space on earth, as we describe a point in space is by 3 numbers, e.g. Latitude, longitude and height above sea level. How can space be 3 dimensional? Imagine it to be 3 dimensions, just like the earth with little bumps all over it.
The 3 space directions and imaginary time would form a space time that closed in on itself without any boundaries or edges like a sphere. The curvature of space time caused by matter in the universe will lead to our 3 dimensional space and the imaginary time line meeting up around the back the back of it.
A very rubbery history of the universe. One that needs plenty of imagination, able to see the multiuniverses in our mind, and subject to change even as you begin to consider it. In fact its histories could be a huge mixture of things from different multiuniverses and imperfections, imagined and real. I don't think we will ever be able to fully write down that history but it's fascinating to think about it.