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I am not a scientist nor do I have a degree in Astrophysics, but I do like to learn new things by asking questions. With that being said, I have read that time is relative to space which began after the big bang. Moreover, before or right at the moment before the big bang, energy was confined to a very very small scale like the tip of the needle or something.

But, I wonder since energy can neither be created nor destroyed, that means energy is infinity with no beginning.

So, when it is said time started with the big bang, is that in reference to our universe? Because in reference to energy, shouldn't time be infinite too? Or time has to be relative to space as well and so there is no concept of time prior to the big bang?

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    $\begingroup$ There’s a logical jump in the statement “I wonder since energy can neither be created nor destroyed, that means energy is infinity with no beginning” that is losing me; what I think you’re getting at is that since energy can’t be created how did the Big Bang come to be, but I’m not sure what that has to with the conclusion that energy is infinite. Could you elaborate? $\endgroup$
    – Justin T
    Jul 19, 2022 at 4:52
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    $\begingroup$ @JustinT well since energy can't be created or destroyed, so wouldn't that mean that it has no beginning and an end translating to infinity? $\endgroup$
    – Ed_Gravy
    Jul 19, 2022 at 10:41
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    $\begingroup$ @Ed_Gravy Energy being infinite would mean that there's unlimited energy. The energy exists forever (as far as we know, and we have good reason to think this is the case), but we have no evidence that it's unlimited. You seem to be confusing the idea of "infinite" with the idea of "eternal". $\endgroup$
    – Hearth
    Jul 19, 2022 at 15:44
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    $\begingroup$ @Hearth and here I thought eternal and infinity mean the same thing or are similar, so your comment is indeed interesting. $\endgroup$
    – Ed_Gravy
    Jul 19, 2022 at 15:46
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    $\begingroup$ @Ed_Gravy Infinite means that there's an unlimited amount of something, whereas with something that's eternal, there may only be a very tiny amount of it, but there will always be that same amount. In principle, something that's infinite now doesn't even have to be infinite tomorrow--there could be as much of it as you want right now, but none to be found tomorrow. $\endgroup$
    – Hearth
    Jul 19, 2022 at 15:48

3 Answers 3

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As far as we can tell, the four-dimensional space-time continuum is unbounded in the space directions (space is infinite) the time dimension seems to have a singularity (about 13.8 billion years before now) and so it is impossible to talk in a sensible way about events before that time. We would normally say that the universe and time itself started at that point, and so any statement about "before the big bang" is completely meaningless. There are other possibilities, such as "eternal inflation", but they are rather speculative, with little positive evidence.

In the other direction, into the future, time seems to be unbounded, though this is less certain, it depends very precisely on several cosmological parameters, whose values are not yet certainly known.

So our current best hypothesis is that time is finite into the past, but infinite into the future.

Energy is locally conserved within the universe, but don't apply such reasoning to the formation of the universe. Indeed the whole concept of physics ceases to operate at that singularity. The basis of nature as cause-and-effect cannot operate at a singularity.

If space is indeed infinite, and the universe is homogenous and isotropic, then the total mass-energy of the universe could be infinite. However the potential gravitational energy might be negative infinite, and so the total energy of the universe might be zero, leading to Hawking's observation that "the universe is the ultimate free lunch".

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    $\begingroup$ "time is finite into the past, but infinite into the future." This is such an interesting statement. $\endgroup$
    – Ed_Gravy
    Jul 19, 2022 at 10:48
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    $\begingroup$ @Ed_Gravy this means each one of us was not born yet a finite time but will be dead infinitetly :( $\endgroup$
    – nerdess
    Jul 19, 2022 at 13:36
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    $\begingroup$ @nerdess, or may be our energy transitions to somewhere else once our bodies (organic machines) are dead. I mean who's to say death/after life is the most certain and mysterious thing at the same time. $\endgroup$
    – Ed_Gravy
    Jul 19, 2022 at 13:41
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    $\begingroup$ It may be meaningless. Feel free to ponder! But ultimately science is not pondering, it's not speculation, it needs actual evidence. At the moment there is no evidence for "existence" prior to 13.8 billion years ago. By the way, while I'm not an astrophysicist (not by a long shot) I do find the fact that you use "orthodox" as an insult very amusing! $\endgroup$
    – James K
    Jul 19, 2022 at 18:33
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    $\begingroup$ Energy is conserved within the universe Not quite. In a non-constant spacetime metric it is not. Which is how CMB photons lose energy. $\endgroup$
    – Jens
    Jul 20, 2022 at 13:18
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Your question, if related to general relativity theory, needs a few explainations. First of all GR describes gravity as a geometry of spacetime; very many geometries are possible - the GR does not tell us which one has been realized in the Universe. While some, higly symmetric solutions of the equations of GR exist, and are thought to describe our universe "well" in the macro scale, e.g. https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric, these solutions are obviously inapplicable to "local inhomogenities" such as galaxies, rotating compact objects, etc. Furthermore "time", or "global time", is not, in general, well defined: at every point of the spacetime, (call it "event"), we can set up the future and past light cones - the "interior" of the future light cone is the set of events that can be reached from current event by a "causal curve" (light ray, or worldline of an observer). Hence, for instance, every observer has his "proper time" (https://en.wikipedia.org/wiki/Proper_time), as measured on the "watch" he carries on his "wrist". Given that "inhomogenities" (galaxies, etc) tend to collapse over time, it is quite generic to expect that large classes of worldlines hit an "end" in finite proper time - hence their time is "finite" in the future direction. In fact, there are general theorems to the effect that such situation is generic in GR - under general conditions there will be a "geodesic incompleteness" in the spacetime (meaning: worldlines cannot be extended indefinitely, see https://en.wikipedia.org/wiki/Penrose%E2%80%93Hawking_singularity_theorems).

A small note on "energy": in general in physics energy is a constant of motion present once the system has time-translation symmetry; in GR this means that there is such a notion (of energy) if the spacetime is at least "stationary" (https://en.wikipedia.org/wiki/Stationary_spacetime); since the spacetime of our Universe is not stationary, there is no such thing as "energy" not to mention its "conservation".

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    $\begingroup$ Your answer certainly is interesting in contrast to the above answer. If it is not much to ask and if you have time, would it be possible to accompany your answer with crude drawings/figures to further understand the concept. $\endgroup$
    – Ed_Gravy
    Jul 19, 2022 at 22:03
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Time is relative so it changes by location and eon, i.e. towards black holes and at the beginning and end of the universe. The speed depends on matter-gravity states. At some places there are dimensional states which are not compatible with our notion of time, just as it was prior to the Big Bang, so we could say "Times" and that every place works on clocks at different speeds. Sometimes those clocks just melt and divide into many clocks.

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