Scivillage.com Casual Discussion Science Forum

https://bigthink.com/starts-with-a-bang/...backwards/

EXCERPTS (Ethan Siegel): In our Universe, time has been progressing forward, for all observers, ever since the inception of the hot Big Bang. There are a few "arrows of time" that coincide with this, including that the Universe has been expanding and, thermodynamically, that entropy has been increasing. If the Universe instead were to contract and collapse, could that lead to time running backward? It's a question that puzzled even Stephen Hawking, but we can answer it today...

[...] Gravitation is still an attractive force, and particles that fall into (or form) a bound structure still exchange energy and momentum through elastic and inelastic collisions. The normal matter particles will still shed angular momentum and collapse. They will still undergo atomic and molecular transitions and emit light and other forms of energy. To put it bluntly, everything that makes entropy increase today will still make entropy increase in a contracting Universe.

So if the Universe contracts, entropy will still go up. In fact, the biggest driver of entropy in our Universe is the existence and formation of supermassive black holes. Over the history of the Universe, our entropy has increased by about 30 orders of magnitude; the supermassive black hole at the center of the Milky Way alone has more entropy than the entire Universe had just 1 second after the hot Big Bang!

Not only would time continue to run forward, as far as we know, but the instant that preceded the Big Crunch would have enormously more entropy than the Universe did at the start of the hot Big Bang. All the matter and energy, under those extreme conditions, would start to merge together as all the supermassive black holes had their event horizons begin to overlap. If there were ever a scenario where gravitational waves and quantum gravitational effects could show up on macroscopic scales, this would be it. With all the matter and energy compressed into such a tiny volume, our Universe would form a supermassive black hole whose event horizon was billions of light-years across.

[...] If this turns out not to be correct, it’s because there’s something profound that remains elusive to us, still waiting to be discovered... (MORE - missing details)

I hope it doesn't run backwards. I can't picture myself being shoved back into my mother's womb!

If the universe expands at >c and contracts just as fast then would time be affected?

If you are driving along in a manual transmission car and then thrust your gears into reverse, you'll hear an almightly crunch and likely have the gearbox try to tear itself apart.

While the universe isn't a manual transmission car, it does tend to have "Causality" and causality is part of the fabric of what makes the universe the way it is. If it was possible to thurst the universe into reverse, at some point you will have an instance where the directions cancel each out to a null (the tearing of the gearbox). The problem is that one that null is reach, it breaks causality as there is nothing to continue on from or to.

So with that in mind I don't think the universe is going to rubber-band back to the future. It will just cease in that instance.

Lucky for us then the universe isn't an auto trans. Or manual trans for that matter. Once Ethan said the obvious thing - a hypothetical contraction phase doesn't magically cause entropy to start decreasing, anything past that comment is just so much fill.
Actually, it can be shown net entropy decrease is possible in a given closed system, but that will not at all alter the forward progression of time.
Recently it was claimed that the direction of time is fundamentally quantum mechanical (entanglement) in nature not thermodynamic per se.:
https://astronomy.com/magazine/news/2022...ns-of-time

(Jul 29, 2022 07:25 PM)Magical Realist Wrote: [ -> ]I hope it doesn't run backwards. I can't picture myself being shoved back into my mother's womb!

It would be time-reversal's version of slow "death", as one's embryo was gradually assimilated by the mother's body. In turn, our version of death would become "birth" -- or the origin point of becoming alive, anyway. Taken from the grave and prepared for resurrection.

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If the potentially specious "flow of time"[1] (treated as an objective property) literally did run backward (for speculative reasons other than a "Big Crunch" scenario), the consciousness and memory of the observing organisms would reverse, also. Due to their dependency on brain processes and structure.

Thus, that inverted cognition would either conceive the "reverse flow" as normal (as always having been the case), or become totally dysfunctional in that temporal "direction" (i.e., reverse consciousness would be the opposite of awareness, experience, etc).

In either case, when the [potentially specious] flow of time resumed the "conventional" orientation again, the applicable organisms would lack knowledge that the strange flip had occurred. No more than the superficial characters in a movie would comment that "time had reversed for a period" when the medium that their information was recorded on was once again playing in the forward direction.

"We never see a broken glass reassemble itself and rise back up on the table, and therefore it cannot occur" is an otiose statement built on the erroneous belief that humans would have the capacity to deem it abnormal if such an anomaly did transpire (due strictly to time-flow reversal) and remember the oddity. Even during the traveling backwards in time stage itself, we wouldn't recall the "just happened" incident because brain memories and environmental records would be erased in that temporal orientation rather than stored. And once switched "forward" so that the incident happens again, our memories and perceptions would reflect the norm of the glass breaking.

- - - footnotes - - -

[1] Paul Davies: In this simple picture, the now of our conscious awareness glides steadily onward, transforming events that were once in the unformed future into the concrete but fleeting reality of the present, and thence relegating them to the fixed past. Obvious though this commonsense description may seem, it is seriously at odds with modern physics...

[...] A number of philosophers over the years have arrived at the same conclusion by examining what we normally mean by the passage of time. They argue that the notion is internally inconsistent. The concept of flux, after all, refers to motion. It makes sense to talk about the movement of a physical object, such as an arrow through space, by gauging how its location varies with time. But what meaning can be attached to the movement of time itself? Relative to what does it move? Whereas other types of motion relate one physical process to another, the putative flow of time relates time to itself. Posing the simple question "How fast does time pass?" exposes the absurdity of the very idea. The trivial answer One second per second tells us nothing at all... [Especially since the non-refined elephant of "second" would be an absurdly inadequate and invented measuring stick for microscopic and subatomic events.] --That Mysterious Flow, SciAm...Feb-2006

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(Jul 29, 2022 08:02 PM)Zinjanthropos Wrote: [ -> ]If the universe expands at >c and contracts just as fast then would time be affected?

Space itself arguably gets away with even FTL.

When cosmological inflation occurred (the dawn of expansion), space supposedly increased or "stretched" vastly faster than the speed of light. Making circumstances what they are. The reverse (suddenly occurring, if that was possible) would apparently thereby convert everything to the equivalent original dense state without intervening developments over an immense time span,

So 'time flowing' is an illusion. Yet what common parlance replaces it? A bit like what common parlance replaces 'sun rise' and 'sun set'.

Regardless of that, the notion cosmic inflation meant 'the universe expanded vastly faster than the speed of light' is for sure meaningless.
Inflation is measured as a rate of expansion da/(adt), where a is the 'radius' of the universe at a given instant (and 'given instant' itself needs careful defining). A ratio rate of change, not a relative speed. 'Moving apart faster than c' only has some sort of meaning by specifying a separation distance.
https://www.preposterousuniverse.com/blo...-of-light/

Hermann Weyl: "The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time."

I very much doubt that "consciousness" is like some substance literally flowing through the worldline of a brain in block-time, either. It's more like each chunk-sequence of neural states corresponding to or responsible for one interval of human cognition is an island devoted exclusively to being "aware" of its own particular installment of information. A kind of solipsistic denial in temporal context that those other "islands" before and after it in sequence also co-exist. Due to there being no overarching brain state that provides integrated manifestation and cognition of one's entire life as a single, non-divided experience.

But that said, Weyl surely apprehended that it was easier for people to grasp "consciousness flowing" (as if water through a river channel), than _X_. Likewise, we venture to "experts" recruiting another allegory... 

Sean Carroll: This isn’t, by the way, one of those misconceptions that rattles around the popular-explanation sphere, while experts sit back silently and roll their eyes. Experts get this one wrong all the time. “Inflation was a period of superluminal expansion” is repeated, for example, in these texts by by Tai-Peng Cheng, by Joel Primack, and by Lawrence Krauss, all of whom should certainly know better.

But despite these "cosmological grammar-Nazi" lectures hounding them, they're probably going to keep figuratively doing that for the expediency of quickly conveying how suddenly the scale of space changed during that tiny interval.

According to inflation theory, during the inflationary epoch about 10^−32 of a second after the Big Bang, the universe suddenly expanded, and its volume increased by a factor of at least 10^78 (an expansion of distance by a factor of at least 10^26 in each of the three dimensions). This would be equivalent to expanding an object 1 nanometer (10^−9 m, about half the 1width of a molecule of DNA) in length to one approximately 10.6 light years (about 10^17 m or 62 trillion miles) long. A much slower and gradual expansion of space continued after this, until at around 9.8 billion years after the Big Bang (4 billion years ago) it began to gradually expand more quickly, and is still doing so. https://en.wikipedia.org/wiki/Expansion_of_the_universe

Space-dot-com not only below indulged in the FTL trope, but also another common act of "seemingly" ascribing the original state of the WHOLE universe to a "sub-atomic" size, when that's only meant to concern the visible portion of the cosmos that we can observe being squeezed into whatever their selected, compact domain is.  

The universe was born with the Big Bang as an unimaginably hot, dense point. When the universe was just 10^-34 of a second or so old — that is, a hundredth of a billionth of a trillionth of a trillionth of a second in age — it experienced an incredible burst of expansion known as inflation, in which space itself expanded faster than the speed of light. During this period, the universe doubled in size at least 90 times, going from subatomic-sized to golf-ball-sized almost instantaneously. https://www.space.com/52-the-expanding-u...today.html

Sean Carroll: 3. There is nothing special about the expansion rate during inflation. If you want to stubbornly insist on treating the cosmological apparent velocity as a real velocity, just so you can then go and confuse people by saying that sometimes that velocity can be greater than the speed of light [the appearance of FTL], I can’t stop you. But it can be — and is! — greater than the speed of light at any time in the history of the universe, not just during inflation. There are galaxies sufficiently distant that their apparent recession velocities today are greater than the speed of light. To give people the impression that what’s special about inflation is that the universe is expanding faster than light is a crime against comprehension and good taste.

Vehicles on an open, ordinary highway and those competing in the Indianapolis 500 may both be exceeding 80 kilometers per hour, but what's transpiring in the latter still occupies a "special" category. This current era isn't "inflation", even with the idea of accelerated expansion tacked-on. 

That is, having some general similarities doesn't equate to having the same identity in terms of specifics.

According to the theory of cosmic inflation, the very early universe underwent a period of very rapid, quasi-exponential expansion. While the time-scale for this period of expansion was far shorter than that of the current expansion, this was a period of accelerated expansion with some similarities to the current epoch. https://en.wikipedia.org/wiki/Accelerati..._inflation

Making me wonder if light has slowed down over the years. I never have an original thought so I figured I’d ask Google….
https://www.vice.com/en/article/8q87gk/l...%20second.