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https://www.pbs.org/wgbh/nova/article/big-bang-may-created-mirror-universe-time-runs-backwards/

This article talks about a new theory where the big bang happened in one direction for us and the other direction where time is going backwards (mathematically). Would that revive the idea of a cyclical universe again? I had this little vision in my head when I read this. A universe chain with a big bang on essentially on either side of the start/end of time and in the next big bang our universe is the one going backward in time and the new one is going forward.

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Not necessarily.

What I believe they are getting at is a theory that, before the Big Bang, there was a universe that was eternally contracting, and then it collapsed into a singularity and then exploded again to form our universe which now will be eternally expanding.

Hence, the "mirror" universe lies, not in some "parallel dimension" to use a kind of mathematically awkward pop-cult term, but in our past, so if you wanted to "go there", you'd have to go back in time. You'd need a time machine to get into it, since it's more than 13.8 billion years (437 Ps) ago. Unless and until someone invents a flux capacitor and gives it 1.21 jiggawatts (JW) [not to be confused with gigawatts (GW), by the way], or perhaps more even, given this is so far back, that ain't happening :)

Now, what was it like in this "mirror" universe? Well, it would likely have been just like ours but, if you view "ours" and its time as proceeding the same space-time axis, what you would see there would be that everything was running backwards: entropy would be ever-decreasing, not increasing. People would walk backwards, stars would be sucking up light, black holes would expand into stars (meaning they would actually be white holes, formed by collapsing Hawking radiation tuned with impeccable rightness to form a white hole for real and not just in sci-fi), rot gas would turn into corpses and dead people would come back to life and so forth as it all winds back to the Gnab Gib which then becomes the Big Bang - but likely not the same people, aliens and other such things as in our Universe and, for them, they would interpret this as a Big Bang and time going normally because their entropy arrow, which determines the direction of their conscious peception, goes the other way and so to them, "we" "already happened" and "our" Big Bang was their Gnab Gib, and someone there is talking goofily about some version of sroticapac xulf whose purpose is to go, from our perspective, forward in (our sense of, backwards in theirs) time to our world, etc. Two universes "joined at the hip", so to speak.

Or to imagine it more simply: Take a movie - any movie. Take a second movie. Reverse it so it plays backwards. Concatenate the first movie to the end of the reversed version of the second movie. Play this pair of movies connected together which will be joined at their beginnings as one single movie. That is what this is like, only on a universe-wide scale.

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  • $\begingroup$ So if we aren't in a cycle and the past was forever contracting and out future is to forever expand then how did the past start and where will the future end? I know those aren't answerable, but they boggle my mind. $\endgroup$
    – furstukin
    Commented Mar 1, 2020 at 4:44
  • $\begingroup$ @furstukin: They both "started" by bursting forth from a single point - the Big Bang. What it means is that there isn't anything wrong to think of time in any of different views: going from an eternal past to an eternal future, an eternal future to an eternal past, or from both to a central place of meeting (the Big Bang) or from a central place of creation to a remote place of dissolution. $\endgroup$
    – The_Sympathizer
    Commented Mar 1, 2020 at 7:35
  • $\begingroup$ As pointed out by physicist Aron Wall (-who has done some work in collaboration with Vilenkin, one of the founders--with Guth--of field-based cosmic inflation), in the last link from my own answer to the question at hand, the inhabitants of whatever local universe where time might appear to pass backward (in relation to the direction in which it appears to pass in our own universe or LU) might perceive its "passage" in very much the same way as we do, because, like time, neurological processes are thermodynamic. (I'm saying "appears to pass" because time's not a substance, but a dimension.) $\endgroup$
    – Edouard
    Commented Mar 1, 2020 at 14:41
  • $\begingroup$ The appeal of all proposals like the OP's, or like those by the well-established physicists cited in my answer, is that those proposals would allow a phenomenon like time to serve in causally-separated regions of doubled spatial extent, regardless of whether time itself has always existed, or arose either spontaneously, or in a simulation, or in some other form of design. $\endgroup$
    – Edouard
    Commented Mar 1, 2020 at 14:59
  • $\begingroup$ Fans of folk wisdom might like the idea of time always having existed, as it can be traced back at least as far as Aristotle, and may consequently have been "supported" by the largest current & past population. $\endgroup$
    – Edouard
    Commented Mar 1, 2020 at 15:32
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Perhaps it does. But I would also stipulate that the two-sided inversion is also overlaid, meaning that we cannot disconnect from the other side, meaning that we need a mirror side of reality to make sense of existence, and meaning that there are likely lesser mirrors (e.g., an actual mirror, the condition called situs inversus, or CP symmetry that appears disconnected from CPT symmetry that is more fundamental)that are somewhat removed from the must fundamental mirror.

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  • $\begingroup$ The failure of early versions of cyclical universes was due to the fact that the density of entropy increases over time (-at least if the currently-conventional description of time as thermodynamic, rather than gravitational or electromagnetic, is valid), regardless of the direction of our passage thru time (which is often described, a little too simplistically, as the direction of time's own passage, which may not happen at all). That's why causal separation of the regions concerned, as described in the two cosmologies discussed in the top 1 of the PSE "related questions", is essential. $\endgroup$
    – Edouard
    Commented Mar 3, 2020 at 17:19
  • $\begingroup$ Black holes are observable only thru their effects, mainly the persistence of orbits still followed by stars that were their pre-collapse partners in binary pairs: Over 90 have been indirectly observed by those means. $\endgroup$
    – Edouard
    Commented Mar 3, 2020 at 18:25
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The OP's link refers, first, to a paper by Carroll & Chen, whose complete version can be seen at https://arxiv.org/abs/hep-th/0410270 . Vilenkin criticized it in a paper at https://arxiv.org/abs/1305.3836, providing reasons ("under very mild assumptions", which he details) to explain why the (local) universe it attempted to describe would be literally "surrounded by singularities", which would prevent it from sharing its Big Bang / Big Crunch with another universe.

The link did not address these difficulties, but, instead, provided another (completely different) possibility based upon an extremely local experiment by Barbour, which, as stated, relied upon Newtonian gravity. Newtonian gravity is entirely adequate for plotting orbits within the solar system, but the fact that it has never sufficed to explain the precession of Mercury's orbit (which is a very slow rotation of the orbit's axis around a point very far outside our system) shows that it will not suffice for any complete cosmology: Actually, as described by Guth with simple algebra on p.296-297 in the 1997 edition of his book titled "The Inflationary Universe", "an infinite distribution of matter under the influence of Newtonian gravity would unquestionably collapse", and, in fact, would collapse as soon as its expansion might've otherwise begun. I imagine Barbour (primarily a mathematician) may have accepted Newtonian gravity for the experiment only because of the current uncertainty about the interaction between gravity and subatomic particles.

It IS possible to hypothesize local universes that do not contain any matter at all, and it is possible that such universes comprise most of reality, but the illustration provided by the link showed the usual assortment of stars within the ones it was hypothesizing.

Concerning cosmologies describing local universes, containing matter during some part of an existence that might either continue indefinitely or recur more-or-less periodically, I've analyzed two (one of them similar to the Carroll & Chen model discussed in the OP's link, but a little older and simpler) at Is the Universe Past-Eternal? .

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    $\begingroup$ This is a bit technical, but are you essentially saying the view from this article is being contradicted by parts of string-theory? $\endgroup$
    – furstukin
    Commented Mar 1, 2020 at 15:59
  • $\begingroup$ No, my take on your question is based on Vilenkin's criticism, cited in the link I provided, of the Carroll & Chen paper, which is based on GR. It remarks on similarities to the less complicated paper by Aguirre and Gratton, which he vetted as compatible with his BGV Theorem (a collaboration with Borde & Guth) and related to Hawking's "No-Boundary proposal": I haven't seen a direct reference to string theory in any of those works. $\endgroup$
    – Edouard
    Commented Feb 12, 2021 at 6:04
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An interesting idea is that at the Big Bang positive matter went forwards, and negative matter went backwards, but time is circular, so we'll meet again.

Since most of space is pretty empty, a collision between the two universes would not wipe everything out instantly. Galaxies would, for the most part, collide with nothing much. But bits of our universe would go forward into their universe, and bits of their universe would go backwards into our universe.

I believe that makes the theory testable, because it should be possible to test whether bits of an antimatter universe are creeping into our universe.

There appears to be some evidence that this is happening, because recent research has shown that the universe is expanding at an increasing speed, when it should be expanding at a reducing speed, because of gravity.

That could be explained by saying that antimatter coming from the future is the fuel to provide the extra energy to keep the universe expanding faster.

That's probably not the true explanation, but it's worth investigating.

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The problem with mirrors in this situation is the question of energy.

What one person might be doing in a situation might be the mirror image of what another is doing, but is the energy the same? In the situation of a mirror universe colliding with our universe, in theory if you put a mirror in the middle, it should be the same on both sides, but once you cross the line, the rules of energy flows are different. Okay, I've got a degree in philosophy, but one of my university mathematics teachers was the guy who figured out the Black Hole equations, and I remember him saying, "heat is a flow" which means it's dependent on time, so in a collision between universes following different rules of time, which way would the heat flow?

And the amount of energy resulting from a collision between just one star of positive matter and another of antimatter would be so huge that nobody really knows what would happen.

But I think it's an area that should be researched.

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