Excerpt from Encyclopedia Mechanica Temporis (18th edition):

The continued existence of the multiverse is, in fact, precarious. As postulated by the many-branch theory of quantum mechanics, every possible existing universe (specifically, every possible combination of different values for the parameters of the standard theory of quantum mechanics) has been created, but, given that only certain combinations of different values for the parameters of the standard theory of quantum mechanics are stable and are able to continue to exist, there should only be a finite subset of existing universes.

In concordance with the colliding brane theory of the Big Bang, each existing universe interacts with adjacent universes, such that, in order to continue to exist, discrepancies in one universe tend to cancel out discrepancies in another. Specifically, while one universe, based upon its particular parameters, would be expected to collapse upon itself in a Big Crunch, whereas another universe, based upon its particular parameters, would be expected to experience Heat Death, their resultant interaction will actually preserve the integrity of each universe, preventing the dissipation of either.

As an aside, this concept (implicit within the colliding brane-theory of the Big Bang) can explain both dark matter and dark energy (see Appendix 18.5), thereby explaining the apparent acceleration of the expansion of the universe (which cannot be adequately explained by the Original Big Bang theory, and is still somewhat problematic even in the inflationary model) as well as the unexpected integrity of large structured features such as galaxies and galaxy clusters despite their apparent inadequacy of mass.

The problem lies in the fact that wormholes, like virtual particles, can form spontaneously, inadvertantly linking two (or more) different parallel universes. If any matter or energy should leak out of one universe into another, the resultant violation of a particular universes’ law of conservation of mass and energy would be enough to doom the entire multiverse, not so much because of the violation itself, but because even one lepton or quark extra would be enough to send a Heat Death type universe into a Big Crunch, and one lepton or quark lacking would do the opposite. In theory, this should balance out anyway, but, in practice, anything that travels through a wormhole loses energy (it is unclear to where) so that the imbalance would likely destroy the current equilibrium of existence.

Nonetheless, our current theories predict that the passage of any particle of matter or wave of energy through a wormhole would be enough to collapse it. Of course, this is assuming that so-called exotic matter does not pass through it (something that generates negative pressure or has negative energy)

Excerpt from The Time Traveler’s Guide to the Multiverse regarding the Campaign for the Preservation of Temporal Sequence AKA the Time Police:

As the Encyclopedia Mechanica Temporis so long-windedly explains, wormholes connecting different multiverses can put all of existence into jeopardy, pooh-poohing the idea with, frankly, lame excuses, and obviously ignoring the large-scale (colossal, stupendous, gargantuan, ginormous, I mean, just huge) projects perpetrated by the travel industry. Parallel Universe tourists (or, in common parlance, Dimension Travelers) are very lucrative to many, many sectors of the multiverse, and, as is expected from rampant capitalists, they don’t really give a damn whether or not the Time Stream gets polluted or if the Multiverse collapses upon itself.

Some would say we’ve just been lucky so far that nothing major has gotten seriously fucked up, but, in fact, someone (or some few, to be exact) has been looking out for us. It is unclear when (seeing as how Time Travelling is affordable to the average citizen of a developed galaxy) or in which universe (I was too lazy to look it up) the Campaign for the Preservation of Temporal Sequence was actually founded, but it has quickly spread to all the other habitable universes, and they are typically known by their less cumbersome name: the Time Cops, naturally, inaccurately depicting the scope of their activities, although as their name suggests, they originally attempted to prevent the pollution of the Time Stream (with mixed success) For some reason, it has been much easier to prevent unlawful immigration or emigration between parallel universes instead. They really should be called Dimension Cops, but no one ever likes my suggestions.