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Could some influence from outside the observable universe be causing the expansion?

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    $\begingroup$ "Outside the universe" is essentially meaningless in physics. The FLRW Metric is a topic I'd direct you to regarding the expansion of the universe. $\endgroup$ – StephenG Jul 5 '18 at 3:49
  • $\begingroup$ @StephenG, et al. Please come discuss the closure of this question on meta: astronomy.meta.stackexchange.com/q/448/6 $\endgroup$ – called2voyage Jul 5 '18 at 14:43
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EDIT - The original question was edited to enable it to be reopened, but my answer below responded to the way the question was initially framed. I've left my answer intact as it may be useful in addressing confusion about the difference between "the universe" and "the observable universe".


Could some pressure from outside of our universe

According to Wikipedia, the Universe is "all of space and time and their contents". So it makes no sense to talk of something "outside" all of space and time.

You might, of course, be referring just to the observable universe, "a spherical region of the Universe comprising all matter that can be observed from Earth at the present time."

However, if the conditions beyond the observable universe are what is causing expansion, then the Universe is neither homogenous nor isotropic. While we have no way of disproving this idea, all the evidence points to a homogenous and isotropic Universe and the vast majority of physicists subscribe to this model.

causing the expansion

The metric expansion of the Universe is a generic property of the Universe under the FLRW metric, an exact solution of Einstein's field equations of general relativity. There is no need to propose an external cause for the expansion.

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Yes, it's possible, but there's absolutely no evidence for it.

String theory -- which is, I emphasize, a theory for which there is no evidence -- has many incompatible variations, and some of them (e.g., the Ekpyrotic theories) suggest that our 4D universe may be one of many that exist on "branes" in a higher-dimensional space.

Collisions between the branes may be possible and would have an impact (pun unavoidable) on our universe -- and may even have been the cause of the Big Bang. Such collisions ought to have left an imprint on spacetime, but searches for the expected fingerprints have come up negative.

None-the-less, String theory and all its many children are flexible enough and unconstrained by observation enough that a solution where the cosmological constant is affected by other brane universes is plausible.

But I wouldn't believe any of that until there's some observational evidence for String theory. At the moment it's basically a modern version of Kepler's Mysterium Cosmographicum -- theory running wild.

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Influences from outside the observable universe explaining dark energy and expansion?

Could some influence from outside the observable universe be causing the expansion?

Outside of the "observable universe" is the portion of the universe which is unobservable.

The whole universe (observable or not) is all that there exists within our dimensions.

Outside of our universe is nothing, our universe is the full extent of existence in our dimensions.

There is no proof that something outside of our universe can influence something within our universe, it's not as though we are contained in something.

Even the rate of expansion is disagreed upon.

In the paper: "Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant" published in the journal of The American Astronomical Society (July 12 2018), by Adam G. Riess, Stefano Casertano, Wenlong Yuan, Lucas Macri, Beatrice Bucciarelli, Mario G. Lattanzi, John W. MacKenty, J. Bradley Bowers, WeiKang Zheng, Alexei V. Filippenko, Caroline Huang, and Richard I. Anderson they wrote:

"... the value of the Hubble constant has been determined to be $H_0 = 73.24 ±1.7\, $km s$^{−1}\, $Mpc$^{−1}$, from R16.". [Note: R16 is Riess' paper: "A 2.4% Determination of the Local Value of the Hubble Constant".]

...

... is inconsistent with the scale needed to match the Planck 2016 cosmic microwave background data combined with ΛCDM at the 2.9σ confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional errors associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance-ladder data raises the current tension between the late and early universe route to the Hubble constant to 3.8σ (99.99%).".

In the "Planck 2018 Results" published as: "Planck 2018 results. VI. Cosmological parameters" (Jul 17 2018), by the Planck Collaboration: N. Aghanim, Y. Akrami, M. Ashdown, J. Aumont, C. Baccigalupi, M. Ballardini, A. J. Banday, R. B. Barreiro, N. Bartolo, S. Basak, R. Battye, K. Benabed, J.-P. Bernard, M. Bersanelli, P. Bielewicz, J. J. Bock, J. R. Bond, J. Borrill, F. R. Bouchet, F. Boulanger, M. Bucher, C. Burigana, R. C. Butler, E. Calabrese, J.-F. Cardoso, J. Carron, A. Challinor, H. C. Chiang, J. Chluba, L. P. L. Colombo, C. Combet, D. Contreras, B. P. Crill, F. Cuttaia, P. de Bernardis, G. de Zotti, J. Delabrouille, J.-M. Delouis, E. Di Valentino, J. M. Diego, O. Doré, M. Douspis, A. Ducout, X. Dupac, S. Dusini, G. Efstathiou, F. Elsner, T. A. Enßlin, H. K. Eriksen, Y. Fantaye, M. Farhang, J. Fergusson, R. Fernandez-Cobos, F. Finelli, F. Forastieri, M. Frailis, E. Franceschi, A. Frolov, et al. (120 additional authors not shown) they wrote:

"The Planck base-ΛCDM cosmology requires a Hubble constant $H_0 = 67.4±0.5\, $km s$^{−1}\,$Mpc$^{−1}$, in substantial 3.6σ tension with the latest local determination by Riess et al. (2018b). The Planck measurement is in excellent agreement with independent inverse-distance ladder measurements using BAO, supernovae, and element abundance results. None of the extended models that we have studied in this paper convincingly resolves the tension with the Riess et al. (2018b) value of $H_0$.".

Wikipedia's webpage "Expansion of the universe" explains:

"The expansion of the universe is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it.

Technically neither space, nor objects in space, move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. Although light and objects within spacetime cannot travel faster than the speed of light, this limitation does not restrict the metric itself. To an observer it appears that space is expanding and all but the nearest galaxies are receding into the distance.

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I think the original question is valid and I have been pondering that question myself for years

First I am having difficulty understanding the logic in stating there is nothing outside the observable universe. The term 'observable universe' by definition limits knowledge to whatever is observable and does not exclude any thing outside of it that is not observable. This has been mentioned in earlier posts.

However there could be observable effects within our universe that may give a clue to the existence of an outside influence upon the known universe.

One of these is the fact that distant galaxies are accelerating, apparently, away from us. One phenomenon that has this ability to cause acceleration is gravity. One hypothesis could be that the distant galaxies are caught in a gravitational field outside our known universe. This would place our known universe within, for want of a better word, a 'bubble' within this gravitational influence causing it to expand in all directions.

Of course this would change the idea of a big bang theory, not in its effect but its origins. One has difficulty in imagining the whole of our universe being crammed into a single point as unstable as that may be. However one could imagine it coming into existence by, for want of a better word, 'squirted' into a void within an outside existence of a meta-universe. This also does away with the need for dark energy as we once thought we needed the ether to carry radio waves.

Karl Popper says that any hypothesis/theory/etc must be falsifiable. The above is certainly one of those things. It only needs to be proven non-falsifiable, possibly, with physics not yet known.

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