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  • The universe began its cosmic life in a big bang

  • nearly fourteen billion years ago,

  • and has been expanding ever since.

  • But what is it expanding into?

  • That's a complicated question.

  • Here's why:

  • Einstein's equations of general relativity

  • describe space and time as a kind of inter-connected

  • fabric for the universe.

  • This means that what we know of as space and time

  • exist only as part of the universe and not beyond it.

  • Now, when everyday objects expand, they move out

  • into more space.

  • But if there is no such thing as space to expand into,

  • what does expanding even mean?

  • In 1929 Edwin Hubble's astronomy observations

  • gave us a definitive answer.

  • His survey of the night sky found all faraway galaxies

  • recede, or move away, from the Earth.

  • Moreover, the further the galaxy, the faster it recedes.

  • How can we interpret this?

  • Consider a loaf of raisin bread rising in the oven.

  • The batter rises by the same amount in between each

  • and every raisin.

  • If we think of raisins as a stand-in for galaxies,

  • and batter as the space between them,

  • we can imagine that the stretching or expansion

  • of intergalactic space will make the galaxies recede from each other,

  • and for any galaxy, its faraway neighbors will recede a larger

  • distance than the nearby ones

  • in the same amount of time.

  • Sure enough, the equations of general relativity predict a cosmic

  • tug-of-war between gravity and expansion.

  • It's only in the dark void between galaxies where expansion wins out,

  • and space stretches.

  • So there's our answer.

  • The universe is expanding unto itself.

  • That said, cosmologists are pushing the limits of mathematical models

  • to speculate on what, if anything, exists beyond our spacetime.

  • These aren't wild guesses, but hypotheses that tackle

  • kinks in the scientific theory of the Big Bang.

  • The Big Bang predicts matter to be distributed evenly across the universe,

  • as a sparse gas --but then, how did galaxies and stars come to be?

  • The inflationary model describes a brief era

  • of incredibly rapid expansion

  • that relates quantum fluctuations in the energy of the early universe,

  • to the formation of clumps of gas that eventually led to galaxies.

  • If we accept this paradigm, it may also imply our universe represents

  • one region in a greater cosmic reality that undergoes endless, eternal inflation.

  • We know nothing of this speculative inflating reality,

  • save for the mathematical prediction that its endless expansion

  • may be driven by an unstable quantum energy state.

  • In many local regions, however, the energy may settle by random

  • chance into a stable state, stopping inflation and forming bubble universes.

  • Each bubble universeours being one of them

  • would be described by its own Big Bang and laws of physics.

  • Our universe would be part of a greater multiverse,

  • in which the fantastic rate of eternal inflation makes it impossible

  • for us to encounter a neighbor universe.

  • The Big Bang also predicts that in the early, hot universe, our fundamental forces

  • may unify into one super-force.

  • Mathematical string theories suggest descriptions of this unification,

  • in addition to a fundamental structure for sub-atomic quarks and electrons.

  • In these proposed models, vibrating strings are the building blocks of the universe.

  • Competing models for strings have now been consolidated into a unified description,

  • and suggest these structures may interact with massive, higher dimensional surfaces called branes.

  • Our universe may be contained within one such brane,

  • floating in an unknown higher dimensional place, playfully namedthe bulk,” or hyperspace.

  • Other branescontaining other types of universesmay co-exist in hyperspace,

  • and neighboring branes may even share certain fundamental forces like gravity.

  • Both eternal inflation and branes describe a multiverse,

  • but while universes in eternal inflation are isolated, brane universes could bump into each other.

  • An echo of such a collision may appear in the cosmic microwave background

  • —a soup of radiation throughout our universe, that's a relic from an early Big Bang era.

  • So far, though, we've found no such cosmic echo.

  • Some suspect these differing multiverse hypotheses may eventually coalesce into a common description,

  • or be replaced by something else.

  • As it stands now, they're speculative explorations of mathematical models.

  • While these models are inspired and guided by many scientific experiments,

  • there are very few objective experiments to directly test them, yet.

  • Until the next Edwin Hubble comes along,

  • scientists will likely be left to argue about the elegance of their competing models

  • and continue to dream about what, if anything, lies beyond our universe.

The universe began its cosmic life in a big bang

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B2 UK TED-Ed universe big bang cosmic inflation expanding

What is the universe expanding into? - Sajan Saini

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    Amy.Lin posted on 2018/11/27
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