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What is the universe expanding into? - Sajan Saini


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The universe began in a Big Bang nearly fourteen billion years ago, and has been expanding ever since. But how does the universe expand and what is it expanding into? Sajan Saini explains the existing theories around the Big Bang and what, if anything, lies beyond our universe.

Additional Resources for you to Explore

A sound scientific theory foretells new experimental results, and Hubble’s observation of galactic recession led to a Big Bang theory that predicted the Cosmic Microwave Background (CMB) and cosmic abundance of hydrogen and helium. This birthed the discipline of cosmology—the study of the life, evolution, and death of the universe.

This TED-Ed video by T. Whyntie briefly describes the early universe, and this online article elaborates on NASA’s cosmic timeline for the era. NASA’s Universe 101 webpage gives a concise introduction to the CMB.

Hubble’s landmark 1929 article infers the relative motion of faraway galaxies by measuring their Doppler Effect: see this Physics Classroom lesson to learn about Doppler redshift.

Ethan Siegel’s science blog explains how the cosmic expansion observed by Hubble can’t happen everywhere in the universe, thanks to the influence of gravity within or between nearby galaxies. This New Scientist article describes how Einstein’s equations of General Relativity (GR) interpret gravity as a literal curvature in spacetime (here’s twelve key insights).

This article and podcast episode by astrophysicist Paul Sutter consider how spacetime curvature relates to the universe having no edge. The TED-Ed video by R. Hlozek examines how the curvature of the universe determines its ultimate fate. The online multimedia textbook Earth and Space Science by astronomy educator Jeffrey Bennett gives an instructive introduction to the observable universe, and our place in it.

Despite the pervasive success of the Big Bang theory, fundamental questions remain unanswered on the formation of galaxies and temperature uniformity in the observable universe. The cosmic inflation paradigm offers an elegant resolution with a profound hypothesis that straddles quantum physics and GR—but it comes at a price, as most of its models imply an eternally inflating multiverse. This Nautilus article by Amanda Gefter deftly reviews how eternal inflation has spurred intense debate about cosmic inflation itself, even as most experts are still in consensus on its likelihood. If you’re feeling adventurous, tackle the introduction to a review paper on eternal inflation by Alan Guth, creator of inflationary theory. And this article at the Early Universe blog predicts characteristics in the CMB for the extreme unlikely scenario of a collision between adjacent bubble universes.

While the inflation paradigm addresses questions on the galactic scale, cosmology’s study of the early hot universe aligns with the subatomic focus of particle physics. Brane cosmology reconciles particle string theories and proposes a multiverse model with potentially interacting brane universes. These interactions may be tested for by turning away from telescopes that look out to the stars, in favor of particle accelerators that focus in on the subatomic realm. That said, this TED-Ed video on detecting dark matter by R. Landua reveals some of the incredible technical challenges in high energy accelerator experiments.

Where might it all lead? The TED-Ed video by C. Anderson briefly explores possible sizes for the multiverse, and two Scientific American articles (by Ellis,Vilenkin & Tegmark) underscore the testable boundaries of these interpretations.

Getting back to our universe: in the 1990s, a more sophisticated version of Hubble’s experiment revealed the rate of cosmic expansion isn’t constant with time. A mysterious “dark energy,” is accelerating the expansion of the universe. The TED-Ed video by J. Gillies reviews attempts to identify dark matter and dark energy. Challenge yourselves further with a Resonance review article by Das Gupta that examines how the ad hoc inclusion of a cosmological constant inadvertently represents the effects of dark energy.

Einstein ruefully called the cosmological constant his “greatest blunder.” Today, it models our accelerating universe, and may help interpret the universe’s relation to what may exist beyond…

Sajan Saini is a former materials scientist and science writer. He directs the educational curriculum for AIM Photonics Academy at MIT. He has written for Coda Quarterly, MIT Ask an Engineer, and Harper's Magazine. Learn about Sajan here.

Raman Sundrum is a professor of Theoretical Physics at the University of Maryland, College Park. His research ranges from subatomic particle physics to cosmology; his best-known work addresses the theory and experimental repercussions of higher-dimensional spacetime. Learn about Raman here.

Jeffrey Bennett is an astrophysicist and former NASA scientist. He is lead author of textbooks in astronomy, astrobiology, math & statistics, books for the general public, and six children’s books that have been read from the International Space Station. Learn about Jeffrey here.

Laura Blecha is a professor in the Department of Physics at the University of Florida. Her research group uses computer simulations to study supermassive black holes at the center of galaxies. Learn about Laura here.

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Meet The Creators

  • Educator Sajan Saini
  • Director Will Mackenzie, Kat Gusarova
  • Narrator Bethany Cutmore-Scott
  • Music Patrick Smith
  • Producer Agota Vegso
  • Content Producer Gerta Xhelo
  • Editorial Producer Alex Rosenthal
  • Associate Producer Bethany Cutmore-Scott, Elizabeth Cox
  • Fact-Checker Brian Gutierrez

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