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Space and Time

Fundamental questions about the nature of the universe, including its shape and ultimate fate, remain unanswered.

Origin and Fate of the Universe

The Big Bang is a widely accepted model of how the universe began. About 13.8 billion years ago, all observable matter and energy expanded from a single point, coalescing into the galaxies, stars, and planets we see today 1. Theories about the long term fate of the universe vary.

TheoryExplanationTime Until the EndNotes
Heat DeathUniverse reaches thermal equilibrium, after which no useful work is possibleUp to 101000 years 2Currently viewed as most likely 3.
Big RipIncreasing repulsive forces overcome all else, including spacetime itself22 billion years 4Could happen with phantom energy, a model of dark energy 4
Big CrunchAll matter gravitationally contracts on itself, reversing the Big Bang100 billion years 5With the discovery of dark energy, this view is generally rejected 6
Steady StateDensity of universe remains constant due to creation of new matterForeverNo longer generally believed 7.

Aside from religious views, the above are some major theories of the ultimate fate of the universe.

Shape of the Universe

Basic questions of the shape of the universe remain unanswered and the best answers are in flux over historical time scales. For example, we have gone from believing the Earth to be the center of the universe, to the Sun, to the modern view of there being no particular center.

The observable universe is now understood to be roughly a sphere of about 47 billion light years in radius. It is defined to be the region of space that we could theoretically observe with a signal that moves at the speed of light. The actual universe is likely much larger; under interpretations of the theory of cosmic inflation, it may be about 250 times the size of the observable universe, or 3 × 1023 times larger 8, or even infinite.

The way that the universe is curved (specified by the cosmological constant Ω), which under general relativity depends on its mass, greatly affects our understanding of the likely size and shape of the universe.

ConditionCurvatureSize
Ω = 1FlatPossibly Infinite
Ω > 1SphericalFinite
Ω < 1HyperbolicPossibly Infinite

Source: 9. Observational evidence suggests that the universe is flat, or close to being flat 10, 11.

Even the dimensionality of the universe is not firmly established. Our universe appears to be four-dimensional (three spatial dimensions and one time dimension). Under some models of string theory, the universe may have 10, 11, or 26 dimensions 12, with all but the familiar four "compactified" and impossible to currently detect. Alternately, the observable universe may be subspace of a higher dimensional space 13.

Observation

Different telescopes observe the universe at different wavelengths, with, for example, Hubble observing primarily in visible light and the James Webb Space Telescope in infrared. Because radio waves are large, a radio telescope must necessarily be large to achieve useful resolution, and thus a radio telescope cannot reasonably be placed into orbit. Nevertheless, radio astronomy is valuable for observing cosmic microwave background radiation, highly red-shifted galaxies and quasers, and possible radio transmissions from another civilization 14.

Problem:
Desire for Scientific Knowledge
Solution:
Radio Telescope of the Far Side of the Moon

References

  1. Max-Planck-Gesellschaft. "Planck reveals an almost perfect Universe". March 2013. ↩

  2. ChemEurope. "Heat death of the universe". Accessed September 15, 2021. ↩

  3. National Aeronautics and Space Administration. "What is the Ultimate Fate of the Universe?". June 2015. ↩

  4. Caldwell, R. R., Kamionkowski, M., Weinberg, N. N. "Phantom Energy: Dark Energy with Causes a Cosmic Doomsday". Physical Review Letters 91: 071301. August 2003. ↩ ↩2

  5. Davies, P. The Last Three Minutes: Conjectures About The Ultimate Fate Of The Universe. ISBN-13: 978-0465038510. January 1997. ↩

  6. Perlmutter, S. "Supernovae, Dark Energy, and the Accelerating Universe". Physics Today 56(4), pp. 53-62. April 2003. ↩

  7. Kragh, H. Cosmology and Controversy: The Historical Development of Two Theories of the Universe. 1996. ↩

  8. Guth, A. The Inflationary Universe. ISBN-13: 978-0201328400. March 1998. ↩

  9. Lachièze-Rey, M., Luminet, J. "Cosmic topology". Physics Reports 254(3), pp. 135-214. March 1995. ↩

  10. Ade, P. et al. "Planck 2015 results - XIII. Cosmological parameters". Astronomy & Astrophysics 594: A13. October 2016. ↩

  11. de Bernardis, P. et al. "A flat Universe from high-resolution maps of the cosmic microwave background radiation". Nature 404, pp. 955-959. April 2000. ↩

  12. Zwiebach, B. "A First Course in String Theory". ISBN-13: 978-0521880329. January 2009. ↩

  13. Randall, L., Sundrum, R. "An Alternative to Compactification". Physical Review Letters 83(4690). December 1999. ↩

  14. Michaud, E. J., Siemion, A. P. V., Drew, J., Worden, S. P. "Lunar Opportunities for SETI". arXiv preprint. September 2020. ↩