The concept of the multiverse—a reality beyond our own universe—has deep roots in ancient philosophy and religion. Yet it emerged as a scientific hypothesis in the 20th century, thanks to pioneering astronomers such as Georges Lemaître (not Fred Hoyle, who actually opposed the Big Bang model) and physicists who explored the structure of space-time and the laws of quantum mechanics.¹ Today, multiverse theories have gained increasing traction among physicists as a way to explain puzzling features of our universe, such as the cosmological constant, fine-tuning of physical laws, and the peculiar nature of quantum observation.
What Is the Multiverse Theory?
The multiverse theory proposes that our universe is but one of a potentially infinite number of universes—each with its own distinct laws of physics, constants, dimensions, and forms of matter and energy. These “parallel,” “alternate,” or “other” universes together form the totality of all existence: a multiverse.²
Some scientists argue that the multiverse is an inevitable consequence of known physical laws, particularly those emerging from quantum mechanics, string theory, and cosmological inflation.³ Others, however, remain skeptical, emphasizing the absence of direct empirical evidence and questioning the theory’s scientific testability. As Nobel laureate David Gross has stated, accepting the multiverse means “throwing up your hands and accepting that you’ll never really understand anything.”⁴
Varieties of Multiverse Theories
Modern physics offers several frameworks in which the multiverse arises. The cosmologist Max Tegmark has developed a useful classification system that organizes these into four distinct levels, each with increasing degrees of speculative content.⁵
Level I: The Spatial Multiverse
At the most conservative level, if space is infinite and matter is distributed roughly uniformly, then identical copies of our observable universe must exist beyond our cosmic horizon. This follows from simple combinatorics—in an infinite universe with finite possible quantum states, patterns must repeat.⁶
Level II: The Inflationary Multiverse
Eternal inflation, developed by Alan Guth, Andrei Linde, and Alexander Vilenkin, suggests that cosmic inflation never completely ends.⁷ Instead, it continues in some regions while ceasing in others, creating an infinite fractal of bubble universes, each potentially governed by different physical laws.
Level III: The Quantum Many-Worlds
Hugh Everett III’s Many-Worlds Interpretation posits that every quantum measurement causes reality to branch, with all possible outcomes realized in parallel universes.⁸ This interpretation has gained renewed support from quantum computing research, as David Deutsch argues that quantum computers effectively perform calculations across multiple universes simultaneously.⁹
Level IV: The Ultimate Mathematical Multiverse
Tegmark’s most speculative proposal suggests that all mathematically consistent structures exist as physical realities.¹⁰ This “Mathematical Universe Hypothesis” represents the logical endpoint of multiverse thinking, though it remains highly controversial.
Quantum Theory and the Many Worlds
Quantum theory—originating with Max Planck’s discovery that energy is quantized—revolutionized our understanding of matter and energy.¹¹ Central to quantum mechanics is the principle of superposition, where particles exist in multiple states simultaneously until observed or measured.
This paradox gave rise to the Many-Worlds Interpretation (MWI), proposed by Hugh Everett III in 1957.¹² MWI suggests that every quantum event spawns a new universe, with every possible outcome playing out in a separate branch of reality. In this view, you and every decision you make split off into countless alternate versions of yourself, each inhabiting a different world.
Recent work by quantum information theorists has strengthened the case for MWI. As Raphael Bousso and Leonard Susskind demonstrated in 2011, the quantum mechanical multiverse and the cosmological multiverse may be fundamentally connected through decoherence processes.¹³
The Double-Slit Experiment
The famous double-slit experiment beautifully illustrates quantum weirdness. When particles like electrons are fired through two slits onto a screen, they produce an interference pattern—like waves—unless observed. If measured, the pattern collapses into discrete impacts—like particles.¹⁴ This dual behavior, dependent on observation, suggests that reality at the quantum level is fundamentally probabilistic and observer-dependent—lending support to multiverse interpretations.
Recent experiments have pushed these boundaries further. In 2019, researchers demonstrated quantum superposition with molecules containing up to 2,000 atoms, showing that quantum effects persist at surprisingly large scales.¹⁵
String Theory and Extra Dimensions
String theory posits that the fundamental constituents of the universe are not particles, but tiny vibrating strings.¹⁶ The different vibrational modes of these strings correspond to different particles and forces. Crucially, string theory predicts extra spatial dimensions—usually 10 or 11 total, including time—which are compactified or hidden at extremely small scales.
These extra dimensions open the door to multiple possible vacuum states, or “string vacua,” each corresponding to a different set of physical laws. This “landscape” of solutions suggests that countless universes, each with its own physical configuration, may exist—a view that feeds directly into multiverse thinking.¹⁷ Leonard Susskind, one of string theory’s pioneers, estimates there may be 10⁵⁰⁰ different possible universes in the string landscape.¹⁸
However, this vast landscape has drawn criticism. Paul Steinhardt, despite being one of inflation’s original architects, now argues that the multiverse has become a “Theory of Anything” that explains nothing while allowing everything.¹⁹
M-Theory: The Mother of All Universes
An extension of string theory, M-theory emerged in the mid-1990s through the work of physicists like Edward Witten.²⁰ M-theory incorporates 11 dimensions and proposes that what we perceive as particles are actually vibrations of higher-dimensional objects called branes (from “membranes”).
According to M-theory, our entire universe might be a 3-dimensional brane floating in a higher-dimensional space. Other branes could represent parallel universes, potentially with radically different laws of physics.²¹ Collisions between these branes have even been proposed as explanations for events like the Big Bang. Lisa Randall and Raman Sundrum’s warped geometry models have shown how gravity might be confined to our brane while other forces extend into the bulk, potentially explaining gravity’s relative weakness.²²
Inflationary Cosmology and Bubble Universes
In the early 1980s, Alan Guth proposed the idea of cosmic inflation—a period of extremely rapid expansion in the moments following the Big Bang.²³ This theory helps explain the large-scale uniformity of the universe and its apparent flatness.
Later refinements, such as eternal inflation, suggest that inflation may not stop everywhere at once. Instead, it continues in some regions, forming a vast “foam” of bubble universes, each with its own unique physical properties.²⁴ Our universe would be just one such bubble among an infinite multiverse.
Recent work by Andreas Albrecht and collaborators at UC Davis has shown that the emergence of classical physics from quantum mechanics in these scenarios is “much less constrained than we thought,” opening new theoretical possibilities for how different universes might evolve.²⁵
Observations of the cosmic microwave background (CMB) lend support to inflation, although direct evidence of other bubbles remains elusive. The Planck satellite’s measurements have confirmed inflation’s predictions to extraordinary precision, while also revealing puzzling anomalies that some interpret as potential multiverse signatures.²⁶
The Quantum Revolution of 2025
A groundbreaking development in January 2025 from the University of Bristol may fundamentally challenge multiverse theories. Professors Sandu Popescu and Daniel Collins have introduced quantum innovations that potentially solve century-old puzzles while eliminating the need for parallel universes. Their work establishes conservation laws for quantum measurement outcomes that could render many-worlds interpretations unnecessary.²⁷
This breakthrough arrives as quantum computing advances strengthen connections to multiverse theory. Google’s Willow processor, achieving scalable quantum error correction, has reignited debates about whether quantum computers leverage parallel universe computation, as David Deutsch predicted.²⁸
Observational Hints and Anomalies
While direct observation of other universes remains impossible by definition, cosmologists have identified several anomalies that might indicate multiverse interactions:
The CMB Cold Spot
The cosmic microwave background contains an unusually cold region spanning approximately 5° in the constellation Eridanus. This “Cold Spot” is 70-150 microkelvin colder than expected, with statistical significance at the per mille level.²⁹ Some theorists, including Tom Shanks at Durham University, suggest this could be a “bruise” from a collision with another universe.³⁰
Hemispherical Asymmetry
The CMB also exhibits a pronounced asymmetry between hemispheres, violating the cosmological principle of isotropy at greater than 3σ significance.³¹ This “Axis of Evil,” as cosmologists have dubbed it, aligns suspiciously with our solar system’s orientation, raising questions about whether it represents new physics or subtle systematic effects.
Dark Flow
Observations by Alexander Kashlinsky and collaborators suggested that galaxy clusters exhibit a coherent flow toward a specific direction in space, potentially indicating gravitational influence from structures beyond our observable universe.³² However, subsequent analyses have yielded conflicting results, leaving this anomaly unresolved.
Challenges and Controversies
While multiverse theories are rich in explanatory power, they also face criticism:
Lack of empirical verification
By definition, other universes may be inaccessible and unobservable. As George Ellis and Joe Silk warned in a controversial Nature commentary, physics risks becoming “a no-man’s land between mathematics, physics and philosophy.”³³
Falsifiability concerns
Karl Popper’s criterion of falsifiability has long been considered central to science. Sean Carroll has argued that multiverse theories represent “normal science” that should be evaluated through “abduction, Bayesian inference, and empirical success” rather than strict falsifiability.³⁴ The late Steven Weinberg went further, dismissing falsifiability as “a silly criterion imposed on physical science by Karl Popper.”³⁵
Anthropic reasoning
The anthropic principle—the idea that we observe this universe because it’s compatible with our existence—is often invoked in multiverse discussions. Critics like David Gross argue this represents giving up on finding deeper explanations.³⁶ However, proponents note that anthropic reasoning has successfully predicted the cosmological constant’s value and the triple-alpha process in stellar nucleosynthesis.³⁷
The measure problem
In an infinite multiverse, how do we calculate probabilities? This “measure problem” remains one of the deepest challenges. As Yasunori Nomura notes, different measures yield wildly different predictions, undermining the multiverse’s explanatory power.³⁸
Philosophical Implications
The multiverse raises profound philosophical questions that extend far beyond physics:
The Nature of Reality
If all possible universes exist, what does it mean for something to be “real”? The philosopher David Lewis’s modal realism, which treats all possible worlds as equally real, finds an unexpected ally in multiverse cosmology.³⁹
Free Will and Determinism
In Everett’s many-worlds, every possible choice is realized somewhere. This has led to debates about whether free will becomes meaningless if all outcomes occur.⁴⁰ Some philosophers argue this actually preserves free will by ensuring all choices are genuinely made.
The Problem of Induction
If infinitely many universes exist with different laws, why should we expect our universe’s laws to continue holding? This represents a modern version of Hume’s problem of induction, with potentially disturbing implications for scientific methodology.⁴¹
Ethical Implications
If every possible action occurs in some universe, does this dilute moral responsibility? Philosophers like David Deutsch argue the opposite—that understanding the multiverse heightens our responsibility to make choices that create better outcomes in our branch of reality.⁴²
Future Prospects
Despite the challenges, several experimental approaches might provide evidence for or against multiverse theories:
Advanced CMB Observations
Future missions like LiteBIRD and CMB-S4 will measure cosmic microwave background polarization with unprecedented precision, potentially revealing signatures of bubble collisions or other multiverse interactions.⁴³
Quantum Computing Tests
As quantum computers achieve greater coherence times and lower error rates, they may enable tests of many-worlds through reversible quantum measurements and interference experiments.⁴⁴
Large-Scale Structure Surveys
Next-generation telescopes like the Vera Rubin Observatory will map billions of galaxies, potentially revealing patterns consistent with multiverse predictions.⁴⁵
Gravitational Wave Astronomy
While currently limited, gravitational wave detectors might eventually detect signatures from bubble collisions or varying fundamental constants across cosmic boundaries.⁴⁶
Conclusion: A Universe of Possibilities
Although there is no definitive proof of a multiverse, many physicists believe that its existence may be a natural consequence of current theories in quantum mechanics, cosmology, and string theory.⁴⁷ Others remain cautious, emphasizing the importance of testable predictions and observational rigor.
The multiverse debate has evolved into a fundamental disagreement about science’s nature and boundaries. Critics raise legitimate concerns about falsifiability and the risk of physics becoming untethered from empirical constraints. Defenders point to indirect evidence, theoretical consistency, and the historical precedent of successful theories initially beyond direct verification.
Whether literal truth, mathematical artifact, or philosophical speculation, the multiverse challenges our most basic assumptions about reality. It invites us to imagine a cosmos far vaster, stranger, and more intricate than we ever thought possible—a reality where everything that can happen, does happen, somewhere. As we stand at this crossroads between observation and theory, between the known and the unknowable, the multiverse reminds us that the universe’s greatest mystery may not be how it works, but why anything exists at all.
Notes
¹ Brian Greene, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (New York: Alfred A. Knopf, 2011), 3-7.
² Max Tegmark, “Parallel Universes,” Scientific American 288, no. 5 (May 2003): 41-51.
³ Alan H. Guth, The Inflationary Universe: The Quest for a New Theory of Cosmic Origins (Reading, MA: Addison-Wesley, 1997), 175-192.
⁴ David Gross, quoted in Dennis Overbye, “A Universe of Universes,” New York Times, October 28, 2003.
⁵ Max Tegmark, “The Multiverse Hierarchy,” in Universe or Multiverse?, ed. Bernard Carr (Cambridge: Cambridge University Press, 2007), 99-125.
⁶ Jaume Garriga and Alexander Vilenkin, “Many Worlds in One,” Physical Review D 64, no. 4 (2001): 043511.
⁷ Andrei Linde, “Eternal Chaotic Inflation,” Modern Physics Letters A 1, no. 2 (1986): 81-85.
⁸ Hugh Everett III, “‘Relative State’ Formulation of Quantum Mechanics,” Reviews of Modern Physics 29, no. 3 (1957): 454-462.
⁹ David Deutsch, The Fabric of Reality: The Science of Parallel Universes—and Its Implications (New York: Penguin Books, 1997), 217-219.
¹⁰ Max Tegmark, “The Mathematical Universe,” Foundations of Physics 38, no. 2 (2008): 101-150.
¹¹ Thomas S. Kuhn, Black-Body Theory and the Quantum Discontinuity, 1894-1912 (Oxford: Oxford University Press, 1978), 92-94.
¹² Peter Byrne, The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family (Oxford: Oxford University Press, 2010), 98-105.
¹³ Raphael Bousso and Leonard Susskind, “Multiverse Interpretation of Quantum Mechanics,” Physical Review D 85, no. 4 (2012): 045007.
¹⁴ Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, vol. 3 (Reading, MA: Addison-Wesley, 1965), 1-1 to 1-8.
¹⁵ Yaakov Y. Fein et al., “Quantum Superposition of Molecules Beyond 25 kDa,” Nature Physics 15, no. 12 (2019): 1242-1245.
¹⁶ Joseph Polchinski, String Theory, 2 vols. (Cambridge: Cambridge University Press, 1998), 1:10-13.
¹⁷ Leonard Susskind, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design (New York: Little, Brown, 2005), 199-201.
¹⁸ Leonard Susskind, “The Anthropic Landscape of String Theory,” in Universe or Multiverse?, ed. Bernard Carr (Cambridge: Cambridge University Press, 2007), 247-266.
¹⁹ Paul J. Steinhardt, “The Inflation Debate,” Scientific American 304, no. 4 (April 2011): 36-43.
²⁰ Edward Witten, “String Theory Dynamics in Various Dimensions,” Nuclear Physics B 443, no. 1 (1995): 85-126.
²¹ Brian Randall Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (New York: W.W. Norton, 1999), 386-388.
²² Lisa Randall and Raman Sundrum, “Large Mass Hierarchy from a Small Extra Dimension,” Physical Review Letters 83, no. 17 (1999): 3370-3373.
²³ Alan H. Guth, “Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems,” Physical Review D 23, no. 2 (1981): 347-356.
²⁴ Alexander Vilenkin, “Birth of Inflationary Universes,” Physical Review D 27, no. 12 (1983): 2848-2855.
²⁵ Andreas Albrecht and Arsalan Adil, “New Thinking About the Multiverse,” UC Davis Department of Physics and Astronomy, accessed January 20, 2025, https://www.ucdavis.edu/blog/new-thinking-about-multiverse.
²⁶ Planck Collaboration, “Planck 2018 Results. VI. Cosmological Parameters,” Astronomy & Astrophysics 641 (2020): A6.
²⁷ Sandu Popescu and Daniel Collins, “Quantum Innovation and the Multiverse,” University of Bristol School of Physics, January 2025, https://www.bristol.ac.uk/physics/news/2025/how-a-quantum-innovation-may-quash-the-idea-of-the-multiverse.html.
²⁸ Hartmut Neven, “Introducing Willow, Our State-of-the-Art Quantum Chip,” Google Quantum AI Blog, December 9, 2024.
²⁹ Patricio Vielva et al., “Detection of Non-Gaussianity in the Wilkinson Microwave Anisotropy Probe First-Year Data Using Spherical Wavelets,” Astrophysical Journal 609, no. 1 (2004): 22-34.
³⁰ Tom Shanks, quoted in Stuart Clark, “The Enduring Enigma of the Cosmic Cold Spot,” Physics World, July 21, 2023.
³¹ Hans Kristian Eriksen et al., “Asymmetries in the Cosmic Microwave Background Anisotropy Field,” Astrophysical Journal 605, no. 1 (2004): 14-20.
³² Alexander Kashlinsky et al., “A Measurement of Large-Scale Peculiar Velocities of Clusters of Galaxies,” Astrophysical Journal Letters 686, no. 2 (2008): L49-L52.
³³ George Ellis and Joe Silk, “Scientific Method: Defend the Integrity of Physics,” Nature 516, no. 7531 (2014): 321-323.
³⁴ Sean Carroll, “Beyond Falsifiability: Normal Science in a Multiverse,” in Epistemology of Fundamental Physics, ed. Richard Dawid et al. (Cambridge: Cambridge University Press, 2019), 65-78.
³⁵ Steven Weinberg, interview by John Horgan, Scientific American Blog Network, May 2015.
³⁶ David Gross, “The Landscape Problem,” lecture at Strings 2005 Conference, Toronto, July 15, 2005.
³⁷ Steven Weinberg, “Living in the Multiverse,” in Universe or Multiverse?, ed. Bernard Carr (Cambridge: Cambridge University Press, 2007), 29-42.
³⁸ Yasunori Nomura, “Physical Theories, Eternal Inflation, and the Quantum Universe,” Journal of High Energy Physics 2011, no. 11 (2011): 63.
³⁹ David Lewis, On the Plurality of Worlds (Oxford: Blackwell, 1986), 1-5.
⁴⁰ David Wallace, The Emergent Multiverse: Quantum Theory According to the Everett Interpretation (Oxford: Oxford University Press, 2012), 299-305.
⁴¹ Simon Friederich, Multiverse Theories: A Philosophical Perspective (Cambridge: Cambridge University Press, 2021), 167-189.
⁴² David Deutsch, The Beginning of Infinity: Explanations That Transform the World (New York: Viking, 2011), 310-312.
⁴³ Eiichiro Komatsu et al., “LiteBIRD: Mission Overview and Design Tradeoffs,” Proceedings of SPIE 11443 (2020): 114432E.
⁴⁴ Dominic Horsman et al., “An Introduction to Many Worlds in Quantum Computation,” Foundations of Physics 49, no. 10 (2019): 1007-1030.
⁴⁵ LSST Science Collaboration, “LSST Science Book, Version 2.0,” arXiv:0912.0201 (2009).
⁴⁶ Neil J. Cornish and Eiichiro Komatsu, “Constraining the Topology of the Universe,” Physical Review Letters 92, no. 20 (2004): 201302.
⁴⁷ Bernard Carr, ed., Universe or Multiverse? (Cambridge: Cambridge University Press, 2007), xiii-xv.
Bibliography
Albrecht, Andreas, and Arsalan Adil. “New Thinking About the Multiverse.” UC Davis Department of Physics and Astronomy. Accessed January 20, 2025. https://www.ucdavis.edu/blog/new-thinking-about-multiverse.
Bousso, Raphael, and Leonard Susskind. “Multiverse Interpretation of Quantum Mechanics.” Physical Review D 85, no. 4 (2012): 045007.
Byrne, Peter. The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family. Oxford: Oxford University Press, 2010.
Carr, Bernard, ed. Universe or Multiverse? Cambridge: Cambridge University Press, 2007.
Carroll, Sean. “Beyond Falsifiability: Normal Science in a Multiverse.” In Epistemology of Fundamental Physics, edited by Richard Dawid et al., 65-78. Cambridge: Cambridge University Press, 2019.
Clark, Stuart. “The Enduring Enigma of the Cosmic Cold Spot.” Physics World, July 21, 2023.
Cornish, Neil J., and Eiichiro Komatsu. “Constraining the Topology of the Universe.” Physical Review Letters 92, no. 20 (2004): 201302.
Deutsch, David. The Beginning of Infinity: Explanations That Transform the World. New York: Viking, 2011.
———. The Fabric of Reality: The Science of Parallel Universes—and Its Implications. New York: Penguin Books, 1997.
Ellis, George, and Joe Silk. “Scientific Method: Defend the Integrity of Physics.” Nature 516, no. 7531 (2014): 321-323.
Eriksen, Hans Kristian, Frode K. Hansen, Amedeo J. Banday, Krzysztof M. Górski, and Per B. Lilje. “Asymmetries in the Cosmic Microwave Background Anisotropy Field.” Astrophysical Journal 605, no. 1 (2004): 14-20.
Everett, Hugh, III. “‘Relative State’ Formulation of Quantum Mechanics.” Reviews of Modern Physics 29, no. 3 (1957): 454-462.
Fein, Yaakov Y., Philipp Geyer, Patrick Zwick, Filip Kiałka, Sebastian Pedalino, Marcel Mayor, Stefan Gerlich, and Markus Arndt. “Quantum Superposition of Molecules Beyond 25 kDa.” Nature Physics 15, no. 12 (2019): 1242-1245.
Feynman, Richard P., Robert B. Leighton, and Matthew Sands. The Feynman Lectures on Physics. Vol. 3. Reading, MA: Addison-Wesley, 1965.
Friederich, Simon. Multiverse Theories: A Philosophical Perspective. Cambridge: Cambridge University Press, 2021.
Garriga, Jaume, and Alexander Vilenkin. “Many Worlds in One.” Physical Review D 64, no. 4 (2001): 043511.
Greene, Brian. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. New York: W.W. Norton, 1999.
———. The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos. New York: Alfred A. Knopf, 2011.
Gross, David. “The Landscape Problem.” Lecture at Strings 2005 Conference, Toronto, July 15, 2005.
Guth, Alan H. “Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems.” Physical Review D 23, no. 2 (1981): 347-356.
———. The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Reading, MA: Addison-Wesley, 1997.
Horsman, Dominic, Chris Heunen, Matthew F. Pusey, Jonathan Barrett, and Robert W. Spekkens. “An Introduction to Many Worlds in Quantum Computation.” Foundations of Physics 49, no. 10 (2019): 1007-1030.
Kashlinsky, Alexander, F. Atrio-Barandela, D. Kocevski, and H. Ebeling. “A Measurement of Large-Scale Peculiar Velocities of Clusters of Galaxies.” Astrophysical Journal Letters 686, no. 2 (2008): L49-L52.
Komatsu, Eiichiro, T. Matsumura, T. Imamura, H. Ishino, N. Katayama, and K. Mitsuda. “LiteBIRD: Mission Overview and Design Tradeoffs.” Proceedings of SPIE 11443 (2020): 114432E.
Kuhn, Thomas S. Black-Body Theory and the Quantum Discontinuity, 1894-1912. Oxford: Oxford University Press, 1978.
Lewis, David. On the Plurality of Worlds. Oxford: Blackwell, 1986.
Linde, Andrei. “Eternal Chaotic Inflation.” Modern Physics Letters A 1, no. 2 (1986): 81-85.
LSST Science Collaboration. “LSST Science Book, Version 2.0.” arXiv:0912.0201 (2009).
Neven, Hartmut. “Introducing Willow, Our State-of-the-Art Quantum Chip.” Google Quantum AI Blog, December 9, 2024.
Nomura, Yasunori. “Physical Theories, Eternal Inflation, and the Quantum Universe.” Journal of High Energy Physics 2011, no. 11 (2011): 63.
Overbye, Dennis. “A Universe of Universes.” New York Times, October 28, 2003.
Planck Collaboration. “Planck 2018 Results. VI. Cosmological Parameters.” Astronomy & Astrophysics 641 (2020): A6.
Polchinski, Joseph. String Theory. 2 vols. Cambridge: Cambridge University Press, 1998.
Popescu, Sandu, and Daniel Collins. “Quantum Innovation and the Multiverse.” University of Bristol School of Physics, January 2025. https://www.bristol.ac.uk/physics/news/2025/how-a-quantum-innovation-may-quash-the-idea-of-the-multiverse.html.
Randall, Lisa, and Raman Sundrum. “Large Mass Hierarchy from a Small Extra Dimension.” Physical Review Letters 83, no. 17 (1999): 3370-3373.
Steinhardt, Paul J. “The Inflation Debate.” Scientific American 304, no. 4 (April 2011): 36-43.
Susskind, Leonard. “The Anthropic Landscape of String Theory.” In Universe or Multiverse?, edited by Bernard Carr, 247-266. Cambridge: Cambridge University Press, 2007.
———. The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. New York: Little, Brown, 2005.
Tegmark, Max. “The Mathematical Universe.” Foundations of Physics 38, no. 2 (2008): 101-150.
———. “The Multiverse Hierarchy.” In Universe or Multiverse?, edited by Bernard Carr, 99-125. Cambridge: Cambridge University Press, 2007.
———. “Parallel Universes.” Scientific American 288, no. 5 (May 2003): 41-51.
Vielva, Patricio, E. Martínez-González, R. B. Barreiro, J. L. Sanz, and L. Cayón. “Detection of Non-Gaussianity in the Wilkinson Microwave Anisotropy Probe First-Year Data Using Spherical Wavelets.” Astrophysical Journal 609, no. 1 (2004): 22-34.
Vilenkin, Alexander. “Birth of Inflationary Universes.” Physical Review D 27, no. 12 (1983): 2848-2855.
Wallace, David. The Emergent Multiverse: Quantum Theory According to the Everett Interpretation. Oxford: Oxford University Press, 2012.
Weinberg, Steven. Interview by John Horgan. Scientific American Blog Network, May 2015.
———. “Living in the Multiverse.” In Universe or Multiverse?, edited by Bernard Carr, 29-42. Cambridge: Cambridge University Press, 2007.
Witten, Edward. “String Theory Dynamics in Various Dimensions.” Nuclear Physics B 443, no. 1 (1995): 85-126.