The argument is a peer-reviewed prediction of inflationary cosmology, developed most thoroughly by the physicist Max Tegmark, then at the University of Pennsylvania, in a 2003 paper titled simply “Parallel Universes,” published in the conference volume Science and Ultimate Reality for the 90th birthday of John Wheeler. It runs as follows. The observable universe is a sphere about 93 billion light-years across, centred on Earth and bounded by the cosmic horizon — the furthest distance from which light has had time to reach us since the Big Bang. The volume of space inside this sphere contains a finite quantity of matter and energy and, by a 1981 result of the physicist Jacob Bekenstein, a finite number of possible quantum-mechanical configurations. The number is enormous — Tegmark estimates it at about 2 raised to the power 10^115, or roughly 10^(10^115) — but it is finite. There are only so many distinct ways to arrange the contents of a region of space the size of our observable universe.
If the universe extends infinitely far beyond our cosmic horizon, as current cosmological observations are consistent with, then space contains an infinite number of regions the same size as ours. Each of those regions has the same finite menu of possible configurations to choose from. By a straightforward counting argument — the pigeonhole principle applied to an infinite set of regions choosing from a finite set of configurations — every possible arrangement must occur, not once but infinitely many times. Including the configuration that produces an exact copy of Earth, an exact copy of you, and an exact copy of you reading these words.
How far away the copies are
Tegmark, in the original 2003 paper laying out the four-level multiverse hierarchy, calculated the expected distances to various types of copy. The nearest region of space identical to our observable universe — your closest exact duplicate of the whole Hubble volume — is approximately 10^(10^115) metres away. The nearest exact copy of Earth alone, or just of you alone, is closer: approximately 10^(10^29) metres, which is the more commonly cited figure when popular accounts mention the idea. Both numbers are functionally unimaginable, but they are also far smaller than the spatial extent of an infinite universe. Even a relatively close duplicate sits at a distance trillions of orders of magnitude beyond the edge of the universe we can see.
The unimaginability of these numbers matters less than the logical structure of the argument. The numbers exceed any quantity that could ever be measured or visited, but they do not exceed the size of an infinite universe, which has no edge to reach. If space genuinely extends without end, then anywhere short of “everywhere” is a finite distance, and every finite distance is reached. The copies are unreachable not because they are distant in any absolute sense but because they sit beyond the horizon of causal contact with us. No signal from them can ever reach us, and no signal from us can ever reach them.
Why this is the simplest of the multiverse hypotheses
The Level I Multiverse, as Tegmark calls it in his 2003 paper and again in his 2009 review of the multiverse hierarchy, is the least speculative of the multiverse concepts that physicists discuss. It does not require any exotic physics. It does not require multiple branches of the quantum wavefunction (that is Tegmark’s Level III). It does not require regions of space with different physical constants (Level II) or all mathematically possible universes (Level IV). It requires only that space extends infinitely beyond our cosmic horizon, with the same laws of physics throughout, and that within that infinite extent the same finite menu of possible configurations is sampled exhaustively.
Tegmark’s argument is that the Level I multiverse is a generic consequence of inflationary cosmology, the leading theoretical framework for the very early universe. Inflation produces, almost inevitably, a spatially infinite universe. If inflation is correct — and it is currently the dominant explanation for the homogeneity, isotropy, and flatness of the observed universe — then the Level I multiverse is essentially unavoidable. As Tegmark has put it in popular writing on the subject, “you can’t have inflation without the Level I multiverse.”
The empirical support for spatial infinity comes from observations of the cosmic microwave background, the residual radiation from the Big Bang. Measurements by NASA’s WMAP satellite, ESA’s Planck satellite, and ground-based observatories have shown that the geometry of the observable universe is “flat” — meaning that parallel lines remain parallel — to a precision of approximately 0.4 percent. A precisely flat universe is, under the standard cosmological models, infinite in spatial extent. The data do not prove this conclusively. The geometry could still be very slightly curved in a way that would close the universe on itself at a scale much larger than the observable region. But the data are consistent with infinity, and the simplest interpretation is that the universe genuinely is infinite.
What the copies are like
The Level I argument predicts not one copy of you but infinitely many. Among them are versions identical to you in every measurable way — same memories, same physical body, same actions in this present moment. Among them are versions differing from you by one atom, one neuron, one breath. Among them are versions where every decision you have ever made was made differently, with every possible variation realised somewhere. The argument is not selective. If a configuration is physically possible, it occurs. If it is consistent with the laws of physics and the initial conditions of the universe, it occurs infinitely many times, scattered across infinite space.
The implications are difficult to digest at first hearing. Every action you have ever performed, every accident you have ever narrowly avoided, every word you have ever said in conversation, every meal you have ever cooked, exists in an infinite number of slight variations elsewhere in the same universe. Some of those variations are catastrophic. Some are trivially different. Most differ from your own life in ways too small to notice. All of them, by the logic of the argument, are equally real, and your own life is one instance in an infinite ensemble of instances.
The case against
Not all cosmologists accept the Level I argument. The most common objection is that it is, by construction, unfalsifiable. The copies in question are forever beyond the cosmic horizon and can never be observed, measured, or tested. A theory whose central claim cannot in principle be checked against any observation is, by the standards some philosophers of science apply, not properly a scientific theory at all. The South African cosmologist George Ellis has been among the more prominent critics on these grounds, arguing that multiverse cosmology of all kinds (including Level I) departs from the empirical standards that have defined physics for centuries.
A more technical critique was offered by the independent researcher Alan McKenzie in a 2017 arxiv paper arguing that conflating the Level I multiverse with the Level III (many-worlds) multiverse, as Tegmark and Aguirre have sometimes done, runs into problems with the eigenstate structure of expanding bubble universes in general relativity. McKenzie’s specific critique does not undermine the Level I argument itself; it pushes back on the further claim that the Level I and Level III multiverses are equivalent. The Level I argument as standardly stated — that an infinite ergodic universe contains identical copies of every finite region — remains the dominant view among working cosmologists who accept the underlying inflationary framework.
The honest summary of the situation is that the Level I multiverse is the prediction of mainstream inflationary cosmology and consistent with all currently available observational data, while remaining empirically unverifiable and philosophically contentious. The duplicates of Earth, if they exist, exist forever beyond reach. The question of whether they exist at all collapses, in the end, to the question of whether the universe is genuinely infinite — and to that question, current cosmology offers a strong “probably yes” rather than a definitive answer. Somewhere, depending on which way that probability resolves, an exact copy of this article is being read by an exact copy of you, on an exact copy of the device you are holding, in a region of space whose distance from us is not just unreachable but unimaginable. Or there is nothing of the kind, and our region is unique. The mathematics that links the two possibilities was published in 2003 in a Cambridge University Press volume. The empirical resolution of the question, if it ever comes, will require a way of looking past the horizon that no current physics suggests is possible.
