The multiverse idea states that there are an arbitrarily large number of Universes like our own out there, embedded in our Multiverse. It’s possible, but not necessary, for other pockets within the Multiverse to exist where the laws of physics are different.
Look out at the Universe all you want, with arbitrarily powerful technology, and you’ll never find an edge. Space goes on as far as we can see, and everywhere we look we see the same things: matter and radiation. In all directions, we find the same telltale signs of an expanding Universe: the leftover radiation from a hot, dense state; galaxies that evolve in size, mass, and number; elements that change abundances as stars live and die.
But what lies beyond our observable Universe? Is there an abyss of nothingness beyond the light signals that could possibly reach us since the Big Bang? Is there just more Universe like our own, out there past our observational limits? Or is there a Multiverse, mysterious in nature and forever unable to be seen?
Unless there’s something seriously wrong with our understanding of the Universe, the Multiverse must be the answer. Here’s why.
Artist’s logarithmic scale conception of the observable universe. Note that we’re limited in how far we can see back by the amount of time that’s occurred since the hot Big Bang: 13.8 billion years, or (including the expansion of the Universe) 46 billion light years. Anyone living in our Universe, at any location, would see almost exactly the same thing from their vantage point. Wikipedia user Pablo Carlos Budassi
The Multiverse is an extremely controversial idea, but at its core it’s a very simple concept. Just as the Earth doesn’t occupy a special position in the Universe, nor does the Sun, the Milky Way, or any other location, the Multiverse goes a step farther and claims that there’s nothing special about the entire visible Universe.
The Multiverse is the idea that our Universe, and all that’s contained within it, is just one small part of a larger structure. This larger entity encapsulates our observable Universe as a small part of a larger Universe that extends beyond the limits of our observations. That entire structure — the unobservable Universe — may itself be part of a larger spacetime that includes many other, disconnected Universes, which may or may not be similar to the Universe we inhabit.
An illustration of multiple, independent Universes, causally disconnected from one another in an ever-expanding cosmic ocean, is one depiction of the Multiverse idea. Ozytive / Public domain
If this is the idea of the Multiverse, I can understand your skepticism at the notion that we could somehow know whether it does or doesn’t exist. After all, physics and astronomy are sciences that rely on measurable, experimental, or otherwise observational confirmation. If we are looking for evidence of something that exists outside of our visible Universe and leaves no trace within it, it seems that the idea of a Multiverse is fundamentally untestable.
But there are all sorts of things that we cannot observe that we know must be true. Decades before we directly detected gravitational waves, we knew that they must exist, because we observed their effects. Binary pulsars — spinning neutron stars orbiting around one another — were observed to have their revolutionary periods shorten. Something must be carrying energy away, and that thing was consistent with the predictions of gravitational waves.
The rate of orbital decay of a binary pulsar is highly dependent on the speed of gravity and the orbital parameters of the binary system. We have used binary pulsar data to constrain the speed of gravity to be equal to the speed of light to a precision of 99.8%, and to infer the existence of gravitational waves decades before LIGO and Virgo detected them.NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer (R)
While we certainly welcomed the confirmation that LIGO and Virgo provided for gravitational waves via direct detection, we already knew that they needed to exist because of this indirect evidence. Those who would argue that indirect evidence is no indicator of gravitational waves might still be unconvinced that binary pulsars emit them; LIGO and Virgo didn’t see the gravitational waves that came from the binary pulsars we’ve observed.
So if we cannot observe the Multiverse directly, what indirect evidence do we have for its existence? How do we know that there’s more unobservable Universe beyond the part we can observe, and how do we know that what we call our Universe is likely just one of many embedded in the Multiverse?
We look to the Universe itself, and draw conclusions about its nature based on what observations about it reveal.
The light from the cosmic microwave background and the pattern of fluctuations from it gives us one way to measure the Universe’s curvature. To the best of our measurements, to within 1 part in about 400, the Universe is perfectly spatially flat.Smoot Cosmology Group / Lawrence Berkeley Labs
When we look out to the edge of the observable Universe, we find that the light rays emitted from the earliest times — from the Cosmic Microwave Background — make particular patterns on the sky. These patterns not only reveal the density and temperature fluctuations that the Universe was born with, as well as the matter and energy composition of the Universe, but also the geometry of space itself.
We can conclude from this that space isn’t positively curved (like a sphere) or negatively curved (like a saddle), but rather spatially flat, indicating that the unobservable Universe likely extends far beyond the part we can access. It never curves back on itself, it never repeats, and it has no empty gaps in it. If it is curved, it has a diameter that’s hundreds of times greater than the part we can see.
With every second that ticks by, more Universe, just like our own, is revealed to us, consistent with this picture.
The observable Universe might be 46 billion light years in all directions from our point of view, but there’s certainly more, unobservable Universe, perhaps even an infinite amount, just like ours beyond that. Over time, we’ll be able to see more of it, eventually revealing approximately 2.3 times as much matter as we can presently view.Frédéric MICHEL and Andrew Z. Colvin, annotated by E. Siegel
That might indicate that there’s more unobservable Universe beyond the part of our Universe we can access, but it doesn’t prove it, and it doesn’t provide evidence for a Multiverse. There are, however, two concepts in physics that have been established far beyond a reasonable doubt: cosmic inflation and quantum physics.
Cosmic inflation is the theory that gave rise to the hot Big Bang. Rather than beginning with a singularity, there’s a physical limit to how hot and how dense the initial, early stages of our expanding Universe could have reached. If we had achieved arbitrarily high temperatures in the past, there would be clear signatures that aren’t there:
- large-amplitude temperature fluctuations early on,
- seed density fluctuations limited by the scale of the cosmic horizon,
- and leftover, high-energy relics from early times, like magnetic monopoles.
Inflation causes space to expand exponentially, which can very quickly result in any pre-existing curved or non-smooth space appearing flat. If the Universe is curved, it has a radius of curvature that is at minimum hundreds of times larger than what we can observe.E. Siegel (L); Ned Wright’s cosmology tutorial (R)
These signatures are all missing. The temperature fluctuations are at the 0.003% level; the density fluctuations exceed the scale of the cosmic horizon; the limits on monopoles and other relics are incredibly stringent. The fact that these signatures aren’t there have an enormous implication to them: the Universe never reached those arbitrarily high temperatures. Something else came before the hot Big Bang to set it up.