Today, looking out across a seemingly boundless cosmos filled with an unimaginable variety of exotic objects, it's easy to forget that the universe we currently admire is the product of a violent event that occurred 13.75 billion years ago.
As we know, the leading theory for universal birth is the Big Bang, where everything came from nothing, in a single energetic burst of inexplicable creation. So, if we turn back the clock 13.75 billion years, what would we see?
My instinct would be to say "energy, the universe was filled with pure, violent energy," but according to some mind-bending work by Jonas Mureika from Loyola Marymount University in Los Angeles, and Dejan Stojkovic from State University New York at Buffalo, the answer may be a little more complicated than that. In fact, it may be so weird that we can't even imagine what it would have been like.
According to an interview with PhysOrg.com, Mureika and Stojkovic have calculated that the early universe didn't only possess a hot, energetic primordial state of matter, but it also had a primordial state of dimensions.
If they're correct, the three dimensions of space and one dimension of time that make the four-dimensional spacetime we live in today isn't how it's always been — the universe may have existed in a lower dimensional state in the past.
The universe, but not as we know it
The thinking goes like this: Shortly after the Big Bang, the Universe possessed only one dimension of space and one dimension of time. It was basically a straight line. As the universe began to cool, and expanded, this one dimension of space became "wrapped up" in such a way to create two dimensions of space and one of time — a plane, like a sheet of flat paper.
The transition from one to two dimensions of space was calculated by the researchers to occur when the universe "cooled" to an energy level of 100 TeV (tera-electron volts, a measurement of energy commonly used in particle physics). A period of time after that, the universe continued to expand and cool until it reached an energy of 1 TeV. At this point, the universe got promoted to a higher dimension; three dimensions of space and one dimension of time, i.e., the universe we live in today.
Mureika and Stojkovic think the universe will eventually be promoted again, to a five-dimensional state, at some point in the future.
Evidence in cosmic rays?
This is all well and good, but isn't it just a fanciful notion that our universal dimensions are evolving to higher and higher states? Even though string theory predicts there could be many dimensions and those weird hypothetical Higgs singlets (yes, the ones that kill grandfathers) need to travel through a fifth dimension for their time-traveling shenanigans, what's the evidence for the universe existing at lower dimensional states?
It turns out that Mureika and Stojkovic may have found some of that much needed evidence: When measuring cosmic ray particles with energies above 1 TeV, they appear to align themselves to a two-dimensional plane. "This means that, above a certain energy level, particles propagate in two dimensions rather than three dimensions," the PhysOrg.com article clarifies.
This effect would suggest these very high-energy cosmic rays originated from a period of time before the universe acquired three spatial dimensions.
The mystery of dark energy
It gets better. As gravity cannot exist in 1- or 2-dimensional space,
if we ever detect a gravitational wave signal
, there should be a very strong cut-off in gravitational wave frequency. This cut-off could represent the transition of when space changed from a 2-dimensional to 3-dimensional state. Gravitational waves can exist only in three-dimensional space!
Also, the particle physicists' Swiss army knife of particle accelerators, the Large Hadron Collider (LHC), may be able to probe this 1 TeV transition when colliding particles beyond these energies — if a two-dimensional signal is received, perhaps that is evidence of this dimension-energy relationship.
So what?
Apart from trying to prove the early universe was a very weird one-dimensional straight line, how else would this research be useful?
There are a huge number of cosmological conundrums that don't seem to "fit" with our current knowledge of the universe (hint: dark energy and dark matter), so the dimensional "evolution" of our universe might be able to help.
But how could we even begin to comprehend what a "lower-dimensional" universe would have been like? Well, that would be like trying to describe a three-dimensional object to a two-dimensional lifeform, in reverse.