We live in a golden age for learning in regards to the universe. Our strongest telescopes have revealed that the cosmos is surprisingly easy on the most important visible scales. Likewise, our strongest “microscope,” the Large Hadron Collider, has found no deviations from known physics on the tiniest scales.

These findings weren’t what most theorists expected. Today, the dominant theoretical approach combines string theory, a robust mathematical framework with no successful physical predictions as yet, and “cosmic inflation”—the concept that, at a really early stage, the universe ballooned wildly in size. Together, string theory and inflation predict the cosmos to be incredibly complex on tiny scales and completely chaotic on very large scales.

The character of the expected complexity could take a bewildering number of forms. On this basis, and despite the absence of observational evidence, many theorists promote the concept of a “multiverse”: an uncontrolled and unpredictable cosmos consisting of many universes, each with totally different physical properties and laws.

To this point, the observations indicate exactly the other. What should we make of the discrepancy? One possibility is that the apparent simplicity of the universe is merely an accident of the limited range of scales we are able to probe today, and that when observations and experiments reach sufficiently small or large enough scales, the asserted complexity might be revealed.

The opposite possibility is that the universe really *is* quite simple and predictable on each the most important and smallest scales. I imagine this possibility ought to be taken way more seriously. For, whether it is true, we could also be closer than we imagined to understanding the universe’s most simple puzzles. And among the answers may already be staring us within the face.

## The Trouble With String Theory and Inflation

The present orthodoxy is the culmination of a long time of effort by 1000’s of significant theorists. In line with string theory, the fundamental constructing blocks of the universe are minuscule, vibrating loops and pieces of sub-atomic string. As currently understood, the speculation only works if there are more dimensions of space than the three we experience. So, string theorists assume that the explanation we don’t detect them is that they’re tiny and curled up.

Unfortunately, this makes string theory hard to check, since there are an almost unimaginable number of how by which the small dimensions might be curled up, with each giving a unique set of physical laws within the remaining, large dimensions.

Meanwhile, cosmic inflation is a scenario proposed within the Eighties to elucidate why the universe is so smooth and flat on the most important scales we are able to see. The concept is that the infant universe was small and lumpy, but an extreme burst of ultra-rapid expansion blew it up vastly in size, smoothing it out and flattening it to be consistent with what we see today.

Inflation can also be popular since it potentially explains why the energy density within the early universe varied barely from place to put. This is significant since the denser regions would have later collapsed under their very own gravity, seeding the formation of galaxies.

Over the past three a long time, the density variations have been measured an increasing number of accurately each by mapping the cosmic microwave background—the radiation from the large bang—and by mapping the three-dimensional distribution of galaxies.

In most models of inflation, the early extreme burst of expansion which smoothed and flattened the universe also generated long-wavelength gravitational waves—ripples in the material of space-time. Such waves, if observed, can be a “smoking gun” signal confirming that inflation actually took place. Nonetheless, to this point the observations have didn’t detect any such signal. As an alternative, because the experiments have steadily improved, an increasing number of models of inflation have been ruled out.

Moreover, during inflation, different regions of space can experience very different amounts of expansion. On very large scales, this produces a multiverse of post-inflationary universes, each with different physical properties.

The inflation scenario relies on assumptions in regards to the types of energy present and the initial conditions. While these assumptions solve some puzzles, they create others. String and inflation theorists hope that somewhere within the vast inflationary multiverse, a region of space and time exists with just the proper properties to match the universe we see.

Nonetheless, even when that is true (and never one such model has yet been found), a good comparison of theories should include an “Occam factor,” quantifying Occam’s razor, which penalizes theories with many parameters and possibilities over simpler and more predictive ones. Ignoring the Occam factor amounts to assuming that there isn’t a alternative to the complex, unpredictive hypothesis—a claim I imagine has little foundation.

Over the past several a long time, there have been many opportunities for experiments and observations to disclose specific signals of string theory or inflation. But none have been seen. Repeatedly, the observations turned out simpler and more minimal than anticipated.

It’s high time, I imagine, to acknowledge and learn from these failures and to begin looking seriously for higher alternatives.

## A Simpler Alternative

Recently, my colleague Latham Boyle and I actually have tried to construct simpler and more testable theories that cast off inflation and string theory. Taking our cue from the observations, we have now attempted to tackle among the most profound cosmic puzzles with a bare minimum of theoretical assumptions.

Our first attempts succeeded beyond our most optimistic hopes. Time will tell whether or not they survive further scrutiny. Nonetheless, the progress we have now already made convinces me that, in all likelihood, there *are* alternatives to the usual orthodoxy—which has grow to be a straitjacket we’d like to interrupt out of.

I hope our experience encourages others, especially younger researchers, to explore novel approaches guided strongly by the simplicity of the observations—and to be more skeptical about their elders’ preconceptions. Ultimately, we must learn from the universe and adapt our theories to it fairly than vice versa.

Boyle and I started off by tackling one in all cosmology’s biggest paradoxes. If we follow the expanding universe backward in time, using Einstein’s theory of gravity and the known laws of physics, space shrinks away to a single point, the “initial singularity.”

In attempting to make sense of this infinitely dense, hot starting, theorists including Nobel laureate Roger Penrose pointed to a deep symmetry in the fundamental laws governing light and massless particles. This symmetry, called “conformal” symmetry, signifies that neither light nor massless particles actually experience the shrinking away of space at the large bang.

By exploiting this symmetry, one can follow light and particles all the best way back to the start. Doing so, Boyle and I discovered we could describe the initial singularity as a “mirror”: a reflecting boundary in time (with time moving forward on one side, and backward on the opposite).

Picturing the large bang as a mirror neatly explains many features of the universe which could otherwise appear to conflict with probably the most basic laws of physics. For instance, for each physical process, quantum theory allows a “mirror” process by which space is inverted, time is reversed, and each particle is replaced with its anti-particle (a particle much like it in just about all respects, but with the other electric charge).

In line with this powerful symmetry, called CPT symmetry, the “mirror” process should occur at precisely the identical rate as the unique one. One of the crucial basic puzzles in regards to the universe is that it appears to violate CPT symmetry because time all the time runs forward and there are more particles than anti-particles.

Our mirror hypothesis restores the symmetry of the universe. If you look in a mirror, you see your mirror image behind it: in the event you are left-handed, the image is right-handed and vice versa. The mix of you and your mirror image are more symmetrical than you’re alone.

Likewise, when Boyle and I extrapolated our universe back through the large bang, we found its mirror image, a pre-bang universe by which (relative to us) time runs backward and antiparticles outnumber particles. For this picture to be true, we don’t need the mirror universe to be real within the classical sense (just as your image in a mirror isn’t real). Quantum theory, which rules the microcosmos of atoms and particles, challenges our intuition so at this point the very best we are able to do is consider the mirror universe as a mathematical device which ensures that the initial condition for the universe doesn’t violate CPT symmetry.

Surprisingly, this recent picture provided a crucial clue to the character of the unknown cosmic substance called dark matter. Neutrinos are very light, ghostly particles which, typically, move at near the speed of sunshine and which spin as they move along, like tiny tops. In the event you point the thumb of your left hand within the direction the neutrino moves, then your 4 fingers indicate the direction by which it spins. The observed, light neutrinos are called “left-handed” neutrinos.

Heavy “right-handed” neutrinos have never been seen directly, but their existence has been inferred from the observed properties of sunshine, left-handed neutrinos. Stable, right-handed neutrinos can be the proper candidate for dark matter because they don’t couple to any of the known forces except gravity. Before our work, it was unknown how they could have been produced in the new early universe.

Our mirror hypothesis allowed us to calculate exactly what number of would form and to point out they might explain the cosmic dark matter.

A testable prediction followed: If the dark matter consists of stable, right-handed neutrinos, then one in all three light neutrinos that we all know of should be exactly massless. Remarkably, this prediction is now being tested using observations of the gravitational clustering of matter made by large-scale galaxy surveys.

## The Entropy of Universes

Encouraged by this result, we set about tackling one other big puzzle: Why is the universe so uniform and spatially flat, not curved, on the most important visible scales? The cosmic inflation scenario was, in any case, invented by theorists to unravel this problem.

Entropy is an idea which quantifies the variety of other ways a physical system might be arranged. For instance, if we put some air molecules in a box, the more than likely configurations are those which maximize the entropy—with the molecules roughly easily spread throughout space and sharing the full energy roughly equally. These sorts of arguments are utilized in statistical physics, the sector which underlies our understanding of warmth, work, and thermodynamics.

The late physicist Stephen Hawking and collaborators famously generalized statistical physics to incorporate gravity. Using a sublime argument, they calculated the temperature and the entropy of black holes. Using our “mirror” hypothesis, Boyle and I managed to increase their arguments to cosmology and to calculate the entropy of entire universes.

To our surprise, the universe with the very best entropy (meaning it’s the more than likely, similar to the atoms unfolded within the box) is flat and expands at an accelerated rate, similar to the true one. So statistical arguments explain why the universe is flat and smooth and has a small positive accelerated expansion, without having for cosmic inflation.

How would the primordial density variations, normally attributed to inflation, have been generated in our symmetrical mirror universe? Recently, we showed that a particular kind of quantum field (a dimension zero field) generates precisely the kind of density variations we observe, without inflation. Importantly, these density variations aren’t accompanied by the long wavelength gravitational waves which inflation predicts—and which haven’t been seen.

These results are very encouraging. But more work is required to point out that our recent theory is each mathematically sound and physically realistic.

Even when our recent theory fails, it has taught us a useful lesson. There may perhaps be simpler, more powerful and more testable explanations for the fundamental properties of the universe than those the usual orthodoxy provides.

By facing as much as cosmology’s deep puzzles, guided by the observations and exploring directions as yet unexplored, we may have the option to put safer foundations for each fundamental physics and our understanding of the universe.