Our understanding of the universe has been completely rewritten in the past century – and the obvious gaps we still have in our understanding mean it is likely to be rewritten again. But what ideas provide possible ways forward? Here we explore some of the most promising.
For the lowdown on our current best theories of the universe, take a look at Reality guide: The essential laws of cosmology and Reality guide: The essential laws of quantum physics
To read more about the outstanding problems in physics today, turn to Reality guide: Six problems physics can’t explain
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IDEA 1: Modified gravity
Our theories of gravity have only ever been tested on small scales
General relativity is a hugely accurate theory of gravity – as far as we can tell. But could modifying gravity perhaps exorcise some cosmic demons?
General relativity’s predictions of the movements of probes and planets are accurate up to the scale of the solar system, and the recent discovery of gravitational waves indicates it predicts spot-on what happens when two orbiting black holes merge, too. But by cosmic standards, these systems have a lot of mass crammed into a relatively small space. What happens to gravity in environments where its strength is weaker?
A tweaked version of gravity called modified Newtonian dynamics (MOND), might describe the anomalous galactic rotations taken to be evidence for dark matter, but there is as yet no theory for how this modification might come about. The DGP gravity hypothesis, meanwhile, named after the initials of its originators, suggests that gravity can leak out from our 4D space-time into a higher-dimensional “bulk”, progressively weakening its effect over time and so producing the illusion of dark energy. We’re yet to find any experimental evidence to back up this hypothesis, however.
IDEA 2: Supersymmetry
More particles can explain why the universe is as it is
Supersymmetry, or SUSY to its friends, is the Swiss army knife of particle theories: it has a tool for everything. It works by suggesting that for every matter-making fermion in the standard model of particle physics there is a force-carrying boson, and vice versa.
Double the particles means more than double the fun. Supersymmetry can solve fine-tuning problems such as the lowish mass of the Higgs boson: interactions with “superpartner” particles rein it in to just the observed level. Calculations done in the 1980s showed that if you assumed supersymmetry existed and extrapolated the strength of electromagnetism and the weak and strong forces back into the early universe, they could be made to unify into one superforce with stunning accuracy. The icing on the cake was that the lightest superpartner, the neutralino, supplied a ready-made identity for dark matter.
Too good to be true? Probably. If supersymmetric particles exist, they should have been created at CERN’s Large Hadron Collider by now. But aside from the odd exciting blip that subsequently went away, there’s been no sign. Perhaps the superpartners are heavier than we thought, or perhaps we just haven’t teased their existence from the data yet. But increasingly, particle physicists are facing an uncomfortable thought: nature might not have supplied as neat an answer as supersymmetry.
Reality guide: How everything fits together
The six principles that rule the universe… and the six big problems we still can’t crack
IDEA 3: A fifth force
Could a quintessence banish cosmic ghosts?
Gravity, electromagnetism, the weak force and the strong force… four seems an arbitrary number of fundamental forces. Why not more?
The most likely fifth force is a weak, long-distance force, a bit like gravity, and would probably interact with it. It might cancel a little of it and explain why the universe’s expansion rate is accelerating, mimicking dark energy. Or it could add to gravity to explain the additional pull usually attributed to dark matter. Such a fifth force would have to be cunningly disguised to explain why we don’t feel it. One suggestion is that the large amount of mass in and around the solar system shields us from its effects. That also makes it a very hard idea to test.
Fifth forces with attendant quantum particles are also sometimes proposed to solve problems such as fine-tuning in particle physics, but there seems precious little evidence for new forces on this scale. The occasional anomaly does keep physicists hoping, however: most recently, aberrations in the expected decay rate of radioactive beryllium nuclei have been touted as first evidence of a whole new “dark sector” of particles and forces.
IDEA 4: String theories
An ultimate theory must subsume quantum theory and relativity
Many physicists’ dreams, including Einstein’s, have been dashed in attempts to formulate a unifying theory of all of nature’s phenomena.
In the past few decades, a popular route to a theory of everything has emerged: string theory, and in particular a variant known as M-theory. In M-theory, matter is made not of pointlike particles as in the standard model, but of one-dimensional vibrating strings living in a universe with 11 space-time dimensions. These strings vibrate in different ways to create various elementary particles – even gravitons to carry a quantised gravitational force.
M-theory implicitly includes supersymmetry. Its extra dimensions are curled up and tiny, explaining why we are not aware of them. Breakthroughs such as the discovery in 1995 of a way to link a theory with gravity in five dimensions with a purely quantum theory in four seemed to indicate it was the right way to unify forces and theories.
But advances similar to this “AdS/CFT correspondence” are yet to be made for space-times more like that of our universe, and string theory has yet to make a single testable prediction. Most perturbingly, it seems to predict the existence of a multiverse of anything up to 10500 different universes.
All that has led some to question whether string theory really counts as science. Rival approaches do pop up from time to time. But all these theories are far away from a theory of everything as most people would understand it: one that can also explain, for example, how properties such as consciousness “emerge” from the workings of inanimate matter.
IDEA 5: The multiverse
The universe is as it is – because every other universe is out there too

Many roads lead to the multiverse. String theory needs it. Inflation creates it. In the many-worlds solution to the measurement problem of quantum mechanics, we are constantly, unwittingly making parallel universes.
The first difficulty is that these multiverses are probably all different. The second is how you get convincing evidence for the existence of any of them.
But multiverses are in general both a blessing and a curse. The string theory or inflationary multiverse, for example, can solve problems like fine-tuning: other universes exist where all other possible configurations of matter exist, and our configuration just happens to be one of those where conditions were ripe for sentient, questioning beings to evolve. But such “anthropic” reasoning absolves us of the responsibility of asking that most probing of questions, “why?”. It may be the best we can do, but by allowing every possibility besides the one you’re probing to play out somewhere in the multiverse, science robs itself of its predictive power.
IDEA 6: Everything is information
Energy and matter don’t matter – bits are where it’s at
When attempting to unify general relativity and quantum theory, it’s generally assumed that general relativity is at fault. It is, after all, a classical field theory of the sort that shinier quantum theories have otherwise nudged aside.
But as long as quantum theory remains largely inexplicable, there’s always the chance it is the wrong ’un, or just an approximation to some deeper theory. What’s truly pulling the strings of , for example? The world embodied by quantum theory is not the most entangled world out there – other, even weirder worlds exist in theory that have even greater degrees of correlation. Why this entangled, and not more?
That might be another fine-tuning issue. But entanglement does seem to be at the root of many mysteries, and there are recent hints that it could be the warp and weft that holds space and time together. In that case, what is entanglement? The best we can say is that it’s some sort of collective information shared between particles.
That highlights a common theme in much of cutting-edge physics: that understanding space-time, and the route to a more unified picture of nature generally, lies in treating information – not matter and energy – as the most fundamental thing in the universe and understanding better how it works.
Perhaps looking at the universe through information-tinted spectacles will open us up to blindingly obvious solutions that make the problems we encounter today melt away.
