Despite claims of a new age of ‘precision cosmology’, the origin of the universe remains a mystery. Confronting cosmic fine-tuning and the multiverse scientifically might help solve it, writes Geraint F. Lewis.
In little over a century, our understanding of the universe has changed dramatically. Driven by theoretical and observational advances in the first few decades of the twentieth century, the seemingly static and eternal cosmos known to the ancients was replaced by a dynamical and evolving universe.
Cosmology is the modern scientific exploration of our evolving universe. Underwritten by Einstein’s general theory of relativity, the mathematics of cosmology have revealed that our universe was born almost 14 billion years ago in a fiery event now known as the “Big Bang”. In its initial moments, the universe was extremely hot and extremely dense, and it has been expanding and cooling ever since. For almost 10 billion years, this rate of cosmic expansion steadily slowed due to the gravitational pull of matter and radiation, but more recently a more mysterious substance has come to dominate. This “dark energy” acts to accelerate the expansion faster and faster.
Astronomers have searched for the stuff of the universe, through counting stars, measuring gravitational pulls, and determining the expansion of the cosmos. They have reached the stark conclusion that normal matter, such as the atoms from which we are made, represents only 5% of the cosmic energy budget. The rest of the matter is “dark matter” which is five times more plentiful than atomic material and dominates gravitation across the cosmos. The remainder of the universe, 70% of it, is in the form of dark energy. But it is the complexity and versatility of normal matter, allowing it to form into gas clouds, stars, planets and people, that makes the universe immensely interesting to our telescopes.
With the laws of physics in hand, cosmologists have been able to accurately chart the evolutionary history of the cosmos. We now know that in the first few minutes the lightest elements hydrogen, helium and a sprinkling of lithium were forged in the fires of the Big Bang. After its spectacular beginning, the universe descended into darkness as it cooled, but in the darkness, matter began to be drawn together under the influence of gravity. Eventually, after about 100 million years, enough gas had accumulated and collapsed for the formation of the first stars, bringing light back to the cosmos.
These first stars were born within dense knots of dark matter which continued to grow as more material rained in. Over the billions of years to come, these sites grew into entire galaxies as generations of stars came and went. About 10 billion years ago, our Milky Way galaxy was formed. Just over 4 billion years ago, our Sun was born, and Earth condensed within the disk of debris that swirled around it. In a cosmic blink before today, we emerged on the surface of this planet, now pondering the nature of the universe around us.
“Just where did our universe come from? We are currently unable to answer this question due to the biggest failing of modern physics”
Cosmologists are proud of this story of success. Some have claimed that we are now in the era of “precision cosmology” with an intense focus on measuring the key properties of the universe to a finer and finer degree. Of course, there are questions left to answer in terms of the growth of galaxies, lives of stars, or the flow of gas, but this is due to the complexity of the physics involved and the answers will soon be in our grasp.
Not everyone is so happy with this seemingly rosy picture. The physical nature of the dark side of the universe dark matter and dark energy remains essentially unknown. Most physicists are content with the notion that dark matter is a fundamental particle currently sitting somewhere outside of the “standard model” zoo of electrons, quarks and other particles. The hope is that experimental searches will yield this missing component of the universe in the near future.
Dark energy, on the other hand, remains truly mysterious. Theoretical ideas have pointed to its existence being related to the nature of the “quantum vacuum”, a background hiss of empty space due to the fleeting existence of quantum particles. Whilst precise calculations have failed to account for the observed energy density of dark energy (by an embarrassingly immense margin), maybe if we keep pushing on the mathematics of quantum mechanics, then one day we will understand it.
But a bigger problem looms over cosmology, namely just where did our universe come from? We are currently unable to answer this question due to the biggest failing of modern physics — the incompatibility of quantum mechanics and Einstein’s theory of gravity. We know that there are four fundamental forces at work in the universe, in which gravity is described by Einstein’s malleable space and time. The remaining three forces - electromagnetism, and the strong and weak nuclear forces are written in the distinctly different language of quantum mechanics. In most of physics, this distinction is not important as we can talk about each of the forces separately, with gravity responsible for the motions of galaxies and expansion of the cosmos, whilst the other forces dominate the lives of atoms. But this distinction is lost in the earliest moments of the universe.
As we continue to push backwards into times where physics becomes more speculative, we still find that gravity and the other forces still dominate over different regimes. Eventually we come to a point where this separation breaks down. Thought to be somewhere about the “Planck time” - about 10-43 seconds after the cosmic beginning - gravity and the other forces are fighting for dominance over the universe, and our mathematical description simply falls apart.
We currently lack a “Theory of Everything” that unites all of the forces within a single mathematical framework. Without one, we cannot keep pushing back to see if time zero was really the start of the universe, or if our universe is part of a larger and more complicated story.
Astrophysicist Catherine Heymans explores the mysteries of the multiverse.
Of course, this does not stop cosmologists speculating about what, if anything, came before. Many find the notion of a “universe from nothing”, a cosmic birth from no time and no space, as unpalatable. Endlessly cycling universes have been proposed, with our universe born from the death of a previous cosmos. Some have grumbled that such cyclical universes will undergo constant degradation due to the continual march of entropy, leaving them eventually dead and sterile. Solutions to avoid this inevitable outcome have been suggested, but eventual demise by entropy is difficult to avoid!
Other cosmologists have been more creative, suggesting that universes are born from violent events in pre-existing universes, such as the creation of a black hole when a massive star explodes. The precise mechanism in forming such baby universes is somewhat hazy, but maybe the formation of a singularity, the infinitely dense heart of a black hole, somehow pierces spacetime to give birth to a new universe. This would mean that our own universe could be giving birth to about a hundred other universes every second!
The notion that our universe is just one of many other universes existing together has given birth to the idea of a “multiverse”, a uber-universe that contains a sea of individual universes The mere mention of this word can raise the hackles on a lot of physicists, so it is important to note that the concept of a multiverse is not a single idea, but more of a grab-bag of thoughts — from causally disconnected regions within a single overall-universe, to distinct individual universes floating like jellyfish in some higher-dimensional structure.
In justifying the idea of a multiverse, some physicists have noted that it might be a solution to some peculiarities of our universe, namely the question of cosmic fine-tuning. To understand fine-tuning, we have to imagine how the universe could have been different, not just different in a rearrangement of the stars and galaxies, but different in its physical properties, such as the strength of fundamental forces, the masses of subatomic particles, the rate of cosmic expansion, and the initial conditions at cosmic birth.
Let’s take an example. Hydrogen is the most common element in the universe, providing the raw materials for stars. It is a simple atom, consisting of a single proton coupled with an orbiting electron. The other particle found in atomic nuclei is the neutron, which is 0.14% more massive than the proton. This tiny mass difference is due to the properties of the fundamental quarks that make up these particles, and if these were slightly different the neutron would be lighter. This would radically change the make-up of the universe, with the neutron being the dominant particle, leaving it inert with no atoms.
Similarly, the quantity of dark energy also appears to be fine-tuned. Our best theoretical models predict that the density of dark energy should be immensely larger than observed and why there is so little in our universe remains mysterious. However, we have benefited from this relatively miniscule quantity of dark energy as if it were larger by about a hundred times (which is tiny compared to theoretical expectation’s 120 orders of magnitude!), cosmic expansion would have accelerated from the outset, emptying out the universe and cutting-off the formation of stars and galaxies. The prospects for complexity and life in such a universe are severely limited.
Many other examples exist, and the conclusions are the same. If the fundamental properties of the universe had been slightly different, the resulting cosmos would be dead and sterile, without the capacity to host life. So why does our universe have just the right mix of physical properties that allows us to be here?
Some have sought a theological solution by demanding a designer who personally chose the physical properties for our existence. In scientific terms, others state that the universe has nothing to answer as the fundamental properties are fixed and immutable, and so asking “why this universe?” is a meaningless question.
“If the fundamental properties of the universe had been slightly different, the resulting cosmos would be dead and sterile”
But to some, cosmic fine-tuning is alluring, even crying out for a scientific explanation. They suggest that in the multiverse individual universes are born with distinct laws of physics, each with a differing mix of fundamental forces, masses and initial conditions. The physics and properties within most of these universes will result in them being dead and sterile, unable to host complexity and life. But every so often, a universe will be born with just the right forces, just the right masses, just the right initial conditions, that would allow for the complexity of life. Our universe must have been one of those lucky few.
Certain physicists object to the very notion of a multiverse, but not necessarily to its possible existence. Their objections are principally to multiverses being discussed at all! They note that, irrespective of which multiverse concept you embrace, any other universes are forever disconnected from us, unable to be reached through our telescopes and microscopes and so outside of empirical testing. This, they claim, squarely places the concept of a multiverse outside of science, and so it has no role in explaining the universe that we see. All that matters is our universe, and everything else is metaphysics or nonsense.
However, this viewpoint is clearly too limited. Physicists constantly play what-if games to tease out the fundamental nature of the universe. Einstein wondered about warping space and time on his path to general relativity. De Broglie asked what it meant for an electron to behave like a wave, taking a crucial step in the development of quantum mechanics. Today, cosmologists are proposing modifications to our picture of a constant dark energy, suggesting it could have changed its properties over cosmic history, and using the cosmological equations to determine how this will influence our observations of the universe.
Of course to be science, such what-ifs have to eventually yield to observation and experiment for us to determine whether they are an accurate representation of reality. Until this point, they are just stories. The multiverse, the critics tell us, will never yield to experimentation because it cannot.
But in reality, it is far too soon to say this is truly the case. We simply lack the theoretical tools to construct meaningful multiverse models, and we will potentially need the sought-after Theory of Everything to do so. However, once we do, not only will this open the door back through the beginning of the Big Bang but could also provide us with new observational tests of our universe’s relation to any multiverse. Maybe something will be written into the cosmic microwave background or the fundamental physical properties of the cosmos, revealing whether our universe is just one in a crowded multiverse or is truly alone. Either outcome will be cosmically mind-bending.