Our observations have now taken us back to the neonatal universe. We have observed the fine details of radiation, called the cosmic microwave background, which emanates from a time when the universe was merely three hundred thousand years old. Our telescopes take us back to the earliest galaxies, which formed perhaps a billion years after the Big Bang, and have allowed us to map huge cosmic structures containing thousands of galaxies and spanning hundreds of millions of light-years across, sprinkled amid the hundred billion or so galaxies in the visible universe.
To explain these features, theorists rely on an idea that arose due to the development of Grand Unified theories. In 1981, Alan Guth realized that the symmetry-breaking transition that might occur at the GUT scale early in the universe might not be identical to the transition that breaks the symmetry between the weak interaction and electromagnetism. In the GUT case, the Higgs-like field that condenses in space to break the GUT symmetry between the strong force and the electroweak force might momentarily get stuck in a metastable high-energy state before relaxing to its final configuration. While it was in this “false vacuum” configuration, the field would store energy that would be released when the field ultimately relaxed to its preferred lowest-energy configuration.
The situation would not be unlike what may have happened to you if you have ever planned a big party and then forgotten to put the beer in the fridge in time. You then put the beer in the freezer and forget about it during the party. The next day you discover the beer, open a bottle, and wham! The beer in the bottle suddenly freezes and expands, shattering the glass, and producing quite a mess. Before the top is taken off, the beer is under high pressure, and the beer at this pressure and temperature is liquid. However, once you open the top and release the pressure, the beer suddenly freezes. During the transition, energy is released as the beer relaxes to its new state—enough energy to cause the expanding ice to break the bottle.
Now imagine a similar situation when you are in a cold climate. On a brisk and rainy winter day, the temperature may quickly drop below freezing, causing the rain to change to snow. Puddles of water on the street may not freeze right away, especially if the tires of passing cars are continually agitating them. Later in the day, when the traffic dies down, the water may suddenly freeze, causing dangerous black ice on the road. Due to the previous agitation by cars and the quick fall in temperature, the water got stuck in a “metastable phase,” namely as a liquid. Eventually, however, a phase transition takes place, and the black ice forms. Because at these low temperatures the preferred, lowest-energy state of water is its solid form, when the liquid freezes, it releases the excess energy it stored in its metastable liquid state.
Guth wondered what would have happened in the early universe if such a behavior occurred during a Grand Unified Theory transition—if whatever scalar field that acts like the Higgs field for that transition remains in its original (symmetry-preserving) ground state for a brief time, even as the universe cools past the point where the new (symmetry-breaking) ground state condensate becomes preferred. Guth realized that this type of energy, stored through space by this field before the transition completes, would be gravitationally repulsive. As a result, it would cause the universe to expand—potentially by a huge factor, maybe twenty-five orders of magnitude or more in scale—in a microscopically short time.
He next discovered that this period of rapid expansion, which he dubbed inflation, could resolve a number of existing paradoxes associated with the Big Bang picture, including why the universe is so uniform on large scales and why three-dimensional space on large scales appears so close to being geometrically flat. Both of these seem inexplicable without inflation. The first problem is solved because, during the rapid expansion, any initial inhomogeneities get smoothed out, just as a wrinkled balloon gets smoothed out when it gets blown up. Pushing the balloon analogy further, the surface of a balloon that is blown up to be very large, say, the size of Earth, could look very flat, just as Kansas does. While this provides two-dimensional intuition, the same phenomenon would apply to the three-dimensional curvature of space itself. After inflation, space would appear to be flat—namely it would be like the universe most of us had assumed we live in already, where parallel lines never intersect and the x, y, and z axes point the same direction everywhere in the universe.
After inflation ends, the energy stored in the false vacuum state throughout space would be released, producing particles and reheating the universe to a high temperature, setting up a natural and realistic initial condition for the subsequent standard hot Big Bang expansion.
Even better, a year after Guth proposed his picture, a number of groups performed calculations of what would happen to particles and fields as the universe rapidly expanded during inflation. They discovered that small inhomogeneities resulting from quantum effects at early times would then be “frozen in” during inflation. After inflation ended, these small inhomogeneities could grow to produce galaxies, stars, planets, etc., and would also leave an imprint in the cosmic microwave background (CMB) radiation that resembles precisely the pattern that has since been measured. However, it is also possible, by using different inflationary models, to get different predictions for the CMB anisotropies (inflation is, at this point, more of a model than a theory, and since no unique Grand Unified Theory transition is determined by experiment, many different variants might work).
Another exciting and more unambiguous prediction from inflation exists. During the period of rapid expansion, ripples in space, called gravitational waves, would be produced. These ripples would produce another characteristic signature in the CMB that might be sought out. In 2014, the BICEP experiment claimed to detect a signal that was identical to what was predicted. This caused incredible excitement in both theoretical and observational communities. Along with Frank Wilczek, I wrote a paper that not only pointed out that such an observation would indicate a symmetry-breaking scale that corresponded nicely to the Grand Unified Theory symmetry-breaking scale in models with supersymmetry, but also that the observation would demonstrate unambiguously that gravity had to be a quantum theory on small scales—so that a search for a quantum theory of gravity was not misplaced.
Unfortunately, however, the BICEP announcement proved to be premature. Other backgrounds in our galaxy could have produced a similar signal, and as of this writing the situation still seems murky, with no unambiguous confirmation of inflation, or quantum gravity.
Most recently, between completion of the first draft of this book and completion of the final draft, the first definitive direct discovery of gravitational waves was made by an amazing set of detectors, called the Laser Interferometer Gravitational-Wave Observatory (LIGO), located in Hanford, Washington, and Livingston, Louisiana. LIGO is a spectacular and ambitious machine. To detect gravitational waves emitted by colliding black holes in distant galaxies, the experimenters had to be able to detect an (oscillating) difference in length between two four-kilometer-long perpendicular arms of the detectors equivalent to one one-thousandth of the size of a proton—like measuring the distance between Earth and the nearest star other than our Sun, Alpha Centauri, to an accuracy of the width of a human hair!
As amazing as the LIGO discovery of gravitational waves is, the waves it detected are from a distant astrophysical collision, not from the earliest moments of the Big Bang. But the success of LIGO will herald the building of new detectors, so that gravitational-wave astronomy will likely become the astronomy of the twenty-first century.
If the successors to LIGO, or BICEP, in this or the next century are able to measure directly the signature of gravitational waves from inflation, it will give us a direct window on the physics of the universe when it was less than a billionth of a billionth of a billionth of a billionth of a second old. It will allow us to directly test our ideas of inflation, and even Grand Unification, and perhaps even shed light on the possible existence of other universes—turning what is now metaphysics into physics.
For
the moment, however, inflation is merely a well-motivated proposal that seems to naturally resolve most of the major puzzles in cosmology. But while inflation remains the only first-principles theoretical-candidate explanation for the major observational features of our universe, it relies on the existence of a new and completely ad hoc scalar field—invented solely to help produce inflation and fine-tuned to initiate it as the early universe first began to cool down after the Big Bang.
Before the discovery of the Higgs particle, this speculation was plausible at best. With no example of any fundamental scalar field yet known, the assumption that Grand Unified symmetry-breaking might result from yet another simple Higgs-like mechanism was an extrapolation that rested on an insecure footing. As I have described, the breaking of electroweak symmetry was clear with the discovery of W and Z particles. But the simple Higgs field could have been a fairy-tale placeholder for some far more complicated, and perhaps far more interesting, underlying mechanism.
Things have now changed. The Higgs exists, and so too apparently a background scalar field permeating all space in the universe today, giving mass to particles and producing the characteristics of a universe we can inhabit. If a Grand Unified Theory really exists combining all three forces into one at close to the beginning of time, some symmetry breaking must have then occurred so that the three known nongravitational forces would only begin to diverge in character afterward. The Higgs demonstrates that symmetry breaking in the laws of nature can occur as the result of a scalar field condensate throughout space. Depending upon the details, inflation thus becomes a far more natural and potentially generic possibility. As my colleague Michael Turner put it jokingly some time ago, aping then Federal Reserve Board chair Alan Greenspan, “Periods of inflation are inevitable!”
That statement may have been more prescient than anyone imagined at the time. In 1998 it was discovered that our universe is now undergoing a new version of inflation, validating some previous and rather heretical predictions by a few of us. As I mentioned earlier, this implies that the dominant energy of the universe now appears to reside in empty space—which is the most plausible explanation of why the observed expansion of the universe is speeding up. The Nobel Prize was awarded to Brian Schmidt, Adam Riess, and Saul Perlmutter for the discovery of this remarkable and largely unexpected phenomenon. Naturally the questions arise, What could be causing this current accelerated expansion, and What is the source of this new kind of energy?
Two possibilities present themselves. First, it could be a fundamental property of empty space, a possibility actually presaged by Albert Einstein shortly after he developed the General Theory of Relativity, which he realized could accommodate something he called a “cosmological constant,” but which we now realize could simply represent a nonzero ground-state energy of the universe that will exist indefinitely into the future.
Or second, it could be energy stored in yet another invisible background scalar field in the universe. If this is the case, then the next obvious question is, will this energy be released in yet another, future inflationary-like phase transition as the universe continues to cool down?
At this time the answer is up for grabs. While the inferred energy density of empty space is today greater than the energy density of everything else we see in the universe, in absolute terms, on the scale of the energies associated with the masses of all elementary particles we know of, it is minuscule in the extreme. No one has any sensible first-principles explanation using known particle physics mechanisms for how the ground-state energy of the universe could be nonzero—resulting in Einstein’s cosmological constant—and yet so small as to allow the kind of gentle acceleration we are now experiencing. (One plausible explanation does exist—first due to Steve Weinberg—though it is speculative and relies on speculative ideas about possible physics well beyond the realm of anything we currently understand. If there are many universes, and the energy density in empty space, assuming it is a cosmological constant, is not fixed by fundamental physics constraints, but instead randomly varies from universe to universe, then only in those universes in which the energy in empty space is not much bigger than the value we measure would galaxies be able to form, and then would stars be able to form, and only then planets, and only then astronomers . . .)
Meanwhile, no one has a sensible model for a new phase transition predicted to occur in particle physics for a new scalar field that would store such a small amount of energy in space today. By sensible, I mean a model that anyone other than those who propose it finds plausible.
Nevertheless, the universe is the way it is, and the fact that current fundamental theory does not make a first-principles prediction that explains something as fundamental as the energy of empty space implies nothing mystical. As I have said, lack of understanding is not evidence for God. It is merely evidence of a lack of understanding.
Given that we do not know the source of the inferred energy in empty space, we are free to hope for the best, and in this case perhaps that means hoping that the cosmological constant explanation is correct rather than its being due to some as yet undiscovered scalar field that may one day relax into a new state, releasing the energy currently stored in space.
Recall that because of the coupling of the Higgs field to the rest of the matter in the universe, when the field condensed into its electroweak symmetry-breaking state, the properties of matter and the forces that govern the interactions of matter changed dramatically.
Now, if some similar phase transition involving some new scalar field in space is yet to occur in nature, then the stability of matter as we know it could disappear. Galaxies, stars, planets, people, politicians, and everything we now see could literally disappear. The only good news (other than the disappearance of politicians) is that the transition—assuming it begins with some small seed in one location of our universe (in the same way that small dust grains may help seed the formation of the ice crystals on our frozen windowpane, or of snowflakes as they fall to the ground)—will then spread throughout space at the speed of light. We won’t know what hit us until after it has, and after it has, we won’t be around to know.
The curious reader may have noticed that all of these discussions relate to new possible scalar fields in nature. What about the Standard Model Higgs field? Could it play a role in all of these current cosmic shenanigans? Could the Higgs field store energy and be responsible for inflation either in the early universe or now? Could the Higgs field not be in its final ground state, and will there be another transition that will once again change the configuration of the electroweak force, and the masses of particles in the Standard Model?
Good questions. And the answers to all of them are the same: we don’t know.
That has not stopped a number of theorists from speculating about this possibility. My favorite example—not because it is better than any of the others, but because it’s a speculation I made with a colleague, James Dent, shortly after the Higgs was discovered—is that perhaps the Higgs does play a role in the observed cosmic expansion. As a number of authors have recognized, the existence of one background field condensate and the particles it comprises can provide a unique window, or “portal,” that may yield otherwise unexpected sensitivity to the existence of other Higgs-like fields in nature, no matter how weakly their direct couplings to the particles we observe in the Standard Model may be.
If the Higgs and other Higgs-like particles exist, perhaps at the Grand Unified Theory scale, the physical Higgs, the particle that was discovered at CERN, may be a slight admixture between the weak interaction Higgs, and another Higgs-like particle. (In this we are guided by the physics of neutrinos, where similar phenomena play a vital role in understanding the behavior of neutrinos measured on Earth coming from the nuclear reactions in the Sun, for example.) It is then possible, at least, to argue that when the weak interaction Higgs field condenses in empty space, this could stimulate the condensation of another Higgs-like field with properties that would allow it to stor
e just the right energy to explain the observed inflation of the universe today. The mathematics required to make this happen is pretty contrived—the model is ugly. But who knows? Maybe it is ugly because we haven’t found the correct framework in which to embed it.
However, one attractive feature of this scenario makes it a little less self-serving to mention it. In this picture, the energy carried by the second field, which would drive the current measured accelerated expansion of the universe today, will likely ultimately be released in a new phase transition to the true ground state of the universe. Unlike many other possibilities for future possible phase transitions in our universe, because the new field can be weakly coupled to all observed particles, this transition will not induce a change in the observed properties of any of the known particles in nature by an amount that would be noticeable. The upshot is that if this model is right, the universe as we know it may survive.
Yet celebration may be premature. Independent of such speculations, the discovery of the Higgs particle has raised the specter of a much less optimistic possibility. While a future in which the observed acceleration of the universe goes on forever is a miserable future for life and for the ability to continue to probe the universe—because eventually all galaxies we can now observe will recede from us faster than light, ultimately disappearing from our horizon, leaving the universe cold, dark, and largely empty—the future that may result because of a Higgs field with a mass 125 times the mass of the proton could be far worse.
For a Higgs mass coinciding with the allowed range of the observed Higgs, assuming for the moment that the Standard Model is not supplemented by a lot of new stuff at higher energy, calculations suggest that the existing Higgs field condensate is teetering on the edge of instability—it could change from its current value to a vastly different value associated with a lower-energy state.
If such a transition occurs, normal matter as we know it changes its form, and galaxies, stars, planets, and people most likely disappear, like the ice crystal on a warm sunny morning.
The Greatest Story Ever Told—So Far Page 28