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
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 store 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.
For those who enjoy horror stories, another, even more gruesome
possibility has been suggested. An instability might exist that would
cause the Higgs field to continue to grow in magnitude indefinitely.
As a result of such growth, the energy stored by the evolving Higgs
field could become negative. This could cause the entire universe to
collapse once again in a cataclysmic reversal of the Big Bang—a Big
Crunch. Happily the data disfavor such a possibility, as poetic as it
might seem.
In the scenario in which everything we now see disappears as the
Higgs makes a sudden transition to a new ground state, I want to
stress that the Higgs mass, as now measured, favors stability but has
sufficient uncertainty in its value to fall on either side of this line—
either producing the apparently stable vacuum that we are now
flourishing in, or favoring such a transition. Moreover, this scenario
is based on calculations within the Standard Model alone. Any new
physics that might be discovered at the LHC or beyond could
change the picture entirely, stabilizing what could otherwise be an
unstable Higgs field. Since we are reasonably certain there is new
physics to be discovered, there is no cause for despair at present.
If that isn’t consolation enough, for those who still fear that the
ultimate future of the universe might be the more miserable one I
have just described, the same calculations that suggest this may
happen also suggest that our current metastable configuration of
reality would persist for not merely billions of years into the future,
but billions of billions of billions of years.
ͥ͟͝
Concerns about the future notwithstanding, now is an
appropriate time to once again emphasize that the universe doesn’t
give a damn what we would like or whether we survive. Its dynamics
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