be responsible for the observed energy scale of the weak interaction.
All of this comes at a cost, however. For the theory to work, there
must be two Higgs bosons, not just one. Moreover, one would expect
to begin to see the new supersymmetric particles if one built an
͟͜͞
accelerator such as the LHC, which could probe for new physics
near the electroweak scale. Finally, in what looked for a while like a
rather damning constraint, the lightest Higgs in the theory could not
be too heavy or the mechanism wouldn’t work.
As searches for the Higgs continued without yielding any results,
accelerators began to push closer and closer to the theoretical upper
limit on the mass of the lightest Higgs boson in supersymmetric
theories. The value was something like 135 times the mass of the
proton, with details to some extent depending on the model. If the
Higgs could have been ruled out up to that scale, it would have
suggested all the hype about supersymmetry was just that.
Well, things turned out differently. The Higgs that was observed
at the LHC has a mass about 125 times the mass of the proton.
Perhaps a grand synthesis was within reach.
The answer at present is . . . not so clear. The signatures of new
supersymmetric partners of ordinary particles should be so striking
at the LHC, if they exist, that many of us thought that the LHC had a
much greater chance of discovering supersymmetry than it did of
discovering the Higgs. It didn’t turn out that way. Following three
years of LHC runs, there are no signs whatsoever. The situation is
already beginning to look uncomfortable. The lower limits that can
now be placed on the masses of supersymmetric partners of ordinary
matter are getting higher. If they get too high, then the
supersymmetry-breaking scale would no longer be close to the
electroweak scale, and many of the attractive features of
supersymmetry breaking for resolving the hierarchy problem would
go away.
But the situation is not yet hopeless, and the LHC has been turned
on again, this time at higher energy. It could be that, in the year
between the time I write these words and the book going into its
tenth printing, supersymmetric particles will be discovered.
͟͜͟
If they are, this will have another important consequence. One of
the bigger mysteries in cosmology is the nature of the dark matter
that appears to dominate the mass of all galaxies we can see. As I
have briefly alluded to earlier, there is so much of it that it cannot be
made of the same particles as normal matter. If it were, for example,
the predictions of the abundance of light elements such as helium
produced in the Big Bang would no longer agree with observation.
Thus physicists are reasonably certain that the dark matter is made
of a new type of elementary particle. But what type?
Well, the lightest supersymmetric partner of ordinary matter is, in
most models, absolutely stable and has many of the properties of
neutrinos. It would be weakly interacting and electrically neutral, so
that it wouldn’t absorb or emit light. Moreover, calculations that I
and others performed more than thirty years ago showed that the
remnant abundance today of the lightest supersymmetric particle
left over after the Big Bang would naturally be in the range so that it
could be the dark matter dominating the mass of galaxies.
In that case our galaxy would have a halo of dark matter particles
whizzing throughout it, including through the room in which you
are reading this. As a number of us also realized some time ago, this
means that if one designs sensitive detectors and puts them
underground, not unlike, at least in spirit, the neutrino detectors that
already exist underground, one might directly detect these dark
matter particles. Around the world a half dozen beautiful
experiments are now going on to do just that. So far nothing has
been seen, however.
So, we are in potentially the best of times or the worst of times. A
race is going on between the detectors at the LHC and the
underground direct dark matter detectors to see who might discover
the nature of dark matter first. If either group reports a detection, it
will herald the opening up of a whole new world of discovery,
͟͜͠
leading potentially to an understanding of Grand Unification itself.
And if no discovery is made in the coming years, we might rule out
the notion of a simple supersymmetric origin of dark matter—and in
turn rule out the whole notion of supersymmetry as a solution of the
hierarchy problem. In that case we would have to go back to the
drawing board, except if we don’t see any new signals at the LHC, we
will have little guidance about which direction to head in order to
derive a model of nature that might actually be correct.
Things got more interesting when the LHC reported a tantalizing
possible signal due to a new particle about six times heavier than the
Higgs particle. This particle did not have the characteristics one
would expect for any supersymmetric partner of ordinary matter. In
general the most exciting spurious hints of signals go away when
more data are amassed, and about six months after this signal first
appeared, after more data were amassed, it disappeared. If it had not,
it could have changed everything about the way we think about
Grand Unified Theories and electroweak symmetry, suggesting
instead a new fundamental force and a new set of particles that feel
this force. But while it generated many hopeful theoretical papers,
nature seems to have chosen otherwise.
The absence of clear experimental direction or confirmation of
supersymmetry has thus far not bothered one group of theoretical
physicists. The beautiful mathematical aspects of supersymmetry
encouraged, in 1984, the resurrection of an idea that had been
dormant since the 1960s when Nambu and others tried to
understand the strong force as if it were a theory of quarks
connected by stringlike excitations. When supersymmetry was
incorporated in a quantum theory of strings, to create what became
known
as
superstring
theory,
some
amazingly
beautiful
mathematical results began to emerge, including the possibility of
͟͜͡
unifying not just the three nongravitational forces, but all four
known forces in nature into a single consistent quantum field theory.
However, the theory requires a host of new space-time
dimensions to exist, none of which has been, as yet, observed. Also,
the theory makes no other predictions that are yet testable with
currently conceived experiments. And the theory has recently gotten
a lot more complicated so that it now seems that strings themselves
are probably not even the central dynamical variables in the theory.
None of this dampened the enthusiasm of a hard core of
dedicated and highly talented physicists who have continued to
work
on superstring theory, now called M-theory, over the thirty years
since its heyday in the mid-1980s. Great successes are periodically
claimed, but so far M-theory lacks the key element that makes the
Standard Model such a triumph of the scientific enterprise: the
ability to make contact with the world we can measure, resolve
otherwise
inexplicable
puzzles,
and
provide
fundamental
explanations of how our world has arisen as it has. This doesn’t
mean M-theory isn’t right, but at this point it is mostly speculation,
although well-meaning and well-motivated speculation.
Here is not the place to review the history, challenges, and
successes of string theory. I have done that elsewhere, as have a
number of my colleagues. It is worth remembering that if the lessons
of history are any guide, most forefront physical ideas are wrong. If
they weren’t, anyone could do theoretical physics. It took several
centuries or, if one counts back to the science of the Greeks, several
millennia of hits and misses to come up with the Standard Model.
So this is where we are. Are great new experimental insights just
around the corner that may validate, or invalidate, some of the
grander speculations of theoretical physicists? Or are we on the
verge of a desert where nature will give us no hint of what direction
to search in to probe deeper into the underlying nature of the
͟͜͢
cosmos? We’ll find out, and we will have to live with the new reality
either way.
No matter what curveballs nature may throw at us, the recent
discovery of the Higgs, the latest and one of the greatest
experimental and theoretical achievements of the remarkable
Standard Model of particle physics, has beautifully capped more
than two millennia of intellectual effort by brave and determined
philosophers, mathematicians, and scientists to uncover the hidden
tapestry that underlies our existence.
It also suggests that the beautiful universe in which we find
ourselves may not only resemble, at least metaphorically, an ice
crystal on a windowpane, it may be almost as ephemeral.
ͣ͟͜
C h a p t e r 2 3
F R O M A B E E R PA R T Y T O
T H E E N D O F T I M E
For the fashion of this world passeth away.
—1 CORINTHIANS 7:31
My own research focus for much of my career has been the
emerging field of cosmology called particle astrophysics. Following
the flood of theoretical developments of the 1960s and 1970s, it was
difficult for terrestrial experiments, limited as they are by our
abilities to build complex machines such as particle accelerators, to
keep up. As a result, a number of us turned to the universe for
guidance. Since the Big Bang implies that the early universe was hot
and dense, conditions existed then that we might never achieve in
laboratories on Earth. But if we are clever, we can look for remnant
signatures of those early times out in the cosmos, and we may be
able test our ideas about even the most esoteric aspects of
fundamental physics.
My previous book, A Universe from Nothing, described the
revolutions in our understanding of the evolution of the universe on
large scales, and over long times. Not only have our explorations
revealed the existence of dark matter, which, as I have described, is
likely composed of new elementary particles not yet observed in
accelerators—although we may be on the cusp of doing so—but far
more exotic still, we have discovered that the dominant energy of
the universe resides in empty space—and we currently have no idea
how it arises.
ͤ͟͜
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
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 33