Lawrence Krauss - The Greatest Story Ever Told--So Far

by Why Are We Here (pdf)

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).

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  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

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  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

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  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

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  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
r />   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

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  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.

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  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

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  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
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  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

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  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.

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  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