Obviously this cannot happen. Information, and hence energy, must be conserved. But many black holes have been detected, and they must evaporate. How is this dilemma to be resolved?
There is an obvious resolution: namely, that all observed black holes are the mass of the Sun or larger and such black holes will last billions of trillions of years before they approach a naked singularity. What if the universe came to an end in a Big Crunch singularity before any black holes had time to evaporate completely?
This resolution of the black-hole evaporation dilemma has a host of fascinating implications. First, it means that the dark energy, whatever it is, will eventually turn off. In an ever-accelerating universe, there will be no Big Crunch singularity, and all black holes will eventually evaporate.
Second, the great Israeli-American physicist Jacob Bekenstein—whose work suggested to Hawking that he should investigate the possibility of black-hole evaporation—has proved mathematically that if event horizons exist, then the entropy of the universe must approach zero as a Big Crunch singularity is approached. But the second law of thermodynamics says that entropy can never decrease, much less approach zero at the end of time. Thus, if the second law holds forever—which it does—then event horizons cannot exist. The absence of event horizons can be shown mathematically to imply that the universe must be spatially finite.
The absence of event horizons also incidentally, almost in passing, resolves the problem of how the information inside a black hole escapes: If there are no event horizons, there is no barrier to getting out. We should keep in mind that the assumption that a black hole is bounded by an event horizon is just that—an assumption, not an observed fact. If the universe ends in a Big Crunch, the information inside a black hole would not get out until near the Big Crunch. No observation today can show that the information is forever bound to being inside the black hole. A claim that event horizons exist is like someone’s claim to be immortal: You would have to wait until the end of time to confirm the claim.
So by merely accepting the obvious resolution of Hawking’s dilemma and applying the standard laws of physics, we infer that the universe is spatially finite, that the dark energy will eventually turn off, that the universe will end in a Big Crunch, and that event horizons do not exist. Various physicists have pointed to each of these facts over the past decade, but the implications seem to have escaped the science journalists. Eventually the information will leak out, hopefully before the end of time.
The Energy of Nothing
Andrei Linde
Author of eternal chaotic inflation; theoretical physicist, Stanford University
Back in 1998, two groups of astrophysicists studying supernovae made one of the most important experimental discoveries of the 20th century: They found that empty space, the vacuum, is not entirely empty. Each cubic centimeter of empty space contains about 10–29 grams of invisible matter—or, equivalently, vacuum energy. This is almost nothing: 29 orders of magnitude smaller than the mass of matter in a cubic centimeter of water, 5 orders of magnitude less than the mass of a proton. If the Earth were made of such matter, it would weigh less than a gram.
If the vacuum energy is so small, how do we even know it’s there? Just try putting 10–29 grams on the most sensitive of scales; it will show nothing at all. At first, many people were skeptical about its existence, but the combined efforts of cosmologists who study cosmic microwave background radiation and large-scale structure of the universe not only confirmed this discovery but allowed measurement of the energy density of the vacuum within a few-percent accuracy. Doubts and disbelief were replaced by acceptance, and by the Nobel Prizes received in 2011 by Saul Perlmutter, Brian Schmidt, and Adam Riess.
The news has shaken physicists all over the world. But is it much ado about nothing? If something is so hard to find, maybe it’s irrelevant and not at all newsworthy.
The vacuum energy is extremely small indeed, but in fact it is comparable to the average energy density of normal matter in the universe. Before this discovery, astronomers believed that the density of matter constituted only 30 percent of the density corresponding to a flat universe. That meant that the universe is open, contrary to the prediction of the inflationary theory of the universe’s origin. The vacuum energy added the required 70 percent to the sum total, thus confirming one of the most important predictions of inflationary cosmology.
The tiny vacuum energy is large enough to make our universe slowly accelerate. It will take about 10 billion more years for the universe to double in size, and if this expansion continues, in about 150 billion years all distant galaxies will forever disappear from our view. This constitutes quite a change from our previous expectation that in the future we would see more and more. . . .
The possibility that the vacuum may have energy was discussed almost a century ago by Einstein, but then he discarded the idea. Particle physicists reintroduced it, but their best estimates of its density were far too large to be true. For a long time they tried to find a theory explaining why vacuum energy must be zero, but all such attempts failed. Explaining why it is not zero but incredibly, excruciatingly small is a much greater challenge.
There is an additional problem: As noted, vacuum energy is at present comparable with the average energy density of matter in the universe. In the past, the universe was small, and its vacuum energy was negligibly small compared to the energy density of normal matter. In the future, the universe will grow, and the density of normal matter will become exponentially small. Why do we live exactly at the time when the energy of empty space is comparable to the energy of normal matter?
Thirty years ago, well before the discovery of the energy of nothing, Steven Weinberg and several other scientists argued that observing a small value of the vacuum energy would not be too surprising: A universe with a large negative vacuum energy would collapse before life had a chance to emerge, whereas a large positive vacuum energy would not allow galaxies to form. Thus we could live only in a universe with a sufficiently small absolute value of vacuum energy. But this anthropic argument, by itself, was not sufficient. We used to think that all parameters of the theory of fundamental interactions, such as vacuum energy, are just numbers that are given to us and cannot change—which is why the vacuum energy was also called the cosmological constant. But if the vacuum energy is a true constant that cannot change, anthropic considerations cannot help.
The only known way to solve this problem is in the context of the theory of an inflationary multiverse and string-theory landscape, which claims that the universe consists of many parts with different properties and different values of vacuum energy. We can live only in those parts where the vacuum energy is small enough, which explains why the vacuum energy is so small in the part of the world in which we live.
Some people are critical of this way of thinking, but in the eighteen years since the discovery of the vacuum energy, nobody has come up with a convincing alternative solution to the problem. Many others are excited, including Steven Weinberg, who exclaimed, “Now we may be at a new turning point, a radical change in what we accept as a legitimate foundation for a physical theory!”*
This explanation of a small vacuum energy has an unexpected twist to it: According to this scenario, all vacua of our type are not stable but metastable. This means that in a distant future our vacuum will decay, destroying life as we know it in our part of the universe while re-creating it over and over again in other parts.
It is too early to say whether these conclusions are here to stay or will be significantly modified in the future. In any case, it is amazing that the news of a seemingly inconsequential discovery of an incredibly small energy of empty space may have enormous consequences for cosmology, string theory, scientific methodology, and even for our view of the ultimate fate of the universe.
The Big Bang Cannot Be What We Thought It Was
Paul J. Steinhardt
Theoretical physicist; director, Princeton Center for Theoretical Science, Prince
ton University; co-author (with Neil Turok), Endless Universe
Two years ago, the scientific community and the press trumpeted the claim by a team of scientists that they had found definitive proof that the universe began with a Big Bang followed by a period of accelerated expansion known as inflation. Their proof was that the light produced in the infant universe and collected by their detectors exhibited a distinctive pattern of polarization that could be explained only if the large-scale structure of the universe was set when the temperature and density of the universe were extraordinarily high, just as posited in the Big Bang inflationary picture.
Over the ensuing year, though, it became clear that the claim was a blunder. In searching for a cosmic signal from the distant universe, the team had not taken proper account of the polarization of light that occurred nearby when it passed through the dust in our Milky Way on the way to their detectors. The new claim from the team, published in recent months, is that there is no sign of the cosmic polarization they had been seeking despite an extensive search with extraordinarily sensitive detectors.
The retraction received considerable attention, but the full import of the news has not been appreciated: We now know that the Big Bang cannot be what we thought it was.
The prevailing view has been that the Big Bang was a violent, high-energy event, during which space, time, matter, and energy were suddenly created from nothing in a distorted, nonuniform distribution. To account for the undistorted nearly uniform universe we actually observe, many cosmologists hypothesize a period of rapid stretching (inflation) just after the Bang, when the concentration of energy and matter was still very high. If there were inflationary stretching only, the universe would become perfectly smooth, but there is always quantum physics in addition to stretching, and quantum physics resists perfect smoothness.
At the high concentrations of energy required for inflation, random quantum fluctuations keep generating bumps and wiggles in the shape of space and the distribution of matter and energy—irregularities that should remain when inflation ends. The quantum-generated irregularities should appear today as hot spots and cold spots in the pattern of light emanating from the early universe—the so-called cosmic background radiation. The hot and cold spots have indeed been observed and mapped in numerous experiments since the COBE satellite detected the first spatial variations in the cosmic-background-radiation temperature in 1992.
The problem is that when the concentration of energy is high, the quantum-generated distortions in space should modify the way light scatters from matter in the early universe and imprint a spiraling pattern of polarization across the cosmos. It was the detection of this spiraling pattern (referred to as B-mode) that was claimed as proof of the Big Bang inflationary picture and then retracted. The failure to detect the B-mode pattern means that there is something very wrong with the picture of a violent Big Bang followed by a period of high-energy-driven inflation. Whatever processes set the large-scale structure of the universe had to be a gentler, lower-energy process than has been supposed.
Simply lowering the energy concentration at which inflation starts, as some theorists have suggested, only leads to more trouble. This leaves more time after the Big Bang for the nonuniform distribution of matter and energy to drive the universe away from inflation. Starting inflation after the Big Bang and having enough inflation to smooth the universe becomes exponentially less likely as the energy concentration is lowered. The universe is more likely to emerge as too rough, too curved, too inhomogeneous compared to what we observe.
Something more radical is called for. Perhaps an improved understanding of quantum gravity will enable us to understand how the Big Bang and inflation can be discarded in favor of a gentler beginning. Or perhaps the Big Bang was actually a gentle bounce from a previous period of contraction to the current period of expansion. During a period of slow contraction, it is possible to smooth the distribution of space, matter, and energy and to create hot spots and cold spots without creating any B-modes at all.
As the news sinks in, scientists will need to rethink, depending on whether forthcoming more sensitive efforts to detect a B-mode pattern find anything at all. Whatever is found, our view of the Big Bang will be changed, and that is newsworthy.
Anomalies
Stephon H. Alexander
Theoretical physicist, Dartmouth College; author, The Jazz of Physics
These days, physics finds itself in a situation similar to that faced by physicists at the turn of the 20th century, just before the dawn of quantum mechanics and general relativity. In 1894, the codiscoverer of the constancy of the speed of light and the first American Nobel laureate in physics, Albert Michelson, stated, “It seems probable that most of the grand underlying principles [of physical science] have been firmly established.” Many, perhaps most, physicists at that time believed that the handful of experimental anomalies that seemed to violate those principles were minor details that would eventually be explained by the paradigm of classical physics. Within a generation, quantum mechanics and Einstein’s theory of relativity were invented and classical physics overthrown, in order to explain those minor experimental details.
In 2012, I spent a year on sabbatical at Princeton, at the invitation of David Spergel, one of the lead scientists of the WMAP [Wilkinson Microwave Anisotropy Probe] space satellite. WMAP had been designed to make the most precise measurements of the ripples in the cosmic microwave background radiation, the afterglow of the Big Bang—ripples that, according to our Standard Model of cosmology, would develop into the vast structures in our universe, to which there is remarkable agreement. Cosmic inflation, our best theory of the early universe, is consistent with the most precise physics known to us, general relativity and quantum field theory.
Despite the success of inflation in predicting the features observed by WMAP, nagging anomalies persist—just as was true at the end of the 19th century. My Princeton colleagues and I spent that year wrestling with those anomalies to no avail. David comforted me with the idea that the WMAP anomalies were probably due to an unaccounted-for experimental factor rather than some strange phenomenon in the sky. “If the anomalies persist with the Planck satellite,” he warned, “we will have to take them more seriously.” Well, in 2014 Planck made an even more precise measurement of the CMB anisotropies and presented us with arguably the most nagging in the suite of anomalies: the hemispherical anomaly.
What’s that?
The undulations in the CMB reflect a prediction from cosmic inflation that aside from those tiny waves, on the largest distance scales, the universe looks the same in every direction and at every vantage point in space. This prediction is consistent with one of the pillars of modern cosmology, the cosmological Copernican principle. During the epoch when the CMB anisotropies were formed, they too were supposed to (on average) look the same in every direction. The theory of cosmic inflation, in which rapid expansion of spacetime smooths out any large-scale directional preference while democratically sprinkling the spacetime fabric with the same amount of ripples in every direction, predicts this feature. This means that if one divides the sky into two arbitrary hemispheres, we should see the same statistical features of the anisotropies in both hemispheres.
However, both WMAP and Planck see a difference in the amount of anisotropies in the hemispheres. With some tweaking, it is possible to modify inflation to account for the anomaly, but this seems at odds with what inflation was invented for—to make the early universe smooth and allow for the tiny anisotropies that later become galaxies. One might think this would provide opportunity for alternate theories of the early universe, such as bouncing/cyclic cosmologies, to rise to the occasion and explain the anomaly, but so far there is no compelling explanation.
Recently, at the Large Hadron Collider in Geneva, the ATLAS and CMS experiments both reported an anomaly when protons collided on energy scales close to 1 trillion electron volts. The experiments saw the predicted production of elementary particles that the standard q
uantum field theory of elementary-particle interactions predicts, with the exception of an excess production of light. If this observation persists with statistical significance, we will need physics beyond our Standard Model to explain.
To me and my theoretical colleagues, both anomalies are a good thing. Will they lead to simple yet less-than-pretty fixes of our current Standard Models? Might one of our supertheories—supersymmetry, strings, loop quantum gravity, GUTs—come to the rescue? Could it be that the anomalies are connected in some yet unseen way? Maybe they will point us in an entirely unthought-of direction. Whatever the case, this is an exciting time to be a theorist.
Looking Where the Light Isn’t
Brian G. Keating
Professor, Department of Physics and the Center for Astrophysics and Space Sciences, UC San Diego
For decades, the search strategy of confining the hunt for your lost keys to beneath the streetlight has been employed both by drunks and neutrino hunters, with no keys in sight and few key insights.
In 1916, Einstein published his final general relativity paper. One hundred years later, using Einstein’s predictions, we are on the brink of “weighing” the last elementary particle whose mass is unknown. Isn’t this old news? Don’t we know all the fundamental particle masses already, after measuring the Higgs boson’s mass? Well, yes and no.
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